Offshore Platform Steel Specification
Authors: Peterson M.L. , Von Rosenberg E.L. 
Published: October 2008

The discovery in the 1980's of local brittle zones (LBZ'S) in and near the heat-affected zone (HAZ) of the fabrication welds in certain offshore platform steels caused great concern among the engineers and welding specialists of the period and led directly to the preparation and adoption of API RP2Z in May 1987 of Recommended Practice for Preproduction Qualification for Steel Plates for Offshore Structures. This recommended practice describes a procedure by which steel producers could prequalify their product in advance of purchase and delivery and thereby reduce the risk of steel plate being delivered to the jobsite which would exhibit weldability problems from the standpoint of low HAZ toughness or HAZ cracking problems when using typical and conventional welding procedures, i.e., the need for extensive fracture toughness testing by the fabricator on each weld procedure was eliminated in favor of a more sophisticated one-time test program in the laboratory of the producer. This recommended practice covers two areas:testing weld HAZ for toughness using CTOD tests anddelayed cracking tests using the controlled thermal severity test (CT'S) and Y-groove test.

This RP was developed for use with API steel specifications 2H, 2Y, and 2W. Most of the effort has been toward the quantification of API 2Y and 2W steels in thicker sections ?4" (100 mm).

This program has been scientifically successful albeit with a significant penalty of time and cost to the steel producer. There has been sufficient standardization that various purchasers have and will accept the same qualification performed previously for another job. This, of course, is exactly what API standards are intended to accomplish. API RP2Z was revised in 1992 and is currently being reviewed for a second revision intended to reduce some of the conservatism built into the original document by our lack of full understanding of the embrittling mechanisms in the HAZ of welded structural steels but now strongly supported by a substantial data base of good material. The paper discusses some of the proposed changes and contains data on the currently achievable fracture toughness levels.

BACKGROUND

For those who are not familiar with RP2Z, the purpose behind the testing program is to establish by means of fracture mechanics testing (CTOD) that any reduction of the fracture toughness of the platform material as a result of welding is within acceptable limits for typical welding procedures likely to be used by the fabricator. The unique feature in this test procedure is that acceptability must be established for the steel before it is produced and delivered to the fabricator. The primary purpose is to avoid the risk of receiving material that cannot be economically welded under typical fabrication yard conditions.

The recommended practice identifies a testing procedure used to qualify a steel producer's manufacturing practice for a particular grade of steel. He must produce welds in the candidate steels high, low, and intermediate heat input and then prepare and test CTOD fracture mechanic specimens in the zones typically found to be sensitive to toughness variations.