As vibratory driving is increasingly used to install offshore foundations, especially for offshore windfarms, an accurate soil model is key to reliable pile driving predictions and thus to improved project planning and execution. During the pile driving, the soil properties, and thus the soil parameters, do not remain constant. For vibratory driving, this effect is generally simplified by the application of the Beta Method, which was first proposed by Jonker in 1987[1]. While this simplified approach could be justified based on the computing capabilities at that time, a more sophisticated soil model should be used as part of pile driving simulations to ensure that the results are more accurate. In this paper, the state-of-the-art soil model for vibratory driving will be described and then illustrated through two brief case studies where the pile driving simulation results are compared with the actual installation records. The paper will show that while the predictions are not perfect, they forecast the pile driving process realistically. To further enhance prediction capabilities, the paper will suggest that pile driving with vibratory hammers is monitored and that the recorded data be used for post-processing to gain a more in-depth understanding of the soil behavior.
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.
The vibratory hammer is one of the tools for the extraction of offshore foundation piles as well as monopiles for the decommissioning of offshore structures. In addition to the standard application, where a pile is driven downward to be installed, a vibratory hammer can also be applied to extract piles. For an efficient and commercially attractive application of vibratory hammers for this purpose, the extraction process needs to be modeled during the planning phase to ensure that the appropriate equipment is used. This paper describes how pile driving simulation software can be used to model the extraction process. This is further illustrated through a case study covering the extraction phase of the 1st (onshore) and 2nd (offshore) part of the Delft Offshore Turbine Project. A monopile with a diameter of 4.0 m was extracted approximate 6 months after installation onshore and then extracted several times offshore shortly after installation in the 2nd phase. The paper will not only present the actual extraction predictions, but also the monitoring data obtained during extraction and the results of the post-analysis.
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