Power dams – Discover Dredging

CEDA Industry News: Snap Back Story out of the Old Stone Box

Cutter wheel dredge ‘Sylvia’ at work on the TIWAG Langkampfen power dam reservoir

You might have read the CEDA Industry News1 publication of September and the LinkedIn announcement2. Or, have been present at Session 10, The Story of Safety3 of the CEDA Dredging Days 2024, where I initially recounted my story of misfortune. For the uninitiated, I’ll shortly recount it over here, although the focus will be more on the design problem back than.

Me telling this story at the Safety Session of the CEDA Dredging Days 2024 (Credit: CEDA)

When I was fresh out of university, working for my previous employer, I was assigned to advise during the commissioning of a bucket wheel dredge in Austria. Here at my website, I’ve already mentioned a very peculiar problem we had with starting the dredge on Monday morning4. The whole dredge was a funny contraption, trying to fulfil the specific requirements set by the customer. The task of the dredge was to clean out the sediment caught in a silt trap5 in front of a power dam to prevent the material flushing through the delicate turbines. Furthermore, the fine sediment should be removed from the system to be sold for beneficial use, but the larger boulders that came along should be rejected from the system. Unless they were very large and stay in place for forming the liner of the silt trap.

General modes of siltation at the usual location in a reservoir

The proposed method was to use a bucket wheel to gobble up the larger stones, but leave the even bigger ones down at the pit floor. Additional advantage of the bucket wheel is that they tend to have a very low resuspension rate which is beneficial for reducing the turbidity in the river and save the dam turbines. However, the larger stones taken by the bucket wheel will not pass the dredge pump. A proven solution to filter out stones for the dredge pump is a so called stone collection box. They come in various executions, but this one was a large cylindrical vessel with a cover on top. Inside was a cage that connected to the inlet and outlet of the vessel. When the cage was filled, the lid was lifted by the crane and the attached cage came out. The cage was swung over to a barge alongside and the stones were unloaded through a trap door. Sounds straightforward, right?

General arrangement of the ladder and suction system on bucket wheel dredge ‘Sylvia’

The thing is, when you have anything in the suction pipe section, it will create a pressure loss and the pressure in front of the dredge pump becomes lower. Or higher vacuum, as you wish. This is already a problem for an inboard pump when you are working at sea level, even more so when working at altitude. The NPSHr is easily reached. In order to have enough NPSHa, even when the stone collecting box was filling up, it was decided to have a submerged dredge pump for increasing the pre-pressure. Consequently, the stone collecting box would also be placed on the ladder, in front of the dredge pump.

Explanation on NPSHr and NPSHa in mountains and submerged, including stone box

All in itself no problem. The disadvantage is that a submerged stone collecting box should be very strong, big and heavy to withstand the under pressure, although there is very little room around the ladder, the ladder itself becomes quite heavy and bulky and the submerged frontal area increased dramatically, increasing the total drag of the dredge in the fast flowing river current. To mitigate this, the ladder winch, the service crane and the 25 ton forward wire winch were all stacked over each other, so each could do it’s job independently. Although none of us ever did a collision check in the design phase of the dredge. In actual operation, they all were in each others way. When lifting the cover of the stone collecting box, it got caught by the forward wire. When lifting it further, it slipped of the side by the strong drag forces on the wire. I was to first to experience our negligence and got hit on my head when I was to eager to inspect the contents of the box. I blacked out, fell in the frigid water in the ladder well and woke up washed up on the ladder. My colleagues rushed me to the hospital, where I ended up between all sorts of winter sports injuries. Fortunately for me, I am able to recount my story and share it here and through the CEDA Industry News.

Annotated overview of the accident site where the wire slipped from the cover onto my head.

References

HYDRO 2018 Gdansk: Selecting A Dredge For Your Reservoir Maintenance

Barrage du Ksob, M’Sila, Algeria with a DOP dredge 350

This week, I am here in Gdansk for a presentation on the HYDRO 2018 Conference1 and assist at the Damen booth at the corresponding exhibition. The paper and the presentation are already prepared and I am very excited to do the presentation, but I can’t wait till tomorrow and I like to share the story now, already. So, you, as my favourite audience, will have my personal spoiler after so many teasers have been floating around2,3,4.

The thing is, dam maintenance and reservoir restoration is something already long on my attention list. Back already in 2008, I wrote a paper on this subject for the CEDA Dredging Days5. Over and over we’ve conveyed the message on various platforms, that dredging might be a viable solution for sedimentation problems in reservoirs. Usually, the solution by dam owners and operators is to flush, sluice or store the sediment. This looks horrible from a dredging perspective, but it is also to the environment. You either smother or starve the downstream river with sediment. As a right minded dredge enthusiast, you see many possibilities to dredge such a project. Immediately we can identify what dredge to use on which location for which purpose.

Selection of applicable dredges for reservoir dredging

If you are very close to the dam and the length of the discharge line allows it, you might even not need a dredge pump. (No wear parts!) It is a so called siphon dredge. But as soon as there is some further transport involved, either distance or uphill, you need a dredge like a cutter suction dredge or a DOP dredge. For even further discharge, you might employ a booster for increased discharge pressure. If the distance becomes very far, you might have to resort to grabs and barges.

Water injection dredging principle and example (this example would be too big for a common reservoir)

As an intermediate solution you might even consider using a water injection dredge. Usually the reservoir is in the mountains and a bottom gradient will be present, enabling the required gravity flow. The actual dredge should have created a silt trap where it can collect the inflowing material from the water injection dredge. Than it can handle the material as usual.

Alternative uses for the dredged sediment a) silt farming as fertile additive b) gravel extraction for concrete

Off course, the dredged sediment belongs to the river and the best thing would be to gradually release the sediment after the dam. But there might be conditions, where it is beneficial to extract the valuable fraction of the sediment and use it for agriculture or as aggregate in the construction industry.

We noticed, that it is often difficult to convey to dam owners and operators which dredge to select for which job. Sediment is seen as a liability and not as an asset and they rather neglect issues associated with the sediment. So, I made an attempt to have a plain and simple selection diagram. That is the core of my manuscript. But my objective is, that we will see many beautiful dredges contributing to a sustainable and viable operation of hydropower dams and reservoirs.

References

DOP Dredges for Power Dams and Reservoir Maintenance

Today, our product group director, Olivier Marcus, gave a presentation1 on the maintenance of irrigation reservoirs and power dams with a range of specially developed DOP dredges. Here, I am happy to elaborate on his message and share with you highlights from his presentation. As you may have noticed from my previous posts, reservoirs are a long standing interest of mine. It was one of my first commissioning jobs at my previous employer. It made me aware, that reservoirs and power dams are not always the clean energy and sustainable solution that they are usually presented for.

Don’t get me wrong, we need these kind of structures to pass on our planet in a better state to the next generation, than we received it ourselves. As with everything: we have to take care of these responsibly.

Dams and reservoirs have mainly two problems. First, if the bottom of the reservoir hasn’t been cleared before filling with water, there is a lot of biomass available. Especially in warm and shallow lakes, the decomposing material can generate a lot of carbon dioxide. The CO2 footprint of the generated electricity might even be higher than from diesel driven power plants2. So much for the green image of hydro power. Second, the interrupted sediment transport will cause the reservoir to fill up. Eventually the storage capacity isn’t enough anymore and the operation has to be abandoned. So much for the sustainable part of the image.

The first problem should have been tackled during the building phase of the power dam. Maintenance during operation for the second problem. The simplest method is to flush the reservoir through the bypass. But you can’t flush all the sediment. The heavy particles up stream will remain in place. And you smother the life of the river downstream. A more effective method is to use dredges. Over the years, we have supplied several of these vessels, each specifically engineered and built for their own location.

Special design reservoir dredges (ul: Cosider, ur:La Mahuna, bl: Djebel Debagh, br: Gross Glockner)

In an initial survey of these dredging opportunities, we found some common features and could develop a range of DOP dredges specific for reservoir maintenance. The transport capacity of the river should be matched by the capacity of the dredge. Often resulting in a range suitable for a DOP pump. Their flexibility makes the dredges adaptable to work on all the different areas in a reservoir.

Various areas in a reservoir with their corresponding DOP dredge solutions

It’s been more than ten years after our first introduction of these nifty little dredges3 and with the experience gained, it was time to present you a next generation that could assist in achieving a sustainable power generation and a more environmental friendly operation. Main innovation is that this new range is also available in an all-electric version. Which should make sense, if it is going to work at a power dam. Always discuss this with the power dam operator, as they are not very fond of plugging in a dredge. They fear distortions and fluctuations on their ‘product’. Than consider alternative clean energy from solar or wind and lastly the old reliable diesel option. With a lot of reservoirs already losing their capacity, there is a lot of opportunity for these type of dredges.

Tag: Power dams

CEDA Industry News: Snap Back Story out of the Old Stone Box

References

See also

HYDRO 2018 Gdansk: Selecting A Dredge For Your Reservoir Maintenance

References

See also

DOP Dredges for Power Dams and Reservoir Maintenance

References

See also