New Welding Technologies for the Thick Plate of Offshore Structures
Authors: Kanatani Fumiyoshi , Matsui Shigetomo , Atsuta Toshio , Toma Shouji , Nagai Hiroyoshi , Nakayama Shigeru 
Published: October 2008

Abstract
For the joining of high performance materials of heavy sectional members for offshore structures, a welding procedure with high reliability and productivity is requested in practice considering the distortion of weldments and difficulty of repairing in deep penetration of weld bead. A narrow groove Gas Tungsten Arc welding process with sequence control and a local vacuum type electron beam welding system were developed, as new joining methods for heavy sections of high strength steels. Welding performance tests of these processes were carried out and it was found that these processes are acceptable for fabrication of heavy sectional members for offshore structures.

Introduction
Recently, it is required for steel structures to have high performances and for their manufacturing processes to be highly efficient. Accordingly, structural members are getting heavier and specialized materials are frequently used. Further the precision requirement in manufacturing process is getting stronger in spite of the enlargement of structural dimensions.
The same tendency also exists in recent offshore structures; the adverseness of working environments such as deep sea water, low temperature in the arctic area, sever meteorological condition requires adequate strength, toughness, corrosion resistance to materials.
Based on these background, manufacturing of these structures needs high technology, especially the selection of suitable welding methods is very important to avoid weld defects such as weld cracking and blowholes, to minimize weld distortion, to ensure strength, toughness and corrosion resistance of welded joint. In some cases, structures themselves become not feasible because of lack of suitable welding method.
In addition to these requirements of performances, the efficiency of welding is also important. From this point of view, authors selected the narrow groove Gas Tungsten Arc Welding (GTAW) and the local vacuum type Electron Beam Welding (EBW) as the new types of welding technologies in which the heat input can be readily controlled, amount of deposited metal can be minimized and, above all, deep penetration welding can be obtained. Reported in the followings are the development of the welding equipments for the narrow groove GTAW and the local vacuum type EBW and some test results of welding thick plates of high tensile steels.

Narrow Gap Gas Tungsten Arc Welding
In welding of thick high strength materials, the following points should be considered;Guaranty of required joint performanceWell controlled quality of weldments without detrimental weld defectsEasiness of welding operationReduction of repair weldingHigh rate of joint production
Considering the points above, narrow gap Gas Metal Arc (GMA) welding is widely applied to large structure of heavy section, because the welding process has high rate of joint production. In this case, the difficulty of repair welding must be considered. The total welding cost including repair and inspection depends on the amount of detrimental weld defects and the manhour of repairing. Unfortunately, narrow gap GMA welding is not always economical from the view point of welding productivity and the repairment.