What this assignment does
The essay takes two 2005 incidents — Buncefield and Texas City — and uses them to interrogate the two techniques the industry most relies on to prevent exactly that kind of event: Hazard and Operability study (HAZOP) and Layer of Protection Analysis (LOPA).
The framing question is doing the heavy lifting. Both sites operated under recognised safety management frameworks and both had, on paper, layers of protection against overfilling. So the essay does not ask what HAZOP and LOPA are. It asks why structured hazard identification and risk assessment sometimes fail to prevent the very scenarios they were designed to address — a question that forces evaluation rather than description.
It works through the COMAH regulatory framework and the ALARP principle, defence in depth, HAZOP applied to a bulk storage tank, LOPA applied to the same scenario, and closes on what the two incidents reveal about the limits of both.
How it is built
Regulation first, then concept, then method, then application, then critique. The COMAH and ALARP section establishes the legal duty and the quantitative anchor (roughly one in a thousand per year as the boundary of intolerability for workers, one in a million as broad acceptability) before any technique appears, so later judgements about tolerability have something to stand on.
The HAZOP and LOPA sections each follow the same internal shape: principles, worked application to a floating-roof gasoline tank, then strengths and limitations. Repeating the shape lets the reader compare the two methods directly rather than holding two differently-organised accounts in mind.
The worked example is a single scenario — the deviation MORE LEVEL on one tank node — carried through both methods. Reusing one scenario is what makes the comparison genuinely comparative.
Annotated extracts
Three passages, each with a note on why it earns marks and what a weaker answer does in the same place — enough to teach the move, not enough to stand in for the assignment.
The first row of Table 1 is significant. A competent HAZOP team should recognise that an alarm driven from the same instrument as the level measurement does not protect against failure of that instrument. In practice, however, HAZOP records often list such alarms as safeguards without questioning their independence. The method is qualitative and therefore depends on the judgement of the team; it does not in itself force the analyst to test whether listed safeguards are truly separate from the initiating cause.
Why this marks well
This is the pivot of the whole essay, and it is only three sentences long. The writer takes one row of their own worked table and uses it to expose a structural weakness in the method — not a weakness in the team, in the method.
Notice the sequence: what a competent team should see, what teams actually record, and then the reason the gap exists (HAZOP is qualitative and does not force the independence test). That third step is what converts an observation into a criticism. Plenty of answers manage the first two and stop, which reads as a complaint about practitioners rather than an analysis of the technique.
It also pays off the introduction. The essay asked why structured methods fail at sites that use them; this is a concrete mechanism for that failure, discovered inside the student's own worked example rather than asserted from the literature.
What a weaker answer does here
A weaker answer presents the HAZOP table as a result — here are the deviations, here are the safeguards — and moves on to LOPA. The table is correct and does nothing, because nobody interrogates whether the safeguards listed in it are real.
Modern process safety is built on the concept of defence in depth, in which a hazard is separated from its potential victims by a series of independent barriers. Reason (1997) represented this as a stack of slices of cheese, each containing holes that represent latent conditions or active failures. An accident occurs only when the holes in all slices momentarily align. The model is valuable because it shifts attention from the last person to touch the equipment towards the design, maintenance and management decisions that created the holes.
Why this marks well
The Swiss cheese model is one of the most over-cited images in safety writing, and most answers reproduce it as decoration. This one earns it by stating what the model is for: it relocates causal attention from the operator to the decisions that created the latent conditions.
That sentence is load-bearing later. Once the essay argues that an alarm and a level gauge share an instrument, the reader already has the vocabulary to see that as a management and design failure rather than a control-room failure. Introducing a concept at the point where it will later do work — and saying explicitly what work it does — is the difference between a literature section and an argument.
What a weaker answer does here
A weaker answer describes the holes and the slices, attributes it to Reason, and never uses it again. The image appears because it is expected in a process safety essay, not because the argument needs it.
The principal strength of HAZOP is its breadth. Because it systematically combines guidewords with parameters, it is good at revealing deviations that designers did not anticipate. However, several limitations have been widely recognised. First, HAZOP is time-consuming and can suffer from fatigue, particularly on large projects, which may reduce the quality of discussion in later nodes. Second, it is primarily oriented to steady-state operation; transient operations such as start-up and shutdown are handled less naturally.
Why this marks well
The strength is stated in terms of the mechanism that produces it — breadth comes from the systematic guideword-by-parameter combination — rather than as a claim. A reader can see why the strength follows from the method's design.
The limitations are then specific and consequential. Fatigue on long projects is not a generic criticism; it predicts where the analysis degrades (later nodes). The steady-state orientation predicts which scenarios get missed (start-up and shutdown). Both are testable claims about when the method underperforms, which is what a marker means by critical evaluation.
What a weaker answer does here
A weaker answer lists advantages and disadvantages as bullet points of roughly equal weight, with no account of why any of them follow from how the method works. The list is accurate and carries no argument, so it cannot connect to the incidents the essay is supposed to explain.
What makes this a strong answer
It is organised around a question rather than a topic. “Why do structured hazard identification methods fail at sites that use them?” cannot be answered by description, so every section has to earn its place by contributing to an answer.
The critique is generated from the student's own worked example rather than imported. The independence problem is discovered in row one of their own HAZOP table, which is far more convincing than citing someone else's observation that HAZOP has limitations.
Claims are anchored quantitatively where the subject allows it — the ALARP boundaries, the capture rates, the failure probabilities — and the essay is explicit that those figures are guidance rather than legal limits. Knowing the status of your own numbers is a postgraduate-level move.
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Model answer written by a Global Projects Help mentor · © Global Projects Help
Published 8 October 2026 · Questions? mia@globalprojectshelp.com