Han Heilig discusses a new method developed to repair a crack on a live gas pipeline in the Java Sea that takes gas from a number of fields in Sumatra.
Artist impression of habitat installation. Images from DCN.
DCN International Diving and Marine Contractors (DCN) recently employed a new method for a client in Indonesia that made it possible to repair a crack on a 32in live gas pipeline 27m below the Java Sea, three times cheaper compared to installing a bypass.
Awarded by Perushaan Gas Negara (PGN), this was the largest engineering, procurement, installation and construction (EPIC) contract in the company’s history. Here, DCN was tasked to seal a tear in the seven-year-old South Sumatra to West Java Phase II transmission line that takes gas from a number of fields in Sumatra to the gas powered electricity generation plant in Bekasi, Jakarta.
Gas from this power station is used to supply electricity to Jakarta. Since switching to another pipeline was not possible, one of the requirements imposed by PGN was that throughout the repair, the gas supply had to be continued without interruption.
To be able to meet the requirement for continued operational availability of the pipeline during the repair, DCN proposed to carry out the repair by performing hyperbaric welding of a sleeve designed to contain the full design pressure of the pipeline, and provide the mechanical strength to reinstate the capacity of the pipe to bear operational loading.
Through the use of a habitat, a sealed working space that offers divers a safe and protected working environment, it allowed the repair to be performed in dry. The company carried out a series of dry runs and tests, and simulated all the elementary welding tasks necessary to ensure the long-term success of the repair at its facility.
The concept was to encapsulate the temporary Plidco repair clamp, isolate the breathable habitat atmosphere from any possible gas leakage from the pipe during the repair and provide equivalent mechanical strength in order to stabilize the crack to ensure long-term integrity of the pipeline.
The temporary repair clamp is not designed to transfer any mechanical loading. It provides a seal to stop gas escaping from the pipeline through the crack. A bolted containment clamp designed to seal around the clamp provided the ability to contain or vent gas that may potential escape from the Plidco, as a result of accidental damage to the Plidco during the repair.
The containment clamp was designed for the full design pressure of the pipeline (1150 psig or 7.93 MPa) and provided a way of isolating any leaks caused by disturbing the clamp. Hoses connected to the containment clamp allowed the detection of leaks from the clamp by passing argon gas through the containment clamp, and to a gas detector without affecting the breathable atmosphere in the habitat.
A stand-off sleeve (SOS), manufactured in two halves to encapsulate the containment clamp was welded to the pipe to provide long-term containment and reinforcement of the pipe. The shape of the SOS consisted of two truncated conical sections, with the small diameter sized to fit the outside diameter of the pipe. The SOS was manufactured from a 40mm thick plate and rolled to precise dimensions to allow fit up to the existing pipe and join the two halves of the SOS along the longitudinal seam.
During the pre-engineering survey, precise measurements of the existing pipe were taken. After removal of sufficient concrete and corrosion coating to allow access to the proposed weld locations, the ovality of the pipe both local to the planned welds and at the sealing location for the containment clamp was accurately determined.
Out of straightness of the pipe across the repair position was also determined. The pipe wall at the weld location was surveyed to determine wall thickness and for potential laminations. Design activities during the engineering phase ensured that the repair method did not impose any undue loads onto the pipeline prior to completion of the repair. Local and global finite element models were used to verify the design of the SOS against all operating and environmental loads.
Global modelling of the pipe crossing included the changes in loading from removal of the concrete; the effect of varying buoyancy when the habitat is placed flooded and then with atmosphere; placement onto the pipe of the containment clamp; and SOS in the dry and subsequent flooding during habitat removal at completion of the repair.
Of course, long term effects of the current and wave loading on the pipe, as well as expected operating pressure variations and temperature changes due to seasonal temperature difference were investigated.
Local finite element models were used to investigate the effect of the global loading on the existing crack and on the repair weld. Crack tip opening displacement measurements on the sample material welded using the welding procedures was used to calibrate the acceptance criteria against the expected weld parameters based on the actual material behavior.
A number of risks were identified early in the project. The obvious are the health, safety and environment risks associated with saturation diving, but the project team had to address also technical risks associated with achieving the required welding on an operating pipeline in the confined space of the habitat.
To ensure the quality of the repair weld and minimize risk for the divers the welds were designed and tested to comply with the requirements for a ‘golden weld’. This required extensive testing during the development of the welding procedures and development of the inspection methods based on time of flight diffraction.
For the mitigation of risk associated with welding under hyperbaric conditions, DCN utilized their hyperbaric testing center to test the welding procedures under simulated hyperbaric conditions. Initially, the proposed weld procedures were qualified under hyperbaric conditions using the same plate that was used to manufacture as the finished SOS.
The test plate underwent the same heat treatment as the final SOS. Subsequently, a full scale mock-up of the SOS on sample pipe sourced from the PGN original project stockpile was welded inside the hyperbaric test chamber.
The sample pipe was chosen to match the chemistry of the two joints the SOS was proposed to be welded to and thus the actual welding conditions were matched as close as practically. All welding machines, cables and umbilical to be utilized in the offshore campaign were first tested in the hyperbaric center.
One of the critical safety risks for the divers was the possibility of contamination of the breathable atmosphere inside the habitat, due to a possible leak from the pipeline caused by, for example, disturbance of the Plidco clamp. The containment barrier described above allowed venting of the gas to a safe area outside the habitat as the primary means of mitigating the risk.
A second mitigation was the development and testing of detailed procedures to ensure that both the containment clamp and the SOS were handled in an efficient and safe way. Each of the half section of the containment clamp and the SOS weighed approximately 2.5 ton.
Training of the divers inside the habitat was part of the mock-up trials. For this purpose, DCN set up the pipe at the angle and elevation as per the pre-engineering survey. The clamp installation procedure could then be tested inside the habitat.
The soil at the crossing locations consisted of 5m thick very soft clay, which overlaid a firmer, older clay layer. Choice of a suitable foundation concept for the habitat was critical in ensuring that the habitat with a dry weight of 80 tonne would remain in place during the estimated month long repair duration.
The chosen concept was suction pile foundations, with four suction piles used to support each of the four legs of the habitat. The suction piles consisted of 6m x 1400mm diameter tubulars that had piping and attachment lugs for connecting the suction pump skid on the top plate.
The habitat was then supported by landing plates that were attached to the top of the pile after removal of the pump skid. A fifth suction pile was planned as a test pile prior to installation of the actual foundation piles, to verify the design geotechnical parameters and confirm the load capacity of the piles.
This pile was placed away from the habitat location and then utilized to provide tie-back of the umbilical for the habitat and welding during the repair. Settlement of the habitat was monitored as the door seals used to seal around the pipe relied on elastomers in contact with the concrete coated pipe for sealing.
Excessive movement would deflect the seals and allow water to flood the habitat. By making the suction piles long enough to penetrate into the firm clay layer at 5m depth the possible settlement was reduced to insignificant levels. Indeed, monitoring the position of the top of the suction piles for the duration of the repair showed that the actual settlement was less than the 2cm accuracy of the Digiquartz used to measure the installed height of the suction piles.
Habitat on Normand Baltic
The implementation of the project itself also called for a carefully planned logistic organization. The first containers of equipment were shipped to Singapore. Here, all the equipment was then placed on the afterdeck of the DP2 vessel Normand Baltic.
The vessel subsequently sailed to the island of Batam to collect the sleeves that were also specially produced for this project. The purpose of the sleeves was to provide mechanical strength and contain the gas. As a result, four sets of sleeves were produced - one set for test welding, another for simulating handling in the habitat, one for actual welding of the cracked pipeline and a fourth as spare.
In preparation for the offshore work 30km off the coast, a pre-engineering survey of the project area was undertaken. This consisted of hull mounted multi-beam echo sounder (MBES) and sub-bottom profile survey, as well as close diver inspection of the existing facilities.
Vibrocore samples were also taken for geotechnical analysis to provide data for design of the suction piles. Similarly, on arriving for the final repair the MBES and diver survey were repeated to ensure that conditions of the pipeline had remained the same. After studying the data, a start was made on installing the five 6m long piles in the seabed made up of silt and clay.
The first pile was used as a trial pile to confirm that the capacity was according to the design expectations. The habitat subsequently was to be placed on the remaining four piles. To ensure installation of the piles within a tolerance of just 20cm, a complete location-finding system was placed on the pipeline. The placement of the habitat with its diving bell went without a hitch.
Following the installation of door seals around the habitat opening through which the pipe passed, it was purged to remove the water from inside the habitat. The gas containment barrier was installed on the pipeline to seal around the existing Plidco clamp, followed by the lower and upper sleeve.
The divers were then able to start on the demanding welding process while the gas continued to be pumped through the pipeline. Over a period of 10 days, welding was carried out uninterrupted, while the welded layers underwent continuous ultrasonic testing. Finally, the repaired section of the pipeline was fitted with an anti-corrosion wrapping. The entire repair project was successfully concluded, well within the agreed timeframe, on budget, and without a single personal injury.
Han Heilig is founder of Practica Productions. He started his career 32 years ago with Wijsmuller Towage & Salvage and was amongst others responsible for the worldwide introduction of the semi-submersible transport concept and the presentation of Dockwise Co.
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