Numerical analysis of cables in the offshore environment

The extraction of mineral resources from deep ocean waters has been made possible by the development of large compliant offshore structures. Mooring cables are crucial components in these offshore facilities and form the basis of this study. The aims of this thesis are: to provide a comprehensive review on all aspects of cables, to determine criteria for numerical modelling, and to ascertain the capabilities of the finite element method for cable analyses using the F.E. package ABAQUS. Difficulties associated with large sag cables arise as a result of their inherent flexibility which causes ill-conditioning of the stiffness matrices. Furthermore, the cable winding configuration causes a nonlinear stress-strain relationship, it's sagged geometry results in nonlinear strain-displacement relations, and the immersion in water leads to nonlinear fluid loadings arising from Morison's Equation as well as uncertainties in the fluid parameters. Various models, starting with the developed. Convergence difficulties basic catenary, have been at start-up, caused by a lack of stiffness in the transverse direction, are avoided by supporting the cable when applying loads. It is further established that numerical analyses of flexible structures are most stable in dynamic analyses and when under tension. In general both displacement based isoparametric and hybrid beam elements were found to be more reliable and applicable than truss elements. Cable whip, ocean floor contact and harmonic motions of cables were analysed. Finally a cable/tower interaction was modelled and subjected to a Stokes's wave. Conclusions and guidelines are presented based on the numerical experiments carried out in this study.

Grantor dc:publisher.institution

Department of Mechanical Engineering

Year dc:date.issued

1988

Author dc:creator

• Davies, Graham John

Advisor dc:contributor.advisor

• Pearce, H T

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/11427/8388

OAI identifier oai:identifier

oai:open.uct.ac.za:11427/8388