Deep Draft Floater Motions - Verification Study

Abstract

The paper presents the main findings from an internal verification study on motions of the Aker Kvaerner Deep draft Floater (DDF). The study employed traditional linear analysis techniques as well as coupled and uncoupled time domain analyses. The coupled analysis is carried out utilizing state of the art software belonging to the Norwegian Marine Technology Research Institute (Marintek) program portfolio and developed further in the DEEPER Joint Industry Program organized by Marintek and Det Norske Veritas (DnV) (ref /1/). The analysis case is based on the functional requirements of a DDF with 56 000 Mt displacement and up to 30 rigid risers. The DDF has been analyzed at 1400 m water depth both in a West Africa and a Gulf of Mexico natural environment. By investigating the effect of including keel plates at large depths below the SWL, the heave response of the DDF can be controlled without significantly increasing the heave excitation on the structure. The 100-year return heave response has been found to be less than 1 m and 2 m for, respectively, the West Africa and the Gulf of Mexico Environment. By comparing coupled and uncoupled analyses of the DDF motions it is concluded that the effect of coupling is slight at 1400 m water depth. With 30 rigid risers present, the fully coupled model will predict approximately 10 % smaller motions than the uncoupled time domain analysis model. Introduction Aker Kvaerner have taken part in the DEEPER JIP development project. As a part of the breaking-in exercise of new deep water time domain analysis software and in order to thoroughly investigate the scope for optimization of the DDF motion behavior, Aker Kvaerner decided to run an advanced verification study on motions of the Aker Kvaerner DDF. The scope of the analysis was as follows:Investigate and optimize first order heave motion of the Deep Draft Floater (DDF) using first order radiationdiffraction analysis. This is carried out employing the WADAM program which is based on WAMIT5 /2/.Investigate the motion of the DDF using the time domain analysis program SIMO developed by Marintek /3/.Investigate the motion of the DDF using SIMO coupled with RIFLEX as described in /1/Verify and optimize mooring line, riser and tensioner design.develop a DDF sizing tool which calculates stability and gives an accurate estimate of first order heave response. The three first points above are addressed in the present paper. The Aker Kvaerner Deep Draft Floater General. The Aker Kvaerner DDF is a general-purpose marine facility for deep water environments. The design is focused on minimal vertical (heave) motions in order to facilitate rigid marine risers with dry trees. This type of design is very little affected by a change in water depth making the DDF ideal for use in deep waters. The DDF as shown in figure 1 can functionally be described as a multi-leg Spar or Truss Spar.