Acergy was responsible as lead contractor for project management and engineering of a subsea hot tap tie-in of the Ettrick field North Sea gas export pipeline operated by Nexen Petroleum UK Ltd. to allow production to be exported to a gas plant north of Aberdeen, UK.
A unique aspect of the hot tap was that the operating pressure of the pipeline was potentially 172.3 barg, higher than any previous hot tap on or offshore. This meant that two custom hot tap machines with a pressure-balance capability of 172.3 barg would have to be designed and manufactured.
Acergy’s scope was to engineer, procure, install, and pre-commission the hot tap assembly, in addition to supplying the temporary and permanent subsea structures. Acergy retained T.D. Williamson SA (TDW) to provide specialist services to support the subsea hot tap operation.
Hot tap paves way
Shutting down a live gas pipeline is costly and time-consuming. Hot tapping was developed as a safe method of trepanning - or drilling into - a pipeline without shutting it down. "Hot tapping" is the process of drilling into a live, onstream piping system without any content release or flow interruption.
The first step in hot tapping is to secure a hot tap fitting onto the pipeline. A permanent valve is attached to the fitting. The hot tap machine then is installed on the valve, and the valve is opened. Finally, the entire assembly is pressure-tested.
Cutting commences using a pilot drill to steady the cutter and to capture the cut pipe - or coupon. The hot tap machine fills with product and all air is purged from the housing. The main tap then is made through the pipeline with the requisite size hot tap cutter, after which the coupon is removed. The valve is closed and the hot tap machine recovered. A spur line then can be installed. When the valve is opened, the new connection is put into service.
In this instance the operator needed to access the line in order to tie-in the gas export line without shutting down the flowing pipeline.
Unique challenges
The pipeline transports approximately 20% of the UK’s gas supply. Any problems arising from the hot tap operation could potentially shut down the pipeline, halting the gas supply and oil production in a large number of oil fields. This would have a major impact on the UK economy. Therefore, it was necessary to demonstrate the highest levels of assurance on all critical elements of the job. These critical elements were:
- Welding and non-destructive examination (NDE) of the branch connection
- Installation of the protection structure
- Hot tapping the pipeline.
Acergy was in the challenging position of satisfying the stringent integrity requirements of the operator, while working against an extremely tight timeline. In preparation, a meticulously-planned schedule of activities was embarked upon to ensure that the subsea hot tap operation would be a success.
Preparations provide assurance
For five months commencing August 2008, equipment development, testing, and operational trials began at TDW’s facility in Nivelles, Belgium. Its top priority was to design and build the custom hot tap machines. It would be the first such pressure-balanced subsea hot tap machine manufactured by the company’s European arm. Following analysis of the targeted pipeline, designs for the machine were finalized. Materials were procured and production was launched. Because the new hot tap machines were to be pressure-balanced to 172.3 barg, TDW hydraulically tested them to 258.6 barg. In addition, the seals were gas tested to the same pressure to ensure that they were tight, and would retain their integrity during the hot tap. These pressure tests met the Factory Acceptance Test requirements. Because the machines represented a new design, a failure modes and effects analysis (FMEA) also was carried out.
| The hot tap machine inside the temporary installation frame at the SIT in Methil. |
The two identical pressure-balanced hot tap machines were manufactured to provide 100% back-up of this critical component. Designed with the requisite pressure balance capability, the machine interfaced with the hot tap valve. The system was powered by a subsea electro-hydraulic unit via umbilical cable housed on a double-drum cable spooler. The hot tap machine is controlled by technicians at an electrical control cabinet on the dive support vessel. In addition, five hot tap cutters and five pilot drills were supplied with each system as contingencies.
| The Acergy Osprey mobilizing for the Ettrick field hot tap operation. |
A two-phase, factory trial was conducted at the National Grid Gas facility in Ambergate, UK, to determine the system operating parameters. Phase 1 involved a series of nearly 24 test cuts to prove the durability and reliability of the cutters and pilot drills. Visual and NDE inspections of the hot tap cutters and pilot drills were done before and after the cutting. The second phase consisted of hot tapping actual sections of the 30-in. (76-cm) diameter (OD) construction pipe to ensure equipment endurance, alignment, and cutting effectiveness.
| The hot tap machine lowered to interface with the hot tap valve during the SIT. |
In conjunction with the Ambergate trials, the actual hot tap dive team underwent an orientation with the hot tap machine and system components.
Subsea operation simulated
To further confirm the integrity of the custom hot tap equipment, a System Integration Test (SIT) simulated a full hot tap operation in Methil, UK. From an equipment perspective, the primary SIT objective was to verify that all custom components fit together and operated according to the design.
The hot tap simulation was carried out in the 90-ton (82-metric ton) custom-made protection frame on a pressurized test vessel. The hot tap machine was set for optimum operational parameters derived at the factory trials. Using a welded hot tap assembly identical to the eventual hot tap branch nipple and sleeve and welded onto a section of the actual pipeline to be tapped, the assembly was positioned in the protection structure. Acergy divers operated the hot tap machine, allowing the pipe section to reach the level of pressure that would most likely be reached in the live pipeline offshore.
The SIT demonstrated that:
- The system did not leak and accurately cut the pipe
- The coupon was retained on the pilot drill
- The hot tap machine remained pressure-balanced throughout.
Subsea hot tap operation goes to plan
The hot tap was to be carried out at approximately 94 m (311 ft.) sea depth on a development in the outer Moray Firth. In June 2008, Acergy installed the hot tap assembly, which consisted of a 12-in. (30.5-cm) bore-welded nipple and bolted reinforcement configuration. Once the protective frame and pressurized dry habitat were placed on the seabed, operations began. In July 2008, the hot tap team mobilized to the hot tap site. Working from the Acergy Osprey dive support vessel, two technicians consulted with the divers around-the-clock via images from cameras on the divers’ helmets and an ROV. The team also monitored for bubbles which could signal a leak.
The entire offshore hot tap phase was nine days. Of that, the actual hot tap was completed in one day. Trepanning, which allowed for all valve tests and pressure testing of the assembly, lasted just over seven hours. The program went as planned with no Lost Time Incidents or production downtime. The cutting operation went smoothly, and technicians removed and recovered the 12-in. (30.5-cm) pipe coupon and hot tap machine to the Acergy Osprey. A piping-valve assembly was attached, providing two tie-in points: a 6-in. (15-cm) tie-in to the adjacent field and a 12-in. for a future tie-in.
Throughout operations a prevailing pipeline operating pressure of 117 barg (1,096 psi) was maintained and hydrocarbons flowed at normal rates.
Considerable challenges were overcome. By producing a hot tap machine with the highest pressure-balance capability ever developed, the team broke the pipeline’s integrity in a controlled manner, allowing pressure to remain steady.
The second major hurdle was to satisfy the high technical integrity requirements of the operator on a very tight schedule. A series of carefully planned and successfully executed tests and trials resulted in a successful subsea hot tap operation, paving the way for the Ettrick field gas export pipeline tie-in.
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