Casey M. Fontana

Sensitivity of the Dynamic Response of Monopile-Supported Offshore Wind Turbines to Structural and Foundation Damping

The prediction of ultimate and fatigue demands for the design of offshore wind turbines (OWTs) requires accurate simulation of the dynamic response of OWTs subject to time-varying wind and wave loads. The magnitude of damping in an OWT system significantly influences the dynamic response, however, some sources of damping, such as foundation damping, are not explicitly considered in design guidelines and may increase damping significantly compared to commonly assumed values in design. Experimental and analytical studies have estimated the magnitude of foundation damping to be between 0.17% and 1.5% of critical, and this paper investigates how increased damping within this range affects load maxima and fatigue damage for a hypothetical 5MW OWT subjected to a variety of wind, wave, and operational conditions. The paper shows that increased damping effects the greatest percentage reduction of ultimate moment demands and fatigue damage when the OWT rotor is parked and feathered. In such cases, the aerodynamic damping is relatively low, allowing for additional damping from the foundation to account for a relatively larger proportion of the total system damping. Incorporating foundation damping in design guidelines may lead to more efficient structures, which is a crucial factor in overcoming the high cost barrier associated with offshore wind development.

Multiline anchors for floating offshore wind towers

Roughly 60% of potentially exploitable offshore wind power is located beyond the range of water depths suitable for fixed foundations, where floating offshore wind towers (FOWTs) moored to the seabed are required. Anchors comprise a critical component of the mooring system. A variety of anchor types are potentially suitable for this purpose, but all have limitations in regards to the types of seabed soils in which they may be deployed and the type of mooring systems (catenary, taut) for which they are suitable. Additionally, foundations for offshore wind make up a large portion of project cost; therefore, minimizing the costs of fabrication, transport and installation of anchors is a key aspect of overall project feasibility. In contrast to offshore oil-gas installations, offshore wind towers are deployed in arrays, which offer the possibility of reducing project costs by attaching more than one mooring line to a single anchor. In addition to direct cost savings, the multiline anchor concept permits a reduced scale of costly offshore geotechnical site investigations. This paper first examines different anchor types that are potentially suitable as anchors for FOWTs, largely within the context of their traditional usage in securing a single mooring line to the seabed. Then, the potential for adapting these anchors to multiline systems is assessed. Anchor types examined include: driven piles, dynamic piles, suction caissons, drag embedded anchors, vertically loaded anchors, pile driven plate anchors (PDPA), dynamically embedded plate anchors (DEPLA), and suction embedded plate anchors (SEPLA). Performance considerations for each anchor include: Soil profile constraints, vertical load capacity, horizontal load capacity, precision of positioning, installation cost, efficiency, performance under sustained loading, potential loss of embedment, as well as other anchor specific considerations.

Efficient Multiline Anchor Systems for Floating Offshore Wind Turbines

A mooring and anchoring concept for floating offshore wind turbines is introduced in which each anchor moors multiple floating platforms. Several possible geometries are identified and it is shown that the number of anchors for a wind farm can be reduced by factors of at least 3. Dynamic simulation of turbine dynamics for one of the candidate geometries and for two directions of wind and wave loading allows estimation of multiline anchor forces the preview the types of loads that a multiline anchor will need to resist. Preliminary findings indicate that the peak demand on the anchor may be reduced by as much as 30% but that anchors used in such a system will need to be able to resist multi-directional loading.Copyright

Comparison of Cyclic P-Y Methods for Offshore Wind Turbine Monopiles Subjected to Extreme Storm Loading

Approximately 75% of installed offshore wind turbines (OWTs) are supported by monopiles, a foundation whose design is dominated by lateral loading. Monopiles are typically designed using the p-y method which models soil-pile resistance using decoupled, nonlinear elastic Winkler springs. Because cyclic soil behavior is difficult to predict, the cyclic p-y method accounts for cyclic soil-pile interaction using a quasistatic analysis with cyclic p-y curves representing lower-bound soil resistance. This paper compares the Matlock (1970) and Dunnavant & O’Neill (1989) p-y curve methods, and the p-y degradation models from Rajashree & Sundaravadivelu (1996) and Dunnavant & O’Neill (1989) for a 6 m diameter monopile in stiff clay subjected to storm loading. Because the Matlock (1970) cyclic p-y curves are independent of the number of load cycles, the static p-y curves were used in conjunction with the Rajashree & Sundaravadivelu (1996) p-y degradation method in order to take number of cycles into account. All of the p-y methods were developed for small diameter piles, therefore it should be noted that the extrapolation of these methods for large diameter OWT monopiles may not be physically accurate; however, the Matlock (1970) curves are still the curves predominantly recommended in OWT design guidelines. The National Renewable Energy Laboratory wind turbine analysis program FAST was used to produce mudline design loads representative of extreme storm loading. These design loads were used as the load input to cyclic p-y analysis. Deformed pile shapes as a result of the design load are compared for each of the cyclic p-y methods as well as pile head displacement and rotation and degradation of soil-pile resistance with increasing number of cycles.Copyright