How to immerse a 60-metre-tall concrete float

Author: Maurice Reijm

Placed on: 20 December 2017

Tags: immersion, floating, techniques, windfarm

Portretfoto Maurice

Maurice Reijm

Strukton International

Maurice Reijms is a Project Manager and Immersion Commander at Strukton Immersion Projects. He travels all over the world. After all, you cannot immerse concrete foundations and tunnel elements from behind your desk. He talks to us about ‘another day at the office’.

Five concrete foundations - 60 metres tall, 30 metres wide - had to be moved from a dry dock in Newcastle to the seabed around 6 km off the coast of Blyth, a town to the north of Newcastle upon Tyne. Each of the five foundations serves as a ‘foot’ for wind turbines. Energy de France will be operating the wind turbines. That means green energy for 34,000 British households.
The immersion job sounds like something that Strukton Immersion Projects would love to get its teeth into. And that is exactly what we did.

_MG_9146 - kopie


Neatly in place

The foundations were built in a dry dock. During the building phase, we installed pipework and sensors in the foundations. The pipes and sensors are needed to allow water and sand to be pumped in and out of the foundations. We also made preparations in the River Tyne and at sea. For instance, we laid temporary moorings and anchors on the seabed.
Originally, the plan was to carry out the immersion between March and May as this is the time the weather conditions are the most favourable. Clearly, setting out to sea with these foundations in a force 8 gale was not going to be an option. However, due to a construction delay, the immersion was postponed until summer.

In June the dry dock was flooded. We pumped water out of the foundations until they started to float. With the aid of winches and a tug we towed the foundations one by one out of the dock and down the River Tyne towards the sea. Ten metres of the foundations remained submerged, leaving 50 metres to tower above the surface of the water. Although they are without doubt enormous structures, these concrete behemoths were kept neatly in place by the three tugs to which they were attached.

_MG_9051


Sand in, water out

We also made use of the three point system at the immersion locations: the foundation in the middle and the three tugs forming a star shape around it. This allowed us to position the foundation as precisely as possible on its location and keep it there. Our colleagues at Geocon were very helpful. Using GPS we were able to follow the position extremely accurately and make any adjustments that were needed. After all, telling someone to move ‘a little to the left’ is a bit tricky at sea.

A fourth vessel, a multi cat, slowly filled the foundation with water allowing the structure to sink in a controlled way to the seabed, at a depth of around 45 metres. To make sure that the foundation was heavy enough to support a wind turbine, we swapped the water for sand. Sand is heavier than water so it sinks downwards, forcing the water upwards.
On 31 August, all five foundations were at their locations, all within 3 centimetres of their ‘targets’. On average it took about four days per foundation, from leaving the mooring to the final location on the seabed.

“All aspects of the project were unique. Take our solution for adding ballast to the foundations: with sand. Considerably more sustainable than concrete”


Unique

A project like this one is anything but a walk in the park. Moreover, all aspects of the project were unique. Placing the foundation on the seabed without driving piles in has a positive effect on marine life. Then there is the solution for adding ballast to the foundations, which was done with sand rather than the traditional concrete. This makes the entire foundation easy to remove when it reaches the end of its service life.

Another unique aspect is the way we pumped the sand into the foundations. Normally this is done with large, expensive hopper barges. We designed and built a bespoke concept: a flat container measuring 100 by 33 metres holding 8,000 m3 of sand which we pumped into the foundations using a ‘conventional’ sand-pumping system. It was the best and most cost-effective solution for a project of this scale.

“Transferring crew from the vessels to the transfer ship in rough seas can be pretty dangerous”


Then there was the safety of the crew to consider. Working out at sea with waves reaching heights of several metres is not without its risks. Transferring crew from the vessels to the transfer ship in rough seas can also be pretty dangerous. This is why we practiced endlessly and why crew members wore special protective suits during the transfer operation. Furthermore, all crew took a Sea Survival course and underwent Working at Height training.

Like family

The nature of the work and the high level of attention to safety combine to create another unique aspect: the trust that we place in each other as a team. We know what to expect from one another and we know each other through and through. It feels like a family as we keep an eye on one another and we’d do anything for each other. That is why our projects run safely and on schedule, whether we are working in Venice, Gothenburg, South Korea or out at sea.

IMG-20170711-WA0045