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Analysing FPSO Topside Structures
G Sankara (PhD), Manager, Structural, Sidvin Core-Tech (I) Pvt Ltd If you have ever had the chance to look over a Floating Production Storage Offloading (FPSO) vessel, then you can't fail to be impressed by how much technology and equipment are placed into such a compact space. Topside Structures play prominent role in the functioning of floating production storage and offloading vessels. This article discusses how understanding the loads and design conditions which these structures are likely to encounter during their life cycle is essential for safe design and maintaining the equilibrium of the vessel.

Variety of major process module structures such as oil processing modules, gas compression modules, power generation modules etc, Flare tower, Vent stack, Helideck, Piperack, Cablerack and Laydown areas etc are built on the deck of FPSO to meet the various functional requirements. Module structures support equipment as heavy as 320T, as lengthy/tall as 17m, skids along with piping, mechanical handling facilities, access ways, electrical networks and instrumentation etc as shown in figure 1. Loads, load combinations, material properties, boundary conditions etc are key inputs for analysis of any structure. Topside structures are subjected to different types 1, 2 and 3 dimensional loads and forces during their lifting, testing, voyage, t and operation. Understanding the loads and design conditions which these structures are likely to encounter during their life cycle is essential for safe design as well as maintaining the equilibrium of the vessel.

Gravity loads, loads due to vessel accelerations, live loads, wind loads and loads due to vessel deformation etc are the various major loads considered in the analysis of FPSO topside structures.

Gravity loads consists of self weight of structural members like columns, beams, plates, grating, static weight of the equipment, piping, electrical, instrumentation. Weights of miscellaneous members such as ladders, stairs, handrails etc are also included in gravity weight. Distribution of gravity loads is very significant to maintain stability of vessel. Weights of piping, electrical, instrumentation, miscellaneous structures are usually added as density to the nearest structural members such as plates and grating for analysis using calculation software. Equipments are modeled in software as per their dimensions, support locations, weight and Centre of Gravity provided in the vendor drawings. These loads act vertically downward. Dry or Empty weights of equipment, piping etc are considered Transit survival and Lifting conditions etc. Wet weights ie, dry weight plus fluid weights are employed in design environmental and design operating conditions etc.

Inertia loads are calculated by multiplying masses of structure elements, equipment, piping etc with accelerations induced by the vessel motions. Vessels are subjected to wave induced motions during their passage and operations in variety of environments. Accelerations in three positive and three negative spatial directions are considered. Though these values vary spatially on the vessel, average values in three directions can be considered for each module. For structures like flare tower, vent stack etc actual acceleration along the height is applied.

As topside structures are above water level and open structures, wind load is applied on all the structures and equipment. 1-minute average wind velocity (ABS 2009) can be considered for calculating wind pressures. Variation in velocity with height is captured by considering velocity at a height of 12-15 m above main deck level for module structures and actual wind profiles for tall structures like Flare tower. Wind load is obtained by multiplying wind pressures with projected areas.

Stresses induced by hull girder bending on columns directly connected to vessel deck causes bending of columns as they are connected at top to pancake deck and at bottom to vessel top deck. In each module typically middle line of pancake columns are fixed to the vessel deck to transfer lateral loads from the module structure as depicted in figure 2. Remaining columns are hinged at bottom to allow the displacement due to hull girder deformations. Forces on these columns due to hull deformation are applied as displacement loads. Displacement of these columns depends on the distance from fixed columns. These loads are to be included in the analysis (ABS, 2009).

Loads representing operating personnel, trolleys, temporary storage etc are applied as uniform or concentrated loads on pancake deck plate and superstructure gratings etc. In lay down areas they represent the weights of temporarily resting equipment.

Normally a uniform load of 4510 N/m2 in walkways and access ways, 9020 N/m2 in operational areas and 13000 N/m2 in storage or lay down locations is applied (ABS 2009). In lay down area, these loads are combined with gravity loads and vessel accelerations are applied on them.

Topside structures are subjected to mild and harsh wind and wave induced accelerations during their transit and operations. Affects of these environmental loads of different magnitude and directions are analysed along with hull girder deformation loads and live loads. Possible worst case scenarios for global and local analysis are arrived by combining loads in different headings such as head seas, beam seas and oblique seas (ABS 2009).

Loads while lifting of structures and hydro test conditions could be different from the environmental loads. Environmental events associated with various design conditions described below are adopted from 5B-3-3/Table 1, ABS2009.

Design Environmental Condition/Field Survival Condition (DEC) This combination defines the intense environmental loadings that may act on the structural components. Severe of 100 year wind with allied wave and current loads or 100 year wave with allied wind and currents are considered. These loads are combined with gravity and vessel deformation loads. Wet weights of piping and equipment etc are used. Typical load combination is as below.
S.No Gravity Accelerations Vessel deformation Wind loads

Design Operating Condition (DOC) Moderate environmental loads that may occur often are considered. Usually loads attached with one-year return period are used. Gravity loads in operating condition, live loads and vessel deformations are associated with environmental conditions. Typical load combination is as below.
S.No Gravity Accelerations Vessel deformation Wind loads Live load

Transit Survival Condition (TSC) Structures need to be analysed for the environmental conditions occurring during voyage. Usually transit from shipyard to drilling fields is considered. If more than one shipyard is employed for building and installing modules on the vessel, passage from one yard to another yard in transit route during is also considered. Normally severe conditions of ten year return period are used in the analysis. Piping and equipment is considered dry.

S.No Gravity Accelerations Vessel deformation Wind loads

Hydrotest Condition During testing of the modules hydrotest loads of equipment and piping are included as part of gravity loads. Environmental loads, vessel deformation loads and live loads are not applied.
S.No Gravity

Lifting Condition Structures assembled with equipment and piping etc need to be intact during their lifting for integration with supports existing on the vessel. Empty weights of piping and equipment etc are considered. Factors applicable to lifting such as dynamic amplification factor sand design consequence factors etc are applied to structural components and weights.

S.No Gravity
In addition to above normal design conditions, specific structures are analysed for other conditions. Helidecks are analysed for impact loading due to helicopter landing, modal analysis etc. Flare stack are analysed for Modal analysis and Thermal loading etc. Fatigue damage/fatigue life assessment at interface of topside structures with vessel deck is also need to be performed.(ABS 2009).

1. Topside structures on FPSOs are subjected to Static gravity loads, Live loads, Wind loads, Vessel motion accelerations and Hull girder deformation loads etc.

2. Direction and magnitude of these loads need to be combined in such a way to generate severe design conditions.