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Sub sea elliptical milling for the Siri platform explained

gantry_millMarred by problems from the start of the project, the Siri platform's costs soared to $617m, almost double the original estimates.

One of the main problems was found during a routine inspection in August 2009 when the company discovered cracks in the nose of the platform’s subsea oil tank. No pollution occurred, but the production was shut down for five months as a safety measure and re-started in January 2010 after implementation of a temporary solution.

The Siri platform continued to require bespoke solutions, tooling and techniques to get it into a better state of repair.

Still profitable

But why did DONG Energy put so much effort into a platform that had fallen into such disrepair? 

Flemming Horn Nielsen, VP responsible for DONG Energy’s Danish oil and gas activities, said to Offshore Mag that the continuous maintenance and repair solutions is valid, despite the cost increase. 

“The Siri area still holds valuable reserves and offers further upside opportunities for new reserves in the wider area. The repair project has been necessary to maintain a high safety and environmental standard for the Siri platform, which will function as a host for a number of fields in the area for many years to come.”

Mirage's input

Situation & requirements

In water depths of 16 metres, significant cracking had been discovered in the main caisson around a vent exhaust duct on the Siri platform. Sub sea elliptical milling was required.

Mirage was approached by the subcontractor to supply a machine to mill a pair of elliptical holes to act as a crack-stop features in the 50mm thick steel caisson.

The machine had to be diver operated, and no secondary damage to the caisson was permitted - including no secondary drilling to locate the machine in place.


Mirage designed a purpose built milling gantry machine, based upon the company's existing gantry equipment, with unique location, restraining and control features to conduct the machining operation.

The machine included an extended stroke milling carriage, bespoke locating 'plug' to centre the machine, a combination of webbing straps and magnets to fix the machine in position and a template guide plate to control the drilling and milling operation.

In addition to achieving the specific machining profile, Mirage tailored the machine for diver operation to ensure it was appropriately robust for offshore operations.

The result

All machining operations were completed first time, to specification and within the customer defined time critical operations window, ensuring no incremental costs were incurred (technical or operational) and there was no impact on the facility operation.

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