Why Over-Conservatism In Line Pipe Specifications Should Be Challenged

Modern line pipe design is a complex business; the various demands of corrosion management, fatigue resistance, deepwater collapse and any other failure modes can place a lot on the robustness of that design. The final details of the design make certain assumptions about the steel pipe that is going to be used to make the pipeline.

These assumptions combine with several other factors and result in the pipe specification. How do you need the pipe made and how do you need it to perform mechanically so as to survive for 40 years in 3,000 meters of water?

The industry’s approach has, for the last 15 years, been to develop standards which can then be modified by the purchaser’s specifications. Examples are ISO 3183 and DNV OS-F101; these in turn have been influenced by design standards such as API 1111, BS 8010 etc. All of them combine to produce a route which results in the right quality and performance of the pipe so that the line can be installed and operated safely to targets for its required lifespan.

A problem that faces the industry is that with so much at stake with multi-million dollar projects, a culture of over- conservatism has taken root in many cases. From a safety and security standpoint, this is a benefit. However, in times of economic uncertainty and concern over resources, it stands as a luxury that should be challenged.

The main element in modern line pipe specifications where over-conservatism manifests itself is in the required Crack Tip Opening Displacement (CTOD) performance of the longitudinal submerged arc weld (SAW). The CTOD test, which basically measures the resistance of material to the spread of a crack, has been developed and refined over many years to become the cornerstone of defining one of the major fracture mechanics characteristics of steel. There are other areas of conservatism in line pipe specifications, but the CTOD is one of the most obvious when considering the application for which the pipe is intended.

With the industry even questioning the over-conservative nature of its own Engineering Critical Assessments (ECAs) in various conferences in 2008-09, the time is right to consider what is actually required of modern line pipe. Most client specifications now require CTOD values of 0.25mm minimum to be met on the parent, weld and heat-affected zone (HAZ) material on the SAW weld. While this is usually easily achievable on the parent material, the weld and HAZ regions have different structures and different concerns.

In the weld, this requirement can drive the need to utilize a welding consumable that delivers the requisite performance reliably, but at the expense of increased hardenability. In the HAZ, the variable nature of the material sampled under the fatigue pre-crack, results in a variable performance of CTOD values. Invariably, line pipe producers experience occasional results below specification in the HAZ, and are driven to use welding consumable that result in high CTODs in the weld, but with higher than desired subsequent hardenability.