Triangular, suction-piled platform offers low capex in shallow waters

Feico Houweling
Contributing Editor

• Installation sequence for SPTs lightweight production platform which features suction pile foundations and pre-tensioned guy wires [19,829 bytes]

Woerden-based Suction Pile Technology has developed a new design for a reusable, lightweight wellhead platform suited for remote, shallow water marginal oil and gasfields. The design was presented to various interested operators in Houston earlier this summer.

SPT is a joint venture of Noordhoek Underwater Contractors and two Volker Wessel Stevin companies, Volker Stevin Offshore and Van Splunder Funderingstechniek. The platform concept has been developed in co-operation with Dutch engineering company Iv Marcon.

The design is based on a steel triangle with suction pile foundations and pre-tensioned guy wires supporting a monopile structure and a 200t deck/facility. Stiffness is provided by three pre-tensioned guy wires attached to the monopile around 5 meters below water level. This allows service vessels to operate near the platform without interference with the wires.

SPT's platform is designed for a water depth of 50 meters and comprises an equilateral triangle with sides of 43m. Each corner is supported by a suction pile, typically 6m in diameter and 8-10m long to provide in situ stability of the platform during production as well as flotation during transportation and installation. Platform dimensions vary according to water depths. Dimensions of the suction piles are governed by soil conditions.

Two 30-in. conductors can be attached either to the outside of the monopile or inside, in combination with a riser for export lines and an umbilical corresponding to production needs and available space. To minimize capital spending, all equipment and features that are not strictly needed should be stripped off the platform.

SPT included the option of pre-fabrication and assembly on a barge near the production site. The structure can be given flotation capabilities by inflating air into the suction piles. Installation can be performed in combination with standard immersion techniques. Application of suction piles also enables the operator to relocate the platform. That was demonstrated earlier this year with a smaller platform of a similar type which was installed in the North Sea near Hoek.

The platform can be installed and secured within 24 hours of arrival at the field. The topsides are light enough to be lifted from a small barge or from the deck of a supply vessel by a drilling rig. It can then be cantilevered over the structure and installed.

According to SPT's general manager, Ton Geul, the application of suction pile technology is patented by Shell. In co-operation with Matech, SPT owns the rights to use the technology for its own projects and third party activities. Platform mooring by suction piles has been performed successfully numerous times. Earlier this year SPT helped Clyde Petroleum install three new jack-up gas production platforms using the technique.

Synthetic ropes-suction anchors offer strong FPSO mooring options

Taut leg mooring using suction anchors in combination with synthetic fibers is becoming a serious option for floating production units, following full-scale tests at sea.

Suction Pile Technology (SPT) has proposed using suction anchors for mooring FPSOs in deep water. A strong candidate for the mooring lines could be DSM's Dyneema rope which is both lightweight and sturdy.

Vertically loaded anchors are an accepted solution. "But handling these anchors may be difficult due to the force needed to stabilize them at the appropriate soil penetration," says SPT's general manager Ton Geul.

"Anchors with a loading capacity of 500 to 1,000 tons should be installed by tugs with 200-250 bollard pull. The costs of mobilizing such tugs are relatively high and with regard to the large number of FPSOs to be installed in the coming years, one might even expect a shortage of tugs at certain times."

The use of suction anchors is a proven technology. SPT has installed numerous such moorings inshore as well as offshore for platform applications. The vertical loading capacity can be extremely high and sufficient for applications such as FPSOs and TLPs.

The simple installation procedure is well known. A typical advantage of suction anchors over other mooring systems is the simplicity with which they can be removed. Pumping air under the anchors allows them to float and they can be towed to shore or other locations by small tugs.

The Dyneema rope, designed by DSM High Performance Fibers, proved superior to polyester rope during tests by Saga Petroleum off Norway in 280 meters of water. The rope has a specific weight slightly less than one and has thus virtually zero weight in water, whatever the length. The DSM fiber is 10-15 times stronger than steel and also stronger than polyester on a weight and diameter basis. The rope also appeared to meet criteria for withstanding very high loads that can occur during severe storms.

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