In dredging fuel is one of the major cost drivers. It can be as much as 30-40% of the weekly cost of operating a dredger. Luckily it is also a cost that an operator can greatly influence by dredging as power efficient as possible.
Now a fair warning, this topic is complicated, but I tried to simplify it as much as possible. Please bear with me, as the benefits of this approach are great.
The lowest fuel consumption is when the dredger is lying idle, but of course then the project will never be completed. So we don’t want to focus purely on the fuel consumption itself, it has to be viewed against the production of the vessel. What we want is to lower the unit rate, the cost per cubic meter dredged, see this Knowledge Update on unit rate for more details.
So we want to lower the fuel consumption, but the same production. How can this be done?
Production is a combination of the amount of mixture discharged and the amount of sand in the mixture, the concentration. Consider the following two examples.
We pump 10 m3 of mixture with a concentration of 20%.
The production is 10 x 20% = 2 m3 of sand.
We pump 5 m3 of mixture with a concentration of 40%.
The production is 5 x 40% = 2 m3 of sand.
However, due to how fluid dynamics work the amount of pump power required to pump 5 m3 is much lower than to pump 10 m3, even when taking the higher concentration into account. And less power used results in less fuel used, exactly what we want.
So Example 2 gives the lower unit rate, but how can you dredge in a way to actually achieve this situation.
The trick is to use the flexibility of the pump revolutions. Now, the amount of flexibility depends on your specific dredger, but diesel driven pumps can generally vary from roughly 80-100% of rpm and modern electric driven pumps can even go from 0-100%. This is one of the reasons an electric driven pump is preferred over diesel driven ones, but that is a different topic.
For pumping it is true that the lower the pump rpm the lower the mixture flow. Flow is the volume of mixture pumped per second, so m3/s. So by lowering the pump rpm the flow automatically shifts from Example 1 to Example 2. Now the question is, will the concentration increase because of this? Sometimes it does, and sometimes it doesn’t.
Usually the cutterhead or draghead is producing a certain amount of cubic meters, which is not affected by the pumping in a major way. So when you lower the flow, the concentration will automatically increase, simply because the sand is there. Unfortunately, there are some situations in which this rule doesn’t apply. So you always have to check the current production. Only a production meter or survey can give the final answer to that question.
A word of caution. There is always the risk of clogging the pipeline with sand, which is one reason why operators tend to use max pump rpm. But a professional dredge operator should realize that lowering the pump revolutions as much as possible is beneficial to the performance of the dredger.
As a rule of thumb, lower the pump speed to just above the speed where the pipeline gets clogged. From there check the production and see if the production increases with a higher pump speed. If not, go back to the lower pump speed.
I challenge you to try this approach on your project and surprise everyone with the fuel savings you have achieved.
As I said earlier this topic is a bit more complicated, so I can imagine you have some questions. So feel free to ask them in below comment box and I will answer them.
To Long Term Great Results,