Observing Cable Laying and Particle Settlement During the Construction of the Block Island Wind Farm


Title: Observing Cable Laying and Particle Settlement During the Construction of the Block Island Wind Farm
Publication Date:
March 01, 2017
Document Number: OCS Study BOEM 2017-027
Pages: 225
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James, E.; Smith, K.; Gallien, D.; Khan, A. (2017). Observing Cable Laying and Particle Settlement During the Construction of the Block Island Wind Farm. Report by HDR Engineering Inc. pp 225.

Key observations, data, findings, and results from two different types of environmental monitoring surveys conducted in and around the Block Island Wind Farm (BIWF) Project Area during its second construction phase are presented in this report. The monitoring was conducted to gather real-time data during the installation of a submarine cable from the mainland at Scarborough State Beach to Block Island’s Fred Benson Town Beach. The cable was installed using a customized jet plowing mechanism. Real-time monitoring included recording visual observations of the installation process and measuring suspended sediment concentrations in the water column.


The data collected during this monitoring will provide additional information necessary for the Bureau of Ocean Energy Management’s (BOEM) evaluation of environmental effects of future facilities and generate data to improve the accuracy of models and analysis criteria employed to establish monitoring controls and mitigations.


Visual monitoring included recording visibility of construction activities from both offshore and shoreline, types of lighting used at the construction site, information on what aspects of the construction activity can be seen from the shoreline, and meteorological conditions. Real-time observations were recorded by dedicated observers from the shoreline and from a vessel adjacent to the cable lay operations. Both still photography and video footage were employed to document the process from Block Island to the mainland at Scarborough State Beach.


The monitoring team was onsite for a period of 27 days. The team arrived on 31 May 2016 and departed 26 June 2016. The initial cable pull on mainland at Scarborough Beach State Park was originally scheduled for 3 June but was delayed until 9 June due to weather and delays in placing the cable mats at crossings. Approximately 105,232 feet (19.93 miles) of the Block Island Transmission System (BITS) cable was successfully laid from Scarborough Beach to Block Island over a period of 16 days. The construction time frame appeared optimal since it appeared to minimize impacts on recreational beach visitors and avoided operational delays due to adverse weather. Scarborough Beach access was not limited as conduit was in place upon team arrival. Access to Town Beach on Block Island was restricted for only two days during winch pull of BITS to cofferdam. The jet plowing operation did not hinder recreational craft traffic. No sediment plume was observed offshore from the observation vessel.


Sediment monitoring included two days of background sampling and one full day of sampling during the cable laying operations. Suspended sediment monitoring was undertaken using two complimentary techniques; acoustic backscatter and optical backscatter. These two techniques respond differently to different sediment grain characteristics such as size, shape, and angularity. Therefore, using both techniques allows for the suspended sediment load to be characterized as robustly as possible. Data were collected at various points around the cable trench footprint from Block Island Town Beach to Scarborough State Beach in Narragansett. A vessel-mounted 600-kilohertz (kHz) Teledyne RDI Sentinel Workhorse ADCP was used to take current and acoustic backscatter (ABS) measurement transects around the cable route and the cable-laying vessel Big Max.


An Idronaut EA89 multiparameter probe was used to collect profiles of turbidity through the water column at 23 locations around the BITS cable route. Eight of these profiles were conducted along the cable route to provide background information while the jet plow was not operational. The remaining 15 profiles were conducted in the vicinity of Big Max and the jet plow during cable laying operations. In addition to turbidity, the Idronaut also recorded conductivity, temperature and depth (CTD) data through the water column. Water samples were collected using Niskin bottles and analyzed for total suspended solids and particle size distribution for calibrating the Idronaut data.


No sediment plume was observed as a result of the jet plow operations. Data obtained during the background and jet plow monitoring remained comparable across all datasets. Sediment concentration estimates derived from high frequency acoustic backscatter measurements were < 1 mg/l at the surface and < 6 mg/l at the seabed. This is supported by the low frequency Multibeam Echosounder (MBES) data that found no significant levels of sediment in the water column. Estimated concentrations from optical backscatter measurements were < 2 mg/l at the surface and < 9 mg/l at the seabed.


Efforts to “calibrate” the backscatter measurements (convert to concentration values) using the on-site water samples were unsuccessful. Therefore, Fugro provided indicative values based on significant previous experience in backscatter calibrations. The estimated maximum concentration derived from the Idronaut data (9 mg/l) can be considered a “worst case.” Suspended sediment levels on site during jet plow activity were found to be up to 100 times lower than those predicted by the modeling work (RPS ASA 2012).


Trench morphology was evaluated at 11 cross section locations to estimate the amount and extent of material deposited outside the trench as an overspill levee. Analyses were performed at locations along the cable route from the mainland to near the mid-point of Rhode Island Sound where the plume monitoring was conducted. Pre- and post-lay MBES data were used in this evaluation. Overspill levees were interpreted to extend 1.5 to 7 meters beyond the trench and were up to 25 centimeters thick. The average distance from the trench and thickness were 3.8 m and 7 cm, respectively. It was noted that overspill sediments may have deposited beyond the interpreted extent of the overspill levee, but likely were too thin to resolve with the MBES data. RPS ASA (2012) estimated that sediments up to 1 cm thick would be deposited within 10 to 30 meters from the trench. This analysis of overspill levee extent suggests that the modeled predictions did not underestimate the distance that overspill sediments deposited from the trench.


The estimated volume of the overspill levees was similar to the volume of the trench scar. This suggests that most of the overspill sediments deposited within the overspill levee. The volume of the overspill levee material was estimated to be 0.1 to 0.6 m3 per meter travelled by the jet plow. The mean value of the overspill levee volumes in the cross sections analyzed is 0.24 m3. For comparison, the modeling report prepared by RPS ASA (2012) estimated that approximately 25 percent of sediment release volume would occur for a 2.28 m3 trench volume. This corresponds to a sediment release volume of 0.57 m3 per meter travelled.


It should be noted that the mean value of our overspill levee volumes is approximately half of what was predicted but the upper limit of the range is very similar to what the modeling predicted. This could be attributed to conservatism in the modeling or more sediment was deposited outside of our interpreted overspill levee body.


The interpretations presented in this report are limited to seafloor elevation differences that could be discerned with MBES data. Near the trench, the levee was more pronounced and exhibited a larger elevation difference from the pre-lay survey. The toe of the levee was interpreted where the two surveyed seafloor elevations converged. A thin drape of overspill sediments may have extended beyond the interpreted levee toe and could not be confidently discerned using the MBES data. However, the OBS and acoustic analysis of the MBES did not reveal a sediment plume that extended upward into the water column and beyond the immediate vicinity of the trencher which supports that most of the overspill sediments deposited within the overspill levee.


The visual and sediment monitoring was conducted for BOEM by the HDR RODEO Team under Contract M15PC00002, Task Order M16PD00011.

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