Which Methods should an Operator Use and How Should they be Optimised?

A method for determining which method to use, and on which pipeline, is a 'Prioritisation Scheme'. These type of schemes are increasingly being used to guide operators on the optimum use of maintenance and inspection methods. For example, if a pipeline's major 3 cause of damage is third party interference, increased surveillance or the introduction of a One-Call System may be appropriate preventative maintenance methods. However, if a pipeline is failing due to internal corrosion, then an internal inspection using an intelligent pig would be appropriate.

A Prioritisation Scheme considers the probability and consequences of failure within a group of pipelines (or sections of a single pipeline) by systematically assessing the pipelines' design, operation and failure history. Points are allocated for design, operation and failure history. High points indicate high risks. For example, the probability of failure due to external corrosion is evaluated by considering the quality of the pipe coating, CP system, etc., and the consequences of failure are considered by estimating the density of surrounding population, security of supply, etc.
The great advantage of this scheme is that it can:
i. rank all the pipelines within a group (or sections of a pipeline) in terms of probability of failure, and consequences of failure,
ii. determine which pipeline (or section of a pipeline) is most in need of some type of maintenance measure,
iii. determine which maintenance measure to use.
There is now no need for a pipeline operator to 'guess' which part of his system needs maintenance, and he now does not to have wait for a section of his system to show signs of deterioration. The Priority Scheme is a proactive method of setting maintenance and inspection schedules. Figure 2a shows a readout of one particular scheme. This scheme has been used to rank seven pipelines. Clearly pipeline number seven is the highest risk pipeline. Figure 2b shows the second analysis the Scheme conducts on pipeline number seven.

Figure 2b shows that this pipeline is most at risk from internal corrosion, therefore an internal inspection is necessary. Figure 3a shows the results of a re-run of the Prioritisation Scheme to quantify the effect of the internal inspection on pipeline number seven. Now, pipeline two is the highest risk, and Figure 3b shows third party interference to be the major risk to pipeline two. The operator can now consider increasing surveillance to reduce this risk.

How to Maintain a Safe Pipeline "What Methods are Available, and What do they Prevent/Detect?"

Pipelines can be, and are, routinely inspected and monitored using many direct and indirect techniques. The methods are well-documented and aim to ensure that:

a. pipelines do not become defective or damaged ('proactive' methods),

b. damage or defects are detected before they cause serious problems ('reactive' methods). An operator should assess the greatest damage/defect risk to his/her pipeline, then select a monitoring/inspection method to reduce that risk:

TABLE : Pipeline Inspection and Monitoring Methods

Why maintain and inspect a pipeline?

Engineering plant follow a 'bath tub' type failure probability curve, Figure 1. This curve shows that during a structure's design life the highest failure probability is when the structure is new, or when it is old. This curve applies to automobiles, aircraft, etc., and pipeline operators will identify with it; pipelines have high failure rates early in life (e.g. hydrotest) and later in life (due to corrosion). An inspection of a pipeline will help to extend the low probability portion of Figure 1 - the goal for the pipeline operator being to extend the design life of his pipeline to 80 or even a 100 years, Figure 1.

Pipeline Inspection

Pipelines are recognised as the safest method of delivering energy. However, pipelines, like all engineering plant can, and do, fail. A number of transmission pipelines have failed recently, with both tragic and spectacular effect. For example in Venezuela in 1993, 51 people were burnt to death when a gas pipeline failed and the escaping gas ignited. More recently (1994), a pipeline failed in New Jersey, USA, and the resulting fireball killed one person, and injured 58 others. There have been other recent reports of pipeline failures in Russia, Pakistan, Argentina, Canada and Britain.

Pipeline failures rarely cause fatalities to the public, but they can disrupt an operator's business, either by loss of supply, or by necessary remedial work. They can be extremely costly in terms of replacement and repair. For example, the BP Forties oil pipeline in the UK North Sea has had to be replaced due to internal corrosion at a cost of $250 million, and a single pipeline failure can cost tens of millions of dollars if it occurs in an environmentally-sensitive area.
An operator needs to maintain a safe pipeline, and ensure it has a long and profitable life. Consequently, he must consider maintenance measures that are both cost effective, and prevent
failures or large repair bills.
Internal inspection of a transmission pipeline using intelligent pigs is increasingly being used by pipeline operators as a means of both maintaining their pipelines and ensuring that their major asset has a long and efficient life. British Gas has internally inspected most of its 18,000 km high pressure pipeline system, and is now able to reduce maintenance bills. This company can now look to the future with confidence in the knowledge that it knows the condition of its (mainly 25 year old system) pipelines, and can demonstrate to Regulatory Authorities their safety, potential for uprating, and potential for an infinite design life.
This Section of the Workshop is aimed at answering the type of questions a pipeline operator should ask himself before he considers an internal inspection. The paper starts by covering maintenance and inspection methods in general.