This paper considers the characterization of clay crusts in deep sea locations off the West African coast. The presence of high strength material at shallow penetration depths is confirmed through laboratory mini-ball penetration and Cam-shear testing. Visual logging of splitcores indicates invertebrate burrows are present within the crust. Fecal pellets, presumably from burrowing invertebrates, are also abundant within the cores. Results from initial Cam-shear tests comparing reconstituted sterile and bacterially-inoculated samples are presented. These tests indicate an inherent structure of natural sediments that is destroyed during reconstitution and sterilization; but no proof that bacteria are the cause of the crust has been observed.
The mechanical properties of deep sea sediment in water depths of between 500m and greater than 2500m off the coast of West Africa (WA) are of interest to oil pipeline designers. Pipelines installed into these sediments undergo several hundreds of thermal cycles during operation and therefore an understanding of the soil-pipe coefficient of friction (m) and the undrained shear strength (su) is required for design. A better understanding of the short and medium-term behavior of these sediments under hot-pipe loading is also required. Penetrometer tests undertaken in offshore locations such as those shown in Figure 1 confirm the presence of ‘crusts’, below which su drops back onto the normally consolidated line following Skempton (1954) and shown modified in Equation 1. (Refer to the full paper) The origin of these crusts, and why they only occur within the top few meters of sediment is currently unknown. Furthermore, this phenomenon has been observed in several offshore oil and gas locations world-wide. The origin and mechanical behavior of deep ocean clay crust, allowing for any changes that must be anticipated during a pipeline's working lifetime, is therefore a significant issue for pipeline designers
