The Oseberg Transportation System

ABSTRACT The Oseberg Transportation System includes a crude oil pipeline placed in water depths up to 357m, connecting the Oseberg Field in the North Sea with a storage terminal in Norway. In the eastern slope of the Norwegian Trench, gravel sleepers were placed and a shore approach tunnel was used through the surf zone in order to reach the shore. A direct subsea pull-in method was developed for the shore approach tunnel. As contingency stand-by equipment, a development of deep water welding technology was undertaken. This is. a remotely controlled mechanized TIG welding process, operated through surface control and without the use of qualified welder/divers. Included were development and verification of compression and decompression profiles for operation to 360m water depth. THE OSEBERG TRANSPORTATION SYSTEM. Introduction The Oseberg Transportation System includes the crude oil pipeline from the Oseberg Field Center, across the Norwegian Trench, traversing the steep and uneven Eastern Slope of the Trench, making a landfall at a group of islands appr 30 miles north of the city of Bergen, and continuing across the islands to an underground oil storage terminal. The pipeline system is a 28inch diameter line, designed for a max. capacity of 600,000bbl/d at design pressure of 154barg. The approximate length offshore is 109km, and onshore incl. the shore approach 6km. It is designed for sour service. Additional pertinent pipeline statistics are shown in Table 1. The Offshore Route The offshore pipeline route can be divided into four sections, see Fig .1. The first being the section immediately to the east of the field, the North Sea Plateau. This area is characterized by a fairly even but gently sloping sea floor at appr. 110msw (meter sea water). The seabed sediment consists of primarily dense sands. The statpipe gas pipeline is crossed here. The second section is the western Slope 1n 110 - 300msw where the seabed slope averages 1%. The sediment consists of medium dense sands changing to silt and soft clay within a short distance from the Plateau. In the Western Slope, extensive pockmarks and local depressions occur. The third section is the Norwegian Trench, a depression between 300 and 310msw, except for the eastern part where the depth increases to 360m. The seabed consists of very soft clay and contains many circular to elongated depressions, and with pockmark occurrence increasing from east to west. The fourth section is the Eastern Slope which is the steepest part of the route. The seafloor ascends from its deepest part of the Trench with an average slope of 5%. In the Iowest part of this slope, sedimentary rock is found covered with soft, silty clay. Towards shore, crystalline rock outcrops form a rough topography, with exposed bedrock arid depressions which are mostly covered with soft sediments. In water depths shallower than 100m, shellsand and gravel are found in the depressions. Clay or other fine grained materials are not present in these areas which indicates a strong exposure to wave action and currents.