FPSO hull structural design concept supporting controlled project execution

Author Names: Timo P.J. Mikkola1), Ingrit Lillemäe1), Grzegorz Mazerski1), Tomi Taponen1), Janusz Ziółkowski2), Lech Tamborski2) & Przemysław Dominiczak2).

1) Deltamarin Ltd, Helsinki, Finland 2) DesArt Ltd, Gdynia, Poland

Abstract

The target of the FPSO structural design is firstly to guarantee structural integrity. At the same time it needs to target in minimizing the limitations on the oil and gas production related functions caused by the structural design. The structural design concept developed for this aim is discussed with emphasis on the practical man-agement aspects. The FPSO steel model is developed within a 3D product model supporting all the other disciplines as well. The structural analyses utilize the steel model as a basis for FEA model development. For the structural design ana-lyses a broad spectrum of programs is available ranging from initial design tools to the high end state-of-the-art analysis software. Spectral based procedures are fre-quently utilized. The load, strength and fatigue analysis capabilities of the design system are illustrated with case examples. The work of the designers is supported by a manage-ment framework and procedures adapted for structural design discipline and the use of state-of-the-art analysis software.

Keywords

FPSO, FDPSO, EWTU, Offshore Installation, Hull Steel Structure, Structural Analysis, Hydrodynamic Analysis, Ultimate Strength, Fatigue, Accidental Loads.

Introduction

Majority of the +150 FPSO units in service and those on order are ship-shaped either conversions from trading tankers or new-builds. Novel concepts have been intro-duced such as the first cylinder-shaped FPSO installed on Piranema Field, Brazil targeting e.g. for improved wave induced motions, higher stability reserves and higher deck load capacity. The future FPSO’s will be heavier for supporting much larger topside facilities including GTL plants or more specialized e.g. early production systems (EPS), ex-

tended well testing units (EWTU) with full drilling deck or FDPSO’s with the real fully fledged drilling capabil-ity. The ship shaped FPSO’s will also in the future re-main the mainstream design concept (Fig. 1): The tradi-tional shipbuilding concepts and technology can be utilized in their design and construction even with the FPSO specific requirements. The ship shape is also a clear advantage for transits.

Fig. 1: FPSO Aoka Mizu. Courtesy of Bluewater.

The future FPSO designs must meet several increased or even completely new challenges. The structural interac-tion between the hull and the topside becomes more demanding due to the new much heavier and larger topside production facilities. The increased topside weight combined with extreme water depths over 1000m introduce further challenges of designing cost-effective mooring and flexible riser systems as well as for the stable station keeping requirement. Dynamic positioning with thrusters is a completely new and tempting challenge especially for the EWTU and FDPSO type FPSO’s. The present paper focuses on the design of the hull steel structure of a new-build FPSO based on direct calcula-tion approaches utilizing advanced numerical analyses. The structural design concept introduced builds on DELTAMARIN and DESART experiences and re-sources in naval and marine design and a wide variety of state-of-the art numerical analysis capabilities.

FPSO-Specific Design Features

A major hull structure failure of an FPSO would direct-ly endanger the safety of the personnel. At the same time it would result in high economic consequences through the potential environmental consequences and lost/deferred production during shutdown. There is therefore a strong case on safety, environmental and economic grounds for robust and reliable hull design where advanced hydrodynamic and structural analysis takes the most important position. Regardless of the design concept, an FPSO is always treated as an “Offshore Installation” rather than as a trading tanker – in terms of design and reliability. Un-like tankers, FPSO’s are intended to operate at a specific site for a number of years without dry docking; so nor-mal maintenance, inspections, and repair have to be carried out onsite. The unit shall withstand the site Me-tOcean conditions covering the operating and the most severe e.g. 100year conditions for strength and the cu-mulative design life conditions for fatigue. The site MetOcean conditions play a major part in the design of FPSO’s hull, mooring, risers and topsides. The extent and complexity of the design process varies greatly depending on the actual project details and site MetOcean severity. The diversity and complexity of an FPSO structural design task becomes clear from the class society rules (ABS 2003, 2008; DNV 2002, 2004; Lloyd’s Register 2008). They offer only a limited amount of simplified easy-to-use approaches for struc-tural design. On the contrary the tendency is to promote the use the state-of-the-art numerical analysis approach-es for the structural design. Designing an FPSO is always a large scale project, where the design process requires solving many multi-disciplinary problems. The structural or any other part of the design cannot be done in isolation but in a close and organized interaction with the other disciplines. This calls for controlled and integrated project execu-tion with disciplined management experience in fast multidiscipline projects, coordination and information flow/information management at all levels of the project. Information management is in crucial position and we have extremely good experience with our Del-taDoris, a web based easy-to-use document manage-ment system. From the structural discipline timely design develop-ment, analyses, problem identification and solving are expected, at the high and reliable technical level re-quired. The design analyses require utilizing a broad spectrum of programs ranging from initial design tools to the high end state-of-the-art analysis software.

FPSO Structural Design Process

The procedures and software applied for the FPSO structural design need to support fast decision making and concept alternative assessment especially during the front end stages. During the later stages the effective-ness is still needed. However the range of capabilities of the software becomes equally important. The broad range of the analyses implies a need for a system com-

posed of a state-of-the-art software modules, many different models and procedures for automatic data transfers. In the present system the FPSO design is developed within a 3D CATIA product model supporting all dis-ciplines. The structural analysis system is developed on using large number of initial design tools of the differ-ent Class Societies. For the FEA Femap and HyperMesh are used for the model and result processing and AN-SYS, NASTRAN and RADIOSS for analyses. Addi-tional software is available for more specialized non-linear and multiphysics analyses. Several programs are available for the naval architectural support such as NAPA and AQWA. For the structural design development the product mod-el provides an up-to-date link to the actual FPSO design enabling reliable interfacing between the various discip-lines. For the structural analysis the product model pro-vides an up-to-date geometry, steel model and weight data as a basis for the structural analysis which at first utilizes various available easy-to-use procedures (Fig. 2 a). This analysis relies on the traditional ship design procedures and it is especially well suited for the pris-matic ship shaped FPSO hull section.

B.C.

DATA

FPSO PRODUCT  MODEL

STRUCTURAL ANALYSIS 

DATA

FPSO PRODUCT  MODEL

LOCAL FEA

LOCAL DESIGNS

LOAD

S

A) B)

CONCEPT

UPD

ATES

UPDATES

GLOBAL FEA

NAVALNAVAL

LOAD

S

Fig. 2: Initial structural design process utilizing A)

easy-to-use procedures and B) FEA.

The FPSO designs include various non-prismatic hull details such as all the topside support foundations on deck, turret/hull interface, a moonpool area and derrick supports. Their initial stage structural analysis can be based on previous project data. A more accurate check-ing of such details, however, requires the use of FEA.

Fig. 3: Sample FPSO steel model and section of cor-

responding FEA model.

The development of the FPSO model within a CATIA product model facilitates effective use of 3D FEA al-ready during the initial design stage (Fig. 2 b). The FEA model is easily produced with the effective modeling routines and utilizing the automatically transferred geometry and property data (Fig. 3). This capability facilitates fast checking of non-prismatic cross sections and other details of the hull design concepts. This even though such a FEA model will inevitably utilize simpli-fied load and weight modeling. During the concept and early design stages the easy-to-use and rapid modeling procedures facilitate repeated reproduction of even the complete global FEA model after major design modifi-cations.

Fig. 4: General Configuration of the Analysis

System.

During the basic design (FEED) stage the global FEA model will become much more complex (Fig. 4). At this

stage several kinds of analyses are performed including hydrodynamic and static or even aeroelastic for defining loads, linear elastic stress/strain for strength and fatigue and more specialized non-linear high speed structural dynamics with large deformation and failure. A large number of models are built and processed; hundreds of load cases are defined, solved and post-processed. Many design iterations are conducted to respond with fast feedback concerning consequences of design modifica-tions. Great care is paid to load and weight modeling utilizing the product model, the hydrodynamic and -static analy-sis software while targeting balanced design load cases. The FEA model is loaded with external hydro- and aerodynamic and buoyant pressures, mass inertial forces due to ship motions and internal pressures in tanks. The idea is to use a common global FEA model which is used for global hull strength analysis and fatigue screen-ing. But this is used as well for producing boundary conditions for all the local FEA models for more de-tailed analyses. At this stage the global FEA model cannot be frequently reproduced. A new revision initiates checking and re-analyzing a series of detail analyses as they rely on the global model for their boundary conditions. As a conse-quence the quality of the global FEA model becomes paramount for the accuracy and reliability of the struc-tural design. The global FEA model needs to be updated throughout the project but in a strictly controlled way. A revision log is necessary tool for keeping track of the various design changes implemented in the model.

Global FEA model

The global A-level hull FEM model shall provide a reliable description of the overall stiffness distribution. The mesh density of the model shall be sufficient to describe deformations and nominal stresses in the pri-mary members of the hull. The model incorporates two other important structural parts of the FPSO, i.e. the supporting interface structures of the topside and the station keeping system (in particular the mooring sys-tem). The model should be built in sufficient detail to describe properly elastic and inertial coupling between the hull and those two other structural systems. This is the reason for building the model with a mesh finer than usually accepted when global ship models are created. It is particularly important when internal turret system is applied having significant impact on the global stiffness distribution of the hull. Actually, the modelling should be as detailed and complete as practicable possible and reasonable within the model management and process-ing limits. In general, the density of the mesh describing the hull structure is similar to the mesh commonly de-veloped when building the so called “3 cargo hold” models. The topside framing and stools are modelled by coarse mesh, but with use of shell elements.

Local detail B-level Structural Sub-models

Where the 3-D global analysis is not comprehensive enough to determine adequately the stress distribution in

the hull girder or in the main supporting structures, additional analyses are performed with use of FEM sub-models. In the fine mesh sub-models, care is to be taken to represent the structure’s stiffness as well as its ge-ometry accurately. Boundary displacements obtained from the global model are transferred to the sub-models as boundary conditions. In addition to the boundary constraints, the pertinent loads are reapplied to the fine mesh of the sub-models. A large number of local struc-tural details need to be considered for this analysis: i ) A large part of a typical midship cargo area contain-ing storage and ballast tanks, typical transverse bulk-heads and web frames – the analysis is oriented on transverse structural strength assessment; external hy-drodynamic pressure is corrected to be more realisti-cally distributed on the hull wetted surface; the interac-tion with topside structures is taken into account. ii) Hull/turret or hull/spread mooring interface struc-tures; loads from mooring lines and risers taken into account. iii) Foundations and hull supporting structures of: top-side modules, deck crane pedestal, riser hang-off plat-form, riser pull-in system, offloading system, helideck, azimuth thrusters of DPS, etc.; all possible load compo-nents with the most onerous combinations taken into account.

Local fine mesh C-level Structural Sub-models

The fine mesh models are specifically dedicated to rela-tively small local structural areas where significant stress concentration effects are expected. They are pro-duced as next level sub-models and are used in the local ultimate strength and fatigue capacity assessment of those special areas. Boundary deformations/boundary forces are transferred from the coarser models (B-level models) as boundary conditions. All the pertinent loads are preserved and reapplied to the new mesh, together with new ones considered as loads specific to the par-ticular local area. The local fine mesh models may be required for exam-ple for the analysis of hopper knuckles, bracket and flange terminations of main girders, longitudinal stiff-ener connections, topside stools, supports of turret bear-ings and other moonpool structural elements, supports of fairleads and chain stoppers.

Defining Design Loads

The FPSO hull structural design needs to consider both the still water loads and the environmental loads. The first vary between the ballast and full load conditions and the latter are described through the statistical MetO-cean data. The still water conditions are defined in the FPSO’s Loading Manual. Application of these load cases in the structural model is straightforward. Howev-er it requires careful and good modeling accuracy. Environmental loads on FPSO hull are induced by waves, wind and current. Already the statistical analysis of MetOcean data alone for the operational and extreme wave and wind conditions as well as for the long term design life conditions can be used for early estimations.

However a seakeeping analysis as described by Mazers-ki & al. (2010) is required for design load analysis al-ready for the early phase concept development. The basis for the environmental design loads of an FPSO are the vessel responses in the site environment. For the great majority of structural details the 1st order wave frequency loads dominate and a spectral based analysis in frequency domain is sufficient. In this case a panel model (Fig. 5) and the AQWA diffrac-tion/radiation program is utilized for producing various Response Amplitude Operators (RAO) which are then processed through the spectral procedures to a number of design load cases. Time domain non-linear analysis is necessary for cap-turing non-linear load effects and low frequency loads. This may be required for predicting effects like bow impact, slamming and green water, for example. a)

b)

Fig. 5: a) FPSO seakeeping model with pressure con-

tours in head sea. b) Wave loads on FE model.

In addition to the site loading conditions, the additional one corresponding to transfer voyage must be taken as well. The transit from fabrication yard to the site is a one-off short duration event. Nevertheless it often yields to surprisingly high design loads which may even be design drivers. The effect of the transport can be miti-gated by route selection. A dry transport would be tempting but is at present hardly a realistic option.

Strength Analysis and Design Development

The ultimate strength capacity of the FPSO steel struc-ture must be checked against yield and buckling limits. Corresponding structural analyses require using the global A-level model for hull girder longitudinal and transverse strength capacity. The use of the B-level local model is usually sufficient for the strength evalua-tion of local primary/secondary structures. The combinations of the various loading conditions and

environmental loads result in a vast number of different load cases. Therefore a selection procedure is applied as developed by Hachmann (1991) and Liu, Spencer & al. (1992) and adopted in ABS (2001). In this approach the criticality of the load cases is described by a Dominant Load Parameter (DLP). For FPSO several DLPs must be applied such as global cross section forces, different acceleration components at various locations and mo-tion parameters yielding to different design load cases. The design driver load cases are determined based on statistical spectral analysis of the values of the DLPs. A set of design wave load cases defined based on the ex-treme DLP values are then applied for the actual strength analysis. The design waves are regular waves selected and scaled to reproduce the environmental wave load part of the DLPs. The actual load cases in FEA models require transfer-ring acceleration and pressure loads from the seakeep-ing model. A significant source of error in the load transfer is the weight data. In practical projects the product model, naval architectural models or the struc-tural models all contribute to the correct weight and weight distribution data. As a result the weight data applied in the analyses is always an approximation and a compromise. For a successful load transfer the seakeeping and the structural models need to use the same weight data. This means that the weight, COG and the inertia terms of the seakeeping model correspond to the weight and weight distribution of the FEA model. A fine tuning between the two models is usually made by modifying the actual FEA weight distribution. Additional important source of imbalance is in the dif-ferent panel and FEA representation of the wetted hull form. Normally this results in only a small imbalance which can be corrected with suitable small modifica-tions in the accelerations. Significant imbalance is usu-ally attributed to errors in models. A systematic control of the weight data and load balance is however impor-tant.

Fatigue Control

An FPSO hull includes a vast amount of potentially fatigue critical details. The main fatigue loads are caused by waves and external forces such as mooring, riser or topside support loads, for example. The fatigue design against the external loads can typically be solved as detail design issues. However the fatigue design becomes easily a demanding challenge with too high global nominal stresses from hull girder wave bending moment (WBM). And this is dictated by decisions made very early in the project. The topside supports causing a stress concentration on the main deck are an example. The local stresses are further increased by the topside support static and dy-namic loads. Meeting the required high fatigue safety factor requires reasonably low level of the hull girder wave bending stresses as the use of locally reinforced scantlings at the main deck i.w.o. the topside supports proves often ineffective. Consequently the WBM becomes the main controlling

load parameter for the fatigue. The WBM response of a ship-shaped FPSO is similar to tankers for which rule values are readily available for fatigue design purposes as given e.g. by (DNV 2003). This data if used for FPSOs results typically in high conservatism (Fig. 6) the main contributing factor being the true FPSO site MetOcean conditions. The MetOcean wave data and the vessel WBM res-ponses are the key data required for the fatigue control of an FPSO design at the early phases. First estimate of the fatigue severity of the FPSO site can be based even on the wave scatter data only, with no relation to the actual vessel structure. In the case example (Fig. 6, 7) the fatigue life at the actual site becomes 6.5 times of that at the North-Atlantic based on the scatter data only (Lillemäe 2009). With the use of the vessel WBM re-sponse the factor is increased to 14 (Fig. 6, 7) as the short wave content is much higher in the site wave data but the WBM for short waves is low. The reduced fati-gue damages due to the site wave data correspond in this case to a stress concentration of 1.9 (scatter data only) or 2.4 (vessel WBM and scatter data).

0 1 2 3 4

ln(W

BM)

ln(ln(1/Q))

North-Atlantic Wave Data

Site Wave Data

DNV 4 6 8 10 12 14 16

Pro

babi

lity

dens

ity (-

)

Sea state period, Tz (s)

North-AtlanticSite

Fig. 6: Long term WBM responses on a Weibull plot.

Wave scatter data shown in small graph.

Fig. 7: The effect of scatter data and vessel WBM

response on the relative fatigue load.

The WBM for the FPSO is processed utilizing the site MetOcean data and the sea-keeping model WBM RAO results. The amount of work required is so modest that this analysis is updated every time the sea-keeping model is updated. Yet this result combined with expe-riences from previous projects provides a good basis for the fatigue control already in the early project phases.

Fatigue Design

The difficulty of fatigue detail design is associated with the complexity of the numerical procedure combined

with the large numerical models and the vast amount of potentially fatigue critical details. The structural fatigue assessment requires a consistent combination of the effects of the global and local responses with systematic employment of spectral analysis methodology, the Ray-leigh model and local nominal hot spot stress S-N curve. The local hot spot stress response can in most cases be evaluated accurately only with detailed C-level 3D FEA models. Their utilization requires simultaneous analysis with the A-level and sometimes even the intermediate B-level models. The load response may consist of two or even more simultaneous load processes which cannot be analyzed with a single structural model within the de-sign process at least. As an example the fatigue loading for the mooring and riser supports are often produced by a mooring/riser designer. Besides, the low frequency component may be significant for example in the moor-ing fatigue loads whereas the hull girder stress response is typically dominated by the 1st order wave loads. Close to the mean water line the stress responses become non-linear due to the effect of intermittent wet and dry sur-faces. Accounting for this effect can be done by intro-ducing corrections for the external pressure distributions at the cost of additionally complicating the analysis procedure. Several of the fatigue critical FPSO details are loaded by two simultaneous load processes. Examples being all foundations loaded by the external support loads and the global hull girder loads. For some structures also the wind loads are relevant for fatigue and their effect needs to be combined with that of the wave loads. In most cases the fatigue damages for the two loads are analyzed separately. There is seldom sufficient data on the actual stress response spectra to support accurate combining the responses for damage analysis. The approximate combined damage equations (Mikkola et.al. 2003; DNV 2005) need to be used as direct damage summation is clearly un-conservative. In practice the fatigue design process of structural de-tails is controlled utilizing a hierarchical set of different analysis procedures and numerical models with increas-ing accuracy. Typically the increased accuracy is asso-ciated with increased complexity and effort. Conse-quently first screening type fatigue analysis approaches are applied which are perhaps less accurate but easy to apply. Their results are utilized for ranking similar structural details based on their fatigue criticality. Theo-retically it is sufficient to conduct more accurate fatigue analysis only for the worst detail of each case. Also the practical design experience often allows reducing the number of details requiring further analysis. The use of the FEA models for fatigue at early design phases is especially important in case of non-prismatic hull details. The moonpool area of an FDPSO is an example where even the preliminary global FEA models serve for analyzing the force flow in this cross section. A good design and increased scantlings will be required at the corner area which is analyzed using a refined FEA model (Fig. 8). For this detail the long term WBM re-sponse alone provides for a reasonably accurate fatigue estimation.

As soon as the global and local FEA models become available they can be utilized for fatigue analysis. A long term stress distribution is produced in Weibull form utilizing spectral based procedures. For the first level fatigue analyses the stress response is taken from the global (A-level) FEA models. The stress response selected is considered to represent the DLP for the ac-tual hot spot detail studied. A corresponding fatigue design wave is selected and scaled based on the Weibull scale parameter at the 10-4 probability level. The fatigue analysis conducted is typically a screening type analy-sis. If a final more accurate fatigue analysis is made then the detailed C-level FEA model must be applied with the developed Weibull distribution. A separate Weibull distribution combined with a limited set of scaled fatigue design waves are required for each detail analyzed. The accuracy of this simplified “fatigue design wave” Weibull-type approach depends heavily on the hot spot area design in the hull and its modeling by the A-level and C-level models. The approach becomes accurate when the selected global stress truly represents a DLP for the local hot spot stress responses. The analysis accuracy can be improved by applying the local stress responses from local detail A-level FEA models directly in the spectral analysis. This approach provides for improved analysis accuracy for cases with complex local stress responses. Its use is however laborious and time consuming and as such its usage should be careful-ly considered.

Fig. 8: Relative fatigue damages i.w.o. moonpool cor-

ner area.

The Load Component or Full Spectral methods are an alternative approach to the direct local stress based spectral analysis (DNV 2003, 2008). The loads are split into clearly defined individual unit load components such as global hull girder bending, unit accelerations and external pressure loads. Load components are represented by their own transfer functions derived by the sea-keeping analysis. The stress response at particu-lar detail location is determined by a linear complex superposition of the stress responses to the unit load component loads. The Load Component approach be-comes practically equally accurate compared to the local stress based spectral analysis provided the stress response is accurately represented by the superposition procedure. This requires e.g. separation of the global and local stress responses utilizing correct boundary

conditions in the analysis. The treatment of the external wave pressure load represents an additional complexity. This can be solved by representing the external pressure distribution with a reasonably coarse panel grid. The pressure distribution in each grid is treated as a separate unit load component. With the Load Component approach the size of the spectral analysis task is greatly reduced as the number of the unit load component loads is much smaller com-pared to the number of unit wave loads.

Special Analysis Challenges

Designing against accidental conditions is an example of the specialized analysis challenges of an FPSO project. The selection of relevant design accidental conditions is dependent on the selected safety philoso-phy of the project and Class guidelines as specified e.g. in DNV (2004a, 2005). The most common accidental events to be investigated are: collision with another vessel, dropped objects including helicopter crash, ex-plosions and fire. Typically the accidental conditions are characterized with high-speed (impact) transient dynamic loads. The structural response caused by the impact energy dissipa-tion involves large deformations and strains far beyond the elastic range. In the case of fire, the material proper-ties should be defined as temperature dependent and a heat transfer coupled analysis involved. In the case of explosion event with an associated blast loading effect, the fluid/structure interaction process should be mod-elled and solved interactively. Often the accidental conditions are taken into account utilizing approximations and applying generally ac-cepted design concepts. This approach cannot necessar-ily guarantee effective or even safe design. The strength of the escape route structures is essential for safety. However the complex nature of conditions during fire or explosion can be revealed only through careful analysis. Simple strengthening of the structures may not be the correct way to increase safety. At present a rigorous numerical analysis of the acciden-tal conditions is possible with the available state-of-the-art software. The FPSO project can benefit from the rigorous accidental condition analyses through in-creased safety of the personnel and the whole facility. The non-linear transient dynamic FEM analysis in an explicit approach is usually applied to solve the prob-lems of this kind. Special FEM models (B-level) dedi-cated to such an analysis are built and processed by explicit dynamics computer programs. In general, the hull structure capacity to resist loads arising during accidental events should be checked with all possible consequences taken into account to ensure that the general structural integrity is preserved and the whole structure remains stable. For example, dropped objects are not critical to the global integrity of the hull but they can damage the topside production modules directly or indirectly by destroying their skids and sup-port structures, also the major pipelines on deck can be damaged. As a consequence of this event more serious

events may occur (explosion and/or fire). Special pro-tection structures can be designed and installed on deck to prevent those situations. Their load-carrying capacity should be checked by computational analysis, (Figs.9,10).

Fig. 9: Structural damage due to impact of dropped object.

The associated load is characterised by kinetic energy governed by the object mass and its velocity at the in-stant of the impact. The kinetic energy is dissipated as strain energy in the impacted structure and in the dropped object. The load-bearing capacity of the struc-ture is sufficient if the response of the structure, devel-oping in time, finally reach the stable balance condition and does not collapse in a more global sense, (Fig.10).

Fig. 10: Typical dynamic response plot of the impacted

protection structure.

Managing Project Execution

The excellence of the structural design and analyses procedures alone does not guarantee success in the hull structural design. The analyses need to focus on solving the practical structural design issues for the benefit of the FPSO design. The structural designers ought to minimize the limitations on the FPSO oil and gas pro-duction related functions caused by the structural design requirements. This calls for not only high quality inter-face and data management but also for strict control of all the structural design and analysis tasks. It is equally important that the structural designers and analysts co-

operate with the experts of all the other disciplines. The structural design discipline applies the matured project management tools common for the design project and all other disciplines. The structural design analyses for an FPSO, however, are highly sophisti-cated, laborious and often time consuming tasks not easy to manage. Therefore working procedures have been developed specially for executing the structural design tasks (Fig. 11).

Fig. 11: Concept for managing FPSO structural design.

Structural analysis plan is a project specific document produced during the early project stage or even in bid-ding phase. It incorporates the rule and project design basis requirements into a project specific description of the design conditions, design load specifications and resulting analysis tasks. It combines the lessons learned from previous projects into the modeling and analysis approaches applied for the project. Developing the anal-ysis plan in close co-operation with the client and class guarantees a good starting point for the design analyses. The analyses are documented utilizing a common doc-ument structure. It is equally important to document the analysis models as well. This is realized through an analysis log which also serves for the need for identify-ing the documented results with the analysis models. The first revision of the analysis document includes a more detailed description of the analysis task basing on project design basis, Class rules and on the projects Structural Analysis Plan. The best practice is to get client approval for this revision at least on informal basis before entering into the laborious modeling and analysis tasks. During the analysis execution the discipline manage-ment needs to follow the progress and review interme-diate results as a routine. The structural analysis and design need to interact, sometimes on a daily basis. The decisions made on structural design must consider ef-fects on other disciplines as well and not forgetting the manufacturing aspects.

Conclusions

The FPSO hull structural design concept introduced consists of an experienced designer and analyst team

working within common project management frame-work and utilizing state of the art modeling and analysis software. The team utilizes a 3D product model com-mon with other disciplines and a full range of FEA and sea-keeping programs. The product model allows for direct interfacing between the structural design and other disciplines. Direct data transfer from the product model to the FEA systems improves the efficiency and model quality. Structural analysis programs and proce-dures are tuned to the requirements throughout the dif-ferent project phases. A management system and adapted working procedures have been implemented supporting the use of sophisti-cated numerical analyses within a safety oriented FPSO design project.

References

ABS (2001). “SafeHull-Dynamic Loading Approach” for Floating Production, Storage and Offloading (FPSO) Systems”.

ABS (2003). “Guide for the fatigue assessment of off-shore structures”.

ABS (2008). “Guide for building and classing floating production installations”.

DNV (2002). “Structural design of offshore ships”, Recommended practice DNV-RP-C102.

DNV (2003). “Fatigue assessment of ship structures”, Classification note no. 30.7, pp. 129.

DNV (2004). “Structural design of offshore ships”, Offshore standard DNV-OS-C102.

DNV (2004a). “Design against accidental loads”, Rec-ommended practice DNV-RP-C204.

DNV (2005). “Safety principles and arrangements”, Offshore standard DNV-OS-A101.

DNV (2008). “Fatigue design of offshore steel struc-tures”, Recommended practice DNV-RP-C203.

Hachmann, D (1991). “The Calculation of Pressures on a Ship’s Hull in Waves”, Schiffstechnik, Vol 38.

Lillemäe, I (2009). “Fatigue load assessment procedure for floating production storage and offloading unit”, Diploma Thesis, Espoo, pp. 70.

Liu, D, and Spencer, J et al. (1992). “Dynamic Load Approach in Tanker Design”, Transactions SNAME, Vol 100.

Lloyd’s Register (2008). “Rules and regulations for the classification of a floating offshore installation at a fixed location”.

Mazerski, G., Mikkola. T.P.J. & Ajanko, R. (2010). “Integrated approach to hydrodynamic analysis in design of offshore floating structures”, paper to be presenter at PRADS 2010 in Rio de Janeiro, Brazil.

Mikkola, T.P.J., Silvola, Arjava, J.-P., Ajosmäki, A. & Kukkanen (2003). ”Fatigue Design of Offshore Floating Structures”, Proceedings of 13th Interna-tional Offshore and Polar Engineering Conference, Honolulu, Hawaii, U.S.A., May 25.-30.