Numerical Modelling and Dynamic Analysis of Offshore Wind Turbine Blade Installation

Abstract

Installing offshore wind turbine blades is very challenging and risky due to large lifting height and high required installation precision. Offshore single blade installation is frequently adopted because of small deck space requirement. Current practice in the industry is to install offshore wind turbine blades by jack-up crane vessels in shallow waters. The typical operational environmental condition is mean wind speed less than 10m/s and significant wave height lower than 1.5_2m. Compared with jack-up crane vessels, floating ones are flexible with respect to operational water depth and efficiency in relocation. The latter might be an alternative for installing offshore wind turbine blades, especially in intermediate and deep waters. Numerical studies of the critical operational scenarios during the planning phase is important aid for planning and execution of a safe and efficient installation. During offshore single blade installation, the final blade mating operation is generally considered to be the critical phase. To assess the dynamic responses of offshore single blade installation systems during mating operations, coupled simulation methods that account for blade aerodynamics, vessel hydrodynamics and crane flexibilities are needed, but are currently limited. In this thesis, a fully coupled method, SIMO-RIFLEX-Aero, is established for numerical modeling and analysis of offshore single blade installation by jack-up or floating crane vessels. It can account for blade aerodynamics, vessel hydrodynamics, structural dynamics and wire coupling mechanics. The leg-soil interaction is also considered for jack-up vessels. The blade aerodynamic loads are calculated in the Aero code developed in this study based on the instantaneous blade displacement and velocity according to the cross-ow principle. The coupled method is used to study offshore single blade installation of the DTU 10 MW wind turbine blade by a typical jack-up crane vessel. The jack-up crane vessel is modeled in details with consideration of wave loads on the legs, wind loads on the hull, structural flexibility of legs and crane, as well as soil-leg interaction. The wave-induced vessel motion and crane flexibility are found to have significant influence on the blade motion. The influence is dependent on site-specific parameters such as soil properties. Detailed modeling of soil behavior using linear springs with dampers is recommended. Simple models using pinned or fixed foundations lead to large overestimation and underestimation of blade motion, respectively, which may affect estimation of operational safety and efficiency. A preliminary feasibility study on single blade installation by using large floating crane vessels is carried out, by comparing their performance with a typical jack-up crane vessel. Two typical floating crane vessels are considered, i.e., a mono-hull and a semi-submersible. They are assumed to be equipped with DP systems that can well eliminate their slowing varying horizontal motions. The results indicate that single blade installation by floating vessels is feasible. The feasibility depends on vessel type and size, as well as site conditions. The semi-submersible vessel is more feasible than the mono-hull vessel. However, the life cycle cost versus benefit needs further assessment. The efficiency of floating vessel installation is higher in short wave conditions. Utilization of weather orientation for floating vessels can greatly reduce the installed blade motion and thus reduce the operational cost. Allowable operational limits in terms of environmental conditions are also evaluated for single blade installation by the semi-submersible crane vessel. They, together with weather forecasts, can assist the planning and decision-making during the execution of installation operation. The critical events, limiting parameters and criteria are firstly identified. The critical events are excessive radial motion of the blade root or bent guide pins at blade root. The corresponding limiting parameters are radial motion and radial impact velocity at the blade root, respectively. For instance, the impact criterion to avoid bent guide pins at blade root which is related to the radial impact velocity, is determined based on nonlinear finite element analysis. Slacks in tugger lines should be avoided and are considered as restrictive events. Fully coupled time domain simulations are then conducted to estimate the characteristic values of the limiting parameters. The operational limits, in terms of wind and wave conditions, are thus derived by using response-based criteria. The impact criterion is considered to be conservative since the turbine hub is assumed to be rigid. In summary, the author develops a fully coupled method for numerical modeling and analysis of single blade installation and conducts a systematic study on installation by jack-up and floating crane vessels. The coupled method can also be used to investigate demounting or replacing offshore wind turbine blades and can be extended to study installation of rotor, and fully assembled tower-rotor-nacelle for offshore wind turbines. In practice, it can be utilized to assist the planning and execution phases of installation and to develop simulators for personnel training.