The aim of this study is to provide an environmental perspective regarding the choice of foundations for offshore windpower, suggesting that differences in environmental impact should be involved in decision-making and development concerning future offshore windpower foundations. The study concerns only the marine environment, excluding seabirds, and is based on the level of knowledge available in 2007.
The study focuses on three different types of foundations; gravity- monopile- and jacket foundations. Also tripod- bucket- and floating foundations are mentioned. The different characteristics of the foundations are discussed based on their environmental impact in five different areas; 1) epifouling and reef-effects, 2) operational noise, 3) changes in hydrographical conditions, 4) noise during construction, and 5) dissolved sediment during construction.
Regarding epifouling, it is noted that the surface texture of the foundation (i.e. steel, concrete) is of less importance in the long run since the initial substrate soon will be covered with organisms, creating a rugged surface for later colonising organisms. It is rather the level of salinity, distance to shore, exposure, depth and turbidity of the water that decide which organisms that will dominate the different foundations after a few years. Generally all foundations for offshore windpower are expected to be dominated by filtering animals, such as blue mussels. A possible exception is if concrete is coated with a silicone product that limits larger organisms to establish on the foundations. This kind of surface treatment has not yet been used by the windpower industry but occurs on other submarine concrete constructions.
The potential for an evident reef-effect (local increased occurrence of mobile animals such as fish and crustaceans) increases with the complexity of the foundation structure. Hence, tripod and especially jacket foundations have better possibility to contribute to the reef-effect than monopile- and gravity foundations.
Reef-effect, as well as epifouling, may be considered negative in some marine environments, such as possible valuable areas without any natural occurrence of hard substratum. In such areas new species may be introduced, changing the local ecological conditions. However, in many areas an increased level of biological diversity is viewed as a positive change, and here reef-effect and epifouling may be considered favourable. To amplify the reef-effect, scour protection devices may be designed to create more habitats.
Operational noise from offshore windfarms has been shown to initially affect some organisms (mussels, fish) during experimental studies in small containers. Whether corresponding operational noise in field and during natural circumstances can cause any environmental impacts is not yet fully understood. Available information indicates that there is a common sound level peak from wind turbines at frequencies of 100 – 200 Hz. In the same frequency range cargo ships emit higher sound than wind power even over several kilometres distance. Based on the present lack of certainty, it can be motivated to minimize the sound at these frequencies in areas with special biological values, such as endangered organisms sensitive to stress. However, there are no indications that operational noise may significantly affect the environment beyond the vicinity of each foundation.
Based on a limited number of measurements it seems as if gravity and monopile foundations emit noise of similar amplitude, but the frequency range of the gravity foundation is generally lower. There are no measurements of jacket foundation but theoretically these should emit less noise, at least within the lower frequency range. Even if little is known about future turbines and foundations, it should be technically possible to decrease the emitted noise level.
The local conditions of the seabed have a large impact on the propagation of the noise, where shallow water and hard substratum allow the sound to propagate longer distances. The background noise is also of importance and in quiet areas there is theoretically a higher risk of environmental impacts than in areas with heavy ship traffic.
Changes in the hydrographical conditions around a foundation are small and are expected to be of importance only in very narrow water passages. The gravity foundation probably has the largest impact on the local hydrography. However, no direct comparisons between the different foundations have been made.
During the construction period extreme noise levels may occur, especially during pile-driving which is needed for most foundations except for gravity foundations. The noise level depends on the diameter of the piles that are driven into the sediment as well as the piling method. This means that the monopile foundation generally emits higher construction noise levels than jackets, while gravity foundations emit the least construction noise.
Since the extreme noise levels from pile-driving, covering large areas, can be harmful to fish and marine mammals it is very important to minimize this disturbance. This can be done by the choice of foundation, by precautionary measures and by adapted methods of pile-driving. It is of great importance not to perform pile-driving during spawning periods of commercially valuable fish species.
Gravity foundations need no pile-driving but require dredging, which disperses dissolved sediment in the water. High concentration of dispersed sediment can disturb or harm sensitive marine organisms such as juvenile fish. The highest risk of negative impact on the environment is dredging calcareous sediments, dredging in stagnant water and where the sediment contains toxic substances. The impact on the environment from dredging can be minimized by precautionary measures and good planning. However, the impact of dredging and sediment transport related to offshore windpower is small compared to other large dredging projects that have been carried out in Sweden without any documented any sustained environmental impacts.
The result of this study is to be applied on local conditions (e.g. hydrography, bottom substrate and ecological circumstances) at every specific site, hereby indicating what type of foundation to prefer from an environmental point of view, and also to state what technical as well as planning adaptations that ought to be applied.