Reliability of Inspection for Root Flaws in Riser Girth Welds

Author(s):  
J. R. Rudlin ◽  
C. R. A. Schneider ◽  
G. R. Razmjoo

The fatigue loading on deep water risers leads to a requirement for the detection of small root flaws during manufacturing inspection. Mechanised welds for offshore pipelines are also subject to extreme loads during laying, leading to a similar requirement. Automated Ultrasonic Testing using zonal methods have been adopted as the inspection method of choice for these inspections, but there is little information in the public domain regarding the expected reliability of the various systems available. Extensive individual inspection qualifications are carried out for each installation. The extent of these could be reduced by the availability of such background information. This paper reviews data from joint industry projects in the area carried out by TWI, and compares results from these with such data as is available in the public domain. An analysis of future requirements and capability of currently available theoretical models for extending the range of qualifications is also given.

Author(s):  
Colum Holtam ◽  
Rajil Saraswat ◽  
Ramgopal Thodla ◽  
Feng Gui

Environmentally assisted sub-critical static crack growth can occur in offshore pipelines exposed to aggressive production environments. Recent advances in fracture mechanics testing methods have shown that slow static crack growth rates can be reliably measured in sweet and sour environments under constant stress intensity factor (K) conditions. This has potential implications for the engineering critical assessment (ECA) of pipe girth welds subject to low cycle fatigue loading with long periods of operation under constant static load between cycles, e.g. lateral buckling. This paper demonstrates the influence of including static (i.e. time dependent) crack growth as well as fatigue crack growth in a modified pipeline ECA approach.


Author(s):  
Andrew Cosham ◽  
Kenneth A. Macdonald

Offshore pipelines experience strains greater than yield during pipelay and in service. Installation by reeling introduces high levels of plastic strain, typically on the order of 2 percent for a 12 in. flowline. Controlled lateral buckling in offshore pipelines, due to high operating pressures and/or temperatures, may also give rise to high strains and large cyclic loads. Similarly, frost heave or ground movement in onshore pipelines can cause high strains. To date, most of the cases involving high strains are to be found in offshore pipelines, in terms of both design and the assessment of accidental states. However, some of the experiences in the offshore industry have relevance to onshore pipelines. Fracture control in this context is designing pipelines to address the implications of these high static and cyclic strains during installation/construction and operation. Pipeline design codes such as DNV-OS-F101 and DNV-RP-F108 give guidance. Two issues to consider are: the degradation of the material properties, and the failure of the girth welds. High strains may cause failure or the growth — by stable ductile tearing — of preexisting flaws in the weld. Subsequent fatigue loading may cause pre-existing flaws to grow to failure. Engineering critical assessments (ECAs) are conducted during pipeline design to determine tolerable sizes for weld flaws. Standards such as BS 7910 and API 579 are primarily stress-based and it is not straightforward to apply them to strain-based situations. DNV-RP-F108 addresses this gap by providing additional guidance derived from UK and Norwegian research programmes. Assessing flaws subject to high strains is at the ‘cutting-edge’ of applied fracture mechanics. ECAs often have a reputation of being ‘over-conservative’. ECAs of pipelines subject to high strains may indicate that only very small flaws would be acceptable, whereas practical experience has shown that the girth welds are highly tolerant to the presence of flaws. It is therefore instructive to consider under what situations might ECAs be too conservative, and when they may be non-conservative. The available guidance for ensuring fracture control in pipelines under high plastic strains is discussed in this paper, and the wider issues are addressed.


2017 ◽  
Vol 33 (1-2) ◽  
pp. 203-231
Author(s):  
Antonio Terrone
Keyword(s):  

The study of Buddhist texts can inform us of the way scriptures were composed, as well as illuminate the reasons behind their production. This study examines the phenomenon of borrowing and reusing portions of texts without attributing them to their ‘legitimate authors’ within the Buddhist world of contemporary Tibet. It shows that not only is such a practice not at all infrequent and is often socially accepted, but that it is used in this case as a platform to advance specific claims and promote an explicit agenda. Therefore, rather than considering these as instances of plagiarism, this essay looks at the practice of copying and borrowing as an exercise in intertextuality, intended as the faithful retransmission of ancient truths, and as an indication of the public domain of texts in Tibet.


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