Seafloor disturbance and recovery monitoring were conducted in and around the Block Island Wind Farm (BIWF) to assess the impact of wind farm construction activities on the seafloor. Previous studies from Europe have shown that introduction of solid structures onto the seafloor, such as the four-legged BIWF turbine jacket foundations, can modify near-bottom current flow processes and induce scour. This in turn may temporarily or permanently alter seafloor characteristics. Changes in seafloor characteristics may result in loss of native benthic habitat directly impacting benthic community abundance and diversity. It may possibly also compromise functionality and physical integrity of the structures themselves
The seafloor can also be affected by several different construction-related activities such as vessel anchoring, life boat legs anchoring, and trenching for laying of submarine power transmission cables. Accordingly, the primary objectives of the seafloor disturbance and recovery monitoring surveys were to identify and characterize seafloor disturbances associated with wind farm construction activities and to monitor recovery times for the different types of disturbance features over time.
This study included developing and field testing a methodology for monitoring seafloor scour around the turbine foundations in real time using innovative scour monitors. Concrete mats were placed on the unburied cable sections for protection. Scour associated with the concrete mats was also evaluated during this monitoring effort.
Data from the seafloor disturbance and recovery monitoring surveys were intended to provide information necessary for the U.S. Department of the Interior Bureau of Ocean Energy Management’s (BOEM’s) evaluation of environmental effects of future facilities, and to improve the accuracy of models and analysis criteria employed to establish monitoring controls and mitigations. Monitoring and testing were conducted under BOEM’s Real-Time Opportunity for Development Environmental Observations (RODEO) Program
The five-turbine, 30-megawatt BIWF is the nation’s first offshore wind facility, and is located 4.5 kilometers (2.8 miles) from Block Island, Rhode Island, in the Atlantic Ocean. Water column depth in the wind farm area is approximately 30 meters (m) (98.4 feet [ft]). BIWF construction was completed in two phases. During Phase 1, five steel jacket foundations were installed on the seafloor. Phase 2 involved installation of the turbines on the foundations and laying of the submarine power transmission cables. Phase 1 construction occurred between 26 July and 26 October 2015, and Phase 2 construction occurred between 13 May and 18 August 2016.
Five rounds of seafloor bathymetry surveys were conducted within a defined construction Work Area from a small research vessel using a Reson SeaBat 7125 ultra-high-resolution multibeam echosounder. The first survey was conducted in May 2016, approximately 7 months after completion of the Phase 1 construction. The second survey was conducted in October 2016, approximately 10 months after completion of Phase 1 construction activities and 2 months after completion of Phase 2 construction activities. The remaining three surveys were conducted approximately 7, 12, and 24 months after completion of Phase 2 construction. Seafloor bathymetry data from the first and second surveys were primarily used to characterize the different types of seafloor disturbance features that resulted from Phase 1 and 2 construction activities. Data from the three rounds of post-construction surveys (Surveys 3, 4, and 5) were used to evaluate the rate of seafloor recovery.
A pair of innovative scour monitors were also field tested during the study. These were installed on the Turbine 3 foundation, and they recorded in real time changes in seafloor elevations up to a distance of10 m (32.8 ft) from the foundation. A near-continuous seafloor elevation dataset was collected over the course of 14 months and 19 days. A seafloor-mounted acoustic wave and current profiler was installed nearby to provide data on oceanographic conditions (e.g., water levels, currents, and waves). Site-specific oceanographic data supported analyses and interpretation of data obtained from scour monitors.
The overall conclusion from monitoring surveys was that 1) a relatively small area of the seafloor off Block Island was disturbed by wind farm construction activities, and 2) much of the disturbed area fully recovered within a relatively short time. The criteria for designating sea bed as “fully recovered” was a condition in which no clear sign of any disturbance was evident as indicated by interpretation of the survey data. A “partially recovered” designation was assigned if only a section of a disturbance feature showed signs of recovery as indicated by interpretation of the survey data.