A review of offshore wind monopiles structural design achievements and challenges

The taller and heavier OWT monopile structures has unique structural engineering design challenges which could lead to catastrophic under-conservative designs or more likely, expensive over-conservative engineering. The need for an efficient engineering design cannot be overemphasised given the current structural configurations and future OWT monopiles novel concepts due to the cost of materials for the increasing towers and foundations diameter and thickness, longer and bigger RNA, which directly impacts the manufacturing, transportation, and installation activities. This increasing trend is projected to continue as the industry utilises shallow to medium water depths for siting new concepts of Offshore Wind Turbine (OWT) monopile structures and development of wind farms. The engineering design through to installation challenges faced by the relatively new OWT industry are exacerbated by the shortcomings of the structural design techniques and practices adopted from the oil and gas industry and in accordance with design codes and standards. This paper presents a concise structural review of the current salient technical aspects, the recent improvements in offshore wind turbine monopile structural design, and the challenges of future OWT monopile concepts considering the increasing monopile structure size and turbine capacity. The review presented in this paper primarily focused on grouted connection between the foundation and tower, damping for monopile structural response analysis, soil scouring, soil-monopile interaction modelling, and corrosion. The aim of this paper is to critically assess, outline, and discuss the current OWT monopile structural design techniques achievements and identify future concepts structural challenges, and to provide structural design direction for OWT monopile research and development activities.