Author(s): H. Imani; K. Wei
Linked Author(s):
Keywords: Scour depth; Jacket-supported offshore wind turbines; Structural response; Natural frequency; Nonlinear finite element analysis
Abstract: Monopiles, as of yet, are considered to be the most feasible supporting structures for offshore wind exploration in shallow waters (0–30 m). However, lattice-type supporting structures, including jackets and tripods, are the primary design basis for offshore wind turbines (OWTs) in intermediate waters (30–50 m). Important drivers for this are technical and economic feasibility of foundation in terms of construction and installation, particularly in regions prone to extreme met-oceanic events such as hurricanes and typhoons (Oh, Nam et al. 2018). Despite vast experience in oil and gas (O&G) industry, several technical issues still exist in the utilization of such space frame substructures for OWTs. Scour phenomena around the foundation is one of these challenges that could have an impact not only on the loading but also the capacity of the foundation and thereby on the response of the structure supported by it (DNV-OS-J101 2013). In this study, the influence of scour depth on natural frequencies as well as nonlinear structural response of a 5MW OWT is investigated. For this purpose, a finite element (FE) numerical model of a full-scale jacket-supported wind turbine (Fischer, De Vries et al. 2010), as the reference model in this study, was developed in USFOS (Søreide, Amdahl et al. 1993). presents the developed FE model. In this model, the tower, transition piece, and jacket supporting structures were modeled using beam elements, while pile foundations were modeled with beam and nonlinear spring elements. Extra rigid beam elements were also used to appropriately simulate the rigid connectivity between the tower and substructure. Rotor-nacelle-assembly (RNA) was idealized through concentrated translational and rotational masses with eccentricity at the top of the tower. The effects of scour depth, both local and global (see Figure 1), on the structural performance of the reference model were evaluated using nonlinear static analyses in USFOS. The analyses were carried out considering the effects of geometric and material nonlinearities in the structure as well as flooded legs, marine growth and buoyancy of the submerged elements. The results indicate that: (1) scour has a minor yet decreasing effect on the natural frequency of the structure, (2) the second vibration mode seems more sensitive to scour compared to the first mode, and (3) ignoring the effect of scour can lead to a considerable underestimation of the structural responses. These findings provide insights which could further improve safe and economic development of OWTs supported by jacket foundations.
Year: 2024