Investigation of Seabed Geohazards and Their Engineering Implications for Deepwater Pipeline Routes Offshore Calabar, Cross River State

The deepwater offshore environment of South East Nigeria presents multiple seabed geohazard risks that must be systematically characterised before pipeline and umbilical route engineering can be finalised for deepwater development projects. This study investigates seabed geohazards and their engineering implications for proposed pipeline routes in deepwater waters offshore Calabar, Cross River State. A geotechnical and geophysical data compilation exercise assembled sub-bottom profiler records, multibeam bathymetry, sidescan sonar imagery, and sediment sample core descriptions from two ocean floor surveys conducted in the study area. Geohazards identified and characterised include submarine slides on the continental slope, active seafloor pockmarks associated with shallow gas venting, mass transport deposits from historic gravity-driven slope failures, and potentially liquefiable sediment facies at water depths exceeding 600 metres. Risk severity classifications are assigned to each geohazard type using the DNV GL RP-F105 free spanning pipeline standard framework. Pipeline route corridor analysis identifies three candidate route options and evaluates each against the identified geohazard constraints. A route avoidance and mitigation matrix is constructed, recommending that the central route corridor minimises combined geohazard exposure while remaining technically achievable at acceptable trenching and rock dumping costs. Recommendations are also made for additional geotechnical investigation priorities in the preferred route corridor. Keywords: seabed geohazards, deepwater pipeline, submarine slide, Calabar, Cross River State.