Monitoring Construction of the Neptune Regional Transmission System in Real Time

By Van den Sype, Jaak Davis, David; Duschang, John

Supervising the Suspended Sediment Plume Created by Trenching Operations for the East Coast’s Largest Undersea Cabling Project The Long Island Power Authority (LIPA) service area, which includes the New York counties of Suffolk and Nassau, as well as Rockaway Peninsula in New York City’s borough of Queens, is home to a growing population of nearly three million people. In spring 2004, LIPA selected the Neptune Regional Transmission System LLC (Fairfield, Connecticut) high-voltage direct current (HVDC) transmission project as the centerpiece of its comprehensive plan to meet steadily mounting energy demands.

The project involved installation of a 660-megawatt, 65-mile- long undersea and underground cable, the longest such transmission project on the U.S. East Coast. The 500-kilovolt HVDC cable extends from a First Energy Inc. (Akron, Ohio) substation in Sayreville, New Jersey, to the LIPA Newbridge Road substation in Levittown, New York. HDVC technology is far superior to conventional alternating- current transmission in moving large blocks of power over long distances.

The majority of the route – 51 miles – is underwater in the New YorkNew Jersey Harbor and Atlantic Ocean. While buried and submarine cable technology eliminated land-side issues like threading cables through the tangled utility infrastructure of Manhattan and Staten Island, it brought challenges of its own. It required in-depth knowledge of the sensitive marine and estuarine water bodies in the area, as well as navigation channels and shipping lanes in one of the busiest commercial harbors in the world.

To guide the project through the complex environmental and regulatory process, Neptune contracted with HDR. The submarine cable route needed to minimize impacts on commercially harvested clams and benthic fauna, avoid areas with sensitive archeological resources and facilitate the crossing of existing utilities and navigational channels. To meet these demands, a variety of engineering and operational controls were employed to minimize potential impacts, including use of “environmental windows,” which limited all in- water construction activities to certain periods to avoid impacting the habitat of species of concern and the transit of migratory fish through the estuary.

Sediment Monitoring

The transmission line consists of a nine-inch encased bundle of three cables that had to be buried up to four feet beneath the ocean floor. Cable burial was accomplished using water-jetting technology, which, unlike mechanical trenching methods, does not rely on removing sediments from the trench. Instead, the seabed is temporarily fluidized by water jets using a hydro-plow, which trenches and lays cable simultaneously. Even though water-jetting methods minimize the amount of resuspension of sediments in the water column, the potential to impact water quality through resuspension of sediment is still possible. This is especially true in New York Harbor, where the seabed is primarily composed of fine and silty sediments.

To meet regulatory requirements for the New Jersey Department of Environmental Protection and New York Department of Environmental Conservation, HDR developed an innovative program that allowed real- time continuous monitoring of the suspended sediment plume generated by the hydro-plow trenching operation. Specifically, the monitoring effort needed to describe the extent of the plume, establish a relationship between total suspended solids (TSS) in collected water samples and turbidity readings from an optical backscatter (OBS) turbidity meter and establish a correlation between TSS water samples and acoustic Doppler current profiler (ADCP) monitoring equipment.

Regulatory requirements stipulated that if at any point as a result of burial activities TSS concentrations exceeded 200 parts per million above background levels, then operational refinements were required to reduce sediment concentrations or, potentially, it would be necessary to cease cable burial operations entirely. As a result, all data from TSS monitoring activities were required to be processed and reported within 48 to 72 hours of collection, posing a significant challenge to the monitoring effort

Monitoring of sediment disturbance focused on defining the extent of the suspended sediment plume using a combination of acoustic instrumentation, optical instrumentation and laboratory analysis of water samples. Acoustical backscatter data was measured using a Poway, California-based Teledyne RD Instruments (RDI) Workhorse 600- kilohertz bottom-tracking ADCP attached to a fixed mount on the survey vessel and positioned approximately one meter below the water’s surface.

Ensuring Quick Turnaround Times

The suspended sediment plume was surveyed by conducting transects at varying distances (generally 100 to 1 ,000 meters) down current from cable burial operations and then comparing the results to up- current, or background, data. A laptop computer running RDI’s WinRiver software package was used to collect the ADCP backscatter data, which was correlated to estimates of suspended sediment concentrations using the Sediview(TM) software package. For Sediview to calibrate the ADCP data, however, it was necessary to collect profiles of turbidity, temperature, salinity and TSS at predetermined intervals along the transect lines, both within the sediment plume and up current. Furthermore, an entire profile, including water samples, had to be collected as quickly as possible to minimize interruption to the ADCP transects and to avoid spatial displacement of the plume.

To this end, a custom water sampling apparatus was assembled, consisting of a Sea-Bird Electronics (Bellevue, Washington) Carousel Water Sampler with six one-and-a-half-liter Niskin water sampling bottles. Attached to the carousel chassis was an OBS 3A optical backscatter turbidity meter and a Falmouth Scientific (Cataumet, Massachusetts) conductivity, temperature, depth (CTD) MicroCTD meter. The CTD and OBS data, along with differential global positioning system (DGPS) positional data derived from a Trimble (Sunnyvale, California) ProXRS GPS receiver, were fed into a laptop computer running Coastal Oceanographies Inc. (Middletown, Connecticut) HYPACK(R) MAX hydrographie survey software. The laptop also ran a custom software application to control the firing mechanism of the Niskin bottles on the carousel. This application allowed real-time control of the carousel bottles, which could be fired in a usercontrolled sequence based on the CTD’s depth. For each water column profile, two replicate water samples were collected at the near surface (three feet), middle and bottom (approximately three feet over the seabed) strata, as well as a continuous profile of turbidity, temperature and salinity.

By far, the most onerous challenge was the need for an extremely quick turnaround time for the data. Given the sheer volume of data generated from the 24-hour monitoring effort, a rapid, organized and efficient post-processing system was required. The first obstacle was simply getting the data off the survey vessel, given the around- the-clock monitoring effort. While data could be offloaded during each crew rotation (once every 12 hours), it would still be a cumbersome solution, as the crews were geographically distant, and this would require another level of management of crew rotation. The solution was a custom data packaging utility that would gather all files from the previous day’s sampling and automatically compress them into a single file that could be e-mailed using a broadband cellular phone card installed on one of the survey laptops.

Custom Data Organization

Once the data were received, the next challenges were organization and post-processing. A custom database application based on Microsoft Access(R) was developed to rapidly process all data and store it in an efficient, easily retrievable format in a centralized location. The database would process the data and automatically generate all the calibration files in the proper format needed for the Sediview calibration, which typically is a time-consuming effort (particularly given the volume of data). The database included automated import routines for the HYPACK raw CTD, OBS and GPS data; laboratory-derived TSS data from the water samples; data sheets recorded in a custom-designed Microsoft Excel(R) file on board the survey vessel; and operational data provided from the cable burial ship (e.g., date/time, positional and jet pressure data).

Additionally, the database included routines to post-process the data and automatically generated reportquality profile plots of turbidity, TSS, temperature and salinity as well as summary data tables for submission to the regulatory agencies.

After the onboard data were received and archived in the database and all requisite calibration files were created, the Sediview calibration was conducted using the Sediview method.1 Data used for the calibration were derived from the gravimetric analysis of the water samples collected and from estimates derived from the OBS turbidity values. Turbidity values obtained from the OBS were linearly regressed against laboratory TSS values for each 24-hour period. TSS values were then calculated for the entire profile based on this regression equation. After the Sediview calibration was completed, TSS estimates based on acoustic measurements were exported in tabular form for each 3D point in space (latitude, longitude and depth) for both background and plume transects. These TSS estimates were then plotted in 3D using Golden Software (Golden, Colorado) Voxler(R) visualization software and interpolated to the spatial extent of the data using an anistropic gridding algorithm to generate a 3D image of the estimated sediment plume. Using the Voxler program, various orthogonal and oblique cross sections, isometric surfaces and other visualizations of the plume could be easily recreated for presentations.

Conclusions

The program HDR developed for the Neptune project proved to be effective in meeting permit requirements for TSS monitoring of a large and complex project with potentially serious environmental impacts in an environmentally sensitive area. The power lines officially became operational in July 2007, carrying enough electricity to meet the demands of approximately 600,000 homes.

Quantifying the potential impacts associated with sediment resuspension is an emerging field with implications for a variety of dredging and bottom construction projects. One of the advantages of the Neptune program is that it can be easily adapted to a variety of similar dredging/plowing projects or wherever rapid processing and turnover of data are required. It is a unique application of acoustic monitoring in a nearshore environment.

Quantifying the potential impacts associated with sediment resuspension is an emerging field with implications for a variety of dredging and bottom construction projects.

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References

1. Dredging Research Ltd., The Sediview Method: Sediview Procedure Manual, ch. 5, 2003.

By Jaak Van den Sype

Environmental Scientist

David Davis

Environmental Scientist

and

John Duschang

Senior Project Manager

HDR

Pearl River, New York

Jaak Van den Sype is an environmental scientist with HDR. He holds a bachelor’s degree in environmental biology from the State University of New York (SUNY) College of Environmental Science and Forestry and master’s degree in applied ecology from SUNY Stony Brook.

David Davis is an environmental scientist with HDR. He received a bachelor’s degree in biology from Siena College and a master’s degree in journalism/science and environmental reporting from New York University.

John Duschang is a senior project manager with HDR. He holds a bachelor’s degree in fishery sciences from the University of Vermont. He leads HDR’s New York environmental sciences and fisheries section.

Copyright Compass Publications, Inc. Jul 2008

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