Deepwater Nonlinear Coupled Analysis Tool
Authors: Ma Wei , Lee Ming-Yao , Zou Jun , Huang Edward W. 
Published: October 2008

This paper presents a consistent analytical approach for the prediction of nonlinear-coupled effects among platform, moorings/tendons and risers. The approach is based on a newly launched Deepwater Nonlinear Coupled Analysis Tool (DeepCAT) which is jointly developed by ABS and ABB. DeepCAT, comprised of two existing numerical algorithms within COUNAT (a time domain motion simulation code) and CABLE3D (a time domain cable dynamic analysis code), is able to simulate platform motions and the associated mooring/tendon and riser tensions, including second-order effects due to wind, current and waves.

The paper identifies primary characteristics of coupled effects, and the necessity and cost benefit of using a coupled analysis for ultra deepwater floating production system (FPS) design. In deepwater, the platforms tend to interact more pronouncedly in relation to their moorings/tendons and risers. The dynamic interaction among platform, moorings/tendons and risers cannot be evaluated using the conventional uncoupled analysis tools, where the platform, moorings/tendons and risers are treated separately, nor can it be accurately determined through model tests due to the limitations of existing model testing facilities and possible scale effects. Evaluation of such interaction requires a consistent coupled solution.

This paper provides sufficient details and results from parametric studies of floating production structures, including two TLPs and a Spar, using coupled and uncoupled tools in a range of water depths up to 10,000 feet. The emphasis of the study is given to the effects of dynamic tensions, with the associated mass and damping of moorings/tendons and risers, on the platform motions. It is concluded that DeepCAT is a valuable tool for the nonlinear-coupled analysis of floating structures, and the coupled effects between the platform and moorings/risers can be and should be included in the design of such structures in ultra deepwater.

Introduction

The Spar, TLP, Semi, and FPSO can be used as floating production systems (FPS) for oilfield development in deep water. Spar and TLP are popular concepts because their natural frequencies are away from the frequencies of the dominant wave energy. However wave and wind loads can excite large amplitude resonant motion and tension responses, most notably due to the second-order low- and highfrequency (the difference and sum of the two frequencies) effects. In the case of a TLP, slowly varying drift motion creates large offset, setdown, and a small air gap while springing and ringing increase tether extreme tensions and fatigue damage considerably. The contribution of the secondorder loads to the motions and tensions plays an important role in the riser design and overall platform sizing. Therefore, dynamic analysis based on reliable and sound techniques should be utilized for these floating production systems.

Second-order wave loads can be evaluated using secondorder diffraction theory (Molin, 1979; Kim and Yue, 1989, 1990; Chau and Eatock Taylor, 1992). The second-order diffraction theory, which gives the interaction terms up to the second-order, can be solved by either constant panel method (CPM, e.g. WAMIT) or higher-order boundary element method (HOBEM).