Retrofitting of monopile transition piece for offshore wind turbines

The monopile foundation for an offshore wind turbine (OWT) has been successfully implemented worldwide, particularly in Germany, Denmark and the United Kingdom. Numerous offshore wind farms have been operational for a large percent of their expected service lives. However, one problematic area of concern is that the transition zone of the structure (the connection between the monopile and tower) relies on a grouted connection which fails to support the axial load of the OWT when it is subjected to wind and wave loadings. This is a major implication for offshore wind farms that are installed and, thereby requires a retrofit solution that is not only adequately-effective, but also efficiently implemented. This paper analyzes a retrofit solution that involves drilling holes through the transition piece, grout, and monopile and installing pins, which would completely prevent vertical movement between the transition piece and monopile. The NREL 5MW reference wind turbine was considered for this study, atop a monopile with a 5m diameter. To adequately address the major issue presented, this study consists of three simulation parts. All three parts implemented a finite element model (FEM) analysis of the transition zone to: 1) simulate the transition zone without any additional supports types (e.g. shear keys); 2) simulate the transition zone with the support of shear keys; and 3) simulate the transition zone with the retrofit pins without the implementation of shear keys. These pins will be spaced based on a general following of the DNV guidelines for shear key design. All three of the models are local models only, and thus have simplified loading conditions applied. Each model is 25m tall and are assumed to be fixed at the base. The retrofit solution will then be compared to the other cases to determine its efficacy in transcending the grout-failure problem. From this study it is anticipated that an effective retrofit solution will be simulated for a general OWT size.