Underwater Welding

The fact that electric arc could operate was 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. Underwater welding is an important tool for underwater fabrication works. In 1946, special waterproof electrodes were developed in Holland by ‘Van der Willingen’. In recent years the number of offshore structures including oil drilling rigs, pipelines, platforms are being installed significantly. Some of these structures will experience failures of its elements during normal usage and during unpredicted occurrences like storms, collisions. Any repair method will require the use of underwater welding.

Classification

Underwater welding can be classified as
1) Wet Welding
2) Dry Welding
 
In wet welding the welding is performed underwater, directly exposed to the wet environment. In dry welding, a dry chamber is created near the area to be welded and the welder does the job by staying inside the chamber.

WET WELDING
Wet Welding indicates that welding is performed underwater, directly exposed to the wet environment. A special electrode is used and welding is carried out manually just as one does in open air welding. The increased freedom of movement makes wet welding the most effective, efficient and economical method. Welding power supply is located on the surface with connection to the diver/welder via cables and hoses.

In wet welding MMA (manual metal arc welding) is used.
Power Supply used : DC
Polarity : -ve polarity
When DC is used with +ve polarity, electrolysis will take place and cause rapid deterioration of any metallic components in the electrode holder. For wet welding AC is not used on account of electrical safety and difficulty in maintaining an arc underwater.

The power source should be a direct current machine rated at 300 or 400 amperes. Motor generator welding machines are most often used for underwater welding in the wet. The welding machine frame must be grounded to the ship. The welding circuit must include a positive type of switch, usually a knife switch operated on the surface and commanded by the welder-diver. The knife switch in the electrode circuit must be capable of breaking the full welding current and is used for safety reasons. The welding power should be connected to the electrode holder only during welding.
Direct current with electrode negative (straight polarity) is used. Special welding electrode holders with extra insulation against the water are used. The underwater welding electrode holder utilizes a twist type head for gripping the electrode. It accommodates two sizes of electrodes.
The electrode types used conform to AWS E6013 classification. The electrodes must be waterproofed. All connections must be thoroughly insulated so that the water cannot come in contact with the metal parts. If the insulation does leak, seawater will come in contact with the metal conductor and part of the current will leak away and will not be available at the arc. In addition, there will be rapid deterioration of the copper cable at the point of the leak.

Hyperbaric Welding (dry welding)
Hyperbaric welding is carried out in chamber sealed around the structure o be welded. The chamber is filled with a gas (commonly helium containing 0.5 bar of oxygen) at the prevailing pressure. The habitat is sealed onto the pipeline and filled with a breathable mixture of helium and oxygen, at or slightly above the ambient pressure at which the welding is to take place. This method produces high-quality weld joints that meet X-ray and code requirements. The gas tungsten arc welding process is employed for this process. The area under the floor of the Habitat is open to water. Thus the welding is done in the dry but at the hydrostatic pressure of the sea water surrounding the Habitat.

Risk Involved

There is a risk to the welder/diver of electric shock. Precautions include achieving adequate electrical insulation of the welding equipment, shutting off the electricity supply immediately the arc is extinguished, and limiting the open-circuit voltage of MMA (SMA) welding sets. Secondly, hydrogen and oxygen are produced by the arc in wet welding.
Precautions must be taken to avoid the build-up of pockets of gas, which are potentially explosive. The other main area of risk is to the life or health of the welder/diver from nitrogen introduced into the blood steam during exposure to air at increased pressure. Precautions include the provision of an emergency air or gas supply, stand-by divers, and decompression chambers to avoid nitrogen narcosis following rapid surfacing after saturation diving.
For the structures being welded by wet underwater welding, inspection following welding may be more difficult than for welds deposited in air. Assuring the integrity of such underwater welds may be more difficult, and there is a risk that defects may remain undetected.

Advantages and Disadvantages of Wet Welding

Advantages

Wet underwater MMA welding has now been widely used for many years in the repair of offshore platforms. The benefits of wet welding are: -
1) The versatility and low cost of wet welding makes this method highly desirable.
2) Other benefits include the speed. With which the operation is carried out.
3) It is less costly compared to dry welding.
4) The welder can reach portions of offshore structures that could not be welded using other methods.
5) No enclosures are needed and no time is lost building. Readily available standard welding machine and equipments are used. The equipment needed for mobilization of a wet welded job is minimal.

Disadvantages

Although wet welding is widely used for underwater fabrication works, it suffers from the following drawbacks: -
1) There is rapid quenching of the weld metal by the surrounding water. Although quenching increases the tensile strength of the weld, it decreases the ductility and impact strength of the weldment and increases porosity and hardness.
2) Hydrogen Embrittlement – Large amount of hydrogen is present in the weld region, resulting from the dissociation of the water vapour in the arc region. The H2 dissolves in the Heat Affected Zone (HAZ) and the weld metal, which causes Embrittlement, cracks and microscopic fissures. Cracks can grow and may result in catastrophic failure of the structure.
3) Another disadvantage is poor visibility. The welder some times is not able to weld properly.

Advantages and Disadvantages of Dry Welding

Advantages

1) Welder/Diver Safety – Welding is performed in a chamber, immune to ocean currents and marine animals. The warm, dry habitat is well illuminated and has its own environmental control system (ECS).
2) Good Quality Welds – This method has ability to produce welds of quality comparable to open air welds because water is no longer present to quench the weld and H2 level is much lower than wet welds.
3) Surface Monitoring – Joint preparation, pipe alignment, NDT inspection, etc. are monitored visually.
4) Non-Destructive Testing (NDT) – NDT is also facilitated by the dry habitat environment.

Disadvantages

1) The habitat welding requires large quantities of complex equipment and much support equipment on the surface. The chamber is extremely complex.
2) Cost of habitat welding is extremely high and increases with depth. Work depth has an effect on habitat welding. At greater depths, the arc constricts and corresponding higher voltages are required. The process is costly – a $ 80000 charge for a single weld job. One cannot use the same chamber for another job, if it is a different one.

Principle of operation of Wet Welding
The process of underwater wet welding takes in the following manner:
The work to be welded is connected to one side of an electric circuit, and a metal electrode to the other side. These two parts of the circuit are brought together, and then separated slightly. The electric current jumps the gap and causes a sustained spark (arc), which melts the bare metal, forming a weld pool. At the same time, the tip of electrode melts, and metal droplets are projected into the weld pool. During this operation, the flux covering the electrode melts to provide a shielding gas, which is used to stabilize the arc column and shield the transfer metal. The arc burns in a cavity formed inside the flux covering, which is designed to burn slower than the metal barrel of the electrode.

Developments in Under Water Welding
Wet welding has been used as an underwater welding technique for a long time and is still being used. With recent acceleration in the construction of offshore structures underwater welding has assumed increased importance. This has led to the development of alternative welding methods like friction welding, explosive welding, and stud welding. Sufficient literature is not available of these processes.

Scope for further developments
Wet MMA is still being used for underwater repairs, but the quality of wet welds is poor and are prone to hydrogen cracking. Dry Hyperbaric welds are better in quality than wet welds. Present trend is towards automation. THOR – 1 (TIG Hyperbaric Orbital Robot) is developed where diver performs pipefitting, installs the trac and orbital head on the pipe and the rest process is automated.
Developments of diverless Hyperbaric welding system is an even greater challenge calling for annexe developments like pipe preparation and aligning, automatic electrode and wire reel changing functions, using a robot arm installed. This is in testing stage in deep waters. Explosive and friction welding are also to be tested in deep waters.

 

Reference:

Joshi, Amit Mukund. –. “Underwater Welding”. Bombay: Indian Institute of Technology. Link: http://www.metalwebnews.com/howto/underwater-welding/underwater-welding.pdf

Deepwater Pipeline Installation

Asle Venas
DNV

Since the 1970s, offshore oil and gas development has gradually proceeded from shallow-water installations up to around 400 m (1,312 ft) to the ultra-deep waters around 3,000 m (9,842 ft) that represent the maximum today. The question is whether the curve will flatten at 3,000 m, or if this is just a temporary pause on the way to even greater depths. There have been plans for a gas trunkline from Oman to India at 3,500 m (11,483 ft) depth, but it is yet to be seen if there will be many such projects in the near future.

 

Pipe wall thickness

The main design challenge for development beyond 3,000 m is related to the high external pressure that may cause collapse of the pipeline. From depths of 900 m (2,953 ft) onwards, external over-pressure is normally the most critical failure mode for pipelines. The risk of collapse is typically most critical during installation when the pipe is empty and external over-pressure is at its maximum.

Many of the world's offshore pipelines are designed and constructed to DNV's pipeline standard DNV-OS-F101, and new concepts such as pipe-in-pipe may easily be accounted for by adjusting the relevant failure modes. (Photo courtesy DNV)

In addition, the pipe will be exposed to large bending deformation in the sag bend during installation that may trigger collapse, and collapse may also be relevant for operational pipelines subject to significant corrosion.

The main manufacturing processes relevant for larger-diameter, heavy-wall line pipes are UO shaped, welded and expanded/compressed (UOE/C, JCOE) and three roll bending. These processes provide a combination of excellent mechanical properties, weldability, dimensional tolerances, high production capacities and relatively low costs compared to seamless pipes.

There are at least six pipe mills that regularly supply heavy-wall, welded line pipe for offshore projects based on the UOE process: Tata Steel, Europipe, JFE, Nippon Steel, Sumitomo, and Tenaris. Research into further improving manufacturing techniques continues in the industry, and we also see several "newcomers" that can produce good quality pipes for deepwater.

This potential failure mode is normally dealt with by increasing the pipe wall thickness. But at ultra-deepwater depths, this may require a very thick walled pipe that becomes costly, difficult to manufacture, and hard to install due to its weight. Currently, there is a practical limit on wall thickness that limits the maximum water depth for 42-in. pipes to around 2,000 m (6,562 ft) while for a 24-in. pipe, this limit is approximately doubled to 4,000 m (13,123 ft).

Three factors have a major influence on the final compressive strength of the pipeline: quality of plate feedstock, optimization of compression and expansion during pipe forming, and light heat treatment. By focusing on these factors together with improving the ovality of the final pipe, it is possible to obtain a collapse resistance comparable to that of seamless pipes.

 

X-Stream

X-Stream is a novel pipeline concept developed by DNV that aims to solve the collapse challenge by limiting and controlling the external over-pressure. In a typical scenario, the pipeline is installed partially water-filled, and is thus pressurized at large water depths. Then, to ensure that the internal pressure does not drop below a certain limit during the operational phase when it is filled with gas, it is equipped with a so-called inverse HIPPS (i-HIPPS).

This system also includes some inverse double-block-and-bleed (i-DBB) valves. It is inverse in the sense that instead of bleeding off any leakage to avoid pressure build up in standard DBB systems, any leakage and loss of pressure is avoided by a pressurized void between the double blocks. This is needed to avoid unintended depressurization by a leaking valve which may not be 100% pressure tight when the pipeline system is shut down. Studies undertaken during the development of X-Stream show that the weight increase due to flooding is more or less balanced by the reduction in steel weight.

X-Stream is still at the concept development stage. Some practical aspects need to be studied, such as how to install large valves in ultra- deepwater. Another aspect is repair procedures and equipment, even though that should not be much different from normal ultra-deepwater pipelines. There are also some optimizations to be performed with respect to pressure loss during operation and equalization of the pressure during shutdown.

However, the potential benefits of the X-Stream concept to gas export and trunk lines at ultra-deep waters are quite significant, such as:

• Reduced steel quantity and associated costs
• Use of standard pipe dimensions, even for ultra-deepwater and large diameters, reduces line pipe costs
• No need for buckle arrestors
• No need for reserve tension capacity in case of accidental flooding.

In addition, a rough cost comparison indicates a 10-30% cost reduction (steel cost, transportation cost, welding cost) compared with a traditional gas trunk line.

 

Installation methods

There are three main methods used to install offshore pipelines: reeling, S-lay, and J-Lay. In ultra-deep waters, the combined loading of axial force, bending, and external over-pressure during installation can also be critical to wall thickness design. A significant external over-pressure in ultra-deep waters sets up both a compressive longitudinal stress and a compressive hoop stress. Both tend to trigger local buckling at less bending compared to a pipe without the external over-pressure.

A common challenge for all installation methods when it comes to deep and ultra-deep waters is the tension capacity. The catenary length before the pipeline rests at the seabed can become quite long, due to the water depth. The pipe needs to be very thick walled to have the necessary collapse capacity; and thus the submerged weight can become high. It is also often required that the installation vessel be capable of holding the pipe in case of accidental flooding (e.g. a wet buckle). However, it is still a topic of discussion whether it is absolutely necessary to be able to hold an accidentally flooded pipe.

The tension capacity of current vessels limits the water depth for 18 to 24-in. pipelines to around 3,000 m, when not accounting for the accidental flooding case. The limit for 30-in. pipelines is around 2,100 to 2,500 m (6,890 to 8,202 ft). New vessels with a tension capacity of 2,000 metric tons (2,204 tons) will be able to install up to 24-in. or maybe 26-in. pipes at 4,000 m (13,123 ft) water depth, while for 42-in. pipelines the maximum depth will be around 2,500 m (8,202 ft).

Another challenge related to deepwater installation is how to detect buckles during installation. Normally, a gauge plate is pulled through the pipeline by a wire at a certain distance behind the touchdown point. In case of a buckle, the wire pulling force will increase to indicate that something is wrong. However, in ultra-deep waters, the length of the wire and the friction between the wire and the curved pipeline may give challenges in detecting minor buckles. Having a long wire and buckle detector inside a pipeline during installation can also be risky. If the pipeline is lost, the water will push the wire and gauge plate inside the pipeline and it may not be possible to get it out again.

 

Suspended installation

The Ormen Lange field is located in a pre-historic slide area, with an uneven seabed at nearly 900 m (2,953 ft) water depth. In its early development phase, a submerged, floating pipeline concept was studied to overcome the challenging seabed conditions. By mooring the buoyant pipeline to the seabed, no seabed intervention work would be required. The concept was left for the benefit of a more traditional concept with the pipeline on the seabed mainly because of the challenges with interference between trawl gear and the mooring lines, but it is still considered feasible both with respect to installation and operation.

Another floating pipeline concept has been developed by Single Buoy Moorings. Here the buoyancy is ensured by a large-diameter carrier pipe to which the smaller pipelines are attached. Buoyancy modules, clump weights, and the end anchoring system ensure tension in the pipeline bundle. A short bundle connecting the FPSO and the spar has been installed at Kikeh offshore Malaysia. However, the maximum length of this concept can be extended by use of intermediate vertical supports. Potential challenges will be hydrodynamic forces, both the steady-state drag and the cyclic ones, including vortex-induced vibrations. The challenge is to balance the need for anchoring with the need for flexibility to absorb the forces. (e.g., by making the attachment to the mooring lines in such a way that it does not cause too concentrated bending deformations).

 

Spiral installation

A future solution for ultra-deep and topologically challenging locations may be to further develop the SpiralLay method developed by Eurospiraal. In this application, the line pipes are joined onshore and wound into a spiral for towing offshore. The spiral can take a quite long length of pipeline which makes it possible to pressurize it. On location, the pipeline is un-wound and installed in a short time. The concept involves installing a pressurized pipeline from a submerged spiral floating at a safe distance above the seabed, thus avoiding the challenges with the combined loading in the sag bend at deep and ultra-deepwater depths. This is a novel concept and needs further development and testing.

 

Seabed intervention

Seabed intervention and tie-in become more challenging with increasing water depth. Some of the equipment, such as fall pipes for rock installation vessels, have practical limitations (e.g. the maximum length of the fall pipe). The same is the case with ROVs and other equipment needed for installation. Some repair methods - such as retrieving a damaged part to the surface or using subsea welding with divers - are limited by water depth, and can only be used in 200 to 400-m (656 to 1,132-ft) waters. For deepwater, repair methods based on remotely controlled equipment are needed.

Recently developed repair methods for deepwater are based on different types of clamps that are fitted over a locally damaged area; or involve cutting and replacing a section with use of end flanges/couplings and spool pieces. In cases with extreme or comprehensive damage, a new pipeline section may be installed. Typically, both the clamps and the end couplings need to be sealed with grouting or metallic seals. Examples are the Oceaneering systems based on Smart Flange/Connector/Clamp and the Chevron deepwater repair system. These are under development, and designed to operate down to 3,000 m water depths. The Statoil-led PRS consortium is also developing a repair system for deepwater based on remotely welded sleeves. This system is based on two lifting frames, cutting the damaged part, then installing some couplings and a new spool piece.

 

Notation fosters innovation

Today, 65% of the world's offshore pipelines are designed and constructed to DNV's pipeline standard DNV-OS-F101. It is the only internationally recognized offshore pipeline standard that complies with the ISO codes. The ISO pipeline standard itself, the ISO-13623, is more like a goal setting standard with basically one hoop stress criterion and one equivalent stress criterion, and with little guidance for engineers on how to actually design a pipeline. Here, DNV-OS-F101 has found its niche, giving more detailed requirements in compliance with ISO-13623.

Another reason for the standard's success is that it is based on the so-called limit state design, where all potential failure modes have to be checked according to specific design criteria with given safety factors. This makes it easy to apply the code to novel designs and outside the typical application range (e.g. in deep and ultra-deep waters, in Arctic environments).

The collapse capacity and the fabrication factor for UOE line pipes may be taken as a good example of the flexibility of the DNV-OS-F101 code. The code contains a clause allowing for upgrading the fabrication factor due to different aspects such as light heat treatment and/or compression, instead of expansion at the end of the manufacturing process. The code is also quite transparent in the way the design criterion is written in order to facilitate and take into account innovation and improvements in the fabrication process. Similarly, new concepts such as the X-stream or various pipe-in-pipe concepts may easily be accounted for by adjusting the relevant failure modes, and adding new ones if relevant.

The most likely deep and ultra-deep potential field development areas known today are Gulf of Mexico, the Brazilian presalt areas, and East and West Africa. All pose challenges that could benefit from technology development and innovation.

Reference: “New installation methods may facilitate ultra-deepwater pipelay” ,http://www.offshore-mag.com/articles/print/volume-73/issue-8/flowlines-and-pipelines/new-installation-methods-may-facilitate-ultra-deepwater-pipelay.html, August 2013.