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The Deltamarin FPSO hull structural design concept sees an experienced designer and analyst team working within a common project management framework and utilising state of the art modelling and analysis software. Direct interfacing between the structural design and other disciplines is achieved through using the Deltamarin 3D FPSO product model and project management concepts. The state-of-the-art analysis software tools are complemented with Deltamarin in-house developments covering design load processing and screening analyses against buckling and fatigue.

FPSO Hull Structural Design Challenge

The majority of the 150 plus FPSO units in service and those on order are ship-shaped, and are either conversions from trading tankers or new-builds. Future FPSOs will be heavier, in order to support much larger topside facilities including GTL plants, or more specialized, in order to include e.g. early production systems (EPS), extended well testing units (EWTU) with full drilling deck or FDPSOs with the real fully fledged drilling (D) capability.

Novel concepts have been introduced such as the first cylinder-shaped FPSO installed on Piranema Field, Brazil targeting improved wave induced motions, higher stability reserves and higher deck load capacity. It is expected, however, that ship-shaped FPSO will remain the mainstream design concept: traditional shipbuilding concepts and technology can be utilised in their design and construction even with FPSO specific requirements. The ship shape offers a clear advantage for transits during commissioning and de-commissioning and especially if the FPSO is to be used at different fields or oil wells during its service life. In hurricane-prone sea areas a ship-shaped, disconnectable FPSO with its own propulsion has clear safety benefits.

The design of an FPSO is always a challenge due to demanding offshore regulations, reflecting the high safety and reliability requirements. The diversity and complexity of an FPSO structural design task becomes clear from class society rules. The new much heavier and larger topside production facilities of future FPSOs, combined with extreme water depths over 1000m introduce further challenges. It becomes increasingly demanding to design cost-effective mooring and flexible riser systems, as well as to meet the stable station keeping requirement. The structural interaction between the hull, the topside and the mooring and riser systems may ultimately require completely new structural concepts. The dynamic positioning with thrusters is a completely new and tempting challenge especially for the EWTU and FDPSO type FPSOs.

The design of offshore structures has traditionally been based on utilising state-of-the-art direct numerical analysis procedures. Designing an FPSO is no exception, even with its traditional ship hull structures. Compared to a highly optimised tanker design an FPSO hull must withstand large loads from topside, mooring and riser systems, moored at site and unable (in most cases) to escape even the severest weather conditions. Preferbly, an FPSO design should complete service life without repairs and definitely without dry-docking. In order to develop a competitive FPSO hull design the extreme and fatigue loads on hull caused by the site MetOcean conditions must be reliably estimated during the front end design stage. The only practical option remains in combining the seakeeping and structural analyses for determining statistically relevant design responses for strength and fatigue analyses.

Structural Design and Analysis Toolbox

The procedures and software applied for the FPSO’s structural design need to support fast decision making and alternative concept assessment, especially during the front end stages. During the later stages the effectiveness is still needed. However, the range of capabilities of the software becomes equally important. The broad range of the analyses requires a system composed of various state-of-the-art software modules, many different numerical models and procedures for automatic data transfers. Speed and reliability in executing a numerical analysis corresponding to the ever-changing design becomes crucial for the project efficiency.

Deltamarin’s solution is based on utilising the CATIA product model of FPSO design supporting all disciplines. The structural model is developed in CATIA Steel. The product model provides an up-to-date link to the actual FPSO design, enabling reliable interfacing between the various other disciplines. For structural analysis, the product model provides an up-to-date geometry, steel model and weight data as a basis for the structural analysis.

The initial structural design analyses utilise various easy-to-use initial design tools of the different Class Societies (Fig. 1 A). At this stage the modelling effort is typically quite minimal. The FPSO designs include various non-prismatic hull details such as the topside support foundations on deck, the moonpool area and derrick supports. Checking such details for strength requires the use of 3D FEA. The same is true for the load cases with unequal and unsymmetric tank fillings. CATIA product model facilitates effective use of 3D FEA during the initial design stage (Fig. 1 B). The FEA model is easily produced with the effective modelling routines and the automatic geometry and property data transfer (Fig. 2). This facilitates fast checking of unsymmetric load cases, non-prismatic cross sections and other non-standard details of the hull design concepts. During the concept and early design stages this often means repeated reproduction of even the complete global FEA model after major design modifications.

During the following design stages the importance of the precision of the analyses increases. The global FEA model becomes increasingly more complex along with the project development. At the basic design stage several kinds of analyses are performed including hydrodynamic and hydrostatic or even aero-elastic for definition of loads, linear elastic stress/strain for strength and fatigue and more specialized nonlinear 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 modifications. Great care is paid to load and weight modelling utilising the product model, the hydrodynamic and static analysis software while targeting balanced design load cases. The FEA model is loaded with external hydro- and aerodynamic and buoyancy pressures, mass inertial forces due to ship motions and internal pressures in tanks.

In order to meet this challenge a partnership has been introduced building on the experiences and resources of Deltamarin and DesArt, Poland, in naval architecture and marine design and a wide variety of state-of-the art numerical analysis capabilities. The Deltamarin/DesArt concept is based on utilising a common global FEA model which is used for global hull strength analysis and fatigue screening. The global FEA model is also used for producing boundary conditions for all the local FEA models for more detailed 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 consequence the quality of the global FEA model becomes paramount for the accuracy and reliability of the structural design. The global FEA model needs to be updated throughout the project but in a strictly controlled way. A revision log is a necessary tool for keeping track of the various design changes implemented in the model.

Controlling Strength

An FPSO’s hull load requirements are governed by the vessel operational profile and the site MetOcean conditions. The rule extreme loads are based on North-Atlantic conditions which make them often far too conservative even as a starting point for an FPSO design. The vessel global hull girder load responses in site-specific operational and extreme conditions are a key result for any FPSO design at concept stage. Deltamarin utilises ANSYS-AQWA for seakeeping analyses and FEMAP/Nastran for structural analyses (Fig. 2 and 3). The design loads based on the global load long term responses in site MetOcean conditions are evaluated using the Deltamarin in-house statistical post processor program Delta FD tool.

Hull strength requirements are dominated by the hull global bending moment and shear force loads. The strength capacity is first checked using Class-provided software like the ABS FPSO Safehull or DNV Nauticus. For simple hull cases like an FSO this analysis is completed with a cargo hold 3D FEA analysis. The use of a more detailed 3D FEA model may become necessary for an FPSO hull with topside modules and additional non-prismatic structural details such as a moonpool. Typically, the same details often carry external interface loads which can be extremely high. Examples are topside supports and their foundations, hull interface to turret, moonpool area or riser platforms and their foundations.

The number of design load cases becomes large as they need to cover typically ballast, full and intermediate loading conditions with varying tank filling rates in each. The vessel should be analysed in operational and extreme sea state conditions under varying wave encounter headings and with a 3D FEA model. The amount of work is greatly reduced when utilising a dominant load parameter (DLP) for the load case severity indicator.

Present analysis tools are readily effective even for large 3D FEA models for strength checking when a specific local stress component values are compared against their allowable values. Buckling analysis, however, too often includes manual data transfer between the FEA model and buckling analysis software. The FEA representation of the structure by elements has usually no direct correspondence to the stiffened panel design. Consequently expert judgement is required in interpreting the detailed FEA stress results for the panel edge stresses needed in the buckling equations. The Deltamarin solution has been to develop an in-house tool for automated buckling screening analysis. A hierarchical buckling analysis with gradually increasing accuracy is applied facilitating rapid automated screening of the whole FEA model for buckling. The analysis process automatically focuses on structural details with highest buckling usage. Typically manual data processing for buckling is limited to few most critical panels.

Controlling Fatigue

Fatigue strength becomes the design driver for several details due to the high fatigue safety factor and the requirement for 20 years or even longer service life without any unscheduled maintenances. An FPSO hull includes a vast amount of potentially fatigue critical details. The main fatigue loads are caused by waves acting directly on the hull or through external forces such as mooring, riser or topside support loads, for example. Fatigue damage development is governed by local cyclic stress history for the complete service life at the structural discontinuities.

The difficulty of fatigue detail design is associated with the complexity of the numerical procedure, 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 Rayleigh model and local nominal hot spot stress S-N curve approach.

It would be tempting to tackle the fatigue design parallel with the detail design at late stages of the project. However the fatigue design can easily become a demanding challenge with too high local nominal stresses caused by the wave bending moment (WBM). The level of the local nominal stresses is dictated by hull sizing decisions made very early in the project. The topside supports causing a stress concentration on the main deck are an example. Here the local cyclic stresses caused by WBM are further increased by the topside support dynamic 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 ineffective.

The WBM response of a ship-shaped FPSO is similar to tankers for which rule values are readily available for fatigue design purposes in class society rules. This data when used for FPSOs results typically in high conservatism (Fig. 4) the main contributing factor being the true FPSO site MetOcean conditions. 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. 4) the fatigue life at the actual site becomes 6.5 times of that at the North-Atlantic based on the scatter data only. With the use of the vessel WBM response the factor is increased to 14 (Fig. 5) as the short wave content is much higher in the site wave data but the WBM for short waves is low.

Fatigue analysis of FPSO hull details requires the use of 3D FEA models which are, at first, relatively coarse. A more realistic analysis requires using local nominal stresses as the DLP. The coarse FEA model and the Weibull model is applied for fatigue screening which ranks similar structural details against their fatigue criticality. The effectiveness of this approach is due to using a coarse FEA model allowing the whole model to be screened. In principal only the worst detail of each type needs to be analysed with a fine mesh model. Naturally the past experiences of similar details are of great value at this stage.

More detailed fatigue analysis becomes necessary when the design process advances. The Weibull model can still in most cases be utilised but combined with fine mesh FEA models (Fig. 6). A more accurate analysis approach may become necessary when designing for severe MetOcean conditions or for exceptionally long service life. This is based on a full spectral fatigue analysis with fine mesh FEA models. For the project to succeed, it is essential that the early stage fatigue analyses have limited the local nominal stresses at reasonable levels. Otherwise the detailed fatigue analyses result in costly redesigns.

Controlled and Integrated Project Execution

The excellence of the structural design and analyses procedures solely do 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 minimise the limitations on the FPSO oil and gas production related functions caused by the structural design requirements. This calls for not only high quality interface 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.

Information management is crucial and we have had an extremely good experience with our DeltaDoris, a web based easy-to-use document management system. The structural design analyses for an FPSO, however, are highly sophisticated, laborious and often time consuming tasks not easy to manage. Therefore working procedures have been developed specially for executing the structural design tasks (Fig. 7).

The structural analysis plan is a project specific document produced during the early project stage or even in the bidding 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 modelling and analysis approaches applied for the project. Developing the analysis plan in close co-operation with the client and class guarantees a good starting point for the design analyses.

The analyses are documented utilising a common document structure. It is equally important to document the analysis models as well. This is realised through an analysis log which also serves the need identifying 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 an informal basis before entering into the laborious modelling and analysis tasks.

The Deltamarin concept for FPSO hull structural design provides the means for successful project execution. The challenges in any FPSO strength design are in finding the balance between the work effort, meeting the time schedules and the analysis correctness. In a practical project one is constantly in a position having analysis results covering the design details only partially, being based on more or less out-dated design revisions and covering design conditions only partially. The level of this uncertainty is greatly reduced with the intelligent strength and fatigue analysis procedures as implemented by Deltamarin.