Conceptual Design and Comparison of Aramid and Polyester Taut Leg Spread Moorings for Deepwater Applications
Authors: Wildel Bob , Kelly Patrick , Librino Filippo , Whitehill A. Simeon 
Published: October 2008

As the offshore industry looks beyond water depths of 3,000 ft for future oil and gas production, the industry standard steel catenary spread mooring (CSM) system used on all floating production systems installed to date becomes increasingly inefficient and costly, An alternative to the CSM is the Taut Leg Spread Mooring (TLSM) system with its characteristic short scope legs and vertically loaded anchors. In water depths greater than 3,000 ft, it has been shown that TLSMS comprised of synthetic mooring lines have performance advantages over systems utilizing steel wire rope and demonstrated potential for significant cost reductions. Early studies investigated the use of aramid fiber rope due to its high strength, lower in-water weight and lower axial stiffness as compared to ropes made from steel. Later studies indicated that the material properties of polyester fiber, primarily it's lower Young's modulus, made polyester mooring lines more suitable for use in TLSMS2'S. As the TLSM knowledge base expanded, aramid rope construction evolved. Earlier efforts to match the characteristics of steel wire rope have given way to new generation 'soft' aramid constructions. This paper compares the mooring performance and total installed cost of a "soft? aram id TLSM to a typical polyester configuration. Both systems were designed for use in the Gulf of Mexico (GOM) with an FPS based on the Aker P45.

Introduction

This study was performed with the objective of investigating and comparing two taut leg spread mooring systems comprised of differing types of synthetic rope. The two types of synthetic mooring line constructions investigated were:Whitehill 36-strand aramid rope.Marlow Superline parallel-strand polyester rope.

Mooring systems using both constructions were designed for the new-build Aker F'45 Semi-submersible Drilling and Production platform (FDPU), The first P45 is currently being constructed for Norsk Hydro for their Njord field development. The vessel's displacement will be 49,600 tons when completed; production facilities will be capable of 70,000 BOPD of crude throughput. The particulars of the Aker P45 are presented in the Appendix to this document.

The study this paper is based upon has been planned and organized such that it would be a suitable extension to work previously presented13

A deep water (3,000 ft) GOM location was chosen as the field location for this work. Three environmental design scenarios were investigated for this site. They are:Maximum Wave Hurricane Event with associated wind and currentMaximum Loop Current Event with associated wave and windMaximum Wave Case -5 year return

The Maximum Wave Hurricane condition noted above was the controlling case and, therefore, was used in the design effort.

The goal of this study is to evaluate whether mooring lines constructed of aramid fiber are cost and performance competitive with polyester for deep water taut leg spread moorings. As part of the scope of the study, installation costs and schedules have been developed for each system. Preliminary component specifications were developed to a level of detail where costs could be estimated for each mooring case.