Abstract
To a certain extent, adhesive bonding of measurement equipment is very common in science and technology, e.g. adhesive bonding of small-scale strain gauges. Adhesive bonding of the entire equipment for a fully autonomous pile driving monitoring of an impact-driven large-scale foundation structure for an offshore wind farm is a completely new application method. Several offshore wind farms are currently under construction in the North and Baltic Seas. Impact pile driving of the large-scale foundations usually causes much louder noise than permitted by regulations, so methods for noise reduction are necessary. Geotechnical engineers of the TU Braunschweig are investigating combined methods for reducing that noise, and in 2014 they had the opportunity to install measurement equipment for the investigation of dynamic pile deflections during pile driving into three of in total eighty monopiles (length: 60 m, diameter: 6 m) of an offshore wind farm in the German North Sea. Due to certification issues conventional methods of fastening such as screwing or welding were not permitted. Instead, adhesive bonding of all parts (sensors, cables, shielding, recorder/computer) was successfully applied and withstood impact driving with several thousand blows of up to 1200 g (earth gravity). The authors would like to present the concept and preceding tests of the adhesive bonding applied within the research project ‘triad’.
Highlights
The Charpy testing machine is recommended for impact testing in adhesive technology in addition to the originally intended testing method to provide material data for metals and plastics by direct destructive testing [3]
The impact tests on a compact specimen design to be mounted directly in a small Charpy test machine showed the well-known ability of elastic PUR adhesives for withstanding much higher impact loads than quasi-static loads
An approach on a similar specific energy application for a larger real mass bonded on a large area of the walls of a monopole, and a small specimen with low mass and small adhesive bond area was calculated
Summary
General technical approach After the basic decision of bonding all components to the monopiles, two main adhesive routes were implemented for the small sensors on the one hand and all other components like cables, protection profiles and recording computer on the other hand. Small sensors were applied by means of thin structural layers of adhesives for best coupling to the structure This method is state of the art for measuring strains and accelerations in laboratory scale and this is adaptable to larger scales. Were installed using thick layers of semi-structural adhesive on a maximized area to provide elastic bedding with excellent adhesion, high damping factor and low failure growth Those two routes of adhesive bonding obviously require completely different materials. The main strategy was to bond all heavy masses on a maximal surface area with a thick layer of soft semi-structural adhesive and break down large structures into smaller pieces to avoid flexural interactions while moving under the accelerations and carry out free movements on the relatively thick polymer beddings. The amount of the impact energy in relation to the impact energy calculated (in Table 1) and the area of adherent are shown in the following Table 2
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