Certification of Fibre Ropes for Offshore Mooring
Authors: Aasland Michael , Sogstad Bjørn E. 
Published: October 2008

Drilling exploration and production of hydrocarbons from deep water will require new types of mooring system. With mooring systems consisting of chain, or a combination of steel wire rope and chain, high line pretensions are required to maintain an acceptable horizontal angle at the fairlead. Furthermore the weight of a conventional mooring system could reduce the pay load of the floating unit.

High strength and high modulus fibre material offers a advantages to deep water mooring. Due to the low weight fibre ropes can be used as segments in a catenary or semi-taut mooring system, or may serve as the main components in a taut leg mooring system. Both a semi-taut and a taut leg ooring system has a small radius at the sea bottom and the stiffness in the system reduces the horizontal low frequency motion compared with a conventional steel mooring system.

This paper specifies requirements for the materials, design, manufacture and testing of offshore mooring fibre ropes, and may serve as a technical reference document in contractual matters between purchaser and the manufacturer. Typical applications covered are permanent moored floating production systems, offshore loading systems and mobile offshore units.

Introduction

Background

The standard for Certification of Offshore Mooring Fibre Rope has been developed to ensure the quality of the fibre ropes and to specify requirements to documentation of important fibre rope parameters.

This standard is made for use in connection with mooring system designed in accordance with DNV's rules (3). However, the standard should also be applicable, when mooring systems are designed according to other recognised mooring standards.

The development of new fibre types, new grades of known fibres, and refinements in rope construction are happening very quickly. So far only polyester fibre has been used extensively by Petrobras (9), but other fibres which have been used in composites materials are being launched for offshore mooring, and new fibres are being developed.

In order to include these other fibres, the standard specifies documentation of parameters which are not critical for polyester, but may be essential for other fibres. Examples of such parameters are: high temperature durability, and creep rupture.

Fibre ropes mooring systems have been designed for floating production and mobile units, but as mentioned above only Petrobras has used those system in a large scale. Permanent moored units and mobile offshore units have different design lives, and very different possibilities for inspection and extraction of test specimens. Fibre ropes for offshore mooring is a new technology, and as all new technologies, the design guides recommend, and the approval standards require, extensive testing prior to approval. Further, of the testing required, it is the fatigue testing which is the most expensive and time consuming and it is also the fatigue loading which is the most difficult to simulate in a laboratory.

However, the DNV standard allows the designer of the mooring system to balance the fatigue testing to be done prior to deployment and the time to extraction of test specimens for further fatigue testing. Test specimens extracted after a time in service is fatigue tested in order to provide documentation of the continued integrity of the mooring rope until the next extraction of specimens.