Abstract
Monopiles are the most common type of foundation currently used in the offshore wind industry. Operators may elect to instrument and monitor foundations to assess the system behavior or support integrity management. For monopile foundations, the region below the seabed is critical since this region sees the largest bending loads in the monopile. The offshore wind industry can benefit greatly from sub seabed measurement systems as they can provide vital information regarding structural health and provide data to optimize future designs. However, installing sensors below the seabed is a non-trivial task that can occupy significant costs to engineer and implement such systems, which is a challenge in an industry that commands low-cost solutions. This paper presents an engineering study of two sensor cover concepts designed for implementation of sub seabed monitoring on a monopile foundation. The two concepts selected are designed to be fabricated from off-the-shelf components, producing a low-cost solution to sub seabed monitoring. Furthermore, the design relies on a combination of sub-scale testing and quasi-static analysis to reduce design costs and help support a low-cost solution.
Both strength and fatigue were considered in the design. Strength analysis was performed using a finite element analysis (FEA) where a series of acceleration loads, intended to represent the acceleration spectrum that would be present during pile driving, were applied to the protection systems in a quasi-static analysis. Additionally, a fatigue analysis is performed for the bolted connections on one concept using the stresses from the FEA. Two concepts were tested via a pile driving test with a sheet pile. The pile driving process was monitored and accelerations for each blow were measured during the testing. One of the two concepts tested survived the pile driving testing with accelerations measuring up to 900g, while the other failed.
The analysis and testing performed for this design showed that the sub seabed sensing on monopile foundations can be built from low-cost steel components, given proper collaboration between with the monopile designers/fabricators and the measurement system provider. Furthermore, this work has shown that sub seabed sensing for offshore wind monopile foundations can be done in a reliable, cost-effective manner that utilizes efficient design practices and is manufactured from commercially available, low-cost materials and components. Additionally, this work serves as an enabling solution for integrity management of offshore wind farms to help reduce operation and maintenance (O&M) cost.
