Prediction and Screening of Truss Spar VIM With CFD

Abstract

The understanding and prediction of spar vortex induced motion (VIM) is an important area for the offshore industry as it affects the integrity of riser and mooring systems related to strength and fatigue loading. With exploration moving to deeper water, there is a desire not only to asses spar VIM early in the design cycle, but also to optimize the design. The methodology to model spar VIM with computational fluid dynamics (CFD) was pioneered by the authors and is utilized in this study to demonstrate how CFD can aid the design process, as well as conducting additional benchmark cases to improve confidence in CFD. Comparisons against experiments of a large scale hull at high Reynolds number are performed to validate CFD and establish an efficient methodology that is subsequently used for screening. This new method utilizes a boundary condition that allows simulation under arbitrary headings. As a result the overall computational cost and setup overhead is reduce by an order of magnitude enabling screening for VIM well within design cycle requirements. Simulations of spar geometries with increasing detail are performed starting from a hull with strakes, adding pipes and mooring components and finally incorporating the truss. The effects of each component are discussed with their impact on VIM response. Overall the computations show that high quality predictions are possible and it is practical to incorporate CFD into the design process. Screening and the generation of response curves can be done during initial design and more targeted assessments during detail design.