H S E&T CO RN ER • D E PA R TM E NT S
3. Development of hierarchical
task analysis diagrams
In order to assess the levels of human
interaction with systems and equipment,
a suitable assessment tool that uses a
systematic approach is needed to identify
potential for human failures within tasks.
Task analysis methods are used to
understand what is being carried out,
where hazards may be present and where
errors may occur.
Hierarchical task analysis (HTA), as the
name suggests, places a hierarchy on the
order of the tasks to be analyzed. It breaks
down a given human-performed activity
into goals, tasks and task steps.
The goal (the main task) is represented
at the top level, and the sub goals (task
steps) are represented at the next subordi-
nate level. The task steps, in turn, may be
broken into more detailed actions that per-
sonnel could take (sub-steps) represented
as nested boxes below each task step.
The method produces a tree structure,
along with an additional output being a
list of tasks outlining their sequencing in
order to meet the overall task goal. This
allows work procedures to be analyzed,
and various human errors and their sub-
sequent consequences can be identified.
Alongside the official company proce-
dures and work instructions for the safety-
critical tasks, other information gathered
from the worksite by the crew, such as
completed walk-through talk-through
(WTTT) templates, can be used as sources
of input toward building the task steps.
The WTTT process consists of an expe-
rienced person demonstrating how the
task is carried out onsite. It helps to under-
stand task steps, identify likely error traps
and aids discussion of how the operator
might typically deal with them. It also
helps in capturing and recording any devi-
ations from or gaps between how the offi-
cial procedures have been written (work
as imagined) and how work is actually
done on the ground (work as done).
4. Human reliability analysis
HRA is a process that uses a set of “guide
words” similar to those used in a hardware
HAZOP. The review team (similar to the
one formed for the safety-critical task
prioritization exercise) applies the guide
words to the lowest level of the developed
Driller Driller
Driller Driller
Driller Driller
Driller Driller
Driller Driller
Driller Driller
Senior Toolpusher
Driller Driller
Driller Figure 1: An example of a hierarchical task analysis diagram.
hierarchical task analysis in order to iden-
tify the possible errors that could arise.
The guide words and questions should
be developed in line with established
methods, such as predictive human error
analysis and the systematic human error
reduction and prediction approach.
This exercise ensures that any devia-
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