Fulmar Submerged Turret Loading System
Authors: Bailey M.J. , Brouwer A.L.J 
Published: October 2008

Shell UK Exploration and Production, operating in the UK sector of the North Sea on behalf of Shell and Esso, haverecently installed and commissioned a Submerged Turret Loading (STL) system for oil storage and evacuation from the Auk, Fulmar, Gannet and Clyde fields.

The STL system replaced the recently abandoned Fulmar Floating Storage Unit (FSU) as a temporary oil evacuation system, prior to the decision being taken on the long term strategy for oil evacuation in the central North Sea.

The STL system is presented in this paper, covering the vessel and the mooring system, the vessel modifications that were necessary to meet UK specljic safety legislation and the operating experience to date.

INTRODUCTION

Oil storage and evacuation at the Fulmar field had, in the past, been through the Floating Storage Unit (FSU) and SingleAnchor Leg Mooring (SALM) system. However, due to the FSU/SALM nearing the end of its useful field life, a temporary system was required, prior to the finalisation of the long termstrategy for central North Sea oil evacuation.

Temporary oil evacuation systems considered, included utilising a CALM buoy, a UKOLS, the Tentech 850, the Petrojarl, and the Statoil STL system. The economicassessment of the options, together with their availability concluded that the Statoil STL system was the preferred option. Consequently, in January 1993, Shell committed to theSTL system on an initial two year lease, as the temporary replacement for the Fulmar FSU/SALM, for oil evacuation from the central North Sea.

STL SYSTEM DESCRIPTION

The STL system consists of a conventional cargo tanker which connects/disconnects to a submerged turret buoy and flexible riser, which in turn are catenary moored to piled anchors (Figure 1).

The STL system, at the time of project realisation, was conceptually designed for the Heidrun field in Norway by Statoil, where two submerged turret buoys, and three continually shuttling tankers will be utilised. However, for the Fulmar application, the tanker operates in a similar mode to the Fulmar FSU, with dedicated shuttle tankers offloading crudevia a stem offloading system.

Mooring System

The STL buoy comprises a steel buoyancy cone, a rotary turret shaft with mooring connections, and an inner crude pipe assembly (Figure 2). The STL buoy at Fulmar is moored to the seabed with a conventional catenary mooring spread of eight 890m long anchor lines. Each mooring line comprises an upper and lower line segment of 112mm diameter spiral strand wire and 120mm K3 Rig chain respectively. The mooring line loads are transferred to the seabed via driven piled anchors. When released from the tanker, the STL buoy, with a dry weight of 80 tonnes, will float approximately 30m below thesurface, in equilibrium with its buoyancy and the verticalforces from the mooring system. The water depth at Fulmar isapproximately 80m.

When the STL tanker approaches the field, the position of the STL buoy is located by means of a Hydro-acoustic Positioning Reference system (HPR) with transponders located on the outside of the buoy and a transducer installed underneath the tanker. hull.