Case Study: Rosetta Channel Pipeline Crossing-Paper

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The Rosetta Channel, a steep submarine canyon offshore Egypt, posed significant geohazards for a 16-inch pipeline crossing. Rough seabed, steep slopes and potential slope instability threatened pipeline integrity, while unmitigated spans were predicted to cause operational overstress and excessive fatigue. Following comparison of routing and mitigation options, pre-lay seabed excavation was selected.<br /><br />Geotechnical design used nearby site-investigation data and sub-bottom profiling to develop soil-strength models for normally consolidated and over-consolidated clays. Stability analyses informed excavation slopes and limits for placing excavated material, helping reduce the risk of local or regional slope failure. The design required excavation at five locations, with tight vertical tolerances and the surrounding seabed left undisturbed.<br /><br />Van Oord adapted a flexible fall pipe vessel and its remotely operated vehicle to deploy clamshell grabs at depths exceeding 600 m. A planned grid-and-layer sequence, frequent multibeam and sonar surveys, and close coordination with pipeline designers enabled progress and as-built profiles to be checked during the campaign. Minor sediment slumping occurred along some steep excavation edges; procedures were adjusted, and the resulting deposits were monitored.<br /><br />Approximately 40,000 m³ of clay was excavated over 94 days, averaging about 425 m³ per day. The large grab achieved an acceptable seabed profile, so the smaller clean-up grab was unnecessary. The work was completed before pipeline installation without affecting the critical schedule, and the campaign recorded no injuries, accidents, environmental damage or asset damage. The case demonstrates that accurately controlled grab excavation can remove and place large volumes of soft seabed soil in challenging deep-water terrain.

Keywords

Rosetta Channel

submarine canyon

pipeline crossing

geohazards

seabed excavation

slope instability

deep-water clamshell grab

geotechnical design

fatigue mitigation

pipeline integrity