Offshore Technology Research Center

 

OTRC Project Summary

Project Title:

Hull/Mooring/Riser Coupled Dynamic Analysis of a Deepwater Floating Platform with Polyester Lines

Prinicipal Investigators:

M. H. Kim

Sponsor:

Deep Star

Completion Date:

August, 2001

Final Report ID#

A126(Click to view final report abstract)

This research will start with developing the governing equation of a new three dimensional finite element model by incorporating nonlinear behavior due to large strain of the rod. The rod is subject to extensible condition and is torque free. The effects of rotary inertia and shear deformations are neglected.

The second step of the research includes the dynamic modulus of the polyester fiber into the governing equation. In this step, a regression equation developed by Fernandes & Vecchio (1998) will be introduced. Note that this equation is a simplified one because it does not consider changes in stiffness over long-term loading and memory effect.

The third step looks into the application of the new governing equation implemented into a new three-dimensional dynamic analysis program. The new program will use the WINPOST source code as a backbone. Changes will be made as necessary and a Newmark time integration scheme will be applied for dynamic analysis, while Newton-Rhapson iterative scheme will be used for static analysis.

As the last step, a series of convergence tests and verifications of the new program will be conducted. The static and dynamic results of the new program should give reasonable results when applied with constant modulus and small strain characteristics. To investigate the influence of the nonlinear dynamic modulus into the motion and tension characteristics, the new program will be used to perform a coupled-dynamic
analysis for a spar in a typical Gulf of Mexico environment. The results of this test will be compared with those obtained from WINPOST.

Since the FPSO is chosen as one of the main objectives, several issues have to be addressed on this research. One of them is related to the motion characteristics of FPSO. Different from spars and TLPs, the FPSO uses turret-mooring lines for weathervaning and station-keeping. A number of mooring lines attached to the turret and it can freely rotate about its vertical axis (yaw rotation).

During extreme weather condition, the yaw angle can be large to change the normal direction of environmental loads toward the vessel. Therefore, wind, wave, and current forces should be computed by incorporating the large yaw-angle rotation. The wind and current forces may be calculated by using the empirical coefficients based on model test data as presented in “Prediction of Wind and Current Loads on VLCCs”, published by OCIMF (1994). For wave loads, the forces are pre-calculated by dividing the yaw angle into several intervals. This means that for all yaw angles, the hydrocoefficients and wave forces are computed and tabulated prior to the simulations.

It should be noted that the fluid structure interaction software called WAMIT is used to compute the linear and second-order wave loads on a floating platform. To create input data for WAMIT, MSC PATRAN software package is required. Once the mesh is created, the grid points data for each panel can be obtained from the MSC PATRAN data base system. Interface programs between MSC PATRAN and WAMIT and also between WAMIT and the new program WINPOST-POLY should be developed to increase the efficiency of the research.

Related Publications: Kim, M.H., Arcandra, Kim, Y.B. (2001). "Variability of Spar Motion Analysis Against Various Design Methodologies/Parameters," Proc. 11th International Offshore Mechanics and Artic Engineering Conference, Rio-de-Janairo.

Kim, M.H., Arcandra, Y.B. Kim, “Variability of TLP motion analysis against design methodologies/parameters” Proc. 11th International Offshore and Polar Engineering Conference, Stavanger, 2001

Kim, M. H., Kim, Y. B., Mercier, R., Ward, E. G. "Hull/Mooring/Riser Coupled Dynamic Analysis of a Turret —Moored FPSO Compared with OTRC Experiment", Proceedings of the International Symposium, Oct 2-3, 2003, Houston, TX; Sponsored by Offshore Technology Research Center, Coasts, Oceans, Ports, and River Institute of ASCE, pp. 239-254.

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