Welcome to the AIChE eLearning course on Layer of Protection Analysis.
Your course instructor iss Bob Johnson,
a longtime member of AIChE who has been involved with HAZOP studies and other
hazard analysis procedures for many years as a facilitator, a trainer, and an author.
I'm a chemical engineer with degrees from Purdue University
and have been a process safety consultant in one position or another
for about 34 years.
I was primary contract author for four CCPS guidelines books with a couple more on the way,
and I also teach the AIChE HAZOPS course and the Advanced Concepts for PHA course.
Layers of Protection are those safety barriers,
"defense in depth" that come in between the hazards of a process
and the people, property, and environment they could affect. They constitute the safety
measures giving this "defense in depth" against process safety incidents.
Some processes have more layers of protection than others, of course. In general the greater the hazard,
the more and better layers of protection you need.
So what is a LOPA anyway?
Here is a good definition that we'll use in this course.
A Layer of Protection Analysis
is a simplified method of risk assessment, intermediate between a qualitative PHA and a quanitative risk analysis (QRA),
using simplifying rules to evaluate scenario impacts,
initiating cause frequency, and independent layers of protection,
in order to provide an order of magnitude risk estimate.
Let's take a closer look at the first part of our definition,
that a LOPA is a simplified method of risk assessment
intermediate between qualitative PHA's and quantitative QRA's.
In the western United States there is a creature rarely if ever seen except on postcards
known as the jackalope, a cross between a jackrabbit and an antelope.
Well, a Jackalopa or just LOPA for short is also a hybrid beast.
On the one hand you have mostly qualitative hazard evaluation procedures,
exemplified by the hazard and operability study method or HAZOP study method,
and summarized in books such as CCPS' "Guidelines for Hazard Evaluation Procedures"
which is now in its third edition.
On the other hand you have fully quantitative risk analysis methods
with components such as fault treat analysis and consequence analysis,
summarized in this CCPS book on CPQRA,
"Guidelines for Chemical Process Quantitative Risk Analysis" now in its second edition.
Layer of Protection Analysis fills a gap between those two and has some elements of both.
A side by side comparison of features of these three basic approaches may be helpful.
A HAZOP study
is intended to examine an entire process, wall to wall, for what can go wrong.
A QRA generally examines an entire process also,
or all the events that might lead up to a specific loss event,
might be called a top event, such as in a fault tree analysis.
LOPA, on the other hand, is a unique beast in that it only studies one incident scenario.
in this respect it really isn't a hybrid of these two approaches
with respect to what is studied.
Also, whereas the HAZOP study generally uses qualitative or category estimates
of the risk parameters,
cause frequency,
loss event impacts, safeguards effectiveness,
on the other hand, QRA conducts a fully quantitative analysis of all risk parameters.
LOPA is intermediate between these two
in that it most often uses order of magnitude estimates of the risk parameters.
LOPA has often been designated as a semi-quantitative approach.
That term, to me, is somewhat questionable; it's like saying someone is half dead.
LOPA really is a quantitative approach
but at a lower degree of resolution than QRA's.
It also tends to err
on the conservative side by the way in which the LOPA methodology is set up.
This order of magnitude approach is included in our definition of LOPA, although it should be recognized that some LOPA's are
done with somewhat of a higher degree of resolution than just orders of magnitude.
This will be discussed further at a later point in the course.
Here is a quote directly from the CCPS LOPA book:
"LOPA typically uses order of magnitude categories for initiating event frequency,
consequence severity,
and the likelihood of failure of IPL's (independent protection layers), to approximate the risk of a scenario.