Stones Artificial Lift System: Extending the Boundaries of Subsea Boosting

Abstract Subsea boosting is now accepted as one of the most competent artificial lift methods to enhance subsea asset productivity. In many scenarios, it is a technical enabler to unlock resources located at a distance or water depth where the reservoir energy is not sufficient to naturally produce and transport the fluids to the surface process facility. The Stones system is currently the world's deepest offshore oil and gas project, operating at a water depth of around 2,900 meters (9,500 feet). The subsea boosting system, which includes two 3-MW single-phase pumps (SPPs), was deployed in the Gulf of Mexico in 2019. The system boosts the production of a complex reservoir and has to counterbalance the effect of the significant hydrostatic column to maintain the production delivered through eight wells. The vital role of the subsea boosting system for Stones production requires continuous monitoring of the equipment to ensure a maximum uptime and detect unexpected events. Maintenance of rotating machinery is always required at some point in its lifetime. Predictability is critical to be able to plan costly subsea interventions and minimize the time required to install spare equipment. Simple modeling of the subsea pump envelope provided too little resolution to be able to make adequate assessments, especially as the field conditions changed. The solution came from using data and modeling capabilities along with integrated surface facility surveillance. The objective was to provide the asset with the optimal recommendation for production well assurance to determine when a subsea pump would require replacement on the basis of performance degradation resulting from solids production and the expected production forecast. The deployment of a condition monitoring system enabled subject matter experts (SMEs) from the operator and supplier to collect, visualize, and analyze critical system operational data together. The connection of production and equipment expertise enabled the SMEs to enrich their understanding of the system during operations and improve the engineering models initially developed to manage the equipment. The strong collaboration between SMEs led to the development of more accurate performance models and enabled the quantification of solids. The solids quantification solution deployed in the condition monitoring system relied on a combination of models that were fine tuned to adequately compare the expected baseline (i.e., a new pump) flowing conditions of the SPP with the actual operating conditions to provide a flow performance factor in terms of percentage of operability and efficiency. Factors considered included suction pressure, well production rates (oil, water, gas), recycling conditions, and densities, in addition to the pump performance curves. Suction pressure and density became key factors along with where the pump was operating within the envelope. A novel solids quantification method was implemented using data from the subsea flowmeters installed upstream of the boosting system to better assess when impacts to the boosting system could be expected. Based on the modeled approach and continuously evolving production profile, pump replacement was able to be deferred, and both SPP units have experienced an extended run life (now at more than 5 years of operation). Proactive surveillance of this system has provided prudent cost savings while still meeting production targets. The Stones production fluids have a bubblepoint between 900 and 1,150 psia. Before 2021, the SPP had been in operation with a minimum suction pressure operating limit of 1,200 psi. Decreasing the suction pressure to operate below the bubblepoint enabled an increase in the overall production through a process that integrated risk assessment, pump performance and flowline dynamic modeling, topside considerations, operating procedures, and a management of change process to approve opportunity and move to execution. Following implementation, an overall 10% production increase was achieved.