The Hybrid Monopile: Design of a novel foundation structure for large offshore wind turbines in intermediate water depths

In the 26 years since the first offshore wind farm was installed in Vindeby, Denmark, the offshore wind industry has undergone remarkable growth. Recently, this growth has accelerated, advancing farms rapidly into deeper water and utilizing larger turbines. As offshore wind farm developers continue to look towards heavier, higher capacity turbines, and harsher sites, improving cost-effectiveness will rely increasingly on support structure optimization. Monopile foundations remain the preferred option, but the “XL” monopiles required in deeper water are sensitive to wave-induced fatigue loads and exceed the weight capacity of many installation vessels. The goal of the Hybrid Monopile support structure, is to provide a cost-effective alternative to the monopile in water depths greater than 30 meters. By providing an open structure in the key wave loading area of the water column, wave-induced fatigue damage can be reduced, allowing for a thinner monopile shell and a substantially lighter support structure. Design of the Hybrid Monopile began with the optimization of the brace member length and diameter for a specific location and turbine in the North Sea. Computations. were carried out using a Matlab-based model in which the structure is represented as a discretized Euler-Bernoulli beam. Extreme load cases were applied in a static analysis to ensure that selected dimensions are able to resist buckling of either the monopile shell, or the brace members. An accompanying finite-element model of the structure was used to verify that the selected brace dimensions were sufficient under extreme loads. To estimate the fatigue life of the monopile shell, and the brace members, wave loads were handled in the frequency domain and wind loads in the time domain. Damage equivalent loads (DELs) for each were then combined using quadratic superposition. Wind and wave roses for the selected site were used in the fatigue analysis to capture the effects of misaligned wind-wave cases on the calculated fatigue life. In this design phase, the Hybrid Monopile was shown to experience sixty percent less wave-induced fatigue damage than the traditional monopile. This baseline version of the design was then modified for 256 different combinations of water depth, turbine size, wave climate, and monopile diameter, to demonstrate the versatility of the concept and identify the design drivers. During the design process, the Hybrid Monopile was updated by altering the monopile shell thickness, brace diameter, and brace length. In each case, a traditional monopile was also designed in parallel, to serve as a basis for comparison. Optimization of each configuration was based on an iterative series of design checks during which the variable design dimensions were increased or decreased based on the structure natural frequency, response to an extreme load, or expected fatigue life at one of the critical locations; the mudline and the individual brace cross sections. Results of the iteration showed that in more than 3/4 of the test cases, which covered water depths from 30-60 meters, and turbine sizes of 8-20 MW, a Hybrid Monopile could be established that offered lower structure weight and improved fatigue performance at an equal or reduced cost compared to a traditional monopile. Viable configurations showed average weight and cost reductions of 27% and 17% respectively. These results were used to establish a limited number of standardized Hybrid Monopile classes, which could be mass produced and assigned to a wind farm, based on only the water depth and turbine size. Looking forward, further work is required to determine the optimum connection detail at the brace-monopile interface, and to assess the fatigue risk at that junction.