Flow assurance challenges increase significantly as wells are situated further away from shore in deep water. Without feasibility studies, pipeline operators would not be able to determine the most viable and safest solutions for offshore cleaning and inspection. ROSEN’s knowledge in flow assurance laid the groundwork for an analysis of a 1,500-meter-deep multiphase flowline system in the South China Sea. Using flow-modelling tools, ROSEN studied the impact of cleaning and ILI pigging on production to provide a pigging plan, support specifications and procedures, thus enabling ILI specialists to inspect the pipeline.
The challenging deepwater project consisted of two flowlines connected to a topside platform. The subsea production wells were tied in to a Pipeline End Manifold (PLEM). One flowline was used for the transport of production fluids to the Topsides platform; the other one supplied recycled dehydrated gas from the Topsides to the first flowline via a crossover at the PLEM. Production had to be maintained during the inspection for commercial reasons. The pig would be driven with the dehydrated gas from the Topsides, traverse the PLEM pigging loop where the production fluid mixed with recycled gas, then be returned back to the Topsides.
Due to the nature of the operating conditions and the pipeline elevation profile, there was a large quantity of liquid holdup in the flowline. Additionally, there was high hydrostatic head, which meant that insufficient driving pressure in the deep water had to be taken into consideration. Furthermore, the pig would be expected to handle dual flowlines operating with dissimilar service fluids while simultaneously maintaining the velocities of fluids with different properties.
No Solution Without Simulation
To estimate the liquid inventory expected in the flowlines throughout the operating life, the dual flowline system was modelled in OLGA, Schlumberger’s multiphase simulator. Pigging simulations for worst-case liquid inventory scenarios helped to find a solution that would minimize the liquid slug generated during pigging: the solution included first sweeping the line with gas at high flowrates, then sending in a pig with a fixed bypass port. The bypass slowed the pig such that it generated the same slug volume but at a slower rate, making slug handling manageable at the Topsides.
During the fixed bypass pre-inspection pigging campaign, a significant amount of fluids would be removed from the line. After that, the inspection would be carried out. Further simulations ensured the flowrates were sufficiently optimized in order to maintain the necessary pig velocity, fully saturate the heavy-wall flowlines and provide the highest inspection resolution.
Achievements in Analyzing
Onshore and offshore, the requirement is operational safety. Beyond promising just that, analyses also provide assurance of pigging and operational readiness.
The flow assurance and pigging feasibility analysis served to reliably and cost-effectively confirm one of the main requirements of this project: maintaining an acceptable level of production during inspection.