API SPEC 2W Standard for Exploration & Production
This specification covers two grades of high strength steel plates for use in welded construction of offshore structures, in selected critical portions which must resist impact, plastic fatigue loading, and lamellar tearing. Grade 50 is covered in thicknesses up to 6 in. (150 mm) inclusive, and Grade 60 is covered in thicknesses up to 4 in. (100 mm) inclusive.
It is intended that steel produced to Grades 50 of the basic API Spec 2W, without Supplementary Requirements, although produced in a different manner and of somewhat different chemical compositions, be at least equivalent in minimum performance and, therefore, in service application, to the corresponding grades listed in Sections 5 through 7 of API Spec 2H. Higher performance (i.e., notch toughness at lower temperatures, or enhanced weldability) typically available with TMCP steel may be achieved by specification of Supplementary Requirements.
API Spec 2W steels are intended for fabrication primarily by cold forming and welding. The welding procedure is of fundamental importance and it is presumed that procedures will be suitable for the steels and their intended service. Because of the characteristic high YS/TS ratio of TMCP steels, users may want to consider welding consumables which avoid under-matched weld metal. Conversely, the steels should be amendable to fabrication and welding under shipyard and offshore conditions.
Post Manufacturing Heating
Due to the inherent characteristics of the TMCP method, plates manufactured to this spec cannot be formed or postweld heat treated at temperatures above 1100°F (595°C) without some risk of sustaining irreversible and significant losses in strength and toughness. If warm-forming is to be required during subsequent fabrication, the tensile and notch toughness properties of the finished component shall be verified and the properties shall conform to the requirements of this specification. The procedure for verification shall be subject to mutual agreement. The plates may be post-weld heat treated at elevated temperatures not exceeding 1100°F (595°C) providing test coupons are subjected to a thermal cycle to stimulate such fabrication operations, as described in Supplementary Requirement S9. Verification or simulation is not necessary for heating at temperatures not exceeding 400°F (205°C).
The primary use of these steels is in tubular joints, stiffened plate construction, and other intersections where portions of the plates will be subject to tension in the thickness direction (Z-direction). Supplementary Requirement S4 provides for through-thickness (Z-direction) testing of plates by the manufacturer and specifies limits for acceptance. Supplementary Requirement S1 provides for ultrasonic examination of the plates by the manufacturer and specifies limits for acceptance.
For applications where through-thickness properties are important but Z-direction testing has not been specified, Supplementary Requirement S5 provides low-sulfur chemistry intended to reduce the size and number of sulfide inclusions in the plate. Supplement Requirement S5 is neither a substitute for S4, Through-Thickness Testing, nor a guarantee of a minimum level of through-thickness ductility.
The notch toughness requirements specified in Section 7 are suitable for applications below water, or above water in areas of temperature climate (14°F [â€“10°C] minimum service temperature). Cold-formed materials have less toughness due to straining than that of the original flat plates, especially in those areas aged by the attachment welding of stubs and braces. The requirements in Section 7 take into consideration typical losses in toughness due to straining and aging. Supplementary Requirements S7 and S8 deal with the strain-aging problem, and consideration should be given to invoking Supplementary Requirement S7 and/or S8 when the strain exceeds 5% or when (Nitrogen × % strain) exceeds 0.040.
For applications with lower service temperatures, lower test temperatures should be considered. Supplementary Requirement S2 provides for impact tests at temperatures other than specified in Section 7 or Supplementary Requirement S12. Supplementary Requirement S2.1 provides for Drop-Weight or Charpy Vnotch testing at â€“60°C. Supplementary Requirement S2.2 provides for such testing at temperatures less than â€“40°C but other than â€“60°C.
Supplementary Requirement S11 and Section 3 of API RP 2Z, dealing with CTOD testing of the weld heat-affected zone and with resistance to hydrogen cracking, respectively, address problems which are not normally dealt with in a â€œcommodity gradeâ€ steel specification. These problems are not unique to TMCP steels, but arise because:
a. Users may be expecting higher performance from TMCP steels than is available with conventional steels (e.g., welding with no preheat, or welding with very high heat inputs while retaining the superior notch toughness), and
b. This is a performance specification which accommodates a variety of different steelmaking practices, rather than a recipe which completely describes all particulars of chemistry, process, and quality control (essential variables).
It is intended that Supplementary Requirement S11 shall apply only when specified in advance by the purchaser. In many cases it may be possible to rely on prior data assembled by the steelmaker, provided no essential variables of the process have been changed.
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