SS: Fiber Moorings, Recent Experiences and Research: Moorings with Polyester Ropes in Petrobras: Experience and the Evolution of Life Cycle Management
Authors: Rossi Ronaldo , Del Vecchio Cesar J.M. , Gonçalves Ricardo Célio F. 
Published: July 2010

This paper describes the experience of Petrobras in the design, installation and operation of deepwater mooring systems incorporating high efficiency polyester fiber ropes. These ropes have been used now for more than 12 years in catenary and taut configurations to moor drilling and production units as well as storage and offloading facilities. The paper provides a history of the development of this technology, highlighting the key issues involved in introducing in mooring systems a component that has non-linear visco-elastic behavior. Differences to all steel systems in terms of: behavior, failure modes and design criteria and procedures are discussed. The paper presents the life cycle management strategy adopted by Petrobras in the early years of the use of this technology and its evolution that have guaranteed that the only failure mode observed over all these years was mechanical damage. It is expected that this paper shows that the use of polyester (polyethylene terephthalate) ropes in deepwater moorings is a mature technology.

Introduction

In the mid 80's Petrobras was drilling in Campos Basin in water depths around 600m and producing in depths around 400m. Mooring systems were all based on chain, wire rope and drag embedment anchors. Increasing water depths indicated that all steel systems would become anti-economical due to the reduction in restoring efficiency.

Initial studies were based in using synthetic fiber ropes as extension wires in the mid-catenary. Since compliance to wave induced motions would be given by the geometry of the steel components the fiber rope should have stiffness similar to a steel wire rope. Therefore aramid ropes were the first to be considered (Riewald, 1986).

Cost considerations led Petrobras and the US Navy (Karnosky & Liu, 1988) to start studying the use of polyester fiber ropes for moorings. It was also devised that, in deepwater, the axial compliance of polyester ropes could provide enough compliance so that a long catenary might not be required (Baxter, 1987).

The use of polyester ropes in permanent moorings was a big challenge for several reasons, the most obvious one being establishing design methodologies and criteria for all possible failure modes. Since most large diameter fiber ropes were not optimized for tension and axial fatigue performance (Parsey, 1983)), it was also necessary to further develop rope constructions and generate enough data on the performance of these ropes.

Results of a comprehensive study on the use of fiber ropes for deepwater moorings were presented in 1992 (Chaplin & Del Vecchio, 1992). Full details of the study, including assessment of failure modes, load-elongation behavior, design methodologies and criteria were produced (Del Vecchio, 1992). Since the study was partially based in model ropes, confirmation of load-elongation behavior in full scale and additional data on fatigue performance were obtained in the following years and incorporated in the type approval process of the ropes. Field trials were conducted in 1995 to assess installation procedures.