Dr Dave McKeown* and Dr David Abson* look at the practicality of repairing damage to hulls without the need to drydock
* Dave McKeown is Manager of Corporate Projects and David Abson is Principal Research Metallurgist at TWI, the research and technology organisation specialising in all joining, surfacing, cutting and allied technologies.
Paper published in Shipping World and Shipbuilder, vol.207. no. 4223, May 2006. pp.24 -26,28.
Underwater structures can be vulnerable to damage. Ships' hulls may collide with underwater objects; quays may be hit; and offshore structures, both platforms and floating production storage and offloading vessels (FPSOs), can develop fatigue cracks. A ship can be brought into drydock, although it is very expensive, but for an FPSO drydocking is an extremely complex operation that is almost impossible to consider. There is therefore a need for in-situ repair.
Coffer dams or local dry boxes can be arranged to allow repair to be carried out in air but such approaches bring their own degree of complexity that increases cost. The simplest solution is to send a diver to the problem and make the repair directly in the water. It can be done, but has resulted in repair welds of dubious quality in the past. This article reviews the possibilities for high quality wet repairing.
There are three main types of underwater welding:
- Coffer dam welding, which is carried out in the dry, in air, where a rigid steel structure to house the welders is sealed against the side of the structure to be welded, and is open to the atmosphere
- Hyperbaric welding, in which a chamber large enough to house the diver/welder and his equipment is sealed around the structure to be welded, and is filled with a gas at the prevailing pressure
- Wet underwater welding, where an arc is struck between the workpiece and an electrode, protected from the water only by the gas generated in the process.
Coffer dam welding is most likely to be employed in harbour works or ship repair. It has the advantage that, once the environment is sealed, the welding is in air at ambient pressure, and so is similar to any other form of structural welding repair. Welding quality can therefore be as good as with a standard, non-marine procedure. There is a practical limit to the depth to which a dam system can be built, and so it is not usually used below 10m. There are also difficulties of sealing against complex shapes, and dams are frequently custom-built to a particular job. All this adds to the cost of the system.
Hyperbaric welding, using MMA (Manual Metal Arc), TIG (Tungsten Inert Gas) or FCAW (Flux-cored Arc Welding), is the preferred process for deep water welds, including tie-ins and repairs in pipelines and risers in the oil and gas industries. Like the coffer dam system, welding is carried out in a dry environment, but the pressure of the gas may be many times higher than atmospheric pressure. This presents difficulties for the diver/welder.
The problem of nitrogen absorption into the blood stream necessitates the use of helium containing 0.5bar of oxygen as the breathing atmosphere. Careful decompression is still required, however, to safeguard the health of the diver/welder. The increased pressure also has an effect on the weld - the denser gas creates faster cooling rates and gives severe arc constriction.
However, a skilled welder is able to produce high integrity work that meets the stringent quality requirements of the offshore industry. The equipment required, together with the decompression arrangements, make this a high cost solution to underwater repair.
Wet underwater welding has been widely used for many years. MMA is the most commonly used process. FCAW has been widely used in Russia and Ukraine, including for repairs to the hulls of sunken ships prior to refloating. Wet arc welding requires considerable skill and, depending on materials and consumables, carries a high risk of fabrication hydrogen cracking. Friction welding, which has the advantage of being relatively insensitive to depth and which lends itself to robotic operation, has the potential for use in deep water repair, but has yet to reach commercial reality.
Progress to date
Whilst satisfactory wet MMA welds can generally be made in the flat position, and with selected electrodes, in the vertical position, welding overhead presents a considerable challenge, and weld quality in this position is likely to be poorer than in the other welding positions. In spite of this difficulty, welds of acceptable integrity can be deposited by this process.
Satisfactory repairs have been reported for US Navy ships and also for offshore structures in the Gulf of Mexico following hurricane damage, and in the North Sea. Welds that meet AWS D3.6 class A requirements (that are the same as for welds deposited in air) have been reported. Ferritic weld deposits generally show only modest ductility and impact toughness, with Ni-base deposits giving better mechanical properties.
The semi-automatic wet underwater welding processes 'water curtain welding' (with the action of a conical water jet containing a gas shield) and flux-cored wire welding (without a gas shield) have been used with some success. The former is capable of producing high integrity welds. Whilst success has been claimed for the latter in the repair of ships and pipelines, the quality of welds has generally not reached that achieved with the other welding processes.
For all wet welds, the dissociation of water by the arc and the rapid interaction between oxygen and the more readily oxidisable elements in the molten weld metal removes oxygen, leaving an environment that contains a significant proportion of hydrogen. The hydrogen diffuses readily through any slag layer to dissolve in the molten weld metal. As the weld cools, the hydrogen begins to diffuse away. However, the hydrogen that remains may be sufficient to cause cracking in the weld metal or in the heat affected zone (HAZ) adjacent to the weld.
Susceptibility to HAZ cracking increases with increasing carbon equivalent (a measure of hardenability based on addition of the effects of carbon and other major alloying elements in steel). It is widely accepted that the upper limit of carbon equivalent to avoid HAZ cracking is 0.40. However, various factors, including the hydrogen levels of the welding electrode employed and the restraint on the joint, will also have a bearing on the risk of cracking.
For structures being repaired by wet underwater welding, inspection following welding is essential, as the risk of hydrogen-induced cracking is so much greater than in dry air. Assuring the integrity of such underwater welds may be more difficult than in air, and there is a risk that defects may remain undetected.
The Certification Scheme for Welding and Inspection Personnel (CSWIP) offers a range of qualifications for divers/inspectors, including one for remotely operated vehicles (ROVs). Whilst the technology may be available to replace the diver with a machine, the positioning of the ROV and the interpretation of the results it sends back still require human skills. TWI has diving tanks in Middlesbrough, UK, and Pinthong, Thailand, where full programmes of training and qualification for underwater inspection are carried out.
For the people involved, the risks can be grouped into three main types. Firstly, there is a potential risk to the welder/diver of electric shock. Secondly, because the arc in wet welding and cutting produces hydrogen and oxygen, precautions must be taken to avoid the build-up of pockets of gas that are potentially explosive. Thirdly, there is the risk is to the life or health of the welder/diver from nitrogen introduced into the bloodstream during exposure to air at increased pressure.
Precautions to avoid electric shock 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 welding sets. The welder/diver is insulated electrically from the arc not only by the electrode holder, but also by wearing a dry suit and rubber gloves.
Danger from stray arcs is avoided by incorporating a double-pole switch in the circuit, so that both the weld power and return lines are interrupted, at the welder's command, when he stops welding. Additional precautions include the use of a DC power supply and a limitation on the arc voltage. An AC power supply is not used because of electrical safety, and the difficulty of maintaining an arc underwater.
When an arc is struck underwater, the heat generated is sufficient to vaporise the water around the electrode tip, creating a cavity that is filled with gas. The water dissociates, creating hydrogen and oxygen. One consequence of this is the evolution of bubbles of mixed oxygen and hydrogen. Provided these bubbles escape to the atmosphere as relatively small, individual volumes, there is no hazard.
However, if the welding is taking place in a restricted area where the bubbles can accumulate to form a substantial gas pocket, there is a real risk that burning slag or another source of ignition could detonate the mixture. Hydrogen and oxygen in the proportions created by electrolysis of water recombine on ignition in a powerfully explosive manner. Care should be taken to ensure that gases escape freely from the area of any repair.
Underwater welding is only different from any other diving activity in that it requires significant periods of time to complete an effective repair. This lays the diver/welder open to dangerously high levels of nitrogen absorption into the blood stream. There is no problem provided a suitable decompression regime is operated. This may either be by strict adherence to decompression stops during the ascent or by use of decompression chambers at sea level.
Use of decompression chambers achieves better time management of the repair team. Stops during ascent need to be carefully controlled as to the depth, and require provision of some means of supporting the diver at the stop, as each may need to be many minutes in duration. Chambers may be on board a back-up vessel or sited at convenient shore locations. These shore-based facilities may be used by several companies, either as a routine means of stabilising the functions of divers or as an emergency when slow ascent has failed. TWI has such a chamber at its Middlesbrough underwater centre.
Wet welding has been in use for many years, but it has usually been considered to give joints of low integrity, prone to porosity and useful only for temporary repairs. Now, with the advent of FPSOs and other structures that cannot easily be brought to drydock, there is greater reliance on wet welding for producing permanent, high quality joints. This is entirely possible in C-Mn steels of up to 0.4 carbon equivalent.
High integrity welding requires control and skill - this is especially true for wet welding where a further range of variables and hazards exists. However, permanent wet welding repairs may be made, as long as experienced welder/divers are employed and an appropriate welding consumable is used.