Gas field developments with long tiebacks into existing infrastructure poses flow assurance challenges due to the liquid drop out causing high back pressure and high energy consumption to transport the production leading to high OPEX, CAPEX, and CO2e emissions. A novel Pseudo-Dry-Gas (PDG) concept was developed as an inline gas-liquid separator which separate liquids from the gas and flows into a separate liquid line, along with new approaches to flow assurance design, to induce hydraulically ‘dry gas’ behavior within a wet gas pipeline over the operating envelope. The work aims to demonstrate the techno-economic benefits from a feasibility study focusing on the flow assurance aspects. Comparisons are made for project economics and integrated life cycle CO2 assessment showing the "energy intensity" for PDG and the previous chosen industry-norm concept.
A widely accepted industry design software is used to generate data for two concepts based on a mid-water (100-200m) gas and oil field development with a 50 km tieback into an existing platform. Advanced flow assurance design technique is developed to evaluate the optimum location for PDG unit along the pipeline for effective subsea separation coupled with mitigation of flow assurance concerns, primarily hydrates. The performance of the PDG unit was predicted by linking the field data with PDG test data. The test was conducted on a 6" flow loop with 6" PDG prototype with pilot scale unit, covering the operational pressure and temperature and anticipated fluid properties. The CO2 assessment was undertaken in accordance with ISO 14044, linking the reservoir production to the power demand to support production, which was then converted into the equivalent CO2e emission.
The results showed that the PDG design removed the need for a new unmanned processing platform over the main field and a new MEG (mono ethylene glycol) regeneration system at an existing platform which also serves as the host tie-in point. The removal of a large jacket with associated topside process equipment and brownfield platform modifications delivers reductions in CAPEX and better economic indicators, NPV (net present value) and IRR (internal rate of return). This beneficial economic outcome is reinforced with a reduction in carbon intensity for PDG concept, with more than 40%, compared to the chosen industry-norm concept. These observations are the net result of changing the internal flow resistance curve of the gas tieback from a quadratic function to a linear function enabling more efficient use of downhole pressure and temperature to reduce the overall carbon intensity of upstream gas production and mitigate flow assurance risks for hydrates and liquid surges.
The work will benefit practicing engineers through the evaluation of novel PDG technology concepts for mixed oil and gas tiebacks, currently in pre-production testing phase, to improve the field economic and to evaluate CO2e emissions over the life cycle of a project in accordance with ISO 14044.
