Dynamic Analysis of the Installation of Monopiles for Offshore Wind Turbines
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- Institutt for marin teknikk 
The installation of offshore wind farms presents great challenges as the industry move farther offshore and into deeper waters, and the turbines and foundations are getting larger and heavier. Current installation methods are all sensitive to weather conditions: lifting the foundations using floating crane vessels, deploying and retrieving jack-ups' legs, and lifting turbine nacelles and rotors at a large lift height. Careful numerical studies of these critical installation scenarios in the planning phase are therefore important to ensure safe executions. Monopiles (MP) are the most commonly used foundations for offshore wind turbines (OWT), but there is little work focusing on their installation phase. In order to predict the responses of the installation system, accurate numerical models and methods are needed. This thesis addresses the modelling and dynamic analysis of two installation phases for monopiles: the lowering into the sea and the initial hammering phases. Due to nonstationarity, current numerical methods used for steady-state conditions are not applicable for simulating the lowering phase. In this thesis, new numerical methods were developed to account for the shielding effects from the floating installation vessel and the radiation damping of the monopile for analysing the nonstationary process. The shielding effects from the vessel are considerable especially in short waves, and the inclusion of the radiation damping of the MP may reduce the predicted responses. These methods also provide more accurate results than the commonly used simplified conservative methods, and they can be extended to apply for other structures. For the initial hammering process, the coupled vessel-monopile installation system with soil interaction was modelled. Dynamic analyses for various monopile penetration depths and soil conditions showed that the responses were sensitive to those factors. These numerical models, methods and dynamic analysis form the basis for assessing the operational limits for different installation activities. The operational limits are essential during the planning phase of the operation, i.e. to size equipment, select installation vessel and optimize the installation method. The allowable sea states together with weather forecasts provide the basis for the decision making during the execution of the operation. Although many studies have focused on obtaining operational limits of specific installation activities, little work has been published on providing a general methodology to establish these limits. Therefore, the thesis also addresses the development of a systematic methodology to assess the operational limits based on the installation procedure, numerical models and safety criteria. To demonstrate the methodology, a detailed procedure for establishing the operational limits is presented for the monopile initial hammering process. First, the critical events and corresponding responses for this operation were identified. The allowable sea states were then obtained by comparing the characteristic responses with their allowable limits. For the monopile lowering process, the allowable sea states were also established using this methodology. An operability analysis at a selected offshore site using different numerical methods was carried out. It was showed that among different modelling parameters, the shielding effect is the most critical factor, followed by the nonstationary analysis approach, wave spreading and the radiation damping from the monopile. The methodology to assess the operational limits is general and can be extended to other marine operations. The original contributions of this work include the development of new methods for simulating the nonstationary lowering operation, and development of a systematic methodology for assessment of the operational limits. These methods provide a basis for further studies on modelling and analysis of other marine operations.