Geotechnical and machinery properties influencing the offshore pile drillability

Foundations of offshore oil and gas production platforms or wind parks are in the majority of cases piled-based. Piles are mainly driven to a certain design depth using hammers. However, there are many situations were driven piles cannot be properly installed. Plug formation or presence of rock layers and/or boulders preclude the correct installation. Since time is a key factor in offshore operations, offshore drilled piles are not very common. Their high level of uncertainty is mainly due to the complexity to assess a pile drillability analysis. For these reasons, assessing the drilling time needed to overcome the problems is very important. The following research compared first of all several well-known equations used for estimating the tunnel boring machine (TBM) excavation rate. Second, a novel approach has been used, which is based on the estimation of the specific energy. After having performed continuous automatic diagraphy tests in Turin subsoils (Italy), values of friction angles and compressive strength were obtained. Specific energy was calculated at the scale of the continuous automatic diagraphy test as a function of ξ, which is a coefficient depending on the geotechnical parameters assumed by Nishimatsu. It is therefore possible to estimate the drilling rate for different values of friction angle and cohesion.