Advantages and Challenges of Perforated Monopiles in Deep Water Sites

Abstract

The potential for developing more efficient offshore wind support structures becomes increasingly critical as the offshore wind sector is faced with challenges such as rising commodity prices and shrinking profit margins. These challenges not only impact the industry’s capacity to meet growing global demand but also hinder efforts to address the imperative challenge of decarbonization. As the offshore wind industry expands, the roles of cost-effectiveness and ongoing research will be pivotal in driving the advancement of offshore wind energy on a global scale.

The support structure is responsible for supporting the turbine, transferring loads to the ground, and allowing access for inspection and maintenance purposes. The environmental loads acting on an offshore wind support structure (OWSS) result from a combination of waves, wind speed, turbulence intensity acting on the shape of the turbine components. These loads can be grouped in a scatter diagrams that depict the probability of occurrence of a given wind wave combined condition which is also known as a "sea state". Different sea states lead to significant vibrations and stresses on the foundation, which can cause fatigue and failure over time.

The perforated monopile consists of a monopile with holes around the splash zone to reduce frontal area, which reduces hydrodynamic loads on the foundation. Assesing the potential of perforated monopiles in deep waters was based on a comparative study of the loads acting on the monopile by developing a model that considers the structural response of the system to different sea states. FEM studies are essential in identifying potential stress concentrations and their effects on the overall integrity and safety of the structure. Thus, analysis focused on assessing the performance of both a reference monopile and a perforated monopile structural models under both parked and power production conditions, including sea states with 50-year and 1-year return periods. The simulations encompassed 35 distinct sea states and computed maximum stresses at critical locations, including the mudline, perforations, and the splash zone.

The study found that the perforated monopile displayed an average reduction of 17% in the maximum stresses found at the mudline compared to the reference monopile. This reduction was attributed to improved flow dynamics facilitated by the perforations. However, the splash zone of the perforated monopile experienced increased stresses around the splash zone of up to a factor of four, attributed to higher overturning moments around the perforated area. Next, the analysis continued with a fatigue life assessment, highlighting sea states that could potentially challenge the structural integrity and longevity of the monopiles over their intended operational lifespan.

The study explored alternative solutions, such as varying thickness parameters at high-stress areas and using different materials at the splash zone. Additionally, the possibility of different perforation geometries was considered, although it could impact natural frequencies and cause resonance with loading frequencies.

To conclude, the research underscores the importance of a comprehensive assessment of monopile designs in offshore renewable energy structures with a discussion and recommendations for further research on the subject. It emphasizes the need for careful consideration of design choices based on specific environmental conditions at the installation site. The findings contribute valuable insights into optimizing monopile designs for long-term performance and structural reliability in varying sea state conditions during power production scenarios.