It’s a real precision job

Author: Bart-Jan Ruesink

Placed on: 25 July 2014

Tags: immersion, measurement systems

Bart Jan Ruesink_vierkant

Bart-Jan Ruesink

Geocon

As survey supervisor at Geocon, a Strukton business unit, I’ve been working in Italy this past summer. Our team has been working day and night on the collection and processing of measurement data.

In order to protect Venice against flooding in the near future, the MOSE project is currently being realized here.

Strukton plays a key role in the MOSE project, since it’s involved in the immersion of the caissons for the flood barrier near Chioggia. For a couple of months now, our team has been working day and night on the collection and processing of measurement data. We know precisely which information the immersion commander needs at which point in the operation.

The ‘surveyor’s shed’

All measurement readings come together in the Geocon Unit – which we refer to as plain old ‘surveyor’s shed’. In this shed, you can find an array of computers that receive and process all the incoming data. About 800 metres further on, you’ll find the caisson that we’ll be immersing next week. In January of this year, we measured this caisson down to the last detail: we mapped out every minor deviation, since the Venice project is based on minimal tolerances. It’s a real precision job. That’s why we’ve fitted reference markers and benchmark disks across the entire surface of the caisson. We have to get a perfectly tight line all the way to the horizon.

Measurement methods for immersion operations

We won’t be the ones positioning the caisson – this is done by the immersion commander and his team. You could say everything changed after Strukton handled the offshore immersion of tunnel segments in South Korea. This project was as colossal as it was complex, since it was realised underwater in turbulent coastal waters. There was no way the optical instruments we normally use for such operations would suffice, which is why we ‘looked over the fence’ and checked out the methods used in the offshore industry. We acquired a whole new series of sensors and completely rewrote our software, which ultimately yielded our new line of specialist measurement systems for immersion operations.

Exploring other markets

You could call the Busan-Geoje project in Korea the cradle of our present-day methods and technology. We are now the foremost authority in immersion methods: no other company in Europe has gained as much experience in this field as we have. But it’s a small market. After Korea, I looked into what else we could do with our equipment and systems. There’s no need to keep everything on hold for current projects, so I regularly visit trade events to rent out equipment that’s standing idle. What’s more, it helps to build our network in the hydrography market, for instance. We need to adopt a modest attitude when it comes to the offshore market though: there’s no way we can position an oil rig for Shell at a depth of several kilometres. However, specifically in ‘dry to wet’ projects, we can definitely bring new knowledge to the table. Particularly when the project involves structures that were built elsewhere and that need to be positioned on location. Like offshore wind farms, for instance, or the positioning of bridges, quay walls or sluice gates in port areas.

Precisely determine the structure’s position

Our job at the site is measuring everything the immersion commander needs to know. Here in Chioggia, we continuously measure the position of the caisson from the shore using total stations – which use traditional surveying methods complemented by so-called distance sensors and inclinometers. In this process, we use prisms (reflectors) on the shaft, the tower and the caisson’s tunnel segment to precisely determine the structure’s position. We continue to monitor jack pressures throughout the immersion, as well as the ballast water levels in the segment – which in turn relate to the tensile forces in the pontoon winches. We rely on a Realtime Kinematic GPS to make an extremely precise measurement of the structure’s positioning within this dynamic environment. Because, although during the summer months the lagoon surface is as smooth as glass, there are actually tide movements under the water’s surface. By constantly making new readings and identifying significant differences with the aid of our software, we are able to determine which corrections to make. And the immersion commander ultimately uses all these data to make his decisions.

Get into the rhythm

The immersion is like a train that needs to get up to steam. The first caissons are already in place – of two different types – and now we’re getting into the rhythm. Well, rhythm… the cycle of the immersion programme is actually irregular: depending on the tide cycle, we have seven to twelve days for preparations. Of course, we’re gradually optimising this process: we’ll only need a few days for the final caissons. For the time being, it’s ‘full speed ahead’. We’ll be entering the operation phase shortly – which means you won’t be able to reach me for a few days.