Service Line Option for Hydrate Management in Single Flowline Tieback

摘要

Many subsea projects worldwide would benefit technically and commercially from using a single flowline solution to replace traditional dual flowlines. However, the combination of a single flowline, downhill flow and high water cut presents several challenges for hydrate management after an unplanned shut down. Depressurization is the most commonly used technique to prevent hydrate blockages. But depressurization at high water cut may not be successful when the geometry of the flowline results in a liquid head in the flowline riser that is higher than the hydrate formation pressure at ambient conditions. Options were examined for managing hydrates during high- water cut conditions using service lines in the umbilical to replace the functionality offered by a second flowline. The options examined included using the service line for dead oil displacement, continuous gas lift, expansion of trapped gas, mono ethylene glycol (MEG) injection after emergency shutdown (ESD) and bleeding subsea side trapped pressure .This paper will present an overview of each concept with respect to feasibility, cost, operability and risk issues. The challenges associated with using the service line options were: 1. Hydrate management during extended operations 2. Impact of the service line on subsea tree design 3. Support required from host facility 4. Operability and risk issues The results of a tradeoff analysis were used to select one concept for detail engineering. The benefits of the single flowline system with the selected hydrate management strategy will be illustrated based on the analysis of a deepwater field development design case.