Weight Optimization of the Veslefrikk Jacket

Abstract The Veslefrikk jacket will be installed in the North Sea in 174 meter waterdepth. It is the largest lifted offshore structure in the world. The lift weight of the structure is close to the capacity of the availablecrane vessels, and weight optimisation has therefore been of greatimportance. This paper describes the jacket weight optimizations and summarizes theactual benefits obtained. Introduction The Veslefrikk field is located west of Sognefjorden in the Norwegian sectorof the North Sea. The field will be developed by a combination of a fixedwellhead jacket and a floating production and quarters platform. The twoplatforms are connected by a telescopic bridge. The partners of the Veslefrikk field are:Statoil 55.0% (operator)Unocal 18.0% (tech. ass.)Deminex 13.5%Norsk Hydro 9.0%Svenska Petroleum 4.5% The Veslefrikk jacket will be installed by lifting. The jacket weight was atan early stage of the project estimated to be close to the maximum capacity ofthe biggest existing lift vessels. The weight optimization would therefore beof great importance for the success of the concept. This paper will discuss the various options evaluated for the Veslefrikkjacket. The actual benefits achieved are evaluated. Jacket Description The Veslefrikk jacket is a four legged structure, shown in figure 1. Thestructure has X-bracing both at the faces and the horizontal levels. The jacketextends from elevation +10 m down to elevation -174 m. The top and bottom dimensions are 18.3 by 18.3 and 54.9 by 54.9 metersrespectively. These dimensions were largely determined by geometricalrequirements during lift. The jacket has four piles around each leg. The 16 piles are each some 100 mlong with an outer diameter of 2.134 m. The top soil layers are very soft, requiring a mudmat'size of 180 square meters on each leg.