Abstract

In this paper, the effects of broken rope components on rope failure axial strain, failure axial load and rope stiffness is studied using 3D finite element analyses. The analyses are focused on small-scale polyester ropes (ropes diameter equal to 6 mm) with different number and distribution of broken components throughout rope cross-sections. These small-scale ropes are members of bigger ropes used as deep water mooring systems. For computational purposes, ropes are subjected to extension with both ends fixed against rotation. Numerical simulations show that the reduction of the residual rope strength and the axial strain at the onset of rope failure due to the presence of broken rope components depend on the degree of asymmetry of the rope cross-section. In addition depending on the location of the broken components inside the rope and due to the accumulation of frictional forces, broken components eventually start contributing to rope response. Around the failure region, simulations demonstrate the existence of strain localization that can cause the premature failure of rope components and reduce the maximum load that a damaged rope is capable of resisting. Simulations obtained by the 3D models are compared with available experimental data obtained from static tensile tests and with the predicted rope responses obtained by using a 2D numerical model extended to include the length effect on damaged rope response. The results of this study should serve as a basis to develop more complete numerical models to predict damaged rope response.

Introduction

The behavior of synthetic-fiber ropes has received a great deal of attention from the offshore industry because they are among the several alternatives for positioning deepwater floating platforms. Previous researchers have shown that such mooring lines provide numerous advantages over typical mooring systems (e.g., steel wire ropes and chain), particularly in deepwater applications in which traditional approaches to mooring become unfeasible due to the large self-weight of the components (Karayaka et al., 1999). The use of synthetic-fiber ropes in above applications has brought to light several unknown aspects of behavior, and one major corcen is the ability of of these ropes to withstand damage.

Damage to ropes is a complicated process that could depend on a variety of factors such as strain range, abrasion, installation procedures, environmental interaction, handling, creep, number of loading cycles, etc. (Lo et al. 1999; Banfield and Casey 1998; Karayaka et al. 1999, among others). The process by which damage occurs can be represented through a degradation of the properties of individual rope components, and it can also include the complete rupture of one or more components. Information on the residual strength and deformation capacity of damaged polyester (PET) ropes is important to the rope industry for developing guidelines for estimating allowable service life and determining whether or not damaged ropes can be retained in service.

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