The Girassol field development, offshore Angola in 1,400m of water, has stringent operating requirements due to flow assurance issues and the need for flexibility. The result was the selection of new riser and flowline concepts with numerous qualifications for materials and equipment. This paper details how these concepts - freestanding riser towers, insulated flowline bundles and a mid-depth export system - met project requirements.
UFL has demonstrated that solutions could be developed in a short timeframe. These concepts will be used as a reference for other deepwater developments and while alternative solutions may be chosen, the challenges will be similar.
The umbilical and flowline network, part of the Girassol development, is made up of the following main systems:
The production system comprises five production loops. Thirteen manifolds are installed in 'daisy chains' along these loops. Up to two production wells can be tied in to each manifold.
Each of the production loops is made up of two sections: the production bundle on the seabed bottom and the production riser integrated within the riser towers connected to the FPSO.
Each of the five production loops consists of several production bundles interconnecting the manifolds (refer to the overall field layout).
In total there will be 11 production bundles with eight installed during the first phae of the offshore operations carried out during 2001.
Each production bundle contains (Fig.5):
Two by 8in production flowlines.
One by 2in service line.
These lines are contained within two semi cylindrical modules of syntactic buoyancy foam, which ensure the thermal insulation of the pipes.
One by 30in carrier pipe housing the insulation foam, the production and service lines.
The carrier pipe ensures the mechanical protection, the corrosion protection and contributes to the overall thermal performance of the system as it avoids exchanges of seawater with the external environment. The carrier pipe is entirely closed and full of inhibited water.
The weight in air is 550kg/m with a submerged weight in production condition is in the range of 90kg/m. Each bundle is equipped at each end with a sled (Fig. 6), which is used for the installation by seabottom tow and also as structural support to perform the tie-in of the spools connecting the bundles to the manifolds.
The length of the production bundles varies from 700m to 3,000m. The lengths have been determined on the basis of the subsea field layout, but are limited by the bollard pull of the towing spread used for installation.
The 8in and 2in risers, which are a continuation of the pipelines in the production bundles, are integrated within freestanding riser towers (Fig. 1 and 3).
Each production riser is equipped with a 3in gas lift line with an entry point at the bottom of the riser tower.