With the increased focus in the US on green technology, there are several offshore wind farms being planned in the US waters to meet the goal of utilizing renewable energy sources for consumers.

The American Wind Energy Association (AWEA), as a standards development organization, sponsored the development of recommended practices (RP) for offshore wind turbine facilities and published its RP report in 2012. The challenges faced by the emerging offshore wind energy industry parallel many of the challenges that were faced by the offshore oil and gas industry in its infancy. These challenges are documented and addressed in the development of the AWEA RP and are described in this paper. The paper discusses how the more mature American Petroleum Institute oil and gas standards were interfaced with International Electrotechnical Commission and other wind turbine and offshore industry standards to provide guidance for reliable engineering design practices of these mechanically dynamic, fatigue-driven offshore wind energy systems. Uncertainties requiring further validation and analysis are also described.

With the increased focus in the US on green technology, there are several offshore wind farms being planned in the US waters to meet the goal of utilizing renewable energy sources for consumers.

As the United States moves closer to constructing its first offshore wind farms, data-based decisions about how to select the most viable project site are crucial. This in-depth study examined both the revenue and costs of three potential wind farms off the East Coast. Multi-year energy time history analyses were performed to determine the energy generation and revenue potential of each site based on local winds and energy pricing. The costs were determined for each site based on characteristics including water depth, distance to shore, and distance from a port. These characteristics formed the basis to estimate the foundation costs, cabling costs, equipment mobilization fees, and other expenses associated with the project. Overall, this study shows that seemingly similar sites can have a very different levelized cost of energy (LCOE) estimates when an in depth analysis is performed.

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.