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PLASTICITY DETECTION AND QUANTIFICATION IN MONOPILE SUPPORT STRUCTURES DUE TO AXIAL IMPACT LOADING
Last modified: 2017-12-12
Abstract
In recent years, the number of offshore wind farms has increased to meet the demand for energy from renewable resources. The most commonly used foundation concept in offshore wind is a steel monopile. Steel monopiles are thin-walled cylindrical structures that are normally driven into the seabed with a hydraulic hammer. Each hammer blow induces stress waves in the pile which help the latter to gradually progress into the soil.
As soon as the pile reaches the desired penetration depth, the wind turbine is installed on top of the support structure. Until recently grouted connections were used, however, bolted connections have become more popular in recent years. Compared to a grouted connection, a bolted connection is more sensitive to material damage inflicted by the pile driving process since the wind turbine is directly connected to the pile head. In addition, material damage in the pile reduces the service life of the support structure. A method to detect and quantify the damage caused by the pile driving process is therefore essential in order to assess the structural health of a monopile.
The detection method for plastic deformation developed in this work is based on the change of the stress wave profile as the stress levels reach the yield stress of the material. With the use of a one-dimensional wave propagation model, which incorporates a non-linear constitutive material law, the influence of the amount of inflicted plasticity on the stress wave profile at a certain distance from the pile head is investigated. In the model the effect of geometrical dispersion is considered as this is can be of significance for large-diameter monopiles. The change in the axial stress wave profile due to plastic deformation is analysed by means of a time-frequency analysis at different locations along the pile length. With the help of the developed theoretical framework, a measurement procedure is presented by which the plasticity inflicted by an axial impact can be detected and quantified. The method is then validated through experimental work.
As soon as the pile reaches the desired penetration depth, the wind turbine is installed on top of the support structure. Until recently grouted connections were used, however, bolted connections have become more popular in recent years. Compared to a grouted connection, a bolted connection is more sensitive to material damage inflicted by the pile driving process since the wind turbine is directly connected to the pile head. In addition, material damage in the pile reduces the service life of the support structure. A method to detect and quantify the damage caused by the pile driving process is therefore essential in order to assess the structural health of a monopile.
The detection method for plastic deformation developed in this work is based on the change of the stress wave profile as the stress levels reach the yield stress of the material. With the use of a one-dimensional wave propagation model, which incorporates a non-linear constitutive material law, the influence of the amount of inflicted plasticity on the stress wave profile at a certain distance from the pile head is investigated. In the model the effect of geometrical dispersion is considered as this is can be of significance for large-diameter monopiles. The change in the axial stress wave profile due to plastic deformation is analysed by means of a time-frequency analysis at different locations along the pile length. With the help of the developed theoretical framework, a measurement procedure is presented by which the plasticity inflicted by an axial impact can be detected and quantified. The method is then validated through experimental work.