Knowledge Centre
Subsea
Introduction
The offshore oil and gas industry has been using subsea technology since the mid-1970s. It entails either ‘tying back’ a well head to an existing platform or connecting it to a floating production, storage and offloading (FPSO) system.
The technology offers significant advantages over fixed production platforms particularly in remote offshore locations where deep water, strong ocean currents and harsh weather conditions may occur, or where export pipelines are difficult to install or uneconomic to run. FPSOs can be floated off at the end of a field's productive life and re-used elsewhere, which has environmental as well as economic benefits, particularly for marginal fields where the production facilities may only be required for a few years.
Subsea technology is allowing the UK’s oil and gas reserves to be extracted more cost effectively and in deeper waters than if platforms had to be built to access each accumulation. In 2008, 43% of UK oil and gas production was accounted for by subsea wells. The demand for subsea expertise and parts is growing around the world and in this area, the UK is seen as the global leader.
FPSO design and construction
The rules and regulations for the design and construction of FPSOs referred to by classification societies are not FPSO-specific and do not fully address the interface between marine and offshore production systems. Therefore, guidance notes have been developed to guide designers, owners and operators when planning and building new FPSOs.
FPSO mooring
The offshore industry has developed highly sophisticated mooring and station-keeping systems which enable FPSOs to operate safely and reliably. In the UKCS, where weather conditions can be extreme, most have a central mooring arrangement located within the hull in a ‘turret’, that allows them to rotate freely around the point of mooring in response to shifting weather direction. This is known as ‘weathervaning’ and allows the vessel's bow always to point into the prevailing wind and currents, minimising the impact of nature's forces. Often thruster systems are also used to supplement the station-keeping and control vessel heading. Mooring Integrity guidelines promote safer mooring of FPSOs.
FPSO production
FPSOs carry on board all the necessary production and processing facilities normally associated with a fixed oil and gas platform, but with the addition of storage tanks for the crude oil recovered from the wells on the seabed below. The hydrocarbons treated on an FPSO are produced through wells located on the seabed.
FPSO tank work
FPSOs contain tanks for the storage of hydrocarbons. There are significant risks to personnel engaged in inspection, maintenance and repair activities within these tanks and also within slop tanks, water ballast tanks and void spaces. A presentation addressing the tasks, risks and methods of prevention of accidents while working in tanks can be found here.
Transport from FPSOs
Untreated liquids are brought to the surface via subsea equipment, comprising valves at the well (a Christmas tree), flexible pipes to connect several wells together and a flexible riser which links to the FPSO. Flexible risers must be able to accommodate the heaving motion of the vessel above and be very resistant to fatigue.
Guidance on monitoring methods and integrity assurance for unbonded flexible pipe is available here. It includes information on risks specific to the use of flexible pipe and on actions which the project or operations manager can take to minimise those risks. Oil must then be transferred from the FPSO to a tanker through flexible hose or hoses. A framework of management, technical controls and procedures necessary to ensure the continued integrity of flexible hose is available here.
The process of loading from the stern of the FPSO to the bow of the shuttle tanker is known as ‘tandem loading’. In order to keep shuttle tanker ‘loss of station keeping events’ to a minimum, guidelines have been developed on managing collision risk. Volume 1 addresses performance standards for controlling risks associated with tandem offtakes involving shuttle tankers and FPSOs. Volume 2 deals with the use of towing assistance during offtake operations.
FPSO repair and maintenance
In 2003, a review of best practice in repairing and maintaining FPSOs was carried out. The report is available here.
FPSO safety
Safety on board any production facility has the utmost priority. Robust safety standards must be in place and rigorously enforced. In the UK, all operating companies must submit a safety case to the Health and Safety Executive (HSE), which demonstrates that the company has:
- In place safety management systems
- Identified risks and reduced them to as low as reasonably practicable
- Put management controls in place
- A safe refuge for personnel in the event of an emergency
- Made provisions for safe evacuation and rescue.
- Features which contribute to the safe operation of FPSOs are described below:
- The hull must be designed for at least the expected life of the field - often 15 to 25 years - and constructed to standards that will permit it to remain at sea during this time without access to dry docking facilities; of special importance is how the vessel will survive a possible collision at sea (normal maritime criteria are used such that the vessel will be able to stay afloat with any two hull compartments flooded)
- Crude oil stage tanks, an integral part of the FPSO hull, are blanketed with inert gas to maintain a safe environment for loading and discharging crude oil
- Oil and gas processing is controlled and monitored remotely; shutdown systems are built in to close off the flow and contain hydrocarbons under pressure in an emergency allowing depressurisation via a flare stack
- Protection against fire and explosion is provided in the form of protective coatings and blast or firewalls; water deluge systems are installed in open areas and sprinkler systems in closed areas
- The accommodation block is mechanically ventilated and pressurised, taking fresh air from a safe location remote from the process equipment, and is thus a safe refuge for personnel
- Emergency evacuation is primarily by helicopter from a helideck situated directly above the accommodation block, or by service craft; other methods use lifeboats, life rafts and standby service craft working in the field
- Personnel are trained to respond to various accident scenarios relating to the process systems, including safe shutdown of operations, de-pressurisation and eventual evacuation; all personnel are provided with personal safety equipment such as survival suits and life vests.
