Consideration of Collision and Contact Damage Risks in FPSO Structural Designs

Transcription

1 OTC Consideration of Collision and Contact Damage Risks in FPSO Structural Designs Ge Wang, American Bureau of Shipping ; Dajiu Jiang, American Bureau of Shipping; Yung Shin,American Bureau of Shipping Copyright 2003, Offshore Technology Conference This paper was prepared for presentation at the 2003 Offshore Technology Conference held in Houston, Texas, U.S.A., 5 8 May This paper was selected for presentation by an OTC Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Offshore Technology Conference or its officers. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Abstract FPSOs have a risk profile different from fixed platforms and commercial trading tankers. Being stationed in one location and routinely visited by supply boats and shuttle tankers. FPSOs can be collided by these ships. In addition, passing ships also pose a collision risk if an FPSO is close to a sailing route. This paper presents a systematic approach to address the risk of collision and contact damage. It is aimed to propose a framework that can be applied in structural designs and risk assessment schemes. The focus is placed on accident scenarios, evaluation approaches and acceptance criteria, the three major issues for a risk assessment and also for a relevant design standard. Accident scenarios and the associated occurrence frequency may be determined through statistics from historical data, expert opinions, and risk analysis, depending on different situations. A spectrum of tools has been developed in recent years that predicts the structural damage of collisions, including simple formulae, simplified analytical methods, simplified FEM, and non-linear FEM simulations. Those based on the advanced structural crashworthiness theory provide powerful and practical tools, and are well suited for design evaluations and risk analysis. Contact damage is more likely to occur than accidents resulting in rupture of shell plating. However, this type of accident has received very limited attention. An analytical approach is presented for analyzing contact damage that can be used during earlier design stage. Acceptance criteria for collision and contact accidents are discussed. The emphasis is on structural integrity including local strength and hull performance. Introduction FPSOs have a risk profile different from fixed platforms and commercial trading tankers. Being stationed in one location and routinely visited by supply boats and shuttle tankers. FPSOs can be collided by these ships. In addition, passing ships also pose a collision risk if an FPSO is close to a sailing route. A collision accident can lead to costly consequences in loss of lives, damage to property and/or environment. As past accidents have demonstrated, the indirect cost of compensation and bad publicity can be even higher. This has been the driving force in recent years for developing standards for design against collision accidents for trading oil tankers. A collision with low kinetic energy can result in permanent deformations of side structures instead of rupture of side shell. This low-energy collision is sometimes called contact accident. Usually, contact damage is not catastrophic and does not pose a threat to the safety of the installation and the protection of environment. Contact damage receives little attention. This paper presents a systematic approach to address the risk of collision and contact damage. It proposes a framework that can be applied in structural design and risk assessment. Discussion is given to accident scenarios and acceptance criteria. Evaluation of Collision and Contact Risks The evaluation of an FPSO s collision and contact damage risks needs some special techniques. The experience from fixed installations can not simply be transferred. FPSOs are unique in configuration, global performance and structural design, etc. Three Main Issues. There are three main issues for a risk assessment and also for relevant design standards [1]: Identification of accident scenarios including frequency of occurrence Procedures for evaluating the consequences Acceptance criteria Whether a design is acceptable or not may be judged by evaluating for some pre-defined accident scenarios if the combination of frequency of occurrence and consequences fall within acceptable limits. Consideration of Collision and Contact Damages in FPSO Structural Designs 201

2 Accident scenarios define the situations having unfavorable influences on the safety of ships and/or the environment. These scenarios should represent situations that are as close as possible to those encountered by the installation. Frequency of occurrence for each scenario should be taken into account. In addition to the worst cases with small likelihood, cases that are more likely to happen but have moderate or small influences should also be considered. Evaluation methods should be applicable to different designs and cover different accident scenarios. Because of the various possibilities of accidents, inherent uncertainties, and complexities in analysis, it is always preferable to have simple, yet reliable, approaches for predicting the consequences of accidents. Simple calculation formulae or simplified analytical methods may be the first choice. In some cases, established engineering practice or complicated analyses packages may also be used when there are only a few scenarios to be investigated. In viewing the structural strength of FPSOs and the possible adverse consequences of an accident, this paper proposes the following definitions: A collision accident causes the rupture of an FPSO s shell plating that opens the inner tanks to the sea environment (Figure 1). A contact accident causes permanent deformations of side structures, but does not cause rupture of side shell (Figure 2). By these definitions, this paper refers to: A collision as a high-energy collision, resulting from the impact of a visiting supply vessel, passing vessel (if close to a shipping route), or shuttle tanker. A contact as a low-energy collision, resulting from the impact of a supply vessel. The criteria for accepting a design provide a means to balance numerous variables in order to achieve an optimal solution. Depending on the situations to be prevented, acceptance criteria can be limits to damage extent, quantity of oil outflow, residual strength, and/or stability reserve. The format of the criteria may be deterministic, probabilistic, or semi-probabilistic. Figure 2. Heavily deformed vertical frame, deck plating and side shell of a car carrier resulting from a collision with a harbor pier (contact damage) Human Errors. Risk is often defined as the product of likelihood and consequences. In order to achieve acceptable levels of safety, managing risks can be realized by reducing the occurrence probability of collision accidents, minimizing or mitigating the consequences, or both. Figure 1. Gashed side shell of a tanker resulting from a collision by another tanker (collision damage) Collision vs Contact Accidents. Studies on historical incidents reveal that collisions occur quite frequently, but consequences are generally limited [2]. Three main scenarios require detailed assessment [3]: Visiting supply vessels (high frequency and low consequences) Passing vessel collision (low frequency and high consequences) Offloading shuttle tankers (medium frequency with high potential consequences) The majority of offshore collision accidents were caused by human error. Minimizing every possible mistake of humans is the most efficient way to prevent accidents from happening. The United States Coast Guard (USCG) has developed a long-term strategy, Prevention Through People (PTP), to re-balance the safety equation by refocusing prevention efforts on casualties caused by human error [4]. With more and more people realizing the importance of human factors, we have seen convincing improvement in safety records in some fields. This paper saves the discussions on human factors for other experts. Though tremendous improvements have been realized, collision accidents continue to occur. As long as human beings operate man-made equipment offshore, accidents will happen [5][6]. 202 Consideration of Collision and Contact Damages in FPSO Structural Designs

3 Table 1. Approaches for Analyzing the Internal Collision Mechanics Methods Analysis Results Modeling Computation Energy Load Stress Simple formulae Fewest Fewest, hand calculations X Analytical approaches Few Few, hand calculable X X Simplified fem approaches Some Some, special programs X X Non-linear FEM simulation Extensive Extensive, expensive software X X X There is always a need to have the installation keep a certain allowance for collisions so that the installation can survive accidents of some severity. The collision and contact damage risks should be carefully evaluated and the installation should be properly designed to account for these risks. Accident Scenarios Accident scenarios and the associated frequency of occurrence may be determined through the following three approaches: Statistics from historical data, Expert opinions, and Risk analysis. Ideally, accident scenarios (including the associated frequency of occurrence) can be defined based on statistics from past accidents. When comprehensive historical data is not available, experts opinions gained from successful and unsuccessful experiences are regarded as valuable resources. The risk analysis approach has emerged as a very powerful tool. With this approach, significant risk contributors can be identified, which help to identify the accident scenarios of primary importance. Accident scenarios and the associated likelihood for FPSO design have not been formalized yet. Incident Data Supply Vessel Collision. Accident scenarios for supply vessel collisions may be determined based on historical incident data. Because of the frequent visits of supply vessels and the accompanying accidents, there are many incident data. Accident scenarios of supply vessel collision can be estimated by analyzing the trends of the incident records. Many incident data are available for the North Sea. Data have been collected and analyzed over many years. Many results have been published by Health and Safety Executive [7], for example, [8]. For the Gulf of Mexico, records of collision incidents can be found from Minerals Management Services [9] and United States Coast Guard [10]. MacDonald et al. [11] presented an analysis on collision risks in the Gulf of Mexico. Classification societies such as LR, DnV and ABS also maintain vessel incident databases. Generally, these databases cover other geographical areas and are not confined to offshore installations only. Unbiased statistics are the most reliable sources for identifying typical and critical accident cases. However, conditions surrounding an accident, such as vessel speed, loading condition, environmental condition and so on, are not always recorded. The following aspects should be born in mind so that the danger of misinterpretation of historical data can be minimized: Statistics are based on past experience. They may not reflect present situations. Statistics of one geographical location can not be used for another location. Statistics from cases with damage may penalize designs that have been able to resist such damage. Analytical Modeling Passing Vessel Collisions. Occurrence probability of passing vessel collisions can not be fully captured using only historical incident data. Very limited data is available. Even there are some, the presence of an installation will absolutely affects the situation when the installation is absence. There is an increasing interest in developing techniques combining historical data with an analytical model, because historical data can not always be used for predicting the future. This is especially true for the passing vessel collisions, because the traffic pattern varies from one location to another. Haugen [2] described a conceptual route based model for collision frequency per year for a given installation. Pedersen [12] presented a risk analysis on a large suspension bridge that can be applied to fixed offshore structures close to high-density shipping lanes. The model is based on division of collisions into a number of different phenomena and subsequent application of mathematical models to quantify the risk from each category. Causation factors and the number of collision candidate ships are taken into account. Traffic of different ship types, length, loading conditions are based on statistical data. Consideration of Collision and Contact Damages in FPSO Structural Designs 203

4 Simulations Shuttle Tanker Collisions. There are reports on collision of offloading shuttle tankers with FPSOs in the North Sea. A few incidents occurred, but the likelihood of this type of accident is very difficult to specify purely from historical data. Chen et al. [13] described a simulation-based approach, using a state-of-the-art time domain simulation code. The relative motion between an FPSO and offloading tanker was calculated, and the probability of collision was predicted by fitting the extreme values of relative distance into statistical models. Available Design Standards. Many design standards (NORSK, UKOOA, DnV, LR) require that an installation will not collapse after a collision by a 5,000 ton supply boat sailing with a 2 m/s speed, which is related to a drifting supply vessel. API- RP2A requires that a jacket type platform should retain, after a collision, sufficient residual strength to withstand the one-year environment storm loads in addition to normal operation loads. The considered collision is a 1,000-ton supply vessel moving at 0.5 m/s speed. An FPSO has a typical tanker shape and applies the structural designs of tankers (Wang et al.[14]). Compared to many fixed installations that are supported by truss constructions, an FPSO has a higher level of structural redundancy, and therefore can survive a high level of collision impact energy. Prediction of Collision Damages Analysis of the accident mechanics can be split into external mechanics and internal mechanics. The external accident mechanics involves simulation of the time dependent rigid body motion of the involved ships with account of accident forces and the effects of the surrounding water. By analyzing the external accident mechanics, the total loss of the initial kinetic energy to be dissipated in damaged ship structures can be computed. The internal accident mechanics includes evaluation of the structural failure response of the involved ships during the accident. Analysis of the internal accident mechanics can be used to obtain the reaction force versus damage curve. By integrating the area below this curve, the absorbed energy for the ship structures involved can be estimated. Considering the principle of energy conservation, the total loss in the kinetic energy at the end of the accident should equal the total strain energy dissipated by the structural failure of the ship structures. By solving the basic equation representing the response of ship structures in a collision or grounding accident, structural damage of the ships during the accident can be computed. External Mechanics Ship Motion. The external accident mechanics deals with the global motion of a ship, usually considered as rigid body, under the action of the collision force and the hydrodynamic pressures. The hydrodynamic forces can be split into inertia forces (added mass), the restoring forces (buoyancy-gravity), viscous drag forces and wave forces. The inertia forces increase the effective kinetic energy of the ships, and are usually represented by a constant added mass. Energy ratio θ =60 deg. θ =90 deg. θ=120 deg. θ =150 deg Collision location ( x/ L) Figure 3. Kinetic energy loss normalized by the initial kinetic energy as functions of collision location and impact angle The loss of kinetic energy in a collision accident is to be dissipated by damaged structures. For some accident scenarios, analytical expressions have been derived for energy dissipated in damaged structures [15][16][17]. Figure 3 is the energy absorption curve varying the collision location along a ship s length [15]. Given the initial kinetic energy, energy to be absorbed by damaged structures can easily be obtained from Figure 3. Internal Mechanics Energy Absorption. The internal mechanics of collisions and grounding are very complex. Deformations many times larger than the structural thickness may take place, and the major part of energy dissipation takes place in inelastic straining. There is a wide selection of tools for analyzing the internal mechanics. They can be categorized as: Simple formulae (Minorsky [18], Pedersen and Zhang [15], Wang and Ohtsubo [19]) Simplified analytical approaches (Wierzbicki [20], Wang and Ohtsubo [19], Simonsen et al. [20], Wang et al. [21], Brown and Chen [22]) Simplified FEM schemes (Paik et al. [16]) Non-linear FEM simulations (Kitamura [23], Glykas et al. [24], Moan and Amdahl [3]) Table 1 summarizes the characteristics of these approaches. Sweeping from the spectrum of these analysis tools, simplified formulae require the least calculation effort and predict global energy absorption; the non-linear fem simulations require tremendous modeling time and computation capacity but give almost every detail of the structural responses. 204 Consideration of Collision and Contact Damages in FPSO Structural Designs

5 Simplified Analytical Methods. Simplified analytical methods capture the basic characteristics of structural crashworthiness. The technological advances in the last decade are represented by the establishment of the structural crashworthiness concept and methodology. A series of methods have been developed using this advanced technology. Some have been yielding results of practical importance. The newly developed simplified analytical methods consist of four major steps [1]: Identify primary damage patterns of structural components according to observation of actual damage (Wang [25]) Develop idealized theoretical models and derive theoretical formulations to capture the main features of the damage patterns Establish global models for the entire damage process of the ship hull Combine the global damage models with formulations for individual structural components. Simplified analytical methods have been applied to a wide spread of accident situations, including head-on collision with large offshore (fixed) installations [19], ship ship collision [16], ship platform collision [19], ship-bridge collision [12], bottom raking [20], and stranding [21]. Comprehensive surveys of published literature can be found in ISSC [26] and Wang et al. [1]. Figure 4 shows the progressive damage process of side structure in a collision. This is based on one of the model tests of Wang et al. [21]. Using simplified analytical methods (line AA BB CC D), the progressive damage process (line abcdefgh) can be reasonably described. As a result, the general performance of structures in an accident can be captured with reasonable effort W-50 TEST PREDICTION representing large bending of local plates, multi-axial stress fields, time-dependent strain hardening and strain rate effects on material properties, etc. Many powerful special-purpose FEM packages, such as DYNA3D, DYTRAN and PAM, are now available that can account for large deformation, contact, non-linearity in material properties, and rupture. For analyzing a collision or grounding accident involving high non-linearity, contact, friction and rupture, the explicit methodology is suitable. The required calculation efforts are fewer than the commonly used implicit methods. Convergence of calculations is much easier to realize. Since structures behave in many complex patterns [25], many special modeling techniques are needed. Challenges involved in analyzing such a high non-linear problem include structural contact, criteria for material s rupture, crack propagation, among others. At present, only a limited group of researchers have accumulated the major key techniques. Simulations of an accident are still not fully transparent to the industry at large. Compared to typical FEM analysis for design purposes, non-linear simulations of an accident use very fine mesh. To properly capture the local large deformation around a plastic hinge, a non-linear FEM simulation of collision may place 16 elements in one stiffener spacing. Using about 4 elements in one stiffener spacing is generally sufficient for a buckling or ultimate strength analysis. A conventional elastic FEM analysis for design verifications often uses 1 element for the same stiffener spacing. Figure 5 shows the side shell of a VLCC penetrated by the bulbous bow of a Suezmax tanker that was predicted using non-linear FEM simulation [23]. Damages to both the struck side structure and the striking bow were predicted with details, and very fine mesh was used. 80 Load (ton) D 60 C' C e h 40 d f g b 20 c B' a A A' B Indentation (mm) Figure 4. Progressive damage of side structure in collision: a - buckling of transverse web, b rupture of side shell, d buckling of intersection of web and stringer. Non-linear FEM Analysis. Non-linear FEM simulations are reliable and provide very detailed information. These are especially efficient in Figure 5. Non-linear FEM simulation of a collision between a VLCC (at left) and a Suezmax tanker (at right) Consideration of Collision and Contact Damages in FPSO Structural Designs 205

6 Calculation of Other Consequences. The hull girder strength can be measured using hull girder section modulus or ultimate hull girder strength. Hull girder section modulus can be calculated directly using engineering programs. The ultimate strength of hull girder corresponds to the maximum bending capacity beyond which the ship will break its back due to extensive yielding and buckling. Figure 6 shows the collapse of a bulk carrier as a result of losing hull girder strength. Many special programs have been developed for predicting hull girder ultimate strength. See [27] and [28]. not break apart as in Figure 6, avoiding loss of the entire installation. Regarding the structural designs for collision risks, acceptance criteria have not been formalized yet. In the simplistic format, the criteria may be: Minimum distance of cargo containment from the shell Limit to ship speed above which a critical event (breaching of cargo containment) happens Allowable quantity of oil outflow Minimum hull girder capacity of the damaged hull Some simple correlations between hull girder strength and the extent of damage are available (Wang et al. [29]) The International Maritime Organization has established a probabilistic methodology in MARPOL for evaluating the oil outflows from a damaged tanker. The intensive calculations may be simplified using rational approaches while keeping the same assumptions for damage extent and similar methodology for oil trapped in double bottom. Michel et al. [30] presented a refined model for investigating the oil outflow performance of tankers. Acceptance Criteria for Collision The consequences of a high-energy collision accident can be: Damages to installations resulting in loss of structural integrity Loss of stability or buoyancy Oil outflow resulting in environmental pollution Loss of entire installation Loss of life Concern with survivability in the event of an accident has been with the loss of stability or buoyancy. Regulatory requirements have been established on subdivision and damage stability. MARPOL specifies requirements for oil outflow performance of tankers in accidents. In the case of an accident, the main objectives of operations are to rescue crews and passengers, maintain the integrity of the ship, prevent or minimize cargo loss, and protect the environment from spilled cargo such as oil. The primary considerations for structures are resistance to the accidental loads, sufficient residual strength, adequate stability, and containment of oil from spilling. In addition to improved tank configurations, FPSOs need to demonstrate the following: Cargo tanks/holds are not breached in an accident so that there will be no danger of pollution. If the cargo tanks are breached, the oil outflow is limited. The installation has adequate residual hull girder strength so that it will survive an accident and will Figure 6. Hull Girder Failure: Collapse of a Bulk Carrier Contact Damage Low Energy Collision The likely form of contact damage is permanent deformation. Usually, a contact damage is not catastrophic and does not pose a direct threat to the safety of the installation and the protection of environment. Only for limited situations (such as the side shell of an offshore supply vessel), criteria for contact damage are specified in design standards. Relevant design criteria are prescriptive, some may be purely rule-of-the-thumb. Usually, the accident scenarios that the design standard intends to prevent from happening are not clearly defined. Some shipbuilders have developed their own standards for re-enforcement of areas prone to contact damage. The design is the working stress format, and is based on assumed impact pressures on the ship s hull. The design impact pressures due to a contact are based on their longterm experiences of successful and un-successful designs, and are usually not related to possible operations of vessels. This makes it very difficult to use these shipbuilders methods in a risk assessment scheme. Contact damage receives less attention in a risk assessment. Often, itis treated as the same category of collision. Because its consequences are small compared to a high-energy collision, a contact accident, sometimes is referred to as a minor collision. It does not receive attention it should, and the related analysis remains not well developed. 206 Consideration of Collision and Contact Damages in FPSO Structural Designs

7 Scenario Definition. Many events can lead to contact damage: Berthing or approaching operations, Sudden roll of the FPSO, Drifting of a supply vessel after losing power or control, Others For the sake of convenience, the following description mainly refers to a berthing operation. However, the conclusions are generally applicable to other events that may lead to contact damage. The following are needed for defining the scenario of a berthing operation: Size (displacement) of the offshore supply vessel Approaching speed Approaching angle Location of contact. The largest supply vessels that may serve the installation may need to be specified for analysis. How fast and at what angle a supply vessel approaches an FPSO depends on many factors, including seamanship, operation procedure, crew s experience, weather and sea conditions. Actual measurements on approaching speed etc. are helpful, but seem very limited. During a ship-toship transfer of oil at sea, the relative speed between two ships are likely in the range of 0.1 to 0.3 m/s, or about 0.2 to 0.6 knots, when a good operation is performed. Analysis Approach. For such complicated problem involving many uncertainties, a simplified analysis approach seems to be good for preliminary design. The analysis is based on the mechanics of a berthing operation and the structural responses. It aims to develop an approach easy enough for design. The problem will be formulated using analytical expressions so that it will be easily incorporated into a risk assessment theme. The analysis consists of calculations of the following: Energy Load/pressure Structural responses of the FPSO s side structure. Energy. Generally, FPSOs are much larger than supply vessels [14]. In a collision with a supply vessel, the movement of the FPSO is usually negligibly small and can be treated as stationary in analysis. Figure 7 shows schematically the berthing problem [31]. The berthing energy to be absorbed by the fenders can be calculated by multiplying the vessel s total kinetic energy by a series of coefficients: E = (0.5mVn 2 ) Ce Ch Cs Where, m is displacement of the supply vessel, Vn is the relative speed, Ce is the eccentricity coefficient accounting for the ship s rotation, and Ch is the virtual mass coefficient accounting for added mass. The eccentricity coefficient Ce may be calculated by the following equation: Ce = (k 2 + r 2 cos(α) 2 ) / (k 2 + r 2 ) Where, k is the radius of gyration of the supply vessel, r is the distance of the point of contact from the center of gravity, and α is the angle between r and the speed vector V. The hydrodynamic mass coefficient Ch may be calculated from the following equation, Ch = 1 +2d/B Where, d and B are the draft and breadth of the supply vessel. National standards (British, Germany, Japan, etc.) have been established that can be followed in calculating the berthing energy. Supply vessel FPSO Figure 7. Analysis model for a supply vessel (at top) coming alongside an FPSO (at bottom) Impact Load/Pessure. Fenders are often installed in offshore supply vessels. They are softer than a ship s hull, and absorb most of the impact energy. Given the maximum impact energy that should be absorbed by the fendering system, the impact loads on the FPSO s side shell can be obtained from the fender s manufacturer. The maximum average hull pressure corresponding to the design maximum energy absorption is generally available, as this is also a design parameter. However, the performance curves of impact pressure versus compression relationship are not always available. Because the contact happens in a very short period of time, the behavior of the side structure may be treated as independent from other loads in normal operations, such as hull girder loads and local pressure loads. The local impact load due to a contact may exceed the normal operation loads on side shell and side longitudinals. Consideration of Collision and Contact Damages in FPSO Structural Designs 207

8 Structural Responses. Permanent deformation occurs when the structure collapses and deforms in plastic range. Analytical solutions to the plastic collapse of plate and beams are available and can be used to determine the occurrence of severe permanent deformations. These analytical solutions to collapse strength should be adjusted to take into account the following uncertainties: Higher localized impact pressure due to the uneven distribution of the impact loads over the footprint, Uncertainties associated with structural modeling, such as boundary conditions, Stresses induced from normal operations of the FPSO, such as the hull girder bending stresses and stresses due to lateral pressure of ballast water. Ratios of the strength over the applied impact load/pressure, or SF factors (safety factors), can be calculated, and can be used for determining the occurrence of permanent deformations. There are no formalized levels of SF factors below which permanent deformations will occur. The side shell may dish-in between longitudinals and transverse frames. Assuming that side shell plate is fully supported side longitudinals and transverse members, the collapse strength of side shell can be calculated following collapse theory. Side longitudinals can be idealized as a beam supported by transverse frames. When acted upon laterally due to a contact, a beam will collapse when its load-carrying capacity is exceeded. Permanent deformation will follow when a beam collapses. Transverse frame, horizontal stringer, and transverse bulkheads are subjected to in-plane loads in a contact. When the in-plane load exceeds the maximum loadcarrying capacity, these structures will develop local large out-of-plane deformations around the loaded area. This failure mode is often called crippling, as the permanent deformations are local and concentrated. Figure 2 shows this typical crippling failure of frames and decks. The crippling strength may be predicted as Pu = 0.85 t 2, where, t is the plate thickness in millimeters, and Pu in tons. This formula has been used to calculate the maximum capacity of slender girders. Acceptance Criteria. Permanent deformations weaken the load-carrying capacity of structural members, or the ship s hull. If these deformations are not properly repaired, over a long period of time cracks may develop and propagate leading to oil leakage. Progressive structural failure and crack initiation can be prevented if permanent deformations do not take place. Though not an immediate threat to the integrity of the installation and the environment, permanent deformations as a result of a contact should be fixed, and this incurs economic burdens, a consequence that can not be neglected. Preventing or mitigating contact damage risks can be achieved through: Reinforcing the FPSO s side structures, or Establishing operation limitations to offshore supply vessels, or Both Operation limitations are aimed to minimize the likelihood of an offshore supply vessel colliding with an FPSO. These limitations may be one or both of the following: Offshore supply vessels are not to be operated close to the FPSO when the weather deteriorates to particular sea-state. Offshore supply vessels should be operated below a certain speed and angle limits when coming alongside and berthing to the installation. Re-enforcement of structures prone to contact damage is aimed to provide strength reserve against a certain level of impact energy that is normally expected from a good operation. The design requirement may be: The side structures of the FPSO should not have permanent deformations in a low-energy collision with an offshore supply vessel. Usually, this design requirement applies to some specified locations, as offshore supply vessels usually come alongside and berth at designated locations of FPSOs. Conclusions This paper presents a systematic approach to address the risk of both collision and contact damage. It proposes a framework that can be applied in designs and risk assessment schemes. The focus is placed on accident scenarios, evaluation approaches and acceptance criteria. Accident scenarios and the associated frequency of occurrence may be determined through statistics from historical data for supply vessel collisions, analytical modeling and expert opinions for passing ship collisions, and operation simulations for offloading shuttle tanker collisions. A spectrum of tools has been developed in the recent years that predict the structural damages in collisions, including simple formulae, simplified analytical methods, simplified FEM, and non-linear FEM simulations. The characteristics of these methods are compared. Those based on the advanced structural crashworthiness theory provide powerful and practical tools, and are well suited for design and risk analysis. Non-linear FEM simulations provide much more detailed information, but generally require expensive special-purpose software and many special techniques that are not fully captured by the industry at large. 208 Consideration of Collision and Contact Damages in FPSO Structural Designs

9 Contact damage is more likely to occur than accidents resulting in rupture of shell plating. However, this type of accident has received very little attention. An analytical approach is presented for analyzing contact damage for preliminary design. It includes simplified solutions to ships motions and engineering models for structural responses. Acceptance criteria for collision and contact accidents are discussed. The emphasis is on structural integrity including local strength and hull performance. Acknowledgement The authors appreciate the valuable comments from J. Card, J. Spencer, D. Diettrich, Jer-Fang Wu and Jorge Ballesio. The authors are indebted to Ms. Jo Feuerbacher and Joan Hauff for editing the manuscript. References 1. Wang G., Spencer J., Chen, Y.J. (2002a). Assessment of ship s performance in accidents. Marine Structures, 15, Haugen S. (1998) A review over ship-platform collision risk modelling. Risk and Reliability in Marine Technology, C. Guedes Soares (Ed.), A.A. Balkema, Rotterdam. 3. Moan, T. and Amdahl J. (2002) Risk assessment of FPSOs with emphasis on collision risk, ABS RD report USCG (1995) Prevention Through People, United States Coast Guard. 5. Baker C.C., McSweeney K., McCafferty D.B. (2002). Human factors and ergonomics in safe shipping: the ABS approach, Proceeding of the Maritime Operations: The Human Element 7 th Annual Conference, Washington, DC. 6. McCafferty, DB, Baker CC. (2002). Human error and marine systems: current trends. Proceedings of IBC s 2 nd Annual Two-Day Conference on Human Error, London. 7. HSE (2003) / Health and Safety Executive webpage 8. OTO (1999) Effective collision risk management for offshore installations. Offshore Technology Report OTO , Health & Safety Executive. 9. MMS (2000) MMS OCS spill database. Minerals Management Service. 10. USCG (1999) Marine casualty and pollution database. United States Coast Gurad. 11. McDonald A., Cain M., Aggarwal RK, Vivalda C, Lie OE (1999) Collision risks associated with FPSOs in deep water Gulf of Mexico, OTC-10999, Houston, TX. 12. Pedersen. P.T. (2003). Collision risk to fixed offshore structures close to high-density shipping lanes. Journal of Engineering for the Maritime Environment. 13. Chen H., Moan T., Haver S., Larsen K. (2002). Prediction of relative motion and probability of contact between FPSO and shuttle tanker in tandam offloading operation. 21th International Conference on Offshore Mechanics and Arctic Engineering, June 2002, Oslo, Norway. 14. Wang G., Spong, R., Shin Y. (2003). Experience based data for FPSO structural design. OTC-15068, Houston, TX. 15. Pedersen, P.T., Zhang, S. (1998). On Impact Mechanics in Ship Collisions. Marine Structures, 11: Paik J.K., Chung J.Y., Choe I.H., Thayamballi A.K., Pedersen P.T., Wang G. (1999) On rational design of double hull tanker structures against collision. SNAME annual meeting, Baltimore MD, 29 Sept ~ 2 Oct Suzuki, K., Ohtsubo, H., Sajit, C. (2000). Evaluation method of absorbed energy in collision of ships with anti-collision structure. Ship Structure Symposium on Ship Structures for the New Millennium: Supporting Quality in Shipbuilding, Arlington, VA, June. 18. Minorsky, V.U. (1959). An analysis of ship collision with reference to protection of nuclear power ships, Journal of Ship Research, 3:2, Wang G., Ohtsubo H. (1999). Impact load of a supply vessel. 9th International Offshore and Polar Engineering Conference & Exhibition. Brest, France, IV, Simonsen, B.C. and Lauridsen, L.P., (2000). Energy absorption and ductile failure in metal sheets under lateral indentation by a sphere, International Journal of Impact Engineering, vol. 24, pp Wierzbicki, T. et al. ( ). Reports, Joint MIT-Industry Program on Tanker Safety. 21. Wang, G., Arita, H., Liu, D. (2000). Behavior of a Double Hull in a Variety of Stranding or Collision Scenarios, Marine Structures, 13, Brown, A. and Chen, D. Probabilistic Method for Predicting Ship Collision Damage, Oceanic Engineering International, 2002, 6:1, Kitamura O. (2001) FEM approach to the simulation of colli-sion and grounding damage. The second International Conference on Collision and Grounding of ships. Copenhagen, Denmark, July Glykas A., Das P.K., Barltrop N. (2001) Application of failure and fracture criteria during a tanker head-on collision. Ocean Engineering, 28: Consideration of Collision and Contact Damages in FPSO Structural Designs 209

10 25. Wang G. (2002) Some recent studies on plastic behavior of plates subjected to very large load. Journal of Ocean Mechanics and Arctic Engineering, ASME. 26. ISSC (2003) Committee V.3 Collision and Grounding. 15 th International Ship and Offshore Structures Congress (ISSC), San Diego, August ISSC (2000) Committee VI.2 Ultimate hull girder strength. 14 th International Ship and Offshore Structures Congress (ISSC), Nagasaki, Japan. 28. Paik J.K., Wang G., Kim B.J., Thayamballi A.K. (2002). Ultimate limit state hull girder design. SNAME annual meeting 2002, Boston, September Wang G., Chen Y.J., Zhang H., Peng H. (2002b). Longitudinal strength of ships with accidental damages. Marine Structures, 15, Michel, K., Moore, C., Tagg, R. (1997) A simplified methodology for evaluating alternative tanker configurations. Journal of Marine Science and Technology, 1: Gaythwaite JW. (1990). Design of marine facilities. Van Nostrand Reinhold, New York. 32. Gilbert R.B., Ward E,G., Wolford A.J. (2001) Comparative risk analysis for deepwater production systems. Offshore Technology Research Center. 210 Consideration of Collision and Contact Damages in FPSO Structural Designs