The export of gas from deepwater production systems can involve a large diameter pipeline with a manifold (PLEM) at the deep end. The PLEM commingles the flow from multiple smaller diameter risers, or flowlines, isolates the pipeline and provides a subsea pig launching and receiving facility. Connecting the PLEM to a large diameter (36” to 46”) gas export line is a major challenge because of the very stiff pipe. Horizontal diverless connection systems can align the pipeline terminations using powerful hydraulic jacks but the size and weight of the spools and equipment for such large diameter tie-ins make them difficult to handle. Therefore, modular designs on a sliding foundation have been developed which significantly reduce the energy required to make up the connection as well as facilitate maintenance and repair.
This paper describes the design aspects of large deepwater PLEM and the methodology which has lead to feasible solutions in 300m to 1300m water depths. Potential cost savings are related to successfully perfoming the subsea tie-ins using smaller subsea construction vessels. The deepwater connections become increasingly expensive for larger diameter pipelines if the risks are not reduced by the design and by performing advanced engineering involving finite element analysis (FEA).