Codes for design and construction of pressure vessels, pipelines, offshore structures, bridges and buildings, process plants etc. contain acceptance criteria that are based on workmanship standards that may be somewhat arbitrary.
During the construction phase or in service, there may be situations where materials properties or observed defects do not meet the strict code requirements. In such cases a fitness‐for‐service (FFS) or engineering critical assessment (ECA) can be applied. Such an approach is nowadays becoming accepted by many codes as it is recognized that the requirements or acceptance criteria inherent in the codes may be unnecessarily conservative.
Using this alternative approach it can be shown that the structure or component can be acceptable if the conditions for failure are not reached within its service life when subjected to many possible damage or degradation mechanisms including brittle and ductile fracture, fatigue, environmental assisted cracking and creep at higher temperatures.
There are now several publications and recommended practices available that describe the FFS and ECA approaches:
- British Standard, BS 7910: 2013 Guide on the methods for assessing the acceptability of flaws in metallic structures
- API 579‐1/ ASME FFS‐1 Fitness‐for‐service, June 2016
- DNVGL-RP-F101 Corroded pipelines, May 2017
- DNVGL-RP-F108 Assessment of flaws in pipeline and riser girth welds, October 2017
- DNVGL-ST-F101 Submarine pipeline systems, October 2017.
Course objective and scope
DNV GL has developed a course programme focusing on the practical application of FFS and ECA particularly for pipelines and structures. BS7910 and API579 are essentially stress-based codes, however, the course also covers strain-based analyses of pipelines with girth weld flaws. It also provides an overview of different methods and software tools for analyzing the tensile strain capacity of pipelines subject to high strain. In addition, the course covers a brief introduction of finite element analysis (FEA) to determine the stress and strain input to the analysis and advanced flaw modelling and J‐integral analysis to derive the crack driving force and limit loads.
The course will cover the theoretical background for elastic and elastic plastic fracture mechanics and the fracture assessment diagram (FAD) methods, fatigue crack growth and tearing analysis. The course will also give insight into the practical aspects of elastic‐ plastic fracture toughness (CTOD, J-integral) testing of weldments and the requirements to non‐destructive testing for flaw sizing and development of alternative flaw acceptance criteria for pipeline installation.
The first three days (days 1‐3) of the course cover applications involving high strain due to lateral buckling, reeling installation and high temperatures in clad pipelines as well as effect of fatigue loading due to vortex induced vibration (VIV) associated with free spans. An optional fourth day (day 4) of the course covers an introduction of finite element modelling, definition of boundary conditions and material models and finite element based fracture mechanics analysis to generate stress/strain input to general design and ECA/FFS assessments.
It is intended that the participants attending this course shall become confident in performing Option 1 and 2 analyses according to BS7910: 2013 and that they also will be introduced to Option 3, which uses numerical analysis to generate a FAD and ductile tearing assessments. The participants will also become familiar with strain based analyses based on DNVGL-RP-F108 : 2017 and alternative methods.
The course is aimed at welding and inspection, structural, mechanical, construction, design and maintenance engineers who require more knowledge of the application of fitness‐for‐purpose assessment methods for structures, piping/pipelines and pressure vessels.
Find more about Jens P. Tronskar, Senior Vice President and Chief Technology Officer of DNV GL's Oil & Gas Technology Centre in Singapore.