Naga 7 suffer a punch-through while positioning on location off Sarawak, East Malaysia
Most like during preloading.
PS> Every Rigmover’s worst nightmare.
I’d really like to know how this happened. I have carried out risk assesments for jack-ups which included punch through and have developed procedures intended to minimise the consequences should the event occur. Did they leave something out, or is there a risk we had not considered? But there is no doubt that working on a jack-up is a risky business, so those taking a job on one should be aware.
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Naga 7 is a fairly new rig, of GustoMSC-CJ46-X100D design, built in China, 2015:
Some pictures from the incident site:
I seem to remember a “Punch Through” in the GOM not too far back. A bit smaller rig. Don’t remember the details.
Punch through did occure in the early days of drilling in the GoM. That is why pre-loading was invented.
One of the early Letourneau rigs, (B.No. 03) the Mr. Cap, blt 1957:
This rig originally had no preload tanks, but had them added on after the danger of punch through was known.
She was the first rig to drill offshore UK (1964) and came out to S.E.Asia in the early 1970 as Chris Segar, for Reading & Bates, drilling for Brunei Shell.
I attended many rigmoves on her and also did Condition Survey on her in 1975.
On inspecting the preload tanks I was surprised to find that the inside bulkhead was corrugated and the bottom part was painted read while the top was gray.
The Drilling Superintendent had been on her since she left GoM and could enlighten me; preload tanks had been added on the outside of the original hull in early 1960s.
(As can be seen on the picture)
PS> She had no maritime certificates and had not been classed at that time. That changed in 1976, during a major alteration carried out in Labuan, 1976. She got water tight doors installed below deck and the original tall concave spudcans were replaced with new that had “North Sea Points”.
The new cans were built inside the old ones, then we took her just outside Victoria Harbour in Labuan, set the legs down and cut the bottom part off while keeping a slight positive buoyancy on the hull.
The lower leg sections were left behind and became a popular fishing spot. (Now inside reclaimed land)
There are two main reasons for punch through; crust and shallow gas. Natuna Sea is known for both.
PS> I have spent MANY hours “bobble watching” in this area.
In this case it was most likely caused by thin crust layer in the subsoil, since it happened during emplacement. (Probably during preloading)
There are many different opinion of the best way to avoid punch through. As many as there are rigmovers. (probably more)
To carry out proper site survey in advance is a must. Interpreting the raw data correctly to identify any punch through danger is next.
Keeping a low airgap during preloading reduce the risk of damages, but difficult in areas with large tidal difference. Off Tierra del Fuego, where the tidal difference is up to 11 mtrs., we invented “single leg preload”. I.e. loading on leg at each slack water, then doing a simultaneous preload to satisfy warranty requirements.
Newer J/Us are able to jack with full preload, thus the loading is done in the water, before lifting the hull out of the water for holding period.
The ultimate is the “Swiss Cheese” method. I.e. drilling several shallow holes at each leg location to break down the crust. This is done with the J/U afloat, or lightly pinned and moved between leg locations. Time consuming, but works very good.
Shallow gas blowout that can wash out the foundation with the rig in drilling position is usually fatal due to the large airgap.
The mitigating method is to drill one or more shallow pilot hole while the rig is lightly pinned on location. Any pocket of shallow gas is allowed to vent off before continuing with emplacement procedure.
If a large gas pocket is encountered the rig may have to be moved away before the ground crater and the drilling location changed, away from the crater.
PS> This is as much to avoid shallow gas blowout when drilling top hole as to avoid punch through during emplacement…
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