IIW White Paper

9 Needs and challenges of major industry sectors for future applications

The 21 st Century for pressure equipment is the century of design by analysis (DBA) to permit much higher allowable stresses for much higher strength materials than were acceptable for design by rule Codes. This quantum leap must be done with assurances of high reliability including documentation with a defensible paper trail of good engineering and quality control. Truly, major challenges and burdens are falling on today’s welding engineers. The increases in allowable stresses may be 50% or more. This more efficient use of materials developed with emphasis on cost effective alloying and controlled processing is a particular problem for the welding specialist. In the last half of the 20 th century, welding specialists could overmatch strength properties and provide a reliable margin of safety against many failure modes. When justification was required, related experience could always be cited. Now, there is no relevant experience and past margins achieved by overmatching are more difficult to obtain. DBA requires elastic–plastic, multi-axial strain limits for weld regions as well as fracture mechanics and time dependent properties of welds and heat affected zones that presently do not exist and must now be obtained through creative specimen design and testing. Design and fabrication cannot be improved after a failure. They must be done right the first time. The objectives of pressure vessel engineers today are remarkably different from those barely 30 years ago whenequipmentwas tobebuilt using proven technologies and very forgiving, provenmaterials. Theemphasis then was on economies of scale i.e. bigger was more efficient (economical). There was the expectation that operating lifetimes would be limited. Replacement due to obsolescence would be in a generation. Today, the pressure vessel engineer must be concerned with prolonging the life of that equipment after it has been operating (and degrading) for 30, 40 or even 50 years. Fitness for service (FFS) or engineering critical (ECA) assessments must be done with assurances of high reliability and with a defensible paper trail. Truly, major challenges and burdens are apparent for today’s pressure vessel and welding specialists that support them. These challenges are occurring in a global economy. This means that a component may be designed in Country A, for installation in Country B, to be fabricated in Country C, of a material developed in Country D, but produced in Country E and welded using consumables produced in Country F. All of this must usually be in compliance with international Codes and regulations by engineers in every country along the way. There must be better understanding and control of essential variables associated with welding and weld materials. An important part of both design and post-construction assessments of suitability for continuing service of pressure vessels is predicting fatigue behaviour of weldments. Fatigue of welds has been one of the most common failure modes. A master S-N curve approach has recently been accepted by ASME and by API for fatigue assessment of welded steel and aluminium vessel construction, and post-construction life prediction in service at ordinary temperatures. Extension of this method to other materials and higher temperatures requires more data and documentation of service experience. Additional work is needed to validate proposed approaches for accounting for environmental effects, methods to improve weld profiles and surface stresses, assigning quality factors, and analysing thermal stress cycles. The master S-N curve approach is based on two key technical advances in fatigue analysis over the last decade: 1. A robust structural stress method that offers good mesh-insensitivity in stress concentration calculation. 2. Definition of an “equivalent” structural stress so that the master curve can represent a large amount of fatigue test data for various joint types, loading modes, plate thicknesses, etc with a relatively tight scatter band. Based on the statistical characterisation of over 1,000 weldment fatigue tests, the design master S-N curve shown below provides both a technically sound and reasonably conservative basis for fatigue evaluation for pressure vessel and piping components ( Figure 9.10 ).

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Through Optimum Use and Innovation of Welding and Joining Technologies

Improving Global Quality of Life