Turret Moorings for Extreme Design Conditions
Authors: Mack R.C. , Gruy R.H. , Hall R.A. 
Published: October 2008

Floating production storage and offloading systems are being designed and constructed for conditions that redefine current thinking regarding the "state-of-the-art". This paper describes four such projects currently underway (by SOFEC) that illustrate some of these extremes, Le., violent environmental conditions both cyclonic and sustained, very deep and very shallow water depths, large numbers of risers, and high operating pressures. These projects clearly demonstrate the versatility of turret moorings and FPSO's and the descriptions herein of these actual projects will provide a useful tool for comparative analysis of potential future projects.

INTRODUCTION

The first turret moorings in the offshore industry were used on "Discoverer" Class drillships developed in the 1960's by The Offshore Company. This background of experience combined with Single Point Mooring (SPM) technology has led to the development of today's sophisticated and elegant turret mooring systems.

The growing demand coupled with proven performance of Floating Production Storage and Offloading Systems has resulted in an ever increasing design envelope. The FPSO concept once thought primarily applicable to the development of marginal fields, has in fact proven itself a viable option for major oil fields located in areas that lack significant infrastructure.

When compared to semi-submersible based developments in such areas, the FPSO, which provides both storage and offloading capabilities in one facility. will nearly always prove to be the more attractive development option.

INTERNAL TURRET SYSTEMS

Quickly becoming one of the most common moorings for extreme design conditions is the internal turret mooring system. Available as either a permanent or a disconnectable system, the internal turret mooring is recognized for its versatility in moderate to deep water depths, for locations having violent environmental conditions, and for applications requiring large numbers of flexible risers. Following are two interesting examples of internal turret systems currently being designed for extreme conditions.

(Available In Full Paper)

The Liuhua 11-1 field is a subsea development having 20 clustered satellite wells and floating facilities which include both a semi-submersible and an FPSO. The semisubmersible supports the control umbilicals for the downhole pumping operations and the FPSO, which is the 141,000 DWT converted Amoco tanker "Mega Eagle", provides final processing, water removal, crude stabilization, storage, metering, and offloading (FIGURE 1) It was decided early in the project to utilize a permanent turret mooring (FIGURE 2) as opposed to the disconnectable type turrets used on two previous "typhoon alley" projects. This meant the FPSO had to remain moored during the 1aO-year typhoon characterized by significant wave heights up to 15 meters, currents in excess of 3 knots, and 87 knot winds. Design forces and motions developed during extensive model tests conducted at the Norwegian Marine Technology Research Institute (MARINTEK) correlated well with those developed analytically during preliminary design

A separate series of wave basin tests was also conducted to determine the effects of "greenwater" over the forecastle deck when the tanker was in a loaded condition. As a result of these tests which showed greenwater as high as 3 meters above the main deck at the turret location.