Abstract

Acknowledgements

Nomenclature

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Abbreviations

1

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3

1 IntroductionIn April 2007 the AHTS vessel Bourbon Dolphin, figure 1, capsized outside the Shetland Islands while working with the deployment of an anchor. Out of the crew of 15 persons only 7 could be rescued, the rest was either found dead or lost in the sea. As a consequence a commission was put together to find out the cause of the accident, analyse it and contingently propose appropriate measures. According to the commission there were several human errors involved in the process leading to the incident, but the capsizing was due to inadequate stability. The commission proposes several measures to prevent similar accidents in the future. Some of those measures are directly connected to the stability of AHTS vessels and the regulation system treating this aspect.

Current regulations are not designed to cover anchor handling operations, why the Norwegian Maritime Directorate investigates whether a new set of national rules regarding such vessels shall be implemented to prevent similar accidents in the future and to improve the safety of these vessels. The proposals from the commission lie as a basis for such contingent rules. However, the commission worked under a lot of time pressure, and further investigation is desirable to make sure the new rules are relevant and compliant with their purpose.

Simultaneously with the directorates investigation DNV wants to evaluate whether a new class notation are to be made specifically for AHTS vessels, or if an amendment to the existing tug notation is enough. This master thesis aims to validate the rules proposed by the commission and to further investigate the stability aspects of anchor handling operations. The purpose is to provide a decision basis for the outworking of new rules for AHTS vessels.

The course of event of the accident and information about Bourbon Dolphin are described in appendix A. Existing stability regulations are described in appendix B.

Figure 1 The Bourbon Dolphin

4

1.1 Anchor Handling Tug and Supply vesselAn AHTS vessel is typically between 50 and 90 meters long, with a breadth between 15 and 20 meters. The bollard pull varies from 65 tonnes and reaches up to 250 tonnes1. At the moment there are even larger vessels under construction with an expected bollard pull of up to 350 tonnes, reaching almost 100 meters in length and 25 meters in breadth.

Typically an AHTS has a high deckhouse in the stem with a high bow in front of it. The deck is low and located aft of the deckhouse, reaching approximately between 2/3 and 3/4 of the total length of the ship. The tugs are equipped with several winches with pulling capacities between 140 and 650 tonnes dependent of the size of the vessel. Consequently the ships can have a winching power that is far more powerful than its bollard pull. This could be a problem in the design process since it cannot be designed based on the bollard pull, which is a conventional design parameter. It is also equipped with one or two cranes, to move heavy equipment on the deck. All this equipment is mostly located right behind the deckhouse. The winches are located high above the deck, often with a KG between 10-18 meters above baseline. When full with wire and chain the total weight of this equipment will be heavy, giving the ship a high centre of gravity located fore amidships. In the aft part of the deck a stern roller is placed to lower the friction between the mooring line and the deck. The stern roller is either one unit or divided in two parts and reaches between 2-3 meters out from the longitudinal centreline. Most AHTS vessels have one or more main propellers, and one or several side thrusters. Some also have an azimuth propeller, which can raise the stated maximum bollard pull during short sessions when

necessary. The 360° rotatable azimuth propeller and the side thrusters increase the vessels manoeuvrability and the ability to maintain a course while subject to strong side currents. Figure 2 shows an AHTS with a layout typical for these vessels.

Figure 2 An AHTS vessel from Farstad Shipping. The layout is typical for these vessels with the high

deckhouse in the forward part of the vessel as well as the highly located winches. This particular vessel is

under construction and will be 88 m long, 21 m wide with a maximum winch power of 500 tonnes and a

bollard pull of 250 tonnes [1].

1 Figured based on a random selection of approximately 20 vessels classed as Tug and Supply vessels of

DNV.

5

1.2 Anchor-handlingAnchor handling is all kind of situations including an anchor. It could e.g. be anchoring a cruising ship only for a short period. It can also refer to anchoring some kind of installation in the water, an installation that is removable but must be made secure in one spot. The purpose of the anchor/anchors is to prevent the installation or boat to move in the water. In this report anchor-handling operations will only refer to anchoring floating platforms or other permanent installations. Information about different mooring systems can be found in appendix C.

1.2.1 Normal procedure of anchor handling operationsAnchor handling operations are not very standardized and before every operation a thorough plan has to be carried out by the operator. The plan should include both the anchor handling and the movement of the rig. The seabed, depth and required length of the mooring line etcetera are considered. All necessary equipment is chosen and respective vessel’s specific task is decided if there is more than one vessel involved which is common in these operations.

Deploying the anchorsThere are several ways to deploy the anchor. Common in all cases is that the AHTS hauls the mooring line from the rig to the anchor position. When the ship reaches the anchor spot the mooring line is connected to the anchor and it is lowered down to the seabed using the anchoring winch and a working wire with the length of about 1.5 times the water depth [2]. During this operation the ship can be exposed to very large forces from the mooring line, depending on how far away from the rig the anchor is to be deployed. Should the weather be bad with wind, waves and currents, the forces can be even greater and it is important that the ship has sufficient stability to handle the current situation. During the last part of the operation it is common that another AHTS vessel assists by grappling the mooring lines a few hundred meters from the operating ship to reduce the weight. According to [3] Bourbon Dolphin had an approximate speed of 0.25 knots while moving away from the rig.

Recovering the anchorsThe recovering of the anchor is basically a reversed procedure of the deploying procedure. The AHTS heaves out 1.5-2 [2] the water depths of working wire and tugs the anchor loose from the bottom of the sea [4]. When the anchor is clear from the seabed the rig starts to pull in the mooring line while the ship slowly winches up the anchor and reverses towards the rig.

6

2 ObjectivesRemovable rigs are placed where the gas and oil are, often in areas with unfriendly weather. Because of this AHTS-vessels must be able to operate in these conditions. When a tug handles heavy anchors and mooring lines the forces acting upon the craft can be very high. Bourbon Dolphin was not designed for the forces she was exposed to, and as a consequence she sank. As described above this thesis aims at providing a decision basis for a contingent new class notation or additions to the existing tug notation. This will be carried out by describing the mechanics involved in anchor handling in a lucid way. The following aspects are to be considered:

1. Determine which forces that are dimensioning for the ship’s stability when performing anchor-handling operations and choose the point of attack and lever arm for those.

2. Analyse the influence of propulsion when deciding upon dimensioning forces.3. Evaluate the commission’s suggestions for additional stability rules.4. Determine what stability parameters to be chosen as part of the dimensioning demands.5. Determine appropriate loading conditions to be included in the book of stability.6. How to handle the weather and the sea state in the dimensioning and how this should be

implemented in the set of regulations. Can the IMO Weather Criterion be modified to be suitable for considering the combined effects of the mooring line and waves?

7. Evaluate if KGMAX curves can be developed to be useful aboard the vessels.8. Evaluate other possible risks that should be taken into account when dimensioning these

vessels.

2.1 DispositionThe first part of the report, chapter 3 – 8, will deal with the first 5 points above. It will start with a description of the influential forces on the ship and the resulting heeling moment during anchor handling, chapter 3 and 4. It will also describe how they can be calculated and where they will affect the ship, i.e. their points of attack. The following chapter, 5, will give a short description of stability rules and how they are based on the ship’s GZ curve. It will also describe what a loading condition is and why it is important. Chapter 6 will in short render the part of the commission report dealing with stability and its conclusions. The following chapter, 7, will evaluate the commission’s suggestions, the heeling moment, importance of loading and the respective influential forces. Finally a discussion about the results will be held in chapter 8.

The second part of the report will deal with the last three points above. In chapter 9 the IMO weather criterion will be presented, and an evaluation will be carried out whether the criterion can be modified to analyse the combined effect of waves and the mooring line. In chapter 10 minimum GM and maximum KG curves will be described and their potential usefulness discussed. The eighth point will not be handled separately, but will be included in the final discussion of the last three points in chapter 11.

In the last part of the report suggestions on how to deal with the discussed aspects will be presented, chapter 12. Finally the whole master thesis will be concluded in chapter 13.

7

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3 Influential forces The first part of this chapter will describe the ships intact stability, which is the ships ability to withstand heeling moments. Thereafter the forces affecting the ship during anchor handling will be identified and their respective impact analysed. The purpose is to create a model that takes all major internal and external forces into consideration.

3.1 Surface ship intact stabilityThe intact stability of a ship can, a bit simplified, be described as its capability to straighten itself back into upright position when exposed to a disturbance. The lifting force of a ship is called buoyancy, B. When the ship is in upright position the action line of the buoyancy is the same as for that of the gravitational force, G. However, when the ship is exposed to a disturbance resulting in a heel, the hull shape makes B moving towards the heeling side of the ship. If the ship is correctly loaded and the cargo is secured G should be constant, resulting in the action lines of B and G running parallel instead. Figure 3 shows a principal sketch of a box shaped ship that has been exposed to a disturbance and consequently has a heeling angle, !.

Figure 3 A box shaped ship with a heel seen from behind. As the ship heels the buoyancy, B, will move

toward the heeling side of the ship creating a righting moment by coupling with G.

As can be seen in the figure B has been moved towards the deeper immersed side of the ship, and consequently changed the lifting force’s line of action so that it is no longer overlapping G. The two forces create a force couple with corresponding lever arms resulting in a righting moment. The lever arm is called GZ and is one of the most important components when considering stability. GZ is calculated as

(3.1)

8

The first half of the equation is dependent on the hull shape and the other half is dependent on G. The larger GZ is the larger the righting moment will be. As can be seen in the figure GZ will grow with increased heel due to B being moved further away sideways from G. If the heeling grows too large, however, B will start to move back and eventually cross G’s action line and consequently create an added heeling moment instead of a counteracting moment. How much the ship can heel and still attain a positive counteracting moment is dependent on the hull design, which varies dependent on the purpose the ship is meant to serve.

The metacentre, MC, is the point where the action line of B crosses the ships centreline, as can be seen in figure 3. Dependent on the heeling angle the position of MC will vary but for small

angles, ! < 10˚, it is typically considered constant [5]. The distance between G and MC is called metacentric height, GM. As can be seen in the figure a positive GM will always give a positive GZ resulting in a moment counteracting any heeling moments. Since MC can be considered constant for small heeling angles, so can GM, which is then called the initial metacentric height, GM0. Thus for small angles, ! < 10˚

(3.2)

which is constant. Furthermore for small angles

(3.3)

Consequently GZ for small angles can be calculated as

(3.4)

Thus there is a linear relationship between GZ and the heeling angle for small angles. The larger GM0 is the larger the initial stability will be. Since all vessels contain liquids there is a free surface movement that has to be considered. It will not be explained here, since it is not necessary for the analysis. However, hereafter GM0 will be referred to as GM and will then be corrected for free surfaces.

Obviously the initial stability is highly dependent on where G is located. From a stability point of view an important measurement of G is the vertical distance from the keel, KG. A low KG will give more stability than a high value, which is demonstrated by (3.1).

3.2 Internal forcesThe following sections will describe the forces that occur from within the ships boundaries, i.e. forces that are not directly connected to the ship’s external environment.

3.2.1 Mooring line The mooring line is heavy and long and therefore most definitely has an influence on the ship. The force is not constant though, but will increase the more line that is paid out. The mooring line will influence the ship in several different aspects like trim and heeling. It will also act as a backward force, increasing the demand on the ships available bollard pull. More information about different types of mooring line can be found in appendix C.

Effective componentsFigure 4 illustrates the ship seen from the side, in the XZ-plane, where all essential forces, components and points of attack are marked in relation to G.

9

Figure 4 A principal sketch of the side of an AHTS vessel. The mooring line is running over the stern

roller with a vertical point of attack at the top of the stern roller. The dimensional relationships in the

figure are not consistent with the reality.

(3.5)

That is the total force from the mooring line in the three dimensional room. "XZ is in the XZ-plane, which gives

(3.6)

(3.7)

Figure 5 illustrates the ship from above, in the XY-plane, where the effective components can be further extracted. Again the figure also presents the essential points of attack and their respective relationship to G.

Figure 5 A principal sketch of an AHTS seen from above. As can be seen in the figure the vertical

component of the force from the mooring line, and the transverse horizontal effective component do not

have the same point of attack. The dimensional relationships in the figure are not consistent with the

reality.

Since the stern is flat and smooth the friction is neglected why there will be no transversal component acting on the stern roller. The force components can then be described as

10

(3.8)

(3.9)

Points of attackWith equation (3.5)-(3.9) the force components along the different axis are determined. However, the points of attack of the forces are not at the same spot, as seen in figure 5. Thus the following equations are added, where consideration is taken both to the direction of the effective component, as well as its point of attack.

(3.10)

where F1 is acting on the top of the stern roller at (xAFT, ySR zDECK). ySR is determined by

(3.11)

# can be assumed to be approximately the same as the yaw angle. ySR,MAX is at the outer edge of the stern roller.

(3.12)

where F2 is acting at (xWINCH, 0, zWINCH)

(3.13)

where F3 acts at (xTP, yTP, zDECK) and F4 at (xWINCH, 0, zWINCH). Thus FML,Y is acting at two different locations, meaning that it will be divided between these. However, as F4 is dependent on

F2 and $

(3.14)

it can be neglected if $ is small enough. $ can be determined as

(3.15)

Considering the towing pins are only some meter from the centre line and located in the aft part

of the ship, while the winches are commonly in the first 1/3 of the ship, $ can be expected to be very small and thereby F4 can be neglected. Thus

(3.16)

3.2.2 PropulsionThe propulsion system of the ship limits its bollard pull. Therefore the propulsion has a direct link to how large anchors and mooring lines the ship can handle. However, the maximum possible bollard pull is not always possible to attain due to use of side thrusters. The bollard pull is dependent on the available engine power as well as the propellers.

Bollard pullThe bollard pull is an indication of the maximum pulling force the ship can exert on another object or ship. It is commonly stated in tonnes, even though it is actually representing a force [6].

11

Hence all forces in this report are stated in tonnes. A ship’s bollard pull is determined by a test, which is normally carried out under the supervision of a Classification Society.

Since the bollard pull is equivalent with pulling force, efficient in the XY-plane, it can be directly connected to FML,X and FML,Y. The bollard pull test is performed with the propulsion pointed along the x-axis. Thus the maximum allowable force from the mooring line will be limited by the bollard pull according to

(3.17)

A loss of pulling power might result in the vessel being dragged backwards. AHTS vessels have very low sterns, which will be even lower due to the weight of the mooring line, and open decks. If the vessel is dragged backwards it might result in water flooding the deck. Naturally this will affect the ships stability and might in worst-case lead to a full capsize if the flooding is large enough. Since the flooding will push the aft deck further down the streaming water will create a negative lifting force, due to the flat deck acting as a lifting surface. A bad spiral is under way where more water on deck will result in even more water on deck [7]. This principle is illustrated in figure 6.

Figure 6 AHTS being pulled backwards. As the wave in the stern is growing there might be water on deck,

which will increase the displacement. Eventually the stern can be entirely submerged resulting in capsizing.

Side thrustThe side thrust is mainly used to counteract side drift during anchor handling operations. It is attained through the rudder, the side thrusters and the azimuth if there is one. The side thrust is efficient along the same action line as FML,Y. Studying blue prints of AHTS vessels shows that the side thrusters and the aft propeller generally is efficient at about the same level along the z-axis. Thus the side thrusters can mostly be assumed to act along a mutual line parallel to the x-axis, zTHRUST. A contingent azimuth will, however, be efficient at a lower height, zAZIMUTH.

There are no exact figures to be found about the magnitude of the side thrust and neither is there any test carried out to determine this. In DNV’s class notation of tugs the applied force from the side thrust is 60% of the maximum bollard pull [8], though without any specification of the reason. According to the bollard pull certificate of Bourbon Dolphin the use of azimuth will increase the total bollard pull approximately 10% [9].

3.3 External forcesThe following sections will describe the major external forces that the ship is exposed to, i.e. forces that origin from the ship’s environment.

12

3.3.1 WindAs the mooring lines have to run in all directions from the rig it is not always possible to avoid side winds. According to [10] the wind-induced force can be calculated as

(3.18)

The coefficient CD,AIR is commonly not known without experimental tests, but is generally in the order of magnitude between 0.5 – 1.5 [10]. VR,AIR is calculated as a function of VWIND, VSHIP,

and %XY,WIND according to

(3.19)

To calculate the projected area &WIND has to be calculated

(3.20) The projected area is calculated as

(3.21)

FWIND acts in the centre of AP,AIR with coordinates (xP,AIR, yP,AIR, zP,AIR).

3.3.2 CurrentsAs with wind it is sometimes unavoidable to sail with the current coming from the side. A side current will act both upon the exposed side of the immersed part of the hull and the mooring line attached to it. According to [11] the force can be calculated with the drag equation

(3.22)

The equation is basically the same as (3.18), but all parameters are calculated for water and the submerged part of the hull instead. VR,CURRENT can be calculated according to (3.19), where

VWIND is changed to VCURRENT and %XY,WIND is changed to %XY,CURRENT. In a similar way

&XY,CURRENT and AP,WATER can be calculated according to (3.20) – (3.21).

To calculate the force due to currents CD,WATER from (3.22) has to be estimated. In 1976 a study was carried out for the US Coast Guard to evaluate the stability criterion for towing vessels [12]. During this evaluation model tests were performed to determine CD;WATER for the vessels being dragged in different directions during towing. Figure 7 is from the report and shows CD,WATER, for small heeling angles, depending on where on the vessel the towing point is located. CD,WATER is in the figure called C1.

13

Figure 7 The graph illustrates the drag coefficient, CD,WATER, for tugs being dragged in different angles.

Dependent on where the longitudinal location of the tow pins is, the tug will be dragged in different

angles. The graph is based on experiments carried out on tugs by the U.S. Coast Guard during the

seventies. C1 is the same as CD,WATER.

CD,WATER should represent the situation where the vessel is being dragged sideways. The corresponding situation from the diagram above should be when the towing pin is somewhere in the middle of the ship since this is most likely to result in a strictly sideways dragging. Consequently CD;WATER is in this case assumed to be approximately 12.

FCURRENT acts in the centre of AP,WATER with coordinates (xP,WATER, yP,WATER, zP,WATER).

2 The error margin can be expected to be fairly high for this coefficient, since the model tests were actually

carried out with models of towing vessels and not AHTS vessels. It is also assumed to have a constant

value, even though the coefficient will alter with the relative direction between the ship and the current.

However, drag test to determine CD,WATER is normally carried out by dragging the ship from the front, and

not from the side. Consequently there is very limited information available about side drag test of any

kind. To make a more thorough calculation of the side forces model experiments or computer analysis for

AHTS vessels has to be done. Because of the complexity of the parameter, and the difficulty to get

relevant values, a simple and conservative approach is chosen with the highest value being constant

regardless of angle.

14

4 Heeling momentThe most important consequence of the large forces involved is the heeling moment. The magnitude of the heeling moment is dependent on the magnitude of the various force components effective in the YZ-plane and their respective points of attack.

To calculate the heeling moment a few assumptions has to be made. The magnitude of the force components from FML are considered constant in the YZ-plane, which is a plausible assumption considering the length of the mooring line in relationship to the ship. Further their respective points of attack are considered constant. The heeling moment will not be constant, but vary with the heel as a consequence of altered lever arms. The calculations below are primarily valid for fairly small heeling angles. Figure 8 illustrates a sketch of a ship with a heel seen from behind.

Figure 8 A sketch of an AHTS vessel seen from behind. The ship has a heel and the respective lever arms

are included. The dashed blue line illustrates where the water line would be without the heel. FCURRENT and

FWIND are pictured approximately at their presumed point of attack. The heel in the figure is exaggerated

to clarify the lever arms, as described above the model is mainly valid for relatively small heeling angles.

The lever arms to the force components from the mooring line can then be expressed as

(4.1)

15

(4.2)

Thus the mooring line’s contribution to the heeling moment can be calculated as

(4.3)

As the thrust will always act at the same distance from G its contribution to the heeling moment will be

(4.4)

Naturally wind and current will influence the heeling moment as well. However, the respective lever arms are somewhat more complicated to determine than those above since it is not obvious where the projected area centre is located. While the centre of the submerged projected area often can be assumed to be located at about half the draught the same simplification is harder to do on the area above the water. This is due to the fact that the projected area is hardly a rectangle seen from the side, which it more or less is under water. Consequently zP,AIR has to be calculated manually for every vessel.

zP,WATER will vary with the heeling angle as the projected area will change, which is due to the fact that the draught will change and not be the same on both sides on the ship. It is calculated as

(4.5)

where T0 is the initial draught, i.e. when the ship is in upright position. ZWL is assumed to be constant for small heeling angles. Naturally it can be wise to estimate whether it is a fair assumption to calculate it as half the draught.

The following equations will calculate their respective addition to the heeling moment.

(4.6)

(4.7)

Thus the total heeling moment is calculated as

(4.8)

Of course the direction of respective added moment has to be considered.

Angles

As can be seen M is dependent on three different angles, "XZ, #XY, and !YZ. As described before

#XY can approximately be considered the same as the yaw angle and is thus a known variable. However, this model is only valid as long as the mooring line is on the stern roller. Thus the

maximum #XY,MAX for which this calculations are valid can be expressed as

(4.9)

!YZ is always known, but as described before the model has a better accuracy for relatively small

heeling angles. However, "XZ is not as obvious to predict. To estimate "XZ with good accuracy a

16

quite complicated calculation has to be performed, where consideration has to be taken to different density of different parts of the chain, their respective length, the speed of the vessel etcetera. Even so currents or sudden waves might change the angle why a more conservative

approach is preferable. Thus "XZ will be estimated as the angle giving the largest possible MML. This can be done by differentiate (4.3)

(4.10)

which leads to

(4.11)

This is the "XZ that will be used through the rest of the report.

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5 Stability criteriaThe rules on ship stability is regulated in the Code on Intact Stability (IS Code. More information about the IS Code and the regulatory system in general can be found in appendix B). The IS Code is based on different criteria that the vessels needs to fulfil. There are general criteria applicable to all vessels exceeding a certain length, and specific criteria applicable for ships with a specified task. The different general criteria can be found in appendix B.

5.1 GZ curveThe stability criteria are generally based on the GZ curve since it describes the ship’s ability to restore from a heel. The following section will describe how the criteria are put into relation to the GZ curve. Figure 9 shows an example of a GZ curve of an example ship.

Figure 9 A GZ curve and added heeling moment. The curve is from Ship 4 with T = 5.6 m, Trim = -2.6

m (trim by the bow), KG = 6.1 m and all consumables are full. The added heeling moment is 1500

tonnes'm, which is converted (5.1) to a lever arm, 0.25 m, in the figure.

From this curve all necessary information whether the ship fulfils the criteria or not can be extracted. The continuous black curve is the GZ curve. The curve displays the maximum GZ value, h, and at what angle this occurs. These are both parameters that criteria are based on. So is the area below the curve. The area is also elucidated by the area curve, the dotted curve in the figure. The area describes the ship’s restoring potential energy at different heeling angles. The dash-dot line is the tangent of the GZ curve at its origin. From this line GM can be extracted, which is the height of the line at 1 radian (not seen in the picture). In this figure there has also been added an external heeling moment on the ship. It is converted to a lever arm that is equivalent to the GZ curve by the following equation3

(5.1)

An added heeling moment alters the equilibrium to the intersection between the GZ curve and the heeling moment lever arm. This will, obviously, reduce the area below the curve and consequently the potential energy at all angles. As today there is no criterion involving a heeling moment for AHTS vessels, in spite of the fact that there is a great chance they will be exposed to it due to the mooring line.

3 The equation is based on the assumption that the heeling moment is expressed in meter·tonnes. In SI-

units a subtraction of 1000·g has to be added.

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5.2 Loading conditionsAs the criteria are based on the GZ curve, it is basically dependent on the ship’s KG and GM as seen in (3.1). Thus the loading of the ship is essential, and also a factor that has to be considered when determining whether a specific ship fulfils the criteria or not.

Loading condition describes how the ship is loaded in terms of ballasting, cargo and consumables (fuel, drink water and oil etcetera) etcetera. It influences the ship’s draught and trim as well as KG and heeling. Every vessel has several standard loading conditions where ballasting and cargo are to correspond to a situation the ship is likely to encounter during normal operation. Each loading condition shall be fulfilled for both arriving condition and departing condition. Arriving condition means all consumables are to be calculated as 10% of maximum capacity and departing condition means 100%. All standard loading conditions are to be presented in a ship specific stability book that shall be onboard the ship at all times.

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6 The commission report

6.1 The commission’s conclusionsAfter the accident a commission was put together to investigate all the circumstances around the accident. The investigation resulted in several proposals for new rules regarding anchor handling. Many of those suggestions do not involve stability, why those will not be represented in this report. The following list summarizes those proposals having to do with stability and is more or less a direct translation of the commissions report [3]. The translated text is presented in italic text and where it is not obvious what is meant the author’s interpretation will be described.

• All loading conditions shall be calculated with 10% respective 100% bunker.

• All winches shall be full of the heaviest possible mooring line type.

• External4 force with the following characteristics:

o Vertical load: the full winch capacity shall be used between the outer towing pins. The winches only have full pull on first layer but as a matter of safety margin the winch is assumed to be fully loaded with chain at the same time. The lever arm shall be calculated from the centreline of the ship to the fore-edge of the stern roller and the vertical load with a point of attack at the top of it. During this vertical load the ship shall have a heeling angle corresponding with a GZ value of no more than 50% of maximum GZ.

This situation would correspond to the vessel performing a very heavy lift, or breaking the anchor from the bottom. It is supposed to be an extreme case where the mooring line is acting on the outer edge of the stern roller. It is to be implemented as a stability criterion formulated as where the lever arm curve intersects the GZ curve, the value shall not be larger than 50% of GZMAX. The calculation is to be carried out as

Criterion 1:

Where GZ = lML

(6.1)

Calculations:

(6.2)

(6.3)

(6.4)

(6.5)

4 In this report the mooring line is defined as an internal force, since it is acting within the ship’s

boundaries. The commission calls it an external force, but it is assumed that it is the same force that is

referred to.

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o Mooring line payout: While paying out the mooring line the maximum force from the mooring line shall be calculated. The maximum force shall be based on static as well as dynamic loads. This force shall be decomposed to a vertical force and a horizontal athwardships force. The lever arm for the horizontal component shall be counted from the height of the working deck by the towing pins to the centre of the forward thrust or the centre of the aft side thrust if this is deeper. The lever arm of the vertical component shall be calculated from the centre line of the ship to the fore-edge of the stern roller and with the vertical point of attack at the top of the stern roller. The mooring line shall have an angle of at least 25˚ from the ships centreline in the horizontal plane. The angle related to the vertical plane shall be calculated so that it gives the largest possible heeling moment. The force from the mooring line shall not result in a heeling moment that gives a lever arm larger than 50% of the maximum GZ. The maximum force from the calculations above will be the ships maximum capacity for these kinds of operations.

The second situation is more or less to correspond with the worst possible situation during an anchor handling operation. The mooring line has an unfavourable angle to the ship, both vertical and horizontal, and the thrust is directed to increase the heeling moment rather than to decrease it. The commission suggests that the angle between the longitudinal centreline of the

ship and the mooring line shall be at least 25°. There is no specific reason given

why it should be 25°. It is to be implemented as a stability criterion.

Criterion 2:

(6.6)

Calculation:

(6.7)

and equivalent to

(6.8)

Consequently FML,2 can be solved as

(6.9)

o If ballasting is necessary to attain the calculated maximum force this condition should be described as an instruction for ballasting during anchor handling operations.

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7 Evaluation of influential forces and the commission’s conclusions

This chapter will evaluate the impact of the various forces and the heeling moment. The analysis of the heeling moment and the force from the mooring line will emanate from the commission’s conclusions, while the focus otherwise will be on determining which forces that has the largest influence on the vessel’s stability and how they affect it. As it is not only external and internal forces that affect the ship’s stability, but also how it is loaded, an evaluation of the impact of loading will be presented as well.

7.1 Type ships The following four type ships will be used throughout the rest of this report when a ship is necessary of any reason. The vessels are existing ships DNV classed as tugs5, but will of confidentiality reasons not be mentioned by name. The tugs are chosen so that they represent several different sizes of AHTS vessels.

Table 1 The four type ships with relevant parameters to carry out the calculations necessary for the

analysis in this chapter.

Ship LPP b D T ! BP FWINCH,MA

XmML,MAXLCGWINCHKGWINCH ySR yTP

1 51.5 15.0 5.5 4.7 2700 69 150 300 33.0 7.50 2.0 0.32 60.5 15.5 7.0 6.0 4300 143 300 400 39.4 13.5 2.5 1.53 68.2 17.2 8.3 6.8 5900 185 400 438 39.8 14.3 3.0 1.64 76.3 18.0 8.0 6.6 6800 237 500 600 37.1 11.2 3.0 1.3

As can be seen in table 1 no vessel has a bollard pull that is more than 50% of the maximum winch pulling capacity.

7.2 Heeling momentAs the heeling moment is so important when considering the stability of the vessels it is essential that it is handled correctly in the regulation system, and presented as such a realistic value as possible. The following section will present the effect on the vessels of calculating the heeling moment as presented by the commission. It will also evaluate what effect the heel will have on the total heeling moment, i.e. if it will influence the ship’s capability of handling the mooring line in any way. Finally the influence of the loading condition on the ship’s ability to handle the heeling moment will be analysed.

7.2.1 Criterion 1To evaluate the first criterion suggested by the commission the four test vessels’ GZMAX are compared with the lever arm corresponding to the calculated heeling moment according to (6.2) – (6.5). The results are presented for arriving condition in figure 10 and departing condition in figure 12. The loading conditions and all exact values can be found in appendix D.

5 Particulars from DNV Exchange

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Figure 10 The first bar is representing GZMAX and shall, for the criterion being fulfilled, be at least twice

as high as the other bar for each vessel. The second bar represents the lever arm calculated as suggested by

the commission. The calculations are done for arriving condition.

As can be seen only Ship 1 fulfils the criterion. Ship 2 and 3 would capsize during the present condition, since the lML is actually greater than GZMAX. Ship 4 will not capsize, but neither will it fulfil the criterion, since the heeling angle will be larger than the one corresponding to GZMAX/2. The bars presented in figure 10 are based on a constant heeling moment, i.e. no consideration is taken to altered heeling moment due to heel. Figure 11 illustrates the corresponding heeling moments calculated according to (4.3).

Figure 11 The heeling moment calculated as suggested by the commission suggests but with consideration

taken to heel. The curves are calculated for arriving conditions.

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As can be seen in the figure the heeling moments will vary a great deal due to the heel. It can either be a strict declining curve, or it can start to incline to a certain degree and then decline. Either way it will have impact on the criterion suggested by the commission. As the lever arm

curve is likely to intersect the GZ curve somewhere around 5° - 20°6 that is the most interesting interval to study. Translated to the bars in figure 10 this will result in the second bar being shorter for Ship 2 and 4, and longer for Ship 1 and 3.

Figure 12 shows the corresponding bars from figure 10 for departing condition.

Figure 12 The first bar is representing GZMAX and shall, for the criterion being fulfilled, be at least twice

as high as the other bar for each vessel. The second bar represents the lever arm calculated as suggested by

the commission. The calculations are done for departing condition.

As can be seen in figure 12 Ship 2 - 4 are closer to fulfil the criterion for departing condition than for arriving condition. Ship 1 will have a smaller marginal than for arriving condition, but will still fulfil the criterion with good marginal. For no ship lML will actually be greater than GZMAX, as for arriving condition. Just as for arriving conditions no consideration is taken to heel. Figure 13 presents the respective heeling moment curves for the ships in departing conditions, calculated according to (4.3).

6 Studying several GZ curves indicates that this is a common interval for the curves to intersect. It is not a

given fact though, why every curve has to be studied individually.

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Figure 13 The heeling moment calculated as suggested by the commission suggests but with consideration

taken to heel. The curves are calculated for departing conditions.

Just as for arriving conditions the heeling moments will vary a great deal due to the heel. For Ship 2 – 4 the shape of the curve will be the same as for arriving conditions, while Ship 1 will, for departing condition, have a strict declining curve instead. Consequently the second bar in figure 12 will be lowered for Ship 1, 2 and 4, where Ship 4 can actually be expected to pass the criterion should the heeling be considered. Ship 3, whose lML is almost exactly as long as GZMAX in figure 12 would probably capsize since the heeling moment will increase while the ship heels.

7.2.2 Criterion 2The second criterion suggested by the commission is supposed to represent a more dynamic situation, where the mooring line is not running vertically between the ship and the bottom but has an angle to the ship. It is supposed to determine the largest allowable force from the mooring line that the ship will be able to handle no matter its direction in comparison to the ship. To determine FML,2 (6.7) – (6.9) are calculated so that (6.6) is fulfilled. The results are presented in table 2. The loading conditions and more specific values are found in appendix D.

Table 2 The table presents the maximum allowable force the ships are allowed to handle calculated

according to the commission’s second criterion.

ShipFML,2

Arriving conditionFML,2

Departing condition

1 150 150

2 83 128

3 112 185

4 240 400

As can be seen in the table all but Ship 1 will have a considerably lower capacity to handle forces than their winches allows. The forces are based on the heeling moment being constant, as

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suggested by the commission. Figure 14 and 15 presents MML,2 for each ship, where consideration is taken to the heel and the initial value is the one required by the commission.

Figure 14 The heeling moment calculated as suggested in the second criterion where consideration to heel

is added. The calculations are for arriving condition.

As can be seen in the figure all vessels but Ship 1 has a strict declining curve, indicating that they would actually be able to handle a slightly greater force and still fulfil the criterion. The difference, however, are fairly small why the importance of the heel can be considered quite small in this case.

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Figure 15 The heeling moment calculated as suggested in the second criterion where consideration to heel

is added. The calculations are for departing condition.

Again all but one vessel have a strict declining curve but with small variations. However, all cases where the heeling moment curve is declining means that the vessel would be able to handle a greater force and still fulfil the requirement, since the lever arm will be shorter where the curve intersects the GZ curve.

7.2.3 LoadingAccording to the commission the heeling moment tests above are to be carried out with the ships’ winches fully loaded with mooring line. Naturally this will influence the ship’s possibility of passing the criterion, since it affects KG and thereby GZ (see equation (3.1)). Table 4 presents how the load on the winches affects KG.

Table 3 The table presents how KG varies for the vessels dependent on whether the winches are full of

mooring line or empty. KG is presented both for arriving and departing conditions.

ShipKG for arriving condition KG for departing condition

Full winches Empty winches Full winches Empty winches

1 4.2 3.8 4.5 4.0

2 6.9 5.9 6.0 5.1

3 7.3 6.5 6.5 5.8

4 7.7 7.2 6.5 6.1

As can be seen in table 3 KG will be reduced between 0.4 – 1.0 m for all vessels in both conditions with empty instead of full winches. Assuming that GZMAX will occur somewhere in

the interval7 20° – 30° heel it will increase between 0.14 – 0.5 m (Equation (3.1). It is assumed

7 Based on several GZ curves for the ships in different loading conditions.

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that KG will be located along the ship’s centreline why TCG is 0). Studying figure 10 and 12 it can easily be understood that this might very well be the difference between fail and pass for the ships.

7.3 Remaining forcesThe following section will analyse the main effects of the remaining forces presented in chapter 3. Since the mooring line is integrated in the heeling moment, and thus evaluated above, there will be no further analysis of the mooring line. Even though all forces will influence the heeling moment they might also affect the ship in other ways, which will be presented and analysed here.

7.3.1 Propulsion and currentThe forces due to currents and the propulsion are closely linked and will consequently be analysed together. The forces will be presented in two sections, first the actual force and its potential magnitude and then how the assumed force will influence the heeling moment.

ForceAs soon as there are side currents use of the side thrusters are likely to be necessary. Figure 16 presents the force, calculated according to (3.18) – (3.21), for three different current speeds and the relationship to the available bollard pull. The ships’ speed is approximated to 0.25 knots and the value represents the side force for the worst possible angle between the ship and the current. In appendix D a more thorough presentation of the side force for different angles are presented, as well as for a higher VSHIP. The respective draught is assumed to be design draught and the trim zero.

Figure 16 The blue bars in the bar chart illustrate the respective vessels maximum bollard pull. The three

other colours represents the side force on the hull due to three different current speeds.

The results in the figure are naturally dependent on the loading condition of the ship. It does, however, indicate that the necessary side thrust will make out quite a large part of the total bollard pull for all four vessels at high current speed. At 2 m/s Ship 2 – 4 will need a side thrust of at least 50 % of the available bollard pull. For Ship 1 a current speed of 2 m/s would require 75 % of the total available bollard pull to its side thrusters. Since maximum bollard pull is synonymous to full use of the ships machinery, use of the side thrusters will reduce the available bollard pull. Thus the ships manoeuvrability with heavy weights will be significantly reduced.

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When BD used 100% of the side thrusters the bollard pull was reduced from 180 tonnes to 125 tonnes [3]. Even though that does not mean that all vessels have the same reduction in available bollard pull due to use of side thrusters, it indicates that the difference might be distinct.

Heeling momentFCURRENT and FTHRUST can mostly be assumed to counteract each other since the side thrust is mainly used to prevent side drift. Therefore the resultant of the two forces can be assumed to be significantly smaller than the forces calculated in 7.3.1. FCOMBINED can be calculated as

(7.1)

Consequently their respective contributions to the heeling moment can also be assumed to counteract so that MCOMBINED can be calculated as

(7.2)

The resulting heeling moments from the heeling moments calculated according to (7.2) are presented in figure 17. FTHRSUT is for all cases equal to and directed opposite to FCURRENT. Their respective values and lever arms are presented more thoroughly in appendix D. Since MCURRENT is not a constant value, but a variable dependent on the heeling angle, a mean value corresponding to 10° heeling is chosen for the calculation. How it varies for small heeling angles can be seen in appendix D. MTHRUST is constant and independent on the heeling.

Figure 17 The resulting heeling moment from the current and the thrust for three different current speeds

for the four test vessels. A positive value means the thrust induced heeling moment is larger than the

current induced.

As can be seen in figure 17 all situations for all ships result in a positive moment, meaning the thrust induced heeling moment is larger than the current induced. This is due to the fact that zTHRUST will always be longer than zP,WATER which is assumed to be half the draught for all heeling angles. Noticeable is that the heeling moment presented in this figure is unavoidable when there are side currents, since the alternative is to simple let the ship drift, which obviously is not an option. Thus this heeling moment will always occur when there are side currents.

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7.3.2 WindForceFWIND is calculated according to (3.18) – (3.21). However, as FWIND is not expected to be important at low wind speeds, relevant VWIND is so much higher than VSHIP so that VR,AIR can be

assumed equal to VWIND. Thus the largest FWIND will occur when & is approximately 90°. This is proven in appendix D. Figure 18 presents FWIND for three different wind speeds, 10 m/s, 20 m/s and 30 m/s.

Figure 18 FWIND attacking from the side for three different wind speeds on the four test vessels.

As can be seen FWIND at 30 m/s is comparable to FCURRENT at 1.5 m/s. Even though the wind by itself only corresponds to a small part of the maximum bollard pull, the combined effect of the wind and the current will make out a large part of it. Wind speeds of up to 20 m/s results in small side forces compared to the bollard pull, and can thus be considered insignificant from a side drift point of view.

Heeling momentJust as the current induced moment the wind induced heeling moment is dependent on the heeling angle. Figure 19 illustrates the moment curves for the four test vessels calculated with the three wind speeds from figure 17. Again the curves are presented up to 20° since the relevance is considered highest at low heeling angles.

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Figure 19 The wind induced heeling moment on the four test vessels for three wind speeds. The graphs

are presented as a function of the heeling angle, between 0° - 20°.

As can be seen in figure 19 the variations in the heeling moments due to heeling are quite small for such small heeling angles. However, the heeling moments, if VWIND = 30 m/s, are comparable with the resulting heeling moment from the current and the thrusters and thus not negligible.

Naturally all heeling moment curves in this analysis can easily be transformed to lever arm curves by using (5.1).

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8 Discussion part 1

8.1 Heeling momentNo matter how one calculates or models the situation around the ship the heeling moment must be considered. It is the heeling moment that will cause the ship to capsize, even though other aspects might reduce the ships capacity to respond to the heeling moment. Therefore the heeling moment is the most important aspect to consider when creating a stability rule for AHTS. Since it in a contingent rule must be represented by some kind of maximum value, it is important that the heeling moment is as realistic as possible. Too conservative and the working capacity of the vessel will be reduced below its actual potential, and the risk is ships will be over dimensioned. Too soft and the ship will be at risk of capsizing.

The tests of the commission’s suggested criteria in the previous chapter resulted in three of the four vessels failing Criterion 1. Criterion 2 showed that two of the vessels would only be able to operate with mooring lines far lighter than their maximum winch pull force. No consideration is taken to heel even though it apparently will influence the heeling moment. The commission suggests that the heeling moment, in Criterion 2, shall be calculated with all force components interacting to create the moment. That is indeed a worst case scenario and should be possible to avoided through cautious planning, i.e. make sure the mooring line is always resting on a towing pin on the down stream side. It is also worth mentioning that the heeling moment due to the thrust will likely be reduced due to the currents that brought the ship to use the side thrust in the first place. Thus the heeling moment will not be as large as suggested by the commission.

8.2 LoadingThe commission requires that all calculations shall be carried out with the winches full of the heaviest possible mooring line, in spite of the fact that the winches will not be able to pull their maximum force if full with mooring line. FWINCH,MAX is only for the first layer of mooring line, and the force will be reduced as the winch is filled up. It is thus questionable whether this is a relevant situation for these vessels at all. To attain the kind of forces the winches are constructed for there has to be a lot of mooring line paid out and already in the sea. Consequently the ship is likely to be positioned on, or close to, the anchoring point. Thus there will be no need for a lot of additional mooring line being loaded on the ship at this moment. As could be seen in the previous chapter the loading resulted in a significantly reduced ability to respond to a heeling moment. Consequently it is a very conservative suggestion and its consequences should be carefully considered before implementing it in a rule.

8.3 ForcesBollard pull and side thrustAs today the marine authorities or a classification society tests the ship’s available bollard-pull. At the same time no test is carried out to establish the maximum side thrust, or the reduction in bollard pull due to side thrusters and the winch system. The maximum bollard pull is based on 100% machinery being available for the forward thrusts. During normal anchor handling the winches are in constant use, why part of the available power will be used for those. Consequently the maximum bollard pull will basically never be available during anchor handling.

Equally important is the available side thrust. Not only because it will reduce the bollard pull, but also because the side thrust is of outermost importance when the ship is exposed to side currents. There where strong side currents when Bourbon Dolphin sank. It was the side current that forced her off course, and further created the increased angle between the mooring line and the ships centreline. Probably the whole situation could have been avoided if the side thrusts would have been powerful enough, or would it have been stated that the ship did not have sufficient power for operating in such weather condition. Neither were there any accurate measurements of the currents at the time according to [3].

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WindAs shown before wind can result in forces too large to be ignored. However it would be pointless to require all vessels being able to handle all kinds of forces while carrying out anchor handling operations. Naturally the wind may not be neglected, but neither should it limit the ship’s capability unless it is necessary, i.e. if the accurate weather situation is windy. Thus the wind shall be considered in the planning stage when the current weather situation is known.

8.4 Stability criteriaThere is nothing in the analysis performed in this report that indicates the suggestion of GZMAX

> 2'lML where they intersect should be bad. However, as the lML curves will vary with the heel, this requirement might not be enough. Even if the ship fulfils this requirement, nothing is said about how the lML curve continues. Dependent on the shape of the curve the remaining potential energy, the area between the GZ curve and the lML curve, might differ much from that should the lML be constant. Thus it might be wise to implement an area criterion as well.

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-".+&0

9 WeatherThe weather will most definitely influence on the ships stability. An external effect on the ship that has not previously been discussed is waves. Waves can e.g. create sudden changes in the heeling angle or alter the direction of the ship. Because of the periodicity and the shape of the waves they will also constantly vary the ships displacement and accordingly its centre of buoyancy and trim. One important thing to consider is the combined effect of waves and a heeling moment due to the mooring line.

To describe the effect of waves is very complex, and is not made easier by adding the heeling moment due to the mooring line. Consequently the most practical way to do this would be to use a recognized method to describe ships motions in waves. The IMO Weather Criterion is an example of such a method. The weather criterion describes the vessels ability to withstand the combined effect of beam wind and waves.

9.1 IMO Weather CriterionThe severe wind and rolling criterion describes a ships ability to withstand the combined effects of beam wind and rolling. As for the entire IS code it is applicable to all vessels with a length of 24 m or more, unless otherwise stated [13]. Its purpose is to ensure that a ship is able to withstand a predefined wind gust while rolling due to a beam wave and constant wind. The assumed scenario is when all those parameters interact to heel the ship in the same direction.

The principle of the criterion is to measure the ships ability to restore to its equilibrium when exposed to the disturbance mentioned above. It is basically an energy balance, where the total amount of energy used to roll the ship from windward to leeward shall not exceed the ships remaining potential energy. The principle is illustrated in figure 20, which can help give a better understanding of the criterion, its principle and how it can be modified to serve the purpose described above.

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Figure 20 The weather criterion is based on the curve illustrates in the picture. The principle is that the

ship is assumed to have an initial heel, (0 due to a steady wind represented by a lever arm, lw1, of the

resulting heeling moment. The ship is then rolling windward due to waves to (1. Finally the ship rolls back

with the combined effect of the waves and a wind gust represented by the lever arm, lw2, of the resulting

heeling moment. (2 is the maximum allowable angle the ship may roll back to, which is either 50°, angle of

down flooding or a major opening, which ever is less. The criterion is that b shall be greater or equal to a.

The curve in figure 20 is the GZ curve. In the weather criterion the ship is assumed to have an initial heel, "0, because of a steady beam wind. This beam wind results in a heeling moment, which is represented by a lever arm, lw1. The equilibrium is where the heeling moment due to the wind is equal to the restoring moment of the buoyancy due to the submersed volume, represented by the GZ curve. Consequently lw1 is equal to GZ where the moments intersect and is calculated as

(9.1)

where P is the wind pressure, A is the projected lateral area of the ship above the water line, Z is the vertical distance from the centre of A to the centre of the underwater lateral area or at

approximately 50% of the draft, g is the gravitational constant and ) is the displaced water in tonnes.

From "0 the ship is assumed to roll towards the wind to "1 as the result of a beam wave. "1 is supposed to be the ships most vulnerable condition and is calculated as

(9.2)

The calculated rolling angle is based on a simplified nonlinear roll theory and takes several ship particulars into consideration. The first three parameters, X1, X2 and k, are damping coefficients dependent on the ships particulars. The first parameter, X1, is dependent on the beam and draught relation B/d according to table 4.

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Table 4 The table presents the damping parameter X1 dependent on the beam/draught relation.

B/d X1

* 2.4 1.0

2.5 0.98

2.6 0.96

2.7 0.95

2.8 0.93

2.9 0.91

3.0 0.90

3.1 0.88

3.2 0.86

3.4 0.82

+ 3.5 0.80

The second parameter, X2, is dependent on the ships block coefficient, CB, according to table 5.

Table 5 The table presents the damping parameter X2 dependent on the block coefficient.

CB X2

* 0.45 0.75

0.50 0.82

0.55 0.89

0.60 0.95

0.65 0.97

+ 0.70 1.00

The third damping coefficient, k, is determined by calculating the relationship between the

projected area of the bilge radius, AC, over the length – beam area, LWL'B. The following values shall be used for k:

k = 1.0 for round-bilged ships having no bilge or bar keels k = 0.7 for ships having sharp bilges k = as shown in table 8 for a ship having bilge keels, a bar keel or both.

Table 6 The table presents the damping coefficient, k, dependent on the shape of the keel.

k

0 1.0

1.0 0.98

1.5 0.95

2.0 0.88

2.5 0.79

3.0 0.74

3.5 0.72

+ 4.0 0.70

For all damping coefficient the intermediate values in the tables shall be determined by linear interpolation.

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The second two parameters in equation (9.2) consider the wave force and motion and the ships rolling period. r is the effective wave slope and s is the wave steepness factor. The effective wave slope is calculated as

(9.3)

where OG is the distance from the waterline to the centre of gravity calculated as

(9.4)

The wave steepness factor is dependent on the ships natural rolling period, Troll, which is given in equation (9.5)

(9.5)

GM is the metacentric height and C is a coefficient for the radius of gyration calculated as

(9.6)

The relationship between the steepness factor and the rolling period is presented in table 7.

Table 7 The steepness factor dependent on the rolling period of the vessel.

Troll s

* 6 0.100

7 0.098

8 0.093

12 0.065

14 0.053

16 0.044

18 0.038

+ 20 0.035

Just as with the damping coefficients the intermediate values shall be obtained by linear interpolation.

The next step is the combined effect of the wave roll back and a gust wind from "1 to "2. A lever arm, lw2, represents the gust wind, which is to be 50% larger than lw1

(9.7)

"2 or "c, whichever is smaller, is the limiting parameter in the criterion. When the ships rolls from "1 to the leeward side the heeling angle shall not exceed the limiting angle. The limiting angle is either 50°, down-flooding (openings in the hull) or the second intercept between the GZ curve and lw2.

As mentioned above the principle is an energy balance, where the energy of the total roll back shall not exceed the potential energy at the limiting angle. The energy is in figure 20 represented by the areas a and b, where a is the roll back energy and b is the potential energy. Consequently area b shall be larger than, or equal to, area a.

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The criterion is based on an extensive experimental research of how vessels with different hull shapes respond and roll in waves. It was developed by the Japanese and the Russians in the middle of the 20th century, both working on a national stability criterion which could later be merged into one more universal criterion.

9.2 Modified weather criterionThe purpose of a modified criterion is, as mentioned before, to predict the combined effect of the force from the mooring line and waves. The useful part of the weather criterion is the rolling prediction, since it is a recognized way of describing how a ship with certain particulars will roll in waves. The modification is basically the added moment, which in the original criterion are the steady wind and the wind gust. Instead of the wind induced moments a single mooring line moment is used. Since the rolling prediction in the criterion is entirely separated from the added moments no other alterations must be done.

The principle is simple, and more or less the same as with the original criterion. The only difference is that there will only be one moment, and it is due to the force from the mooring line instead of wind. The lever arm of the added moment is calculated according to (5.1).

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10 Limiting curvesA limiting curve is a curve that illustrates the highest or lowest value, for a certain parameter, at different draughts or trim for which a specific stability criterion is fulfilled. It can be either maximum KG or minimum GM, and it can be made for all stability criteria. A limiting curve can be very helpful from a loading perspective, since it gives a good and clear picture for what draught and trim the vessel will be able to handle the highest KG/lowest GM and still fulfil the requirement.

The curves could be made for either one criterion or for several criteria. Should it be several criteria the value of the curve will always be for the criterion resulting in the extreme value, i.e. maximum KG or minimum GM. The following part will describe a flowchart to create limiting curves for Ship 4.

1. Determine criterion. Here the chosen criterion is Criterion 1 suggested by the commission. It is chosen since it involves a heeling moment and is fairly simple to analyse. Due to the added heeling moment it will also be far more limiting than any existing stability criteria.

2. Determine heeling moment.

As FWINCH,MAX is 500 tonnes and ySR,MAX is 3 m the applied MML is 1500 tonnes'm.

3. Choose draught interval.Design draught for Ship 4 is 6.6 m. However, as anchor handling often do not result in such a vast draught, it is more likely when acting as a supply vessel that kind of draught is attained, the chosen interval is 5 m – 7 m.

4. Choose trim interval.The chosen interval is -2 m – 2 m where negative trim is trim by the bow.

5. Create curves.Iterating the current value, KG or GM, for every draught and trim until the criterion is exactly fulfilled, does this. In other words KG or GM is altered until GZMAX is exactly twice as large as lML where it intersects the GZ curve for the current draught – trim position. This is preferably carried out in a computer program such as NAPA where the iterations are carried out automatically.

Figure 21 illustrates the maximum KG curve for Ship 4 and figure 22 the minimum GM curve.

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Figure 21 The figure illustrates the limiting KG-curve for which Ship 4 still fulfils the commission’s

Criterion 1.

Figure 22 The figure illustrates the limiting GM-curve for which Ship 4 still fulfils the commission’s

Criterion 1.

A strong pattern can be seen in figure 21 and 22, where a small draught and trim by the bow is clearly preferable. Considering the high bow on these vessels this is quite understandable since

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there is so much displacing area in the front. However, too much trim by the bow and too little draught might result in reduced steering capacity, which naturally is very negative. Thus there has to be a balance between stability and drift when ballasting the ships. Nevertheless a curve like this is a very good basis for how to ballast the ship for anchor handling and the same analysis of the other test vessels shows the same pattern for them.

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11 Discussion part 2

11.1 Modified weather criterionEven though the weather criterion is an existing method of predicting the response of ships in waves it is under a lot of discussion at the moment. The discussion is about its validity on modern ships, while the criterion is based on statistics of ships from the first part of the 20th century. Consequently there are a lot of research and experimenting going on right now, to determine whether the criterion needs to be updated with more information about modern ship hulls [14] and [15]. Thus it is possible the weather criterion does not give a perfect prediction of an AHTS vessels response to waves. However, as long as the weather criterion is in use as it is now it must be considered just as good as any other method. Should the criterion be updated, so can the method being presented here.

Another aspect to consider here is that no analysis has been carried out of the effect of the criterion and when it should be used. To determine that more thorough studies has to be carried out, at which significant wave height the criterion is necessary and in what extent it shall be implemented in the regulatory system. The analysis presented in this report is merely to determine whether the modification could be carried out at all, i.e. analyse the criterion to see if the wind induced heeling moment can simply be replaced by a mooring line induced heeling moment. As it turned out there is nothing that implicates this could not be done. Since it is only relevant when waves occur, there is no reason to make it a general criterion.

11.2 Limiting curvesThe commission suggests that a KG curve for onboard use shall be mandatory for every vessel. It is not mentioned exactly how it should be done, but a possible variant would be to use the same kind of KG limit curves as described in the previous chapter. However, at the sea when the ship is exposed to a certain heeling moment there is not very much to do about KG even if the crew knows the situation is critical and KG instantly has to be lowered. It is therefore preferably to account for all possible scenarios in the planning stage, where the ship is to be loaded as advantageous as possible for the specific task. The use of such curves for the crew seems consequently rather limited.

An alternative is to implement a computer that constantly calculates the heeling moment as described in this report. The computer should be connected to all variables presented, ballast

condition, consumables, force from the mooring line, thrust and direction, currents, wind, KG, # and draught etcetera. By constantly comparing the heeling moment to the accurate GZ curve the crew would be constantly updated about the current risk for the ship. Even though the computer cannot replace papers it will provide a very good compliment.

11.3 Avoid over dimensioningWhen design criteria are created a lot of aspects have to be considered, they have to be fairly simple, easy to apply, be general and they shall serve a purpose. Three out of four tested vessels failed Criterion 1. Criterion 2 resulted in the same vessels having a significantly lower working range than they have winch capacity. Yet all three vessels are in operation, and have as today not capsized. Loading all winches full of mooring line during anchor handling operations is basically very bad loading, just as it would be bad loading to put all oil in the starboard tanks on an oil tank or all containers at the port side of a container vessel. Yet it is possible to do so, but it is not done. The regulation system does not have to, and cannot be, fail-safe. Thoroughly stressed remarks about acceptable loading of the winches are probably a far better and more dynamic way to go.

Another factor to consider is the over lapping of safety margins and its effect on the final result. The commissions’ suggestions are conservative in several ways. The moment is calculated for an extreme condition, the effective components from the mooring line are effective at the most unfavourable angles creating the highest possible heeling moment. In addition to this the loading

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condition of the ship is assumed to be unfavourable. On top of that the safety margin of the ratio between the moment and the GZ curve is 100%. The risk is that this will require very large ships for quite small operations. That is not only inefficient and expensive, but also bad for the environment since larger ships makes larger resistance in the water and therefore consumes more fuel.

As with all safety aspects there are always several aspects to consider. After a terrible accident, like the Bourbon Dolphin, it is always easy to think one-dimensional and see only to the perfect safety of the seamen. That is, of course, a natural way of responding to such an accident, and at large also a correct response. The final outcome, however, must take more aspects into consideration, where the safety of the ships has to be put in relation to the efficiency of the ship, its environmental friendliness, the total cost of the ship and a general realism of how a contingent rule can be formed.

In times of global warming and financial crises it is impossible not to mention those aspects here. Even though money can never be compared to human lives, it is a fact that companies, and consequently potential ship owners, are dependent on earning money. Over dimensioned ships are definitely more expensive to build and buy, and will also be more expensive in use. Of course this is not a reason to build unsafe ships, but things has to be put in relation to each other. If the rules are so restrictive so that those ships are getting financially indefensible costs has to be cut elsewhere and that could mean a downturn in the industry, with unemployed people as a consequence. Alternatively cost can be saved in other building or operating related aspects that are not regulated, meaning the safety could be reduced in another end instead. The result would then in fact be contra-productive.

An even more tangible consequence of over dimensioning is the environmental aspect. A larger vessel requires more fuel to operate, more material to build and in the end more disposals. Even though the aggregated emission from the worlds AHTS fleet is probably quite modest it can never be neglected. The ships are powered by diesel and thus contribute to the green house effect. Larger ships require larger engines, which results in more emissions. The increased cost, described above, can also result in a lowered interest from the buyers and builders to invest in more expansive environmental friendly technology.

The conclusion is not that new stability rules will automatically cause the scenarios described above. There naturally has to be regulations to guarantee the safety of the seamen working aboard these vessels. The conclusion is simply that all aspects have to be considered, and the regulations must stand in proportion of the potential risks.

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-".+&1

12 SuggestionsIt may be obvious that the best way to avoid capsizing is to avoid heeling. Thus it is preferable to prevent any situation that gives root to any great heeling moments. Consequently great care should be taken to preventively actions, which minimizes the risk of the ship being exposed to great heeling moments. Considering the large forces involved in anchor handling it will never be possible to fully avoid this, why the ships still need sufficient stability. It is, however, preferable to have a regulation system that both include stability criteria and mandatory operational procedures to avoid dangerous situations.

The following part will present some suggestions how the safety during these operations can be raised. It will include recommendations both on how to control the ships suitability and how to handle the vessel. The focus will, however, still be entirely on stability. Thus the operational recommendations will only be such that is directly connected to stability.

Heeling moment• The contingent largest heeling moment shall determine the largest allowable force from

the mooring line.• The heeling moments shall be calculated so that the heeling angle is accounted for.• FTHRUST,Y shall never to be pointed away from F3.

• The mooring line shall always run along the downstream side of the vessel so that at least one force component will always counteract the others considering the heeling moment.

Propulsion and currents• Establish the maximum side thrust and the remaining available bollard-pull at full use of

the side thrusts during the bollard pull test.• Mandatory analysis of the accurate sea state, including the direction and force of the

currents. The current situation shall also be constantly updated during the operation.

Loading• The maximum allowable heeling moment shall be determined for several different

loadings on the winches. E.g. for 0%, 25%, 50% and 75% full winches. • The ballasting should be based on limiting curves, such as presented earlier in this

report.

Wind• The wind induced heeling moment shall be added to the total heeling moment when the

wind exceeds a wind speed of 20 m/s.

Stability criteria• GZMAX > 2'lML shall be complemented by a criterion with a minimum area requirement.

Modified weather criterion• The modified weather criterion shall be considered when the weather is rough and the

significant wave height exceeds a predetermined limit. More analysis has to be carried out to determine this wave height.

Limiting curves• Requirements of onboard KGMAX curves are unnecessary if sufficient planning is

accomplished. • Recommend a computer that constantly calculates the stability situation of the ship.

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13 ConclusionsThe eight objective points can be concluded as

1. The major forces that will act upon the ship during anchor handling are FML, FTHRUST, FCURRENT

8 and FWIND. The largest force acting on the ship is the mooring line. Consequently it is the force from the mooring line that is the largest contribution to the contingent heeling moment the ship can be exposed to. However, all forces will affect the ship, and FCURRENT and FTHRUST,Y will always create a heeling moment.

2. Use of the side thrusters reduces the bollard pull, why the test of the available bollard pull should include a test of the available side thrust at the same time. No matter how good stability a ship has, the best thing is always to avoid extreme situations. With sufficient bollard pull and side thrusters, the ship has a lot better chance of avoiding situations where the heeling moment grows large since it can avoid side drift due to strong currents.

3. The commission’s suggestions are altogether rather conservative and should, as shown in this report, lead to several vessels not being allowed to carry out the kind of operations they are designed for. The loading requirement shall be more flexible and the heeling should be considered.

4. In addition to the suggestion of the commission about GZMAX > 2'lML there should be a criterion based on minimum area below the GZ curve. This is due to the fact that the shape of the moment curves can be very different and thus an area criterion is necessary to guarantee a certain remaining potential energy for the ship.

5. The loading of the ship has a great influence over its capability of handling heeling moments. Because of the large bow a trim by the bow is preferable, since it increases the ship’s ability to up-right itself from a heel. Further the ship has highly located winches, which results in a high KG when fully loaded. The high KG reduces the GZ curve, why care should be taken when loading the winches. There shall be special loading conditions for anchor handling, where the ballasting is based on limiting curves.

6. Wind and currents are to be added to the moment equation when necessary. Waves can be accounted for by altering the IMO weather criterion, where the moment due to the wind is exchanged to, or complemented with, the moment due to the force from the mooring line.

7. A requirement of KGMAX for onboard is regarded unnecessary as long as the operation is carefully planned. A computer that takes all kinds of forces and loads during anchoring operations should be recommended as standard equipment on those vessels.

8. By creating a non-flexible regulation system, e.g. regarding loading, the risk is great that the vessels will be over dimensioned in the future. There is also a risk that perfectly fit ships have to be taken out of operation, since they do not fulfil the requirements with fully loaded winches even though there is no necessity for this. Larger ships require more fuel and consequently are both more uneconomical and environmental unfriendly.

8 It has after the completion of this report come to the author’s understanding that the current and wind

will be implemented in the regulation as extra safety margins rather than directly added to the heeling

moment. This is partly due to the fact that zP,WATER and zP,WIND are to complicated to implemented in a

rule, and must therefore be even more simplified than suggested here to be useful.

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14 Appendixes

14.1 Appendix A – Bourbon DolphinBourbon Dolphin (BD) was a DP2 AHTS vessel built at Ulstein Verft in 2006. It was part of a three-ship contract of this type with the shipping company Bourbon Offshore. BD was built to perform anchor-handling operations, tug operations and to act as a supply ship in deep water. It was delivered to the shipping company in early October 2006 and was immediately put into operation. Table 8 presents BD’s particulars.

Table 8 Bourbon Dolphin’s particulars and maximum bollard pull respective winch power.

Ship Lpp (m) b (m) D (m) BP (tonnes) MWP (tonnes)Bourbon Dolphin 65 17 8 180 400

OperationsBefore the accident the ship had finished 16 working missions. At the end of March 2007 BD was contracted to help the oil company Chevron to move their semi-submersible deep-water drilling unit Transocean Rather. BD was involved in the operation as a main vessel as well as an assisting vessel. An assisting vessel is there to e.g. grapple the anchor chain when all chain is out and the anchor is about to be lowered into the sea. At this point of the operation there is a lot of heavy chain and wire hanging loose in the sea, resulting in very large forces on the rig and the ship operating the anchor. To ease the pressure on the rig and the operating vessel, the assisting vessel grapples the chain a few hundred of meters from the spot where the anchor is about to be lowered into the sea.

As a main vessel BD was involved in both recovering anchors as well as deploying anchors. It was during on of the latter operations, when the last anchor was to be deployed, that the accident happened.

The shipwreckAt approximately 9 am the 12th of April 2007 BD started its work with putting out the last anchor (#2) to steady the TR. The anchor was to be put out north/north west from the rig. According to different weather reports from this day the commission has concluded wind speed of 30-35 knots from the south west, significant wave heights around 3.5 meters and currents between 0.6-3 knots. The latter numbers are quite unsure, since no real measurement of the current was made at the time. The operation of putting out anchor #2 began with 84 mm chain from the rig being attached to the ships winch. Thereafter BD started heading towards the anchoring point and the rig started paying out the 914 m long chain. When all 914 m chain was out it was attached to the 76 mm chain aboard the ship. BD continued towards the anchoring point while paying out the rest of the 900 m chain from its own winch. The first part of the operation went smoothly, but as the first part of the chain was out BD started to drift towards the east. This drift worsened the more chain that was paid out, and at the time when all chain was out BD had almost drifted over to anchor line #3. An attempt was made by another vessel to grapple the chain and help BD coming on course again but it failed and almost led to a collision. With this much drift BD had started to heel approximately 5˚ to the port side due to the increased angle of attack between the chain and BD’s centreline. In spite of this fact BD steered to the west to get away from anchor #3, resulting in an even higher angle of attack for the chain. During the whole operation the anchor chain, weighing over 200 tonnes in the water, had been resting towards the inner starboard towing pin. At about the same time as BD started steering up west the inner starboard towing pin was lowered which caused the chain to drag over to outer port towing pin. This switched the transverse and vertical forces to the port side of the ship, at the same time as the angle of attack was increased as mentioned above. The consequence of this manoeuvre was fatal and the ship capsized in a matter of seconds [3].

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14.2 Appendix B - Existing rules on stability for AHTSIMOThe maritime sector is a very international industry, where a great deal of all sea transports is carried out between different countries. Thus most ships will spend a lot of their time at other jurisdictions than that of their own flag state. For this reason it is essential to have an international set of regulations for the shipping industry, so that the same rules applies for a specific ship regardless of what port it is about to enter. In 1959 the newly established UN organ International Maritime Organization, IMO, held its first meeting. IMO is the international organization that sets those rules. The main purpose is to “develop and maintain a comprehensive regulatory framework for shipping and its remit today includes safety, environmental concerns, legal matters, technical co-operation, maritime security and the efficiency of shipping”. Today 168 states are members of IMO and another three are associate members [16]. While IMO sets the regulations it is the individual member states national maritime government agencies that are responsible to make sure the regulations are obeyed by ships entering its territory. In Norway the Norwegian Maritime Directorate (NMD) regulates the rules for ships operating Norwegian waters. As most states are, Norway is part of IMO why the Norwegian set of regulations is primarily based on the various IMO conventions. Even though the IMO regulations are only minimum requirements, and the individual member states are free to apply stricter rules, it is praxis to use to follow IMO regulations. Because of Norway’s strong position as a maritime nation its ambition is to be a leading country when it comes to safe and environmental friendly shipping. Thus Norway is highly involved when it comes to developing new regulations and revising the existing ones [17].

IS CodeRequirements regarding stability are formulated by SLF in the Code on intact stability (IS Code). SLF is a subcommittee to SOLAS, which is the most important IMO convention. The criteria in the IS code are supposed to be based on “state-of-the-art” research available at the time [13]. Since ship design and experimental methods for research etcetera is constantly developing the code is to be periodically revised and updated. The SLF holds regular meetings where the member states participate and are free to raise questions of interest regarding general safety and stability. The issues discussed during a meeting might then be object for further investigation and research. This research is carried out and paid for by one, or several, of the member states. Since IMO is an UN organization, without any means of its own, it is totally dependent on this contribution from the member states to develop proper regulations. The regulations are based on the material presented at those meetings.

The existing IS code is only recommending, that is there is no demand from IMO that ships sailing under one of the member states flag must fulfil the criteria presented. However, a new code, which will be mandatory, is under development and will be in use soon [18].

Even though the IS Code is only recommended from IMO today, it is implemented and statutory in most member states maritime departments. To get classed by DNV, all parts of the code applicable to the specific ship are also required, why the code in praxis is basically already mandatory. The IS Code is applicable on all vessels above 24 meters of length unless otherwise stated [13], where the first part covers all vessels above this size. The second part is divided in several different parts, where more purpose specific requirements are presented. As today there is no part specific for AHTS vessels in the IS Code. The code does however have additional recommended criteria for offshore supply vessels, which is part of the tasks AHTS vessels are constructed for. These criteria do not in any way take anchor-handling operations into consideration. In the Norwegian building regulation there are requirements specific for AHTS vessels regarding the equipment in terms of winches, wire breaks, towing pins etcetera. There are also additional requirements for tugging operations where the vessel is exposed to a heeling moment due to the tugging [3]. There is no requirement from the NMD that this condition should be fulfilled when considering anchor-handling operations. It is common, however, that these vessels have a tug notation when classed by DNV. To be notated as a tug according to DNV rules the ship must fulfil certain special demands, among other things the requirement of a heeling moment, (B.1), due to towing [8]. The criteria required to be classed as a tug is presented in short below. For a more thorough explanation of the IS code please refer to [13]. All

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requirements are related to the GZ curve in different ways. For more information about the GZ curve and how it is related to the criteria please refer to chapter 5.

• The area under the GZ curve shall not be less than 0.055 meter-radians up to ! = 30°

and not less than 0.09 meter-radians up to ! = 40° or the angle of down flooding if it is

less. Additionally the area between the heeling angles 30° and 40°, or the angle of down flooding if it is less, shall not be less than 0.03 meter-radians

• GZ shall be at least 0.2 m at a heeling angle of 30° or more

• GZMAX shall occur at a heeling angle not less than 25°. Special practices may be applied at the approval of relevant maritime authority

• GM0 shall not be less than 0.15 m• IMO weather criterion, which is detailed described in chapter 5.4.1• The ship shall be able to handle a heeling moment corresponding to

(B.1)

where BP is the continuous bollard pull and a is the distance between the towing pin and the centre of the lowest thrust.

All four test vessels and BD are/was classed as tugs, and consequently fulfils the requirements presented above.

14.3 Appendix C - Mooring systemsGenerally there are 4 primary anchors and 4-8 secondary anchors when anchoring a platform. The weight of the anchors varies between 12-65 tonnes [4]. To couple these between the sea bed and the rig mooring lines are used. Basically there are three different types of mooring lines; chain, wire rope and synthetic fibre rope. Chain is the heaviest form of mooring line but is still widely used, often with wire rope added to the chain to lengthen the line without increasing the weight to a critical load for the rig [19]. Fibre rope is a lot lighter and therefore not relevant in this investigation since the tugs must be designed to handle all kinds of mooring lines. The length of the mooring lines is dependent of the weather as well as the water depth at the current anchoring spot, and there are no thumb rules for how much mooring line that is used for a specific depth. Every single operation is carefully analyzed and the amount of mooring line is based on this analyze. There are basically two ways to moor the platform, catenary mooring system and taut leg system [4].

Catenary mooring systemThe catenary mooring system typically consists of mooring lines running from the platform to the bottom at an angle and thereafter continues on the bottom for several hundred meters to the anchors. The anchors are mostly of the more conventional kind with a shank and flukes and can only hold horizontal forces. To achieve the drag force the mooring lines have to be long and heavy and run horizontally across the bottom. Figure 23 shows a sketch of a catenary mooring system.

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Figure 23 The mooring lines are hanging from the rig and then runs on the bottom to create the necessary

horizontal force. The heavy mooring lines ensures that all forces that will act upon the anchor will be

horizontal [4].

Taut leg mooring systemWhen using the taut leg system the mooring lines can run straight from the rig to the anchors, without any chain running over the bottom. The taut leg system has the ability to handle both horizontal and vertical forces. There are different anchor types to achieve this, e.g. suction anchor or vertical load anchor. Both penetrate into the soil of the bottom and can consequently withstand loads in all directions. Since the holding capacity is due to the penetration into the bottom there is no need for long heavy chains on the bottom and the mooring lines can be a lot shorter making the radius of the footprint9 much smaller. As a consequence the weights involved when using this system is a lot lower. Figure 24 shows how a taut leg system might look, with the image of the footprint for a catenary system as a comparison.

Figure 24 The mooring lines runs at an angle from the rig to the bottom where they are immediately

anchored to the bottom. The drag force form the anchors are achieved by sucking anchors [4].

9 The circle of anchors and mooring lines at the seabed.

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14.4 Appendix D - CalculationsThe following chapter presents the exact results and loading conditions to evaluate the forces and heeling moment.

14.4.1Commission criterion 1

Table 9 Loading condition and added heeling moment for calculating criterion 1 in arriving conditions.

Ship ! TACTUAL Trim KG zDECK zTHRUST GM MML,1 GZMAX lML Ratio

1 2260 4.2 -1.3 4.7 0.8 -5.0 2.2 300 0.94 0.13 7.2

2 3190 4.6 2.2 7.6 -0.6 -8.0 0.6 750 0.14 0.23 0.58

3 4770 5.5 1.8 7.4 0.9 -7.6 0.5 1200 0.15 0.25 0.59

4 4950 4.9 1.5 7.7 -0.9 -7.9 1.0 1500 0.42 0.30 1.4

Table 10 Loading condition and added heeling moment for calculating criterion 1 in departing conditions.

Ship ! TACTUAL Trim KG zDECK zTHRUST GM MML,1 GZMAX lML Ratio

1 2740 4.7 -0.3 4.4 1.1 -4.7 3.1 300 0.63 0.11 5.7

2 4160 5.7 -0.2 7.2 -0.2 -7.6 1.1 750 0.20 0.18 1.1

3 6190 6.9 0.7 6.6 1.7 -6.8 1.2 1200 0.19 0.19 1.0

4 6660 6.3 -0.3 6.7 -0.5 -6.9 1.7 1500 0.41 0.23 1.8

Negative trim is equivalent to trim by the bow. As can be seen in table 2 and 3 Ship 2 – 4 will be closer to fulfil the criterion in departing condition. This is due to the fact that KG will be lowered for all three vessels, resulting in a larger GZ. At the same time the displacement will increase, resulting in a smaller lever arm from the heeling moment. zTHRUST is an approximation where care has been taken both to the azimuth as well as the thrusters.

14.4.2Commission criterion 2

Table 11 Loading condition and added heeling moment for calculating criterion 2 in arriving conditions.

Ship ! TACTUAL Trim KG zDECK zTHRUST GM MML,2 FML,2 lML GZMAX

1 2250 4.1 -1.7 4.6 0.9 -4.9 1.8 459 150 0.20 0.90

2 3310 4.8 0.7 6.6 0.4 -7.0 0.6 321 83 0.08 0.16

3 4490 5.4 0 7.3 1.0 -7.5 0.1 515 112 0.10 0.20

4 4580 4.7 -0.6 7.7 0.3 -8.0 0.4 1067 240 0.18 0.36

Table 12 Loading condition and added heeling moment for calculating criterion 2 in departing conditions.

Ship ! TACTUAL Trim KG zDECK zTHRUST GM MML,2 FML,2 lMLGZMAX

1 2680 4.7 -0.8 4.3 1.2 -4.6 3.2 459 150 0.13 0.56

2 3940 5.5 -1.9 6.0 1.0 -6.4 0.9 496 128 0.10 0.20

3 5890 6.8 -0.8 6.5 1.8 -6.7 1.1 851 185 0.14 0.28

4 6360 6.2 -1.7 6.6 1.4 -6.9 1.5 1778 400 0.23 0.46

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14.4.3Current and propulsionForceFigure 25-28 presents the resulting side force due to currents for different current directions and three different current speeds; 1 m/s, 1.5 m/s and 2 m/s. The calculations are carried out according to (3.22) – (3.25) for two different VSHIP; 0.25 knots and 1.25 knots. Table 13 presents the loading conditions and projected area for the ships.

Table 13 The table presents the loading conditions for the ships in terms of draught, trim and KG. It also

presents the projected submerged area.

Ship T Trim KG AP,WATER

1 4.7 0 4.4 250

2 6.0 0 6.5 380

3 6.8 0 6.5 480

4 6.6 0 6.4 525

Figure 25 The figure presents FCURRENT from the side at different & for different VSHIP and VCURRENT.

These specific curves are for Ship 1.

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Figure 26 The figure presents FCURRENT from the side at different & for different VSHIP and VCURRENT.

These specific curves are for Ship 2.

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Figure 27 The figure presents FCURRENT from the side at different & for different VSHIP and VCURRENT.

These specific curves are for Ship 3.

Figure 28 The figure presents FCURRENT from the side at different & for different VSHIP and VCURRENT.

These specific curves are for Ship 4.

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As can be seen in the figures the largest side force will occur around 95° - 115° dependent on the speed of the vessel. Naturally the magnitude of the force is highly dependent on both VSHIP and VCURRENT.

Heeling momentFigure 29-32 presents the current induced heeling moment for the ships, calculated for the same current speeds as the forces above and with the vessel speed 0.25 knots. The heeling moment is calculated for the angle giving the highest force in the previous section. Table 14 presents the lever arms for both the current force and the thrust. Since zP,WATER alters with the heeling angle it is presented as an interval.

Table 14 The table presents the lever arms used to calculate the heeling moments presented below. zTHRUST

is a constant value, independent of the heeling angle, while zP,WATER will change due to altered heeling

angle. The presented interval represents the values at 0° of heel and 20°.

Ship zP,WATER zTHRUST

1 2.1 – 3.2 4.7

2 3.5 – 4.6 6.9

3 3.1 – 4.4 6.7

4 3.2 – 4.5 6.7

Figure 29 The heeling moment due to current with different speeds for Ship 1.

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Figure 30 The heeling moment due to current with different speeds for Ship 2.

Figure 31 The heeling moment due to current with different speeds for Ship 3.

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Figure 32 The heeling moment due to current with different speeds for Ship 4.

As can be seen in the figures the heeling moment will increase due to the heel, since zP,WATER will increase due to the deeper submerged hull side. As described in the report the model to calculated zP,WATER is primarily valid for small heeling angles, why the curves above are only

presented up to 20°.

Table 15 presents the corresponding heeling moments due to the thrust, where the thrust is assumed to have the exact same value as the side current force to counteract side drift. The thrust induced moment is independent on the heeling angle, why it is only presented as a single value for every current speed.

Table 15 The table presents the thrust induced heeling moment where the thrust is equal to the current

induced side force. The calculations are carried out with VSHIP being 0.25 knots and the direction between

the ship and the current being the one resulting in the highest possible side force.

ShipThrust induced heeling moment

Current speed 1 m/s Current speed 1.5 m/s Current speed 2 m/s

1 62 139 247

2 139 310 550

3 171 381 675

4 187 416 738

14.4.4WindForceFigure 33 - 36 illustrates how the maximum side force is influenced by the angle between the wind and the ship for the four test vessels. The tests are carried out for three VWIND, 10 m/s, 20 m/s and 30 m/s and for three VSHIP, 0.25 knots, 1.25 knots and 2.25 knots. The loading

56

conditions are the same as for the current calculations but with different projected areas. These areas are presented in table 16.

Table 16 The vessels’ respective projected area above the waterline.

Ship AP,WATER

1 310

2 470

3 565

4 600

Figure 33 The figure illustrates how FWIND is dependent on &. There are 9 curves in the figure, three of

each sort. The continuous curve illustrates the force at VWIND = 10 m/s, the dashed curve at VWIND = 20

m/s and the dashed-dotted curve at VWIND = 30 m/s. For each wind speed the force is calculated for three

different VSHIP, where black curve is the force when VSHIP = 0.25 knots, the red curve when VSHIP = 1.25

knots and the cyan curve when VSHIP = 2.25 knots. The curves represent the wind force on Ship 1.

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Figure 34 The figure illustrates how FWIND is dependent on &. There are 9 curves in the figure, three of

each sort. The continuous curve illustrates the force at VWIND = 10 m/s, the dashed curve at VWIND = 20

m/s and the dashed-dotted curve at VWIND = 30 m/s. For each wind speed the force is calculated for three

different VSHIP, where black curve is the force when VSHIP = 0.25 knots, the red curve when VSHIP = 1.25

knots and the cyan curve when VSHIP = 2.25 knots. The curves represent the wind force on Ship 2.

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Figure 35 The figure illustrates how FWIND is dependent on &. There are 9 curves in the figure, three of

each sort. The continuous curve illustrates the force at VWIND = 10 m/s, the dashed curve at VWIND = 20

m/s and the dashed-dotted curve at VWIND = 30 m/s. For each wind speed the force is calculated for three

different VSHIP, where black curve is the force when VSHIP = 0.25 knots, the red curve when VSHIP = 1.25

knots and the cyan curve when VSHIP = 2.25 knots. The curves represent the wind force on Ship 3.

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Figure 36 The figure illustrates how FWIND is dependent on &. There are 9 curves in the figure, three of

each sort. The continuous curve illustrates the force at VWIND = 10 m/s, the dashed curve at VWIND = 20

m/s and the dashed-dotted curve at VWIND = 30 m/s. For each wind speed the force is calculated for three

different VSHIP, where black curve is the force when VSHIP = 0.25 knots, the red curve when VSHIP = 1.25

knots and the cyan curve when VSHIP = 2.25 knots. The curves represent the wind force on Ship 4.

As can be seen in the four previous figures VSHIP is negligible, as the force maximum is more or less unchanged when the ship speed increases from 0.25 knots to 2.25 knots. It can also be seen

that the largest force occur when & is approximately 90°.

Heeling momentTo calculate the heeling moment the maximum force from the respective graphs above are used. Table 17 presents the assumed lever arms.

Table 17 The assumed zP,AIR for the four test vessels with the present loading condition from table 6.

Ship zP,AIR

1 4.5

2 4.4

3 5.7

4 5.6

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15 Reference list

[1] http://www.farstad.no/default.asp?menu=30&id=312 (2009-02-10)

[2] Wintermark, P., Bjønnes, P. & Lienes, V. (2002) M.A.D- Mannskap Av Dekk. Konseptutvikling for løsninger i tilknytning til ankerhåndtering og bedret sikkerhet for mannskapet om bord i fartøyene. Trondheim: Master thesis, NTNU

[3] NOU, Norges offentlige utredninger. (2008) Bourbon Dolphin forlis den 12. april 2007.

[4] Vryhof. (2005) Anchor Manual. The Netherlands

[5] Rosén, A. (2008) Hydrostatik, trim & tvärskeppsstabilitet. Stockholm, KTH

[6] Zahalka, Captain P. (2008) Bollard Pull. Association of Hanseatic Marine Underwriters. Germany

[7] Hancox, Captain M. T. (2008) Anchor handling vessel safety. Nautical Institute, London

[8] DNV Rules for Classification of Ships Pt.5 Ch.7 Sec.2

[9] DNV, Certificate of Bollard Pull. (2006) Bourbon Dolphin. Ulsteinvik

[10] Huss, M. (2007) Fartygs stabilitet . Stockholm: JURE förlag AB

[11] Karlsson, A. (2006) Formelsamling i strömningsmekanik. Stockholm, KTH

[12] Miller, E. R. & Ankudinov, V. (1976) Evaluation of current towing vessel stability criterion and proposed fishing vessel stability criteria. Department of transportation United States Coast Guard, Washington

[13] Resolution A.749(18) Code in Intact Stability for all types of ships covered by IMO instruments. IMO. London, Great Britain 1993

[14] Neves, M & Belenky, V. (2008) A Review of the Ninth International Conference on the Stability of Ships and Ocean Vehicles (STAB 2006). Marine Technology, Vol 45, No. 3 July 2008

[15] Larsson, A & Ribbe, G. (2008) Analysis of IMO Weather Criterion & Determination of a GM Limit Curve Based on a Model Test. Göteborg: Master thesis, Chalmers

[16] www.imo.org

[17] http://www.sjofartsdir.no/en/Legislation_and_International_Relations/ (2009-04-07)

[18] Adoption of the international code on intact stability. Draft MSC Resolution. IMO. London, Great Britain (2008)

[19] Captain on an AHTS vessel