OCTG basically serves to maintain the well hole integrity during drilling, completion and production lifecycle of wells. Among them, the Casing, in particular, provides resistance to collapse caused by high-strength external load.The source of external load includes, pressure coming from stratum or reservoir flow in process of exploration and development, pressure from poor drillability mud and salt layer, mud system pressure, cement or seawater pressure under conductor or line pipe. What is Collapse Collapse resistance is a property of pipe body and defined as the limit exceeding which pipe fails in the circumferential direction when external pressure is applied. Collapse calculation formulas were addressed in the API 5C3 or ISO 10400. Four failure modes have been defined based on failure pattern observation and mechanical rules (refer to figure below). These four domains are continuously positioned as a function of the ratio of the outside diameter to the wall thickness, or D/t ratio. The WPB is not short hand for any process. It is simply an identifier developed by ASTM. The fittings in question are manufactured in the same manner as the companion pipe for this specification. In very large sizes, the fittings may be manufactured in pieces and welded together as is the case for many large fittings. The term used for the manufacturing is called [wrought" which means it is worked as opposed to being cast or forged. As to the W standing for weldable, I`m not sure that is correct since that are many, many pipe fittings that are weldable but carry designations other than a W. Note the reference below. Forging is the manufacturing process of ASTM A234 WPB pipe fittings. W = WELDABLE / wrought P = RELATED TO P NUMBER OF ALLOY MATERIAL B = REFER TO MINIMUM YIELD OF MATERIAL LATEST STANDARD IS: ASTM A234/A234M-07 Standard Specification for Piping Fittings of Wrought Carbon Steel and Alloy Steel for Moderate and High Temperature Service This standard covers wrought carbon steel fittings of seamless and welded construction which are manufactured to the dimensional specifications of ASME B16.9 and B16.28. These fittings are primarily for use in pressure piping and in pressure vessel fabrication for service at moderate and elevated temperatures. RELATION INFORMATION OF STEEL Materials shall consist of forgings, bars, plates, seamless or fusion welded tubular products with filler metal added, and shall be produced by open-hearth, basic-oxygen, or electric-furnace process. Forging or forming operations shall be performed by one or a combination of two or more of the following procedures: hammering, pressing, piercing, extruding, upsetting, working, bending, fusion-welding, or machining. All welding shall be completed prior to the austenitizing heat treatment, which shall be executed in the normalized, normalized and tempered, annealed, or quenched and tempered conditions. Steel specimens shall conform to required values of chemical composition, tensile strength, yield strength, elongation, wall thickness, and Charpy V-notch impact value. All fusion-welded butt joints shall undergo radiographic examination, while hydrostatic testing of fittings is not required in this specification. Repair welding shall be permissible for parts made to dimensional standards.