Offshore Renewable Energy Installations: Impact on navigation and marine safety.

Excerpt from U.S. Coast Guard Proceedings of the Marine Safety & Security Council magazine by Mr. George H. Detweiler, Jr., marine transportation specialist, U.S. Coast Guard Marine Transportation Systems Management Directorate.

To reduce our dependence on foreign energy supplies, alternative or renewable energy sources are being pursued. These sources exploit a wide range of technologies:

• solar photovoltaics or power plants;
• hydroelectricity (dams);
• ocean thermal energy conversion facilities;
• offshore renewable energy installations, which may include “wind farms,” marine current turbines, and wave generators (hydrokinectics).

All these technologies have the potential to affect marine navigation and safety, and although no offshore renewable energy installations presently exist in U.S. waters, several are contemplated following successful trials in other countries.

Of the technologies being considered, wind farms and hydrokinetics pique the Coast Guard’s interest because their developers propose to locate them in U.S. navigable waters.

Navigation Impact

All offshore installations, regardless of type, will have impact on vessel navigation and safety in their vicinity.

Location. An offshore site could affect navigation based on the traffic volume, types of waterway users, and other vessel characteristics such as speed capability, navigation equipment, and number of passengers.

Spacing. To make best use of the wind, turbine spacing is proportional to rotor size and the down-wind wake effect created. As such, wind farm turbines are generally spaced 500 meters or more apart.

Visibility. These structures could also block or hinder the view of other vessels, the coastline, or other navigational features such as aids to navigation, landmarks, or promontories used by mariners to navigate.

Electronic impact. Larger structures could produce radio interference with respect to any frequencies used for aviation, marine positioning, navigation, or communications, including automatic identification systems.

Effects of tides, tidal streams, currents, seabed changes. Current maritime traffic flows and operations in the area of an offshore renewable energy installation are affected by the depth of water in which the installation is situated at various states of the tide.

Additionally, the structures themselves could cause changes in the set and rate of the tidal stream or direction and rate of the currents. Also, structures in the tidal stream could produce siltation, deposition of sediment or scouring, and other suction or discharge aspects, which could affect navigable water depth.

Mitigating the Impact

While these offshore renewable energy installations have many potential benefits, it’s important to recognize the equally potential negative effects mentioned and to devise plans to mitigate them.

The Coast Guard will review the assessment to develop a “safety of navigation” opinion and associated mitigation measures that it will forward to the appropriate lead permitting agency.

Full article is available at http://www.uscg.mil/proceedings/spring2011/.

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