Record offshore hot tap in Australia

Connecting a 30 inch line carrying the bounteous flow of gas from subsea wells to an existing 40 inch trunk line lying 127 m below the surface poses numerous technical and environmental concerns.

For dive teams and equipment operating in an extreme deep water environment, the obvious goal is for the task at hand to go according to plan – but a lot of things can easily go wrong. Ultimately, the key to success hinged on a brilliant and determined group of T.D.Williamson (TDW) engineers who were charged with designing custom adaptations to the TDW SubSea 1000 XL tapping machine and solving several challenges presented by a combination of unique factors.

The customer had very strict requirements. First among them was 100 per cent system redundancy on the project site. Redundancy included identical tapping machines, a full compliment of spare parts with backup technicians and divers. When a job site is 160 km from the nearest land and a problem occurs that stops the tapping process at a critical point, it could take days or weeks to move a new tapping machine to the job site, even longer if a replacement tapping machine has to be manufactured from scratch. The cost of remaining on site waiting for replacement equipment could become extreme.

The second requirement was that all machine fluids used to operate the SubSea 1000 XL had to be completely environmentally friendly. If the machine were to leak any fluids, the leakage could not be allowed to inflict harm to the ecosystem. The environmentally friendly fluids that were selected for the job had somewhat different characteristics that the engineering team had to factor into the final design.

There were many hours of design and redesign and more long hours of test and retest. Starting at the TDW manufacturing plant in Tulsa, Oklahoma, the engineering team studied every aspect of the project and worked closely with customer representatives. Identical tapping machines were built and test vessels were prepared. Test vessels duplicated the size and thickness of the 30 inch cast hot-tap flange that the tapping machine would be required to penetrate.

Cutting through a 3 inch thick metal membrane at depth presents a series of considerations that must be addressed to assure unanticipated situations do not arise. Metal hardness, temperature, pressure and ocean currents are all factors that affect machine stability, torque and cutting rate. Engineers tested and refined the teeth on cutters to assure that, once the tapping operation started, the cutter would complete the tap. All preliminary design and testing focused on being able to mount the tapping machine to a pre-existing hot tap flange and conduct the tapping operation, start to finish, without stops or delays.

Running equipment and systems through tests on dry land is one thing. The question then becomes, how will everything work once it is under water and under great pressure? To answer that question, a second phase of testing was arranged. All equipment and technicians made a journey to the Naval Surface Warfare Division at Carderock, Maryland. Inside a pressurised under water hyperbaric chamber, equipment repeated the previous testing process in conditions duplicating those that were expected to be encountered at the project site. All aspects of equipment performance were examined including possible leakage of any fluids into the water. After achieving clean test results, the set of fully redundant tapping machine systems was shipped to Australia for system integration testing.

Upon arrival in Australia, the TDW equipment made its way to a towering assembly facility where the SubSea 1000 XL tapping machine was mated with a tapping tool deployment frame designed to descend onto a lower protection frame straddling the pipeline on the ocean floor. The entire lifting and lowering process was performed inside the assembly facility so that divers and engineers could observe the process that would shortly take place in dark water at depths of over 120 m. Once on site, divers would control the descent of the frame and tapping equipment. The lowering process is highly versatile and allows divers to move the frame in virtually any direction to accurately align pins and bolts that secure all equipment to the lower protection frame and to the future tap flange on the pipeline.

Inside the assembly facility divers worked closely with TDW technicians to operate the SubSea 1000 XL. Once in the water and at pipeline depth, divers operating the tapping machine would have TV cameras on their diving helmets to allow TDW technicians to monitor every phase of equipment operation.

Exact measurements are critical. The cutter must penetrate the metal membrane in the hot tap flange without touching the pig guide bars located just below. Pre-testing with precise valve simulators and tapping flanges allowed TDW engineers to produce SubSea 1000 XL tapping machines that would perform to specifications once on site and at depth.

As preparations were nearing completion, a delay was encountered. Because of back-to-back cyclones that struck the area earlier in the year, additional time was required until offshore conditions were acceptable. As a result, all equipment was crated and stored. Technicians were sent back to America to await a final go-ahead to perform the sub-sea tap.

TDW technicians were called back to Australia and left the United States on 24 August 2007 to prepare equipment for transport to Karratha. Equipment was reassembled and an extra test tap was performed to assure that the machines were ready for the task and to allow divers additional rehearsal time before they worked with the tapping machines in the dark, artificially lit depths with the added challenge of shifting currents.

Equipment was loaded onboard a dive support ship which set to sea from a dock in Dampier on Sunday 9 September. After an eight hour boat ride, the ship reached the project site. On 16 September a team of three divers descended to co-ordinate placement of the deployment frame that was lowered with the valve and tapping machine pre-assembled and ready for attachment to the future tap flange already in place on the pipeline. The same team of divers started the hot tapping process at 12:50 p.m. and established penetration by the pilot drill before their shift was over when they were replaced by three more divers. The second dive team successfully completed the tapping process at 10:22 p.m. that same night. Helmet mounted cameras allowed TDW technicians on the ship to watch every move and communicate directly with each diver during the entire operation.

The tapping operation was conducted exactly as rehearsed. TDW completed the subsea hot tap in just over six hours with a single cutter. No problems were encountered and the TDW SubSea 1000 XL tapping machine performed flawlessly in extreme conditions. Upon completion, the cutter was retracted and the valve closed. The tapping machine, with the coupon still inside, was raised with the frame assembly and loaded back on board the ship. Equipment was then offloaded from the ship at the dock, crated and sent back to Perth.

Once back on land, TDW technicians were informed that they had just performed the largest verified subsea hot tap on a class 900 system at full operating pressure. The operation was a total team effort that included TDW design engineers working closely with their counterparts on the customer side, the technicians who directed the operation from topside, and the outstanding dive teams who operated TDW equipment like seasoned professionals.

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