Article
Ultimate Axial Load Prediction Model for X65 Pipeline with
Cracked Welding Joint Based on the Failure Assessment
Diagram Method
Jianping Liu
1,2
, Hong Zhang
1
, Shengsi Wu
1
, Xianbin Zheng
3
, Dong Zhang
1
and Xiaoben Liu
1,
*
Citation: Liu, J.; Zhang, H.; Wu, S.;
Zheng, X.; Zhang, D.; Liu, X. Ultimate
Axial Load Prediction Model for X65
Pipeline with Cracked Welding Joint
Based on the Failure Assessment
Diagram Method. Appl. Sci. 2021, 11,
11780. https://doi.org/10.3390/
app112411780
Academic Editors:
Alberto Campagnolo and
Alberto Sapora
Received: 7 November 2021
Accepted: 8 December 2021
Published: 11 December 2021
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4.0/).
1
National Engineering Laboratory for Pipeline Safety, MOE Key Laboratory of Petroleum Engineering,
Beijing Key Laboratory of Urban Oil and Gas Distribution Technology, China University of
Petroleum-Beijing, Beijing 102249, China; liujp@pipechina.com.cn (J.L.); hzhang@cup.edu.cn (H.Z.);
2020210831@student.cup.edu.cn (S.W.); 2021310325@student.cup.edu.cn (D.Z.)
2
PipeChina Engineering Technology Innovation Company, Tianjin 300456, China
3
Natural Gas Marketing Company, PetroChina Company Limited, Beijing 100101, China;
xianbinzheng@petrochina.com.cn
* Correspondence: xiaobenliu@cup.edu.cn
Abstract:
Crack defects in the girth welds of pipelines have become an important factor affecting the
safe operation of in-service oil pipelines. Therefore, it is necessary to analyze the factors affecting the
safe operation of pipelines and determine the ultimate load during pipeline operation. Based on the
failure assessment diagram (FAD) method described in the BS 7910 standard, the key factors affecting
the evaluation results of the suitability of X65 pipeline girth welds are analyzed, and the effects of
crack size, pipe geometry, and material properties on the evaluation results are investigated. The
results indicate that the crack depth is more crucial to the safe operation of the pipeline than the crack
length. While the effect of wall thickness is not significant, the misalignment can seriously aggravate
the stress concentration. In general, the higher the yield ratio and tensile strength of the pipe material,
the more dangerous the condition at the weld. The ultimate axial load that a crack-containing girth
weld can withstand under different combinations of the above factors was determined. Furthermore,
a data driven model via the optimized support vector regression method for the ultimate axial load
of the X65 pipe was developed for engineering application, and the comparison results between the
FEM results and the predicted results proved its accuracy and reliability.
Keywords:
ultimate axial load; girth weld; failure assessment diagram; fracture assessment; X65 pipeline
1. Introduction
For in-service pipelines, internal inspection can identify possible defects which may
endanger pipeline operation. However, we can control the maximum axial load during
service to ensure that the pipeline continues to operate. Take the Mo-Da line as an example,
which is the first large-diameter crude oil pipeline in China passing through the alpine
permafrost. This pipeline is made of APL X65 line pipe steel with a diameter equal to
813 mm. The pipeline passes through the mountainous island permafrost in the northern
part of the Greater Khingan Mountains, where there are numerous swamps and well-
developed river systems. Frost heaving in winter and thaw settlement in summer can
easily cause damage to the pipeline (e.g., pipeline folding, bending, and even rupture), and
can seriously affect its safety [
1
]. In recent years, several accidents have occurred in the
pipe due to the weld cracks [2].
In general, geometric defects, such as cracks and misalignments, as well as the discon-
tinuity of materials, can lead to the concentration of stress at the girth weld joint. This can
result in the reduction of the deformation-bearing capacity of girth welds and even fracture
failure under the axial load. Due to such accidents, girth weld cracking has become an
important factor affecting the safe operation of oil and gas pipelines. Therefore, there is
Appl. Sci. 2021, 11, 11780. https://doi.org/10.3390/app112411780 https://www.mdpi.com/journal/applsci
