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A S Jessup-Bould:

Assessment of Pressure Relief Devices for Onshore and Offshore Natural Gas Treatment Facilities







Project Background





End Client: BG, Tunisia







The assessments were completed as part of a Process Safety in Design (PSiD) review of two of BG’s natural gas facilities in Tunisia. These were for the Miskar platform and the Hannibal gas treatment plant.

The Miskar field is operated by BG Tunisia and comprises the Miskar offshore gas production complex (Miskar platform) located 120 km offshore Tunisia, a sub-sea pipeline and the Hannibal gas treatment plant, onshore, 20 km to the south of Sfax.

Hannibal was initially installed and commissioned in 1995 with a significant revamp in 2005 when a gas turbine driven compressor and additional production separator were added. Further changes have since been made to maintain production levels.



Assessment of over-pressure and under-pressure relief devices







Summary of Process















The Miskar offshore installation comprises 3 platforms (production, accommodation + utilities, flare). The production platform has three separation streams with total rated capacity of 340 MMSCFD of gas. There is a compression train and two glycol dehydration trains.

Photo source GM-RAM Limited




The feed gas to Hannibal arrives via a 24” pipeline and is first admitted to a slug catcher. Gas from the slug catcher is processed in an Amine unit to remove carbon dioxide and hydrogen sulphide. The gas is subsequently dehydrated by glycol and a molecular sieve. Nitrogen is removed in the final stage of gas treatment prior to compression and export via pipeline. Stabilised condensate is exported from the site via pipeline and recovered acid gases are processed in the sulphuric acid plant or incinerated.

Photo source Middle East Online


















  • BG approval to work on PSiD for pressure relief.
  • Undertaking calculations for scenarios involving supercritical fluids.
  • Use of Hysys simulation package.













Unit Operations

Pressure vessels including slug catchers, pig launchers / receivers, separators

Pumps and compressors.

Shell and tube heat exchangers.

Air blast coolers.

Compabloc heat exchangers.

Fired reboilers.

Glycol and amine contactors.

Stripping columns and fractionator.

Molecular sieves.

Nitrogen recovery towers / cold box.

Gas filters.

Condensate stabilisers

Storage tanks

Plant and instrument air

Materials of Construction

Carbon steel.

Documents Produced

Required relief rate calculations.

Relief device sizing calculations.

Internal notes.

Overall summary spread sheets.

Final reports.

Over and Under Pressure Scenarios Considered

External fire.

Control valve failure.

Blocked discharge.

Thermal expansion and vaporisation due to solar gain.

Heat exchanger tube rupture.

Non-return valve failure.

Inadvertent valve opening.

Equipment failure (eg pump/compressor)

Abnormal heating.

Other abnormal conditions.

Compressor / pump blocked outlet.

Utility failures (coolant, instrument air, fuel gas, electricity).

Tank filling and emptying.

Tank thermal breathing.


ABB PSiD team.

Client's engineers.

Standards etc.

BG standard BGA-ENG-PROC-TS-0015 “Process Engineering – Process Safety in Design”.

BG standard BGA-ENG_PROC-TS-0003 “Relief, Blowdown and Flaring”.

API standard 520 Part 1 “Sizing, Selection and Installation of Pressure Relieving Devices in Refineries – Sizing and Selection”. 7th Edition.

API standard 520 Part 2 “Sizing, Selection and Installation of Pressure Relieving Devices in Refineries – Installation”. 5th Edition.

API standard 521 “Pressure Relieving and Depressurising Systems”. 5th Edition.

API standard 2000 “Petroleum, petrochemical and natural gas industries – Venting of atmospheric and low-pressure storage tanks”. 6th Edition.