No abstract

Upon recently, most offshore foundations were based on a number of driven piles that were installed with an impact hammer. With the increase in offshore windfarms two major developments have occurred: the shift from jackets to monopiles, leading to increase of the diameter of the foundation piles, and a shift from impact hammers to vibro hammers. As the use of vibratory hammers is becoming more and more common practice, the need for accurate vibro-driving simulation software has increased, which requires that the soil modelling is enhanced to address soil fatigue during pile driving and to predict reliably the soil behavior and resistance during pile driving. In addition pile driving monitoring, which is routine for piles driven with an impact hammer, needs to become common practice. This paper addresses advances in soil modelling that allows more accurate pile driving simulation as well as the application of Vibro Driving Analysis (VDA) or Monitoring (VDM) to validate the simulation results. This is illustrated by a case study of the test pile for the Delft Offshore Turbine project, a 28 m long monopile with a diameter of 4 m that was driven 15 m into dense sand layers using a vibro hammer. After some 6 months the pile was extracted, and pile driving simulations and VDA were done both for the installation and extraction phase.

When assessing the driveability of a pile into the soil using a vibro hammer it is important that all factors that affect that process are recognized. For a particular project, the soil conditions are obviously fixed as they are determined by the project location, and the only true variables are the selection and operation of the hammer, as in many cases the pile is already designed without taking into account all aspects that are related to the driveability with a vibro hammer. The paper explores the soil related aspects (so that the hammer can be selected to generate the movement in the pile to enhance soil penetration), the pile related aspects (to illustrate how pile parameters can affect the penetration process), and the operational aspects of the pile driving. These aspects are then illustrated through simulation results and actual installation records of the monopile installation for the NAM L13-F1-1 platform on the Dutch sector of the North Sea.

No abstract

For an open pipe pile installed on the North Sea to serve as the foundation for an offshore gas production platform the capacity was determined in various ways. First the Cone Penetration Test data obtained at the location were used to simulate the vibro driving process. Then the monitoring data obtained during vibro driving were used adjust the soil model used for the initial simulation and to derive the pile capacity. Next the monitoring data for the Beginning of Restrike and the End of Impact Driving were used to assess the capacity yet again. The results as well as the soil models used for the simulations and the capacity estimates are presented to demonstrate that there is no need to drive a pile to its final penetration with an impact hammer to assess its bearing capacity, which when adopted in the industry will greatly reduce pile driving duration and thus cost.