Decommissioning has become a new buzzword in the offshore and nuclear industries. Dwindling oil prices and a shift towards more renewable energy sources has turned many operators’ attention towards decommissioning their ageing assets.
To say that dismantling and recovering these huge installations is difficult would be an understatement. The process is expensive, time consuming and can be hazardous. Any equipment used should be robust and reliable and be safe to operate. Thankfully, there’s portable machine tools available which have been designed specifically to ensure these projects are completed efficiently and safely.
Mirage has responded to the challenges presented by customers with purpose designed products – some which have now become part of the standard Mirage product line up.
Let’s take a look at some examples of decommissioning projects, and the Mirage products used.
Cutting Subsea Pipelines
Subsea pipelines are laid in very long sections, so when the need to decommission arises many cuts need to be made along the pipeline, enabling sections to be brought up on deck. These cuts can be made using a Mirage Diamond Wire Saw. These are ROV compatible, but can also be lowered into position from above and clamped into position by remote operation.
Drilling out flange studs
In the extreme conditions encountered offshore – flange studs can be prone to corrosion, making separation of flange joints incredibly difficult. One solution is to drill out the studs using a high torque portable drilling machine - such as those in the Mirage HT range. These powerful tools can be fixed to a job by a number of methods; including bolting, switch magnets and chain clamps. The powerful 3.7kw (hydraulic) drive motor on the HT50 allows studs up to 5” diameter to be drilled out.
Flange Bolt Cutting
An alternative to drilling out seized flange studs is to cut through them using the purpose designed Mirage Bolt Saw. This clever design used a circular saw blade, which is fed through the gap between the two raised face flanges. After each cut the machine is simply repositioned by loosening the ratchet strap, rotating and re-tightening. The latest model – (the MBS1360) has been designed to fit onto a wide range of flange diameters all the way up to 1810mm (71.25”).
A wellhead can be removed in both topside and subsea situations using one of the three Mirage portable band saws. The smallest saw in the range will cut diameters from 9” to 24” and the largest machine capable of cutting from 32” up to 48” can be severed easily. Typical applications include cutting conductors, caissons and multiple grouted strings.
Drilling Casing Lift Holes
1. Using a standard double drill unit
Removing tall cylindrical offshore items in the vertical orientation requires lift holes to be created. This then allows the item to be removed after it has been severed using one of the Mirage band saws. The standard DDU (double drill unit) uses a swing gate arrangement, allowing it to be placed into position and then secured using adjustable jaws. Following this, two lift holes are drilled on opposite sides of the casing simultaneously – reducing cutting time and eliminating the need for repositioning.
2. Using a horseshoe double drill unit
When access is difficult, the horseshoe double drill unit (DDHS4263) can come to the rescue. Instead of using the swing gate type of mechanism used in the standard DDU, this open machine is offered to the casing and then secured into position using hydraulically powered clamping legs. As with the standard DDU the machine drills both left holes simultaneously.
3. Using an internal casing cutter
In some situations, external access to create lift holes in the casing isn’t possible. In these tricky instances, the ‘DDI’ machine provides a solution. This machine is designed to be inserted inside the casing, allowing two lift holes to be created simultaneously from the inside.
Wharf decommissioning: Cutting through piles
Decommissioning projects aren’t just restricted to the offshore oil and gas sector. One example was when a wharf in Australia was being decommissioned as part of a regeneration project. The requirement was to remove 200 piles made from various materials such as timber with concrete sleeves, hollow steel, and concrete filled steel piles. The machine used was the MDWS1638.
A diamond wire saw is more than capable of cutting through a steel mooring chain. The Mirage manipulator saw is a smaller and quicker to deploy than the standard MDWS This making it ideal for this type of work. Please be aware though, that this method is not classed as ‘cold cutting’ - so a coolant will need to be applied when used topside.
Cutting into a Master Valve
One customer came to Mirage with a difficult decommissioning challenge. A lower master valve from a wellhead Christmas tree contained seized locking screws which meant it could not be removed from the platform. The solution was to use a Mirage clamshell/split frame cutter to cut through the 90mm thick valve casing. Before the actual offshore valve was cut the process was tested on a similar master valve at an onshore workshop.