FPSOs are an evolving breed

FPSO design and construction has evolved over time to respond to the industry?s demands for safe and efficient facilities to operate in an ever more challenging environment. However, there is always room for improvement and opportunities to learn from experience in the design, construction and operations of FPSOs built to date.

There are about 15 FPSO and FPS units operating off the UK and 70 worldwide. Oil and gas is being found and produced in ever-deeper waters and harsher environments, which affect the type of production facility to use. Floating units are a cost effective solution for developing smaller, satellite or marginal fields in shallower water as they can be floated off when reservoirs are depleted, and re-used elsewhere.

The UK Offshore Operators Association (UKOOA) highlights that the structural loading spectrum encountered in FPSO service is higher than that experienced during relatively long periods as trading tankers, particularly in the way of fatigue. It is for this reason that it is important to improve local detail design at the conversion stage, thereby enhancing fatigue performance.

The UKOOA says that service experience has shown that there is little to choose between the overall performance of purpose-built FPSOs and those developed from speculative new-build hulls. In the case of the hull, both new and old vessels have experienced structural failures that would indicate that site-specific environmental loadings are in excess of those predicted by either the design or the hull strengthening report.

When it comes down to vessel selection there are three basic choices:

l building a new vessel

l converting an existing vessel

l intercepting a newbuild.

The newbuild offers the ability to customise the design to suit the needs of a particular field, which allows for the hull size to be optimised, so saving on hull/mooring costs. The hydrodynamics, structure and systems can also be optimised for long-term operation, minimising operating costs.

Converting a tanker involves less capital expenditure and generally gets the unit on station quicker than a newbuild. Conversions are more suitable for short- to medium-terms on station where the environment is not too hostile. However, this does not preclude their use for longer life or in hostile environments as long as they have been suitably upgraded.

The third option is to intercept a newbuild during construction and equip it for duty as an FPSO. This can reduce the cost of a newbuild but still offer a vessel that has not used up part of its fatigue life.

Converting tankers for use as FPSOs has become big business and a number of ship repair yards have become specialists in the business. One of the leaders in this field is Keppel Shipyard which has completed 42 FPSO/FSO conversions and upgrading projects. The yard recently completed the fast track conversion of the 307,431dwt FPSO Serpentina 10 days ahead of schedule for Single Buoy Moorings (SBM) Inc.

FPSO Serpentina is the second generic FPSO that the yard has converted for SBM ? the first being FPSO Falcon in June 2002.

The generic FPSO is designed to cope with a wide range of field properties and water depths up to a maximum of 2,100m, and has the versatility to relocate between fields during its lifetime.

The vessel was converted from the tanker, Amazon Eagle and work began in May 2002 on a full refurbishment for life extension, installation and integration of topside modules and the fabrication and installation of the external turret, two topside modules, flare tower, helideck and tandem berthing of the export vessels.

FPSO Serpentina, which will be deployed off Equatorial Guinea, will be able to produce up to 100,000 barrels per day of oil.

The yard also completed the conversion of FPSO Four Vanguard after a nine-month stay at the yard. The 94,225 dwt vessel (ex-Four Lakes, Isola Blu-97) was converted for the Four Vanguard SNL joint venture between Italian-based Premuda and British-based Vanguard. The scope of work included fabricating and installing a comprehensive topside system, helideck, flare tower and the installation of a disconnectable single point mooring system, which can quickly disengage the FPSO when faced with cyclone weather conditions.

The vessel is now deployed in the Woollybutt field offshore western Australia, operated by ENI – Australia, which owns the block in partnership with ExxonMobil and Tap Oil. The Woollybutt field has been developed with two subsea wells (approximately 1.7km apart) tied back to a mono hull FPSO.

Four Lakes entered Keppel Shipyard in Singapore on June 6, 2002 and was completed in February 2003 and accepted its first oil on May 5, 2003. The conversion work was carried out in accordance with the leading rules of the Australian Government with regard to safety, procedures, etc., where the FPSO will work for the duration of the project.

The double hulled Four Lakes was equipped with an eight point spread mooring and was fitted with modular deck-mounted production facilities, including gas lift and water treatment. A deck-mounted generator also provides power generation of 4MW.

Tandem offloading is provided for and the unit is equipped with two six inch production flowlines, two x subsea direct hydraulic control umbilicals and two, two inch gas lift flowlines, which may be included in the umbilical. Premuda explains that this is a low risk solution because “standard shipbuilding and marine technology has been used.”

This means that no turret or swivels are required and that modular production facilities have been used, so that the FPSO can be relocated. Easy abandonment is provided by a simple under vessel buoy mooring.

The disconnectable spread mooring solution combined with flexible risers and umbilicals will be connected to the FPSO riser balcony in such a way that they can be quickly disconnected when required. Premuda explains that on receipt of a cyclone warning the risers will be lowered to a mid-depth or sea-based ?wet store? position. The mooring lines will also be provided with a quick-disconnected system so that it will be possible to ensure that all risers and mooring lines are disconnected in a timely manner.

Preliminary calculations indicated that the FPSO would be required to be disconnected 1.5/2.0 times per year due to weather/cyclones. The disconnection period is estimated at 6-8 days/years.

The offloading tanker will be moored aft of the FPSO and will offload production via a 16 inch floating hose. To prevent the offloading tanker from weathervaning and tangling with the FPSO spread mooring, the tanker will be held by the stern with a hold back tug.

A helideck is located on the port side aft adjacent to the bridge and at bridge deck level. It can accommodate a Super Puma L332, which has a maximum take-off weight (MTOW) of 9,150kg.

A pedestal mounted 10t marine crane has been installed on the main deck starboard side at midship on a new pedestal foundation. A 55m high flare tower is located on the port side forward.