The Enicom North fibre optic cable was installed m 2001 to provide broadband communication facilities to platforms at the Troll, Veslefrikk, Oseberg, Huldra, Kvitbjern, Gullfaks and Snorre Fields in the Norwegian Sector of the North Sea. There had previously been many investigations for subsea developments in this region including for the platforms themselves and a number of export pipelines. A detailed desk study of the proposed fibre optic cable route was undertaken using the available geodata from these previous investigations. This indicated that there might be some local hardground but it was unlikely to present a hazard on a significant scale; the phenomenon had been identified only at a few locations on the Norwegian Continental Shelf. A route-specific geophysical survey was also undertaken but acoustic sub-bottom profiling was not expected to be capable of detecting very shallow hardground. A limited amount of geotechnical testing was carried out.
During the installation works the trenching tool experienced widespread resistance to excavation in areas of otherwise relatively soft silts and sands. This had a significant impact on the speed of burial operations and the depth of burial achieved. Following a detailed review of the installation records, in particular ROV video tapes of the trench, extensive cemented sedimentshardgrounds were identified This paper discusses the nature of the hardgrounds and their impact on the installation works. Suggestions are made for avoiding similar problems on future cable projects.
Gas-cemented hardground (GCH) has previously been described from the Norwegian Continental Shelf by a number of authors (e.g. Hovland et al. 1985, Hovland et al. 1987, Lien 1988) but it was thought to be of only local lateral extent. The phenomenon is encountered at very shallow depths below seabed and is hence a potential hazard to seabed engineering work.
During the installation of the Enicom North fibre optic cable m the Norwegian Sector of the North Sea in 2001, GCH was extensively encountered Considerable problems were experienced during the trenching works with the hardground having significant impact on both the speed of burial operations and the depth of burial achieved.
Post-lay ROV video records of many kilometres of trench were examined. The generally excellent quality images provide a comprehensive and detailed visual database that reveals a variety of hardground development stages and forms the basis for this paper. The presence and degree of GCH can be correlated with the trenching performance and an assessment made of its impact on the installation works.
The Enicom North cable was installed to provide fibre optic communications between a number of oil and gas developments in the Norwegian Sector of the North Sea The cable route runs E-W from Troll A m the middle of the Norwegian Channel to the Veslefrikk platform on the upper Western Slope, with a spur S to Oseberg, which lies on the Central Plateau.