FPSO Tank Ventilation – Air Change Rate

While building regulations worldwide are quite prescriptive when considering the volume of ventilation required for different spaces, there is a distinct lack of guidance for the ventilation of confined spaces, in terms of air changes per hour or even what constitutes “adequate ventilation.” To a layperson or to a professional in the field this just doesn’t seem correct.  

An underground carpark requires 20 – 30 air changes per hour and it is easy to understand the risk to health if the ventilation is not effective. A restaurant requires 8 – 12 air changes per hour where there is not really a health or safety risk.  

Most FPSO operators’ procedures consider that any level of ventilation is sufficient to make a confined space safe, subject to gas testing before entry. There are no regulations applicable to confined space entry offshore, certainly nothing that will drive any operator to deviate from its goal-setting approach. The only outline regulation is to provide a safe working environment, which the duty-holder is obligated to provide under the Health and Safety at Work Act. 

In practical terms, very often the standard procedure for ventilating a confined space is a venturi air mover or 110V electrical axial fan. It is not uncommon to see a venturi blowing through a deck hatch into a 10,000 m3 tank without any ducting routing to where people will be working. If 30 – 40m of ducting is attached, then the ventilation efficiency drops significantly to the extent that 1 air change per hour may be achieved in a water ballast tank of 2,000 m3, using a single ventilation unit. Doubling up to 2 ventilation units for a cargo tank of 8,000 – 12,000 m3 is only going to provide less than 0.5 air changes per hour. If the gas test is OK, then is this adequate? 

Configuration of space

Perhaps the reason there isn’t stringent regulatory guidance is because the level of ventilation which is adequate depends on the activity being carried out, and the configuration of the space.  

In very complex spaces with limited openings, higher pressure ventilation is required to overcome the frictional losses in the ducting, to ensure air reaches the extremities of the space. If there are only 2 people carrying out visual inspection, then the ventilation requirements will clearly be less than where a team of 6 are gouging and welding steel – producing welding fumes, paint fumes and even ozone. It would be impractical to stipulate where hotwork operations are being carried out in a confined space, the ventilation rate per hour should be 20 times the confined space volume even though this would be the minimum requirement for a fabrication shop. The key reason is that in a cargo tank with 10,000m3 capacity, fume producing fabrication is only occurring at typically 1 location rather than throughout the space, as you may expect in a factory. In all cases, the fumes produced by welding/gouging should be locally extracted or filtered rather than just displaced by general ventilation or relying on Respiratory Protective Equipment (RPE). 

It isn’t just welding which drives the need for effective ventilation. Blasting and spray painting within confined spaces creates a significant problem with the breathable atmosphere, which is usually addressed by using line fed breathing apparatus. Sufficient ventilation is still required so that visibility can be maintained such that personnel within the tank can safely evacuate when required. We have even seen a situation in a cargo tank where the evolution of volatile organic compounds (VOCs) from stripe coating using paint brushes was enough to displace sufficient air to cause low oxygen alarms to sound on personal gas detectors.  

The need for effective ventilation goes further than meeting the minimum requirements of legislation or avoiding improvement notices, it actually increases productivity. Where the air is fresh, visibility is good, the temperature is ambient and there is no foul smell from any processes, remaining liquids or residues in the tank, people will be more efficient. Without effective ventilation more break periods would be required, and it may be necessary to take extended breaks to allow the atmosphere to clear, extending the project duration and cost. 

Significant experience

High volume tank ventilation introduces a new risk which also needs to be considered. For Tankers and FPSOs, the ship’s deck is always considered a hazardous area, and as a result there is always a risk that gas from leaking pipework or smoke from a fire can be blown through the ventilation system to the location in the confined space where people are working. This situation is similar to the risks seen in accommodation ventilation systems and electrical local equipment rooms in hazardous areas. In these spaces smoke and gas detection systems have executive actions to shutdown the ventilation system, alarm and isolate equipment in the event of a hazard being detected. With the current focus on Human Factors throughout offshore and marine activities, it is essential these actions are automatic and reliable rather than reliant on people to react appropriately in an emergency when this can be avoided.  

Marine Technical Limits has been managing FPSO tank repair works for 20 years. We have a range of ventilation equipment specifically to support confined space cleaning, blasting, painting, welding, inspection and maintenance activities in water ballast tanks, cargo tanks, slops tanks, void spaces and cofferdams on producing FPSOs. We are experienced in defining the most effective ventilation plan for the work, delivering it, and reviewing if there were any areas for improvement/optimisation for next time.  

Looking back at our extensive portfolio of work, we typically ventilate tank spaces with 2 – 6 air changes per hour. The required ventilation level varies based on the type of work activities, number of work sites and the physical arrangement of the tank and ventilation openings. Sometimes we need to create additional deck openings to support increased ventilation and material handling for large repair projects while on other occasions we have to limit the number of worksites to stay within the effective ventilation limits achievable. 

Perhaps the lack of clearly defined ventilation specifications in the offshore regulations is intended to make duty holders properly consider what is suitable in each scenario. If that is the case then a specific ventilation plan should be prepared to support a set of confined space activities within a particular space, rather than the generic statement of “ventilate tank.”  

Useful references for further reading: 

IACS Confined Space Safe Practice No. 72, Rev 3 (Dec 2018) 

OCIMF Cargo Guidelines for F(P)SOs, First Edition 2018 

OCIMF International Safety Guide for Oil Tankers and Terminals (ISGOTT), 6th Edition  

HSE Safe Work in Confined Spaces ACOP L101, Third Edition 2014