Adhering to best practice in areas such as process optimization and piping, electrical and structural design will help to deliver safe and operable FPSO topsides that minimize capital and operating expenditures. Alliance Engineering’s Norb Roobaert, Juan Campo, Howard Newman and Alan Phillips outline their approach to topsides design optimization.
Successful topsides design requires effective decision making in five areas: process optimization, and module, piping, electrical and structural designs. Key to success is experienced engineers and designers who understand best practices for the structural interface between hull and topsides modules and for optimized equipment location, safety, maintenance and operability.
FPSO topsides projects developed by utilizing best practices benefit from the application of systematic detail design based on experience and proven results. The result is improved quality and consistency. Cost and schedule are reduced through efficient engineering and design that minimizes equipment and simplifies fabrication, installation and commissioning.
Process optimization
Optimizing the process before beginning detail design is a critical component of best practice and delivers measureable benefits to operators.
An FPSO was designed to accommodate oil production of 100,000b/d, associated gas production of 250mmcf/d, produced water of 100,000b/d, and water injection of 275,000b/d at 5000psig surface pressure. The process optimization goals were to meet HSE requirements, export maximum oil quantities, reduce capital expenditures, minimize deck space and operating weight, minimize process complexity, maximize reliability, minimize operating cost and maximize net present value.
The process optimization resulted in the utilization of an oil stabilization process that maximized oil recovery and operated at higher pressures to minimize compression. The resulting facility consisted of 10 modules and reduced topsides weight by 7000 tons compared to conventional stage separation, providing an approximate savings of $300 million.
Layout
Best practices help define the location of living quarters, safe access/egress, flare, hazardous and non-hazardous process systems, ship loading and unloading activities, and supply vessel and helicopter operations.
The layout of the FPSO topsides is driven by the method of mooring, sea state factors, and placement of the modularized process and utility systems, with allowance for operability, maintenance and, above all, safety. Vessel topsides orientation should be determined early as it effects the configuration of all topsides elements.
The FPSO discussed in this example is a spread moored vessel. Other vessel mooring types will have different requirements for topsides orientation. In this case the flare is placed downwind of the prevailing wind direction and as far away as possible from the accommodations. Helicopter approaches should be at quartering angle in the direction of predominant wind and above the accommodations. The approach angle must take into account obstructions from the flare, vertical equipment above the module structure, the destabilizing effects of exhaust gases from turbine-powered equipment and visual aids for helicopter touchdown.
Metocean data and dropped-object risk factors were considered in determining supply boat approach to allow for primary and optional offloading scenarios. An offloading buoy is the primary means of delivering cargo and the supply boat approach should always be up-current so the boat will drift away from the FPSO if it loses power. Laydown areas should be adjacent to the supply boat access to allow for ease of loading and offloading supplies and equipment. Riser locations should be on the opposite side from the supply boat access to reduce risk from dropped objects during transfers between vessels.
The FPSO layout best practice utilizes an arrangement of process and utility modules alongside a central pipe rack. The pipe racks must be of a similar length to their adjacent modules.
The most hazardous modules, based on process system, should be placed farthest from the accommodations to maximize safe distances. Lowest risk modules, such as the utility modules, are placed closest to the accommodations. A 20m gap, or a safe zone, should be maintained among accommodations and all process and utility modules regardless of the perceived risk or classification. Laydown areas, if not used to store hydrocarbons/ chemicals for long periods of time, are the only exception.
Equipment within each module should be arranged with heaviest items closest to the hull centerline and on the lower levels to reduce the effect on the FPSO’s center of gravity. Towers and tall vessels should also be located in modules as near as possible to the hull centerline to reduce the affect of sea state conditions on equipment performance and support structure.
Roll onboard an FPSO has the most variation from horizontal due to sea state conditions. The shaft of rotating equipment should be orientated parallel to the longitudinal axis of the FPSO hull. This mitigates the effect of the ship’s roll, reducing impact on rotating equipment performance and increasing reliability and uptime.
Structural
Modular systems simplify overall shipyard construction and reduce integration cost and schedule. Modules can be independently fabricated, tested and delivered to the shipyard as complete packages ready for integration, commissioning and start-up.
Generally accepted spacing between modules is 1m clearance; this can be reduced to 600mm between module/rack. Modules should be designed with a threemeter gap from edge of hull to outermost edge of module, excluding stairs, to allow overhead access to the ship’s main deck and to provide a perimeter egress zone.
FPSO modules are designed along a standard general ratio of the ship’s length divided by 10. This means for modular sections the length between outer module supports is a maximum of one tenth of the length of the hull being used.
Arranging the pipe rack along the longitudinal centerline allows process and utility equipment to be arranged on either side. This allows for later introduction of equipment, packages or modules with minimum risk to previously installed topsides components. The floor of the pipe rack should be at the same elevation as the lower deck level common to all modules to provide primary maintenance access.
FPSO module sizes are also restricted by the fabricator’s lift capacity.
Module support designs allow for the behavior of the hull and topsides. Increased hull modulus and complex flexibility issues require the link between the topsides and hull to be broken. Modules designed to be on a sliding stool system are able to move and reduce stress to the hull. Although two or three longitudinal frames usually are utilized, typically only one stool support per module longitudinal frame is pinned to the hull; the other stools are sliders. The ‘pin’ stool allows for free rotation but restricts translation in up to three directions. The sliding stools prevent movement only in the vertical downward direction, although some are designed to limit translation in the hull transverse direction as well. This minimizes the effects of hull hogging and sagging on both the module and the hull while keeping the module secured to the hull. All structural interfaces with the topsides modules or riser porches are designed to be directly over a hull bulkhead or frame.
The design of FPSO modules includes many load cases resulting in braced structures and less access to equipment. Sea state conditions greatly affect the design of FPSO topsides. Structures must allow for lateral bracing at every change in pipe rack direction regardless of its size. All long, straight-run pipe racks need to be tied back to module columns and have racking bracing in multiple locations along their length.
The in-place analyses of the modules include operating and extreme environmental conditions. The modules are also designed to be transported from the shipyard to the production site and lifted from the fabrication yard or transportation barge to the ship. Spectral fatigue analyses incorporating both the in-place and transit conditions are performed.
A free zone is established between hull, marine and topsides systems. All systems should be excluded from this zone unless recognized to be part of the integration or hook-up. A minimum 3m air gap is used between the hull deck centerline and the underside of the modules. This gap is kept as small as possible to maintain the lowest possible center of gravity on the FPSO hull. Top of module support stool is the battery limit between the topsides and marine structures.
Piping
Key to success for piping design is controlling interconnect piping between module/pipe rack and module/hull. Dedicated areas should be defined as having the least amount of differential movement. Interconnect piping on modules with sliding supports should be located in the module corner closest to the pinned-end support. The piping should be designed to have a fixed point in the module before crossing to the pipe rack or module-to-hull gaps to reduce dynamic stresses in equipment nozzles. Sufficient horizontal and vertical offset should be present to obtain greater pipe flexibility.
Piping should not cross the module-tomodule gaps but should be routed via the pipe rack to reduce vessel-induced stress.
Electrical
FPSO electrical systems should be designed to utilize more than one electrical building. Having a main electrical building aft and an auxiliary electrical building forward reduces the need to route a large number of trays throughout the vessel from one location. The addition of an auxiliary electrical building distributes the cable tray runs more evenly over the pipe rack, allowing for more efficient use of space, smaller electrical buildings and lower module weights.
Power generators, main transformers, main electrical building and large motors such as water injection pumps must be located in close proximity to reduce the amount of medium- and high-voltage cabling. These loads should also be as close as practical to the main generation station to reduce voltage drop due to cable resistance/reactance.
Medium- and low-voltage switchgear and medium-voltage motor control center should be routed longitudinally due to height-to-depth ratio.
Low-voltage motor control centers should be routed laterally, again because of the height-to-depth ratio.
Conclusion
Best practices for FPSO topsides design begins with optimizing the process, followed by equipment selection and layout according to established guidelines for safety and operability. The piping, structural and electrical designs all should follow established best practices guidelines.
Following these best practices will increase the safety and operability of the topsides design while reducing costs. OE
Norb Roobaert, PE, chairman of Alliance Engineering, has 40 years of engineering and project management experience in the oil & gas industry. Roobaert holds degrees in chemical engineering and chemistry from the University of Michigan.
Juan Campo, PE, manages Alliance Engineering’s structural department and has more than 40 years of experience in the oil & gas business. Campo is licensed in 13 states, has been granted three US patents and holds degrees in civil engineering from Louisiana State University and Tulane University.
Howard Newman manages the piping department at Alliance Engineering. He has over 32 years of experience in the oil & gas industry and is pursuing a chemical engineering degree at the University of Houston.
Alan Phillips, PE, a senior electrical engineer at Alliance Engineering, has worked on 13 FPSO topsides electrical designs and holds a degree in electrical engineering technology from the University of Houston.
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