Clearing up blockages in slurry transport pipelines

Blockages in long pipelines

Computer simulations have been used to study the behaviour of slurry in long pipelines and the formation of blockages. These obstructive and slowing blockages can extend up to 500 metres. They don’t only occur with sand slurry from hopper dredgers but also with much coarser sediment from cutter suction dredgers, as well as with (wet) mining slurry, involving the pumping of coarse minerals. De Hoog states: “In all cases, the risk of blockages increases as the pipeline gets longer.”  

Blockage effects

During his doctoral research, De Hoog focused on two different clogging effects. “The first, already well known in the dredging industry, involves the erosion of a stationary sand layer in a horizontal pipe. It becomes silted up at too low a velocity. This process has never been fully analysed but causes instabilities in the flow.”

The second clogging effect was investigated during an experiment in a 130-meter-deep old mine shaft in East Germany. “There, we suspended a vertical pipeline circuit. It turned out that, despite the continuous movement of the slurry (which contained up to 20% sand), an increasingly thick accumulation formed. Compared to horizontal slurry transport, this was a fundamentally different symptom. We had never seen anything like it, and neither had many of the academic experts we spoke to at a conference shortly afterwards.” 

“With these two outcomes – and thanks to a variety of experiments conducted by the two other graduates –  I was able to create models for the many different aspects of a simulation, making them increasingly realistic and accurate.”

Solutions on the horizon

De Hoog's research has provided sufficient new knowledge and perspectives to realistically work towards practical solutions for tackling pipeline blockages. De Hoog’s is firmly convinced that these solutions are not only highly feasible but also bring significant benefits to the dredging industry. “The more sediment in the pipeline, the higher the production and the more efficient the process, but with an increased risk of blockages.”

Monitoring and adjustable pumps

“My research shows that blockages during pumping are strongly related to the conventional steady-state pipeline design method, in which designers assume the transport process remains stable. However, if more sediment needs to be transported with a lower water content, instability comes creeping in. The solution primarily lies in adjustable pumps and smart slurry monitoring.”

Intelligent pumping 

Longer pipelines require multiple boosters, especially as particle sizes increase. Furthermore, pumps need a controller to adjust the power in real time. This controller increases output when necessary to prevent a drop in speed. This way, the slurry's velocity remains constant, stabilising the pumping process as well as the flow. You only need a single sensor for this—usually placed at the first pump—regardless of pipeline length.” 

 

Less water, more sediment

“To get the water concentration down and, consequently, increase the concentration of sediment, the cutter suction dredger must be able to dig more. Redundant cutting capacity can be used to get more sediment into the pipeline. On the other hand, a trailing suction hopper dredger offers the great luxury of being able to work in the hopper by fluidising less with jet water. This saves costs.” 

Many tangible benefits

Better monitoring of the slurry and adjustable pumps result in lower costs per cubic metre of transported slurry, without any production loss—in other words, higher production at the same cost.

• Less water is needed to jet and fluidise the sediment in the hopper.
• In some cases, a smaller pipeline can be used while transporting the same amount of material at a lower velocity (due to higher concentration).
• This allows for the use of smaller pumps. A new pump design is not required.
• Lower velocity = reduced energy consumption and wear.
• Smaller pipelines and pumps, along with reduced jet pump power, result in lower costs and less onboard weight. This makes the dredging vessel both cheaper to build and more cost-effective to operate.

Pump test circuit

Both in the field and in the laboratory, extensive physical testing has been carried out, including in the relatively new pump test circuit in Kinderdijk. “I built an experiment in our laboratory to measure the cumulative effects using very fast sensors. This provides valuable data to validate the new design methods. I am now working on a new version of the simulation model, which allows for further modelling of the pump and controller behaviour.” 

The next step: practical tests

“I am convinced that the simulation results produced to date accurately reflect real-world physical situations. However, the market demands tangible results.” To substantiate his research findings with practical tests, a cutter suction dredger and a hopper dredger would be particularly welcome. “There are upcoming opportunities for practical testing within Royal IHC. If dredging companies are interested in participating in field tests, I would be happy to hear from them.” 

Conclusion

Edwin de Hoog's doctoral research shows that blockages in slurry transport pipelines aren't necessarily inevitable. With improved monitoring and adjustable pumps, blockages can be prevented, leading to more efficient and cost-effective slurry transport. Initial lab test results are promising. The challenge now lies in practical validation, for which collaboration with the industry is crucial. “The technology is there; now it’s time to put it into practice.”


The image at the top of this article was generated using AI.

Predicting density waves

Edwin de Hoog has used the 1D-Driftflux-CFD modelling technique as a basis. This technique enables the modelling of solid materials and particles as a liquid and allows for the calculation of velocity differences. This makes it possible to predict the amplification of density waves. Thanks to the experiments conducted by Oscar van der Ven, now employed at Boskalis, and Tjalie van de Voort, working at Van Oord, this model could be developed. Unlike traditional 2D or 3D CFD simulations, which may take hours or even days to complete, the 1D method delivers results within minutes. This provides significant advantages for quickly analysing different scenarios and optimising design choices.