The Influence of Foundation Modeling Assumptions on Long-term Load Prediction for Offshore Wind Turbines

In evaluating ultimate limit states for design, time-domain aeroelastic response simulations are typically carried out to establish extreme loads on offshore wind turbines. Accurate load prediction depends on proper modeling of the wind turbulence and the wave stochastic processes as well as of the turbine, the support structure, and the foundation. One method for modeling the support structure is to rigidly connect it to the seabed; such a foundation model is appropriate only when the sea floor is firm (as is the case for rock). To obtain realistic turbine response dynamics for softer soils, it is important that a flexible foundation is modeled. While a single discrete spring for coupled lateral/rotational motion or several distributed springs along the length of the monopile may be employed, a tractable alternative is to employ a fictitious fixed-based pile modeled as an “equivalent” cantilever beam, where the length of this fictitious pile is determined using conventional pile lateral load analysis in combination with knowledge of the soil profile. The objective of this study is to investigate the influence of modeling flexible pile foundations on offshore wind turbine loads such as the fore-aft tower bending moment at the mudline. We employ a utility-scale 5MW offshore wind turbine model with a 90-meter hub height in simulations; the turbine is assumed to be sited in 20 meters of water. For a critical wind-wave combination known to control long-term design loads, we study time histories, power spectra, response statistics, and probability distributions of extreme loads for fixed-base and flexible foundation models with the intention of assessing the importance of foundation model selection. Load distributions are found to be sensitive to foundation modeling assumptions. Extrapolation to rare return periods may be expected to lead to differences in derived nominal loads needed in ultimate limit state design; this justifies the use of flexible foundation models in simulation studies. INTRODUCTION Nominal loads for the design of wind turbines in ultimate limit states are generally established from time-domain aeroelastic response simulations. The accuracy of these derived loads depends on the number of simulations and on how realistically the models used to represent the turbine, support structure, and foundation describe the true structural response. One potential shortcoming in modeling foundations relates to their flexibility. A single pile (often referred to as a monopile) is the most common type of foundation used today for offshore wind turbines; the support structure connects to such a pile foundation that extends some depth below the mudline. One way a monopile foundation could be modeled is by means of a rigid connection at the mudline. This model ignores the soil profile and the associated soil-pile stiffness and, as such, would not account for the pile’s expected lateral/rocking movement. Such simplifying assumptions could only adequately simulate the behavior of a monopile founded in rock. Many offshore wind turbines, however, are founded on softer soils where the monopile experiences at least some movement at and below the mudline. It is therefore worth assessing the accuracy of the use of a fixed-base model versus a flexible foundation model. In the present study, we carry out fixed-based model simulations and study turbine loads (specifically, the fore-aft tower bending moment). These are compared with loads derived using a flexible foundation model. This latter model utilizes stiffness properties derived from the soil profile at the location of the turbine by means of a conventional pile foundation analysis and appropriate p-y lateral load-deflection relationships. The flexible foundation model involves derivation of an “apparent fixity length” representing a distance below the mudline where an equivalent cantilever yields the same lateral movement and rotation as the monopile experiences in the pile analysis with the true soil properties. The mass per unit length of the equivalent cantilever is adjusted to match the sub-soil mass of