The Girassol field development, a full subsea production system and an FPSO, has included the implementation of a number of technological innovations, such as wet syntactic bundles and riser towers. Water depth, cold environment and specific Girassol fluid and reservoir characteristics set new challenges to flow assurance discipline and demanding constraints for field operation.
A limited time frame for project development, uncertainties on fluid and reservoir knowledge and a level of innovation favored interdisciplinary work and integration of operability aspects in design requirements. Flow assurance strategies developed to handle the main risks and significant findings are presented.
Girassol is a deepwater (1,400m) field development, located 210 km off the coast of Angola. The reservoir is shallow (1,200m) with a large horizontal extent. Initial bottomhole pressure and temperature in the main reservoir were estimated at project start at 268 bar and 58-69°C, respectively. API gravity was approximately 32 and GOR 110-130 Sm3/m3.
Production from the field is gathered through five 8in ID piggable production loops with up to 23 deviated or horizontal wells connected through ‘daisy chained’ manifolds. The wells are open hole gravel pack or natural sand pack with high productivity up to 40,000b/d. Maximum tieback distance from the FPSO is approximately 6km.
Production flowlines are gathered in flowline bundles and riser towers and gas lift can be injected at riser base for slug management and increased oil recovery purposes. Methanol may be injected subsea through a 2in service line and delivered to the wellhead and subsea manifold main headers.
Wells are equipped with downhole chemical injection line for continuous injection and at each xmas tree for batch treatments.
During the pre-project phase, fluids from exploration wells were analyzed in order to evaluate flow assurance risks and set preliminary requirements for subsea production system design and operational strategies. This early assessment established the requirements for extensive thermal insulation of the subsea system to cover both hydrate and wax formation risks, but also for the injection of chemicals and methanol at the xmas tree and well level.
Evaluation of new information, made available during the project phase, and development of operational procedures generated design changes and improvements. Early testing of thermal performance of equipment, RAM and QRA analysis performed on production and chemical injection systems proved to be very useful and justified implementation of system modifications to improve availability and operability. Close cooperation between the field operation engineer, flow assurance engineer, and system engineer facilitated the implementation of operability aspects and late modifications to the system design.
Hydrate risk was considered from early on in the project phase as a key parameter for system design. This resulted in substantial efforts to improve reliability of topsides and subsea components, extensive qualification program of insulation material for Girassol conditions and verification of system design through thermal tests.