The requirements of the Danish authorities regarding the 36" Ekofisk-Emden gas pipeline created the need for the development of new engineered backfilling methods. The actual backfilling was to be performed in the open sea in the middle of the North Sea. Water depth was 40 to 70 meters ". The required minimum cover over the top of the pipe was 50 em. The total length of the pipeline to be covered was 36 km.
Due to the short time available it was decided to develop two independent systems to have a safeguard in case one of the systems did not perform properly.
One system was based on taking material from the seafloor by suction hopper dredgers and pumping it through a floating hose to a DI drilling vessel positioned above the pipeline. The material was transferred through a vertical fallpipe down to the gas pipeline.
The other system was based on a converted 20 000 tdw bulk-carrier equipped with a DP system. This vessel utilized material taken from shore and placed above the pipeline through a vertical fallpipe equipped with a guiding thruster at the lower end.
The total cost of this covering operation represented more than twice the initial cost of the same length of laid and trenched pipeline. In spite of this no significant improvement in safety is likely to have been obtained by the increased cover.
The 36" Ekofisk-Emden gas pipeline is going from the Ekofisk field in the Norwegian sector of the North Sea. As shown on Figure 1 the pipeline is crossing the Danish sector on its way to Emden in West Germany. The pipeline was in focus during the summer 1977 in relation to the start-up. The need for additional cover launched a huge sandbagging operation where 776,000 sandbags were used. Further new techniques were developed and altogether 36 km have been given added cover.
The laying of the pipeline was mainly performed in 1974 as shown in Figure 2. Trenching started the same summer with air still in the pipeline. Especially in the sandy areas towards the German coast, it proved necessary to increase the pipe weight by utilizing water-filling. Altogether the trenching was performed by more than 3 passes in average. Over some difficult areas nine passes were performed. The aim of all this trenching was to lower the pipeline according to the requirements of the different authorities as specified in Figure 3. The requirements spelled out in 1974 described a minimum distance between top of pipe and mean seabed level. Further, the pipe was required to be fully covered.
The survey in 1976 showed that parts of the pipe were exposed, especially in the Danish sector. Table 1 shows the coverage obtained as early as in 1976 with 9% of the total pipeline length exposed. Most of this exposed pipeline was situated, however, in a clay area covering the southern part of the Norwegian Sector and the northern part of the Danish Sector.
With the jet sled trenching method the bottom of the trench is not levelled at all places due to variations in soil condition, trenching speed, etc. This created a certain number of unsupported pipe conditions in the clay areas with limited natural infill of the trench. Figure 3 illustrates the locations of the clay areas.
