An Integrated Motion and Structural Analysis for Tension Leg Platforms

Abstract The tension leg platform (TLP) concept has become increasingly popular as oil reservoirs, once considered too deep to recover, now contain the financial incentive to justify their development. The effectiveness of the TLP will be determined partly by the extent to which wave-induced motions and structural responses 1imit the operational performance of the platform. To this end, a newly developed analytical method utilizing modern hydrodynamic theory and finite element techniques is presented for the analysis of a TLP. This method, based on advanced potential fluid theory, offers the following advantages:The structural analysis is integrated with the motion analysis via an elaborate process which ensures coherence between the hydrodynamic forces and inertial forces appl ied to the structure.The hydrodynamic interactions among TLP members, such as columns and pontoons, are explicitly included in both the motion and structural analyses.Advanced statistical methods have been applied to enhance the prediction of maximum stress and fatigue life of the structure. This procedure has been applied to design the structure of a TLP platform. It has also been used to analyze the tie down mechanisms of a platform deck during transportation. The results indicate that hydrodynamic interactions have significant effect on both the motion and structural responses of the TLP. These results al so demonstrate that for a structure like the TLP, the consistency between the motion and structural analyses is a critical factor which affects the accuracy of the structural analysis. Results of the foregoing appl ications wi11 be presented to illustrate the advantages of this method. Specifically, the procedure used in developing a macro space frame representation of the TLP structure will be discussed, as well as the methods for designing localized structure for column/pontoon joints. The stress response ampl itude operators and the predicted maximum stress value at a selected location will also be presented. Introduction A tension 1eg platform behaves 1ike a moored semisubmersible, therefore, it must be treated in the same manner as a floating vessel. Thus, for purposes of design, the seakeepi ng ability of the TLP must be the first concern. This fact is substantiated by realizing that a vessel's seakeepi ng characteristics are related directly to the vessel's operational performance. Second, the structure must be considered in terms of its structural strength in relation to environmental forces. In addition, the cumulative structuraleffects of repeated excitations must be considered. It would be a simpler matter if the goal is simply to design a TLP which satisfies both seakeeping and structural criteria. However, this would be a trivial task. Realistically, a third criterion or constraint, cost, must be considered. Therefore, for preliminary design considerations, an optimization process must be utilized so that an economically feasible solution can be found.