Feature ArticleDrilling Tools for Installation of Offshore Wind Foundations
More than 15 offshore European wind facilities with turbine ratings of 450 kilowatts to 4 megawatts exist today in very shallow waters of 5 to 15 meters. According to the European Wind Energy Association, during the first six months of 2012, 270 foundations (an increase of 141, or 109 percent, from the same period last year) were installed in 10 wind farms: Thornton Bank 2 in Belgium; Lincs, London Array, Sheringham Shoal, Gwynt y Môr and Teeside in the U.K.; Anholt and Avedore 2 in Denmark; and BARD Offshore 1 and Riffgat in Germany.
In general, three types of foundations are used to support offshore structures: gravity-based, monopile and jacket or template structures. Driven piles continue to dominate the German offshore wind turbine market, particularly steel pipes driven open-ended into the seafloor. They provide the most common form of North Sea and Baltic Sea offshore foundations.
Pile sizes for a standard conductor, ranging from 0.76 to 2.5 meters in diameter, are driven routinely for oil and gas platforms to depths of 100 meters or greater in a variety of geotechnical settings. In exceptional cases, piles 5.1 meters in diameter have been driven successfully in the North Sea. A 2007 review by Shell U.K. Ltd. (London, England) of its North Sea piling operations shows a trend for platforms designed since 1996 to employ mid-sized piles (0.660 to 2.134 meters in diameter, with 26 to 87 meters penetration), for which the rated axial compressive capacities fall between 14 and 100 meganewtons.
However, there are many situations where piles cannot be driven to full penetration without drilling-and-driving techniques. Pile-driving difficulties can occur due to the piles being dented or otherwise damaged during offshore handling, hammers performing poorly, soil conditions leading to harder driving than expected, or encountering rock layers or boulders. The American Petroleum Institute states that if no other provisions are included in the construction contract, pile-driving refusal is defined as the point where the driving resistance exceeds either 248 blows per 250 millimeters for 1.5 consecutive meters or 662 blows per 250 millimeters for 0.3 meters penetration. Therefore, if stiff clays are encountered during installation, bored piles are preferred, such as in the Great Lakes area of Canada where mobile rotary drilling machines were developed for installation.
To enable drilling in hard to soft soils for offshore foundation construction, BAUER Maschinen GmbH (Schrobenhausen, Germany) is developing the Dive Drill, which could be used to support pile-driving and alternative pile installation methods.
Calculating Sound Exposure Level
In some soil profiles (e.g., sensitive clays), the use of driven piles might not be advisable. Impact pile driving also causes strong impulsive underwater noise that is harmful to the marine environment, in particular to marine mammals. As part of the approval process for construction off German coasts, for instance, a noise limit of 160 decibels from a distance of 750 meters must be complied with for pile driving.
The researchers Rainer Matuschek and Klaus Betke have calculated the noise for pile driving as sound exposure level (SEL) with the following equation:
T1 and T2 are the arbitrary time boundaries of the sound event (i.e., the pile driving blow), and T0 is 1 second. Normally, the sound event between T1 and T2 is about 0.05 to 0.40 seconds. Reference sound pressure p0 is 1 micropascal.
Essentially, the SEL equation includes the square of the observed time-variable sound pressure p(t), takes the average over time T1 and T2, and divides by the squared reference sound pressure p02 (energy averaging).
It is important to point out that the driving noise depends not only on the intensity of the pile ramming but also on the averaging time and the pause between the pile driving blows. The single-event level of a sound pulse (pile driving blow) is therefore the level of a continuous sound of 1-second duration and same sound energy as the impulse.
Different measurements during pile driving of research platforms and foundation structures of offshore wind turbines in the North Sea and Baltic Sea give SEL values of 10 decibels higher than the 160-decibel limit. Therefore, BAUER Maschinen is developing drilling systems for alternative offshore pile installations.
BSD 3000 Drilling Tool
In the case of the successful installation of a 1-megawatt tidal energy turbine in the European Marine Energy Centre test area off the island of Eday in Orkney, Scotland, in July 2011, a monopile was chosen as the foundation structure due to the large loads. At a water depth of about 33 meters, the seafloor consisted of sedimentary rocks from the Silurian and Carboniferous periods that were generally sandstone and siltstone of medium strength (up to 150 micropascals) in alternating strata.
The turbine axis was located about 15 meters above the seafloor. The rotor blades had a diameter of 16 meters. The monopileís pipes had a 2-meter outer diameter with 60-to-90-millimeter wall thickness. A transition piece was welded at the pile tip. The connection with the rock strata was 11 meters long and had a diameter of 2.3 meters.
The monopile was installed by means of BAUER Maschinenís BSD 3000 drilling tool. The drilling rig consists of three main components: drilling template with legs, including leveling and weight plates, and the middle piece, including tubing clamp and clamp; a guide tube with casing shoe and internal consoles, and bars for supporting the drilling unit; and the drilling unit with a rotary drive, rag and pinion, drill pipe and the roller bit drilling head.
The drilling was performed with a conventional rotary drill with a full-face roller bit with heavy weights. Drill cuttings were transported by airlift. The air siphon tube ended just above the drill. The drilling took about 24 hours, and the drill pressure had to be adjusted continually due to the varying rock ground conditions. To continue this article please click here.
Dr. Giovanni Spagnoli gained his bachelorís and masterís in geological sciences and geotechnologies at the University of Milan Bicocca and received a Ph.D. in engineering geology from the RWTH Aachen University. He previously worked in the field of soil mechanics in Trevi, MARUM and Fugro before joining BAUER Maschinen GmbH as a product manager in the department of maritime technologies.
Leonhard Weixler studied mechanical engineering at the University of Applied Science in Augsburg, graduating in 1985. He started his career at BAUER Maschinen GmbH, where, until the end of 2010, he was responsible for mechanical design and development. Now, he is an executive director for the department of maritime technologies.
Stefan Finkenzeller has a degree in mechanical engineering from the University of Applied Science in Regensburg. In 2000, he started his career within BAUER Maschinen GmbH in the design department, specializing in drilling tools, kelly bars and special projects. He is now the head of design and development in the department of maritime technologies.