POAC09-140

ICEBERG COLLISION WITH OFFSHORE UNIT

Z. Mravak 1, S. Rudan 2,V. Tryaskin 3, D. Coache 1, J. de Lauzon 1, A. Dudal 1 1 Bureau Veritas, Paris, FRANCE

2 Faculty of Mechanical Engineering and Naval Architecture, Zagreb, CROATIA 3 State Marine Technical University, St Petersburg, RUSSIA

ABSTRACT The increase of world demand for the energy resulted in a high energy price, which have reinforced the interest in the development of new oil and gas fields in deeper sea or in harsher environment area. Due to a significant reserve of petroleum and gas north of the Arctic Circle, further field development and increase of exploration in Arctic region are expected in the incoming decades. In Arctic region, the harsh environment leads to serious challenges in the design of off-shore units. Between different scenarios of unit hull and ice interactions, a possible collision with the icebergs requires particular attention. This scenario is discussed in the paper for the case of floating moored off shore unit. Depending on the iceberg properties and its size and mass relative to the offshore unit dimensions, the effects of collision on the unit are discussed and analyzed on two levels: global loads on the unit and local hull structure loading. The external collision mechanics is analyzed taking into account parameters of the collision, mass and drifting speed of iceberg. The energy absorption during internal mechanics of collision, due to iceberg crushing and unit structural deformation is calculated using non-linear FEM explicit procedure. The energy absorption curve during the collision is calculated and discussed.

POAC 09 Luleå, Sweden

Proceedings of the 20th International Conference on Port and Ocean Engineering under Arctic Conditions

June 9-12, 2009 Luleå, Sweden

INTRODUCTION The estimation is that hydrocarbon reserves in the Arctic area is huge, up to 25% of the world’s undiscovered resources. Large oil and gas developments are taking place at several locations, e.g. Offshore Sakhalin, Pechora Sea, North Slope of Alaska, Grand Banks of Newfoundland and Barents Sea. Due to presence of significant reserves of petroleum and gas, further field development and increase of exploration are expected in the coming years. Observations in the last few years show rising temperatures and more ice melting in the Northern Hemisphere. Regarding the main causes, the opinions varied from atmospheric pollution to the effects of natural cycles solar radiation levels. Not only the extent of ice reduces, but there is also evidence that the thickness of the ice is diminishing. Even though the summer sun might melt a large part of the ice sheet in the North, the Arctic Ocean will freeze in winter and into spring. In addition, more icebergs are generated nowadays, measured by volume. This can be clearly linked to increase in ocean temperature and increased number of days with positive air temperature causing more surface melting on the ice sheet. From surface melted water flows down to the base of the glacier lubricating the ice/rock contact surface, resulting in accelerated glacial flow and iceberg production. The same oceanic factors that are triggering greater iceberg production also are responsible for their more rapid destruction. The seasonal presence of icebergs, bergy-bits and growlers, is an environmental and operational hazard to oil exploration, production and shipping on the Arctic region. In the future raising number of offshore object in service will increase the probability of a unit striking an iceberg. Both recently and in the past, there have been a large number of occurrences of vessels being impacted by an iceberg, a bergy-bit or a growler, Figure 1.

Each year on Arctic numerous icebergs appear ranging in size from very large, having a mass of one million tonnes or even greater, to small growlers. For the oil or gas production field the system of iceberg management is developed. In practice these system includes: surveillance & ice detection; data collection, reporting, forecasting; decision-making; avoidance or deflection. Strong winds, occasionally storm-force wind and wave conditions, and frequently poor visibility in ice season can reduce visual and radar detection of icebergs and the

effectiveness of their towing and deflection. Therefore certain risk of offshore unit collision with iceberg is always present.

Figure 1 MS Explorer sinking after collision with iceberg,

2007/11/12 Antarctic Bransfield Strait [Ref. AC, 2007]

FLOATING OFFSHORE UNIT AND ICEBERG COLLISION The collision of floating unit with glacial ice has the effects on global and local level. The collision loading on global level affects the mooring system of the unit and global hull strength, while the hull structure response in the region of collision is usually analyzed as the local phenomena.

Figure 2 Offshore unit and iceberg collision

The mechanics of floating unit impact with glacial ice can be assessed as problem of conservation of energy and momentum. In the analysis of collision, two main elements need to be considered. The first is external collision dynamics i.e. the change in external energy and momentum of the vessel and iceberg involved in the collision. The second element is the internal dynamic, which considers the energy absorption by the hull damage and ice crushing. The kinetic energy of iceberg before the impact will be transformed in: iceberg and offshore unit kinetic energy, energy dissipation due to objects moving in the sea, cumulated energy in mooring lines and absorbed energy by hull structure deformation and ice crushing.

Icrushing

Udefhull

Umooring

Undissipatio

Indissipatio

Ukinetic

Ikinetic

Ikinetic EEEEEEEE

o++++++= (1)

The following parameters and objects properties define the collision: offshore unit mass and hull shape; mass, dimensions and shape of iceberg; iceberg collision speed, position of collision and incoming direction; ice crushing parameters; hull structure design; mooring system properties, Figure 2. The external dynamics include consideration of unit and iceberg motions during the collision { } { })(,)( tt IU ξξ and contact force between the bodies )(tF coll

r, Figure 3. In the point of

contact, collision force could be presented as: )()()( tFtFtF coll

Tcoll

Ncoll

rrr+= (2)

The friction coefficient between the ice and steel is 06.003.0 −=frictionf . When the following

condition is satisfied )()( icoll

Nfrictionicoll

T tFftFrr

⋅≥ , the contact point will move along the hull.

Figure 3 Collision external dynamics

The deformation of the hull δU and the iceberg crushing δI absorb the kinetic energy during impact. Total energy dissipation due to ice crushing and structural deformation is WI+WU, as shown on Figure 4. The amount of vessel’s local structural damage in collision with iceberg depends on the ice load magnitude and the strength of the hull structure. The methods for calculating internal structural damage in collisions can be either simplified empirical methods or numerical FE analysis.

Figure 4 Absorbed energy by hull structure deformation and iceberg crushing

If the iceberg mass is relatively small comparing with the mass of unit, the energy absorption will be mostly internal and during the impact the displacement of the unit will be

{ } 0)()(6,1 ≅== tt UUi ξξ .

But if the mass of iceberg is significant the absorption of energy during the collision will occur

on external and internal level, resulting with the unit movement { }⎭⎬⎫

⎩⎨⎧

⎭⎬⎫

⎩⎨⎧ •••

)(,)(,)( tttUU

U ξξξ .

∫=I

oI

collII dFW

δ

δδ )( ∫=U

oU

collUU dFW

δ

δδ )(

COLLISION LOADING ON MOORING SYSTEM The external dynamic of offshore unit collision with the iceberg is analyzed. The analyzed offshore unit is FPSO having a ship shaped hull with the following main dimensions: L = 320m, B = 58m, T = 16.5m, ∆= 240 000t, Figure 5. The unit has turret type mooring system, with 9 mooring lines. The line length is 1500m, thereof 700m laid on sea bottom. The sea depth is 300m and the pretension force in each mooring line is 300kN.

a) FPSO model

b) mooring system general arrangement

c) mooring line configuration

Figure 5 Numerical model of FPSO hull and mooring system

The medium size iceberg with the tabular form (vertical sides) and the following main dimensions: L= 60m, B= 30m, H= 20m, T= 17.6m and tmiceberg 00030= is considered, Figure 6.

Figure 6 Iceberg geometry, top view

The simplified assessment of collision assumes that the whole iceberg kinetic energy is absorbed in the mooring system resulting with additional tension forces in mooring lines. The energy that could be absorbed by the mooring system is limited and defined with the maximal unit

displacement which corresponds to maximal allowable tension force in one of the mooring line, Table 1.

Table 1 Mooring system capacity until failure

Displacement until failure

[m]

Max. total force [kN]

Capacity of mooring

[MJ]

tmiceberg 00030= ,

needed icebergv [m/s]

smviceberg /5.1= , needed

icebergm [t]

No environmental load 125.6 18.80·103 819.64 7.4 728.57·103 With environmental load 79.5 18.80·103 748.85 7.0 665.73·103 Even assuming that the whole energy will be transferred to the mooring system, only the icebergs with big mass or high speed present the potential danger for the mooring system. Assuming moderate environmental conditions acting in the same direction as incoming iceberg: wind 30=windv m/s, current 1=currentv m/s and waves 0.3=SH m; 14=PT s, the iceberg mass and speed, needed to damage the mooring system, remain significantly high. For the given offshore unit and iceberg dimensions, above mentioned environmental conditions, the iceberg speed 5.1=icebergv m/s and incoming direction °= 45β , the dynamic analysis is performed assuming the collision on the hull near the turret supporting structure. The hydrodynamic coefficients of the unit and iceberg in 300m deep sea are calculated (BV- HydroSTAR software), and transferred to program for the analysis of mooring system (BV- Ariane 7). The collision is analyzed in time domain using step by step procedure, calculating the dynamic of the unit { }Uξ ,

⎭⎬⎫

⎩⎨⎧ • U

ξ , ⎭⎬⎫

⎩⎨⎧••U

ξ and iceberg { }Iξ , ⎭⎬⎫

⎩⎨⎧ • I

ξ , ⎭⎬⎫

⎩⎨⎧ •• I

ξ in each time step, as well as the contact

force itcontactF . To define the force-penetration relation in the point of contact, the curve calculated

in the next chapter is used. As a result of the external dynamic analysis, the time history of turret displacement in horizontal plane and time history of collision force )(tFcontact are obtained, Fig. 7.

FPSO turret displacement

118

119

120

121

122

123

124

125

126

127

-30 -25 -20 -15 -10 -5 0 5 10 15East [m]

North [m]

0

5000

10000

15000

20000

25000

30000

35000

458 460 462 464 466 468 470t [s]

F con

tact [

kN]

Figure 7 Displacement of turret and contact force during collision

The contact force during the collision reaches the maximum of about 30.5MN, Figure 7. The displacement of turret structure remains relatively low 23.6m comparing with breaking line limit, therefore the additional tension forces in mooring lines are low as well.

ICEBERG LOADS The interaction between the ice and the structure is mainly defined with their shape and size. The global ice action during the collision is the combination of stress and energy limiting mechanisms. Beside the impact parameters, the iceberg crushing failure characterized with ice strength properties determines the collision contact force. When ice crushing occurs against structure, the global ice load normal to the surface )(tFcontact can be expressed as:

)()()()(

tApdAtptF otA

contact ⋅== ∫ (3)

where: op is averaged ice pressure and )(tA is contact area. The pressure associated with the global action is influenced by the ice temperature, the nominal contact area, the shape or aspect ratio of the contact area, the nature of the contact, the relative speed and displacements between ice and structure as well as the compliance of the structure. Within the nominal contact area there can be many areas that are subjected to higher local pressures. This local pressure should be used in the design of structural elements like shell or stiffeners. For the massive ice formations large scale measurements data could be used, in determination of local ice pressure. The following equation defines the local pressure as a function of contact area (API RP 2N): ]}[]{[5.0)( n

In

II AmpabsAmppAp −+−−= (4) with parameters: 5.0;1.8;5.8 −=== nmpI . For the larger area 29≥A m2, the pressure

5.1=op MPa is to be used. In general collision situation, where iceberg speed is icebergv and collision angle isβ , impacted hull surface will deform crushing the ice during penetration and at the same time two bodies will relatively rotate. For the given iceberg geometry and pressure area defined, it is possible to calculate total contact force as a function of penetration, )(δcontactF . The function )(δcontactF could be calculated for different shape of impacted surface, iceberg shape and collision angle. The ice crushing energy

IW as a function of penetration is: ∫=δ

δδo

contactI dFW )( (5)

For the iceberg geometry defined previously, assuming flat rigid hull surface and collision angle o90=β the function )(δcontactF is calculated and presented in Figure 8.

y = 150.6x5 - 470.92x4 + 573.83x3 - 353.41x2 + 134.54x + 20.5

y = 1E+09x5 - 2E+08x4 + 9E+06x3 - 245190x2 + 3463.7x - 2E-09

0

10

20

30

40

50

60

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00

Penetration, m

Con

tact

forc

e, M

N

Figure 8 Contact force including ice crushing

FE COLLISION ANALYSIS OF OFFSHORE UNIT HULL To analyse the internal dynamic of collision, hull structure deformation under collision loading an explicit non-linear finite element analysis is performed using Abaqus software. Collision scenario, as described earlier in external collision analysis, consists of a 30 000t iceberg colliding with a moored FPSO ship in the part near the turret supporting structure. Iceberg speed 1.5m/s with different collision angles are considered. The results for the incoming direction normal to the hull shell are presented hereafter. Figure 9 presents the finite element model of the entire FPSO ship (left) and the finite element model of the hull part close to turret structure (right) used in analysis. Colours indicate plate thicknesses.

plate thickness

Figure 9 FE model of offshore unit

Being a part of the entire FPSO model, turret finite element model is rather large and consists of 129778 finite elements and 888 396 degrees of freedom. Iceberg is considered to be rigid so that entire collision energy is absorbed solely by the FPSO structure. Iceberg was set in motion by an initial velocity of 1.5m/s; no other displacement control was set to model. In this way a part of kinetic energy was restored to the iceberg after maximum penetration occurred. Boundary conditions are set to fix the position of the model extreme edges. The element size in the contact region is of the order of 100mm to 120mm. Stiffeners were modelled with plate finite elements in the contact zone. Reduced integration shell finite elements and beam finite elements (Abaqus nomenclature S4R, S34 and B31 respectively), were used throughout the model. Material properties considered refer to common engineering steel used in shipbuilding. True stress-strain curve of steel is defined using the power law curve that fits the form [S. Ehlers et al., 2008] :

neK ⋅=σ (6) has the strength coefficient value K=730 and hardening exponent n=20. Failure mode considered is shear failure criteria, according to Abaqus definition of the equivalent plastic strain required for the element to fail, Ef. Evaluation of the critical plastic strain was performed using IMO recommendation [IMO, SLF 46/INF.10, 2003]:

eegef l

tl εεε +=)( (7)

where Ef is failure strain, Eg = 0.056 is uniform strain and Ee = 0.54 is necking strain for plate elements, t is the plate thickness and le is the element size. The average value of element length is

considered to be 100mm and the average thickness leading to critical failure strain of 0.16mm/mm. The steel-ice friction is not considered in this analysis. Damage of the structure after collision is presented in Figure 10.

Figure 10 Hull structure deformation

The force-penetration curve and the absorbed energy in structure deformation is presented in Figure 11.

Figure 11 Force penetration curve and absorbed energy

Penetration force reaches the maximum when the penetration of rig body in a hull is 0.8m and the value of contact force is then close to 71MN. The energy absorbed by hull structure reaches a maximum value of 33MJ after 0.9s of collision. External dynamic contact force-penetration curve, Figure 7 (right) differs. Difference between these curves is expected since in FEM analysis ice crushing effect is not taken into account, leaving so more energy for a hull to absorb. External dynamic takes into account not only ice crushing, but a movement of entire mooring system as well, damping the collision energy in this way. It also extends the collision time. FEM collision analysis results, with fixed offshore hull structure and iceberg defined as rigid body are therefore conservative. More complex failure criteria, steel-ice interaction effects and proper ice material crushing model are needed to improve the analysis so that it can capture the real-life collision with iceberg.

However, even so it provides the insight into ammount of dammage and collision energy expected and could be used to compare the different levels of ice class structure reinforcement, for the offshore units aimed to operate in the iceberg populated waters. CONCLUSION Even with modern iceberg management system, developed and organized for the oil and gas production field in the Arctic region, certain risk of iceberg collision with the offshore unit remains present. The mooring system of big offshore units, e.g. FPSO analyzed here with length of 320m and displacement of 240 000t, dimensioned for the open sea conditions is relatively strong. Possible collision with the iceberg is not a real danger for the mooring system, off course assuming that direct contact between the ice formation and mooring lines is avoided. Iceberg collision is not a dimensioning design scenario for the mooring system. Other scenarios and interactions with ice formations may have to be considered. The local loads of icebergs on the hull structure bring a challenge. Small experience and only limited amount of measurement data is available for the icebergs local loading on the moored or even fixed offshore units. The description of ice properties and behavior for the numerical tools nowadays is not properly developed. Therefore the numerical tools for the direct analysis of iceberg collision is not enough suitable and mature. Further research and development effort are needed to properly capture the important physical phenomena. A combination of experimental data and numerical analysis is needed for the design of the offshore units intending to operate in the harsh environment of Arctic region.

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AC Associated Content, 2007. Will the MS Explorer Sinking Bring Titanic Thoughts to Antarctic Tourism?, Article published on www.associatedcontent.com

API RP 2N, 1995. Recommended Practice for Planning, Designing and Constructing Structures and Pipelines for Arctic Conditions.

Bureau Veritas, 2003. HydroSTAR For Expert, v2.0 - Reference Guide and Tutorial for Naval and Offshore Hydrodynamic Application.

Bureau Veritas, 2009. Ariane, v7.0 - Mooring Software, Theory and User’s Manual.

Bureau Veritas NR 527, 2007. Rules for the Classification of Polar Class Ships.

Guichard, A., 1988. Impacts d'icebergs sur une structure massive., Thèse de doctorat de l'Université Paris VI, Paris

ISO/DIS 19906, 2009. Petroleum and natural gas industries - Arctic offshore structures.

Jiang, H., Zhao, G. X., Gu, Y., 2007. Damage analysis of FPSO due to collision. ICCGS Conference, Hamburg

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S. Ehlers, J. Broekhuijsen, H. S. Alsos, F. Biehl and K. Tabri, Simulating the collision response of ship side structures: A failure criteria benchmark study, International Shipbuilding Progress 55 (2008) p. 127–144.