Emergency generator – Exhaust gas dispersion

Client:
Operator
Year:
Business Unit:
Consultancy

Exhaust gases from the platforms emergency generators were suspected to be entering the main heating, ventilation and air conditioning (HVAC) air intakes through air handling units (AHUs), leading to reports of a diesel exhaust smell inside the modules and uncomfortable living conditions. Our client obviously wanted to identify and fix this issue as soon as possible so a quick turnaround was required.

Through modelling, it was found that for certain combinations of exhausts, wind speeds and wind directions, the exhaust fumes were blown into the HVAC inlets. Potential solutions were then discussed with the client and ranked, based on their cost and feasibility. The top solutions were then simulated to determine their effectiveness. Following this, a firm recommendation was made to eliminate the problem, where a novel dual-routing exhaust was proposed. This solution was subsequently designed by Apollo and successfully installed by our brownfield modifications sister company, Global E&C.

The Problem

  • When the platform emergency generators were running, there were reports of a diesel exhaust smell coming from the module vents. Although toxic levels of exhaust had not been detected, this exhaust smell was linked to headaches and uncomfortable living conditions in the platform accommodation.

  • Our client approached Apollo due to our extensive consequence modelling experience to assess and confirm this possibility and consequently recommend the solution and the best way forward.

Our Solution

  • The first step was to recreate the conditions where the air supply was contaminated. This was to confirm the source of the contamination and verify that problem could be accurately modelled. A computational model of the platform was created, and the release and atmospheric dispersion of the exhaust gases were simulated. A computational fluid dynamic (CFD) modelling approach was the most suitable for this problem, as CFD allows the important gas behaviour and platform geometry to be modelled.
  • A range of wind conditions were modelled, and it was confirmed that the exhaust gases could enter the HVAC inlet for certain combinations of windspeed and wind direction. In certain wind conditions, the exhaust plume was blown into the platform modules and engulfed the HVAC inlets. These results tied up well with the observations from the client.

  • The next step was to assess a range of credible solutions. These were discussed extensively with the client, and a shortlist was decided on, which were each evaluated using the CFD model.
  • The final solution consisted of a novel dual-routed exhaust, where the exhaust gases could be directed below the platform in addition to the original exhaust location. This allowed the client to select the exhaust routing depending on the weather conditions, and completely avoid the contamination of the HVAC.
  • Throughout the project, the client was kept up to date and informed about the progress of the project. This was through formal meetings and progress reports, but also by routinely speaking directly with the Apollo engineers working on the project. We pride ourselves on the close relationships our engineers form with our clients, keeping them up to date on the scopes at all times. This allowed the work to be performed efficiently.

And for all you engineers who want more details:

The methodology

  • Our client suspected that the air supply to the HVAC plantroom of the platform was being contaminated by exhaust gases from the emergency generators. To confirm this, Apollo has reviewed the results of a natural ventilation study and performed an extensive computational fluid dynamics analysis of the exhaust gas dispersion.

The solution

  • All modelling was performed using the latest version of Ansys Fluent, where the species transport model was used to capture the atmospheric dispersion of the exhaust gases. Mesh adaption was used where the spatial discretisation of the gas plume was dynamically improved as the simulations progressed. This allowed the gas plume to be efficiently resolved for a range of weather conditions while keeping the simulations at a manageable size.
  • A range of steady-state conditions were considered, where changes in the wind speed and wind direction were assessed. An atmospheric velocity profile suitable for the platform’s location was imposed at the model boundary. The modelling results confirmed the client’s reports of the HVAC contamination and clearly showed that for certain wind conditions exhaust gases were entering the HVAC inlets. This confirmed the source of the contamination and allowed potential solutions to be tested.

What did the client think?

  • A total of 9 solutions were prepared by Apollo and presented to the client, and the best way forward was agreed based on their pros and cons. A dual routing of the exhaust piping was decided on, allowing the client to choose the exhaust gas route depending on the weather condition. This dual routing solution was designed by Apollo and installed offshore by Global E&C. As part of the design process, a laser scan of the area was performed and the proposed exhaust piping marked up on the scan results, shown below. This enabled any clashes with existing structures and equipment to be negated and allowed the client to readily see the proposed piping.

  • The final solution was successfully installed, and the contamination of the HVAC supply was eliminated.
  • The client, whom we have worked with closely over the past 5 years was very pleased with the execution of the project and the end result combining our analytical skills with offshore execution.

If you have a pressing issue and need engineering support, let us know on consultancy@apollo-oe.com

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