A probabilistic approach for estimating the theoretical net drilling rate for offshore bored piles

Driven piles, installed by using hammers, are the most common offshore foundation type. However, in certain cases driven piles can incur in a premature refusal (due to plug formations, hard layers, presences of rock and boulders or soil set-up). Drilled piles are therefore used as a back-up solution. However, the technology is not always preferred as time is a key factor for offshore operations and a pile drillability analysis is generally more complex and less reliable than a drivability assessment. This paper extends the results of previous researches, which considered a theoretical drilling rate model based on the estimation of the specific excavation energy, applying a probabilistic approach. To consider the uncertainty of the geotechnical parameters of soil/rock used in drilling pile projects, a statistical method – that is, the Monte-Carlo method, has been applied to two cases of weak rock/cemented soil. It was possible to obtain the theoretical drilling rate values as a probabilistic distribution in order to assess the reliability of the estimation. It is important to understand that the drilling rates are based on a theoretical approach and they have to be considered as such.