When working in difficult, challenging and sometimes unpredictable environments, thinking ‘outside of the box’ is a necessity. Problem solving quickly, innovatively and above all safely is a must when the unexpected happens, and yet many organisations in the oil and gas industry often fall into the “one size fits all” category of problem solving.
That isn’t the case for us at Marine Technical Limits (MTL). Through careful planning and our safe systems of work, we consistently innovate to develop pioneering solutions for our clients. That was certainly the case when we were presented with an FPSO which had a crack into the bilge keel plating in a ballast tank, resulting in an ingress of water into the tank. Due to the location and vertical orientation of the crack, there was a genuine and immediate risk to the integrity of the FPSO, particularly as the winter season loomed. Our team of Naval Architects and Engineers worked tirelessly to identify a suitable temporary repair without the need for diving.
The issue had originated from vertical notches that had been introduced at numerous locations in the bilge keels during a previous modification scope of work carried out by the organisation. These notches had created discontinuities in the bilge keel arrangement and introduced specific areas of stress. As a result, these notches had initiated cracks into the bilge keel ground bars. At some locations these cracks had propagated into the bilge keel plating in the ship’s hull, although only one location was found to be a through thickness crack.
To ensure the stability of the crack, 3rd party analysis was initially conducted to identify practical methods to reduce the risk of brittle fracture resulting in crack escalation. The recommendation from this analysis was to alter the vessel loading to increase hogging moment and put the crack into compression. When our recommendations were implemented by the vessel crew the ingress of water was immediately arrested, undeniably proving the calculations performed during the analysis were accurate!
After further inspections and review of the problem, our Naval Architects proposed several solutions to our client and their classification society. Due to the reluctance for diving excluding a traditional cofferdam type repair, we agreed that the focus would be to deliver a world-first repair to the FPSO side shell; the use of well-established hot tapping equipment commonly used to perform interventions on live pipework or vessels. A repair of this kind has never been carried out to the side shell of an on-station or in production FPSO before.
The definition of hot tapping can be summarised as:
“Hot tapping, or pressure tapping, is the method of making a connection to existing piping or pressure vessels without the interrupting or emptying of that section of pipe or vessel. This means that a pipe or tank can continue to be in operation whilst maintenance or modifications are being done to it.”
Our teams immediately got to work on the extensive planning and preparation process. At MTL we are the industry leaders in risk-based integrity management an on-station repairs for FPSO’s and floating structures. We firmly believe a good job is delivered through careful planning and having a sound understanding of the complexities of performing hotworks in confined spaces onboard operational FPSO’s. Ensuring we address all aspects of the workscope, recognise and put in place contingency plans for all eventualities, while maintaining our stringent safe systems of work is of the utmost importance to us.
During project planning it was established that the standard hot tapping cutters would not be suitable for performing the hot tap to the bilge keel arrangement. Further to this, the hot tap machine, which as standard is suitable for pressures up to 100 barg, had an oversized tapping adaptor which would lead to on-site clashes with immovable structure within the tanks. Working alongside the hot tapping subcontractor, we sanctioned bespoke cutters and a custom tapping adaptor suitable only for the anticipated pressures expected at this worksite.
An onshore trial of the hot tapping equipment was conducted over two days to verify the capability of the equipment and establish any issues which may prevent a successful project conclusion. The trail was performed on two fabricated mock-ups of the on-site bilge keel plating arrangement.
Once planning and engineering had been finalised, and the equipment mobilised offshore, our team of offshore construction supervisors mobilised to the vessel to carry out the repairs whilst the vessel remained on-station and in production.
Initially, our industry leading ‘Safe System of Work’ equipment was set-up to provide a safe means of ventilating the tank atmosphere and providing power for hotwork tools and equipment. Bespoke pressurised habitats and equipment were erected at the tank hatches to provide a positively pressurised environment within the tank, capable of initiating an automatic hotwork shutdown in the event of loss of positive pressure.
Using our specially qualified water-backed welding procedures, the hotwork team carefully welded the pipe stub to the internal side shell plating, ensuring the full extents of the crack was captured within the internal diameter of the pipe stub. A pressure test was performed to validate the water-tight integrity of the welded connection.
Upon completion of the pipe stub welding, a Class approved shipside gate valve was installed. This valve would later facilitate the isolation of the hot tap machine from the seawater once the hot tapping operation had been completed, allowing the removal of the machine without the risk of water ingress.
The modified hot tap machine was installed to the gate valve, the valve was fully opened, and the hot tapping operation commenced.
Mitigating the Threat
During the project planning, and as a direct result of the onshore trials performed, a risk from a full coupon release was established which could have created difficulties with retracting the hot tap cutting head. As such, the hot tapping operation was engineered to cut fully through the bilge plating, but only partially cut into the bilge keel arrangement. By engineering the operation to ensure the coupon would not be accidently released, we mitigated the structural integrity threat of the crack propagating any further into the hull plating and ensured the hot tapping equipment could be fully isolated and removed upon scope completion.
Once the hot tapping operation had been successfully completed, the hot tapping cutter was retracted into the hot tap machine, the gate valve was closed and a blank flange was fitted to safely secure the overboard pipe stub arrangement.
After the execution of the first successful hot tap repair, this solution was undertaken at two further locations where similar defects had subsequently been identified.
The benefits to using hot tapping technology in this situation was to deliver a suitable on-station temporary repair without the need for diver intervention, whilst ensuring the repair could be executed at the Clients convenience (not weather dependent).
The initial project, which took 16 days to complete, was a perfect example of how we strive to explore the technical limits of what can be achieved, and creatively innovate to ensure the success of every project we’re involved in.
We are committed to delivering certainty through our stringent planning processes. Each project is treated uniquely, with the specific asset constraints understood from the outset. Our rich experience with problem solving ensures no problem is too difficult to overcome safely and efficiently.