Sea Technology

APR 2018

The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community

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12 ST | April 2018 Under the Massachusetts Chap- ter 91 law, which dates back to colonial times, proposed projects cannot obstruct the public's abili- ty to "fish, fowl, and navigate." In recognition of the need to support innovative projects, the state re- cently implemented changes to the Chapter 91 regulatory program to allow the issuance of a permit for test projects through an expedited process. MRECo applied for and received a one-year, renewable permit to construct and operate the BTTS. Once its benefits and impacts can be studied, MRECo will seek a 30-year license for the facility. A key element in the permitting pro- cess was the support of state and lo- cal regulators who recognized that impacts of new technologies could not be determined unless they were tested, and, therefore, these agen- cies were cooperative in applying appropriate requirements. The BTTS also required a com- prehensive license from the U.S. Army Corps of Engineers, which manages and operates the Cape Cod Canal. The license included authorizations under the Rivers and Harbors Act of 1899, the Clean Wa- ter Act of 1970 and the Endangered Species Act of 1973. It also includ- ed a real estate license designating the property, both in the canal and on land, and uses that would be leased to MRECo as part of the proj- ect. The Corps issued its lease after all other approvals were obtained, allowing for construction to com- mence. Structural Design Considerations In recognition of the challenges of developing devices for use in the ocean, the U.S. Department of En- ergy has encouraged a deliberate development and testing regime and has applied the concept of technology readiness levels (TRLs), first developed for the aerospace industry. The BTTS was designed to meet a missing piece of the test infrastruc- ture: "TRL-5 – Testing of partial scale devices in relevant environment." While the size of partial scale will depend on the size of full scale, MRECo determined that a 3-m-di- ameter axial turbine would have the greatest application for scaling and selected it as the ideal-sized test de- vice for the BTTS. Testing a device requires many considerations. For devices intend- ed to operate "in stream," one of the first considerations is blocking. Any turbine causes resistance in a flow. In a hydroelectric dam, the design forces the water into the turbine, and this increases the energy con- version. In the case of a tidal "in- stream" device, the resistance caus- es flow to move around the turbine and decreases energy conversion. However, if the device is tested in a constrained flow, the blocking of the sides and bottom can force more water into the turbine and artificially increase the conversion efficiency. To properly test an in- stream device, blocking must be minimized. For the BTTS, this was done by ensuring the width (7 m) and depth (7 m) of allowed flow around the entire turbine. A second consideration is ac- cess. New technologies are expect- ed to encounter problems identified and corrected during testing. Easy access to the device for mainte- nance and upgrades is critical. The BTTS allows this by providing a lifting arm capable of raising the turbine out of the water. Addition- ally, the structure is close to shore, facilitating installation and removal, as well as the addition of sensors or other data acquisition components. Access for cabling or wireless com- munications is also simplified. A third consideration in any test- ing environment is control. Open- ocean testing must be the ultimate test environment, but using it for scale testing can add confounding factors that can obscure issues influ- encing turbine efficiency. In particu- lar, waves can change performance if the periodic surge aligns with the direction of current flow, alternate- ly increasing or decreasing the flow on the turbine. Turbulence can also impact performance. The BTTS is particularly well suited because it is shielded from wave action and it is relatively turbulence free.

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