The Yme Jack-Up with Skirt Foundation

Abstract The Yme field is developed using the Maersk Giant jack-up as a permanent drilling and production platform. This paper will present the structural and geotechnical design aspect of jack-up with skirts to document this new way of utilising jack-up structures. It will also outline the design of the riser system and installation aspects of the rig, The skirt foundation is an innovative cost effective foundation concept offering substantial opportunities for extending the use of jack-up structures compared to traditional spud-ears foundations. The Yme field development would not have been economically attractive without skirted foundations on the jack-up. Introduction The Yme field is located approx, 100 km south-west of Egersund in Block 9/2 offshore Norway. The water depth is 93 m. The field is marginal with an estimated economical lifetime of 70 months. The producible reserves are approx, 10.4 mill Sm3 oil. The maximum production rate is stipulated to 8000 Sm3 pr day. The drilling and production facilities are provided by means of a converted jack-up platform, Maesk Giant. The rig was equipped with process weighing 1150 tonnes. In addition the hull was modified to provide support for eight production risers. A converted oil tanker with capacity of 150000 m' oil is utilised as a Floating Storage Unit (FSU). The vessel is located about 2.500 m from the jack-up platform, and offloading is done by shuttle tankers connecting to the stem of the FSU, ref. fig 1. Jack-up rigs are widely used as mobile drilling units for the exploration of hydrocarbons. At present jack-up rigs have been placed in water depths of up to 125 m, but there is interest in extending their use to greater depths and larger deck weights, To demonstrate the feasibility of standard jack-up rigs for the Yme field Statoil found it necessary to account for the rotational stiffness of the footing in the structural analysis. Small skirts have previously ken used on spud-cans in shallow water to reduce scour. At Yme, the skirts were used to achieve restraint (bottom Fixity) and thereby reducing the leg utilisation drastically. This new Statoil approach of utilising skirts have previously been used in connection with the Sleipner Vest development in the North Sea, but was then used mainly for increasing the bearing capacity ref. /1/. Foundation Skirt Geometry. The spud can-skirt arrangement is shown on figure 2. Each spud-can is equipped with skirt extending 4,8 m down from the circumference of the spud can. The skirt was built up of six 30 mm thick half cylinders, welded to the extension of the three chords and to bottom of the thickness of the lower part of the skirt was increased to 60 mm to resist the forces from the installation phase.