Buoyancy Supported Offshore Wind Energy
By Christopher D. Barry, P.E.

There is a significant energy resource in the form of wind offshore of much of the United States. For example, a recent study done at Stanford by Dvorak, Jacobson and Archer suggests that there is an exploitable wind resource of up to 200 TWh off the coast of California. Unfortunately, 90% of it is in waters more than 50 meters deep, so a lot of this energy is not regarded as economically viable with the current monopile tower technology.

Floating Turbine Prototype floating in Brindisi harbour. Blue H Technologies BV

In general, the offshore oil industry has found that cost of a bottom founded platform is a function of depth cubed so the cutoff depth is a function of the value of the energy asset (whether it's oil, gas or wind). This is cubic function is because if the water depth doubles, the platform height (from the bottom to the water line) doubles, so the amount of steel in it doubles. Then if the platform doubles in height, the base dimensions have to increase, roughly linearly, because the overturning arm on the base increases at least linearly. (The weight the lower section supports also increases as well, which adds some more steel cost.) As the platform gets larger, the wave forces also get larger, and the overturning force gets larger. The wave forces are largest in near surface zone, so they don't increase as much all the way down, but at the end of the day, all this tends to a rough depth cubed relationship, especially for Pacific platform, which are also subject to seismic loads. Seismic loads don't fall off with depth the way wave loads do because in an earthquake, the base of the platform jerks sideways and the inertia ("added mass") of the water surrounding the platform tries to resist the movement of the rest of the tower.

Fortunately, the offshore oil faced this problem in the late 80's, especially when the price of oil fell dramatically, and developed a number of options to economically exploit small oil fields in deep water. Most of these concepts are even more applicable to wind power, because another critical problem of many oil platforms is the high payload weight (which, even worse, can vary substantially during operation) needed to support equipment to condition and control reservoir fluids. Wind turbines do not represent such high payloads, and they don't vary, so many of the concepts developed for offshore oil can be simplified.

Floating Wind Turbine Prototype on quay with counterweight along side. Blue H Technologies BV

Floating platforms represent the most straight forward solution. The basic concept behind a floating system is that most of the wave load comes from the water plane, where the body pierces the water surface, but the buoyancy can be anywhere, so the motions of a platform due to waves can be reduced by having most of the body well submerged with only small members piercing the surface. To be a bit more specific, the natural period of a floating body, and hence its response to wave energy increases as the mass of the body (including the “added mass” of the water closely surrounding it, which moves with it), and decreases with larger waterline. In addition, the acceleration of the fluid in the waves produces a force in a submerged body opposite to the rise of the wave – under the crest of a wave there is a downward component of force, and under the trough, an upward force. As a result, a body with a relatively small waterplane and larger submerged bodies has a range of wave periods where the net heave (vertical) forces are minimal or even zero. This gives us the two basic forms for fully buoyant platforms, either a semi-submersible or a spar.

A semi-submersible has two or more submerged hulls connected to the platform with multiple columns. One common configuration is a tripod of slender cylindrical vertical columns with much larger diameter shallow sections, “cat food cans”, at the bottom. This configuration is now rare for offshore oil systems, because it has a limited payload, but it is probably well suited for wind turbines. The spread of the columns and their diameter produce the stability to resist overturning forces from the wind, and the volume of the “cat food cans” provides the buoyancy to support the platform and the mass to increase the natural period. It is worth noting that the shallow shape produces a large added mass as well – the water above and below the can moves with vertical motions of the cans, so it increases the effective mass with requiring a larger body. The shape also provides damping, which further reduces motions. The design of this type of platform is an optimization of required buoyancy and stability, while tuning the ratio of column diameter to can shape to a period that minimizes wave motion in the typical wave environment on site, but the process is well understood.

Another alternative is a spar. This is a long single cylinder, ballasted at the bottom. The cylinder diameter generally increases toward the bottom, again to provide buoyancy without adding waterplane, which would cause increased wave forces. In this case, stability comes mainly from the ballast very low down. With the very small waterplane a lot of very low ballast is required to keep the spar vertical, so the volume of the spar is much higher than required to just support the payload, but on the other hand, the structure is strong and simple – no framing is required to interconnect columns, for example and some promoters are considering reinforced concrete instead of steel.

In either case, the mooring system must also be considered. In general, floating systems, especially those in fields of multiple turbines will require multi-legged catenary mooring systems, and even then, the platform will still move around somewhat. The mooring system and its installation also add to costs. Floating systems will generally have more motions than bottom fixed systems, and there have been some concerns about interactions between the platform motions and turbine and rotor dynamics. Some researchers are looking into specially designed turbines that can take increased forces from relative motions of the platform and rotor.

There are also three essentially “hybrid concepts”, guyed towers, buoyancy supported towers, and tension leg platforms. These generally have much reduced motions, though probably at higher costs.

Some of the problems of bottom founded towers, especially with relatively light payloads, can be addressed simply by taking up the overturning forces with guy wires, also connected to the bottom. A number of guyed towers are in service now. The amount of steel they require still goes up faster than depth increases, but they may represent a solution for specific sites, especially in intermediate depths. One advantage of guyed towers is that the relatively slender tower produces minimal wave loading. These towers are also frequently designed as relatively light truss work instead of a large diameter tube, which may reduce production costs.

Another possibility is a buoyancy supported tower. In this case, the tower structure only carries a portion of the total weight, and one or more fully submerged buoyant hulls near the surface provide the remainder of the lift. The couple between the bottom foundation and the buoyancy also produces large stability to resist overturning forces. Buoyant towers are often designed with flexible links to the bottom foundation. This is especially useful for seismic loads.

Taking a buoyant tower to the extreme produces a tension leg platform. In this case, the total buoyancy is greater that the weight of the system, and flexible tendons, in some cases cable, pull the platform down to a heavy foundation, usually a large concrete box, set on the bottom. The tension in the tendons produces excellent stability and reduced motions. This can even provide enough stability to allow a structurally simple spar configuration, but with minimal ballast. The tension leg platform is installed by floating out the platform and foundation separately. The foundation is flooded so it sinks, the platform is connected to it, and the tendons are tensioned. The tendons, the heavy foundation (including separate installation processes) and the excess of buoyancy over weight represent increased costs compared to fully floating systems, but the reduction in volume required for stability may balance this out. Again, the specifics of the site, including water depth, bottom conditions, available wind resources and the normal and extreme wave environment will dictate the applicability of this concept.

Another consideration for offshore wind is extreme wave height. It would probably be unfortunate for a moving turbine blade to be struck by a wave. In most cases, extreme waves will be associated with severe wind, such that the blades will be stopped, but occasionally, large swells can form from a distant storm. These will be long period waves, so a floating platform with a long period will be able rise and pass safely over these swells, but a bottom fixed system will not.

Finally, the power generated must come ashore. This poses two issues, cable dynamics in the case of a floating platform, and power. A flexible power cable is required, and it will subject to motions from the platform at the top end, as well as effects from ocean current throughout its length, so it has to resist fatigue damage for twenty years or more, as well as marine environmental effects such as corrosion and marine growth. The technology for flexible oil hoses used in some offshore systems is available for power export cable design, but it is likely that some research effort will be required to adapt these components for the high current and voltage requirements of export power cables. Cables with alternating current will also produce severe inductive losses if immersed in a conductive medium, so it is likely that high voltage, direct current will be required for transmission. The equipment to produce this current will have to be in each individual platform, which increases complexity of the platform, but also provides opportunities. Instead of a single large generator, offshore wind systems have been proposed with multiple smaller generators driven by a bull gear off the rotor. These would generate AC and each one would have its own step up transformer and rectifier to produce high voltage DC which would then be combined. This might reduce cost and dynamic stresses in the drive train, and would produce redundancy and easier maintenance (an especially important consideration offshore).

Developers around the world are working on various applications of these concepts for offshore wind energy. Some notable programs include Norske Hydro efforts on spar systems, significant research on the dynamics of various floating systems at MIT under Professor Paul Sclavounos, and a semi submersible platform from Marine Innovation and Technology in Berkeley, California. ( http://www.marineitech.com/index.php?option=com_content&view=article&id=46&Itemid=82 ) Blue H Group, in the Netherlands, has a set a tension leg wind turbine test platform in 108 meters of water off Italy . http://www.bluehgroup.com/company-newsandpress-0712062.php

It is clear that there is a significant energy resource in offshore relatively deep water wind sites, and thanks to this accidental synergy between offshore oil and renewable energy that there are many viable concepts to exploit these resources.

Contact Chris Barry, cdbarry12@hotmail.com