Scour Sensitivity Study for Large-Diameter Monopiles

Abstract

MMI Engineering performed sensitivity studies for a wind farm operator to evaluate the consequences of scour around offshore wind turbine (OWT) monopile foundations. The depth of scour raises two issues; firstly a potential reduction in the overall stiffness of the foundations, which could in turn lead to problems due to resonance of the turbine and structure; and secondly their overall stability may be compromised. MMI gathered data and performed analyses to demonstrate the design margins relating to resonance and stability. The collected data included; the original design data, accelerometer outputs from the turbine's condition monitoring systems and bathymetry survey data. MMI modeled the structures using fully-coupled, nonlinear finite element software and then calibrated the analysis results using observed data to give increased confidence in model accuracy. The results from the resonance analysis were used to define scour limits such that the vibration of the OWT remained within allowable limits. The strength analysis defined a further set of scour limits such that the foundation retained the required capacity. Introduction MMI Engineering has completed a series of studies to address the impact of scour on the structural performance of monopile foundations for two different wind farms. The periodic observations of the ground elevation indicate high levels of scour and sand bank movement around the monopile foundations at both sites. Excessive scour that occurs immediately adjacent to the monopiles increases the unsupported length from the foundation to the nacelle and also reduces the effective fixity due to lost lateral bearing contact with the supporting soils. These conditions can increase the likelihood of lost operation and structural failure. During the studies, the operators deployed an instrumentation program, recording motions on multiple turbine support structures, to provide data for use in the scour sensitivity analysis. The records were provided to MMI for detailed processing to support the assessment of scour impact. This assessment also included a rigorous analysis of the strength and stability of the monopile to address the performance of the monopile foundations for conditions that exceed the limits used during the original design. The dynamic properties of the turbine support structures were estimated using structural analysis models that are based on collected site data (e.g., water depth, soil properties) and the known physical properties of the structure (e.g., weights, member diameters and thicknesses, pile penetration depth). The accuracy of these models is affected by bias and errors in both the data and in the modeling procedures (e.g., soil-pile interaction algorithms) that are used. A key objective in this series of studies was to utilize in-situ recordings of the gross dynamic response of some of the offshore turbine systems to address the aforementioned errors and bias in order to calibrate the mathematical models and therefore more closely represent the actual dynamic properties of each of the turbine systems at the sites.