Manmade Fiber Ropes in Deepwater Mooring Applications
Authors: De Pellegrin Ivan 
Published: October 2008

The paper deals with the several offshore floating units that have been installed in Campos basin, offshore Brazil, making use of polyester ropes in their mooring lines. A description is made of the technical specifications of these ropes, such as minimum breaking load, specific strength and dynamic modulus. A series of real-scale laboratory tests have been performed to obtain the approval and certification of the ropes. The paper describes the testing procedures adopted and discusses the results obtained. In sizing up the polyester installations, it has been observed that the loads on mooring linesreach levels somewhat lower than those obtained with traditional solutions (wire ropes and chain catenary systems). The paper points out and discusses this and other advantages that were observed when polyester ropes are employed. The paper highlights recommendations on how to handle and care for the polyester mooring lines, as well as the proper installations procedures to be followed. Finally, the paper points out some of the developments being sought to improve the performance of synthetic mooring structures while stressing the opportunities for new applications of man-made fiber ropes in the offshore industry.

Introduction

Oil exploitation in deep and ultra deep water has been developed at quick pace in Campos basin, offshore Brazil. It is estimated that over 33% of Brazilian reserves are located at water depths between 300 and 1000 meters and that over 35% at water depths beyond 1000 meters1.

Several technologies, in different areas, have recently been developed or improved to enable oil and gas production in these depths. One of the areas in which the Brazilian State owned company - Petrobras - concentrated research efforts was the area of anchoring systems and one of the technologies that was improved was the application of man-made fiber ropes in such systems. Technical specifications were issued for mooring systems using segments of polyester rope and manufacturers of man-made fiber ropes developed new products for these applications.

The first units in which polyester ropes were employed in all anchoring lines were installed in Campos basin from 1997 on. With the success of the first installations, a series of semisubmersible platforms, FPSOs, FSOs and monobuoys have been installed with polyester anchoring ropes both in taut leg and catenary systems - the latter being a mixed system using bottom chain and polyester rope. Fig. 1 and Fig. 2 illustrate both kinds of anchoring systems.

A list with the units already installed which use polyester ropes in their anchoring system as well as some projects under way in Campos basin in which polyester ropes may be used is shown in table 1.

Technical specifications of the polyester ropes used in Campos basin

In 1995, Petrobras made available to its suppliers one of the first technical specifications for anchoring polyester ropes. This specification is currently known as ET-3000.00-6651-962-PGT-001 - Polyester Rope for Mooring Line2.

The basic requirements in this technical specification are:The minimum yarn specific strength of 0,78 N/tex. It is the ratio of the yarn breaking strength per its linear density in tex (grams per 1000 meters), measured according to equation E-1.The minimum core specific strength of 0,47 N/tex. It is the ratio of the actual breaking strength of the rope per the lineardensity of the rope core in tex (grams per 1000 meters, at 2% of MBL), measured according to equation E-2.