This website uses cookies primarily for visitor analytics. Certain pages will ask you to fill in contact details to receive additional information. On these pages you have the option of having the site log your details for future visits. Indicating you want the site to remember your details will place a cookie on your device. To view our full cookie policy, please click here. You can also view it at any time by going to our Contact Us page.

Sour gas hazards in decommissioning projects

12 March 2014

Hydrogen Sulfide (H2S) is unlocked as a result of drilling and well-servicing operations.  Oil and gas fields, tankers and production facilities all have the potential to contain significant amounts of H2S.  It is a hugely challenging gas to deal with.  Allan Cameron, Technical Director with H2S specialists Sabre Safety, takes a look at the safety challenges in dealing with this gas in decom work and irrespirable environments.

H2S is a gas that acts as a broad spectrum poison when inhaled by mammals.  Broad spectrum means that it can affect many different systems in the body – from the respiratory, pulmonary and circulatory to the digestive system.  It has claimed the lives of many workers around the world in incidents that might have been prevented had they known more about the properties of H2S.

The gas occurs naturally in geological formations, with certain geological periods being more likely to contain it than others, with the Triassic, Permian and Carboniferous the highest risk.

H2S is colourless and therefore invisible – there are no visual clues to its presence.  It has a distinct odour of rotten eggs but will quickly impair a victim’s sense of smell, even at low concentrations, so it could be fatal for a person to rely on their nose as a detection device. It is so toxic that it will quickly overwhelm the nervous system.

In terms of toxicity, 5 parts per million (5 PPM) is the long-term exposure limit.  A dose of 500 – 700 PPM will cause loss of consciousness and death within 30 minutes to 1 hour.  A dose of 1,000 PPM (the equivalent of one tenth of 1%) will cause immediate unconsciousness and death within a few minutes – even if the casualty is removed to fresh air. 

All petroleum industry jobsites are potential H2S locations and as the gas is heavier than air, it tends to settle in low-lying areas such as pits, cellars or tanks. 

As well as being a poison, it is also potentially explosive.  Mixed with the right proportion of air or oxygen, H2S can ignite.  The ignition range is 4.3 and 46% of atmosphere, and it can auto-ignite at temperatures around 260°C.  Add to that its highly corrosive qualities, and the fact that it is soluble and can therefore be present in any container or vessel used to carry or hold fluids, and you have a formidable adversary.

H2S does not discriminate and is as prevalent in decommissioning as it is in drilling work.  In the North Sea, pumping seawater into rock formations as part of workovers to maintain pressure often leads to the build-up of H2S as organic matter degrades and produces the gas in the absence of oxygen.

As can be imagined, there is absolutely no margin for error when working with this toxic and highly dangerous gas. It means that everything must be done not only to meet, but also to exceed safety requirements, with multiple backups to ensure that there is no interruption to breathing systems under any circumstances.

The challenges of decommissioning

As the North Sea reaches maturity, so the need for decom work is rising - particularly in the Norwegian and the Dutch sectors of the North Sea.  There are currently around 40 decom programmes under consideration, and the costs and the nature of the work are truly formidable.

The approach for decom work is often more complex than for drilling.  In most of the work carried out for exploration firms, we can predict the equipment we will require for a project.  In decom, however, although we have tried and tested techniques, we usually have to design a system that is highly job-specific and work our way back to ensure we provide maximum protection for the teams involved.

Decommissioning work has meant that many new skills have had to be developed.  So, for example, preparing storage cells to ensure that they are empty before removal from the seabed has presented some major obstacles to access and retrieval.  Getting people into these areas down platform legs at a 45 degree angle poses real challenges for rope access.  In this highly hazardous and difficult environment, planning has not only to be innovative, but also meticulous, to ensure that people can be retrieved quickly from areas of danger. 

There have to be multiple backups for systems so that there is always a contingency in the event of any potential failure. We have to be sure that we have many control measures in place so that even in the unlikely event of something happening we are still able to manage it.  Quality systems have to work together, but the most important  element is mutual trust within the team.

Team building and collaboration

As in other oil and gas industry operations, close collaboration within the team is the key to a safe and successful outcome. In the context of decom operations, and particularly irrespirable environments, this is even more fundamental.  Much of the work involves confined space work, in platform legs or elsewhere,  and personnel must be aware of their own and other team members’ situations at all times.

It is no coincidence that so many people involved in this line of work come from a forces background where people have to count on others at all times.  People know each other not just professionally, but personally. 

Operations have to be structured around people – the right people for the team and the right team for the job.  Often the same personnel will work together on a number of jobs, so for example, the rope access technicians will have built up relationships with the other specialists, an important factor when the team members work together 12 hours a day and live cheek-by-jowl 24 hours a day offshore.

The composition of the teams really matters.  In this environment close working relationships are created and so sending someone new out on the job can have quite a detrimental effect on how a project runs.  Team members might find themselves with someone they are not familiar with who is looking after their air, and that could sow the seeds of doubt. 

The benefits of involving safety specialists at the start of the project

Decom work involves a major environmental challenge too – one that once again affects how people need to work.  Oil companies are obligated by governmental agreements to restore the seabed to its original state in many cases or at least to have a fully worked-out plan in place that will meet the environmental conditions.  This represents a real challenge in terms of safety and cost management. Quite simply, oil companies are going to need really close collaboration with all their major suppliers if decommissioning is going to meet these joint objectives of safety and cost-effectiveness.

All of this underlines the fact that specialist companies such as those supplying safety solutions should no longer be seen merely as suppliers of a service, but rather as advisers on the planning and execution of projects.  Sabre Safety, for example, is currently involved in a project with an oil company to formulate a plan to remove platform legs and deal with a complicated cell structure.  Sabre has been involved at the early stages of the project to discuss different options for dealing with H2S, which is likely to be far less expensive and more effective than calling in gas safety specialists when the project is already underway.


This kind of approach in decom also mirrors what is happening in exploration, where new and much more marginal fields are being developed and where oil companies face rising costs.  Time and cost savings are ever more critical, but they have to delivered without any compromise in terms of safety and quality standards.  In the past, specialist safety providers were brought in at an ‘appropriate’ stage – often when fundamental decisions had already been made.  Now, they are brought in at the planning stage where they can contribute to the methodology and maximise efficiency and safety by working as part of the team.

The future

Looking ahead and as decom work expands, collaboration across the industry looks like it will influence construction work in a fundamental way.  Decom will almost certainly be increasingly considered at the design stage of a platform.  It may well also be the case in future that consortia of suppliers will come together to offer a complete and flexible package to oil companies for decom work – rather than oil companies initiating this in all cases.  In everything though, safety considerations will rightly be the prime driver of innovation.

The last few years have taught the industry much that will be highly useful for future decommissioning projects in the North Sea and beyond.

Print this page | E-mail this page

CSA Sira Test