ABSTRACT

This paper addresses the problem of evaluating the elastic deformations and motions of towlines and cables where bending and torsional effects are present. For such problems where large deflections and finite rotations of sections may occur, it is important that the kinematics are properly developed and consistent with the constitutive relation between torque and twist of the towline. Two alternative techniques are compared, one based on an extension to the classical theory of elastic rods and the other on a more established method that uses convected coordinate systems that translate and rotate with the centreline of the pipe. Both are implemented within a finite element framework and applied to a series of elementary problems for the purposes of evaluating the respective methods.

INTRODUCTION

The majority of pipeline and riser problems arising in offshore engineering do not require the calculation of torque and twist in the line. However, exceptions may arise with respect to flexible risers (O'Brien, McNamara and Grealish, 1992), cables and steel catenary risers, particularly during installation and under non-orthogonal loading conditions, and deep-ocean mining applications. The influence of torque and its coupling with axial forces and bending moments is extensively investigated in a series of papers by Cheng and Chung (1997), Chung, Cheng and Huttelmaier (1995, 1994a, 1994b). For such problems where large elastic deflections and rotations of sections occur, the action of torque and its interaction with bending may be important. The inclusion of torque in the pipeline equations for large elastic motions is not trivial and two such methods are investigated here; both are developed within a finite element framework. A general solution method for the nonlinear statics and dynamics of marine pipelines, based on an updated Lagrangian approach, is detailed by Chung, Chang and Huttelmaier (1994a).

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