The Pipeline Technology Conference in Ostend (see our report on page 59) showcased 117 papers over three days, making it difficult to listen to each presentation. This article discusses three papers presented at the Conference. Each paper presents different subjects, and while acknowledging that there could be as many equivalent selections made as there are reviewers available, the below papers are considered to be of particular interest.
X80 pipe
X80 pipe is being used more and more widely due to its higher strength and thinner wall thickness. A further advantage over thicker materials is that less longitudinal and girth welding is required.
Authors Dallam et al. examine the criteria for X80 pipe welding, in terms of the weld performance in wide-plate tests. As they point out, considerable resources and efforts are expended in the design and fabrication of safe and economical pipelines. While differing design philosophies may be employed, the suitability for service and the risk of failure ultimately depend on the material’s behaviour in actual service conditions.
The trend in design, and consequently pipeline material development, is to use higher-strength materials to take advantage of either less material performing the same function, or the same amount of material supporting greater operating loads.
One approach involves ‘overmatching’ the weld metal yield strength, making the weld metal stronger than the host metal of the pipe. The motivation for this is to achieve higher performance and safety at a lower total project cost.
With increased pipe strength, and the potential for thinner wall pipe, cost reductions can be made due to the decreased pipe weight; thinner pipe requiring less welding; and, the lower cost of transporting the pipe to the job site. For example, switching from X70 to X80 pipe could reduce wall thickness by 12 per cent because of the ratio of pipe strength, while the weld metal volume could be decreased by 25 per cent.
Testing at a small scale has evolved to prove the materials and the design. Tensile tests and Charpy V-notch impact tests are normally performed on small specimens, and consider the most extreme set of expected service conditions. On the other hand, large-scale tests more accurately simulate service conditions, and this is a recommendation where higher-strength materials and welding processes are being investigated. The authors point out, however, that as the required strength increases, the number of possible processes, procedures, and consumables that are suitable for welding decrease.
The drop-weight tear test
Dr Andrew Cosham and co-authors continued the theme of testing in their paper on the drop-weight tear test (DWTT). For over 40 years, linepipe specifications have stipulated minimum requirements for the shear area in a DWTT to ensure the arrest of a long-running brittle fracture, and the test has been very successful in preventing such fractures. However, many current pipeline engineers do not understand the background or importance of the test – this can lead to problems, as the test is an essential requirement for many types of pipeline.
Pipelines that transport gaseous fluids, two-phase fluids, dense-phase fluids, or liquids with a high vapour pressure, are susceptible to propagating fractures. Once initiated, a fracture can spread for long distances in either the brittle or ductile mode. Toughness specifications for linepipe have been developed to ensure that any propagating fracture is arrested within an acceptable length. The DWTT shear area requirement ensures that such fractures will not occur. Tests show that fractures will not propagate if the shear area measured in the test is 85 per cent or higher at the minimum design temperature.
Following original research in the 1960s and subsequent testing in a variety of conditions, the DWTT requirement was incorporated into API 5L in 1969, and has proven to be very successful in preventing in-service long running brittle fractures. The introduction of the test also led to significant improvements in the manufacturing of linepipe steel, and for many years linepipe has met the shear area requirement without difficulty.
However, the background to the development of the DWTT is in danger of being lost in time, and there is now a risk of complacency. The authors of this paper point out that in a number of recent projects, the linepipe supplied does not meet the DWTT shear area requirement.
Additionally, there are inconsistent and arbitrary limits on DWTT – such as it only being required for welded pipe of 20 inch diameter or larger – which now need to be reviewed and revised.
The authors conclude that, even 40years after its introduction, the background to the development of the DWTT still needs to be recognised and fully understood, as the DWTT shear-area requirement is just as relevant to today’s pipeline designers as it was at its introduction.
New welding process
It is widely acknowledged that there will be a great increase in pipeline construction in the coming years, and upwards of 280,000 km of new pipeline construction has been announced worldwide. We can reasonably assume that these new pipelines will require approximately 20 million welds to join the pipes – an unparalleled challenge on welding operations in terms of productivity, quality assurance, employee safety, and environmental impact.
Traditional manual or semi-automatic welding techniques have a number of limitations in relation to this challenge:
* The higher strength steel grades that are becoming more commonly used (X80 and above) are more difficult to weld with conventional technologies;
* Increasingly stringent quality requirements demand constant, reproducible, welding quality, which only automatic welding can guarantee to provide;
* Relatively slow joint completion rate due to the slow process of traditional welding and number of passes required, which also require higher energy consumption and give rise to increasingly unacceptable carbon dioxide emissions; and,
* The difficulty of finding skilled welders and to encouraging them to work in often remote and inhospitable areas.
In order to meet these challenges, several automatic welding processes have been developed by the industry. Friction welding is a forge welding process in which the heat is generated through the friction between two surfaces rubbing against each other under controlled axial pressure. However, conventional friction welding is not suitable for joining long components, such as pipes, since these cannot be rotated for obvious practical reasons.
To enable pipelines to be welded using the friction-welding process, an innovative variant of the conventional process has been created. Called the FRIEX process, this welding process was developed by Denys NV in co-operation with the Soete Laboratory of Ghent University and the Belgian Welding Institute, with the financial support of the Institute for the Promotion of Innovation by Science and Technology in Flanders.
The major difference between the FRIEX process and conventional friction welding is that a ‘filler material’ in the form of a solid ring is used. This ring is placed between the pipes and is rotated under an axial load that generates the required friction and associated heat (see Figure 1). After the two adjacent pipes are brought into contact with the rotating ring, the friction between the ring and the pipes increases the temperature in the contact area until the forge temperature is reached. At that moment, the rotation of the ring is rapidly stopped and the axial force is increased to the final forge force.
Forging is done using either a hydraulic or pneumatic force, and after welding, the remaining welding ring material and welding flashes are removed using automated turning and milling machines. As an example of the speed of this operation, two 20 inch diameter X65 grade steel pipes were completely welded together in around 12 seconds.
The developers of this process recognise that further testing is required, but the feasibility of their system has clearly been effectively demonstrated. Among its many advantages over other methods are its speed, reliable weld quality consistency, and a minimum requirement for highly-skilled labour, all of which will be welcome to contractors and pipeline owners alike.
