Diving ROV with TMS from DP Vessel in shallow water

[Abdelnasser]

Hi Guys

I have been asked today if it safe to launch ROV with TMS from a DP vessel in 11 m water depth!!

I answered Yes it is unsafe but question is there a minimum water depth to carry out rov dive from DP vessel to avoid the hazard of vessel thrusters itself in case of TMS or free swimming ROV

[Ray Shields]

Can the job be done with the ROV still in the TMS?

Hazards are:- Lost of power/control of ROV or TMS, DP run off/failure, LARS failure

Controls/mitigations that can be used: ensure ROV is deployed "down current" of vessel so that in case of failure ROV drifts away; restrict length of tether out so that in case of failure ROV cannot reach vessel thrusters, ballast sub heavy so that in case of failure it sinks down, how likely is the vessel DP to fail? How likely the ROV?

There is no hard and fast rule, it all depends on many factors including size of vessel and ROV (obsrov or workclass), currents, weather, etc. 11m of water, although maybe shallow, is not that bad.

[T-Boy]

11m is not that shallow! Even for a WC. If it’s below the surface, then it’s good.

There is no hard and fast minimum depth..depends on distance of LARS head from surface, SoW, Speed, wave height, current, launch point location relative to the screws, height of stacked system, depth of keel, size of testis’ amongst others.

If the system is to remain stacked, then you really should keep the TMS underwater for cooling obviously, you don’t want to laterally load the LARS for example, so must be smooth seas, low current, minimum wash etc etc.

These are just a handful of things to consider when mitigating some of your safety concerns in a RA.

From personal experience, it can be done from a lot shallower, proceed with caution though, make sure you have a good set of eyes outside, and real ‘mill pond’ conditions.

I feel rescue #3 this year coming on 😆

P.S I didn’t copy you Ray, I didn’t read your post – doh!

[Paul Bond]

I have done work like that and decided it was ok as
a) the ROV was ballasted negatively buoyant
b) tether was heavy
c) TMS was kept as low as possible

At the end of the day you have to decide on each individual occurrence. It is your neck on the block if it all goes wrong – make sure you risk assess it all and get it approved by the adults!

Also, make sure your opposite number is singing from the same hymn sheet or it leads to problems.

[Craig Thorngren]

Unsafe??? I’m guessing you can’t use divers because of the DP vessel, so the only real alternative is using an ROV correct? Using some of the techniques others have talked about helps mitigate some of the risks but you will never eliminate all of them. I just came off a job where we were flying up to 4 ROV’s off of a DP vessel to place some underwater structures in very precise locations. Some of the structures were shallower and some were deeper than 11M like your situation. In our case there was absolutely no way divers were going to be used, we looked at what we thought were all of the risks and tried to mitigate them, but at the end of the day we and the client were fully prepared to right off all of the systems as a loss so long as no one was hurt.

Chief

[Des_b]

ROVVOR,

Have you spoken to your office yet? There may be a clause in the insurance that says something about minimum water depth etc. You may need/want something in writing from the office that covers you and the system in case something goes wrong, although as the others have said its not that shallow…

Just a thought… 😉

[Abdelnasser]

Thank you all for replies, The order to do the job not received yet but it was like what if question

I may contact office to take their advice in case I have to do the job

I just wonder why there are no ROV for Dummies to help in such situations

[Haddock]

Shallow water work is increasing rapidly with all the windfarm work.

Some good comments made above, but in terms of the ROV spread the main points are:

1) ROV temps, depending on where abouts in the world you are the ambient temps can be significantly higher so the hydraulic system and motor temps should be monitored.

2) Umbilical temps, particularly if a lot of power is being taken. All those layers remaining on the drum with no cooling – look at using a spraybar and potentially monitoring the temps via a DTS on a spare fibre.

Regards

[panandtilt]

Why did you say it was unsafe? As long as you have proper procedures in place there is no problem at all in diving in 11M of water. Current will be very important at that depth.

[Author]

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