Don’t rock the boat, don’t tip the boat over

DOP Dredge ‘Roanoke’, Long Island, USA

We were quietly enjoying our dinner on a relaxed evening in our vacation. Suddenly, we were rudely disturbed by rumble and clatter from across the valley. For our eyes developed a rock slide. Just as sudden as it started it was already over. Perplexed, we were too slow to capture the event and put it on social media. Afterwards, I took some pictures of the rubble. As you can see, it was not even a proper rock slide, more the collapse of a retaining wall.

Retaining wall collapse, Sóller, Mallorca, Spain, June 21, 2018

Come to think about it, it was not the first collapse I witnessed. Back in 2006, I was visiting our DOP dredge at Roanoke on Long Island, NY in the USA. I had to do some measurements and general inspection. I was below decks connecting the data recorder to the drive system and had to check something with the dredge master. Just when I climbed on deck, he yelled at me to hold on. Immediately a torrent of water and sand was flung horizontally over the dredge. Some stones cracked a window in the control cabin. Within seconds a tsunami lifted the dredge for about a meter and we kept rocking until the reflecting waves in the pit eventually subsided.

DOP Dredge Roanoke with pit bank in the foreground, before it collapsed. Older bank collapses in the background.

That was one big bank collapse to me. A bank collapse is a known, although undesirable phenomenon in dredging1. It is a result of dredging methods, relying on the development of an active bank to produce a heavy slurry, that is sucked up. However, the sediment does not consist of a uniform block of sand. Usually, the sediment is deposited in different layers, each with their own geo technical properties. These result in varying propagation velocities of the active banks. When a ‘faster’ sand is under a ‘slower’ sand, the upper layer is not supported anymore and collapses. As the bank slumps down, it displaces an enormous volume of water and this often causes a tidal wave of its own. At Roanoke, the effects were aggravated by the fact, that the upper bank ran all the way to above water level.

Progression of an active bank and bank collapse

As this bank collapse can be expected when dredging with active banks and different sand layers, dredging companies are very keen on predicting these nasty consequences. Not only for the safe working condition of the crew, but also to prevent material damage and eventually for a stable and reliable delivered profile. Exactly this is what is being investigated by dr. Askarinejad in the Laboratory of Geo-Engineering at the Technical University Delft2. He has a beautiful rig, where exactly those conditions can be simulated and measured. With a neat trick he tips the whole test facility to form an instable bank. This makes the bank collapse on demand3.

Static liquefaction tank TU Delft (Credit: dr. A. Askarinejad)

Basically, this is exactly what we can demonstrate with the ‘breaching exhibit ‘ in our dredging experience4. Of course you are welcome to come over. For those who are not in the circumstance to visit us, you can also visit the National Dredging Museum as they now have a breaching exhibit of their own5.

Handover of our old breaching exhibit to the National Dredging Museum

References

  1. Breaching Process OE 4626, van Rhee, TU Delft
  2. Amin Askarinejad, TU Delft
  3. Statische liquefactietank , Delft Integraal
  4. Loose sand, how hard can it be?
  5. Baggermuseum krijgt model van Damen Dredging, Binnenvaartkrant

See also

Book Review: Moddergeuzen

Front cover Moddergeuzen by C. Baardman (Credit: JN Voorhoeve, Den Haag)

Summer is in full progress. People are already taking holidays. So, it is time for another book recommendation to take with you and pass your leisure time. Another favourite book from my dredging library is ‘Moddergeuzen’, or ‘Mud Beggars’, by C. Baardman1. However, the English translation doesn’t convey the right feeling about it. In Dutch the ‘Geuzen’ were patriotic nobleman, who fought against the Spanish oppression in the 80 year war2. Beggars sounds so derogatory.

In contrast to ‘Mannen van Sliedrecht’, from my previous review3, ‘Moddergeuzen’ is more describing the personal life of the crew on board the dredges, instead of the big historical picture. Baardman is able to bring all the crew members to life. At least to me. Either by reminding me of stories I heard from my family, or even by similar events I experienced personally. Still, there are some main characters. Most notably Leen Hoornaar and his son Jan. There are also other family ties between the crew members, as was and still is often the case on the Dutch dredges. Somehow, all those stories by and discussions with dad at the kitchen table do convey more knowledge insight and experience, than education at school does. (You need school! What your parents can tell you is your bonus to stand out from the crowd.)

That is also one of the topics Baardman addresses in his book. The old trade handed down from father to son, versus the new developments in culture, where education and knowledge gains momentum in the dredging industry. Another topic is the continuous inequality between the crew members on the one hand and the people from the office on the other hand. Regularly, socialistic thoughts are proclaimed by the crew members, but Baardman voices his idea about social improvement through the characters of Leen and Jan Hoornaar. They are the first glimmers of a work council in the industry. Another social phenomenon is the ever present religious group, that tends to be very conservative. Still, Baardman would rather identify himself as a religious writer. However, his concern to get the message across, but be careful not to look over zealous often made his books read like a regional novel4.

Well, regional is hardly applicable. Just as the dredging industry expanded all over the world, the stories in the book take place in all the now familiar dredging markets: China, Germany, Belgium, France, Rumania. Particularly the storyline in Hamburg was very interesting, as this is also the same time and region where my great grandfather has been working for some time (another story for later). The descriptions about how they must have travelled to and from the project where very illustrative.
Also the pranks they pulled on each other, or more precise on the youngest person on board where very familiar. I’ve heard the story of the coxswain being ordered to fry the cucumber, before. I don’t know if pranks are banned today. At least I don’t hear them so much anymore.

Paper model I received after building my first dredge

Times have changed. So, were the changes described at the story time in the book. Shure, there were a lot steam driven bucket ladder dredge in the adventures. And they do bring most of the exciting adventures. However, modern equipment was on the rise. Plain suction dredges and the first hopper dredges also feature in the book. Remember, hopper dredges at that time were still stationary. Free sailing trailing suction hoppers came much later. The dredge on the cover is a nice example of early twentieth century dredge design. As it happens, the National Dredging Museum in Sliedrecht has model, that looks very similar to the one depicted.

Suction hopper dredge ‘Nautilus II’ for Prins Aannemingsmaatschappij build by J&K Smit

References

  1. Moddergeuzen, Amazon
  2. Geuzen, Wikipedia
  3. Book Review: Mannen Van Sliedrecht, Discover Dredging
  4. C. Baardman: De schrijver en zijn werk, Historische Vereniging Hardinxveld-Giessendam

See also

A Visit to the Oresund Link with the CEDA Dredging Management Commission and 23 Years Ago with DOT

The CEDA Dredging Management Commission on site visit at the Oresund Link

Edit (11/08/2018): It appears, that my memory fails me on the details. It must have been late 1996 or early 1997.

This week, there was a meeting of the Dredging Management Commission of the CEDA. In this case, the venue was the offices of Femern A/S in Copenhagen1. The objective of the Dredging Management Commission is to provide expertise and guidelines to successfully manage dredging projects. A first publication was the ‘Checklist for Successful Dredging Management’ introduced at the CEDA Dredging Days last November. Currently I am involved in the Working Group on Innovative Dredging Technology2.
One of the items on the agenda was a visit to the artificial island of Peberholm. This island was reclaimed in 1995 during the Oresund Link Project, a tunnel and bridge connection that links Denmark and Sweden over the Oresund. A massive complex project, involving two countries, multiple contractors and numerous suppliers. Unlike most of the complex international government projects, the Oresund Link was delivered ahead of schedule and within budget3.

View from Peberholm on the Oresund Bridge and Sweden

At the time, I did my graduation thesis project with Skanska, one of the main contractors. There were also two other students working with Skanska. And as there were a lot of other Dutch contractors involved, the student club of Dredging and Offshore Technology organised a study tour to the project and have a first-hand experience with this landmark project. Returning to these sites with the CEDA was interesting to see the final product after it’s been working for several year in use.

Students of Dredging and Offshore Technology on study tour in 1995

On the Swedish side, a bridge was a good solution. On the Danish side, the link would be too close, to Copenhagen International Airport to build a bridge. It would interfere with the flight path of approaching planes. Therefore, a sunken section tunnel was chosen. Peberholm was created with fill material, dredged from this tunnel trench. Digging through hard limestone rock, the cutter suction dredge ‘Castor’4 consumed about 62,000 pick points. In the end, they had to exchange all the teeth every 15 minutes, sometimes too hot to handle.

Cutter head of the CSD ‘Castor’

Each pillar and the two pylons of the bridge have a foundation that reaches to the hard bedrock of the Oresund. For this dredging work, amongst others, the back-hoe dredge ‘Rocky’5 from Skanska was deployed. One of the contractual clauses was that turbidity had to be reduced to an absolute minimum, never seen before in any contract. Although protesting, the contractors complied and did a great job on reducing turbidity. The loose material from the bottom and the rubble from the bedrock created visual plumes anyway and Skanska had to modify their procedures and equipment to reach the stringent standards.

Left, Back hoe dredge ‘Rocky’. Middle, bucket of crushed limestone. Right, Spill plume.

It was fitting, that we visited the location of this impressive project as inspiration to advice how to successfully manage a dredging project. The next step to test the management skills of the Danish people will be the Fehmarn Belt Project6.

Panoramic view from Peberholm. Denmark left, Sweden right and Saltholm in the middle.

References

  1. Femern A/S
  2. CEDA Dredging Management Commission, CEDA
  3. Oresund on time and within budget, Øresundsbron
  4. Castor, Van Oord
  5. Rocky (after modification), Boskalis
  6. Fehmarn Belt Project, Wikipedia

See also