Decom Engineering buoyant after success of new C1-32 Chopsaw on Brent Charlie project

Decom Engineering had developed a pioneering “neutrally-buoyant” Chopsaw which has proved its capabilities on a landmark North Sea’s decommissioning project.

The subsea cutting specialist was tasked by offshore contractor Allseas with designing a Chopsaw specifically for the unique challenges posed by the conductor removal scope for the Brent Charlie platform decommissioning campaign, which required 40 well conductors to be cut at various water depths. 

The challenge was that the Chopsaw needed to be positioned and powered from a ROV subsea, with several cuts required below the conductor guide-frame.

As a result, the tool had to be neutrally buoyant in seawater and easily manoeuvrable with a single ROV, while the Chopsaw width was restricted to 3 metres due to the complexity of the conductor layout on the seabed.

Decom Engineering managing director, Sean Conway, said: “We successfully designed, manufactured, assembled, and tested the new Chopsaw within just six months, ready for offshore deployment from Allseas’ heavy lift vessel Pioneering Spirit.

“The C1-32 Chopsaw has demonstrated its effectiveness in the field, completing numerous conductor cuts with an average cut time of approximately one hour. The tool's capability to perform multiple subsea cuts before needing to return to deck has saved both operational and vessel time.

“With the buoyancy functioning as designed and tested, the ROV was able to easily navigate the Chopsaw to and from each conductor, even at depths as shallow as 20 meters below sea level.”

Allseas’ Pioneering Spirit safely removed the 31,000-tonne Brent Charlie topsides in a single historic lift on 9 July. After delivering the topsides to Hartlepool for dismantling and recycling, Pioneering Spirit returned to the Brent field to complete the conductor removal scope.

The C1-32 Chopsaw was engineered to cut conductors up to 32” outer diameter (OD), with a blade diameter of 2,100mm. Each of the 40 Brent Charlie conductors had an OD of 30 inches, consisting of both welded and Talon conductors, as well as some flexible risers.

With its neutrally buoyant design, the Chopsaw weighed approximately 6,700 kg in air but only about 50 kg in seawater, making it easily manoeuvrable by a ROV and allowing the tool to be flown into position and manoeuvred between conductors with precision.

To facilitate safe deployment and recovery, the Chopsaw was used in conjunction with a "deployment frame" which allowed the tool to be quickly deployed to the correct subsea depth and safely through the splash zone. Once at the desired depth, the ROV connected the hot stabs, opened the clamps, and removed the tool from the frame. 

All of Decom’s Chopsaws can be fitted with replaceable tips, allowing operators to change individual tips rather than the entire blade - a feature which significantly reduces labour and enhances safety during blade replacements on deck.

Sean Conway added: “We have a strong relationship with Allseas, having worked for them on a previous North Sea platform removal project. Completion of the Brent Charlie workscope further cements our credentials as a major provider of complex subsea conductor cutting services.

“Ongoing investment in R&D has been a critical factor in securing these projects and we will continue to invest and innovate so that our technologies