Given that ULCF is defined by cyclic loads which generate plastic effects, stress based design criteria for structures which are subjected to extreme loads do not yield good results. Therefore, advanced analysis and modelling of structures or components subjected to extreme loads is presently done using strain-based design criteria which are based on limit state design and displacement control load.
The primary areas where strain based design is expected to be applicable, taking into account the current industrial application focus of the thesis, pipelines, are in design of reeled laying of offshore pipelines, in thermal design of arctic pipelines, in design of types of offshore pipe lay systems, in design and assessment of pipelines in areas with significant expected ground movement, and in HT/HP pipeline designs. Some pipelines may also have some applications of strain-based design where cyclic loadings cause occasional peak stresses above the pipe yield strength. Here, the cyclic lifetime assessment is improved by using strain ranges for the cycles, instead of stress ranges. However, the cyclic lifetime assessment needs further development and validation, mainly for LCF or ULCF load conditions. Some important drawbacks can be pointed out to the current state-of-the-art strain-based design [30]:
• The current use of strain-based design has many project-specific compon-
ents. This limits the ability of a cookbook approach where each step can be laid out as part of common design sequence to apply to all areas of pipe strain-based design. This situation would indicate that taking the cur- rent state-of-the-art methods and creating a code or standard would be ineffective at covering the range of needs for future pipeline designs.
• Past design practices have asked designers to determine whether a par-
ticular loading was load-controlled or displacement-controlled without any other possible choices. Designers today need to recognize that there are a range of intermediate cases between full-load control and full displacement control. The behaviour of the pipe, particularly its buckling resistance, can
2.3. Strain-based design method 11
change significantly depending upon the designers choice of the appropri- ate intermediate case for design. Guidance on local buckling compression resistance of pipelines appears to be well founded when using the critical strain. The additional strains that can be achieved under partly or fully dis- placement controlled loading should be more thoroughly studied to allow more specific guidance.
• The methods for assessing tensile failure resistance of pipelines by Engineer-
ing Critical Analysis (ECA) become fewer when the plastic strain exceeds 0.5% and fewer still as the strain increases to 2% or more. More validations trials are needed in the open literature to support the use of the few existing methods up to high axial strains.
• Further study is needed on the effects of prior strain history on the resist-
ance of pipeline materials to different failure modes.
• Methods of assessing cycles of loading that include plastic strain are avail-
able. But the limited number of tests on which they are based may mean that these methods are conservative for many pipeline design situations to which they might be applied. Additional testing and analysis of cyclic behaviour of pipelines is needed to improve the method currently available.
• Design of pipelines to resist ratcheting has become more important recently
because of thermal cycle effects on high-temperature pipelines and flow lines. As for other types of cyclic loading, the current design methods are relatively conservative, but have been shifting to allow more cycles of plastic strain. Additional testing and assessment is needed in this area to improve the current methods.
• Although some workable strain-based design methodology and the support-
ing engineering processes and models have been achieved and validated, some improvements and enhancements are needed, especially as we move to high pressure, high strength pipe and large-diameter pipelines [61].
Under cyclic action, the initial loss of strength can be produced by plastic be- haviour (or other non-linear phenomena) coupled with the loss of strength by fatigue, which also conduces to the high reduction of the residual strength, caus- ing a decrease of the residual life under very low number of cycles. The capturing of this life reduction is possible by means of a new constitutive model based on the residual strength of the solids under fatigue loads coupled with other nonlinear models (ex: plasticity, ductile damage).
Plastic design is applied in two areas: for structures subjected to static loads and for structures subjected to loads varying with time. Extreme transient loading conditions involving widespread yielding may lead to monotonic ductile fractures or to fatigue failures after a very short number of load fluctuations (Nf in the order of 102 cycles). The latter failure mechanism is often called ULCF to distinguish it from the well-known low-cycle fatigue process (Nf in the order of 103 cycles or greater) since it involves distinct micromechanisms, not fully understood and characterized until now.