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.
Inevitably, when an ECA is conducted, the requisite value of the longitudinal weld CTOD actually needed to ensure a safe design is usually very low; around 0.06-0.10mm. Some slight variation in the ECA results does occur depending on the subtleties of the design. This is predominantly due to the fact that it is the girth weld where the greatest concern is manifested; the inconsistencies present in all welds are usually orientated favourably in longitudinal welds compared with the least favourable orientation in girth welds.
It would appear that the ‘safe’ approach is to determine the CTOD value required for the more critical girth weld, and impose this on the longitudinal weld at the same time. It is this lack of definition between the requirements of girth and longitudinal welds that results in over-conservatism. However, it is noted that some designs are more concerned with internal pressure fatigue rather than the usual installation and spanning issues. In these very rare cases, longitudinal weld fatigue can be a greater concern; however, it is still the girth weld fatigue that controls the vast majority of line pipe design.
When one considers that the ECA approach is itself conservative, and the final result is still below the values being specified for line pipe suppliers to meet, the stacking nature of this ‘overkill’ approach deserves to be reviewed. A better solution would be to understand the factors that give the best possible performance in terms of CTOD. These are well understood by modern line pipe manufacturers; a clean steel with low carbon, low sulphur, low phosphorous and a suitable micro-alloying content to control weldability. Combining this with a robust specification for non-destructive testing (NDT) control and defect sizing in the SAW weld would be a more sound and reasonable approach. This would also allow the pipe manufacturers to balance the property requirements.
As mentioned previously, a high CTOD requirement can drive the need for an SAW welding consumable that will deliver a high CTOD value, but that is more susceptible to hardening when being repaired or on interaction with the girth weld. Economic production of SAW line pipe usually requires methods of pipe forming known as UOE (where the steel is pressed into a “U” shape then an “O” shape and expanded) or the slower but more flexible JCOE (where it is pressed in to “J”, “C” then “O” shapes and expanded).
Both of these techniques then utilize single pass inside and single pass outside SAW welding. A multipass approach would yield better CTOD values with less hardenable consumables, but that would become uneconomic and unsupportable from most scheduling perspectives.
So the question is one of what is best; specify over-conservative CTOD requirements and deal with the complex contractual wrangles that usually follow, or specify what is needed and what is practical and control the contributing parameters to ensure that the best possible balance of performance and cost is achieved.
Martin Connelly joined Corus Group in 1993 after graduating with a degree in Metallurgy and Engineering Material from Strathclyde University. He worked in several technical, quality and operational roles before being promoted to technical manager.