A Systematic Investigation of Girassol Deepwater-Field Operational Data To Increase Confidence in Multiphase Simulation

Abstract

Summary After more than 5 years of 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 floating-production, storage and offloading vessel (FPSO) at 1,350-m 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, watercuts, and gas-lift rates, including flow-stability tests performed both on upward- and downward-sloping flow-lines. These data were compared to the results obtained from dynamic simulations performed with the simulation code OLGA, 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 to achieve the best compromise between model accuracy and computation speed. The optimum was met when the model, run with the Slug Tracking option, was able to reproduce the transition to unstable flow observed onsite. 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.