Abstract: The study compared domestic and existing submarine pipeline damaged pipe repair dry and wet methods, the domestic waters of Bohai Bay oil special geological conditions and determine the characteristics of dry hyperbaric welding repair is - kind of quality, reliable and easy to expand its use of deep-sea repair methods. Describes the design and manufacture of complete specifications of the largest domestic and only dry hyperbaric welding experimental system, the system is equipped with complete automatic control unit, able to complete the arc welding characteristics of high-pressure pipelines and submarine high-voltage dry-type automatic welding equipment and technology research . The paper also discusses the experimental system of the environmental gas selection, acquisition and visual signal devices, all-position welding experiment-related issues.
Keywords: dry hyperbaric welding; subsea pipeline repair; underwater welding

Foreword

Subsea pipeline repair is a complex and ambitious project, the development of foreign repair subsea pipeline technology started in the 1960s, initially relying on divers into the water for inspection and implementation of operations, but with the laying of pipelines and the laying of the pipeline depth and length of increasing maintenance staff alone can not meet the need, and therefore from the seventies onwards, Britain and the United States began to develop a variety of underwater pipeline repair equipment, are now maturing. China's offshore oil and gas exploration and development started late in related marine engineering services, compared with other countries in technology, equipment, etc. There is not a small gap. At present, China has not been able to implement underwater pipeline repair plant, the corresponding underwater pipeline repair work almost all the works entrusted to foreign companies, and expensive.

Underwater pipeline maintenance repair environment by wet and dry sub-two kinds of wet in the water environment, maintenance is directly welded to the pipe or mechanical repair. As low quality wet welding, wet repair flange or hoop to use more mechanical repair method. In deep water, because people can not be safe and effective operation, the foreign key using unmanned remotely operated vehicles (R0V) for wet maintenance. Private First Class submarine pipeline repair tools (the tools currently used in most foreign countries have patent protection).

Dry maintenance is in the water to create a dry environment, a direct pipeline maintenance personnel. In turbid water conditions, ROV, etc. No one method has been largely restricted operations, personnel directly involved is essential, and when the water depth of less than 60 meters, can be air diving, and can work safely and effectively, so a dry repair method is necessary and feasible. Dry conditions, can be flange or hoop mechanical maintenance methods, but also can be used welding method for dry welding quality can be guaranteed, and many more welding based. Maintenance and dry sub-atmospheric pressure and high pressure, technical requirements for high-pressure chamber sealed, the pressure is small, the operational safety and efficiency of personnel affect the case of small, multi-use high-pressure dry maintenance method. Dry hyperbaric welding technology and equipment research is of great significance.

A high-pressure test chamber of the overall design of welded

1.1 design and features

Hyperbaric welding JEM determine the characteristics of the work must meet specific requirements. Cabin air pressure of 0.11 ~ 0.70 MPa, stepless adjustable; chamber gas composition can be changed (based on compressed air explosion experiments may be); must be easy to open and close, and out of favor welded specimens; to be able to case closed cabin to observe the interior of the welding process: during the test chamber can not be welded into one, automated test equipment requirements; TIC into a separate protective gas welding; cabin inside and outside the electrical signals, gases and welding the specimen volume and exchange frequently.

1.2 Test system components

Where a system consists of the following components. High pressure welding test chamber and the chamber of the hyperbaric welding and orbital welding systems welding power supply EVA; test chamber environment to deploy the gas tank: Gas supply system should be required to deploy tanks and high pressure gas welding test chamber supply nitrogen, argon , carbon dioxide and compressed air: control system by the central console, operating system module, orbital welding machine control systems, welding power control system and camera system. System structure shown in Figure 1, the experimental system is built as shown in Figure 2.

 1.3 System Works

First, the deployment of tanks in the ambient gas within the compressed air and nitrogen ratio in accordance with the appropriate mix to the required pressure, then the mixed gas chamber filled with high pressure welding test. Welding test required by the blowing of argon gas to provide independent. Spools (wire feeder) placed on the track welder, welding and orbital welding power supply controllers are placed in high pressure welding test chamber. Each welding process test is completed, the first high-pressure gas welding test chamber through a dedicated pipeline to export gas emissions EVA, until the cabin pressure dropped to 0 MPa, and then open the test chamber. Control system by the central control system and the sub-control system, which is to deploy high-pressure welding environment control systems, rail welding and welding power control system control system. Each subsystem by an independent real-time control computer, while the parameters sent through the communication cable to the central computer. Can be observed on the center console high-speed camera arc welding, equipment areas within the scene and the scene camera test chamber, and through the cabin air system operating action button to complete the test chamber opening is closed, you can also perform light source switch and the electric heating pad warm-up temperature control and other auxiliary tasks.

2 dry hyperbaric welding experimental system to other key issues

2.1 Dry welding shielding gas with ambient gas

According to the existing pipeline depth of the Bohai oil field, in the dry cabin overall system design to be used in conventional air-diving technology-based, independent gas mask diver from the umbilical cord, because the side of the chamber entrance to the cabin-style pattern of operations. Configure the appropriate surface diving vessel system, with the dry cabin systems independent of each other. Because in the dry cabin overall system design, the independent gas mask diver from the umbilical cord, so the choice of gas pressure on the diver breathing gas is no longer considered the impact of the system, while the key consideration explosive combustion, welding quality and economic efficiency. Nitrogen (N2) cheap and does not burn the explosion, but the quality of welding pressure welding test to be adopted in the tests conducted to examine the cabin. There are three possible gas pressure options that pure air, air + Ar, air + N2. Thus, we must first determine the critical explosion of pure air medium pressure Pr, if Pr <7bar, they have decided to add Ar or N2, is more appropriate. If you choose to air + Ar as a pressurized gas, you need to experiment to determine: and with the Pr-7bar match the concentration of a critical explosive argon Ar%. If you choose to air + N2 as pressurized gases, also requires a match with the Pr-7bar explosive series of critical nitrogen concentration N2%. Experiments show that in the controlled atmosphere in the proportion of combustible components of the premise of arc, detonation does not occur.

Still use conventional welding shielding gas used in TIG welding argon gas.

2.2 visual problems

As the welding experiment was carried out in sealed high-pressure cabin must be arc welding cabin scene and image acquisition and guide to EVA, in order to facilitate observation. As the cabin pressure increased, the visual system of mechanical components and electrical components must withstand the test pressure, such as capacitors may be crushed. To ensure the normal operation of CCD device, you can have three options. First, the camera placed in the pressure within the shield; second rigid optical path (mirror) or flexible optical (fiber) to the light guide to EVA; a certain pressure range allows the camera directly to the home in cabin use.

2.3 the cabin all-position welding tooling

One of the goals is to develop the subject of a submarine pipeline for all-position welding process, so you can design a complete all-position welding experiment chamber and pipe welding test tooling.

2.4 Welding and Pipe Welding Electrode Technology

Manual TIG welding with welding in irregular and partial rendering pipeline repair welding joints. Pressure and humidity in the seabed dry cabin with high-frequency arc discharge is very unsafe for divers, computer-controlled welding power source can weld current and weld voltage for precise control of complex, tungsten and the workpiece through direct contact It is possible to achieve the arc. Significant pressure on the effect of TIG welding arc voltage to increase. Operating voltage of the increase is due to the increase in high-pressure environment, heat-absorbing ability. More easily because the heat loss from the area outside the arc, the arc diameter can be reduced by reducing heat loss. In fact, this requires cross-current of the arc area is small, thereby increasing the current density, ionization level and temperature. Additional energy demand led to the increase of arc voltage. If the arc voltage from the power supply in the mother ship, you need to add an extra voltage to meet the power and connect the cable resistance welding tank requirements. Because of environmental impact on the TIG arc voltage, welding process in the establishment of a suitable operating voltage limits before, these factors require special consideration. For TIG welding, the efficiency of an atmospheric pressure from 90% to 6bar (6 × 102kPa) at 70%, while the 8bar (8 × 102kPa) time and recovery to 75% after remained constant. For constant current, arc voltage increases with depth faster than the welding speed and reduced efficiency. As environmental pressures increase, TIG welding arc stability was reduced.

3 Conclusion

Design and manufacture of successful experiments dry hyperbaric welding and arc welding system with a Aluminium Welding Electrode test the full functionality, currently only applies to 1-7bar within 12 "double-pipe high-pressure study of the TIG welding process, high-voltage arc can also be physical study, the ratio of two or more ambient gas effects on the arc and the research process, two or more gas on the arc and protect the process of impact studies. After appropriate transformation, the device can be used for vacuum welding research.