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Weighty advantages for FPSO conversion

Posted: 6 June 2007

Technology providing significant space savings and typically 70-80 per cent weight reductions have been used on an FPSO conversion for Prosafe at the Keppel Shipyard, Singapore, to connect high pressure piping and equipment on the vessel.

The FPSO is being deployed for the Tui oil field project, offshore New Zealand (off the Taranaki Peninsula), by operator Australian Worldwide Exploration (with a 42.5 per cent interest).

The SPO Compact Flanges from compact technology and sealing specialist Vector International were selected for the FPSO because of the extreme space restrictions on the former tanker, which could not accommodate traditional ANSI flange connections.

Over 50 Class 1500 SPO Compact Flanges in a range of sizes, predominantly 8 and 10 inches, were used in the conversion project.

An 8 inch Class 1500 SPO Compact Flange weighs only just over 36kg, as against a substantial 125kg for the equivalent rated ANSI flange – representing a 71 per cent weight saving, and giving an indication of the significantly smaller size of the SPO Compact Flange.

Unlike a traditional flange, which relies on bolting to hold the two seal faces together and compress a gasket between them (placing the loading on the bolts, with integrity largely dependent on bolt torque), Vector’s SPO Compact Flange incorporates an internal metal-to-metal sealring.

This replaces the gasket of a conventional flange and works with the seal faces to ensure face to face mating so that only low seating forces are required.

The SPO Compact Flange uses radial compression, like an ANSI flange, but thanks to a slight convex bevel to the face it combines this with a wedging action.

When assembled, the flange faces first make contact at the sealring, then at the heel as the pre-tension increases, until full face to face contact is achieved, placing 90 per cent of the loading on the heel area.

"This design minimises the bolt force required, enabling smaller bolts and nuts to be used, in turn requiring a smaller diameter bolt circle for tightening," said KimSeng Ng, Vector International business development manager Asia Pacific.

"Equally, this allows smaller flange sections to be used. The result is a significantly smaller, compact flange connection.

"Space and weight savings are key for all offshore developments of course, but are particularly significant for FPSOs," he said.

A further consideration on FPSOs comes when high capacity topsides lead to situations where maintenance access is difficult, or critical equipment may have to be installed in a location where it becomes vulnerable to corrosion.

In these circumstances the SPO Compact Flange’s minimal maintenance requirements offer added value – requiring no periodic tightening.

Moreover, while the minimised size of the SPO Compact Flange helps address these issues, a further advantage is that the internal sealring is protected against corrosion, the design creating an environmental seal at the outer edge – unlike an ANSI flange in which the gasket is vulnerable to corrosive elements.

Additionally, the SPO Compact Flange offers high performance joint integrity.

"Once pressure-tight the SPO Compact Flange will remain pressure tight. These are no-leak connections," said KimSeng.

The Prosafe conversion project at Keppel Shipyard saw a former tanker converted into an FPSO capable of handling 50,000 b/d of oil, 120,000 b/day of fluid, and with a storage capacity of 700,000 bbl.

The Tui Area Oil Field project is being developed via four subsea wells tied back to the FPSO. Proved and probable recoverable reserves are estimated at 28 million barrels.

The use of this latest compact connection technology on the Prosafe FPSO is indicative of a growing trend as the high value savings and performance advantages to be gained are increasingly recognised.





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