A floating production, storage and offloading (FPSO) unit is a floating vessel used by the offshore industry for the processing of hydrocarbons and for storage of oil. A FPSO vessel 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 transported through a pipeline. FPSOs are preferred in frontier offshore regions as they are easy to install, and do not require a local pipeline infrastructure to export oil. FPSOs can be a conversion of an oil tanker or can be a vessel built specially for the application. A vessel used only to store oil (without processing it) is referred to as a floating storage and offloading vessel (FSO).
Oil produced from offshore production platforms can be transported to the mainland either by pipeline or by tanker. When a tanker is chosen to transport the oil, it is necessary to accumulate oil in some form of storage tank such that the oil tanker is not continuously occupied during oil production, and is only needed once sufficient oil has been produced to fill the tanker. At this point the transport tanker connects to the stern of the storage unit and offloads oil.
In the early days, the storage units consisted of decommissioned oil tankers, which were stripped down and equipped with process/production facilities (becoming therefore FPSOs), and were connected to a permanent mooring point. Today, there are two main types of FPSOs, those built converting an existing oil tanker, and those that are purpose-built. The FPSO design will depend on the area of operation. In benign waters the FPSO may have a simple box shape or it may be a converted tanker. Generally (but not always) the production lines (risers) are connected to a major component of the vessel, called a Turret, which allows the vessel to rotate in order to head into the wind and reduce environmental forces on the moorings. In relatively calm waters, such as in West Africa, turrets can be located externally to the ship structure, hanging off the bow of the FPSO. For harsher environments like the North Sea, the turret is generally located internally. The turrets and the mooring systems can be designed to be disconnectible or to remain permanently moored. Most ship-shaped FPSOs in the North Sea are purpose-built and are permanently moored.While most FPSOs are ship-shaped, some FPSOs have a semi-submersible type hull with storage (very rare), or have a cylindrical hull. The inherent symmetry of these FPSO configurations makes turrets unnecessary, so the platforms remain in a fixed orientation.
An FPSO has the capability to carry out some form of separation process. If the unit does not have such facilities, it is generally referred to as a Floating Storage and Offloading unit (see below), and would be operated in conjunction with a production platform. Process plant on FPSO is a core component on facility and forms a key part of production process. Production is usually conducted in 3 phases:
- Separation of Gas.
- Separation of Water.
- Separation of oil.
Gas recovered/separated during production may be used as fuel on Marine energy resource units ( MRU) fitted on board. Gas may be flared off in some cases if MRU is not fitted. Water separation may be carried out using Dehydrators or Hydro Cyclones.
Floating production, storage and offloading vessels are particularly effective in remote or deepwater locations where seabed pipelines are not cost effective. FPSOs eliminate the need to lay expensive long-distance pipelines from the oil well to an onshore terminal. They can also be used economically in smaller oil fields which can be exhausted in a few years and do not justify the expense of installing a pipeline. Once the field is depleted, the FPSO can be moved to a new location. In areas of the world subject to cyclones (northwestern Australia) or icebergs (Canada), some FPSOs are able to release their mooring/riser turret and steam away to safety in an emergency. The turret sinks beneath the waves and can be reconnected later.