Tazerka Multiwell FPSU: Design of Production Facilities Integrated With the Tanker
Authors: Rietveld R.J.C. , Biss N.C. , Gelderblom D. 
Published: December 2007

This paper will review the design of the production facilities on the multiwell floating production and storage unit (FPSU) operated by Shell Tunirex in the Tazerka field, offshore Tunisia, on behalf of a joint venture with AGIP (Africa) and Entreprise Tunisienne d'Activits Petrolires. Economics and product specification dictated the application of special techniques in the production and utility designs regarding process flexibility, gas disposal, power generation, and pollution prevention. The field, containing estimated reserves of 8 to 10 million bbl [1.3 × 10(6) to 1.6 × 10(6) m3] of recoverable oil, is located in deep water (459 to 984 ft [140 to 300 m]). It was desirable to use a single-process train with as near a 100% on-stream factor as possible. While this is not the first application of a flow station on a weathervaning unit, it is believed to be the first to process crude directly from several subsea wells.

Introduction

The Tazerka field lies 35 miles [56 km] offshore the northeast coast of Tunisia in the Hammamet Grand Fonds permit. It has been in production since Nov. 1982 and currently produces some 10,000 B/D [1590 m3/d]. Oil is produced from four subsea completed wells in water depths ranging from 459 to 607 ft [140 to 185 m], flowing by Coflexip (TM) flowlines to the single anchor leg system (SALS) that holds the FPSU on location in 459-ft [140-m] water depth. Oil flows through swivels on the SALS to the FPSU-i.e, to the converted very large crude carrier (VLCC) tanker Murex (Fig. 1). The entire crude production facilities are integrated onto the converted unit, which then stores up to 140,000 tonnes [140,000 Mg] of crude. The system is completely independent of platforms and pipelines to shore, with off-take achieved by discharging, using the original VLCC cargo pumps, to trading tankers moored alongside. This approach to developing the field, instead of conventional methods such as steel or concrete platforms connected by pipeline to shore, was dictated by economics, which showed that conventional methods would have been too expensive.

Facilities Description

The initial field development plan called for reservoir natural depletion with provisions for future installation of water injection and gas lift facilities to enhance recovery. Fig. 2 shows a block flow diagram depicting the deck-mounted facilities/utilities that have been designed and engineered for the Tazerka FPSU.

Crude Oil Production

The Tazerka FPSU development is designed to receive 30,000 B/D [4770 m3/d] of produced liquids from up to eight subsea completed wells. All wells have provisions for future gas lifting. Three of these wells also have been designated for water injection. At present, the well fluids from the four installed subsea wells flow through 4-in. [10-cm] Coflexip seabed lines to the SALS manifold, then through a six-bore product swivel, and finally, through the mooring yoke to the receiving manifold on the FPSU. Well subsurface safety valves and wing valves are controlled hydraulically from a control panel on the FPSU deck by control lines routed through a 20-bore swivel on the SALS.

Oil/Gas/Water Separation

From the receiving manifold on the FPSU, the incoming stream is routed to a single-process train, consisting of a separation unit (HP, LP, and test) and a stabilizing unit. The possible combinations of the process train and the valve switching provide for an on-stream factor close to 100%. Oil can be produced through any one of the six product swivels to the receiving manifold. All separators are of identical design and are capable of handling the full production flow, therefore enabling any one separator to be out of service for maintenance or repair while still maintaining production through the others. The test separator also can be used as a stabilizer. Fig. 3 shows the changeover philosophy in detail. Produced water can be removed from the crude oil in two stages.1. Bulk separation in the three-phase separators ispossible in either series mode or parallel mode.2. Final separation of water occurs in the FPSU's dedicated crudereceiving tanks, where water can be taken off the bottom by theoriginal VLCC's stripping pumps. A dehydrator can be installed ifhigh water production or stable emulsions occur.

JPT

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