1. Introduction
Due to the difficult working environment in the ocean, marine engineering structures are exposed to numerous challenges, including structural loads, storms, waves, tidal forces, corrosion from seawater, erosion from sand streams, and the threat of fire and explosion. of oil and natural resources. gas.
Additionally, most marine engineering structures are submerged, making inspection and repair of welded joints difficult and expensive after they are in service. Any significant structural damage or rollover accident can result in serious loss of life and property.
Therefore, strict quality requirements are imposed on the design, manufacturing, material selection and welding construction of marine engineering structures. With the development of the maritime, oil and natural gas industries, offshore gas pipeline projects are increasingly venturing into deeper waters.
Therefore, carrying out research and improving the application of underwater welding technology is of great importance in the development of the shipping industry, the exploration of offshore oil fields and the utilization of abundant marine resources for the benefit of humanity.
At present, underwater welding technology has been widely applied in marine engineering structures, submarine pipelines, ships, shipyards and port facilities, river engineering and nuclear power plant maintenance.
Underwater welding has become a key technology for assembling and maintaining large-scale offshore structures such as oil drilling platforms and pipelines.
2. Classification and characteristics of underwater welding methods
2.1 Classification of Underwater Welding Methods
Currently, there are various types of underwater welding methods being applied and researched around the world. It can be said that almost all welding technologies used in terrestrial production have been tried underwater.
However, the most mature and widely used methods are various arc welding techniques.
Underwater welding can generally be classified into three categories based on the welding environment: wet underwater welding, dry underwater welding, and local dry underwater welding.
However, with the development of underwater welding technology, new methods have emerged, such as underwater stud welding, underwater explosive welding, underwater electron beam welding and underwater exothermic welding.
2.2 Characteristics of Underwater Welding
Underwater welding processes are much more complex than welding processes on land due to the underwater environment. In addition to welding techniques, factors such as diving operations also come into play.
The characteristics of underwater welding are as follows:
(1) Poor visibility:
Water absorbs, reflects and refracts light much more strongly than air, resulting in the rapid degradation of light as it travels through water. Furthermore, during welding, a large number of bubbles and smoke are generated around the arc, significantly reducing the visibility of the underwater arc.
In areas with muddy seabeds or sediment-laden waters, underwater visibility becomes even worse. As a result, underwater welding has traditionally been considered blind welding, severely affecting the performance of diving welders and contributing to the high occurrence of defects and poor quality of welded joints.
(2) High hydrogen content in welds:
Hydrogen is a major concern in welding, as exceeding the allowable hydrogen content can easily cause cracking and structural damage. Underwater arcs cause thermal decomposition of the water around them, increasing dissolved hydrogen in the weld.
Generally, the diffusible hydrogen content in underwater welding is 27-36 mL/100g, several times higher than that in acid electrode welding on land. The poor quality of welded joints in underwater shielded metal arc welding is closely related to the high hydrogen content.
(3) Fast cooling rate:
During underwater welding, seawater has greater thermal conductivity compared to air, approximately 20 times greater. Even fresh water has a thermal conductivity several times greater than that of air.
When wet or dry spot underwater welding is employed, the part is directly in contact with water, resulting in a rapid and significant cooling effect on the weld, which can lead to the formation of highly hardened, tempered structures.
Therefore, only dry underwater welding can avoid the effect of cold.
(4) Pressure effects:
As pressure increases (0.1 MPa increase for every 10 meters of water depth), the arc column becomes thinner, the weld width decreases, and the weld height increases.
Furthermore, increasing the density of the conducting medium makes ionization more difficult, leading to higher arc voltage, reduced arc stability, and increased spatter and smoke.
(5) Difficulty in achieving continuous operations:
Due to the influence and limitations of the underwater environment, continuous welding is often challenging. In many cases, welding has to be carried out intermittently, resulting in discontinuous welds.
3. Applications, metallurgical characteristics and design of underwater welding electrodes for wet underwater welding
3.1 Applications of wet underwater welding in marine engineering
Wet underwater welding is performed by divers in an aquatic environment, as shown in Figure 2. Due to poor visibility underwater, diving welders cannot clearly see the welding process, leading to the occurrence of blind welding. It is difficult to guarantee the quality of underwater welding, especially tightness.
Therefore, achieving high-quality welded joints with this method is challenging, especially for welding structures used in critical applications.
However, due to its simplicity, low cost, flexibility and adaptability, wet underwater welding using coated electrodes and manual arc welding still continue to be researched in several countries. Other applications of these methods are expected in the future.
Wet underwater welding has been widely applied in the United States, with the American Welding Society's AWS standard (AWS D3.6) being the most influential document guiding wet underwater welding design.
The most commonly used methods in wet underwater welding are shielded metal arc welding (SMAW) and flux-cored arc welding (FCAW). When welding, scuba welders use waterproof coated electrodes and welding tongs specifically designed or modified for underwater welding.
Although significant progress has been made in wet underwater welding, it can be said that achieving high-quality welding joints in water depths greater than 100 meters is still a challenge and therefore cannot yet be used to weld critical engineering structures. naval.
However, with the development of wet underwater welding technology, many problems associated with wet underwater welding are being overcome to a certain extent.
The use of well-designed electrode coatings and waterproof coatings, together with rigorous management and certification of the welding process, led to successful applications of wet underwater welding in the repair of non-essential structural components in the North Sea in 1991. Wet underwater welding has now been successfully applied to the repair of auxiliary components on North Sea platforms.
Furthermore, wet underwater welding technology is widely used in shallow water areas with favorable marine conditions and for welding components that do not require high stress resistance.
Currently, the Gulf of Mexico is the most used region for wet underwater welding and wet underwater welding electrodes. Wet underwater welding technology has been used for the repair of bubbler tubes in Gulf of Mexico nuclear reactors and for underwater welding repairs at a depth of 78 meters on the Amoco Trinidad oil platform.
Research into this technology is of great practical importance for the future repair of underwater pipelines in Bohai Bay and Liaodong Bay, China, as well as the repair of non-critical components such as the replacement of sacrificial anodes.
Table 1: Gas composition of shielded metal arc welding flux (volume percentage)
| Electrode types | H2 | CO | CO2 | Other |
| J422（E4303） | 45~50 | 40~45 | 5~10 | <5 |
| J507（E5015） | 20~30 | 50~55 | 20~25 | <5 |
As the water depth increases in underwater welding, the volume of the arc bubbles gradually decreases due to compression.
However, insufficient arc bubbles can lead to an increased tendency for weld metal porosity. When the arc bubbles become too few, the arc is easily extinguished, making it difficult for the welding process to proceed smoothly. The growth of arc bubbles must satisfy the following physical conditions:
pg ≥ pa + ph + ps
In the equation:
- pg represents the pressure inside the bubble,
- pa represents atmospheric pressure,
- ph represents the hydrostatic pressure around the bubble,
- ps represents the additional pressure caused by the surface tension of the bubble.
During land welding, the pH is close to zero. However, in underwater welding, ph increases with water depth, while pa and ps can be considered unaffected by water depth.
Therefore, to ensure smooth welding, it is necessary to increase the pg. One way to increase pg is to increase the arc temperature, which can be achieved by adjusting the welding current. This is because a higher arc temperature can dissociate enough hydrogen and oxygen. Another way is to improve the gas production function of the electrode coating, so that more CO2 and CO gases are generated during the combustion of the electrode coating.
However, a high proportion of hydrogen in the arc bubbles can lead to the generation of two types of hydrogen-related defects: a greater tendency for weld porosity and a greater susceptibility to hydrogen-induced cracking in the weld metal and weld zone. affected by heat.
Therefore, in formulating the electrode coating, it is necessary to ensure sufficient pressure on the arc bubbles and at the same time try to reduce the proportion of hydrogen in the arc bubbles. Adding an appropriate amount of CaF 2 and SiO 2 to the coating can achieve this objective, as these additives help to reduce the hydrogen content.
SiO 2 +2CaF 2 + 3(H) = 2CaO + SiF + 3HF
or
SiO 2 +2CaF 2 = 2CaO + SiF 4 CaF 2 +H 2 O(g) = CaO + 2HF
The chemical and metallurgical reactions involving the products CaO, SiF or SiF 4 MnO, SiO 2 and TiO 2 as flux in the weld pool during underwater welding are important. These reactions result in the formation of gases such as HF, which do not have harmful effects on the weld metal and also contribute to increasing the pressure in the arc bubbles. Floating slag contains CaO, SiF or SiF 4 MnO, SiO 2 and TiO 2 , which help remove impurities from the weld pool. The HF gas also helps to increase the pressure in the arc bubbles.
Underwater welding has a greater susceptibility to hydrogen-induced cracking compared to land-based welding. This is due to the strong cooling effect of water on the part, causing phase transformation and martensite formation in the thermally affected zone of low carbon steels. When the carbon equivalent in steel exceeds 0.4%, the hardness in the heat-affected zone can exceed 400 HV.
In addition, if the hydrogen content is high during welding and the weld absorbs a significant amount of hydrogen, it may lead to the formation of hydrogen-induced cracks under the influence of welding thermal stress and phase transformation stress. Therefore, it is essential to reduce the proportion of hydrogen in the arc bubbles to mitigate the risk of hydrogen-induced cracking.
3.3 Electrode Coating Formulation Design
(1) Slag System Selection
Slag is the protective layer formed on the surface of the welded joint during the welding process, consisting of the fusion of the welding core, electrode coating and base material through high-temperature metallurgical reactions.
The properties of slag, such as its oxidation reduction ability, fluidity and permeability, directly affect the protection of the weld metal and the formation of the weld joint.
In this experiment, a slag system composed of SiO2 – TiO2 – CaF2-CaO was chosen, which falls between the acidic and alkaline slag systems. This choice guarantees good performance of the welding process and effectively reduces the harmful effects of hydrogen in arc bubbles. The corresponding minerals and chemicals were selected to meet the compositional requirements of the slag system.
(2) Optimization of Coating Formulation
Table 2 presents the results of 10 formulations that were tested based on the metallurgical characteristics of wet underwater welding.
The content of each substance in the formulations is as follows:
- TiO 2 in hematite: 52%;
- CaF 2 in fluorite: 98%;
- CaCO 3 in marble: 98%;
- Mn in low carbon ferromanganese: 85%;
- Ti in ferrotitanium: 75%;
- Si in ferrosilicon: 45%; and SiO 2 in feldspar: 93%.
The optimization process involved carrying out performance tests during the formulation of new formulations. All welding tests were carried out in a pressurized container simulating water depths of 70 to 100 meters.
Apologies for the confusion. Here is the corrected information:
Table 2: Composition and test results of different formulations
| NO. | Hematite | Fluorite | Marble | Low carbon iron manganese | Ferrotitanium | Ferrosilicon | Cellulose | Feldspar | iron powder | Arc Bubble Features |
| 1 | 20 | 10 | 20 | 10 | 5 | 5 | – | 12 | 18 | Reduced arc extinction with fewer bubbles |
| two | 20 | 10 | 25 | 10 | 6 | 6 | – | 10 | 13 | Reduced arc extinction with fewer bubbles |
| 3 | 20 | 15 | 20 | 10 | 7 | 7 | – | 13 | Reduced arc extinction with fewer bubbles | |
| 4 | 15 | 12 | 25 | 10 | 6 | 6 | 3 | 10 | 10 | Stable bubbles |
| 5 | 15 | 12 | 25 | 10 | 6 | 6 | 5 | 13 | 8 | Stable bubbles |
| 6 | 15 | 12 | 25 | 10 | 6 | 6 | 7 | 15 | 4 | Stable bubbles |
| 7 | 10 | 18 | 25 | 10 | 6 | 6 | 5 | 10 | 10 | Stable bubbles |
| 8 | 10 | 16 | 30 | 10 | 6 | 6 | 3 | 12 | 7 | Stable bubbles |
| 9 | 10 | 15 | 30 | 10 | 5 | 5 | 5 | 15 | 5 | Stable bubbles |
| 10 | 10 | 15 | 35 | 5 | 5 | 5 | 5 | 15 | 5 | Stable bubbles |
3.3 Process Performance and Mechanical Performance Test
A small quantity of 4.0 mm diameter welding rods were produced using formulations 1-10 on a 25-ton hydraulic cladding machine. The following tests were performed:
(1) Porosity and Formability Test
For the test, 6 mm Q235-C metal sheet was used. When welding was carried out underwater at a depth of 70 m using formulations 1-3, the lack of sufficient gas-forming materials made it difficult to stabilize the presence of arc bubbles, resulting in severe porosity. The welding process could not go smoothly.
Formulations 4-10, which included increased gas-forming materials and reduced hydrogen content, showed no porosity. Among them, formulations 7-9 exhibited good formability. The morphological characteristics are shown in Figure 2.
(2) Determination of diffusible hydrogen content in weld metal
Diffusible hydrogen content is a key indicator of welding rod performance. In this study, the glycerol method specified in GB 3965-93 was used to determine the diffusible hydrogen content of formulations 4-10, which showed satisfactory initial performance.
The results measured for formulations 4-10 were as follows (mL/100g): 15.5, 16, 18.2, 7.2, 6.7, 6.9, 7.2. It can be seen that formulations 7-10 meet the requirements of GB 5117-95 (diffusible hydrogen ≤ 8 mL/100g).
(3) Mechanical performance test
Based on the comprehensive process performance test results, it can be analyzed that the welding rods formulated with 7, 8 and 9 meet the requirements for underwater welding. Although formulation 10 meets the requirement of diffusible hydrogen content, the weld seam formed using this formulation has low formability and is therefore not adopted.
Welded test plates were prepared using welding rods formulated with 7, 8 and 9 (in 19 mm thick 16Mn plates) for weld metal tensile tests and V-impact tests. The test results are shown in Table 3.
Table 3: Mechanical Performance of Welded Metal
| NO. | Tensile strength (MPa) |
Elongation Rate (%) |
Section Contraction Rate (%) |
Impact Absorption Energy (Akv/J) |
| 7 | 525 | 23 | 38 | 85 |
| 8 | 496 | 24 | 41 | 125 |
| 9 | 516 | 24.5 | 43 | 130 |
According to Table 3, it can be seen that the mechanical performance indicators of No. 7-9 welding rods fully meet the requirements of GB 5117-95 for low-carbon steel and high-strength low-alloy steel, making them those suitable for underwater welding of low carbon steel and low alloy steel.
4. Application of dry underwater welding technology
Dry underwater welding is a method in which the welding area is completely or partially dried using gas to remove surrounding water, allowing the underwater welder to work in dry or semi-dry conditions. When performing dry underwater welding, it is necessary to design and manufacture complex pressure chambers or workstations.
Depending on the pressure inside the pressure chamber or workstation, dry underwater welding can be divided into high pressure dry underwater welding and atmospheric pressure dry underwater welding.
4.1 Application of high pressure dry underwater welding technology
High pressure dry underwater welding is shown in Figure 2. With the increase in underwater welding projects, the depth of underwater engineering and the higher requirements for welding quality, high pressure dry underwater welding is gaining more attention due to to its advantages of high welding quality and good joint performance.
Wet underwater welding and localized dry underwater welding are generally only used to repair non-critical structures at depths of several meters to tens of meters, with practical application depths generally no greater than 40m.
In order to adapt to the development of offshore engineering in deeper waters, many countries have increased the research and application of high-pressure dry underwater welding technology.
Currently, for underwater maintenance operations, high-pressure track TIG welding systems are widely used. Well-known systems include the PRS system and the OTTO system. The PRS system was developed by Statoil, a Norwegian company, with the aim of welding in water depths of 1000m. Successful pipeline welding was carried out at a water depth of 334m, achieving a -30°C impact energy of 300J and a weld seam microhardness below 245HV.
To date, this system has successfully completed more than 20 underwater pipeline repair tasks. The OTTO system in the UK mainly consists of a welding chamber and a tracked TIG welding machine. Experimental results showed that the weld seam at a water depth of 135m achieves a -10°C impact energy of 180J and a fracture strength of 550MPa. This system operated continuously underwater for 4 weeks, completing a total of 18 welds, and the welding procedures and quality were certified by the Norwegian Lloyd's Register.
In China, in October 2002, high-pressure dry underwater welding technology was planned as a significant part of the “Key Technologies for Exploration and Development of Bohai Oil Field” under the National 863 Program. This project is led by the Institute of Petrochemical Technology in Beijing.
At present, the first high pressure welding laboratory in China has been designed and established, equipped with a high pressure welding test chamber for carrying out welding testing and research at different pressure levels. Subsequently, annual plans for high-pressure welding process experiments and process evaluations were implemented.
High-pressure dry welding was first proposed by the United States in 1954 and was used for production from 1966. It can weld underwater pipelines with diameters of 508 mm, 813 mm and 914 mm.
Currently, the maximum practical water depth is around 300m. In this welding method, the bottom of the gas chamber is opened and a gas pressure slightly higher than the water pressure at the working depth is introduced to discharge water from the bottom opening of the chamber, allowing welding to be carried out in dry gas. chamber.
Generally, welding methods such as stick arc welding or inert gas shielded arc welding are used. It is one of the best welding methods in terms of quality in underwater welding and can reach a level close to that of onshore welding. However, there are three issues that need to be addressed:
(1) Due to limitations imposed by the shape, size and position of the engineering structure, the gas chamber has significant limitations and is less adaptable.
Currently, it is only suitable for welding structures with simple and regular shapes, such as underwater pipelines.
(2) A set of life support, humidity control, monitoring, lighting, safety assurance, communication and other systems must be provided.
The auxiliary work time is long, requiring a large surface support team, resulting in higher construction costs. For example, the welding device (MOD-1) from the company TDS in the United States, which can weld pipes with a diameter of 813 mm, is valued at up to $2 million.
(3) The issue of “pressure influence” also exists.
When welding at great depths (tens to hundreds of meters), the characteristics of the welding arc, metallurgy and welding process are affected to varying degrees as the gas pressure around the arc increases. Therefore, it is necessary to carefully study the influence of gas pressure on the welding process to obtain high-quality welds.
4.2 Application of atmospheric pressure dry underwater welding technology
Welding is carried out inside a sealed pressure chamber, where the pressure inside the chamber is equal to the atmospheric pressure on land and independent of the water pressure in the surrounding environment, as shown in Figure 4.
In fact, this welding method is not affected by the depth or presence of water, and the welding process and quality are similar to land welding.
However, the application of atmospheric pressure welding systems in offshore engineering is limited. The main reason for this is the difficulty in ensuring the sealing of the welding chamber in structures or pipes and maintaining the desired pressure inside the chamber.
An operational system of this type, developed jointly by Petrobras and Lockheed, was applied in the Amazon Basin. Atmospheric pressure dry welding equipment is even more expensive than high pressure dry underwater welding and requires a greater number of welding support personnel.
Therefore, it is generally only used for deep-water welding of critical structures. The biggest advantage of this method is its ability to effectively eliminate the influence of water on the welding process. Welding conditions are identical to those on land, ensuring the highest welding quality.
A special case of dry underwater welding at atmospheric pressure is the use of cofferdams in shallow water areas. The unstable work environment in shallow water areas, caused by waves, tides and significant changes in water depth, presents challenges.
Some companies have addressed this issue by connecting the welding chamber to the water surface via a bucket-like structure equipped with a ladder, creating an atmospheric pressure working environment, as shown in Figure 5.
The pressure difference in this construction environment is minimal, allowing effective sealing methods to be employed. Although ventilation and safety procedures must be considered, this technology has proven to be practical in certain specialized applications, particularly for the maintenance of offshore engineering structures in flat tidal areas.
5. Local Dry Underwater Welding
Local dry underwater welding technology uses gas to artificially displace water in the welding area, creating a localized dry gas chamber for welding. The use of gas guarantees a stable arc and significantly improves welding quality.
Currently, the preferred method for welding offshore steel structures is underwater dry spot welding with partial drainage and gas shielded metal arc welding.
Underwater dry spot welding was first proposed by the United States and later used in production by multinational companies in the United States and the United Kingdom. It is a portable cylindrical gas chamber, the end of which is sealed, while the other end has an opening with a flexible sealing gasket that adapts to the geometry of the welding area. The gas shielded welding gun is fixed on a flexible neck and extends into the movable cylindrical gas chamber.
The gas chamber is pressed over the welding area and a gas of a certain pressure is introduced to displace the water (forcing the water in the gas chamber to pass through the semi-sealed joint) and provide protection for the weld.
The diver carries the cylindrical gas chamber with the welding gun along the weld seam for welding. This dry gas chamber system can adapt to welding in any underwater position, and the joint strength is not inferior to that of the base material, with a cold bending angle of up to 180°.
It has been reported that qualified welds can be obtained at a water depth of 29m, and welding has been carried out at a depth of 27m in the UK. This method was used to repair two pipes with a diameter of 350 mm, located at a water depth of 7 m, on the Ekofisk drilling platform on the Norwegian continental shelf, and after magnetic particle tests, no defects were found.
In addition, there is the application of large-scale local dry underwater welding using a removable transparent cover. This device is installed or placed around the underwater steel structure to be welded. The bottom of the hood is opened and inert gas is introduced to displace water and keep the welding area dry. The diver extends the welding gun from below and performs MIG welding in a dry environment.
After welding and inspection is complete, the hood is removed. This method mainly uses solid wire or flux-cored wire for semi-automatic gas shielded welding, tungsten inert gas (TIG) welding and shielded metal arc welding.
In the United States, this method was used to repair a 406 mm riser on an oil production platform at a water depth of 12 m, which passed the water pressure test and met the requirements. Underwater spot dry MIG welding has also received attention as a promising underwater welding method.
By studying the fundamental theory of gas shielded welding, mathematical models have been established, nozzle structures and suitable airflow velocities have been designed, and the relationships between water pressure, shielding gas, process behavior, arc behavior and rate of deposition were explored.
Doppler velocimetry has been used to test and analyze airflow distribution and phase distribution in local voids, and the relationship between the hood and heat and pressure transfer has been studied. Based on the understanding of the principle of radon vacuum pumps, a new type of drain cover was designed, reducing the gas pressure in the welding area.
Experimental results showed that the welding performance achieved with this drainage cap is comparable to that of air. Wang Guorong et al. studied a local dry underwater welding technique.
Fluid mechanics theory has been used to calculate and test the drain cover, determining the appropriate structure and size. Local dry welding experiments were conducted and the results showed that this method has lower cooling rates, diffusion hydrogen content and maximum HAZ hardness in the welded joint compared to wet welding methods.
The welds produced are free from defects such as porosity, cracks and slag inclusions. The mechanical properties of V-groove welded joints meet the requirements of API 1004 and ASME standards. This method is easy to operate, requires simple equipment, is low cost and achieves satisfactory joint quality.
Tsinghua University has conducted experimental research on underwater laser welding. 304 stainless steel was used as the base material, ULC308 was used as the filler wire, and the laser power was 4 kW. The results showed that the gas flow rate had a significant impact on the weld quality.
At low gas flow rates, the oxygen content in the weld reached 800ug/g, while at high gas flow rates, the oxygen content decreased to 80ug/g. The tensile strength of the weld metal did not change with the gas flow rate, but the ductility decreased with decreasing gas flow rate.
The nozzle shape had a significant influence on the welding protection environment, and appropriately increasing the nozzle diameter resulted in a more stable gas cavity and satisfactory welding quality. Local dry underwater welding can achieve a joint quality close to that of dry welding.
Furthermore, due to its simplicity, low cost and flexibility comparable to wet underwater welding, it is a promising underwater welding method. Currently, several local dry underwater welding methods have been developed, some already being used in production.
5.1 Underwater water curtain welding method
This method was first proposed by Japan. The welding gun has a two-layer structure. High-pressure water jets exit in a conical shape from the outer layer of the welding gun, forming a rigid water curtain that blocks external water intrusion.
The inner layer of the welding gun introduces shielding gas to displace water directly beneath the welding gun, creating a localized and stable gas phase cavity within the water curtain. The welding arc is not affected by water interference and burns stably within the gas phase cavity.
The water curtain has three purposes: protecting the welding area from the surrounding water, utilizing the suction effect of the high-speed jet to remove water from the welding area and forming a gas phase cavity, and breaking large air bubbles that escape of water into many small bubbles to maintain stability within the gas cavity.
This method ensures that the strength of the joint is not lower than that of the base material, and the front and back bending angles of the welded joint can reach 6708. The welding gun is light and relatively flexible, but the visibility problem has not been solved .
The presence of shielding gas and smoke agitates the water in the welding area, making it cloudy and impairing the diver's visibility, causing the welder to essentially work blindly. Furthermore, there are strict requirements for the distance and inclination of the nozzle relative to the workpiece surface, requiring high operational skills from the welder.
Combined with the reflection of the steel plate in high-pressure water, this method is not effective for welding lap joints and fillet joints, and manual welding is challenging. Therefore, it must be developed in the direction of automation.
5.2 Wire brush underwater welding method
This method was developed in Japan to overcome the shortcomings of the water curtain method. It uses a 0.2mm stainless steel wire “skirt” instead of a water curtain as a localized water drainage method. This method can be used for both automatic and manual welding.
To reduce the gaps between the steel wires and increase the stability of the gas cavity, a copper wire mesh (100-200 mesh) is added to the steel wire skirt. To prevent spatter from adhering to the steel wires, a layer of 0.1mm diameter SiC fiber wire is coated on the inner side of the steel wire skirt. This method has been used to repair welded joints in steel piles corroded by seawater at depths of 1 to 6 m.
5.3 Hood underwater welding method
This method involves installing a clear cover on the workpiece, using gas to displace water within the cover, and having the diver extend the welding gun into the gas phase area within the cover for welding.
The welder observes the welding process through the hood. This underwater welding method can be used for spatial positioning welding of different joint shapes, mainly using gas shielded metal arc welding, but also tungsten inert gas (TIG) welding and shielded metal arc welding.
The maximum practical water depth for this hooded spot dry welding method is 40m. This dry and hooded spot underwater welding method is a large-scale dry spot method with higher welding quality compared with the small-scale dry spot method.
However, it has less flexibility and adaptability. Furthermore, the welding time is prolonged, resulting in increased smoke inside the hood, which impairs the diver's visibility. Proper exhaust ventilation is necessary to keep the gas clean within the range hood, making it a problem that must be resolved.
5.4 Mobile chamber underwater welding method
This method was first proposed by the United States in 1968 and later applied in production by multinational companies in the United States and the United Kingdom. It is a movable chamber with an open end that allows both water drainage and gas protection.
The moving chamber is pressed over the welding area to displace the water inside, creating a gas phase cavity where the welding arc burns. The chamber diameter is only 100-130mm, making it an underwater dry spot welding method.
During welding, the open end of the chamber comes into contact with the workpiece, and a semi-translucent sealing gasket and a flexible sealing gasket for the welding gun are installed in the opening.
The welding gun extends sideways into the chamber and the drain gas displaces the water, allowing the welder to use the chamber's internal lighting to clearly observe the position of the groove and then initiate the welding arc. The welder moves the chamber segment by segment along the weld seam until the entire weld is completed.
This method allows welding in any position. Due to the stable gas phase cavity inside the chamber, the arc and welding quality are improved, resulting in a joint strength no lower than that of the base material. The welds are free from defects such as slag inclusions, porosity and undercuts, and the hardness in the welding area is also low.
The mechanical properties of welded joints meet the requirements of the American Petroleum Institute and are used in maximum water depths of 30-40m. However, this underwater welding method also has some limitations:
(1) Does not effectively remove the influence of welding smoke.
(2) There is still a layer of water between the chamber and the diver's face mask. Although it has little effect on visibility in clear water, visibility problems remain unresolved in murky water.
(3) The welding gun is flexibly connected to the chamber, and the welding process is interrupted each time the chamber is moved, resulting in discontinuous welding and potential defects in the welding pass joint.
In summary, the rational application of partial drainage measures can effectively solve the three main technical problems in underwater welding, thereby improving arc stability, improving weld formation and reducing welding defects.
The underwater welding methods currently used have limitations, with the quality of the welding being influenced by working conditions and the depth of the water. However, from the perspective of offshore development prospects, research into underwater welding falls far behind the needs of the industry. Therefore, strengthening research in this area is of great importance, both now and in the future.
6. Research progress on underwater welding technology
6.1 Application and Development of Underwater Welding Technology
Underwater welding first appeared in 1917 when the British Navy's Shipbuilding Institute used underwater arc welding to repair leaks in riveted joints and rivets on ships. In 1932, Khrenov developed special underwater welding electrodes coated with a waterproof layer on the outer surface, which improved the stability of underwater welding arcs to a certain extent.
At the end of World War II, underwater welding technology gained importance in salvage operations, such as the salvage of sunken ships.
In the late 1960s, especially with the development of offshore oil and gas, there was an urgent need for underwater welding repairs to offshore engineering structures to address fatigue, corrosion, or accident damage while ensuring a good welding quality. The first report in this regard was in 1971, when Humble Oil Company performed underwater welding repairs on drilling rigs in the Gulf of Mexico.
In 1958, the first group of certified commercial divers were trained and wet underwater welding processes for water depths of less than 100 m were established. In 1987, wet underwater welding technology was applied to the repair of stainless steel pipes in nuclear power plants. In the 1990s, as the number of underwater engineering structures requiring repairs increased and the cost of shipyard repairs increased, there was further development of wet underwater welding technology.
Underwater welding technology has also received attention and applied in China. As early as the 1950s, underwater wet welding with electrodes was employed. In the 1960s, China independently developed special underwater welding electrodes. Since the 1970s, South China University of Technology and other institutions have conducted extensive research on underwater welding electrodes and metallurgy.
In the late 1970s, with the help of the Shanghai Salvage Bureau and the Tianjin Oil Exploration Bureau, the Harbin Welding Research Institute developed LD-CO2 welding technology, which is a local dry underwater welding method. The semi-automatic welding gun specially designed for underwater welding effectively removes welding smoke, allowing the diver to clearly observe the position of the groove and ensuring the welding quality. Over the past 20 years, many construction tasks have been completed using the LD-CO2 welding method.
The main factors that affect the quality of underwater welding are the depth of the water, the corresponding environmental pressure and the humid and harsh working environment. Ensuring the quality of wet underwater welding is a challenge, and improving the quality of wet underwater welding is a main focus of research. The United Kingdom and the United States have developed several high-quality underwater welding electrodes.
Normally, the water depth for wet underwater welding does not exceed 100m. The current focus is to achieve a breakthrough in underwater wet welding technology at a depth of 200 m. Research on welding process monitoring using advanced technology has made some progress, particularly in the automation and intelligence of dry and partially dry underwater welding. Automated track welding systems and robotic underwater welding systems with automated process monitoring have been developed, resulting in improved welding quality, reduced working time and reduced workload for divers.
The use of remotely controlled automated welding allows you to overcome the depth limitations of manual divers. Rail welding systems have modular structures, simplifying maintenance. Rapidly developing robotic underwater welding systems provide greater flexibility and are capable of achieving satisfactory welding quality in high-pressure dry underwater welding such as gas tungsten arc welding (GTWA), gas metal arc welding (GMAW) and flux-cored arc welding. (FCAW), even in water depths of 1100m.
Underwater robotic welding systems guided by laser devices provide more flexibility to detect and control welds and defects, contributing to improved welding quality. The wire feeding system is a challenge in underwater welding due to the depth of the water. A new type of high-reliability, wire-based underwater feedback system has been applied.
Overall, there are still many problems with current underwater robotic welding systems, including flexibility, size, operating environment, sensing and monitoring technology, and reliability, which need to be further developed and improved.
