Case Study – How a TSHD dredged to 30cm accuracy

Strategic Dredging is assisting in the execution of this dredging project, which is still ongoing. In this Knowledge Update we share the most challenging aspect of this project.

The dredging project is part of a much larger project of constructing a new offshore windfarm project.

A completed offshore windfarm nearby

The dredging part consisted of clearing sand dunes along the proposed cable route. After dredging was completed a special large plough would come and bury the cable in the clay layers beneath the sand dunes.

The plough, even though it was big, could only handle small deviations to the sea bottom so the contractually agreed accuracy was a maximum over dredge of 30 cm. Imagine a TSHD in waves, dredging up to 15 m deep, with a draghead weighing dozens of tonnes and a width of about 7 m and you get the general gist of how challenging this accuracy was.

Preparation

Survey equipment suspended above the moonpool.

It was anticipated that when dredging closer and closer to the design a lot of high spots needed to be removed, instead of a steady layer across a large stretch. When trying to remove very small layer it becomes important to get feedback on the seabed after dredging as quick as possible. In addition, there was the potential for rough seas, so a small survey tender might not be able to perform accurate surveys. To solve both issues it was decided that the TSHD itself would be equipped with multibeam survey equipment. In that way the TSHD could dredge a high spot, survey directly afterwards and get almost immediate feedback on the new seabed level.

The second big preparation was coming up with a dredging plan, that would allow the vessel to get to the required design level, without over dredging more than 30 cm. It was decided that the dredging would be done in layers. First the bulk with a dredge level of design+75cm. Once that was (nearly) completed the next layer would be design+50cm and so on. This would ensure that any remaining high spots weren’t too high above the design, which would make it difficult the remove them later.

Other preparations included making sure enough bunkers and stores were available to last the entire anticipated dredge period, as bunkering would entail a lot of sailing time to and from a port.

Execution

The cable route was split in various Zones. The zones had been surveyed earlier, but because the sand dunes were moving on the seabed it was required to get a new survey done. This was first order of business.

The TSHD used on this project had a hopper capacity of more than 11,000 m3. A large TSHD like this can easily remove more than 50 cm of loose sand in one pass. If you try to dredge to 30cm accuracy that is a real challenge. So after the survey was done trials were performed to determine how to dredge without removing too much material in one pass. Parameters such as jet water use, mixture flow rate and draghead offset from the bottom were all tested to get the best possible outcome.

The last one might need a bit of explaining. By not placing the heel of the draghead on the sea bottom, but keeping it suspended 10-20cm above the seabed it would suck in a lot of false water. Normally you don’t want this, as it lowers your production. But in this case it helped a lot in making sure there was no over dredging.

Once the best working method was established the “bulk” dredging could commence. I put bulk in quotes, as you can hardly call it bulk dredging when you were still artificially limiting the production of the vessel. But we needed some way to distinguish between the two dredge phases, so this is what we used.

Over the next few weeks the dredging, punctuated by periods of surveying, took place.

Differential chart of the trench, showing high spots to be dredged.

Hand-Over

As expected the production was not even close to the capabilities of the vessel, but a necessary evil to get the required accuracy. Even with all the above it proved very difficult to get to the required accuracy. Luckily, in discussions between the client and the dredging contractor a practical approach was taken. After clearing some last spots which really weren’t maneuverable by the plough the dredged areas were accepted, even though not everywhere the tolerances were met.

This completed the first campaign. more campaigns were to follow in the coming months for other cable routes.

What did you learn from this case study? Let us know in the below comment section.

To your success,
Timon Vinke

Leave a Reply

Your email address will not be published. Required fields are marked *