DEFINITION OF WELDING
Underwater welding is the ideal solution for restoring repairing damage to vessels with ruptures, cracks, etc. because it provides speediness with low repair costs. BEVALDIA (Manager of the branches PSOMAKARA & GAMSRO) guarantees the high quality of performance of underwater welding due to the long term experience of its professional divers, who are certified by the State of Spain and by Lloyd’s Register of Shipping for their high level of knowhow for underwater welding which they possess. Underwater welding is used for ships for temporary or permanent repairs of cracks in the hull of the ship, for the installation of anodic protection, for temporary repair of cracks on rudder and rudder blades. Underwater welding is divided into two areas wet welding and dry welding.
Wet underwater welding is the direct exposer of the diver and electrode to the wet environment and is widely used for many years for underwater repairs. The diver is dressed with proper diving suit, depending on the method of diving that he carries out and depending on the water temperature that he will dive. He also wears rubber gloves for protection from the electricity and special filters on the diving mask for eye protection from the radiation of the electric arc. The equipment required are the same as those used on the surface.The only difference is that in the wet welding the electrode must be waterproof. Commonly used method for wet welding is the shielded metal arc welding (SMAW) also known as manual metal arc welding (MMA or MMAW), which is done with fully insulated waterproofed electrode holder designed for water and (waterproofed) stick electrodes’ coated with flux that releases gases to preserve the integrity of the weld. Welding power supply is connected to the welding equipment through cables and hoses which give direct current around 300 – 400 amps to power the electrode. The electric arc heats the work-piece and the welding rod, whilst the molten metal is transferred through the gas bubble around the arc. The gas bubble is partly formed from the decomposition of the flux coating on the electrode but it is usually contaminated to some extent by steam. Current flow induces transfer of metal droplets from the electrode to the work-piece and enables positional welding by a skilled diver.
CAUSES OF SHIP DAMAGE
Weld defects such as hydrogen cracking is primarily dependent on the type of consumables used. There are two ideal wet-stick welding electrodes one is the nickel based with rutile or acid oxidizing coatings and the other is ferritic with the same flux coating.
BASIC WELDING TECHNIQUES
With the drag technique, the electrode is dragged across the work and the diver applies a slight downward pressure, whilst the electrode is being consumed. The diver maintains the appropriate lead and slope angle, together with a suitable travel speed; the electrode will produce a regular bead of suitable profile. Drag technique: the diver applies downward pressure the electrode is dragged across the work.
- Oscillation (Weave)
With the oscillation technique the electrode is oscillated so that the point closest to the holder, and not the end closest to the work, is raised and lowered in a vertical direction, i.e. the slope angle is constantly changed whilst welding. This aids metal deposition and prevents non-concentric burning of the electrode, especially when welding in the root of a joint. Weave technique: the diver uses little or no pressure best used for the root run in fillet welded joints
- Back step
This technique involves a continual stalling of the forward travel motion by traveling backwards slightly, into the previous deposited weld. This technique has two particular functions; it helps improve the final microstructure, by reducing the solidification rate of the weld puddle and increases throat thickness. Mostly used is the drag technique because in practice this technique is in constant contact between the electrode and the work.
ADVANTAGES OF WET WELDING
- The low cost
- The flexibility of the diver, who can reach inaccessible parts of the ship, with which other methods is impossible.
- Faster execution of work.
- Time saving, since the equipment required is minimized and does not require a chamber to be structured.
- The possibility for non-destructive testing (NDT) * of welding and the ability to monitor the entire process from cameras placed underwater (NDT inspections).
- Inspection of damage to determine whether the welding can be performed safely and effectively
- Cleaning of marine growth and coating or other foreign matter on damaged area and proximity where the welds will be placed
- Cropping and smoothing out of damaged sections
- Welding of double plate with specially treated waterproof electrodes and fully insulated waterproofed electrode holder
- Welding of flat bars
- Application of underwater epoxy resin on all the welds for protection in order to reduce metal wastage
The development of the techniques of underwater welding in a dry environment makes it possible to create high quality weld at greater depths. In the dry underwater welding, the welding is carried out at the same pressure or slightly higher than the ambient or atmospheric pressure (1 atm) in a special chamber, by having first removed the water. Depending on the size and configuration of the chamber, the diver can be either completely inside or only partly.The main methods commonly used in dry underwater welding are, coated welding electrode (SMAW), with tungsten electrode and gas protection (GTAW), tubular electrode (FCAW), plasma arc (PAW). The welding with tungsten electrode and gas protection gives very good weld quality despite the fact that it is slightly slower compared to other types. The advantages of dry underwater welding are: 1. The greater safety of the diver, since the welding is done within the chamber, without the influence of sea currents and fish. There is good lighting and the required temperature. The chamber has its own, autonomous environmental control system. 2. A better welding quality, compared with the wet welds, because there is no presence of water and the hydrogen levels are much lower. 3. The ability to carry out non-destructive testing (NDT) *of the welding and the ability to monitor the entire process from cameras mounted in the chamber. 4. The preheating ability, maintaining proper temperature between the cords and reheating for the relief of the weld.
Preparation is the key to stopping crack spreading. Attempts to stop a crack by filling in with weld metal most often result in failure as the crack is very likely to open up again through the weld. Therefore, small patches are welded on. These patches can be readily attached to the hull of a ship by fillet welding because the groove of the fillet provides a guide for depositing the electrode. 1. The ends of the crack are located and NDT (non-destructive testing) is carried out . 2. Small holes (stoppers) are drilled at the extreme ends of the crack to prevent further crack development. Alternatively, the holes may be burned . 3. An area of approximately 2 inches is scraped or wire brushed shiny clean around the crack where the patch is to be fixed. 4. Preparation of patch, so as to extend beyond the crack 6 inches in all directions.
Vertical underwater welding is already one of the most widespread welding techniques in the section of shipping for the repair and damage restoration of the ship and most preferred technique for “butt” and “fillet” welding. BEVALDIA (Manager of the branches PSOMAKARA & GAMSRO), having to its potential experienced and appropriately trained divers, certified by Lloyd’s Register of Shipping, performs vertical underwater welding where requested and necessary for the restoration of damage to the ship, guaranteeing precise and excellent results. The execution mode of the welds applied for ship repair varies according to the position which is to be welded and the sensitivity of the vessel’s area, which affects the penetration and adhesion of the weld metal where the weld is to be made, and the stress there of.
VERTICAL DOWN WELDING
Vertical–down welding is likely better that the vertical-up welding since the pressure of the arc gives vertical support to the liquid metal and fills the cavity made by the arc to form acceptable weld beads.
The technique of the vertical underwater welding contains 2 separate methods of welding, “vertical-up” (vertical underwater welding directed upwards) and “vertical-down” (vertical underwater welding directed downwards).
VERTICAL UP WELDING
The vertical-up welding the liquid puddle does not have adequate support to form the weld bead since the liquid metal runs down due to gravity. In both cases experienced and well trained diver-welders are required for both methods.
CERTIFIED WELDING SERVICES
The underwater welding up to 1983, where the first standards of evaluation were published, there were no rules and no clear guidelines for their inspection. This made the need to identify the welding process and evaluate the suitability of the diver. The organizations that have set the standards are, the International Institute of Welding, the American Welding Society (AWS), the American Society of Mechanical Engineers, the British Standard Institute and Bureau Veritas (BV) and Det Norske Veritas (DNV). Then the other societies were incorporated . The American Welding Society (AWS) in 1974 began to develop the ANSI / AWS D3.6, Specification for Underwater Welding, which established four classes for underwater welds, because the characteristics of each welding varies due to the pressure of the cooling rate of the method used. There is also the prerequisite trial welding, which is evaluated before the work commences
The overhead underwater welding is one of the most difficult welding techniques, because the diver–welder requires to possess great skill and precision during this operation, parallel fighting with the gravity that is created between the diver and the work-piece (underwater). BEVALDIA (Manager of the branches PSOMAKARA & GAMSRO) provides well trained and certified divers to accomplish the overhead underwater welding, whenever it is requested or judged necessary, by providing guaranteed results.
Mr. Peroukidis George (Mechanical Engineering – Educator) is his notes states that: “The current volume must be as low as possible. To achieve a great penetration until it reaches the root of the rupture, there must be a preparation for the peak; even to the smallest and thickest metal (higher than 3mm).” “The movement of the diver – welder must be circular. The electrode must form an angle 80° degree, almost vertical, compelling the electro welder to be carefully protected. A drop of molten metal reaches the temperatures of 2000 °C”
For best results for underwater overhead welding, it is required to keep an upward direction of the electrode and a short arc seam, in combination with low work rate, as to achieve:
- Motion control, against gravity
- A better possible penetration to the cracks (rupture)
- Avoidance of porous seam welding
Overhead Welding Underwater Services
This technique is usually preferred to any material heavier than sheet metal. In contrast, the welding can also be done by descent of the electrode, when the metal plate is to be welded, and does not need much penetration. It is worth noting that the difficulty of underwater overhead welding, which is mainly located is in the fight against gravity, by experienced divers, since the technique of underwater overhead welding requires that welding is performed just above the height of the head of the diver and with very low welding speed, increasing working time to achieve the desired and accurate result.