Addressing the challenges of offshore wind turbine installation and maintenance in high winds

Robust calculation model for offshore wind turbine cost reductions

Robust calculations

The cost of the wind turbine installation setup is an important factor in the quest to reduce the levelised cost of energy (LCOE). Research and development within new technologies is key to making installation quicker, more reliable and safer.

But in order to fully understand the implications of reducing installation setup costs, it is necessary to perform robust calculations. And these calculations must be based on correct assumptions and figures that are as accurate as possible.

Robust calculation models are important tools in the industry, according to Alun Roberts of BVG Associates.

He says, “Installation contractors need to be able to understand and explain the value they add by reducing installation times, not only for turbines but also for cables, substations and foundations.”

So how do you effectively measure the reduction in time? What calculation models can be used to determine the offshore wind turbine installation cycle?

One such model is applied in a recent investigation available on this site. The investigation considers four main parameters: metocean data, wind farm specification, installation vessel and the installation cycle. I’ll attempt to explain each of them briefly here:

Metocean data

Wind and waves are two of the most obvious constraining factors. It is therefore vital to consider them when planning installation or other lifting operations at an existing or prospective offshore wind farm.

Such metocean conditions can be modelled based on key data. The Crown Estate has an excellent resource for wind farm data, the Marine Data Exchange, which is well worth bookmarking.

Wind speed data and wave data can be analysed using a time-series installation algorithm. This wind and wave data can be used to produce wind and wave joint occurrences and assumptions made based on average wind speeds at defined hub heights that represent a likely wind farm construction. Multiple wind and wave series can then be generated.

For further considerations and calculations examples of metocean conditions, see Jelte Kymmell’s discussion.

Wind farm specification

This involves specifying a representative wind farm based on parameters such as number, capacity, blade length and hub height of the turbines.

You also need to consider the distance from installation port, wind farm lifetime, capacity factor and discount rate.

Installation vessel

As a point of comparison, it can be useful to make assumptions based on existing vessels or vessels soon to be launched. For example, you could base your assumptions on DEME’s Offshore Heavy Lift DP2 Jack-up Vessel Apollo.

Here, it is important to consider the number of turbines of a certain size that can be transported and lifted by the vessel as well as its transit speed, jacking limits and, of course, the day rate.

Installation cycle

The industry best practice for installation is based on a five-lift strategy. This covers the tower, nacelle and hub and the three blades.

Around these core activities, you need to consider the time required for mobilisation, loading, transit to and from the wind farm site, positioning, jacking up and down, lifting operations and demobilisation.

Assumptions need to be made about the time required for the activity, taking into account the limiting metocean conditions.

Applying the data

Such data can be used to calculate the advantages or disadvantages of various scenarios. For example, BVG Associates applies this model to calculate the reduction in installation time, particularly in high winds, when using an intelligent guidance tool.

The calculation model resulted in a highly interesting business case describing the advantages of installing offshore wind turbines in wind speeds of 16m/s compared with the industry standard of 11m/s.

Whatever modelling techniques are applied, it is vital that the assumptions are made for the right reasons, because the cost implications are potentially enormous.

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