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Advanced Hybrid ROV For Marine Renewable Technologies
A System for Offshore Wind and Wave Power Installations

AUTHORS:


Joshua Elvander

Graham Hawkes



Marine renewable energy structures such as offshore wind turbines and marine hydrokinetic devices will require advances in subsea technologies for pre- and post-installation survey and maintenance activities.
Offshore renewable energy continues to gain viability. Installations in Europe have been online for several years, and activity in the United States is beginning to ramp up. Regulatory hurdles for the Cape Wind effort offshore Cape Cod, Massachusetts, and a proposed 14 percent increase in the federal budget for the U.S. Department of Energy’s Water Power Program are evidence that development of these resources in the U.S. will accelerate. Installation of marine renewable energy structures such as offshore wind turbines and marine hydrokinetic (MHK) devices are predicted for the mid-Atlantic and Northeast coasts. These will require a variety of visualization and monitoring equipment for contractors and authorities to survey the seafloor properly for initial installation, lay cable and hardware, and conduct post-installation monitoring and maintenance tasks.

With these demands come challenges for current subsea technology. For instance, shallow-water installation sites exhibiting high ocean currents and tides necessitate remote systems that can execute station-keeping in water flows up to 6 knots. In addition, traditional optical solutions such as divers or cameras are not feasible due to visibility, necessitating forward-looking sonars, laser imaging or similar tools.

ROVs have become a dominant tool for subsea operations in support of the marine renewables sector, continuing their extended presence in offshore oil and gas industries for the past 30 years. ROVs have limitations, however. They are generally limited to a speed of 2 knots and cannot execute station-keeping activities in higher currents. Areas of low visibility decrease the effectiveness of the primary feedback sensors, namely video cameras. This can hamper certain types of operations, decreasing the pace and increasing cost. In addition, ROVs are generally deployed off dedicated surface craft, increasing overall operations costs.

The AUV was initially used 10 years ago in the oil and gas market and gradually applied into similar markets. AUVs do not explicitly compete with existing ROV designs as they perform different tasks. ROVs are traditionally used to replace divers at depths or in environments not conducive to human operation.

The various classes of ROVs execute tasks ranging from simple observation or in-situ data collection to light work-class systems with electrically powered manipulators to heavy work-class systems requiring hydraulically driven end effectors. AUVs, by contrast, are used for broad-area survey. AUVs directly compete with sensor systems traditionally mounted on towed devices that provide data from sensors, such as sonars with an extremely high level of positional accuracy.

Bluefin Robotics (Quincy, Massachusetts) and its partner company Hawkes Ocean Technologies (HOT), in Point Richmond, California, have developed a hybrid ROV called the U-4000 to extend the complementary nature of ROV and AUV operations by increasing the speed capability and positional accuracy of a medium-scale ROV platform. This will allow it to operate in the highest currents experienced in the shallow waters of marine renewable technologies, implement forward-looking sonar systems that eliminate the issues with low-visibility water, and implement sophisticated navigation found heretofore only on AUVs.


Marine Renewables Physical Environment
There are a multitude of offshore marine renewable energy concepts at various stages of development, from the conceptual projects to multistage, long-term evaluations in the subsea environment. They can be generally divided into three categories: offshore wind turbines, MHK devices and offshore thermal energy converters (OTEC). The first two installations have historically occurred in waters of 20 meters or less, although there is a trend in offshore wind to drive to deeper water depths. OTEC technologies generally occur at a sea wall, where cool water from very deep locations (1,000 meters or more) is pumped to a shallow location for thermal energy conversion.

The very nature of MHK and offshore wind installations finds them located in environments where wind, waves, tides and currents are consistently energetic enough to make conversion into electricity both viable and, ideally, cost-effective. While beneficial from an energy standpoint, these locations can be extremely challenging from an installation and maintenance perspective.

Modern ROVs used for support cannot sustain station-keeping beyond approximately 2 knots, far below the currents of up to 6 knots observed in some candidate deployment areas. In addition, the strong winds and high tidal and ocean currents found in littoral waters favored by these technologies are often accompanied by extremely poor water clarity. The primary sensory feedback, video data, is obscured to the point of rendering divers or ROVs useless in certain conditions. This leads to costly delays while the support resources wait for favorable conditions to resume operations, despite support vessels being more than capable in surface environments.

Support activities for MHK and offshore wind installations can be grouped into four main categories: initial site survey, installation, cabling, and both final and maintenance surveys. These support operations require a variety of subsea assets on site to execute the installation and maintenance, ranging from specialized vessels to commercial divers to towed-survey sleds to ROVs. Such operations are often both time consuming and expensive. While MHK and wind turbine installations are relatively shallow, high currents can reduce the effectiveness of towed systems and ROVs, and reduced visibility can challenge the effectiveness of divers. Robotic systems, such as AUVs and innovative ROVs, can address these challenges by providing highly accurate survey capability and ROV station-keeping in high-current environments with forward-looking sonars such that the operational availability is significantly higher than has been observed to date, reducing overall installation costs and improving the cost-effectiveness of the system over its lifetime. To continue this article please click here.



Joshua Elvander has been a program manager at Bluefin Robotics for six years. In that time, he has led a variety of efforts to develop and deliver advanced AUV technologies to academic, military, government and commercial customers. Prior to joining Bluefin Robotics, he spent 10 years in aerospace working on advanced propulsion concepts.

Graham Hawkes, an internationally renowned ocean engineer, has been responsible for the design of a significant percentage of all manned (and more than 300 remote) underwater vehicles built for exploration, research and industry worldwide, including the Deep Flight series of winged submersibles. He has founded several companies and is currently chairman and chief technology officer of Hawkes Ocean Technologies.







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