Dual Stiffness Approach for Polyester Mooring Line Analysis in Time Domain

Polyester mooring lines have been used in the offshore industry since the late ’90s. With increasing oil exploration and production in deeper waters, using polyester lines provides greater benefit than using traditional steel wires and chains. Some advantages of using polyester include a reduction of mooring line weight, a reduction in vessel offset and a reduction in the dynamics of the line tensions. However, unlike steel, polyester lines exhibit axial stiffness characteristics that are nonlinear and vary with time and loading history.Tahar (2001) developed a comprehensive theory and numerical tool to capture this behavior. The formulas allow relatively large elongation and nonlinear stress-strain relationships, as typically observed in polyester fibers. The mooring line dynamics are based on a rod theory and finite element method (FEM), with the governing equations described in a generalized coordinate system.Since this theory is computationally intensive, the benefits outweigh the costs less than they do for the practical approach recommended by API. Therefore, the fully coupled dynamic analysis tool CHARM3D has been modified to incorporate the API-recommended approach. Two axial stiffnesses (EA), post installation (static) stiffness and storm (dynamic) stiffness, have been convoluted into a dual stiffness to represent the total response of the floating platform in a single run. In the traditional method, the analyses are done twice, one run for each stiffness. Then, the extremes from each run are used as governing values for design. This paper presents the global performance comparison between the dual stiffness method and the traditional method. The effect of motions on SCR strength is also investigated using ABAQUS software.Copyright © 2012 by ASME