Girassol: The Subsea Production System Presentation and Challenges
Authors: Barnay Gilles 
Published: October 2008

This paper focuses on the subsea production system portion of the Girassol development. It will note the main challenges - water depth, thermal issues and flexibility to cope with reservoir uncertainties. It will describe how the technical issues were solved using design reviews, exhaustive qualification program and comprehensive verification testing.

The experience during the first months of production indicate a high availability of the SPS equipment confirming that the measures taken were invaluable.

Introduction and history

In 1996 TotalFinaElf and its partners discovered the Girassol oilfield in 1,400m water depths approximately 100km west of the Angolan coast in the Gulf of Guinea.

Two appraisal wells were drilled, confirming the presence of commercial quantities of high quality oil. While appraisal studies were on going, concept selection for the development was initiated leading to the choice of an FPSO and subsea scheme. The preliminary studies ended in June 1997.

A project definition phase then started covering the SPS, basic engineering and a funded design competition. The first objective of the design competition was to prepare the predevelopment plan in relation to the expertise of the contractors, together with parallel works, such as subsurface appraisal and reservoir studies. The ultimate objective of the design competition was to be able to make a recommendation on the contract award for the SPS. The decision was taken in July 1998 to develop the field and the SPS contract was awarded to FMC Kongsberg Subsea, covering engineering, procurement and construction of the SPS. The contract included 30 xmas trees (plus options for an additional 10), 11 manifolds (plus options for two), jumpers between the xmas trees and manifolds, two completion/workover systems, the subsea control system and its associated topside components.

Installation of well-related equipment would be carried out by the rig contractor with the remainder handled by the UFL (umbilical/flowline) contractor.

Technical requirements

The main requirements of the SPS are:

Environment

Water depth: the system has to be designed to sustain the very high external pressure (up to 145bar).

Soil conditions : the system has to cope with very soft soil conditions

Basic data

Maximum wellhead shut-in pressure = 235bara

Maximum operating temperature =70°C

Average liquid production per well: 5,000-40,000b/d

Injection trees: maximum operating pressure = 300 bara

Flow assurance

Hydrate prevention

The temperature shall remain above the hydrate formation temperature (20°C) during normal and low flow conditions. After an unplanned shutdown, the production fluids in the SPS shall remain above 20°C for more than eight hours. During production shutdown and startup, methanol injection will be used. Methanol injection may also be used for hydrate removal in certain instances.

Wax deposition

The wax formation temperature is estimated to be around 39°C. In order to avoid or reduce wax deposition, the fluid temperature should remain above 40°C during normal flow condition. Roundtrip pigging is also required.

Sand control

Sand production is prevented by sand control completions and monitoring is ensured by sand detectors installed at production xmas tree level.