Discussion of the Modified RP-2Z Pre-Qualification Test for Deepwater Pipeline and Riser Applications
Authors: Devall Mark Randall , Huddleston Robert C. , Hayes Micheal D. 
Published: May 2011

The weldability of steels for deepwater applications must be determined long before welding procedures are qualified. Deepwater structures and pipelines are subjected to currents which result in high cyclic stresses. It is imperative that steels selected for this service have high CTOD fracture toughness values in the base metal, heat affected zone (HAZ) as well as the weld metal. Strategies are discussed for determining the fracture toughness of the HAZ in these steels using API RP 2Z as the basis for prequalification of the steels prior to construction. The European equivalent to API RP 2Z is EuroNorm 10225 (2009). Alternate prequalification schemes will be discussed in relation to cost, schedule and applicability to different material.

Introduction

API RP 2Z was written to allow purchasers of structural steel plates for demanding applications to determine the weldability of the steels prior to purchase. Of particular importance are the changes that take place very close to the weld in the region defined as the heat affected zone, or HAZ (for a complete description of HAZ definition, see commentary in section 3.6 of API RP 2Z). By utilizing the API RP 2Z methods and making them a condition of purchase, material degradation issues associated with the HAZ can be identified and corrected early at the point of material manufacture, thereby, avoiding costly delays that could occur if left to be discovered during welding procedure qualification.

While RP 2Z has been used quite extensively in the past as a basis for prequalification of pipeline steels, the absence of a standardized testing program specifically for offshore pipeline materials has led to situations where RP 2Z has been either in whole or partly specified for use without regards for the differences between structural steel plate and pipeline tubulars, especially those with outside diameters between 8?? and 21?? as is typically used for highly fatigued SCR, TTR and DR applications. These testing programs have in some circles been referred to as RP 2Z " modified?? or RP 2Z " lite?? or by some similar moniker. A need for standardization exists.

This paper is not intended to be a discussion on fracture mechanics. The weldability program discussed is meant to establish a material's weldability over a specified heat input range using existing relevant standards for all testing. Furthermore, the testing program discussed is not intended to evaluate the performance of production joints, weld metal toughness, weld metal mechanical properties, nor shall it be used to qualify production welding procedures.

Pre-Qualification Methodology

In order to fully establish a material's weldability over a specified welding heat input range, API RP 2Z presents a relatively onerous welding and testing plan. This is especially true when it is compared to the testing requirements most pipeline contractors are accustomed to encountering when tasked with qualifying welding procedures to such standards as API 1104, DNV OS-F101, or ASME Section IX. Table 1 below presents an overview of the testing requirements of API RP 2Z and compares this to the " modified?? testing program proposed here for the basis of standardized testing for offshore pipeline materials.