First Jack-Up Production Platform in the North Sea

Abstract

Introduction A jack-up gas production platform by Amoco International Oil Co. was installed in Aug. 1972 in 75 ft of water in the Leman field, British North Sea. It was the first jack-up production platform installed in the North Sea. production platform installed in the North Sea. The main advantage of using the jack-up platform rather than the conventional template-type platform is that all production, storage, and living facilities can be installed, tested, and checked out in the fabrication yard before it is removed for installation. As a result, considerable expensive offshore work is avoided, and more important, much less time is required to commission the production facilities and begin operation. Design Basically, the platform is designed as a flat-bottom, flush-deck barge with vertical sides and raked ends. Between the flat bottom and flush deck, the barge is subdivided into thirty compartments by six athwartships full scantling bulkheads, four longitudinal bulkheads and several division bulkheads. The barge is supported by six caissons 71-in. OD and 195 ft long, each extended through a well in the platform. There are three wells on the port side and three on the starboard. The dead weight of the barge and caissons is 3,070 kips. The dry weight and wet weight of the production facilities are 1,550 kips and 1,840 kips, respectively. The soils encountered in the boring to the maximum penetration explored (203 ft) are primarily fine sand. penetration explored (203 ft) are primarily fine sand. A laterally loaded pile computer program was used to develop soil-resistance caisson-deflection characteristics for the caisson design. The design penetration for the six caissons is 55 ft, with a 20-ft-long grout plug placed on top of the natural soil plug. The design analysis was performed with a staticelastic space frame analysis computer program. Stokes' fifth order wave theory was used to generate the wave forces on the caissons. The maximum lateral deflection of the barge under storm condition is 10 in. The natural period of the platform is computed by the Rayleigh method and is found to be 2.5 seconds, which is much less than the period of the storm wave. Thus the dynamic amplification effect is negligible. The limiting angle of roll of the barge under tow was 9.25 degrees, at which position the stress in the caissons would be equal to 80 percent of yield. The draft of the barge is 5.1 ft. The period of roll of barge under tow was calculated to be 4.8 seconds. Construction The platform, built in Holland during 1971 and 1972, was constructed in three sections - one each in Amsterdam, Sliedrect, and Rijn. The three completed sections were then assembled at DeGroot and Van-Vliet yard at Slikkerveer and then towed to the DeGroot International N.V. Yard at Zwijndrect. JPT P. 323