PREVENTING FIRES AND EXPLOSIONS ON OFFSHORE INSTALLATIONS

The explosion and fire on the semi-submersible drilling rig “Deepwater Horizon” on Tuesday 20th April 2010, with the loss of 11 lives, and its subsequent capsize and sinking, has prompted me to write something about the manner in which jobs of this sort are usually carried out, and how the related risks are minimised. Of course we have no idea what went wrong on the rig, but almost certainly there was a leak of hydrocarbons from the well which ignited at deck level What follows is not intended to suggest what might have gone wrong on the rig. It just provides some information for those who have an interest, but who do not have detailed knowledge of the work.

Those who have followed the content of my website, www.shipsandoil.com over the years will be aware that in a previous life I was a safety consultant and technical author with a marine background, and part of what I used to do was to assist my clients to develop safety cases for mobile drilling units. I started doing this work in 1993, five years after Piper Alpha, and have only recently given it up, but of course I have retained much of what I have learnt.

Maybe the first thing is to identify different types of offshore installation. There are platforms are nailed to the seabed and which stand up on their legs above the waves, there are jack-ups which are used for drilling the holes and which stand above the waves on legs, but which can be lowered into the sea until they float and then moved from one place to another. And then there are semi-submersibles which float on pontoons. The pontoons can be submerged when they are drilling, to provide a stable platform, so that the vertical columns and the decks on top of them are visible, which prompts many journalists to describe them as if they are some-how standing on the seabed. Semi-submersibles can be anchored or dynamically positioned, using thrusters. There are also drill ships which look like large tankers which are also usually dynamically positioned. It is conventional for the dynamically positioned vessels to drill in deep water, although rigs have been anchored in water depths of 3000 metres (Nearly 10,000 ft).

 Strangely these exploration rigs have the task of probing the earth’s crust in the hope of locating a sufficiently large deposit of hydrocarbons, oil, gas or a mixture of the two, to make it worthwhile  bringing it to the surface. But oil and gas under pressure is dangerous stuff, so they have to be able to drill into a reservoir while at the same time keeping it under control. I’m sure we have all seen film, either real or fictitious, of a wooden drilling rig being engulfed in a fountain of oil and people rushing about throwing their hats in to the air. They had found oil, but had as yet not found a way of containing it. There are also pockets of gas near the surface which should be avoided as far as possible, and ideally this is done by carrying out surveys to locate them, and then putting the rig at a distance from them.

 It is also necessary to emphasise the difference between drilling rigs and production platforms. Production platforms are usually collecting points for the hydrocarbons from a number of wells. The oil is processed and then pumped on, either to the shore or to some other storage facility. Of course on a production platform there is always a quantity of hydrocarbons, the inventory, being processed and there is therefore the possibility of a leak in the pipework or in some form of containment. This was the case in Piper Alpha. On a drilling rig, in ideal circumstances, the only time there are hydrocarbons (well fluids) at deck level is during well testing.

 Without going into too much detail, the rig is connected to the seabed by a large diameter pipe, known as the riser, and at the bottom of this is the blowout preventer. The well is drilled through this, and the resulting hole, and the riser is filled with  drilling fluid which is known in the business as “mud”. The mud is the first line in defence against the pressure in the reservoir. The fluid is a mixture of chemicals, based on water or a form of oil and is weighted with the mineral baryte. The whole business of dealing with mud is a science in itself and the weight must be just right, heavy enough to keep the oil down there, but not so heavy that it fractures the formation and disappears into the ground.

Of course it is possible that the pressure in the reservoir will exceed the hydrostatic head provided by the mud and so the drillers may find the mud coming back up the hole. That or some other sign, often known as a “kick” will result in the activation of the blowout preventer (BOP). You will remember that the BOP is installed at the seabed on the top of the well, and at the bottom of the riser. The BOP is an enormous lump of steel about fifty feet high and weighing up to 150 tonnes. It contains a number of rams of different sorts which can either encircle the pipe passing through, or else in extremis cut the pipe.  

Picture taken from the first edition of "Supply Ship Operations" showing the relationship between the rig, the riser and the blowout preventer.

In the first case the pipe can be encircled and the well isolated, but work can still be done on the well. Heavier mud can be pumped down the drill string and control be regained. However if it seems that control will be lost the pipe can be cut and the well completely isolated. Of course once this is done, getting things sorted out is a complex operation, but everyone remains safe.

This is a description in the simplest possible terms, and there are other circumstances where gas or oil can get to the surface during the drilling operation, and in order to minimise the possibility of this being ignited much of the equipment on the deck of the rig is constructed in a way which will minimise the possibility of ignition. There are gas detectors positioned in all the most important areas and these are usually monitored from the control room. If gas is detected then things can be done, and finally if it seems that all control will be lost it is possible to get everyone into the lifeboats and to evacuate the rig.

There have been many blowouts over the years all over the world, but only a few of them have been investigated in a way which has resulted in the findings being available in the public domain. Hence those helping with the development of emergency procedures, and providing guidance for the guys out there, have only a limited amount of information available, and it may be that there are still new situations which are unaddressed. It is also true that different legislations have different approaches to the whole business.

After Piper Alpha and the resulting public enquiry, the whole process of offshore safety in the UK was put in the hands of the Health and Safety Executive, with the requirement that all offshore installations including drilling rigs be provided with a safety case. There is no doubt that Lord Cullen’s intent in recommending the safety case regime was well intentioned, with the idea of making complex structures and activities more transparent, but even after 15 years it is still a work in progress.

So, keeping people safe offshore is still a difficult business. The guys out there are at sea, and have lots of other stuff to contend with. For me just being at sea was difficult enough.

Vic Gibson April 2010

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