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Risk Analysis applied to Subsea Pipeline Engineering 285

it is necessary to determine how the probability value is to be assigned, when using the FTA
and ETA.

Reliability analysis is used as the main method of determining the probability of failure
caused by physical aspects of a pipeline i.e. corrosion, trawling impact, vortex-induced-
vibrations etc. The theory and application of reliability analysis are discussed in Chapters 13-
15.

Failure events that are not caused by physical failure of the pipeline may not be compatible
with the reliability method of analysis; an example of this is the probability of human error.
This type of failure requires deeper analysis using techniques such as historical data analysis
or using comparable circumstances from other industries.

16.5.2 HOE Frequency
Humadorganization error (HOE) probability is an area of pipeline risk analysis that is rarely
quantified with reasonable accuracy, this is primarily due to physical and mental distance
placed between individuals designing, constructing and operating the pipeline. A justifiable
basis for a risk evaluation can be established by implementing an assessment of HOE. The
purpose of a HOE evaluation is not to predict failure events, rather it is to identify the
potentially critical flaws. The limitation of this is that one cannot analyze what one cannot
predict.

There is little definitive information on the rates and effects of human errors and their
interactions with organizations, environments, hardware and software. There is even less
definitive information on how contributing factors influence the rates of human errors.

Lack of dependable quantitative data that is currently available on HOE in design and
construction of pipeline structures can be compensated for using the following four primary
sources of information, presented in work by Bea (1994).

1. Use of judgement based on expert evaluations
2. Simulations of conditions in a laboratory, office or on sites
3. Sampling general conditions that exist on site, laboratory and office
4. Process reviews, accident and near miss databases

Considering the quantity of conclusive data which is available, the principle mode by which
to quantify assessments is judgement method. As investigations into pipeline failures should
eventually lead to comprehensive and reliable databases of HOE, these databases will
compliment judgements and allow a more justifiable quantification to be arrived at.

It is necessary that any results that are deemed to be meaningful are qualified and unbiased.
Investigations by Bea (1994) gives a number of biases that can distort the actual causes of