On the Design and Construction of Statpipe Pipeline System

ABSTRACT During the past two years, the laying and testing of approximately 842 kms of submarine pipelines in the North Sea was successfully completed which involved twice crossing of the 300m deep Norwegian Trench for the first time in its history. The longest and deepest project et undertaken in the hostile waters of the North Sea was expectedly highly demanding with regard to engineering and construction planning preparations. All project specifications, contractors' operation procedures and manuals were diligently prepared, reviewed and implemented in a consolidated team approach. Extensive studies and field/laboratory tests were carried out on the performance of numerous materials considered for use in the project, particularly that of field joint materials and coating systems to verify that the stringent project requirements were adequately met. INTRODUCTION The Statpipe Gas Transportation system involves 882 kms of pipeline, of which 842 kms are offshore as illustrated in Figure 1. The rich gas from Stratford will be transported to Karsto through the 289 km long 30 inch pipeline in dense phase. The pressure in this line must be kept above 105 barg at all times to maintain the required flow characteristics. Two subsea branch connections are made to this pipeline. One subsea tee is installed in block 30/6 for future connection to gas fields in this area, whereas the other connection is downstream of the Statfjord B platform for tie-in to the Statfjord A and C and the Gullfaks field via Statfjord C. After processing into dry gas and gas liquids at Karsto plant the dry gas is trans- ported offshore to riser platform 16/11–8 in block 16/11 via the 207 km long 28-inch submarine pipeline. Here the stream will be combined with the dry gas coming in the 36-inch line from the Heimdal field and transferred into the second leg of the 36-inch pipeline tretching to riser platform 2/4-8 connected by a bridge to Ekofisk production complex onward to Emden terminal in West Germany. The total length of the 36-inch pipeline is 346 kms. HISTORICAL BACKGROUND TO STATPIPE SYSTEM The technical feasibility of crossing the Norwegian Trench by submarine pipelines has been studied for more than a decade (Lund, 1983). In 1972, the Norwegian Government waivered the requirement of landing petroleum in Norway by approving piping Ekofisk crude oil to UK due to lack of technology to cross the Norwegian Trench. In the following years governmental committees were formed with the primary objective of establishing the technical requirements of deepwater pipe laying. In 1974, it was concluded that laying of small diameter pipes in the trench would be feasible. Although transporting some volume of the Frigg gas to Norway by pipeline was considered at the time, the idea was abandoned on economical grounds. Following the discovery of rich Stratford field in the same year, the investigations into the trench crossing and deep water pipe repair methods intensified.