A few years ago an offshore oil company situated in Cameroon decided to remove 2 old wellheads and one vertical signalling pipe from their shallow water field.
I was therefore sent to Douala to prepare and execute the work.
In the past I have already had the occasion to blow up some abandoned wellheads with bulk charges composed with a well known 2 component liquid explosive, and I thought I could use this explosive here too. Unfortunately once on location, I had to choose another solution because 1° this kind of explosive was not available in Cameroon and 2° the customer wanted the work completed as soon as possible so I did not have enough time to order these products.
The only explosives the local retailer had to offer were SIGMAGEL 605 a watergel or NC2 dynamite in small cartridges diameter. By experience I knew that to cut metal pieces with explosives, it was better to use a Product with a VOD over 5.000 m/sec. otherwise the effect was to deform the steel without severing it. My choice went therefore for the NC2. To initiate the explosive, I also ordered a few electrical detonators and a spool of 12 g/m detonating cord.
The first wellhead was composed with a 30'' outer casing and a 10 '' internal casing.
The second one was made of a 30'' - 10 '' and 7'' casing, the inner spaces were filled with concrete. As for the vertical tube, it was made of a 30'' pipe.
For the blasting of the two wellheads I decided to use an internal bulk charge because it was not possible for me to prepare a collision charge due to the scattering in the initiation time between normal electrical detonators.
To make the calculation the next formula was used:
C = S. K. 3
Where C = quantity of explosives (kg)
S = area of the cut (cm²)
K = factor depending of the nature/ width/shape of the piece to be cut (in our case K was equal to 20 and 25)
3 = majoration factor when the explosive is not in contact with the target.
Which gave respectively a consumption of 50 kg for the first wellhead and 55 kg for the second one. For the cutting of the vertical tube, it was not possible to use an internal charge because the pipe was sealed at the top. It was therefore decided to cut the tube as low as possible with an external circular plaster charge.
The quantity of explosive was calculated with the same formula except that here it was not necessary to use the majoration factor due to the fact that the charge would be in contact with the steel, which gave then a minimum theoretical charge of 15 kg.
A few days later after having received all the necessary clearance, the diving team composed partly with French and Cameroon's divers embarked on board of the supply boat that was to be our working vessel for the entire operation.
After a few hours of mobilisation we were ready to leave the port when a little surprise waited for us. In fact, just before dropping the last hawser we saw a nice little black and yellow snake slide on the after deck and promptly disappear under one of the diving container's.
Immediately on behalf of the African divers, we closed all the gangway doors to prevent the snake entering the accommodations, and with much care we tried to chase the animal from under the container. After a while the frighten reptile left his place to find another refuge into the .... Diver's umbilical. As the time went on and we were tributaries of the high tide to leave the river, we finally decided to remove the snake overboard with the help of the vessel fire fighting equipment.
During the 12 hour of journey, we prepared 2 metallic charge's supports with a diameter slightly smaller as the internal casings.
Next morning, we were in position on the oil field. As the supply boat was too small to take the wellheads on board, we had to tow them one by one to a deep dropping place. The first day was therefore devoted to remove some items from the wellhead structure and also to secure the casings so as to keep them together during the towing.
While some men were working on the structure, I started the preparation of the first bulk charge with the help of one of my college's. As it is well known, dynamite as 2 rates of detonation, a low decomposition with a VOD less than 3.000 m/sec and a fast rate with a VOD higher than 5.000 m/sec. To obtain the second decomposition rate, it is quite important (and this even more under water) to have a powerful initiation. As we only disposed of 12 g/m detonating cord, I decided to recover the metallic shaft of the charge support with several rows of cortex which would then act as a good booster with a shock wave moving from the centre to the periphery.
As the first blasting was programmed for the next day at 8 AM. We had informed the field manager that we were going to have a radio silence since 7 AM, which would allow us to install the charge and the firing circuit. After a good night sleep, we were ready for the operation. The weather and sea conditions were perfect with no risk of thunderstorm. We could begin operations.
Seen that the water depth was only 6 m deep, it was necessary to calculate if there was enough water above the charge to avoid an acoustical shock wave. The used formula was:
RB = 1,5. C1/3.
R.B = Bubble boundary (m)
C = quantity of explosives (kg)
As we had intended to fix the charge at 3 m below the mudline we could expect a bubble expansion of about 5,7 m diameter so there was no risk to have such a nuisance.
Secondly once the towing cable was connected to the top of the casing I wanted to place the supply boat at a distance of about 300 m from the blast, therefore the theoretical pressure on the hull of the vessel was calculated with the help of the Cole formula
PM = K (C1/3/ D)a
PM = peak pressure (bar)
K = constant depending of the explosive
C= instantaneous charge weight (kg)
D = distance from the explosion (m)
a = Exponent depending of the explosive
The calculation indicated that the theoretical pressure at 300 m would therefore probably be less than 4 bar, pressure that the hull could easily withstand.
As planned, the charge was lowered and fixed at 3 m under the mud line and afterwards, after having connected 2 electrical detonators to the firing line, they were fastened to the emerging cortex. Everything was ready; the only thing left to do was to leave the structure and to uncoil the firing line from the structure to the vessel where we could hide ourselves from any accidental flying shrapnel.
Unfortunately, while we were lowering the explosive inside the wellhead casing, the sea became crowded with dozen's of pirogues which probably informed by the bush tom-tom waited the blasting time with delight.
What could we do? Wait until they went away? No way, we would still be there. We inform them of the danger, with the vessel's loud speaker. No reaction, we informed them in French and in Pidgin, those damned fishermen didn't move away from the area.
Could I shoot with all these guys dozing in their boats with sometimes their hands or feet hanging in the water?
Here again I used a good old formula learned in one of my technical books to verify what would the minimal safety distance be for a swimmer.
SD = K . Q1/3
SD = the safe distance (m)
K = a factor depending of the charge depth
Q = the quantity of explosive (kg)
With a charge hanging at a depth of 9 meters K was equal to 150 so that the safety distance had to be equal to 570 m.
As nearly all the fishermen pirogues were inside this zone, I decided to break the radio silence to inform the field manager that to disperse them, we needed the assistance of the Cameroon's Navy together with a few speedboats working on the field.
Once on the site, the dissuasion task force could only chase the '' enemy ''with the help of speedboat water guns. And it was so that between all that confusion I had to choose the best moment the fire the bulk charge.
As foreseen there was no acoustical wave, only a huge column of water burst some 20 m in the air.
Immediately after the blast as we could imagine, every one around the area went to rush for the dozens of dead fishes drifting onto the water.
As for us, the only thing left to do was to tow the first wellhead to the deep dropping zone.
The blasting of the second structure went as well as the first one, except that this time we had informed the Navy and the speedboats to be in position at the beginning of the operation.
The cutting of the vertical signal pipe was made differently. As explained earlier, it was impossible to install a bulk charge inside the pipe, therefore, at first the divers removed the mud with a waterjet and an airlift in order to fix the external charge some 2 m under the mudline. The charge (20 kg) was made with a series of cartridge wrapped into a waterhose. Here again several rows of detonating cord were fixed on the opposite side of the target in order to obtain the best efficiency from the explosive.
For some technical reasons, it was necessary that after the cutting, the pipe fell in a well precise direction. As for this severing I was using a circular cutting charge, I could not predict which way the pipe would fall. Therefore, I decided to leave a small bridge of unexploded metal on the due falling side. This small uncut piece would therefore keep the pipe in a vertical position after the blast.
After the explosion, everything went on as forecast, a huge water spray, plenty of fish and a standing tube. The only thing left to do was to crack the small bridge with a nice little pull from the supply boat and the tube fell in the expected direction.
After another few hours towing, the third and last structure rejoined the deep where it now serves as a refuge for some nice and delicious crayfishes.
After seeing the results, it seems that the consumption of explosives was a bit to high for the severing of the 2 wellheads because the several casings were destroyed over more than 2,5 m which of course was unnecessary.
As for the external cutting we found a quite nice sharp cutting with only a relatively small internal depression of the tube.
(Translated by the author from a reading at the Belgian Professional Diving School 1993)