Vessel Utilisation – Part 1: The analysis of on hire times


Marine operations often require multiple vessels which are assigned specific tasks to perform.  Often the tasks performed by the vessels are independent of each other (i.e. the vessels work alone) but are linked in such a way that one vessel is dependent on the other completing work.  In this series of posts we are going to consider cable lay and trenching operations using two assets.  We’ll also look at the consideration of the simulation results both in Mermaid and as part of a bespoke analysis outside Mermaid.

Analysis set up

The operations under consideration here involve the laying and trenching of 35 cables within an array of marine energy devices.  We have a number of ports which we work from and can seek shelter in, and we have a large (30+ years) metocean data set of wind and wave height which we will model the operations against.

Shown below is the map for this analysis, here we have a mobilisation port (Milford Haven), an operating base (Falmouth) and a shelter location (Hayle).  We have assumed that the cables are being laid for an array of devices at WaveHub and we have imported a chart for this location.


Two vessels are to be used, an “Installation Vessel”, which will perform the cable lay operations, and a “Trenching Vessel”, which will post lay trench.  Both vessels will mobilise at Pembroke Dock in Milford Haven and will operate from Falmouth.  Falmouth is set as the home port for both vessels and we have set the storm duration thresholds as shown below, meaning that:

  • For a storm of less than 1 hour the vessels remain at the offshore location, but cannot work;
  • For a storm of greater than 1 hour, but less than 6 hours, the vessels leave site and seek shelter in the nearest available location (in this case Hayle, on the north Cornwall coast);
  • For storms longer than 6 hours the vessels return to their home port.



We’re interested in maximising our vessel utilisation.  We know that our trenching vessel works independently of our cable lay vessel, but that it cannot trench a cable before it is laid.  This means we probably don’t want to take the two vessels on hire at the same time as the trencher will sit in port waiting for the cables to be laid, wasting money on day rate expenditure.  Therefore the question we want to answer is:

“When should we take the trenching vessel on hire, relative to the installation vessel?”

Before we can begin to answer this question we need to define the operations to be performed.  These are outlined below:

Overview Flow Diagram

Clearly we need to add a lot more detail to these operations before we can perform a meaningful simulation in Mermaid; we’ll look at this next time, including considering how we cope with different length cables.