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Supporting the DONG Siri project with portable gantry milling machines


Located in the Danish sector of the North Sea, client DONG (Danish Oil and Natural Gas) required the permanent repair of the field which holds recoverable reserves of 60 million barrels.

Through Subsea 7, Mirage Machines has helped to stabilise and re-establish the integrity of the DONG Siri platform following the discovery of cracks in the structure in 2009 - cracks that were severly reducing the load bearing capacity of the platform.

This is the case study of how we used one of our gantry milling machines to begin the repair process.


Subsea 7 required a pair of elliptical holes to be used as crack-stop features in the main caisson of the Siri platform.


220km from the coast of Denmark in the North Sea.


Mirage supplied a portable gantry milling machine which was adapted for Subsea 7 to include unique control location, extended stroke milling carriage and a locating plug to the centre of the machine.

The 50mm thick steel tower had experienced significant cracking in the caisson around a vent exhaust duct. As well as producing a machine that matched the profile of the job, the bespoke gantry mill had to be adapted for diver operation, 16 metres below sea level, and ensure it was robust enough for more offshore operations.

Due to the severity of the crack, no secondary damage was permitted, including drilling the machine in place.

Completion date

Due to the severity of the cracks, and the repairs being made 65 metres under sea, the project is expected to finish by the end of 2014.

Gantry mill range

Mirage's portable gantry milling machines modular rail kit can be used to convert our 2-axis milling machines into a large area 3-axis maching in situ.

This modular assembly provides great flexibility throughout the range, whilst both axis are drive on variable feeds which offer high precision in use. The quick set-up of all machines can also be achieved with the jacking system included, and has previously been used for repairs to heat exchangers, pump and motor pads, as well as ship building and turbine split lines.


This multi-purpose and multi-location machine also comes with an extension kit which includes:

  • 2 off linear rail and blocks
  • Sub frame
  • Rack feed
  • Mounting plates
  • Jointing kit
  • Manual and tool kit
  • CE certification
  • Shipping and storage box

As the repair project for the Siri platform comes to an end this year after a 5 year absence, clients Subsea 7 and DONG can be assured that their platform, using the machine, is ready to take advantage of the 60 million barrels.

Download the Mirage Machines Linear Milling Buyers Guide

Photo Credit: Offshore Technology

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