Installation of Monopiles for Offshore Wind Turbine Foundations
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It is now well accepted that the global warming, caused by society's dependence on fossil fuels, needs to be solved. In order to do so, and to cope with the increasing energy demand and the rising oil and gas prices, the world is turning towards renewable energy resources, with sustainable development policies being initiated across the Europe and spreading out beyond this region, rapidly increasing in China, Japan and the US. In the global energy landscape, wind energy appears as a clean alternative with an enormous potential onshore and offshore. Although the offshore sites offer a considerably higher productivity, many technical and economical challenges are encountered, among them a high cost of installation, accounting for approximately 20% of the total offshore wind capital cost. For the offshore wind to become competitive with other energy sources, further research is required in the manufacturing process, installation, and operations and maintenance procedures. Due to its rapid development in Europe, the offshore wind industry is moving toward larger turbines and deeper waters, imposing new challenges on the foundation structures. This thesis is focused on the monopile foundations and their installation by impact driving as they are, by far, the most common foundation concept in the offshore wind industry. The increased size of monopiles generates the need for larger pile driving hammers, with energies sufficient to drive the piles to their target depths, while avoiding any structural damage, limiting the fatigue of steel and keeping the noise levels in the allowable range. The years of experience on small diameter, long piles used in oil and gas industry cannot easily be extrapolated to account for the behaviour of large diameter monopiles nowadays, thus challenging the traditional empirical soil resistance models, that are the basis of any driveability analysis. The scope of this PhD work involves several topics regarding installation of monopiles; one of them dealing with pile driving in chalk. Experience of installations in chalk is generally low and can range from easy penetration to very hard driving and refusal. A back-analysis study of three wind farms located in the Southern North Sea identified the different characteristics of the material and indicated its complex behaviour that is difficult to predict based on a set of geotechnical parameters derived from a limited number of laboratory tests. The study demonstrated a large scatter in the results even at the same site, and revealed the challenges of correlating soil resistance in chalk directly to the CPT measurements or UCS values. Assumptions regarding low shaft resistance developing in low density chalk due to remoulding of the zone adjacent to the pile, were confirmed with dynamic pile testing on several piles at a nearby location. The presented work also identifies challenges associated with the uncertainties in the data available for the analysis. Another topic presented is a novel concept of large-diameter monopile installation, developed to reduce the noise impact and the fatigue damage during driving. The piles were easily installed using low energy blows delivered at higher frequency; the reduction of 30% to 60% in blow energy did not affect the pile set per minute. To investigate the effects of impact frequency on a possible temporary reduction in soil resistance that occurred during driving and subsequent bearing capacity, a field experiment was planned and executed at a sand site in Perth, WA. The experiment did not reveal major differences in the observed installation resistance between several installation methods (vibration, low- and high-frequency impact driving), however it revealed some un-expected results that sharply contrast the current understanding of significant gains in pile capacity with time. The inconsistency prompted the re-examination of factors influencing set-up, combining the data from that study with the existing pile test databases. It is shown that the number of installation blows or jacking increments has a critical effect on the `short term' shaft capacity; the `long term' shaft capacity is similar to the `short term' capacity for lower number of installation cycles. However, piles with a large number of installation blows evidently create significant disturbance to the sand around the pile shaft and the recovery from this disturbance (or ageing) can lead to `long term' capacities that are greater than the `short term' capacities of piles driven with a small number of blows. The disturbance caused by installation greatly affects pile's ageing effects and should be accounted for in the pile capacity estimations.