SHIP STABILITY IN PRACTICE

JAN BABICZ

Consulting Naval Architect & Ship Surveyor

BAOBAB NAVAL CONSULTANCYwww.betterships.com

GDAŃSK 2011

Foreword

The main purpose of the present publication is first of all to make ship designers and operators realize that stability rules are not perfect and their fulfillment does not mean that the ship will be safe in all con-ditions. Secondly, to present Onboard Stability Documentation standard which enables better under-standing of ship stability and increasing its safety.

Each year new rules are developed aimed at increasing safety of ships’ operation. It concerns also the ship stability rules. During the 52nd session of IMO Sub-Committee on Stability and Load Lines and Fish-ing Vessels Safety (SLF52) held in January 2010, i.a., development of dynamic intact stability criteria and damage stability regulations for ro-ro passengers ships was discussed.

New rules always increase labour consumption related to a new ship design and number of documents to be approved. Unfortunately, it does not affect the quality of stability documentation which is prepared imperfectly due to the wrong assumption that in practice the master will use the stability instrument only. Very often the documentation is difficult to understand and of little practical value. It is no surprise that it is just put aside.

The solution which may lower costs of stability documentation preparation, increase its quality as well as practical value is Onboard Stability Documentation standard. Easy to understand and logical standard will significantly facilitate and accelerate documentation approval process in Classification Societies but above all it will make officers’ work easier and increase ship stability safety. Proposal of such a standard for dry cargo vessels is presented in Part II of the publication.

Part I includes basic knowledge on ship hydrostatics and stability, presentation of current rules require-ments concerning stability as well as information on ship’s behaviour in waves. Furthermore, it contains descriptions of stability accidents and proposals of design solutions which increase ship collision safety and survivability after damage.

The book is addressed to a wide circle of professionals involved in ship design, approval of stability calculations and documentations, survey and operation of ships. It should be of interest to ship design offices, ship manufacturers, shipping companies (owners and operators), education institutes and others concerned with ship stability. However, it does not replace any regulations.

Jan BabiczConsulting Naval Architect & Ship Surveyor

CONTENTS:FOrEwOrD ...........................................................................................................................................5

PART 1 SHIP HYDROSTATICS AND STABILITY .................................................................... 9

1. SHIP GEOMETRY .....................................................................................................................101.1 HULL FOrMS ...........................................................................................................................101.1.1 SHIP COOrDINATE SySTEM ..................................................................................................121.1.2 GrAPHIC DESCrIPTION OF HULL FOrMS ..........................................................................131.2 MAIN DIMENSIONS ..................................................................................................................151.3 COEFFICIENTS OF FOrM .......................................................................................................22

2. FLOTABILITY ...........................................................................................................................232.1 ArCHIMEDES’ PrINCIPLE AND SHIP EQUILIBrIUM ............................................................232.2 SHIP MASS AND CENTrE OF GrAVITy ................................................................................252.3 IN SErVICE INCLINING TEST SySTEM (ISITS) .....................................................................312.4 DEADwEIGHT AND CArGO DEADwEIGHT ..........................................................................322.5 MEAN THEOrETICAL DrAUGHT “dH” ....................................................................................342.6 DrAUGHT MArKS ...................................................................................................................362.7 FrEEBOArD AND LOAD LINE MArK .....................................................................................382.8 TErMS AND DIMENSIONS rELATED TO THE LOAD LINES CONVENTION .......................412.9 FrEEBOArD PLAN ..................................................................................................................482.10 CAPACITy PLAN ......................................................................................................................48

3. INTACT STABILITY ..................................................................................................................503.1 DEFINITION OF STABILITy .....................................................................................................503.2 INITIAL TrANSVErSE METACENTrE ”M” AND METACENTrIC HEIGHT “GM” ..................503.3 METACENTrIC DIAGrAM .......................................................................................................533.4 rIGHTING ArM GZ ..................................................................................................................543.5 CUrVE OF STATICAL STABILITy ............................................................................................553.6 CrOSS CUrVES OF STABILITy .............................................................................................583.7 LOLL ..........................................................................................................................................593.8 DESIGN FACTOrS AFFECTING THE SHAPE OF THE GZ CUrVE .......................................613.8.1 ANGLE OF DECK IMMErSION ................................................................................................613.8.2 DEPTH D ...................................................................................................................................653.8.3 MAxIMUM DrAUGHT ..............................................................................................................663.8.4 BrEADTH B ..............................................................................................................................673.9 OPErATIONAL FATOrS AFFECTING SHIP STABILITy .........................................................703.9.1 LIST ...........................................................................................................................................703.9.2 FrEE SUrFACE EFFECT ........................................................................................................713.9.3 HANGING LOADS .....................................................................................................................733.9.4 Dry BULK CArGOES ..............................................................................................................743.9.5 ICING .........................................................................................................................................743.10 ANGLES OF DOwNFLOODING ...............................................................................................763.11 wIND HEELING ArM ...............................................................................................................803.12 INTACT STABILITy CrITErIA ..................................................................................................823.13 MINIMUM OPErATIONAL METACENTrIC HEIGHT GMMIN CUrVE ......................................893.14 MAxIMUM PErMISSIBLE VCGMAx CUrVE ............................................................................933.15 STABILITy DUrING HEAVy LIFT OPErATIONS ....................................................................97

4. DANGEROUS SITUATIONS IN ADVERSE WEATHER AND SEA CONDITIONS ...............1024.0 INTrODUCTION .....................................................................................................................1024.1 wAVES PArAMETErS ..........................................................................................................1034.2 PHENOMENA OCCUrING IN FOLLOwING AND QUArTErING SEAS ..............................1044.2.1 SUrF-rIDING AND BrOACHING-TO PHENOMENON .......................................................1044.2.2 rEDUCTION OF STABILITy IN LONGITUDINAL wAVES .....................................................1054.3 SyNCHrONOUS rOLLING MOTION ....................................................................................1054.4 PArAMETrIC rOLL rESONANCE ......................................................................................1054.5 SUCCESSIVE HIGH-wAVE ATTACKS ...................................................................................1094.6 OTHEr HAZArDS AND rISKS ..............................................................................................109

5. CAPSIZING .............................................................................................................................1105.0 GENErAL ...............................................................................................................................1105.1 CASE STUDIES ...................................................................................................................... 1115.1.1 CAPSIZING OF THE CONTAINEr SHIP DONGEDIJK .......................................................... 1115.1.2 FOUNDErING OF KArIN CAT .............................................................................................. 1115.1.3 CAPSIZING OF THE ANCHOr HANDLING TUG STEVNS POwEr ....................................1125.1.4 CAPSIZING OF THE HEAVy LIFT SHIP STELLAMArE ........................................................1135.1.5 CAPSIZING OF THE GENErAL CArGO SHIP OMEr N ......................................................1135.1.6 CAPSIZING OF THE rO-rO SHIP FINNBIrCH ....................................................................1145.1.7 CAPSIZING OF THE rO-rO FrEIGHTEr rIVErDANCE ....................................................116

6. DAMAGE STABILITY .............................................................................................................1186.0 INTrODUCTION .....................................................................................................................1186.1 CASE STUDIES OF COLLISIONS AND GrOUNDINGS .......................................................1186.1.1 COLLISION OF THE SMALL COASTEr JOANNA wITH THE rO-rO FErry STENA NAUTICA ..1186.1.2 HEAD COLLISION BETwEEN SKAGErN AND SAMSKIP COUrIEr .................................1196.1.3 FOUNDErING OF THE 70,000DwT BULK CArrIEr FU SHAN HAI ...................................1206.1.4 rOCKNES - CAPSIZING AFTEr GrOUNDING ...................................................................1226.2 HULL wATErTIGHT SUBDIVISION ......................................................................................1236.3 POGrESSIVE FLOODING .....................................................................................................1396.4 DAMAGE STABILITy CALCULATIONS ACCOrDING TO SOLAS 2009 ...............................1396.5 DAMAGE STABILITy OF CHEMICAL AND PrODUCT TANKErS .......................................1416.6 DESIGN FOr DAMAGE SUrVIVABILITy .............................................................................1436.6.1 PrOTECTION OF THE ENGINE rOOM AGAINST FLOODING ..........................................1446.6.2 SAFEr BOwS .........................................................................................................................1466.6.3 FOrEPEAK .............................................................................................................................1476.6.4 LIMITING PrOGrESSIVE FLOODING ..................................................................................149

PART 2 ONBOARD STABILITY DOCUMENTATION ........................................................... 153

7. ONBOARD STABILITY DOCUMENTATION .........................................................................1557.0 INTrODUCTION .....................................................................................................................1557.1 SCOPE OF DOCUMENTATION ..............................................................................................1567.2 ONBOARD INTACT STABILITY DOCUMENTATION ...........................................................1607.2.1 LOADING AND STABILITy MANUAL (LSM) ...........................................................................1607.2.2 CArGO SPACE INFOrMATION ............................................................................................1827.2.3 TANK SPACE INFOrMATION ................................................................................................1827.2.4 HyDrOSTATIC DATA TABLES ...............................................................................................1827.2.5 CrOSS CUrVES OF STABILITy TABLES .............................................................................1827.3 ONBOARD DAMAGE STABILITY DOCUMENTATION ........................................................1927.3.1 SUBDIVISION AND DAMAGE STABILITy CALCULATIONS .................................................1927.3.2 DAMAGE CONTrOL INFOrMATION (DCI) ...........................................................................1967.3.2.1 DAMAGE CONTrOL PLAN (DCP) .........................................................................................1977.3.2.2 DAMAGE CONTrOL MANUAL (DCM) ...................................................................................1987.3.2.3 ExTErNAL wATErTIGHT INTEGrITy PLAN .......................................................................2017.3.2.4 INTErNAL wATErTIGHT INTEGrITy PLAN ........................................................................201

INDEx .................................................................................................................................................202

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3.5 CURVE OF STATICAL STABILITY

The righting arm of a typical cargo vessel increases as the angle of heel increases to some maximum value GZMAx, then GZ decreases as the ship progressively heels. The plot of the righting arm GZ calcu-lated as function of the heel angle, at constant displacement and vertical centre of gravity KG values is used to measure the ship stability at large angles of heel. It is called the curve of statical stability or the righting arm GZ curve. For symmetric forms like ships the curve will be symmetric with respect to φ = 0 and only right half of this curve is presented as in the figure below.

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Righting arm GZ curveSuch diagrams are used to evaluate the ship stability in a given loading case.

3.8 DESIGN FACTORS AFFECTING THE SHAPE OF THE GZ CURVE

Stability of any loaded ship depends on its main dimensions, a shape of the submerged hull and on the actual location of her centre of gravity KG.

Investigations of effects of a change in freeboard on the GZ curve can be found in many books on ship stability. However, two ships can have different freeboards only as a result of different depths or different maximum draughts. Decisions on depth and maximum draught of a vessel are made at the design stage. In operation of a ship, a change in freeboard might arise as a consequence of a change in draught result-ing from loading and discharging of cargo, ballast or stores.

For these reasons, investigating the effect of various factors on the GZ curve it is necessary to divide them into factors defined at the design stage and operational factors which can be defined either by master or by environment during ship operation.

Among design factors affecting ship stability are hull shape, angles of deck immersion, depth, maximum draught and beam, tank arrangement as well as location of unprotected openings such as engine room air intakes.

Master has influence on the final mass of the loaded vessel and the location of the centre of gravity defining amount of cargo, stores and ballast water as well as their locations. Suspended weights, free surface effect, shifting cargoes, icing and water absorption are factors which affect location of the centre of gravity during ship operation.

In this section most of the various factors are discusses separately, but it must be understood that in practice the GZ curve representing ship stability will be affected cumulatively by a number of these fac-tors.

3.8.1 Angle of deck immersion

Photo courtesy of Wärtsilä Corporation

Offshore support vessels have low angle of deck immersiondue to small depth in way of open working deck

3.10 ANGLES OF DOWNFLOODING

There are many openings (hold accesses, air inlets, cargo hatchways, etc.) through which water can enter and endanger a vessel. Usually such openings must be provided with closing arrangements of ad-equate strength to ensure watertightness and structural integrity of the surrounding structure. However, there are also unprotected openings such as air inlets to the engine room, which must be always open in order to enable work of the propulsion plant. Some dangerous goods require continuous ventilation and the cargo hold ventilation must work all the time.

According to the Load Line Convention the lower edge of the ventilation opening that cannot be closed shall be placed at least 4500mm above the freeboard deck or 2300mm above the superstructure deck.

The angle of heel at which the lower edges of any openings in the hull, superstructure or deckhouse which lead below deck and cannot be closed weathertight submerge is called the angle of downflood-ing for intact stability. In cases where the ship would sink due to flooding through any openings, the stability curve is to be cut short at the corresponding angle of downflooding and the ship is to be consid-ered to have entirely lost its stability.

Diagrams of downflooding angle as the function of draught covering the intended trim range shall be a part of onboard stability documents. The opening for which the downflooding angle had been calculated shall be clearly defined and the diagram shall be accompanied with a sketch showing position of this opening. Usually, there are several such critical openings. Therefore several diagrams shall be present-ed in the Loading and Stability Manual.

Ventilation of cargo holds for reefer containers

Cargo holds of container vessels are not provided with high ventilation such as used on multi-purpose vessels. However, more and more reefer containers are transported in holds and closing of ventilation openings can be a choice between ship safety and cargo damage.

In such situations diagrams of downflooding angles for hold critical ventilation openings should be part of the LSM.

Photo J.Babicz

Ventilation of holds for reefer containers

Example of unprotected openings which, for proper ship operation, cannot have closing appliances

High ventilation ducts between cargo holds

Emergency Generator RoomVentilation Outlet

Engine Room Ventilation Inlet

Photos J.Babicz

3.13 MINIMUM OPERATIONAL METACENTRIC HEIGHT GMMIN CURVE

As explained in the previous section the intact stability criteria regard properties of the righting arm GZ curve.

During designing of a ship many calculations are made in order to define values of the minimum meta-centric height necessary to meet all intact stability criteria. Values of GM which ensure compliance with damage stability criteria are defined by a designer.

Basing on these values a set of the minimum operational metacentric height GMMIN curves can be prepared for draught range from lightship draught to maximum draught. The limiting envelope curve presents minimum operational metacentric heights which meet all criteria resulting from intact and dam-age stability for draught range from lightship draught to maximum draught.

In cases where a vessel is intended to operate with trim, the minimum operational GMMIN curves shall extend over the full range of operational trims. The trim values are to correspond with those presented for hydrostatic particulars and cross curves.

To obtain accurate guidance to the stability of the ship it is enough to calculate the actual metacentric height GM corrected for free surface effects and check if it is higher or equal to miminum operational GMMIN value required for the actual trim.

Very often tabulated values of GMMIN are used instead of curves. Values from lightship draught to above maximum draught with steps not exceeding 0.10m shall be available. However diagrams are necessary to understand how GMMIN values change with draught.

Some examples of real GMMIN diagrams are presented on the following pages. It is necessary to under-stand the diagrams are valid only for specific ships and are presented for educational purposes only.

The diagram on page 90 shows the minimum operational metacentric height GMMIN for a 1200TEU ves-sel. For draught range from 6.20m to 8.70m the minimum metacentric height necessary to meet dam-age stability governs. Only for smaller and larger draughts and 4, 5 or 6 tiers of containers on deck the weather criterion requires higher GMMIN values.

The vessel without trim at draught of 8.0m meets all statutory stability requirements with very low value of GMMIN = 0.283m. At the same draught a longer vessel CV1500 needs GMMIN = 0.337m to meet stability rules, (see diagram on page 91).

The Panamax container vessel without trim at design draught of 12.00m needs only GMMIN = 0.30m to fulfill statutory rules. For lower draughts GMMIN is larger and depends on the number of container tiers on hatch covers.

However, having in mind inaccuracies in calculations of the vertical centre of gravity, it would be a mis-take to use such small operational metacentric heights in practice. with very low error margin of +-1% in the VCG value the error for this ship could be as large as +-0.13m which is a considerable fraction of the minimum permissible GMMIN.

Minimum operational metacentric height GMMIN in metres

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6.6.1 Protection of the Engine Room against flooding

Step by step a double-skin hull in the cargo space area is going to be a common practice. Unfortunately this is not the case in the way of machinery spaces where only double bottom is provided. As a result even a minor damage to the ship’s side in this area can lead to the flooding of machinery spaces or even to the loss of the vessel - see the case study 6.1.1. Due to the hull shape, providing a full double skin in the way of the engine room can be impractical. However it is possible to close side spaces in the upper part of the engine room, i.e. in the areas where damages are more probable. Even such partial double skin can be valuable protection of the engine room against flooding. In addition, void spaces at ship’s sides can be used for cables and fire-pipes not allowed to be led through machinery spaces.

Engine Room protected by voids, tanks and cofferdamsDesigned by BNC

Protection of machinery spaces by a double hull is even more important for all ro-ro passenger vessels. Unfortunately most of roPax vessels have a double hull in the way of lower cargo hold only.

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6.6.2 Safer bows

when a ship strikes another ship in the side-shell structure the most structural damage will be found in the struck ship. The striking ship’s bulb and bow may be severely damaged, but usually not as critically as the side-shell of the struck ship.

Ship designer has no influence on time and place of a collision. However, he can have influence on the collision results by using more favourable bow geometry and stiffeners layout which increase a percent-age of the collision energy absorbed by the striking ship. Experience shows that transversely stiffened bow has a significantly lower resistance and in case of collision will be crushed absorbing more energy.

Meanwhile, some bulbous bows look as if they were specially designed for ramming other ships. Espe-cially dangerous bows are those ice-strengthened and with a bulb. Being hit by such a bow may end in the sinking of the rammed ship, whereas the striking ship may suffer a relatively small damage. An exam-ple of such a case may be Coscos’s FU SHAN HAI foundered after being hit by the small multi-purpose vessel GDyNIA outside Bornholm in May 2003 – see 6.1.3.

It would be good to give more chances to the struck ship developing a bulbous bow that will deflect or crumple in a collision, before it has penetrated the inner skin. A bulb structure stiffened by transverse ring frames rather than longitudinal framing has been shown to collapse progressively frame by frame. Decreasing section size at the bow root is expected to be another useful feature, since the bulb is then easily bent under horizontal or vertical pressure.

Photo J.Babicz