This document summarizes a safety risk analysis of a natural gas compression plant in Italy. It describes the qualitative and quantitative risk assessment methods used, including HAZOP, FMECA, ETA, and FTA. The analysis identified unacceptable risks from potential jet fire and UVCE events. Recommendations include routine maintenance of pressure sensors, safety radii around the plant, and continuous maintenance to reduce vulnerability.

1. Safety Risk Analysis on a Natural Gas Compression Plant in Italy
Okoroma Justice (MSc.), Milan Kumar (MSc.), Olapade Olushola (MSc.), Mohammed Al-Salman (MSc.)
Petroleum Engineering, Politecnico di Torino, Italy
1.0 Abstract
This work considers the risk analysis of a Natural Gas Compression facility and
associated Fire Fighting system. Different analyses were carried out on the system in the
course of the project. The risk assessment was approached both qualitatively and
quantitatively. The Hazard and Operability (HAZOP) Study and Failure Mode Effect and
Criticality Analysis (FMECA) make up the qualitative analyses carried out on the natural
gas plant and the fire fighting system respectively. The quantitative approach involved
the Event Tree Analysis (ETA) and the Fault Tree Analysis (FTA); ETA for the natural gas
plant, and FTA for the fire fighting system.
The normal operational configurations were selected for both systems. The failure
modes were identified for the components and their probabilities were estimated based
on whether unavailability, unreliability or both is/are being considered. The TOP (or
minimal cut set) obtained from the FTA was used in the determination of the failure
probability for the fire fighting system which is a combination of the TOP for
unavailability and unreliability of the system. Same was done for the shutdown system.
Consequence analysis was performed using the Quick Method and the damages due to
the Reference Initiating Events (RIE) obtained from the ETA were evaluated based on the
results obtained from the frequency calculations of events and accidental sequences in
the ETA. The results revealed unacceptable risks implying that additional risk reduction
measures must be pursued. Results also showed risks belonging to “as low as reasonably
practicable” category. In both cases, recommendations were suggested to ensure safer
plant operation.
2.0 Process Description
It is a simple compressing process of natural gas. Gas, coming from separators where liquid
phase is removed from gaseous phase, arrives to the suction scrubber for further separation of
entrained liquid that could be potentially dangerous to the compressor. After sufficient
scrubbing, the gas is compressed by a turbo-compressor and cooler in a discharge cooler before
going to a discharge scrubber. At the end, natural gas is ready to be fed to the grid.
There is also a fire-fighting system which uses two substances: foam and water. Foam operates
on temperature and also on oxygen removal, thanks to its’ sweltering effect that brings about a
temperature decrease and the separation between fuel and oxidant, blowing out the flame.
3.0 Hazard Identification: FMECA & HAZOP Study
5.0 Probabilistic Analysis: ETA & FTA
7.0 Conclusion & Recommendations
The risk assessment revealed that upon an event leading to jet fire, a total of 17 fatalities
are foreseen within the high lethality area, while 49 people will be affected within the
irreversible injury zone. As expected, the damages done by UVCE are reduced as
compared to jet fire (uncontrolled and with domino effects), 6 fatalities and 15 casualties
or irreversible injury victims. The size of affected area predominantly fell within the plant
layout (above 70%) while a smaller part was outside the plant.
Routine maintenance should be performed on the pressure sensor low instrument
installed on the pump’s common discharge line and control system. Also, scheduled
check of the mechanical sealing system of compressor is recommended. Safety radius
should be set up around the plant to prevent damages to buildings or potential
industries producing or using flammables/explosive chemicals, since a disaster could
have domino effect on the environs. Continuous and proper maintenance of the plant
should be strictly carried out to reduce the vulnerability of the area.
References: [1] Carpignano A., Risk Assessment, Risk Analysis / Safety and Risk Analysis Methodologies, Part 1, 3 and 5. Dipartimento Di Energetica, Politecnico di Torino.
[2] Ganci F., Risk Analysis lecture slides – Hazardous substances, FMECA and Criticality Analysis, HAZOP, Selection of Initiating Events, ETA, FTA, Consequence Analysis.
4.0 Reference Initiating Event
Failure Mode Effect and Criticality Analysis (FMECA)- sheet 1 of 10 Hazard and Operability Study (HAZOP)- sheet 1 of 25
Distribution of Risks
Nodes| Description / Components
1 Suction Scrubber including process gas line from upstream separator, and scrubber’s liquid leg
2 H.P. Compressor, including suction line, discharge line, BDV line and PSV line
3 H.P. Compressor Discharge Cooler
4 Recycle (line and valve) from HP Compressor Discharge Cooler outlet to Suction Scrubber inlet
5 Discharge Scrubber including process gas line from compressor, and scrubber’s liquid leg
6.0 Consequence Analysis & Risk Assessment
Accidental sequences and calculated frequencies – Sheet 1 of 2
Fault tree analysis (FTA) diagram– Sheet 1 of 2
Event tree analysis (ETA) diagram – Sheet 1 of 2 Computation of damages, frequencies and risks
Frequency vs Damage plot for Risk Acceptability
Areal view of damage with respect to plant
Grouping of IE’s and
Selection of RIE