The inspection of onshore and offshore pipeline is mostly performed based on given constant intervals. By using a risk based approach the inspection frequency can be adapted to the current status and the potential failure consequence of the specific pipeline.
This paper describes some of the major aspects of the methodology developed to estimate the risk to onshore & offshore pipelines. Based on a risk assessment an optimised inspection strategy can be developed. The determination of the risk includes the identification of the different failure modes, the probability of failure and the potential consequences. The interaction of different failure modes is also considered. Finally, the current risk can be estimated.
The paper presents the results of a general risk assessment of 22 offshore pipelines. Although only basic data has been available this was sufficient to work out reliable results within a semi-quantitative approach. Depending on varying wall thickness, diameter, etc. a pipeline was divided in different segments. The assessment reveals different governing threats and failure modes, like ageing, free span or corrosion. In a final step the results of the risk assessment are used for the optimisation of the inspection intervals within the proposed risk based inspection framework.
This paper describes some of the major aspects of the methodology developed to estimate the risk to onshore & offshore pipelines. Based on a risk assessment an optimised inspection strategy can be developed. The determination of the risk includes the identification of the different failure modes, the probability of failure and the potential consequences. The interaction of different failure modes is also considered. Finally, the current risk can be estimated.
The paper presents the results of a general risk assessment of 22 offshore pipelines. Although only basic data has been available this was sufficient to work out reliable results within a semi-quantitative approach. Depending on varying wall thickness, diameter, etc. a pipeline was divided in different segments. The assessment reveals different governing threats and failure modes, like ageing, free span or corrosion. In a final step the results of the risk assessment are used for the optimisation of the inspection intervals within the proposed risk based inspection framework.
1. INTRODUCTION
During the last years the number of very old pipelines (age over 50 years) has increased significantly. Especially these pipelines require a detailed assessment of their integrity in combination with special inspection planning.
The inspection of onshore and offshore pipelines follows principles defined by
companies and national regulations. Mostly constant intervals are used. However,
the question arises why no or little benefit is taken from the already performed
inspections. This can be changed by introducing a condition based determination of
the next inspection interval. Here, the actual condition of the pipeline which
corresponds to the assumed failure probability is taken into account. However,
should a small infield line be inspected by using the same condition based
assessment as for a mayor transport pipeline? By also taking credit for the
consequence of a potential failure the risk based inspection scheme is introduced.
This approach allows the focus on inspections of the so-called “high-risk” elements.
The risk assessment is carried out before defining the new inspection interval. For
determining the risk the probability of failure of pipelines and the corresponding
consequence have to be estimated. There exist different procedures to estimate the
failure probability and consequences. More details are provided in the following
section.
2. RISK ASSESSMENT OVERVIEW
Risk assessments consist of the following steps; cf. [1], [2], [3]:
1. Data gathering
2. Identification of failure modes
3. Assessment of the consequence
4. Assessment of the failure probability
5. Assessing the risk
6. Determination of the optimised inspection frequency and type
2.1 Data gathering
Before performing a risk assessment data gathering has to be carried out. This is an
important step as the quality of the determined risk strongly depends on the data
collected. This step mostly involves the pipeline network operator. He has to check if
there are still existing design documents which can be used, if all inspection data is
available, operational data like pressure changes have been monitored and are
available, etc. In most cases some of the data is only available in paper format and
some of the data does not exist anymore. Based on the amount and quality of data
available, the most suitable risk assessment procedure can be chosen:
to perform a semi-quantitative approach by calculating suitable key performance
indicators and make assumptions where data is missing. These assumptions have to
be listed, so that they can be replaced with “real” values if they are available in the
future.
2.2 Identification of failure modes
The first real step of the risk assessment is the identification of the relevant failure
modes. Procedures like the HAZIDs or existing code like DNV-RP-F116 [5] can be
used to perform this step. Also historical data from the operator or historical data
available in data bases like PARLOG [10, 11], UKOPA [8], CONCAWE [4], EGIG [6],
ESMAP [7] or data provided by the US department of transportation [14] can be
used. Examples for onshore (see Fig. 2) and offshore (see Fig. 3) are provided
below.
important step as the quality of the determined risk strongly depends on the data
collected. This step mostly involves the pipeline network operator. He has to check if
there are still existing design documents which can be used, if all inspection data is
available, operational data like pressure changes have been monitored and are
available, etc. In most cases some of the data is only available in paper format and
some of the data does not exist anymore. Based on the amount and quality of data
available, the most suitable risk assessment procedure can be chosen:
- Nearly no data: highly qualitative approach
- Most of the data, however incomplete: semi-quantitative approach
- All data, incl. distribution functions: fully quantitative approach
to perform a semi-quantitative approach by calculating suitable key performance
indicators and make assumptions where data is missing. These assumptions have to
be listed, so that they can be replaced with “real” values if they are available in the
future.
2.2 Identification of failure modes
The first real step of the risk assessment is the identification of the relevant failure
modes. Procedures like the HAZIDs or existing code like DNV-RP-F116 [5] can be
used to perform this step. Also historical data from the operator or historical data
available in data bases like PARLOG [10, 11], UKOPA [8], CONCAWE [4], EGIG [6],
ESMAP [7] or data provided by the US department of transportation [14] can be
used. Examples for onshore (see Fig. 2) and offshore (see Fig. 3) are provided
below.
2.3 Assessment of the consequence
The second step of the risk assessment is the estimation of the consequence of
failure. The consequence of failure should consider the impact on the environment
and human safety. Sometimes additionally the financial impact should be taken into
account. The consequences can be estimated qualitative, e.g. by considering the
location, diameter and content as described e.g. in DNV-RP-F116 [5] or can be
based on detailed spill or dispersion calculations. The method to be used depends on
the data available and also on the location. For offshore pipelines a more general
approach is suitable, while for onshore pipelines the approach should be more
detailed. For onshore pipelines there should be e.g. taken into account the houses in
the vicinity of the pipeline; a procedure is provided in [9].
2.4 Assessment of the failure probability
The failure probability can be estimated based on a highly qualitative procedure or a
fully probabilistic approach or a combination of both extremes. A combination of
qualitative assessments with some calculations as proposed by Muhlbaur is widely
used. For the fully probabilistic approach the input data like distributions of material
properties, weld qualities, pipeline dimensions etc. are normally not available,
especially for older pipelines. Therefore, the practical approach using scorings
systems is more often used. For the failure probability the following influencing
factors should be considered:
Design data
Manufacturing data
Installation data
Operational data
Maintenance data
Inspection data
Damage history
Failures due to design errors typically occur in an early stage of the intended design
life. The same is true for fabrication and installation errors. These types of failure
normally occur during this first 2 years of operation. Third party threats, e.g. due to
anchors, can play a major role and should therefore definitively be regarded, see Fig.
2 and Fig. 3. Additionally it can be concluded from Fig. 3 that leakage due to
extensive corrosion may also be a governing factor and should be considered in the
risk assessment.GL uses a semi-quantitative approach based on Muhlbaur [9] for
onshore as well as for offshore pipelines. The Muhlbaur approach consists of a
scoring system for the main failure sources:
- Design Corrosion
- Third Party Impact
- Operation
These four topics are divided in sup-topics and each sub-topic is rated. This procedure gives the user a general overview of the condition of the pipeline.
In addition to this general overview the time dependence of the failure probability is considered by using a remaining life time approach. The remaining life time for offshore pipelines considers: free span, corrosion, design and operational fatigue.
2.5 Assessment of the risk
Risk is the combination of failure probability and consequence. It is normally represented in a matrix. This matrix can be 3x3, 4x4 … or un-symmetric as e.g. 5x7. The GL risk matrix is shown in Fig. 4
In addition to this general overview the time dependence of the failure probability is considered by using a remaining life time approach. The remaining life time for offshore pipelines considers: free span, corrosion, design and operational fatigue.
2.5 Assessment of the risk
Risk is the combination of failure probability and consequence. It is normally represented in a matrix. This matrix can be 3x3, 4x4 … or un-symmetric as e.g. 5x7. The GL risk matrix is shown in Fig. 4
Reference: "Application of risk based methodology to offshore pipelines", G. Stadie-Frohbös, J. Lampe, http://www.pipeline-conference.com/download/1121/ptc_2013_Stadie-Frohboes.pdf?redirect=node/1480, february 2015
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