How Subsea Processing Impacts Flow Assurance and Field Architecture in Ultra Deepwater

Abstract

Abstract Subsea processing covers different functions: flow boosting, gas-liquid separation, gas-oil-water (and sand) separation, and gas compression. These functions offer increasing opportunities along with increasing levels of complexity, and timeframe. Subsea processing offers new possibilities for flow assurance and production practices. Accordingly, this will modify field architecture, as we know it. All components in the chain from subsea to surface will be affected: manifolds, flowlines, subsea templates, and risers. Noticeably, this will also affect the service and construction industry that will have to adapt to these new requirements. The paper will describe the flow assurance options made available with flow boosting, gas-liquid, gas-oil-water separation and screen current flowlines and risers options to identify how they are likely to evolve to meet the requirements of subsea processing in ultra-deepwater. Introduction For oil producing fields, subsea processing can be classified primarily into flow boosting, gas-liquid separation, and gas-oil-water separation. Subsea compression is applicable primarily to gas producing fields, and at this juncture considered for relatively shallow water projects and not for deepwater or ultra-deepwater. This paper concentrates on ultra-deepwater and therefore subsea compression is not developed further, even though production of gas at elevated pressure can be beneficial as evidenced by Compressed Natural Gas (CNG) concepts. This paper successively includes a review of oil production subsea processing, a typical ultra-deepwater case, and how the available technology of risers and flowlines covers the needs of these new features. Subsea Processing Oil production requires heavy topsides production equipment. This equipment includes the oil production trains, the associated gas compressors for gaslift and gas compression, the water injection system, power generation systems, storage, utilities, and living quarters. The combined weight of all these systems is quite large (typically 25 000T for a 200 000 BOPD FPU). The impact of subsea processing on the size of the topsides is limited and the benefit is not to be gained from a drastic reduction of topsides but from additional production. Additional production can be achieved through two main technologies:Flow boosting: increase of pressure of the oil-and-gas wellfluid by a multiphase pumpSeparation of the gas from the liquid phase (water and oil) and pumping in liquid phase of the well fluid A typical oil production train includes three separators in series. The pressure stages are optimized to meet the oil specification such as the Reid Vapor Pressure (RVP). A typical high pressure separator operates in the range of 500 psi (35 bar). As water depth increases, the benefit of subsea processing increases for a given tubing size (usually 5-1/2??).