Natural gas transmission pipelines are being laid at water depths in excess of 2,000 meters (6,562 ft). At these water depths, small pipewall penetrations can provide a path for water to enter the pipeline, form hydrates, and block gas flow. This paper discusses a test in which water was "leaked" into a 200-bar (2901-psi) "pipeline" and the resulting hydrate blockage was safely and quickly cleared by pressure reduction on one side of the blockage. The formed blockage contained gas pockets and the measured blockage permeability was low " indicating that reducing the pressure throughout a line, including blockage-trapped gas pockets, can take a long time. Executive Summary A small penetration of a deepwater gas transmission pipeline can lead to water ingress (due to high external pressure) and hydrate blockage of the pipeline with large differential pressure across the blockage and partial flooding of the line. In the test discussed in this paper, water was leaked into a flowing natural gas stream at deepwater pipeline pressure and temperature. This resulted in formation of a hydrate blockage system consisting of at least two blockages that entrapped one or more high-pressure gas pockets. In a system with entrapped gas pockets, using traditional "two-sided" pressure reduction (without calculations and evaluations) to clear a blockage can be unsafe. After appropriate calculations and evaluations, "one-sided" pressure reduction was used to clear the formed hydrate blockage safely, quickly, and effectively. Based on the test results, the estimated time for clearing a similar blockage in a large (300-km long, 24-inch) deepwater line, while maintaining pressure uniformity across the system, is over a year - resulting in over a year of lost gas delivery. With "one-sided pressure reduction," the blockage cleared safely with a 138-bar (2,000-psi) pressure difference across the test section. The pressure difference across the test section dropped to a few bar within 4 seconds of blockage freeing. The clearing time for a similar blockage in a large (300-km long, 24-inch) deepwater line with "one-sided pressure reduction" (including the time required to drop the line pressure on one side of the blockage) is much less than the clearing time with "two-sided pressure reduction." Introduction Natural gas transmission pipelines are being laid along the floor of deepwater seas. Some of these lines run for hundreds of kilometers at water depths in excess of 2,000 meters (6,562 ft). The deepwater environment is typically cool (2°C to 10°C), and the deepwater pipeline external pressure is in excess of 200 bar (2901 psi). In this deepwater environment, the pipeline external pressure may exceed the internal gas-stream pressure. Small pipeline penetrations - for example, those resulting from corrosion - can result in water leaks into the pipeline gas stream. Water in a natural gas stream at these conditions may form hydrates. Hydrates. Hydrates are ice-like solids with guest molecules occupying cages in a crystalline structure formed by water molecules.
Recent completion of the marine pipeline linking Algeria and Spain across the Mediterranean Sea, with a 24-inch pipeline, as part of the Medgaz Gas Transmission project, successfully overcame the challenges of 2,155 metres water depths, deep water cable crossings and sea bed post trenching activities. This paper summarizes the various key design considerations addressed to meet the demanding requirements of the ultra deep Medgaz pipeline together with the engineering challenges met by Saipem in planning and executing the laying of the 24-in. gas pipeline in the Mediterranean Sea ultra deep water. Design issues discussed include route optimization, efforts in relation to mitigating geohazards and challenging seabed topography characterising both Algerian and Spanish continental slopes. The intrinsic difficulties in performing intervention works in ultra deepwater, stimulated the application of advanced analysis methodologies for the assessment of in-service buckling and long term freespan fatigue. The installation of the Medgaz marine pipeline presented numerous difficulties which were of more complex nature than for the normal laying of a trunk line. Crossing of the Alboran Sea (the particular area of the Mediterranean Sea), (see Fig. 1) with a 24-inch pipeline that reached a depth of 2,155 metres was, without doubt, the most complex part of the entire works, involving two large semi submersible pipelay vessels: Castoro Sei and the Saipem 7000. The paper presents an overview of the main aspects of pipelay technology facing long trunk lines in ever great water depths and outlines the many engineering challenges faced as well as the solutions adopted that contributed to an effective development in pipelay technology. The presentation reviews also the post-lay intervention work using the Beluga deepwater trencher, the Innovator Remotely Operated Vehicle guiding ultra deep water laying and crossing of five existing seabed cable. Resolution of the many technical problems faced during the execution of the Medgaz Marine Pipeline project has proven the soundness of the engineering solutions adopted for the implementation of this u Fig. 1 -Route map of Medgaz Pipeline ltra deep water pipelay project.
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