By Martin Connelly
Modern linepipe 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.
However the industry faces a problem – a culture of over-conservatism has taken root.
It’s easy to see why this has happened, we’re operating in times of economic uncertainty and despite the improvements we’ve seen over recent months, many are still concerned about resources.
Nonetheless, this over-conservatism should be challenged. Not least to ensure a better balance of cost and performance.
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 3000m of depth?
The industry’s approach has, for the last 15 years, been to develop standards which can then be modified by the client’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.
The most obvious element in modern linepipe specifications where over-conservatism occurs 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.
With the industry even questioning the over-conservative nature of its own Engineering Critical Assessments (ECAs) in various conferences in 2008/2009, the time is right to consider what is actually required of modern linepipe.
Most 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 utilise 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, linepipe 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 around the 0.06-0.10mm; some slight variation 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.
When one considers that the ECA approach is itself conservative, and the final result is still some way below the values being specified for linepipe 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 linepipe maufacturers; a clean steel with low carbon, low sulphur, low phosphorous and a suitable microalloying 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 linepipe 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 utilise 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; a technical manager, Tata Steel
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