Clear answer for muddy questions

At what point does water become mud? This near-philosophical question has solid financial implications if you are guaranteeing clearance for incoming vessels at a port like Rotterdam. But, while a high-tech sediment profiler will provide answers it also needs to be easy to operate – and quite often it needs to fit whatever vessel comes to hand.

As Koen Geirnaert of dotOcean explained the Dutch Rijkswaterstaat (the authority in charge of much of the Netherland’s infrastructure), has to put hard numbers to the “soft, blurry interface” between sediment and water column. However, since the possibility of grounding is such a critical matter, clearance issues usually place conservative limits on shipping.

So the Rijkswaterstaat’s recent upgrade to dotOcean’s DensX profiling equipment will tell the organisation which points along the bed of a fairway measure up to salt water’s 1,020g of solids/m3 or if they are in fact mud’s 1,200g of solids/m3. It does this by using an X-ray probe to penetrate the silt, a huge advance over the previous generation’s reliance on radioactive isotopes which are thankfully on the way to being banned.

However, as no single-point reading would mean much on its own the probe is deployed across a 100 point-plus grid pattern; this gives a visualisation of the water-mud density gradient for the sampled area, providing RWS with “a clear, readable picture” explained Mr Geirnaert.

The action has to be capable of being initiated automatically with a controlled winch, although the operator will often just position the vessel and hit the start button. Therefore it’s a fairly complex business requiring a range of failsafe responses: for example if the needle tilts on meeting a hard layer the device has to stop and retract, likewise it will react if it senses higher than expected torque on the cable or the bottom reaches a density that would be problematic for navigation.

As this means the profiler needs to be knitted very closely together with its deployment device, dotOcean got together with MacArtney to customize a CORMAC M modular winch. This has enough capacity to handle the 70kg DensX profiler and has a maximum speed of 64m/p/min, driven by a fairly standard 230V single phase motor, all held by a corrosion resistant stainless steel structure.

This particular winch has two characteristics that make it attractive for the joint venture: one of them “is that it’s robust and field proven” said Jeroen Vercruysse of MacArtney – a necessity given the high end kit on the end of the wire - plus alongside the more usual remote controls it has the kind of open-architecture software that allows dotOcean’s profiler to hook up with the winch’s software and direct it.

The power source and all-important data connection are twinned in an electrical and 100mb ethernet combination cable: this last is based on a standard broadband connection which means that it can be controlled from any PC interface on the vessel (including a laptop). However, the line has to provide a clear signal despite the changing depths and lengthening or shortening cable: it has been achieved via a slip-ring with brushes that retains the broadband connection, retaining a high bit-rate connection throughout deployment. Although slip-rings are quite common, “wiring one to accommodate ethernet is quite clever” pointed out Mr Vercruysse: “This slip-ring brings the network right down to the sensors.”

Ease of transport is a MacArtney speciality so the kit has been housed in a frame that’s around 1.2m2 and block-pallet mounted. This should have made it pretty easy to deploy, but there was one extra (quite literal) twist to the project. “The system has to be fitted onto whatever vessel is available at the time, and because they have very different deck layouts the entry of the cable into the winch could be rather difficult,” he said.

This meant tailoring the design so it could cope with anything from a horizontal level-wind to almost vertical as the only appropriate space could be up close to the vessel’s A-frame. Mr Vercruysse explained that this has been overcome by allowing four different positions to fix the level wind (with four bolts on each side of the winch). However, he pointed out that while this may sound simple, “the challenge has been to make the level wind motor moveable as well, and still protect the slip-ring underneath it”.

The market for this combination tech looks like growing with both port authorities and consultants expressing interest. Certainly the solution yields a clear picture – even when the water isn’t.