Mirage has recently completed a subsea window machining project that entailed the repair of a conductor in Apache North Sea, Forties Delta, in 115m water depth. The conductor, a 26-1/2” installation required a 1.3 x 0.45m window machining with 50mm radii 9m from the seabed so a mud dump vent clamp could be fitted successfully.
As part of the initial project research, cold cutting was specified as the application to use, purely down to the conductor housing casing and a power line that could not be damaged by cutting, being close to the conductor repair project.
Mirage’s flexibility in creating portable machine solutions for clients has meant that they are proficient at adapting their current stock of machinery for other uses. In the case of this project, Mirage modified their gantry milling machine, GM2000, and their milling rail, MR1500, for subsea use. A pipe clamp with chain tensioning was also produced allowing the diver to clamp the machine onto the conductor to begin the project.
The four corners were pilot drilled and then the four 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 were retained by cross member clamps with drilled and tapped holes in the conductor.The left-hand picture above shows how the gantry system chain clamp and deliver operation were able to keep to a minimum by using hydraulic stops and auto feeds on the main axis to avoid over-machining.
Meanwhile, the photo on the right-hand side shows the milling rail mounted onto the gantry system. This includes a large hand wheel for the diver to control the feed while the auto stop hydraulic valves controlled the size of the window and spindle position.
As part of the project, Mirage also ran a full mock up FAT trials in their UK HQ, as well as recording a demonstration of the removal of the gantry system and machined window and a full diver training DVD, so the project was completed safely, efficiently and successfully
After adapting and testing the machinery, Mirage along with the engineering team managed to both drill and complete trepanning of each hole within – 45 minutes. Combined with machining the top, bottom and sides through the conductor thickness, total machining time took just two days.