An innovative installation approach was adopted this summer for a Dutch riser access tower constructed under Shell's SWEEP gas accumulation development programme. David Morgan reports.
The 800t riser access tower (RAT) was fabricated by Mercon Steel Structures under a framework agreement with Shell UK and NAM that could potentially see up to nine such structures built as the operators seek to mop up smaller gas accumulations in the UK and Dutch sectors of the North Sea.
Netherlands-headquartered ALE, which has made something of a specialty of lifting heavy and awkward loads offshore in recent years, was called in to plan and execute the loadout of this new monopile structure and its installation alongside NAM's existing K15-FA-1 production platform. Mobile cranes were employed to assemble the RAT sections and then the structure was loaded out onto a cargo barge using SPMT trailers.
With seafastening complete, the barge, equipped with a heavy duty grillage for the skidding system, was towed to location. This grillage also contained the main rotation point of the upending frame and the connections for the upending strand jacks and cylinders.
A bespoke upending frame developed by ALE's design engineers was used to support and guide the RAT during skidding, upending and lowering. The skidding was performed by a hydraulic compensated and self propelled skidding system. After upending the RAT was lowered on the seabed by means of two strand jacks.
ALE executive director Kees Kompier says his team, assisted by suction pile foundation specialist SPT, maintained full control during the structure's positioning, self penetration, suction and final positioning phases. ‘We strive to find new and increasingly efficient solutions rather than constricting ourselves and our clients by following the norm,' he adds.
According to Shell's head of projects for the southern North Sea, the rationale behind SWEEP is to employ a single RAT in combination with subsea trees and existing production platforms to develop multiple small accumulations, rather than develop each gas field individually. ‘By using one design repeatedly, these smaller fields can be exploited in an economically viable way,' he adds. ‘It will help us to unlock ever smaller gas fields.'
Mercon's managing director Willem Griffioen says the RAT installation methodology developed in conjunction with ALE worked out ‘considerably cheaper' than more traditional methods employed offshore. ‘Besides the convenient installation, the tower can be decommissioned very easily as well and thus used again for new wells.' OE
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