Over the past 18 months there has been a growing interest among industry suppliers and developers for supersize monopile foundations - the so-called XL and even larger and heavier XXL types. But the jack-up and floating vessels that are capable of hoisting and handling monopiles that can weigh up to 1,500 tonnes remain in short supply and are already booked for projects for years to come.
Monopiles are by far the most popular offshore foundation type, with their ease of manufacture making them a cost-effective solution for turbine sizes up to about 5MW in water depths as deep as 30 metres. Until recently, most offshore wind experts expected the future for foundations of large turbines built far from shore in deeper waters to be dominated by steel jackets. A less prominent but significant role was also foreseen for concrete gravity-base and tripod designs.
Jacket restrictions
The reason for this expectation was the advantage jackets have over monopiles because of their reduced mass, derived from their open, wide-based spaceframe structure. However, this design feature turns the serial manufacture of jackets into a complex, time-consuming and therefore costly process, despite sustained industry efforts to rationalise processes by using modular design, advanced automation, and the use of cast nodes to eliminate complex welds and allow the introduction of robotised welding.
In contrast, the experience of manufacturing increasingly larger monopiles has been reported as very positive, raising confidence in the prospects for even bigger sizes. Installation experts for the latest 6MW turbines, fitted with 150-metre-plus rotor diameters and in water depths of up to 45 metres, are quoting outer diameters of up to 10 metres, with a total mass of the cylindrical pile and the transition piece of between 1,500 and 1,800 tonnes.
Despite such optimism, the dimensions and weight of the XL cylindrical piles already represent a formidable supply chain challenge in terms of adapting manufacturing processes to meet the shifting demand.
Equally challenging are the new installation requirements, including the application of much bigger and heavier next-generation handling and stowage equipment, supersize hydro-hammers and suitable vessels.
The anticipated advent of XXL-type monopiles for 6-7MW turbines and 40- to 60-metre water depths, poses even more exacting demands. In particular, the limited availability of installation vessels with sufficient crane-hoisting capacity is likely to create a bottleneck, even if only the smaller and lighter XL foundations enter the market.
The absence of competition among suppliers of such vessels is also predicted to have further negative consequences on project capital expenditure.
A comparative study by Dutch civil engineering contractor Ballast Nedam indicates that there are currently 56 vessels available for wind turbine foundation installation, excluding planned vessels. When the search is narrowed to vessels with a hoisting capacity of more than 400 tonnes, this number reduces to just 54. These figures exclude heavy-lift vessels destined for the oil and gas market with correspondingly high daily rates.
Newer vessels
According to Ballast Nedam, maximum specifications for monopiles traditionally include a total length of 90 metres, a pile mass of 1,500 tonnes, and a foundation height measured from the seabed of 60 metres. For far-shore monopile installation of weights between 1,000 and 1,500 tonnes, fabrication capacity is the main constraint and key limiting factor.
Analysing this industry segment, the specialists found that 13 vessels qualify for the 1,000-tonne category, a further seven for 1,200 tonnes, but only three vessels for the heaviest 1,500-tonne class. In short, there are only 10 vessels capable of meeting 'average' hoisting demands for XL monopiles, and only three that could manage the much heavier XXL types.
Ballast Nedam's floating heavy-lift vessel Svanen can hoist a maximum load of 8,700 tonnes. The (semi-standard) installation method for monopiles involves floating individual piles, with each of the open ends closed with a plug, by tug from a supply port to Svanen. The main crane picks a complete pile out of the water ('up-ending'), after which it is put into the installation frame for the actual pile ramming. Svanen is also capable of installing transition pieces but not nacelles.
Crane capacity
It is not surprising that first-generation installation vessels were not always designed in anticipation of future product scaling demands. The pioneering six-legged MPI Resolution self-propelled, jack-up vessel design was, for example, based on the outcome of a concept evaluation at the turn of the century, when offshore wind was still at an infant stage.
The MPI Resolution entered commercial service in 2004 with only a 300-tonne main crane, the design team taking the then state-of-the-art Vestas V80-2.0MW as its product reference. However, bigger turbines, notably the 3.6MW Siemens unit, soon started entering the market, and monopile foundation mass started to exceed existing crane capacities.
To address these new demands but without opting for a larger crane, a 500-tonne monopile up-ending tool and guidance frame was designed, and attached at the MPI Resolution' stern. With this advanced tool, individual piles stowed at the vessel deck could be horizontally moved into the device equipped with advanced hydraulic grippers and closing cylinders. In the next installation step, the pile is tilted from horizontal into vertical position, after which the grippers are loosened.
Gravity plus limited friction force allows pile-guidance support during controlled pile lowering into the seabed. The conversion of the MPI Resolution took six months to complete after contract award and commissioning, after which the vessel left the yard to install 60-metre long and 500-tonne monopiles at Barrow, in the UK.
Adaptations
MPI Resolution's existing 300-tonne crane was replaced with a custom-designed 600-tonne pedestal crane during 2010 and early 2011. This is an inherently complex and time-consuming process. One possible design measure is the incorporation of large, roundor square-shaped steel reinforcement sheets around the crane for strengthening the vessel's internal structure.
The fact that even most of the recently built installation vessels lack sufficient hoisting capacity to meet new XL and XXL demands is surprising. But, as the example of MPI Resolution demonstrates, the offshore industry is capable of adapting and devising alternative options when new developments occur. Besides expanding the reach of existing crane — or choosing a crane upgrade — there are further avenues to explore.
One option being investigated is to extend the length of the transition piece and thus reduce pile length at the same time. However, such an underwater connection requires divers and entails additional technical challenges. Another option is to use a split monopile, consisting of two halves joined by a slip joint comprising one half with an outer and the other with an inner matching coning surface. This technique also eliminates the traditional grout connection.
New offshore foundation challenges continue to be met by clever solutions, ranging from onshore and offshore transport to the actual installation process. But, given the paucity of suitable heavy-lift vessels, the industry will have to be particularly innovative if it is to continue down the supersize monopile route.
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