ABSTRACT

This paper aims to suggest more accurate global time domain analysis of flexible risers and to investigate the effect of nonlinear bending model on global analysis results. The suggested global time domain analysis procedure, unlike to the previous studies, preliminarily estimate the bending hysteresis curve of each riser segment with different pressure loads. Then, the calculated curves are implemented in the global time domain analysis to reflect the nonlinear bending behavior of flexible riser. During the analysis, the resultant stress in a tensile wire is calculated through the theoretically derived stress model. Throughout the 6 different model, a comparative study is conducted to investigate the effect of nonlinear bending model. As a result, significant differences in both the structural responses and stress time history are observed when implementing a more detailed bending model in global time domain analysis.

INTRODUCTION

Technological developments in flexible risers have been, in parallel, followed by advances in their design and the analysis techniques for the mechanical behavior. A multi-layered section itself can prevent many undesirable accidents such as flooding, buckling, and fatigue failure. Among the many layers, tensile armor layers mainly contribute to the structural integrity of a flexible riser. Multiple tendons are helically wound on the internal layers, providing sufficient bending flexibility with high tensile strength, thus the riser can be easily utilized in various conditions. The unique characteristics of tensile armor layers offer some advances in the use of offshore floaters, however, unlike to other types of risers, they also present a difficult challenge to evaluate structural integrity.

One of the main problems arises from non-linear bending responses of risers. Helically wound wires of experience slip between themselves and adjacent fabric layers when they exposed to a large bending curvature, consequently the Euler beam equation is not valid for the riser analysis. This problem leads to an introduction of nonlinear finite element method and development of new analytical methods in the riser design processes.

This content is only available via PDF.
You can access this article if you purchase or spend a download.