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Design of FPSO Topside Structure
Ozgur Ozguc, Assoc. Professor
Istanbul Technical University
Offshore platforms are designed to carry out exploration and production in offshore oil fields. A structurally dividable offshore platform into two sections such as the topsides and the hull. Floating output storage and offloading systems (FPSOs) are ship-shaped oil and gas processing units, capable of working under a wide range of water depths and conditions. Usually, FPSOs collect unprocessed fluids from subsea wells; extract and handle oil, water and gas; and store the oil in the storage tanks of the vessels for discharge to shuttle tankers. A typical FPSO vessel with topside and turret arrangement is shown in Figure 1.
Figure 1. A typical FPSO vessel with topside arrangement
Topside substructures are essential to support the “sitting” of the topsides at an elevation safe above the surface of the ocean. In general, the top and substructure of the platform are designed and manufactured separately. The integration methodology depends on the topsides and the design of the substructure, while the chosen integration method has a strong impact on the design of the platform, in particular the design of the topsides [1].
The structural behaviour of the FPSO topside module and its supporting hull depends on the interface structure that connects them, and the interface structure consists of a combination of individual unit support structures. Types of interface structures are various and, accordingly, the basic design of the FPSO topside module structure is greatly influenced, so various design methods should be considered from the initial design phase. Structural design of FPSO topside module requires consideration of the number of seats, connection type, and structural analysis options such as the range of finite element models, load conditions, and boundary conditions for verification of structural strength [2].
The topsides layout is influenced by followings:
- The amount of topsides equipment (process, utilities), and the associated facilities to be installed on the topsides (flare, cranes, technical rooms, accommodation, etc.),
- The overall size of the FPSO vessel (length and width),
- The safety requirements (escape ways, firefighting, blast walls, etc.),
- The requirements for laydown areas, maintenance and handling,
- The type of FPSO vessel (second-hand or new built).
FPSO hull topside interface details is shown in Figure 2.
Figure 2. FPSO hull topside and turret interface and details
Multipoint Support Columns on Main Deck
In this type arrangement, the topsides structure is supported by vertical columns that are welded directly to the vessel structure. The column sections may be tubular or square hollow sections, or prismatic I sections. The columns may need to be more slender with minimal diagonal bracing in the longitudinal direction in order to adequately absorb the vessel deflections. Please see Figure 3.
The advantage of this type of arrangement is that it is relatively simple to fabricate and reasonably well suited to integrate with vessel structure, which is important consideration in tanker conversions. The topsides deadweight can be maximized and fairly well distributed and eventually to underdeck reinforcement could be limited.
The disadvantage of this type of arrangement is that the high numbers of supports involved could make difficult for the accurate prediction of support reaction loads without a very detailed analysis under many load cases.
Figure 3. Multipoint support columns shown
Sliding Support Stools on Main Deck
Compared to multipoint column design, this arrangement has less support points, typically, around four to six per module. Each individual support will typically have a stool structure located on the vessel deck. This type of supports needs to be considered for large size modules in large new buildings. Please see Figure 4.
To better compensate for vessel flexibility, two or more of the supports will be designed to provide a laterally flexible connection. This usually achieved using composite elastomer material applied between the stool and module mating surfaces. The elastomer pads are stiff in compression but flexible in shear. With a suitable arrangement of stoppers and clamps, the module can be decoupled from the hull’s deck deformations.
The advantage of this kind of supports is that the clearance between topsides and vessel deck can be greatly increased for the access and maintenance because the number of supports can be fewer.
The disadvantage of this type supports is that the distribution of weight of topsides structure on the hull may be less uniform, and some of the resulting support reactions can be relatively high. On the other hand, significant reinforcement may be required below vessel deck. For many new buildings, this is not generally big problem, but for the conversion project could be a case.
Figure 4. Sliding support stools and elastomer shown
Barrier: Blast Protection
A barrier should be provided at main deck to prevent any hydrocarbon liquid release from spreading to other areas (i.e. living quarters). Providing a barrier at the lifeboat embarkation to protect personnel from potential radiation during evacuation. Please see Figure 5.
Figure 5. Blast protection on FPSO hull vessel
Topside Interface – Finite Element Analysis
In order to get a realistic picture of the strength sensitivity (i.e. fatigue, buckling, yielding) for each topside stool location, it is necessary to include the stiffness of topside modules in the cargo hold or full ship FE model of FPSO. Mass balance is not necessary as long as the stiffness is correct if the model is just used for the screening purposes. Please see Figure 6.
On the FE model the loads are applied such as vertical & horizontal hull girder bending moment / shear forces in the sagging and hogging conditions, vertical / horizontal / longitudinal accelerations acting at the CoG of topside module, deck deformations, internal and external pressures. It should be noted that still water values shall not be less than the permissible values stated in the loading manual [4].
It is important to consider topside stool using shell elements in order to extract the stress components easily. The topside modules can be model using beam model needs to be sufficiently rigid to avoid large unrealistic deflections, which lead to unrealistic results [5].
Figure 6. FPSO topside interface FE analysis
The assessment of topside-hull interface in terms of yielding, buckling and fatigue capacities can be done by means of integrated hull-topside analysis and separated local models for hull and topside structures [3].
If topside module is analyzed separately from the hull, hull deformations induced by bending moments and reaction forces from the topside model shall be applied to the hull model at a location which will have ignorable impact on the stress distribution in the hull model.
Ideally the topside support should be designed in a way that the longitudinal stresses from wave bending moment will not interact with the stress from the inertia forces from the topside module. This is usually achieved by using large soft brackets at the topside support to main deck
References
[1] Liu G, and Li H. (2017) Offshore Platform Topsides and Substructure. In: Offshore Platform Integration and Float over Technology. Springer Tracts in Civil Engineering. Springer, Singapore.
[2] Jang BS, and Ko DE (2018). A study on the structural behaviour of FPSO topside module by support condition, Journal of the Korea Academia-Industrial Cooperation Society, Vol. 19 Issue 11, pp. 18-23.
[3] DNV Norske Veritas (DNVGL). Buckling strength analyses, DNVGL Classification Notes No.30.1, 2020.
[4] Recommended Practice DNVGL-RP-C206, Fatigue Methodology of Offshore Ships, 2020.
[5] Recommended Practice DNVGL-RP-C102, Structural Design of Offshore Ships, 2020.
