This report presents methods, data, observations, results, and conclusions from analyses of underwater sound monitoring data that was collected during the construction of the Block Island Wind Farm (BIWF). The facility is located 4.5 kilometers (km) (2.8 miles [mi]) southeast of Block Island, Rhode Island. Construction was completed in two distinct phases. Phase 1 construction began in August 2015 and was conducted over an 18-week period. It included installation of wind turbine foundations on the seabed. Phase 2 construction was completed in two steps. In Step 1, which was initiated in January 2016, submarine power cables were laid on the seabed. In Step 2, which was completed over a two-week period (3 August–18 August) in 2016, a turbine tower, a nacelle, and three blades were assembled on each of the five wind turbine generator foundations. The nacelle is a case that houses all of the generating components in a wind turbine, including the generator, gearbox, drive train, and brake assembly.
During Phase 1 construction five wind turbine foundations were installed on the seabed within the BIWF Project Area. Unlike in Europe where the majority of the offshore wind turbines have monopile foundations, the BIWF turbine foundations consist of a jacket structure, which is tailored to accommodate the complex aerodynamic and hydrodynamic loading of deep waters. The four legs of the jacket structure are raked at an angle of 13.27° to the vertical. During construction, each steel jacket was lowered onto the seabed by a crane and then individual piles, which measured between 1.4 and 1.7 meters (m; 4.6 and 5.6 feet [ft]) in diameter, were placed into the guide holes at jacket corners. Impact (percussive) pile driving was used to drive the piles incrementally into the seabed. The piles were driven to their final penetration design depth of 76.2 m (250 ft) or until refusal, whichever came first.
Underwater acoustic monitoring was conducted during pile driving to detect and record underwater acoustic and sediment-borne signals generated by the pile driving impacts. Several stationary and a towed platform equipped with varying number of hydrophones were deployed for the data collection. The stationary mooring provided information at the deployment location (one range from the pile driving activity) whereas the towed array collected information across a variety of ranges during selected pile driving events. The towed array data was useful in addressing the question of the range at which a pile driving signal transitions from an impulsive signal to a non-impulsive signal. Knowing the transition point improves understanding of the potential effects to marine animals.
Results from preliminary data analysis1 indicated that underwater pile driving sound was above background sound levels at ranges in excess of 20 km (12.4 mi) and that the received levels were approximately 120 decibels (dB) relative to 1 micropascals (µPa) root mean square (HDR 2018). Background sound levels at distances of 20 to 30 km (12.4 to 18.6 mi) from the construction site were recorded from 97.7 dB to a 125.7 dB (mean of 107.4 dB). Based on models calibrated with empirical data, the sound levels were a function of water depth, which varied based on direction away from the pile.
The key conclusion from preliminary data analyses was that underwater sound levels were lower in deep waters and higher in shallow waters; the difference between the two could be as large as 10 dB re 1 µPa root mean square. Sound levels were also shown to be dependent upon the orientation of the pile to the recording sensor. The piles were driven at an angle (13.3° relative to perpendicular). A 10 to 15 dB difference in sound levels resulted depending on whether the pile was angled towards or away from the measuring sensor. Particle motion, which is important to demersal fish and megabenthos, was greater at the seabed compared to higher in the water column.