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Cyclic lateral design for offshore monopiles in weak rocks

Abstract : The European Union sets ambitious agreement of producing 20 % of its total energy needs from renewable sources by 2020. In response, the French government has launched tenders for offshore wind. The consortium that includes EDF Renouvelables won three of them for a total of 1.5 GW. Two of these projects concern wind turbines founded on monopiles installed in soft rock. One of the main objectives for EDF Renouvelables is to secure offshore wind turbines (OWT) design as well as the design of their foundation. Although monopiles represent 75 % of foundations installed offshore, their design can be optimised especially for the type of ground encountered in these particular projects. OWTs are subjected to specific design requirements such as tight tolerances concerning their natural frequencies and permanent rotation at the end of their lifetime. The monopile response plays an important role to assess these requirements. Therefore, in situ pile tests were carried out in a former quarry with similar soft rock properties as the ones encountered in the offshore projects.The pile dimensions are selected to reproduce the ratio between the pile embedded length and the pile diameter of typical monopiles. The methodology used to define the testing programme accounts for a good representativeness of the tests compared to offshore loads. The stiffness evolution in due course of the cyclic loading is analysed as it is a key factor for the natural frequency requirement. The accumulated rotations are looked at for the long-term rotation requirement. Since the piles are installed in soft rock, two phenomena are highlighted: the creation of a crushed zone around the pile due to the driving process and the onset and propagation of cracks in the surrounding rock mass.Based on these observations, a semi-analytical modelling is developed. The most commonly used procedure for prediction of the behaviour of laterally loaded piles is the P−y curves formulation which gives an efficient framework to predict the response of the pile. The semi-analytical modelling is based on this framework and is extended to take in account the particularities of both monopiles design requirements and the fact that monopiles are installed in soft rock. Emphasis is given to the modelling of the response at small lateral displacements. To account accurately for the initial response, the soft rock zone appears to play an important role. The classical P–y curves framework accounts neither for multi-directional loading nor for irreversible displacement and accumulated displacements due to cyclic loading. Unloading paths with or without gapping are introduced to account for irreversible displacements. To account for multi-directional loading, we propose to model several springs around the pile circumference. Analytical solutions are given in order to calculate the P−y curves for multi-directional loading from the various existing P–y curves for unidirectional loading. Similarly, to creep tests, cyclic loading exhibits three main types of response: stabilisation of accumulated displacements, ratcheting and unstable increase of accumulated displacements up to failure. Therefore, we use existing creep models for simulating the cyclic response. This procedure is validated by comparing the numerical results with data recorded in field pile tests performed in soft rock.Last of all, numerical finite element modelling is implemented using Code_Aster. The different phenomena are first analysed in a 2D configuration. This helps to understand and quantify the impacts of each phenomenon: the creation of the crushed zone, the gapping behind the pile and the onset and propagation of cracks. Then the same phenomena are analysed in a 3D configuration to understand the changes of size from the piles of field testing to the offshore monopiles
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Anaïs Lovera. Cyclic lateral design for offshore monopiles in weak rocks. Environmental Engineering. Université Paris-Est, 2019. English. ⟨NNT : 2019PESC1016⟩. ⟨tel-02527176⟩

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