Turrets & FPSOs

To my mind, the Turret to an FPSO is what a Heart or Lungs to a human body!It is central to the functionality of an FPSO. A number of statutory requirements pertaining to a wide range of issues including health, technical safety, work place safety, lifting operations, environmental protection and pollution prevention and control have evolved over a period of time ever since the first FPSO (Petrojarl I) entered UK waters in 1986.The safety assessment methods for the turret may typically include usage of approximate reasoning and evidential reasoning approaches.The first generation FPSOs were spread-moored with the risers feeding into a simple porch at mid-ship. Today, in most locations spread-mooring is not practicable and a single point mooring is desirable to allow the vessel to weathervane, adjusting to wave and climate conditions. A turret-mooring system has thus become an integral part of most of the modern FPSO systems, and the method of providing single-point-mooring. This swivels to provide effective weathervaning effect, maintain the system on station and allow fluid transfer from the risers to the process plant onboard regardless of the external environmental situations such as severe weather and wave conditions.Complex multiple swivel assemblies are used in recent designs of FPSOs; in some cases, production fluids are handled from multiple wells. Such designs of turret system are relatively complex and may typically include multiple flexible riser connections, pipes for process fluid and well injection, valves, initial separation assemblies, pig launchers and securing bolts.The internal turret mooring system consists of a turret, suspended from a bearing in a moon-pool at the bow of the vessel, with the lower end of the turret anchored to the seabed via 3x3 composite wire/chain legs attached to piles. The arrangement allows the FPSO to weathervane freely through 360 deg around the turret in response to varying weather and tidal conditions. This provides a passive mooring system, enabling the FPSO to be retained on station without any aid from thrusters or external sources. However, a single azimuth thruster is fitted to facilitate offloading operations and minimize green-water conditions.The main components of the mooring system are the mooring lines, which are secured to anchor piles. The mooring lines are grouped in 3 bundles. The bundles are spaced 120 degrees apart and the mooring lines within a bundle have a relative spacing of 2 degrees.The turret system performs four main functions in a typical FPSO: Maintaining the vessel on station through single mooring. Allowing weathervaning or rotation of the vessel to adjust to climate conditions. Fluid transfer from the risers to the process plant. Providing transfer of electrical, hydraulic and other control signals.There are four main interfaces separating the turret system from the vessel, important from a safety and operational standpoint as they are both points of load and/or hydrocarbon transfer. They are: Fluid Transfer System to ship. Main turret bearing. Lower turret bearing and cavity containing moon-pool or turret cavity. Mooring spider or disconnectable mooring buoy.

Usually flexible risers connect the swivel to the FPSO/turret interface. In cases where the turret includes a separate mooring spider, or mooring buoy in the case of disconnectable systems, there will be significant load transfer across the structure in way of the moon pool. The main bearing is the primary load transfer point from turret to the vessel including both axial and radial loads. The lower bearings, usually pads, take much lower loading. The vessel structure adjacent to the turret cavity can encounter significant ovality and severe loads and therefore is prone to development of cracking.The complication of the technique makes it technically one of the most exhilarating field of engineering..