Value Case of an Integrated Subsea Seawater Desulfation and Injection System

Abstract Seawater is typically pumped into oil wells for pressure support or to increase hydrocarbon recovery during extraction. Disinfection and elimination of bacteria and other invasive species are necessary to avoid reservoir souring. Moreover, removing the sulfates in the injection water can help avoid scaling and pore blockage in the reservoir. This paper describes a fully integrated subsea seawater treatment, desulfation, and injection system for deepwater and shallow applications. A lean, flexible, and robust subsea desulfation and water injection (SDWI) system is defined, any technology gaps are defined as per API RP 17N (2023) and API RP 17Q (2023), a technology qualification plan (TQP) to close the gaps is proposed, and mitigation measures to lower project execution risk are provided. This paper describes the technical work undertaken to prove the viability of the SDWI system, focusing on reduced complexity, improved reliability, and cost-effectiveness. A system integrator and a seawater treatment technology provider collaborated to conduct the work, with key input from the field operator throughout the study. This paper builds on a conceptual SDWI study conducted from 2023 to 2024 for a deepwater field application in Brazil. In the current study, the initial phase focused on process system selection and sizing of all modules, followed by preliminary mechanical design, modularization, and subsea layout. In later stages, the study examined the selection and dimensioning of a suitable topside and subsea power and control system, as well as a proper umbilical to power and control the SDWI. The study also evaluated the required modification on the FPSO to install the topside power and control module. During the system design, subsea system monitoring and instrumentation were investigated. A detailed technology maturation assessment (TMA) with input from subject matter experts was conducted, in addition to a technology qualification plan to close the gaps that were identified. A Failure Modes, Effects, and Criticality Analysis (FMECA) and reliability, availability, maintainability, and safety (RAMS) analysis was performed to support the estimated life of the individual modules and intervention intervals for the system. Detailed system testing, delivery schedule, and execution strategy were also investigated. The value of the SDWI system is based on its application in operational FPSOs in deepwater environments, where constraints on seawater treatment capacity, space, and weight, in addition to the complexity and downtime of modifications, limit further expansion. A decoupled and independent subsea system increases capacity while minimizing onboard brownfield modifications. Moreover, this robust, streamlined, and cost-effective subsea seawater treatment system has a lower carbon footprint compared with that of traditional topside water treatment systems.