Standing Up to the Perfect StormLarry Maloney | February 04, 2016
Most vessels aren’t built to survive the confluence of powerful weather systems described in Sebastian Junger’s The Perfect Storm. But a unique testing lab at the University of Maine promises to go a long way toward helping engineers design more rugged ships, wind turbines, offshore oil rigs and other marine structures.
Dedicated in November 2015, the Harold Alfond W2 Ocean Engineering Lab features a test basin nearly l00 feet long and 30 feet wide, equipped with a massive mobile wind tunnel and wave generator. Place a 1/50th scale model of a wind turbine or merchant ship in the basin, and engineers can simulate winds of more than 200 mph and waves as high as 115 feet.
And that’s not all. Later in 2016 the lab will add more testing capability, including a tow carriage to simulate moving vessels and an adjustable floor that will allow a greater variety of tests.
The advantage for companies in all this: A faster, less expensive route to design validation and prevention of late-stage development problems that could cost tens of millions of dollars. “This is a state-of-the-art facility that UMaine has built from scratch,” says Neal Brown of San Diego, Calif.-based Float, Inc., which tested a scale model of its wave energy converter at the W2 lab in mid-January. “It’s the most advanced wave test basin that I have ever seen.”
Filling a Global Need
The lab is the latest in an expanding array of test facilities at UMaine’s Advanced Structures and Composites Center. Since its founding in 1996, the center has expanded into a 100,000-sq. ft. laboratory that evaluates materials and structures for companies worldwide. Industries served range from aerospace and defense to civil infrastructure, nanocomposites and ocean engineering. Development of the new W2 lab stems in part from the center’s frustration with the lack of suitable test basins for validating its own design work in renewable energy systems, says Habib Dagher, a structural engineering professor and executive director of the UMaine Composites Center.
“As we began looking around six years ago, we couldn’t find any facility in the U.S. that could do both wave and quality wind testing that we required for floating turbine model testing,” says Dagher, a principal investigator in the government-funded DeepCwind research consortium for offshore wind energy development. Dagher’s UMaine team moved much of its floating wind turbine testing to the Maritime Research Institute Netherlands (MARIN), where the Maine engineers helped with the design of a new wind generator for that facility.
It didn’t take long, however, before Dagher’s engineers were yearning for a state-of-the art test facility closer to home. The solution: Build a world-class facility at UMaine. Besides drawing on their experience at MARIN and visits to other test basins, the Composites Center surveyed prospective clients to gauge their interest in a new ocean engineering lab with full wind and wave generator capability. “These were firms in such fields as oil and gas, shipbuilding and renewable energy, and the answer we got back was very positive,” says Dagher.
Anatomy of a Wind-Wave Basin
Designing a truly “holistic” test environment for ocean engineering involved a range of strategies -- from computer simulation and modeling to scale-model testing in a 15-ft.-long prototype of the lab. “What is unique about this effort is that much of the major equipment we needed didn’t exist,” says Dagher. “Other than the wave generator, we had to design and build everything ourselves.”
That work involved designing a wind-making assembly that features 32 individually controlled fans, stacked four high and eight wide, which combine to produce wind flows as high as 28 mph at a flow direction relative to waves of 0° to 180°. That translates to a maximum wind speed exceeding 200 mph for a 1/50th scale model under test.
The movable wind machine design lets researchers change wind direction and wind speeds with height, just as occurs in the natural ocean environment where wind speeds are lower near the surface and higher farther above it. In addition, the wind machine can rotate so that wind can be directed toward a scale model from different directions. Currently, a crane moves the fan assembly around the basin. The UMaine team is evaluating whether to use PLC control to automate the movement of the wind assembly, including possible use of a rail system above the water surface.
“Being able to move the wind tunnel lets you simulate unusual squall and storm events, where winds can frequently change direction,” says Anthony Viselli, a structural engineer who manages the W2 lab. “Think of a hurricane where the wind swirls, then reverses direction as it crosses the eye.”
Creating the wave action in the basin is a 16-paddle system supplied by UK-based Edinburgh Designs. This assembly can create waves up to 2 foot 4 inches high and at varying periods and angles. For a 1/50th scale model, this allows simulation of waves as high as 115 feet. Built-in software includes typical spectra associated with offshore wave modelling, as well as the ability to generate directional waves, waves moving at angles off the sides of the tank, or even random events like rogue waves.
To absorb the waves, the UMaine engineers, led by mechanical engineering professor Krish Thiagarajan, designed a “beach” at the end of the tank opposite the wave maker. It consists of a smooth, parabolic-shaped composite structure with aluminum ribs. While CFD software played a key role in the beach’s design, Viselli emphasized the importance of validating computer modeling with data from a scale model of the structure tested in a 15-foot tank. The same was true for the wind machine, where CFD modeling was followed by construction of a prototype one-third the size of the final design.
Together, the basin’s wind and wave generation capabilities allow the W2 lab to simulate some of the harshest ocean environments on earth. But the facility is planning still more features. Coming on line later in 2016 will be the ability to vary water depth in the tank from a few inches to 16 feet via adjustable concrete slabs, as well as installation of an overhead towing carriage.
Andrew Goupee, a mechanical engineering professor involved in designing these new features, says that the movable floor consists of floating concrete slabs with foam on the underside and connected together into one large unit. Winches pull the floor down to achieve varying water depths.
“This allows us to tailor tank depth based on a client’s particular application,” says Goupee. “Oil and gas platforms, offshore wind turbines and aqua-culture fish pens all call for different depth requirements, which can affect the wave behavior and mooring systems.”
The overhead motorized towing carriage will target applications ranging from high-speed vessels to slower, large displacement hulls. In the preliminary specs, the system calls for a maximum towing speed in the tank of 16.4 ft/s for models measuring up to 13 feet long and 13 feet wide with a draft of 2 feet. In addition to testing vessel hydrodynamics, the towing function also can simulate ocean currents and their impact on scale models of oil and gas platforms and other ocean engineering structures.
Each of the W2 lab systems – wind machine, wave generator, movable floor and towing carriage – has its own individual control system. In addition, the UMaine team is designing a master PLC to ensure smooth, simultaneous control of the total test operation. “This is critical, both from a safety standpoint and to prevent one system from interfering with another,” says Dagher. “For example, if you’re doing a towing test of a boat model with a certain draft, you need to make sure that the control system for the movable floor does not exceed that draft. Also in a tow test, this master controller can ensure that the wind does not affect the particular wave spectrum you are measuring. All these pieces interact.”
From Ships to Wind Turbines
As Dagher sees it, the new lab will serve potential clients that include ship and pleasure boat builders, offshore oil and gas operators, designers of ocean energy systems, aquaculture firms, and builders of such civil infrastructure as piers, bridges and breakwater systems. Add to this mix, academic researchers investigating such issues as rising ocean levels and impacts on coastal areas.
Depending on the client’s application, tests can last from one to three weeks, with costs ranging from $10,000 to $250,000 or more. Clients can supply their own scale models, but the Composites Center offers full model-building capability, based on client specs, including 3D printing equipment to produce large-scale components. Fabrication services at the Composites Center will expand this summer with the opening of the robot-equipped Alfond Advanced Manufacturing Lab. Together, the W2 lab and this new manufacturing facility represent a $13.8 million investment. Staffing these operations are 50 engineers and technicians, plus 130 students.
Constructing scale models with hydrodynamic properties similar to an actual ocean engineering structure, such as a ship, involves the use of Froude scaling laws, which ensures that the loads and subsequent motions are properly scaled to generate meaningful results.
“You are building these models out of materials, such as plastics and foam, that are not the same as the end product,” says Viselli. “But they still need to embody the geometry, mass and weight distribution properties of the full-scale design.” Plus, they must be strong enough to stand up to the tank tests. The result, Viselli says, is “quite a design and manufacturing challenge.”
In a typical model test, client companies provide the lab with test specifications. Test instruments may include:
- Load cells, either placed on the model itself or on the mooring lines that anchor the model to the bottom of the tank.
- Accelerometers to measure the model’s movement in six degrees of freedom as it is hit by wind and waves.
- Wave probes, which can be placed anywhere in the tank, to measure water elevation.
- Anemometers placed near the model to characterize the wind.
- A Qualisys 3D motion-capture camera system, similar to those used to create video games or analyze athletic movements. To determine motion and acceleration, reflective dots are placed on the scale models, and the cameras track how the dots move down to a fraction of a millimeter.
In addition, the team plans to install particle image velocimetry (PIV) equipment to map the wind environment above the waves, as well as water speed. Together, these instruments give clients a realistic picture of how structures will perform in virtually any ocean environment.
Testing the Waters
The first companies to use the new facility were renewable energy firms testing new wave energy converters (WEC) as part of the U.S. Energy Department’s Wave Energy Prize program. This design-build-test competition aims at doubling the output of current WEC systems while keeping material costs in check. The top three competitive teams will take home $2.25 million in prize money to invest in their technology.
“The W2 facility is world-class,” says John Rohrer, whose RTI Wave Power Team of York, Maine, used the basin for testing its WEC in November. “It has the water depth and accurate digitally controlled wave making paddles to produce virtually any realistic ocean state.”
Rohrer says that relying purely on CFD software to model wave energy converters would be difficult. “In WECs, you need to take account of the waves, plus two bodies moving relative to year other, each of them in six degrees of freedom. You can develop a computer characterization of your device, but you can’t calibrate it without putting the device in the water.”
Tim Mundon of Seattle, Wash.-based Oscilla Power, another Wave Energy Prize team, agrees that tank tests can help provide a definitive measure of how a device will perform, while validating numerical data from computer modeling. In the case of his firm’s two-body WEC, the tests in the W2 lab provided a close match to Oscilla’s power and damping predictions for the system.
UMaine engineers also cite similar benefits from scale model testing of their own renewable energy projects, such as the VolturnUS floating wind turbine platform. “Without model testing, we would have spent a lot more money and a lot more development time,” says Goupee.
The bottom line: While computer simulation plays an important role, physical model testing is essential, especially in complex ocean engineering structures that must operate in demanding, ever-changing natural environments.
“Model testing is an established way of doing business in ocean engineering,” says Dagher. “Whether you’re a large company developing a multi-million-dollar offshore oil platform or an entrepreneur designing a renewable energy device, there is no substitute for this kind of testing. You need tank tests to validate a computer model.”
For More Information
University of Maine Advanced Structures and Composites Center: http://composites.umaine.edu/
W2 Ocean Engineering Laboratory specifications: http://composites.umaine.edu/product-development-and-testing/equipment-and-facilities/w2/specifications/
UMaine Ocean Engineering Lab Model Wind Turbine Basin Testing video: https://www.youtube.com/watch?v=ZDLPzRwKZCU
Dr. Andrew Goupee’s video discussion of model testing of wind turbines: https://www.youtube.com/watch?v=lWKRCovJpvU
UMaine wave generation video: https://www.youtube.com/watch?v=YkyIo_fXawU
DeepCwind Consortium: http://composites.umaine.edu/our-research/offshore-wind/deepcwind-consortium/
DOE Wave Energy Prize: http://waveenergyprize.org/
Wave generators from Edinburgh Designs: http://www.edesign.co.uk/
Harold Alfond Foundation: http://www.haroldalfondfoundation.org/
Wind Testing at MARIN: https://www.youtube.com/watch?v=GnFDrX4unf8
Oscilla Power Triton WEC information and video: http://oscillapower.com/wave/
Qualisys motion tracking system for marine applications:
RTI Wave Power: http://waveenergyprize.org/teams/rti-wave-power