Interface shear tests undertaken with a Cam-shear device have been conducted on very soft clay core samples from the Gulf of Guinea. These tests have been completed at normal effective stresses applicable to pipelines and suction caissons. Pipeline tests were conducted on rusted and coated steel, and on smooth and rough polypropylene interface material at speeds of 0.050mm/s and 0.001mm/s. Tests relevant to suction caisson installation and extraction were conducted on a rusted steel plate at shear rates of 1mm/s and 5mm/s. It is shown that the interface friction value is strongly influenced by both the shearing rate and the interface roughness, leading to the following conclusions. Firstly, rough and smooth interfaces produce comparable peak interface friction values. However, shearing on rough interfaces can produce lower residual friction values than on smooth interfaces. Secondly, slow shearing can produce residual interface friction values that are significantly higher than obtained during fast shearing. Thirdly, the apparent roughness of an interface is likely to be a function of the existing natural soil structure, which includes robust faecal pellets ranging in proportion from 20% to 50% by dry mass. The paper concludes by highlighting relevant implications for the design of pipelines and suction caissons located in similar sediments.

1. Introduction
1.1 Background

Very soft offshore sediments within the Gulf of Guinea present sizeable challenges for the design of pipelines and suction caissons that are to be installed into these sediments. Undrained shear strength profiles of these sediments typically exhibit high values at shallow depths, corresponding to what is commonly referred to as a ‘crust’. During operation, thermally induced cycling of the pipelines due to hot product flowing through initially cold pipelines causes pipe walking and ratcheting in the axial direction, and pipe buckling in the lateral direction.

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