Comparative Evaluation Of Disconnectable And Permanent Moorings For Fpsoâ??S In Harsh Environments
Authors: Delepine Y.M. , Engh I.B. , Dove P.G.S. 
Published: October 2008

This paper discusses the factors that operators considering FPSO developments in harsh environments should evaluate when selecting between a permanent or disconnectable Single Point Mooring system. The paper first reviews the state of technology for both mooring types, focusing on turret-type systems. A global approach is presented by defining the quantitative and qualitative factors that must be considered in the selection and how each one can be addressed. These factors include mooring system size, green water, tanker hull longitudinal strength, system cost and schedule, reliability, safety, risks to project schedule and cost, and risk of operating downtime. The sensitivity of mooring system feasibility, size and cost to water depth is quantified. The importance of obtaining input into the selection from the operator is emphasized.

INTRODUCTION

In the last seven years, offshore production in typhoon prone areas has required the development of FPSO Single Point Mooring (SPM) systems capable of disconnection in heavy seas. Extreme events, referred to as typhoons, include any extreme climatic event that greatly exceeds normal conditions and can be forecast with sufficient warning to allow escape. The disconnection concept has been applied in the South China Sea and the Timor Sea, offshore Australia, where a total of six disconnectable moorings will soon be in operation. By eliminating the need to design for extreme weather, disconnectable mooring systems can be less expensive than their permanent alternatives. However, depending on water depth and the severity of typhoon conditions, the cost advantage may not be decisive. Consideration must then be given to a large number of design and operational issues.

The purpose of this paper is to provide operators a rational and comprehensive approach to the mooring selection based on FPSO feasibility and cost studies performed for several geographical areas. The use of converted tankers is assumed but the impact of using a new-build tanker is also briefly discussed.

STATE OF MOORING TECHNOLOGY

SPM Design Types

Many variations in both permanent and disconnectable mooring systems have been proposed by the Single Point Mooring (SPM) industry.

Permanent Mooring Systems

Two main categories of permanent mooring systems have been designed and installed for harsh environments: Single Anchor Leg Mooring (SALM] systems with rigid yokes, and turret moorings, internal to the hull or externally-mounted. Other designs have also been developed such as the Emerald Field FSOâ??S soft yoke.

SALM systems are applicable in shallow to medium water depth range but become very costly in deeper water due to fabrication, handling and installation costs for the long riser. Externally-mounted turrets have been used extensively in milder environments, but are less attractive in extreme conditions due to the high loads applied to the external structure and the greater exposure of the equipment to waves. Internal turret systems (Figure 1) offer better equipment accessibility and protection.