Design and Installation of the Veslefrikk Jacket

ABSTRACT The Veslefrikk A jacket, standing in 174 meter water depth in the North Sea, was lift installed in May 1989. The jacket is the heaviest lift installed offshore structure in the world. The jacket is relatively tall and slender, and the lift weight was close to the capacity of the available heavy lift crane vessels. Structural design and weight optimisation were thereforeimportant. This paper gives a general description of the jacket and highlights some of the special features and lessons learnt in the design and installation of the structure. INTRODUCTION The Veslefrikk field is developed with a two platform concept. The drilling equipment is placed on the Veslefrikk A jacket, while processing equipment and quarters are located on the semisubmersible platform Veslefrikk. The field development is further described in reference 1. Being 184 metres long and with a lift weight of 9086 tonnes, the jacket was lifted and installed by the world's largest dual crane semi-submersible lift vessel. The lift took place inshore at the mouth of Sognefjorden. The jacket was then towed in a partly submerged position to the field for the upending, mating and piling operations. Various special design methods and features were introduced in the jacket design. Most of these were introduced since the weight of the structure during lift was critical, others because thejacket is slender and dynamically sensitive. JACKET DESCRIPTION The Veslefrikk jacket is a four legged structure, as shown in fig 2. Both the faces and the horizontal levels are X-braced. The jacket extends from the sea bed up to elevation +9.8 metre. The top and bottom dimensions of the jacket are 18.3 by 18.3 and 54.9 by 54.9 metres respectively. These dimensions were largely determined by lift clearance requirements. A separately installed deck support frame extends up to elevation +25.5 metres. The legs have an outer diameter of 1.3 metres between elevation +25 and -60 metres. The legs in this area are reinforced by 1.1 metre diameter internal tubes grouted to the outer legs. Below -60 the leg diameter increases from 2.2 to 3.5 metres. The face bracing varies from 0.8 to 1.6 metres diameter at the top and bottom of the structure respectively. The plan level bracings are 1.1 metre in diameter. The jacket is supported by four piles at each leg. The jacket was installed over a template having six pre-drilled wells. The following criteria were used in the design :(Available In Full Paper) SPECIAL CHALLENGES IN DESIGN The decision to design a liftable jacket was taken after feasibility studies of both lifted and launched structures. The lift operation determined the geometry of the jacket and put emphasis on weight optimisation. The jacket is slender and light and founded on relatively weak soils. This also put special emphasis on design for temporary and permanent foundation strength and dynamic behaviour of the jacket.