OTRC Project Summary
|Hull/Mooring/Riser Coupled Motion Simulations Of Thruster-Assisted Moored Platforms
|M. H. Kim
|Minerals Management Service and Industry Consortium
Final Report ID#
|A134(Click to view final report abstract)
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.
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.
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.
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. To 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.