Fiber Rope Damage Assessment and Acceptance Criteria - New Guideline
Authors: Åhjem Vidar R. , Colby Craig A. , Flory John F. , Lee Ming- Yao , Petruska David J. 
Published: October 2008

A joint industry project was conducted to establish the first industry guideline on how to assess external mechanical damage on large fiber ropes.

The damage assessment guideline requires damage tolerance tests. It provides a method for comparing the findings from inspection of a damaged rope with the damage tolerance documentation. Application of the guideline produces revised strength and fatigue considerations for the damaged rope.

Caution

For the purpose of performing a damage assessment, the full damage assessment guideline should be used and not this paper. This paper presents the utility of the new-developed guideline, but simplifications have been made to suit the format of a technical paper. The guideline also contains useful hints and examples to assist in conducting a damage assessment.

Introduction

This paper presents the new-developed guideline for fiber rope damage assessment. The work has been largely carried out by DNV; with important technical input provided by TTI and the rope-making industry. The project was supported by 10 oil companies.

Prior to the work summarized in this paper, no industry standard existed for damage assessment of large rope of the types used in deepwater moorings.

The incentive was that such an industry standard would increase the confidence in fiber-rope mooring systems. The only previous work on the damage issue had been carried out by BP at SES, in preparation for the use of polyester ropes on rig Ocean Confidence.

An industry project was formed with technical basis in the "summation principle". This had already been developed together with the rope-making industry as the most suitable basis for a damage assessment method.

Ropes of different subrope construction and splicing pattern were tested. The output from the project is a project report and a guideline for damage assessment, with support sections.

This paper describes how the guideline should be utilized as a working tool. Inspection techniques and the approach for quantifying damaged-rope properties are discussed in this paper. The findings by BP are discussed separately in the light of the later developed guideline. The robustness and versatility of the approach are explained.

Application of the guideline

The damage-assessment guideline is applicable to offshore mooring fiber ropes that have been subjected to mechanical damage. It covers any make of rope of the parallel-subrope type as shown in Figure 1. The guideline provides a systematic assessment of the extent of damage, which permits requantification of the strength of the rope. If the re-calculated strength is deemed sufficient, "temporary service" is recommended. If the fatigue performance is deemed sufficient too, "prolonged service" is recommended. In either case, jacket repair must be carried out. The guideline has been developed using polyester test ropes. Other fiber materials should also be covered if this is verified by testing.

Technical basis

As can be seen from Figure 1, the type of rope covered by the guideline consists of a parallel assembly of subropes. The subropes are the structural building blocks of the rope. The bundle of subropes is held together by an external jacket.

Figure 1. Principle sketch of a parallel-subrope rope.(Available in full paper)