The Rosetta channel is a submerged canyon located approximately 50 km north of the Egyptian coast in the West Nile Delta. It was required to cross the channel with a 16” rigid pipeline as part of the Giza, Fayoum and Raven gas development. The channel was approximately 3 km wide at the crossing point with large embayments 0.5 km to 0.7 km wide, gradients of up to 40° and the seabed was comprised of normally and over-consolidated clay. The pipeline design required large seabed interventions to reduce spans and mitigate against pipeline fatigue. After a detailed assessment it was decided that pre-lay excavations would provide the optimum solution. An approximate total of 40,000 m3 of seabed sediment was excavated across five locations, in over 600 m of water. It was considered that subsea excavation on this scale had not been undertaken before in such an environment. The full excavation campaign was successfully completed prior to arrival of the pipeline installation vessel without impacting the critical path. This paper describes the geotechnical design undertaken and the challenges faced during the planning and offshore execution of the excavation campaign.
David Kay, David Rushton, Frans Van Herpen, Aad Van Es, Sanjay Shinde, Martyn Chapman and Stelios Panayides
Keywords: geohazard, slope stability, excavation, free-spans, very soft clay
Publisher: DFI