Downhill Pipe Welding—Heavy-wall and Large Diameter
The author originally planned to add a new chapter about downhill pipe welding, addressing the changes that have taken place over the
years in the construction of cross-county pipelines, including the needed changes in welding procedures. After considering ways to incorporate the original text on downhill welding, it seemed the best approach was to complement existing material with this section,
The alloys used in pipelines today have led to larger diameters and increased wall thickness. Pipelines are expensive to construct. Longterm planning indicates the necessity for future capacity, and it is possible that the pressure at which Іащег volumes of natural gas and crude oil flow will also demand higher strength, ductility, and toughness of the pipeline material.
The pipe materials most encountered today for pipeline construction are the Lx60 and Lx65, as classified by the API standard. Tensile strength, hardness, ductility, and toughness have become the focal point of changes in the welding procedure. The piping materials are classified as low alloy materials. Attaining this higher strength, ductility, and toughness will be risky in terms of higher carbon and manganese content. Under bead cracking has been experienced when working with Lx56, which contains carbon and manganese near maximum acceptable limits. This pipe grade has a lower strength than the Lx65 and Lx60. Therefore, the composition of the steel used for pipelines must not only factor in weldability under hydrogen-induced conditions, but also the influence on toughness when the carbon and manganese content increase.
Certain steels, and all of the higher strength steels, are produced by some manufacturers by adding small amounts of columbium and vanadium to gain the required strength. These take the place of an increase in the carbon content, knowing that carbon has a detrimental effect upon fracture toughness. Fracture toughness is a much needed mechanical property that helps prevent running cracks from developing. This property is quite important in pipeline steel, where the initiation of a running crack can lead to destruction of miles of pipeline. Adding columbium or vanadium, or both, along with a relatively low, control finishing temperature in hot rolling, produces a fme-grain, stronger steel with adequate toughness.
The principle guide for welding pipelines is the API Standard 1104, a document written by representatives of the API, AGA, PLCA, AWS, and SNT. This standard provides the requirements for obtaining weld joints of adequate quality for gas and crude oil transmission pipe lines, or other high pressure services, using skilled welders and commercial - ly-available material and equipment. API 1104 also outlines the methods of testing and the test requirement for qualified welding procedures, as well as materials for a specific kind and size of pipe joint.
When a contract is awarded, the contractor is required to prepare a document called a welding procedure. In this document, the contractor stipulates all variables, methods, and an overall plan for executing the work in order to achieve the objective of the client and responsible agencies. The following items must be stipulated:
All these items and many more must be substantiated by the contractor in accordance to the code and specifications related to the job, in API 1104.