A 26-1/2” conductor required a 1.3 x 0.45m window machining with 50mm radii 9m from the sea bed for a mud dump vent clamp to be fitted.
Cold cutting was specified due to the 26-1/2” conductor housing 13-3/8” casing and a power line that could not be damaged during cutting.
Mirage Gantry (GM2000) and Milling Rail (MR1500) were modified for sub-sea use and a pipe clamp with chain tensioning produced to allow the diver to clamp the machine onto the conductor.
The four corners were pilot drilled and then the 4 off 100mm diameter holes were trepanned at fixed centres with hydraulic valves acting as datum point stops. The gantry system was then used to mill between the corners to produce the window and the cut out portion of the conductor was retained by cross member clamps with drilled and tapped holes in the conductor.
Drilling and trepanning –45 minutes per hole, machining top, bottom and sides through conductor thickness –total machining time –2 days.