Application of underwater welding processes for subsea pipelines

While welding repairs or hot tapping onshore pipelines is a common occurrence, welding repairs on subsea pipelines is most often never even considered. However, the risks involved with welding subsea in-service pipelines underwater is present and needs to be managed by ensuring that, when conducted welding is performed in a reproducible and consistent manner.

The fact that electric arc technology could operate underwater has been known for over a 100 years. The first ever underwater welding was carried out by British Admiralty Dockyard for sealing leaking ship rivets below the water line in the early 1900s and the specific waterproof electrodes and the methods to use underwater were developed in Holland by ‘Van der Willingen’ in 1946.

Increasing the underwater welding practice

In recent years, the number of offshore structures, pipelines, and platforms being installed in deeper waters has increased. Some of these pipelines and structures will experience failures. Any repair for these on location will require the use of underwater welding.

When confronted with the issue of underwater welding, we often question: “Why should we consider underwater welding in the first place?” The immediate answer is “Why not?”

It’s true; it is not a commonly-used technique and it does require meticulous planning, availability of highly-skilled tradesmen and tenacity to be successful. Even then, it is still a viable technique.

If the issues are analysed, there is no valid reason not to consider underwater welding, especially if production losses due to outage for repairs is punitive. Sunsea welding generally needs specialised welding knowledge combined with diving skills, which is more demanding than run-of-the-mill commercial divers can offer. Subsea welding covers areas of repairing pipelines, offshore oil platforms and ships.

Subsea welding also reduces the cost for the company by directly carrying out the welding work on location, saving time lost in production to the company.

Furthermore, because of the offshore exploration, drilling, and recovery of gas and oil in deeper waters today, it is necessary to have the capability to repair pipelines and the portion of drill rigs and production platforms which are deep underwater.

Risks and precautions

Welding underwater can be a dangerous profession if precautions aren’t taken. The main risks are electric shock and the possibility of producing in the arc mixtures of hydrogen and oxygen in pockets, which might set off an explosion. The other common danger is breathing nitrogen in the air mix, which is absorbed into the blood but not metabolised by the body at depths under pressure. This could turn into bubbles on ascent and paralyse the diver. Curiously, the risk of drowning is not considered in commercial diving because that is the first hurdle to overcome in this profession.

The quantity of dives, dive repetitiveness, depth of the operations, time spent underwater and the exhausting nature of a specific task increase these risks significantly. Appropriate safety measures are provided to the diver via emergency air or gas supply, stand-by divers and decompression chambers. The diving-related health and safety procedures are managed by strict governing guidelines and work procedures.

When subsea welding is completed, both the welder and the structures being welded are at risk. The welder has to be very careful to avoid receiving an electric shock. For this, adequate precaution is taken by insulating the welder and limiting the voltage of welding sets. Continuous control of hydrogen and oxygen build-up is managed by removal and kept away from the arc to minimise any potential explosion.

Lastly, the welder’s time under water is controlled by using saturation diving chambers and regular rest periods in between. Inspection of an underwater weld is very difficult and complicated when compared to surface welding, but as it is the only controlling process of the quality of the weld, it is always done. The weld is inspected very carefully to confirm that no defects remain.

There are many underwater welding schools located in different parts of the world, including Australia, to train commercial divers. Historically, underwater welding was restricted to salvage operations and emergency repair work with limited depths of less than 9 m.

Wet welding – the way to go

There are two well-developed major categories of underwater welding process: one is welding in a wet environment; the other is welding in a dry environment.

Working underwater to weld serves to provide a number of benefits. Firstly, there is no need to pull the structure out from under water to perform work. In addition, many structures like oil rigs and ship hulls may become damaged at sea, necessitating the need for immediate work below the surface.

Because of the poor quality and difficulty in the process of welding underwater in the past, welding in the wet environment was used primarily for emergency repairs in shallow water. For example, to weld a patch for short duration until a complete repair could be performed in dry docks. With more experience and the advent of special welding rods and the persistence of some ambitious individuals and companies improved results were achieved, which has made wet welding a common occurrence.

Today’s underwater arc welding is accomplished in much the same manner as ordinary arc welding the only variations being that the electrode holder and cable is well-insulated to eliminate any possible current leakage and electrolysis of the surrounding water and the coated water-proof electrodes are used so that the electrodes do not get wet.

The most commonly used wet welding technique is shielded metal arc welding, informally known as stick welding. The main differences in wet welding equipment versus onshore welding equipment is that wet welding uses DC current only.

AC is not used as it can electrocute the diver and it is difficult to maintain a welding arc underwater with AC. The inclusion of a single or dual circuit breaker switch and the use of double-insulated cables protect the diver from electrocution. The power source should be a direct current machine rated at 300 or 400 amperes. Motor generator welding machines are most suitable for underwater welding.

Typical pipeline repair methods

Typical repair methods, as described in DNV RP F113, are to use fittings for repairs and tie-in of submarine pipelines.

These fittings include: couplings, clamps, T-branch connections and isolation plugs. Mechanical means are used to connect fittings such as sleeves/couplings and T-branches to the pipeline and welding subsequently used to make the repair permanent.

The section on the strength of the mechanical attachments is also applicable to pipeline recovery tools. Couplings connect pipes by direct attachment to the pipe walls via mechanical means and welded. Flange joins pipes via thick, machined pieces of additional material that is welded to the pipe ends prior to installation. Clamps are fitted externally to the pipeline to prevent leaks or add strength. Hot-tap T-branch connections are fitted externally to the pipeline assembly even during operation. A pressurised pipeline is machined open to allow fluid flow through the branch. Pipeline isolation plugs or smart plugs are pumped with the pipeline fluid to the repair site and then activated to form an isolating barrier that can resist differential pressure.

The pipe itself represents the key member of the repair assembly with consequential limitations such as, but not limited to, pipe wall strength, surface irregularities, and deviations in shape. Fittings for sub-sea repair must be installed with caution to reduce the likelihood of damage. Coupling strength should be sufficient in resisting stresses from all relevant loads, within a factor of safety as defined in the standard.

Avoiding pipeline damage

Pipeline damage after installation may be caused by internal and external corrosion, hydrogen-induced stress cracking, unstable seabed conditions, anchors, and dropped objects from the surface.

The risk of damage depends on the intensity of surface activities such as ship transport and offshore operations, depth, seabed conditions and the design of the pipeline itself. The extent of possible damage will vary from insignificant to a fully buckled or parted pipeline. Consequently, the repair and repair preparedness strategy depends on this.

The following steps have to be taken to select an appropriate repair method:
• Detailed selection criteria – this can be location type and strength requirement of sleeve, sleeve design and fabrication mechanical attachment requirements, handling requirements; and,
• A pipeline repair procedure manual.

This manual should include:
• Avoiding burn-through, factors affecting burn through such as wall thickness, heat input, and operational parameters such as pressure, temperature and flow characteristics;
• Prevention of hydrogen cracking concerns factors;
• Selecting an appropriate procedure – welding procedure options, predicting required heat input, inter-pass temperatures, pre-heating and maintaining;
• Proper electrode handling, welding sequence, and control of heat-input level;
• Welder/procedure qualification;
• Welder training;
• Code and regulatory requirements (changes to API 1104);
• Code requirements for weld deposition repair; and,
• Inspection and testing.

Final thoughts

My experience has shown that defects found in sub-sea pipelines can be permanently repaired by using mechanical intervention and underwater welding technology more safely, quickly and economically than any alternative technique. However, significant amounts of time and resources are still being applied to test and research programs to provide proven solutions for new repair applications. These will in turn provide the worldwide pipeline community with the correct answers and operational security required in the future.

Sumber:http://pipeliner.com.au/news/application_of_underwater_welding_processes_for_subsea_pipelines/063794/, (11 Februari 2014)