Jet Trencher Vs. MFE

You've got to love the guys at HIA Energy - Connecting great people with great offshore projects , no sooner had I hit the publish button on LinkedIn for my article about how MFEs suck (see here) (although I think SEAJET, an OEG Renewables company might prove me wrong later this year), than one of their new agents got in touch with me and asked me to explain the difference between Jet Trenchers and MFEs.

They really are committed to learning the technical aspects of this industry and I respect that.

So, an MFE hangs suspended above the seabed and blasts a jet down to blow away sediment. They historically have a lot of inherent issues: mid water column stability, an inability to discern the natural seabed, and therefore unable to prove depth of burial of the cable, a poorly defined trench with little to no protection of the cable, slow backfilling and more. Seajet, part of the increasingly omnipresent OEG Offshore , has worked hard to mitigate all of these and promises to revolutionise our perception.

Jet trenchers, on the other hand, sit on the seabed astride the cable.

The first time I saw a jet trencher in action I was on a vessel as the online surveyor, about 15 years ago, and it was an unmitigated disaster. The trencher mobilising team couldn't get the buoyancy right and the jet pressure. Pretty much every time they powered up the jets the entire thing blasted off the seabed like the lunar landing module, did a corkscrew around its tether, needing a re-termination more times than I can count (each re-termination taking about 24 hours).

It was NOT an AssoJet system by the way.

Jet trenchers have become, and normally are, a lot more reliable than that. Again, I think a lot of the issues related to a very poorly recruited team.

Anyway, most times since then jet trenching goes pretty well.

On each side of the cable the jet trencher lowers down a blade, this is a long steel arm with several jet nozzles along its length pointing down parallel to the cable. The jets then basically power wash the sediment away and carve a pretty deep and defined trench affording, in theory, very reliable depths of burial and a good rate of cutting. As the trench gets deeper the blades can be lowered into it to control and define a better, deeper trench.

They tend to be on tracks, like a tank on land, and they move along the length of the cable slicing through the seabed like two hot knives through butter, allowing the cable to gently nestle down into the newly formed trench.

I've seen that Asso.subsea has a new model coming up promising depths of burial down to 3.2m. That's deep. Just trying to remember the depths we could track cable down to with different instruments. You see, when a cable is buried and can't be seen from the surface of the seabed (and you want it to be buried), you have to use various instruments to work out where it is and how deep it is so you can find it in the future, aside from the initial depth of burial surveys.

We often use a TSS440, the Teledyne Pipetracker designed to track pipes, not cables. It's limited in scope because it is not designed to do this task, but if well calibrated can track a cable down to around 1.5m burial, topping out at about 2m if you have a low ROV flying altitude. If you use tone with a TSS350 then perhaps you can get up to about 2.5m and a cleaner signal. The Innovatum Smartrack follows the signature of magnetised cables and I found was quite good down to 2m, perhaps 2.3m, as well, but it tracks the magnetic field rather than the actual cable. I think the Pangeo system can go deeper.

Anyway, I'm labouring a point. I've done thousands of kilometres of laying, ploughing, trenching, DoB surveys and remedial works. Pretty much every export cable I have worked with is designed for a relatively consistent 1.5m burial with a tolerance on either side, so 3.2m of burial is crazy depth. I'm sure there are instances where more than 1.5 or 2m is defined. If it can go down to 3.2m that gives me a sign that it has powerful jets, blades that can go as low as you'll ever need and the Assojet III Mk 2 is a nice looking piece of kit with some good specifications. Why is "III Mark 2" and not just AssoJet 4, ah I see, they have an AssoTrencher IV mark 13, who names these things? I guess there's some logic somewhere.

I suppose there are instances where if there is a mound on the seabed you want to cut deeper to get through the hump and maintain some consistency, Anyway, I like the look of it. If anyone has used their kit can you drop a comment below and let me know if you think it's worth using? Thanks.

So, HIA Energy - Connecting great people with great offshore projects , I hope that answers your question. Keep it up. And if you can use one of these and are looking for a good role in trenching, get in touch with the legendary Stephen Hanlon .