Design and Analysis for the Lift Installation of the Veslefrikk Jacket

ABSTRACT In order to provide a cost efficient development for the Veslefrikk field, a hybrid concept consisting of a floating production facility and a lift-installed wellhead jacket was chosen. This paper describes the design, analysis, and model testing work involved in the preparation for the lift installation of the jacket. INTRODUCTION Statoil's development concept for the Veslefrikk fleld 1n the northern North Sea, consists of a fixed wellhead platform adjacent to a floating production faci1ity in 174 m water. This innovative field development plan contributes to reduced investments making Veslefrikk economically viable in a low oil ' price scenario. In compliance with the overall cost conscious development philosophy, a decision was made to lift-install the wellhead platform, which is among the tallest jacket structures in the North Sea. The Jacket has been designed to accommodate an offshore lift as well as an inshore lift followed by a wet tow. Being 184 m long and with a lift weight of 9060 tonnes, the Veslefrikk jacket will be the largest Jacket to be 11ft-1nstalled and the heaviest marine lift in the world. This operation has only become Possible due to the emergence of larger capacity 11ft vessels, a light topside and an innovative jacket design. The lift installation condition has had significant impact on the jacket design criteria. Accordingly, considerable time has been spent on analysing and designing the' jacket for this condition ensuring a light structure that can be lifted horizontally off the transport barge, lowered into the water and then upended. A rigorous weight optimization program and an innovative design approach was implemented to satisfy the lift weight requirements. The overall design, including the use of a deck support frame, insert legs, antifouling coating, conductor shielding effects, X-braces, rotated topside and lightweight materials is presented in a separate 1989 OTC paper by Baerheim and Fossan (Ref. 1), and elsewhere by Lefranc et al. (Ref. 2 and 3). The purpose of this paper is to describe in more detail the aspects of the design and analysis particularly relating to the lift installation itself. DESIGN BASIS The jacket is to be lift installed using a large crane vessel. Two vessels are able to perform this lift: the DBI02 having two 6000 tonne cranes and the M7000 equipped with two 7000 tonne cranes. In addition, Balder and Hermod may lift up to 12000 tonnes in parallel. However, all these figures are maximum, unfactored loads, and so to maintain competition between the installation contractors, the maximum allowable weight of the jacket was somewhat less than 10000 tonnes. The jacket weight for structural design purposes was taken as 9800 tonnes. The estimated lift weight at the time of writing is 9060 tonnes. To be able to utilize the maximum capacity of the crane, the distance from the centre of gravity of the jacket to the crane vessel shall be a minimum.