Comparative Assessment of Steel and Polyester Moorings in Ultradeep Water for Spar- and Semi-Based Production Platforms
Authors: Kota Ravikiran S. , Greiner William , D&apos;Souza Richard B. 
Published: October 2008

The purpose of this paper is to compare the performance and cost impacts of polyester taut leg and steel semi-taut leg moorings in deep water applications. Motivation for this study is to provide a basis for selection of mooring system in conceptual and preliminary engineering phases. Mooring systems are designed for semi-submersible and spar based floating production and drilling units (FPDUs) in a typical Gulf of Mexico environment. Water depths used for mooring design range from 1,000 m to 3,000 m. Effect of horizontal load from steel catenary risers (SCRs) is also considered. Based on the results of the design, comparative performance and costs assessments of polyester and steel are made.

Introduction

As the offshore exploration and production moves to deeper waters, the design of stationkeeping systems for floating production systems becomes increasingly challenging, driven by the need to maintain tight watch-circles for risers and to minimize vertical mooring loads on the platform. Conventional steel catenary mooring systems are either not feasible or are not cost-effective in ultra-deep water depths.

Traditionally, mooring systems for vessels operating in shallow to intermediate water depths have most often been simple steel catenary systems. Generally, these systems are chain-wire-chain moorings with drag-embedment anchors. As drag anchors are not designed to take vertical loads, it is necessary to use a very long and heavy section of ground chain. Anchor radii and horizontal excursions in such systems are normally high. For shallow waters, suspended weight of catenary mooring systems is generally not large enough to significantly impact the vessel payload. However, for deeper water depths, line sizes and pre-tensions required to meet the safety factors and offset limits, increase significantly. Also, relatively long sections of ground chain are necessary to keep the anchor uplift angle to a minimum. This causes a significant vertical load on the platform and makes mooring installation increasingly difficult. In some instances, it is possible to use a submersible buoy close to the surface to reduce offsets and line tensions. Typically, these buoys are sized to hold the suspended line weight in the pre-installed condition. Therefore, feasibility of using such buoys is limited by water depth. Large buoys introduce cost, installation and reliability concerns.

Development of anchors capable of withstanding large vertical loads, over the past few years, has given rise to the concept of taut leg and semi-taut leg moorings. By definition, mooring lines in taut and semi-taut leg systems do not lay any line on bottom when loaded. As a result, the mooring footprint is significantly reduced, which is particularly useful to avoid fouling with existing pipelines or other sub-sea structures. Reduction in ground chain length renders the mooring system stiffer which in turn reduces the watch circle. Steel semi-taut systems typically comprise of platform section chain, an intermediate wire followed by a relatively short ground section of chain.

As an alternative to steel, there has been a considerable interest in the use of synthetic ropes for deepwater permanent mooring systems. This is motivated by the need to use a lightweight material with high breaking strength. While various candidate rope materials and rope construction techniques are being studied, it appears at this time, that polyester is the best suited for the purpose because of its high strength to weight ratio, low elasticity and good fatigue properties. Polyester rope has been recently employed in several semi- and monohull based production platforms in Brazil.

However, the design procedure for synthetic mooring syste