Effect of Monopile Installation on Nearby Breakwater

Wind energy generation (both onshore and offshore) has gained wide popularity across the globe because of the emerging need for renewable energy. Monopile is a reliable foundation that can be adopted for wind turbines at deeper water depths, usually up to 25m. Monopiles are massive structures weighing around 1000t and 6m in diameter. The proximity of the port facility ensures better connectivity to the turbine location, helping faster construction and hence providing economic benefits to the wind farm project. This also safeguards and helps immediate repair the wind turbine, if necessary. The monopile located close to the port enables easy installation of the wind turbine and its sophisticated system. The main goal of this work is to study the effect of monopile installation on the breakwater located closer. The three-dimensional finite element software Plaxis 3D is used to model monopile and the adjacent breakwater using the soil and structural details obtained from the site. The water depth at the site is 20m. The influence of underlying soil is studied by conducting numerical analysis with varying soil strata- clay and sand. The soil-pile interaction is reproduced via contact elements. The soil properties are described via the nonlinear Mohr-Coulomb model. The steel pile having 6m diameter and 65mm thickness is driven by Hydrohammer S-2300. The 115t hammer can produce maximum blow energy of 2,300kNm when dropped at its maximum stroke of 2.018m. The hammer drop is set to 200mm height, and the frequency varies from 10 to 30 blows per minute. The breakwater that borders the port consists of the inner core and outer armour layers. The inner core is made of quarry run with a weight ranging 10-500kg, and the armour layer consists of rock weighing 1-3t. The core and armour layer permits the lateral flow of water and is created in the numerical model using a drained Mohr-Coulomb model with horizontal permeability. The safe distance of monopile is determined by placing it at 10m from the breakwater, and consecutive monopiles are placed at 10m spacing each. The vibration on the nearby breakwater is analyzed using the peak particle velocities (PPV) along with the three directions. The peak vibration at the breakwater toe closer to the monopile, top of the breakwater, and toe away from the monopile is measured simultaneous to pile driving. The vertical PPV is found to dominate and hence plays a significant role in determining the optimum hammer loading. The study is conducted with varying monopile distance and hammer frequency, ensuring the vibration limit suggested by the British standard (BS 7385-2:1993) is not exceeded. The distance from the breakwater is varied as 10, 20,30,40&50m and the safe distance to restrict the vibration at the breakwater is within the limit suggested by the Indian standard DGMS (Circular, 1997) is reported.