Offshore Technology Research Center

 

OTRC Project Summary

Project Title:

Deepwater Mooring and Riser Analysis

Prinicipal Investigators:

Jun Zhang

Sponsor:

Industry Consortium

Completion Date:

August, 2002

Final Report ID#

A117(Click to view final report abstract)

OBJECTIVE:

Springs are often employed in model tests of moored (especially deep-water) floating structure in wave basins. Existing numerical methods assume mooring lines to be inextensible or of small elongation. The assumption restricts their application to mooring lines with inserted large elongation elements, such as spring cells. The proposed research eliminates this restriction and makes the computation of mooring-line dynamics more accurate. The corresponding numerical codes will be applied to:

1) the design of truncated deep-water mooring-line model; and

2) the integration with the existing numerical schemes for computing wave loads on offshore structures to quantify the low-frequency damping induced by mooring lines.

APPROACH:

The newly developed numerical scheme can be applied to the simulation of small elongation elements with prescribed bending stiffness, such as risers and steel chains, as well as long elongation elements with negligible bending stiffness, such as spring cells. For studying the responses of moored deep-water offshore structures, a coupled analysis is required, which links the numerical scheme for dynamic mooring line/riser simulation with that for the prediction of hydrodynamic loading on a floating structure. Hence, the newly developed mooring line/riser numerical scheme will be integrated with 1) the numerical schemes based on wave diffraction theory, such as WAMIT and 2) the numerical schemes based on nonlinear wave kinematics (HWM) and the Morison equation. The numerical results based on our numerical scheme will be examined against the corresponding experimental results, such as the measurements of mini TLP and FPSO recorded in the OTRC wave basin.

DEPLOYMENT OF RESULTS:

The numerical scheme for simulating mooring line/riser has many applications, which are crucial to the development of deepwater offshore structures.

1) It can be used to design truncated deep-water mooring lines to be tested in the OTRC wave basin. The static and dynamic characteristics of truncated mooring lines may better match those of undistorted mooring lines, not only statically but also in the magnitude of low-frequency damping.

2) Together with other numerical schemes for computing responses of a floating offshore structure, it can be used to simulate the responses of a moored deep-water offshore structure. Hence, the simulation can be used to examine and compare with the model tests conducted in the OTRC wave basin.

Scope of Work: Integrating the numerical scheme with other schemes such as WAMIT for simulating the responses of a moored floating offshore structure. Comparing the simulated results with corresponding wave basin measurements. The integrated numerical code will be delivered to the OTRC wave basin.

Related Publications: Chen, X., Zhang, J., Johnson, P., and Irani, M. (2000) “Studies on Dynamics of Truncated Mooring Lines,” Proc. 10th International offshore and Polar Engineering conference, Vol. II, 94-101.

Chen, X. and Zhang, J. (2001) ‘’ Numerical Simulations of Truncated Mooring Lines with Inserted Springs,” Proc. 11th International offshore and Polar Engineering conference, VI, Stravanger, Norway, pp. 635-642.

Chen, X., Zhang J., Johnson, P., and Irani, M. (2001) ‘’Dynamic Analysis of Mooring Lines with Inserted Springs’’, Applied Ocean Res..Vol. 23, 277-284.

Chen, X., Zhang J. & W. Ma (2001) ‘’Study on the Coupling Effects Between a Spar of and its Mooring System’’, Ocean Engineering, Vol.28, No.7 863-888.

Chen, X., Zhang, J. Liagre, P., Niedzwecki, J. and Teigen, P. (2002)” Coupled Dynamic Analysis of A Mini TLP: Comparison with Measurements,” Proc. 21th International Conference on offshore Mechanics and Arcticr Engineering.

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