This paper highlights thermal performance of subsea production systems based on Shell's Gulf of Mexico deepwater field experience. Cool down time during an unplanned shut down or ESD is an important design and operational consideration for deepwater subsea systems to avoid hydrate formation.
While considerable experience has been gained in the thermal design to insulate subsea trees, well and flowline jumpers, manifolds sleds and associated equipment, there is very little published data on field experience. This paper highlights the thermal design philosophy, describes the thermal insulation materials and provides a summary of the thermal analysis performed to support the design. Furthermore, based on field measured pressures and temperatures, the actual cool down performance during production shut-ins experienced in the field have been collected and analyzed to benchmark the design. The results of the benchmarking analysis will be presented.
Lessons learned from the benchmarking included:
Cool down performance in many cases are different than the design
Some of the reasons include variations in flow conditions, flowing wellhead temperature, fluid properties, performance of the materials (i.e. water absorption), geometry of the insulation, etc.
Operational procedures have been ‘fine-tuned’ based on actual cool down performance The paper will also provide some recommendations or options for consideration in the thermal design of subsea hardware for future projects.
As production operations continuously expand in recordbreaking water depths using subsea technology, flow assurance continues to be a critical part of system design and operations. A common method of hydrate management combines thermal insulation for the subsea production system, chemical injection combined with specific operating procedures such as blow down after a shut down event combined with optional circulation of the flow lines. However, subsea trees, well jumpers and manifold branch piping typically are not configured for round trip circulation. To plan for an unplanned shut down, these equipments are generally specified with a minimum of 8 to 12 hours of cool down time, which requires wet thermal insulation. This approach is combined with chemical treatment after the shut down event. In deep water applications, blow down alone is generally not sufficient to prevent hydrates. Therefore the integrity and reliability of the insulation to the subsea production equipment is critical.
If hydrate blockages form inside tree piping, valve cavities and manifold branch piping, it makes it difficult or impossible to open/close tree valves, manifold valves, direct the well flow to different manifold headers, injection of chemicals or circulation through certain flow paths. In extreme cases, well bore isolation and well control could potentially be jeopardized. Therefore, highly reliable thermal insulation is critical for the subsea production equipment similar to flow lines and risers. While there have been numerous papers and industry focus on wet insulation and pipe-in-pipe flow lines and risers in the industry, there has not been as much attention given to the thermal design of subsea trees.