Hull/Mooring/Riser Coupled Motion Simulations Of Thruster-Assisted Moored Platforms

Summary

Objective

Reduction of large motion responses of offshore platforms in a harsh environment in deep waters is the most important consideration for the analysis and/or design of the platforms. DP systems can be applied to increase motion performance of the platforms, but thruster-assisted FPSOs are not fully understood in the sense of hull/mooring/riser coupled analysis. The goal of this study is to develop and recommend a system, based on coupled dynamic analysis, that will provide better motion response in a deep water environment.

Approach

By applying thrusters, total dynamic responses can be improved in terms of mooring line/riser top tensions and operation radii of platforms. Thrust must be added and adequately controlled in the following equation of motion in the horizontal plane (i.e. surge, sway, and yaw).

mX= F E + F M + F H + FT

where m represents platform’s mass plus added mass, X displacement vector in x-, y-, z directions, double dot 2nd time derivative, E F environmental forces and moment, M F mooring force, H F hydrodynamic reaction forces and moment due to platform motions, and T F thruster forces and moment acting on the platform. To minimize the error i.e. the difference between desired and measured location and heading angle of the platform, the thruster force T F needs to be calculated, and a controller needs to be designed.

Project Plan

Scope of Work:
A coupled dynamic program, WINPOST-FPSO, has been developed, and it has been determined that the program is suitable for the analysis of FPSOs (Arcandra, 2001). The following hypotheses were tested:

1. Motion responses of thruster-assisted structures give smaller value of riser/mooring line stresses than those without a thruster.
2. The watch circle of a moored platform becomes smaller so that the structure has better global motion responses.

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To test these two hypotheses, the following scope of work was addressed:

1. To develop and implement thrust control algorithm in the hull/mooring/riser timedomain coupled dynamic analysis program (WINPOST-FPSO) by tuning each gain of the P(I)D (Proportional-Integral-Derivative) controller.
2. To conduct case studies of hull/mooring/riser coupled motions of thruster-assisted moored platforms. The FPSO and the spar used for the DeepStar Project are adopted for this study.
3. o investigate the global motion response by means of spectral analysis and statistical analysis by comparing two cases: (1) A floating platform with a thrusterassisted system and 2) a floating platform without the system.
4. To determine how and why a thruster-assisted moored platform can (or cannot) be an effective solution in terms of mooring line stress reduction and improvement of the global motion response for deepwater system types.

The hydrodynamic loading, floating platform dynamics, and relevant numerical approaches for a solution of the derived mathematical formulation are reviewed along with a review of position-keeping systems. Mooring line dynamics is addressed, and thruster-assisted position-keeping system is discussed. Coupling between a hull and legs is briefly discussed.

Related Publications

S. Ryu and M. H. Kim, 2003. “Coupled Dynamic Analysis of Thruster-Assisted Turret-Moored FPSO,” OCEANS 2003 MTS/IEEE, San Diego, California.