Paper No.
08130
INTERNAL CORROSION DIRECT ASSESSMENT OF
BURIED STEEL PIPELINE IN CHINESE OIL INDUSTRY
Lu Qimin
PetroChina Planning & Engineering Institute
Beijing, China
Huang Guibai
CNPC Research Institute of Engineering Technology
Tanggu, Tianjin, China
ABSTRACT
The substance and the progress of the study on Internal Corrosion Direct Assessment (ICDA)
technique in buried steel pipeline are described. The technical route of ICDA in buried steel pipeline as
well as the internal inspection technique and assessment of buried pipeline are studied in combination
with the actual application practice in Chinese oil industry. The ICDA study achievement will result a
trade standard of Chinese oil industry.
Keywords: oil, pipeline, corrosion, assessment, inspection
INTRODUCTION
The internal corrosion of buried pipeline is the cause of the problems in safety, environment protection and
economic loss, but it is always hard to be controlled in oil industry. Since 1990s, we have started the study on
internal corrosion inspection and assessment technique in pipeline and a related trade standard of Chinese
oil industry has been issued. Recently, we have a study on ICDA technique in buried steel pipeline in
according to the internationally prevailing management concept of pipeline integrity. The technical route and
related substance of this study are described in this paper.
Copyright
©2008 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole must be in writing to NACE
International, Copyright Division, 1440 South creek Drive, Houston, Texas 777084. The material presented and the views expressed in this paper are
solely those of the author(s) and are not necessarily endorsed by the Association. Printed in the U.S.A.
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STUDY ON ICDA OF PIPELINE
Development of Internal Corrosion Inspection and Assessment Technique in Buried Pipeline
Since 1995, our institute has started the study on inspection and assessment technique for
corrosion and prevention of in-service buried pipeline and storage tanks and the trade standard of
Chinese oil industry SY/T0087-95 “Standard of Survey Method for Corrosion and Prevention of Steel
Pipeline and Storage Tank” was issued, in which a set of inspection and assessment technique is used
for the internal corrosion of pipeline (refer to Fig. 1). It is the first technique used in China for the
inspection and assessment on internal corrosion in pipeline.
Study on Technical Route of ICDA in Pipeline
Recently NACE introduced the standard of ICDA of natural gas pipeline (water cut less than 5%)
and recommended ICDA technique for oil pipeline (no NACE standard is formulated at present). The
ICDA technique has not been established for the oil pipeline with water cut more than 5%. However, the
internal corrosion is serious relatively in such kind of buried oil pipelines in Chinese oilfields. Therefore,
we focus the study on ICDA technique to be used for the oil pipeline with water cut more than 5%. Under
the new management concept of pipeline integrity and in combination with the actual operation practice
in Chinese oil and gas pipelines and the development of inspection technology in China, we study and
put forward the technical route in ICDA, which is applicable to the actual internal corrosion condition in
the buried oil pipeline in China (Fig. 2). This assessment technique consists of several steps as follows:
Preparation
Routine Survey
Key Point Survey (excavation
inspection and assessment)
Collect basic and historical corrosion
and leakage information and present
the requirement of instrument
Monitor media corrosivity and
inhibitor effectiveness in service
Make excavation inspection in
suspected points and assess metal
corrosion and internal coating
effectiveness
Have comprehensive analysis
based on routine survey and key
point survey result
Comprehensive Assessment
FIG.1 - Frame Diagram of ICDA of Pipeline
Pre-assessment. The preparation before the indirect inspection and assessment and direct inspection
and assessment shall include followings:
Collection of information and data;
ICDA feasibility study;
Division of ICDA pipeline;
Inspection method, selection of equipment and technical requirement.
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Indirect Inspection. A primary above ground examination will be made for the pipeline in this step to
find the most potential internal corrosion area of buried pipeline, which is used as the guidance to determine
the excavation location for a detailed examination.
By the basic ICDA method, it will try to find the lower points of the natural gas pipeline and the locations
where water will accumulate are thought as the potential areas of internal corrosion of the buried pipeline. But
obviously such basic ICDA method is not applicable to the buried oil pipeline in view of its characteristics. It
might not necessary to find the major corrosion areas by the inspection route of analyzing the flow
characteristics of the media in pipeline and the features of location. Therefore, we select pipeline
instantaneous electromagnetic method (to measure the average metal loss of buried pipeline body) in our
study in combination with the above ground inspection technique such as coating holiday detection and
corrosion activity examination.
Collect basic information, survey
media corrosivity, divide ICDA area,
ICDA feasibility study
Pre-assessment
(preparation)
Above
ground
pipeline
instantaneous
electromagnetic
inspection, coating holiday detection,
active point inspection
Indirect (above ground)
inspection
Determine excavation location and
number; corrosion test at excavation
point; assess remaining strength
Detailed inspection and
assessment
Analyze metal loss in combination
with routine survey data, present
repair and service suggestion
Present repair and
service program
Calculate residual life, determine
time of re-assessment, assess ICDA
effectiveness with routine survey
data
Post assessment
Keep improving
Through assessment, feedback
ICDA each step with substance
FIG. 2 - Technical Route of ICDA in Chinese Buried Pipeline
The specific steps are as follows:
First: For the areas where IDCA is required, it will apply the pipeline instantaneous
electromagnetic method to find the location where pipeline average metal loss is relatively great (both
internal and external corrosion are possible). Or pipeline wavemaker detection method could be used to
measure the pipeline body metal loss rate if the condition is allowed.
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Second: For the locations found by the first step, it will apply an above ground examination such
as external coating holiday detection and corrosion activity examination. It is aimed to find the locations
where the external corrosion is more serious. In this way, it could make a judgment to find the locations
where the internal corrosion is serious in the buried pipeline.
The indirect inspection technique is applicable to the pipeline to transport the media with various
water-oil ratios.
Detailed Inspection and Assessment. During this period, it will determine whether corrosion is
existed in the pipeline locations where potentially high internal corrosion is found by the indirect
inspection and the degree of corrosion if any. The specific steps are as follows:
First: Determine the number of excavation points in the locations where internal corrosion might be
high.
Second: Make pipeline internal corrosion examination at the excavated points, that is, it will apply
wavemaker method to determine the location of internal corrosion and apply automatic ultrasonic
method or X-ray radiophotography to examine the depth and shape of the defect. In the same time, the
external corrosion defects will be examined, too.
Third: Select a related assessment method and calculate the remaining strength and minimum
residual wall thickness at the corroded locations of the pipeline.
Fourth: The cause of corrosion will be analyzed in combination with the oilfield routine survey data
such as media corrosivity and inhibitor effectiveness and repair suggestions will be raised.
Post Assessment. The purpose of the post assessment is to determine the time of next
assessment and make an assessment on the effectiveness of ICDA.
Determination of the remaining service life and the time of re-assessment
A method to be known as an extreme value statistics for prediction of maximum wall loss is put
forward, which is used to determine the minimum wall thickness of the pipeline. The corrosion rate of the
pipeline is determined by the actual measurement and the recommended value method. Both equation
(1) and related formulas are used to calculate the time of re-assessment.
Time of Re-assessment = (T1 – T2) / FCA
(1)
Where, T1 - estimated minimum residual wall thickness of the pipeline after survey and repair
T2 - minimum safety wall thickness from risk assessment;
FCA – corrosion rate of pipeline
Assessment on effectiveness of ICDA
The assessment shall follow the track of the effectiveness of IDCA on the basis of the routine
operation survey data of buried pipeline.
CONCLUSION
1.
According to the management conception of pipeline integrity and in combination with the
application and practice in the oil and gas pipelines in China, it is the first time to put forward the
technical route and related technical substance to make a direct assessment on the metal loss
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2.
3.
inside the oil pipeline with water cut more than 5%. The breakthrough is achieved in view of the
indirect inspection, direct inspection and assessment technique for the metal loss inside the buried
pipeline.
The study makes the metal loss inspection and assessment technique inside buried pipeline more
scientific and more operational. It will play an important role in corrosion prevention and safety
management of oilfield buried pipelines.
The technical route and related substance of this study are being further studied and eventually it
will result a trade standard in Chinese oil industry.
REFERENCE
1.
Chinese Oil & Gas Industry Trade Standard SY/T0087.1-2006 “Assessment Standard of Steel
Pipeline and Storage Tanks – Direct Assessment of External Corrosion of Buried Steel Pipeline”.
2.
NACE RP0502-2002 “Pipeline External Corrosion Direct Assessment Methodology”.
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