Turret-Mooring-System Experience and Enhancements in the Atlantic Frontier
Authors: Carpenter Chris 
Published: February 2015

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 166560, ’From Schiehallion to Quad 204 FPSO: Turret-Mooring-System Experience and Enhancements,’ by Lionel Fromage and Jean-Robert Fournier, SBM Offshore; Pieter Drijver, BP; and Alwyn McLeary, BP North Sea, prepared for the 2013 SPE Offshore Europe Oil and Gas Conference and Exhibition, Aberdeen, 3-6 September. The paper has not been peer reviewed.

The Schiehallion floating production, storage, and offloading vessel (FPSO) is moored by a turret-mooring system (TMS) located in a water depth of 400 m in the Atlantic in very challenging environmental conditions. The Schiehallion FPSO went onstream in 1998. As part of the planned field development, the Quad 204 FPSO currently under construction will replace the producing unit in 2015. This paper compares the main features of the Schiehallion and Quad 204 TMSs, reporting the performance of key components over 15 years of operation.

Introduction

A contract was signed in June 1995 between BP and an alliance of partners to develop an FPSO for Schiehallion. Sanction was given to the project by BP and their partners in February 1996.

The harsh environment encountered in the Schiehallion field, offshore the Shetland Islands in the Atlantic Frontier area, required a new dimension of a mooring system. An internal turret was chosen to allow the vessel to freely weathervane and to transfer the fluids and services from subsea to the vessel and vice versa. The required turret diameter to accommodate 24 risers precluded the use of a conventional slewing-ring arrangement, as was used in previously built North Sea turrets.

The new Quad 204 FPSO that will replace the Schiehallion will be a double-hulled vessel 260 m in length designed for harsh weather and continuous operations in the west Shetland area.

BP Schiehallion FPSO

The Schiehallion is at this time the world’s largest purpose-built vessel, capable of storing 950,000 bbl of oil, with a length of 245 m, a width of 45 m, and a depth of 27.25 m (Fig. 1). A simple barge-type hull was chosen, incorporating a design life of 25 years in situ, with a fatigue life of 50 years and the capacity to survive the 100-year-storm condition. A high forecastle was designed to protect the forward process area and the turret. Accommodation astern of the vessel provides full temporary refuge facilities for 80 persons.

For stability performance, ballast water tanks are segregated and arranged to form a double-sided hull. Two aft-mounted thrusters are implemented for heading control during offloading operations to tandem-moored shuttle tankers.

Mooring System. The mooring design is based on an entirely passive mooring (i.e., no assistance of a dynamic-positioning system is required for station keeping). The natural FPSO weathervaning capacity ensures minimum environmental loading on the catenary mooring arrangement.

The mooring system was conceived of by analytical methods validated by a series of model tests in a 1:80 scale representing the 400-m water depth at the site.