The full from of the FPSO is Floating, Production, Storage and Off-loading vessel. The Floating production system is a floating oil platform which may be equipped with gas compression and water injection system, in addition to oil processing equipment. After processing, the stabilized crude oil is stored in the in the ships cargo tanks prior to being transferred in to shuttle tankers via mooring hawser and hose, reeled from the stern of the vessel.
Gas produced as by-product of oil production is normally used as fuel for the gas turbine powered electricity generators or where significant volumes are produced and an export pipeline is not available, it may be re-injected in the formation or flared when production exceeds demand.
FPSO is designed to receive hydrocarbons produced from nearby platforms or subsea template, process them, and store oil until it can be offloaded onto a tanker or, less frequently, transported through a pipeline
There are some significant design challenges which have had to be overcome to make the FPSO a viable proposition for long term field development goals. Whilst these relate primarily to the mooring system, turret and the flexible risers, the basic hull and the topside processing equipment all must be designed to a higher specification than those of a conventional tanker or offshore structure.
Main Parts of the FPSO
1. Hull
2. Topside
3. Turret
4. Mooring
5. Risers
Hull
A number of existing tankers have been converted for service as FPSO’s and in some cases they have retained their original propulsion system. Where operators have opted for new purpose built vessel, main propulsion systems are generally omitted, the vessel relying on the electrical powered thrusters units to assist in station keeping. The hulls are built in accordance with rules provided by the ship classification societies and are designed to remain on station for a period of 10-20 years without dry-docking.
Topside
The topside oil and gas processing equipment is designed and constructed in accordance with fixed offshore platform and refinery standard, specification as per American Petroleum Institutes, American society of Mechanical Engineers (ASME). How ever the code and standards must be adapted to take account of vessel motion. The additional loading due to wave action can be considerable and have a serious effect on the fatigue life of the equipment, particularly the process vessels and pipe works.
The Process equipment is normally per-assembled into skids, the skids being mounted on a framework on pillars 3 meter above the main deck of the vessel, the air gap providing some protection against green seas, large waves breaking on the deck and to provide adequate hazardous area boundaries.
Turret
The turret consists of a large diameter around 16-32m vertical cylinder which sits within the hull of the vessel, mounted on heavy duty roller bearings. The mooring wires and the flexible sub-sea risers are attached to the turret which prevents it from rotating. The vessel is thus free to rotate through and angle of 270 deg around the fixed turret under the influence of wind, waves and currents, the extent of rotation being controlled by the vessel’s azimuth thrusters unites.
Mooring
A spread of 8 to 14 anchors ensures that the FPSO remain on location whilst producing oil, the anchor spread typically consisting of a combination of wires and chains which are tensioned by winches within the turret. Convectional anchors, suction anchors or piles may be used to make the connection with the sea bed.
A mooring system is made up of a mooring line, anchor and connectors, and is used for station keeping of a ship or floating platform in all water depths. A mooring line connects an anchor on the seafloor to a floating structure. We will focus on mooring Mobile Offshore Drilling Units and Floating Production Systems.
The mooring line can be made up of synthetic fiber rope, wire and chain or a combination of the three. Environmental factors - wind, waves and currents - determine which materials make up the mooring system.
Risers
To permit the FPSO to move both vertically and laterally the connections to the sub-sea wells are made through flexible, steel reinforced risers. The risers are designed to absorb any wave induced motion which might affect the position of the vessel, often being draped over submerged mid-line buoys which help in reducing the loads on both the risers and the turret the oil being transferred from the turret to the process equipment through a swivel stack.
It is the swivels and to a lesser extent the risers which are currently the limiting factors in the development of ultra deep-water FPSOs, gas export lines in particular being difficult to install successfully and limited to a diameter of less than 16 inched at operating pressure of 340 bar. Another problem which increases with water depth is the risk of hydro-static collapse of the risers, a typical maximum water depth for a steel-reinforced riser of 8-10 inch diameter being approximately 800-1000 m.