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Guide to subsea cutting and machining projects using portable machine tools


The type of subsea problems operators need help with can vary enormously, and with oil and gas decommissioning projects on the increase there will be a huge amount of new machining challenges to face over the coming years.

So what are the kind of projects where portable machine tools have helped to deliver successful outcomes for subsea operators? 

We've put together some examples to provide you with an insight into the typical problems faced and how they are resolved.

Many on site machining tasks demand a successful outcome first time. This may be due to costs, time constraints, or simply because of changing weather and tidal conditions. Therefore it is advisable to carry out workshop trials in advance which can help to minimise unforeseen issues on location. These can normally take place either in the Mirage workshop or at the customer’s onshore locations.

Below are some example projects demonstrating how some tricky subsea problems were solved using portable machine tools.

Machining a window into a conductor to fit a mud dump vent clamp


Solution and machine used:  For this  project, cold cutting was carried out to prevent damage to the conductor housing casing and a power line. The equipment used consisted of  the Mirage gantry mill  and milling rail MRY1500 used with a chain pipe clamp.

Project Outcomes:

  • Machining completed within 2 days.
  • Operation was  time kept to a minimum using hydraulic stops and auto feeds on the main axis.

How to remove a 512 metre length of Super Duplex pipe from the sea bed.


Solution and machine used: The Mirage MDWS 620 portable diamond wire saw was used with a 3 circuit hydraulic controller. The diamond wire travelled at  200 metres per minute and with a feed rate:of 8mm per minute.The  Pipe specification was super duplex material with a 12" diameter and wall thickness of 1"

Project Outcomes

  • 22 cuts made within 72 hours allowing
  • Pipe sections each around 23m long to be raised to deck.
  • With the pipeline removed the customer was able to construct a new pipeline.

How to remove a longitudinal weld seam from a pipe to allow fitting of a ‘Morgrip’ connector.


Solution and machine used: Workshop trials were carried out in advance using the Mirage MR1500SS milling rail with a chain pipe clamp.

Project Outcome

  • Successful machining allowed fitting of the,‘Morgrip’ mechanical connector. 

How to terminate a crack in a main caisson 


Solution and machine used: 

Cracking was discovered in the main caisson around a vent exhaust duct on the platform leg. To terminate the crack, the customer's requirement was to machine an elliptical hole into a 5m diameter platform leg.  Workshop trials were carried out in advance using a Mirage 3 axis gantry milling machine.

Project Outcome:

  • All machining operations were completed first time, to customer’s specification and within time critical operations window

Removing plates from tidal turbine monopoles to improve water flow


Solution and machine used: 

Plates dimensions were (1.8m x 150mm x 30mm). These were removed using a slitting saw. The remaining material was removed using face milling. Quick release tooling was used to allow fast project completion..

Project Outcome:

  • All machining operations were completed first time, to specification and within time critical operations window. Diving time was severely restricted to 20 minutes due to tidal direction change and ‘Neap Tide’.

Removing a seam weld over 3m length and anode doubler plates


Solution and machine used: 

Cold cutting recommended to prevent cutting into the parent pipeline. The Mirage MR1000SS was used with a pipe chain clamping system, fixed feed, and depth stops.

Project Outcomes

  • Milling of seam: less than 1 hour.
  • Doubler plate removed in 3 hours.

Solving a mechanical failure in a subsea tree’s latch lock removal system


Solution and machine used: 

A different type of project which did not require a machining tool, but the design and development of products to fix the problem. The successful solution incorporated a lathe chuck, with ROV handles to deploy the latch onto the inconel rod.

Project Outcomes

  • Three different tooling solutions were provided in 10 days.
  • The client’s problem was resolved.
  • Tool was designed to prevent the problem from occurring in the future.

How to machine an injection point into a trace heated supply line


Solution and machine used: 

Mirage provided a complete machine/offshore service package. A process was identified to series trepan the external of the supply line. Extensive workshop testing carried out in advance. Material: Super Duplex, with heavy wall thickness. The machine used on this project was a HT50 portable drill.

Project Outcomes:

  • Successful completion of the subsea operation. Client testimonial:“Did a great job offshore and everyone was very pleased”
  • No swarf ingress or tool dwelling.
  • Success achieved despite just one chance on offshore application.


  • Project Engineers tasked with subsea decommissioning projects can be reassured that no matter what cutting and machining challenges lie ahead, there are plenty of products available for purchase or rental to complete the job successfully.
  • For some of the more complex tasks it is advisable to carry out workshop trials to make sure everything goes to plan on location.
  • Remember that if you cannot see an suitable machine online a bespoke one can often be purpose built, or an existing machine adapted to suit the requirement.

 if you need any advice for a machining project please click here to email us. You are also welcome to download our Subsea Case Study Pack in pdf format.

Download the Subsea Case Study Pack

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