New tools and directions in offshore site investigation

The offshore environment provides a number of challenges for geotechnical site investigation, among which are the high costs associated with vessel day-rates and an associated need for equipment reliability. New oil and gas developments are increasingly remote, in terms of distance from land and water depths, and seabed conditions often comprise extremely soft sediments within the depth range relevant for infrastructure such as pipelines, subsea foundations and anchoring systems. These factors have combined to provide a gradual shift away from ship-based drilling tools towards seabed-based robotic equipment for drilling, sampling and in situ penetrometer testing. In parallel, a variety of free-fall samplers and penetrometers have been developed, particularly for preliminary investigations to characterise the seabed sediments. An important aspect of the soil response is the extent to which it may be considered drained or undrained during particular operational events. For example, design calculations for the stability and operational movements of pipelines are relatively sensitive to the consolidation response of the soil. Even interpretation of penetrometer data requires an assumption with respect to the degree of consolidation occurring during penetration. The paper provides an overview of recent developments in offshore site investigation equipment, and then focuses on evaluation of consolidation properties.