Analysing Wind Farm Foundation Installation – Part 4: Improve Performance – Split Installation

In the last post in this series we identified that even with a larger carrying capacity, our work force was still unable to fully exploit the preferable summer weather and that all start options performed at least some work during the winter.  In this post we’re looking at splitting our installation process into two halves.

The Simulation

To see if we can gain a performance improvement by better exploiting the summer season we are going to install the foundations in two batches of 36.  We’re using the base case vessel, so only carrying one foundation set per trip from port and have modified the flow diagram accordingly, this is shown below.  The changes made are:

  • We’ve duplicated the foundation installation process so that we have two large groups. Each group is set to repeat 36 times.
  • We’ve modified the task prior to the second installation to be remobilisation of the vessel and this task has a start date and a dependency on the previous demobilisation. This start date is set exactly the same as the first but one year later.  Having the start date and the dependency means that the second installation will start at the later of:
    • The specified start;
    • The completion of the previous batch of foundation installations.

This means that if there is substantial delay (and none of sufficient magnitude is expected) the second mobilisation will be delayed, rather than work erroneously overlapping with the previous batch.

Flow Diagram Overview - Offhire

The Results

The project progress burn up chart is presented below.  The total duration is significantly longer than any seen previously due to the large pause in the middle of the project.  It’s worth noting that the winter starts will be adversely affected by this change as we’ll be off hiring the vessel during the summer, however this is not of concern since we are trying to better exploit the summer season by working only then from appropriate start dates.

Presented further down is the median installation progress for all three options we’ve considered so far.  Here we can see the improvement obtained by increasing the carrying capacity.  We can also see that the completion of the foundations with a split installation takes much longer, but that when we are working the gradient of this (grey) line is steep and therefore progress is fast.

Offhire Progress Burn Up

Base-Larger-Split Duration Comparison

Comparing based on durations is not a particularly valid approach due to the length forced pause.  A better consideration is the cost associated with the operation.  The figure below shows the expenditure due to the vessel day rate during the project, again these are median values for operations starting in March.  We can see that the base case is the most expensive option and that the increased carrying capacity is the cheapest.  In these cases we are using the same vessel more effectively due to the reduced transits.   The split installation represents an improvement on the base case due to not incurring cost during the winter period when we have off hired the vessel. This is a benefit and, if we can’t increase the carrying capacity, a way to reduce the cost of this operation.

Base-Larger-Split Cost Comparison

The Next Steps

This option reduces the cost incurred and reduces the risk associated with the operation by moving all the work to the summer months. However, there is a significant downside.  Using this approach significantly delays the completion of the last 36 foundations by deliberately delaying for a substantial period of time.  This delays the completion of the wind farm and delays the point at which revenue is generated.  Whilst this option is cheaper, with regard to capital expenditure, the delay to revenue may be unacceptable.  This is something that can be determined through further cost modelling, with the Mermaid outputs forming a key part of this process.  We are going to assume that delaying the completion of the installation so much is not acceptable and discard this option.

Previously we saw the benefit of maximising the working time by reducing the transits. If we could remove almost all transit we may be able to improve our performance further; we’ll take a look at this next time.