How shallow water depth for DP drilling?


#1

Does anyone know how shallow water depth a DP semi-submersible and drill ship can safely drill?

I’ve heard there is a minimum depth due to the footprint it can keep compared to the height above the seabed. There is no clear information on search engines.


#2

Yes its riser angle issue and acoustics.
If someone built a DP rig and forgot to allow for DP assisted working mooring winches to be installed so they can work in shallow water, well they are the ones that need scrapping.


#3

I just happen to have the spec of a DP semi to hand. It says that the minimum water depth in which it can operate is 100m


#4

I guess this is in reference to a moored semi. It would be nearly impossible for a DP drillship or semi to perform an EDS sequence to disconnect BOPs and LMRP in an emergency before reaching equipment limits in that water depth. There was mention of acoustic being a limiting factor, but that is not the case. You would certainly need to choose the correct style transponder heads (180), that had a full range including baseline for LBL.


#5

No it isn’t - of course what I have is a spec, probably produced by the designers who would be optimistic about everything. I did have a look at the spec of a moored semi which gave the minimum operating depth as 80m.


#6

That may be top hole drilling, minus BOP.


#7

More information is needed to come up with anything. The ‘spec’ values mentioned seem to be a reasonable starting point for a minimum, but more information would need to be available to determine if those were safe values. The type of operation (open hole or with BOP), shallow gas potentials, prevailing Metocean conditions, and the actual properties of the vessel which determine the vessel drift path all come in to play if the vessel were to have a propulsion loss. The HPR issue can be dealt with if using the proper equipment and techniques, I doubt that it would be the limiting factor. The chief concern would be if there was a drive off, due to the minimal to nearly non-existent reaction time to safely disconnect whatever was connecting the vessel to the seafloor. The rate of acceleration, the point of disconnect before equipment gets damaged, and time required to disconnect all impact this. If the vessel had any sort of drive off, It’s very doubtful that any BOP disconnect sequence that I’m aware of would finish in time to prevent damage in 100 meters of water.


#8

100 m. would be the absolute minimum in benign and/or semi protected waters, like the Java Sea, where anything over 3 m. seas and 40 kts. sustained wind is rear.
Even so it would be difficult to justify it, if any possibility of thruster of positioning fault is taken into the safety planning.

I know that an anchored drillship worked in very shallow water (40 m.) by digging a "cellar " in the seabed and keep a very high work tension in the mooring system.

PS> This was the SEDCO 601, which could drill with surface stack, or sit on bottom in even shallower water.