The challenge

Deploying, locating and retrieving seabed sensors can be a challenge. Choosing reliable sensors that will work for the whole duration of the deployment and provide robust reliable results is just the start. Accurate seabed positioning data that are delivered in real-time to enable dependable acoustic release of the sensors is also required.

An additional layer of complexity is introduced for these kinds of operations when the site in question has a significant tidal range and current regime.

This is just what independent physical oceanography consultancy DynamOcean has been doing in the Rance estuary in Brittany, France. The Rance estuary – where the River Rance enters the English Channel – has one of the largest tidal ranges in France. It averages around 8m between low and high tide. During spring tides this figure can jump to as high as 13.5m!

DynamOcean has been studying sediment transport through the estuary with the aim of measuring current, waves and turbidity in the Rance estuary. To do this, they’ve been using an acoustic Doppler current profiler (ADCP). The measurements are being taken over six months, with recovery and re-deployment of the ADCP – in exactly the same position – every two months. This requires both a reliable acoustic release and accurate seabed positioning in real-time.

The solution

For this project, DynamOcean chose our RT 6-1000 acoustic release transponder. Their choice was guided by several years’ experience in using our Lightweight Release Transponder (LRT) and deciding to upgrade to our newer RT 6-1000.

The RT 6-1000 is our entry-level release transponder with features that are far from entry-level. The 1,000 m depth rating, 15-month battery life, optional rope canister and – crucially – the reliable release mechanism made this an ideal choice for the deployment, retrieval and redeployment work DynamoOcean needed it for.

Alongside the RT 6-1000, DynamOcean chose our Micro-Ranger 2, the smallest USBL system we have available at Sonardyne.

It’s easy to set up and use – even from a RIB or uncrewed surface vessel (USV) – and enables tracking of up to 10 targets out to 995 m range. For this project, it means DynamOcean can instantaneously position its seabed ACDP platform and inspect it using a mini-ROV, which can also be positioned via a Nano beacon using the same Micro-Ranger 2 topside.

Because the RT 6-1000s, Nano and Ranger USBL systems work with the same 6G platform, they can be used interchangeably and seamlessly. The time needed to coordinate each part is minimal and integration with the topside platforms is simple.

DynamOcean’s managing director Eloi Droniou explained the three key benefits they enjoyed when speaking with us about the setup for the Rance estuary project:

“The RT6-1000 integrated within a pop-up buoy on the seabed platform has multiple purposes,” he says, “one, release the pop-up buoy for the recovery of the seabed platform without the need for divers at the end of the measurements; two, measure inclination to make sure the platform is more or less level; and three, position accurately the seabed platform using various Sonardyne 6G topside options.

“With the release of Micro-Ranger 2 and Mini-Ranger 2, DynamOcean was thrilled to be able to not only instantaneously position the seabed platform but also to inspect it using a mini ROV, [which was] positioned using the lightweight and compact Nano transponder.”

The results

“This allows us to make sure that it has landed properly, without needing any divers,” adds Droniou. “If the landing location is not satisfactory, the seabed platform can be raised from the seabed and moved to another nearby location. The seabed platform can also be inspected with the mini ROV just before releasing the pop-up buoy with the RT 6-1000 to check that everything is in order and verify the instrument’s height above seabed.

“Having the positions of the vessel, the seabed platform and the mini ROV, simultaneously and on the same survey window (Ranger 2 software), is essential for these operations.”

DynamOcean were able to simplify their operation, thus cutting costs and the time needed to position, deploy, retrieve and redeploy sensors for the estuary study. On top of this, because the need for divers was removed the risk associated with the study has also been reduced.