batanony, defaced and 3.2 Inspector,
I don't think it's necessary a fax to describe the welding sequence I've used successfully back in my days of erector engineer to join the two halves of a large diameter petrochemical tower. I'll describe it hereinafer and you'll tell me if you've understood it.
1st case. The two pieces are in the vertical position and the weld is horizontal. The two ends have been already fit up.
First thing, you should apply tacks welds. The quantity and length of tack welds depends on the tower diameter. Your good judgment will tell you how many and how long. For batanony's 5 meters diameter pipe I would apply , say, 12 tacks 8 inches long each. Most important, the tacks should be applied by two welders working at the same time in positions 180º apart from each other, or by four welders working at the same time in positions 90º apart from each other. The tacks will remain in place during welding, so the necessary precautions must be taken.
After tacking, the first pass will start. The first pass should be applied by two or four welders working at the same time. If there are two welders, they'll be 180º apart; if there are four, they'll be 90º apart from each other. For batanony's 5 meters pipe I'd use four welders. They will start welding at the same time going clockwise or counterclockwise, and they should finish at the same time, or with a few seconds difference. So, the welding speed should be controlled.
After the first pass is complete, let it cool down, unless the WPS calls for a minimum interpass temperature.
The subsequents passes are given in the same way.
2nd case. The two pieces are in the horizontal position and the weld is vertical. The two ends have been already fit up.
The tack welds are applied in the same manner already described.
Now the first pass will start. In this case, you can only use two welders. The first welder will start at the 6 o'clock position and the second at the 9 o'clock position, both going upwards and working at the same time. At approximately the same time (or with a few seconds difference), the first welder will arrive at the 3 o'clock position and the second at the 12 o'clock position. For this, the welding speed should be controlled.
At this point, the first welder will keep on welding upwards to the 12 o'clock position, while the second will go down to the 6 o'clock position and start welding upwards to the 9 o'clock position. They should arrive at their respective final points at the same time (or with a few seconds difference). In this manner, the first pass will be completed. Most important: the welders should always work at the same time, don't let one being at work while the other is not.
Allow the first pass to cool down, unless the WPS calls for a minimum interpass temperature. The subsequent passes are given in the same manner.
Now take a pencil and a piece of paper and draw the welding sequence described above. You'll see that it's easeir than it seems to be when you read it for the first time.
Any doubt, Gentlemen?
Giovanni S. Crisi
I've used the dearman on a hyperbaric 24" pipeline repair. The pipe was out of round about 1.5 inches, straightened right out, dearman says that it will make a pipe that is up to 2 or so inches out of round, round again. For the application that we used it for, we had to fabricate arm extensions because of the particular preheat pads and insulation that had to be used. I remember that there was a couple of other companies out there that offered similiar clamp, but the names escape me at the moment. 
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