The simulation software FAST is an open source CAE package maintained by the National Renewable Energy Laboratory. A new module of FAST for assessing the dynamic response of offshore wind turbines subjected to ice forcing is presented. This paper describes the recent addition of capabilities for analyzing the response of wind turbines subjected forces resulting from ice impact on the turbine support structure. Several models are presented which involve both prescribed forcing and coupled response. For conditions in which the ice forcing is essentially decoupled from the structural response, ice forces are established from existing models for brittle and ductile ice failure modes. For conditions in which the ice failure and the structural response are coupled, such as lock-in conditions, a rate-dependent ice model is described, which is developed in conjunction with a new modularization framework for FAST.
The ice loading module includes ice mechanics models, that incorporate ice floe forcing, deformation, and failure. For lower speeds, forces slowly build until the ice strength is reached and ice fails resulting in a quasi-static condition where the frequency of the forcing is about an order of magnitude lower than the response of the structure. For intermediate speeds, the ice failure response can be coupled with the structural response resulting in response where the ice feature failure period and the response period of the structure approximately coincide. A third response regime occurs at high speeds of encounter in which brittle fracturing of the ice feature in contact with the structure occur in a more or less random pattern, which results in random vibration excitation of the structure. These three conditions are specifically addressed in the ISO standard 19906:2010 for consideration in the design of arctic offshore structures. Special consideration of lock-in vibrations is required due to the detrimental effects of such response with regard to fatigue and foundation/soil response. The use of FAST for transient, time domain simulation with the new ice module is well suited for such analyses.