The Chinese National Offshore Oil Corporation (CNOOC) and Husky Energy Inc. (Husky) have developed Phase I of the Liwan 3-1 Deepwater Gas Field in the South China Sea in which natural gas and condensate flow onshore to the CNOOC Zhuhai Gaolan Terminal via a 30-inch subsea, two-phase pipeline. Forecasts of the gas and condensate flow performed by multi-phase simulation tools predict steady-state liquid slugs arriving at the terminal during normal operations as well as extreme liquid slugs during pigging operations. This is largely due to a combination of the forecasted condensate production and the pipeline's final onshore elevation 190 meters (623 ft.) higher than the offshore elevation. These factors, as well as minor changes in elevation along the pipeline's route, will cause large volumes of liquids to build up in the pipeline. As these liquids move onshore through the pipeline, " slugs" are created. To receive large amounts of liquid in short periods of time, a 7,000 cubic meter (44,000 barrel) capacity was specified.
Although not uncommon to have large, multi-pipe slug catchers at onshore receiving stations, the sheer volume of the predicted slugs during pipeline pigging presented unique challenges to the design and construction of this slug catcher, which is currently deemed to be the largest in the world. A total of 28 finger sections (storage tubes) of 56 inches in outside diameter (OD) each with a length of over 175 meters (575 ft.) were required to store the large volume of arriving condensate. The 28 parallel fingers, coupled with their 175 meter length, required a plot area of over 15,000 square meters (161,400 sq ft.), equivalent to several soccer fields, making this slug catcher the largest single piece of equipment on the entire project.
Since all of the gas and condensate feeding the onshore development and gas processing plant must first pass through the slug catcher without interruption, the criticality of design with regard to flow assurance was paramount to the overall project. The engineering and design of the process and separation capacity in the header systems also had to consider elevated future flow rates and pigging operations. The engineering design was subsequently verified through the use of Computational Fluid Dynamics (CFD).
Several other issues were targeted for special design consideration including large thermal expansions, earthquake loading, slug forces, modularization, construction tolerances and flow assurance.
This paper describes the special circumstances leading to various design decisions which were employed to successfully complete the project.
