Before a single turbine can be placed in an offshore wind farm, reams of marine data must be gathered.
By Robert Mecarini
Wind farms are like any major construction project – developers must have a thorough knowledge of every aspect of the site before breaking ground. But in the case of offshore wind farms, there is the added element of difficulty created by a site submerged under water. Before a single turbine can be placed, developers must discover potential obstacles and hazards and understand the geology of the site. Gathering these data requires specialized vessels and tools to capture and analyze information.
Detailed and accurate data sets are required for many aspects of offshore wind farm development, including site selection, cable routing (between turbines and to shore), foundation design for turbine support structures and ongoing inspections. Most project developers start with a desktop study (DTS) to determine the best site for a wind farm. They use publicly available information such as marine charts, published data sets, and consultation with various authorities to determine a potential site. Information is also gathered on the presence of potential archeological relics, marine mammal species and sensitive habitats that may preclude a site or require implementation of specific processes during data collection, construction and maintenance. DTS data may be incomplete and/or out of date, and does not provide enough detailed and site specific information for the actual design and permitting process, so once developers select a preferred site, they bring in a marine surveying company to conduct a detailed benthic, marine mammal, oceanographic, bathymetric, geophysical and geotechnical survey. Other contractors specializing in wind, avian and bat monitoring will also be party to the data collection efforts.
Following are some of the major elements required for marine data collection and surveying for offshore wind farms:
- Oceanography: These data include the measurements of current, tides, waves, and water quality at and around the wind farm site.
- Hydrography: The equivalent of topography on land, a hydrographic survey provides information on the water depths, seafloor slopes, outcroppings, and other natural and manmade topographical features that may be present on the seafloor.
- Geophysics: These data provide information on surface and subsurface geological features, as well as imaging of the seabottom to detect manmade and natural hazards. The data outline the subsurface geology, and provide key information for the geotechnical drilling campaign. This allows the geotechnical ground truthing, which is a far more expensive process than gathering geophysics, to be targeted to areas of concern or importance to the project. Data are collected using acoustic tools such as sub-bottom profilers, boomers, and sparkers that use sound to determine changes in sediment density and create an “image” of the sub-bottom stratigraphy. These systems come in many flavors, including multi-channel systems that are essential for achieving a clear picture of deeper geology; surveyors choose systems based on required seafloor penetration, soil conditions, and water depths. The data are used to detect the presence of buried channels (which may have archeological significance), faults, gas pockets, rock, clay and sand layers, etc. that would impact cable and turbine foundation installation. Other tools used include side scan sonar, which provides detailed images of the seafloor to detect wrecks, debris, utilities, rock outcrops, sand waves, and changes in surface sediment types; and magnetometers/gradiometers, which detect magnetic objects just below the sea floor, and are key to determining the presence of things such as buried utilities, wrecks, debris, and ordinance.
- Geotechnics: These data provide the detailed soil information that cable installers and turbine foundation designers need. Cable installers are usually interested in the first three meters of the sediment to determine burial methods and insulating properties. This information is usually gathered using vibracore sediment samplers, in-situ thermal probes, and, in instances where soft sediments are suspected on the sea bottom, a seafloor-deployed cone penetrometer (CPT), which measures in-situ geotechnical parameters such as soil density and shear strength.
For foundation design, the requirement in the US is to collect data 50 ft below the foundation depth. Standard penetration tests (SPT) use drill rigs, and measure how many blows of a standard weight it takes to move the drill down a set distance, and retrieve physical soil samples for laboratory analysis of the exact quality of the sediment at the boring location. In most cases, engineers and permitting agencies will also require CPTs, which can either be taken down-hole by a drill rig at set intervals, or in cases where the sediments are softer, with seafloor-deployed systems. Generally speaking, cone penetration is the most reliable and accurate way of measuring the properties of the seabed for engineering purposes.
Geotechnical data are collected at discrete intervals along the cable route and at turbine locations. They are then overlaid on the geophysical data, enabling geologists to extrapolate information on the geology that exists between geotechnical data collection sites.
Collecting these marine data requires special equipment as well as special vessels that are able to transport and deploy tools and crew. Vessels with living quarters for crew increase production as they avoid the need to transit back to port every night. On-board data processing capability is also key to project efficiency: fast turnaround on data analysis creates the opportunity for in-field decision-making to adjust the project design to the existing conditions.
Floating platforms for offshore geotechnical drilling operations need to stay on location for extended periods of time and compensate for ocean swell while drilling. Depending on floating plant availability and local sea conditions, jack-up drills can provide a stationary and stable platform for drilling in shallow-water sites. Truck-mounted drill rigs are often a less expensive solution than employing a geotechnical drill ship. However, it is very important to make sure that the jack-up is sea-worthy and can handle the sea conditions expected during the operation, or the consequences could be tragic. Project safety and success in deeper waters requires a dynamically positioned drill ship that stays on location using a combination of thrusters tied into a computerized navigation system, and which is outfitted with a heave-compensated drill rig.
While the list of data seems extensive, marine surveyors often cite their mantra, “You can never have too much data.” If a developer does not collect enough data upfront, it will wind up paying the cost on the back end. Developing an offshore wind farm is a high-stakes project, so collecting the right marine data is a small investment for success. And, since the specialized tools, equipment, and manpower can be costly, it’s wise to work with marine surveying firms experienced with offshore energy projects.
Robert Mecarini is president of Alpine Ocean Seismic Survey Inc., located in Norwood, N.J. For more information, visit www.alpineocean.com.