A Systematic Investigation of Girassol Deepwater Field Operational Data to Increase Confidence in Multiphase Simulation
Authors: Zakarian Erich , Larrey Dominique 
Published: February 2016

After over 5 years experience as operator of Girassol, the earliest deepwater field put in production offshore West Africa, Total has recorded a large amount of operational data. The production system includes several conventional subsea loops connected to a FPSO by 1350m water depth, with gas lift injected at the bottom of the risers for activation and flow stabilization.

A systematic review of the operating parameters of the subsea production loops over the past years gave the opportunity to extract series of measurements representative of a wide range of flow rates, water-cuts, gas-lift rates, including flow stability tests performed both on upward and downward sloping flowlines. These data were compared to the results obtained from dynamic simulations performed with the simulation code OLGA2000®, originally used for the design of the subsea production system. The comparison focused on the overall pressure drop between manifolds and topside and on the transition between stable and unstable flow with decreasing gas lift rate.

The work was conducted in two steps, first updating the model of each flowline to implement the details of the as-built geometry, then performing extensive numerical simulations and post-processing of the selected operational cases. Particular attention was paid to the first step in order to achieve the best compromise between model accuracy and computational speed. The optimum was met when the model, run with the Slug Tracking option, was able to reproduce the transition to unstable flow observed on site.

In order to investigate future operating conditions of the Girassol field, this methodology will help to establish a confidence level in multiphase simulation. This work can also serve as a reference for other deepwater field developments.

The Girassol field

Girassol is a deepwater oil field development, located 210km northwest of Luanda in Angola, and about 150 km from shore [1]. The reservoir is shallow (1,200 m) with a large horizontal extent: pressure and temperature are about 250 bar and 70°C, respectively. The oil has an API gravity of approximately 32° and the GOR is in order of 110–130 Sm3/Sm3.

Production from the field is tied-back to a FPSO through 8" I.D. piggable production loops: cf. Figure 1. The 24 production wells are connected to the production loops through 2-slot manifolds at a maximum 6km distance. They are equipped for chemical injection, continuously at downhole and possibly at Christmas tree for batch treatments.

Flow assurance issues such as wax deposition or hydrate formation are primarily covered during normal production operations by an extensive thermal insulation of the subsea production system (SPS) [2]. Thermal performance is achieved with the gathering of the production flowlines (right and left lines of a production loop) within seabed bundles and riser towers (two production loops per riser tower). During shut-down conditions, the full SPS is designed to be preserved from hydrate formation (after a no-touch time) through:methanol injection at wellheads, jumpers and manifolds from 2" service lines;displacement of live oil with (stabilised) dead oil circulation from the FPSO into the production loops.

Gas-lift can be injected at the base of the production risers for activation and flow stabilization.