Selection of Subsea System Architectures for Offshore CCS

Compared to hydrocarbon production developments, subsea CO₂ injection fields are characterised by a simpler subsea architecture and present lower power and low to no chemical demand. Such specification provides opportunities to reduce CAPEX by using simplified subsea injection and control systems. The extent of the subsea carbon capture and sequestration (CCS) system simplification, however, depends on the availability of technologies (including those currently under development) and the regulatory requirements that will apply to CCS developments, which may vary among regions. In this work, the elements that contribute to a technically optimised and economically efficient subsea control system for offshore CCS developments were examined.

Different subsea control system architecture scenarios that can be used to inject CO₂ offshore from a carbon capture plant onshore were assessed. Each scenario represents a qualification step toward a minimalist and fit-for-purpose configuration, with the final system being a very simple and all-electric subsea architecture. We adopted a holistic approach to a hypothetical subsea CCS project, evaluating the impact of the subsea control system equipment technical readiness, CCS regulatory requirements, umbilical sizing, and installation strategy.

The CCS subsea equipment specification can simplify the profile of the subsea architecture required for carbon injection. Leaner umbilical and subsea equipment can be considered, leading to a significant saving on the project CAPEX. The subsea system CAPEX (with exclusion of the CCS pipeline) is predominantly influenced by the price and length of the umbilical and the injection tree design. The project execution profile is also positively impacted by simplifying the logistics and opening new asset opportunities (e.g., access remote reservoirs for CO₂ injection).

After definition of the technical solutions and analysis of the budgetary price estimation, an all-electric scenario has been identified as the most cost-efficient architecture, with technologies that will be available and qualified for projects awarded in the near future.