A Drill Floor after a shallow gas blowout. The shear rams were remotely triggered without knowledge of the position of the tool joints.

I am a visitor to the gCaptain forum where mariners of all sorts discuss things that are of interest to them, and have witnessed the recent intelligent discussion about the Deepwater Horizon. However, it is obvious that many people lack the detailed knowledge of the subject - specifically that the BOP is not the first point of defence against blowout, it is the last. So here is the introduction to the blowout section of my book "A Catalogue of Disasters" in which a number of blowouts are described as well as many other major accidents to offshore units. 

Blowouts may not actually be the most serious events to affect oil rigs. Indeed, back in the days of oil exploration in the southern states of the USA the sight of a plume of black stuff rising into the air was a sign of success, rather than a sign of danger. And in the early 1900s when wells were being drilling from platforms in Lake Catto and elsewhere over water, sometimes the wells caught fire, and were just left to burn.

But when the drilling rig became a floating object, blowouts became more of a problem. To start with the drillers had to cope with shallow gas which initially could not be detected, and hence they would start drilling the hole and unexpectedly be engulfed in gas which would enter the apertures on the rig, and be ignited by the first hot surface it came across. Over time it became possible to detect possible hot spots using shallow seismic and so they could be avoided, or precautions taken in case there was a release. When drilling open hole, the term used for the process before  the Blowout Preventer (BOP) is put in place, they used to set crew members to look over the side, or into the moonpool so that if there was a release of gas the bubbles would be detected. Now it has become common to set an ROV to observe the hole so that as soon as any gas is released the people on the rig will receive fairly early warning.

The action taken on the detection of shallow gas differs, depending on the type of unit doing the drilling. Jack-ups, whose legs rest on the seabed, just have to keep their fingers crossed that the gas does not create a crater, which could result in the rig falling into it. Therefore, as soon as they can, they will put the first pipe from the rig to the seabed in place. This is known as the conductor, probably because it would under the best circumstances allow them to conduct the released gas over the side using the diverter.

Semi-submersibles affected by shallow gas have the option of moving the unit out of the way. But both types of vessel have to consider the possibility of ignition. And if they are keeping equipment operating there is always the possibility of ingestion of the gas into the engines, or ignition due to sparking. Even dowsing the deck in water might not be the best thing because of the possibility of sparks due to static electricity.

But assuming we get past the shallow gas problem and the well is being drilled into greater depths, with casing in place and so on, blowouts are prevented by use of drilling fluid. “mud” as it is known. This is a combination of the mineral baryte and water, or oil of some sort. Today oil based mud (OBM) is generally used since it has advantages by lubricating the drill bit and coating the hole. The weight of the mud is extremely important, and is determined during the well plan.

So during the drilling operation the mud is pumped down the drill string and comes back up outside the drill string but inside the riser. If all goes well, the same amount of OBM comes back, as has been pumped down. The simplest change which might be observed is that more is returning than has been pumped down. This will mean that in all probability the well is flowing and that therefore the BOP should be closed in and action taken to prevent the flow, probably by increasing the weight of the mud.

But if the mud is too heavy it may start to leak away into the substrata, known in the business as being “lost to the formation”. On detection of the loss more mud has to be made up and injected into the well. Hence the column of mud is the first line of defence against blowout. The last line of defence is the BOP, which if closed up will prevent the release of oil or gas at the deck of the rig.

The BOP itself is a fairly complex object containing as it does a number of different components all of which are supposed to be able to close around the drill string, or other tubulars which may be passing through it. There will be one or two preventers which are compressed about the tubular, and at least three sets of rams, some of which will close around the tubular as long as the hole in the middle is the same size as the tubular. Or in some cases the BOP may be fitted with variable pipe rams which can adjust their size to fit any pipe passing through, and in extremis there will be one set of shear rams which under some circumstances will be able to cut the pipe.

Once a well has been completed, or activity within it terminated, there is a possibility that it might be plugged and abandoned, or at least plugged for later re-entry, and two of the events described in "A Catalogue of Disasters" are the result of failed cement jobs.

The difficulty which those on the Drill Floor, and those on the rig managing the emergency, have to face, is determining when control of the well has been lost, and as a result oil and gas is being released either under, or on board the rig. Once this has happened there is a serious possibility of an explosion and fire. The enquiry into the Piper Alpha disaster determined that in an emergency there should be somewhere safe for the crews of offshore installations to muster, known initially as a Temporary Safe Refuge (TSR) although realising that once there has been an explosion or fire on an oil rig nowhere is actually safe, the name was changed to Temporary Refuge (TR). And as a result of this in the UK sector of the North Sea all installations (which include mobile units) must be provided with a TR and emergency procedures which include its use, particularly in blowout or other hydrocarbon release situations. Elsewhere in the world it is likely that once a week people will muster for ten minutes at the lifeboats and the person in charge will record the fact in some sort of formal document.

It could be that the TR is ony required for worst case events where the ignited hydrocarbons create such high levels of radiation that people at the boats would be shrivelled up in the heat without protection, so things won’t be this bad in all cases. But why take a chance we would say.

The accidents described in the book have been chosen from the many available for analysis because they illustrate the different forms of hydrocarbon release. As well as the reports in the public domain, there are also many which remain cloaked in mystery. If not much pollution has resulted and no westerners have died in the accident, the operators and the rig owners may just put it down to experience and move on, although it may not always be quite as bad as this. The author has had access to a number of blowout investigation reports which have not made it into the public domain and therefore cannot be used in the book. But on the plus side the rig owners chose to carry out their own investigation and therefore, hopefully, were able to learn from the experience.

Also there are known blowouts about which limited information is available. Maybe the most famous of these is the IXTOC accident in Mexican waters of the Gulf of Mexico where the Sedco 135F was drilling in 1979. 

From the available information with the well at 3600 feet (1097 metres) the casing was set, it seemed that the drilling fluid was being lost to the formation, possibly in large quantities and so it was decided to plug and abandon the well. Probably during the recovery of the drill string the well started to come in and so it was decided to operate the BOP. But unfortunately the drillers closed the shear rams on the drill collars in the drill string and so the well was not closed in. The release ignited and destroyed the rig, and the resulting oil spill was the largest in the Gulf of Mexico up to the time of the Macondo disaster. Yes, difficult as it is to believe, the ultimate limitation for the BOP is the fact that the shear rams cannot shear all possible types of pipe passing through it. Drill collars, which are thick lengths of drill pipe used to provide weight to the string where considered necessary, are too thick for the shear rams, as are “tool joints” the connections between the thirty foot sections of pipe. The Driller keeps a record of where the pipe is and therefore whether he has to move it up or down to shear the thinner sections of it. But this does not prevent someone in panic pressing the button to shear the pipe from somewhere else, and once fired you cannot reload the shear rams.

But if the rams in the BOP are successfully closed around the drill string or other tubulars passing through the BOP the choke and kill lines can be used. The choke and kill lines are connected to the BOP and are piped in between or beneath the rams, so as to give the rig the maximum flexibility in their operations. These lines run up the sides of the riser and under the Drill Floor of the rig, they are connected by flexibles to something called the standpipe manifold, which itself is connected to the mud pumps and often the cement unit pump as well, because of the higher pressures which this pump can usually achieve. Using these lines the drillers can control a kick, an excess of pressure in the well. If everything works out, the pressure is detected and some of the rams in the BOP will be closed so that the overpressure is contained. From this point the choke lines and valves can be used to bleed off the pressure and the kill lines can be used to inject heavier mud into the well to control the problem. Once equilibrium is achieved the job can continue. In the reports in the book the choke and kill system features in more than one, and from the initial report of the congressional investigation it seems that the drill crew on the Deepwater Horizon may have done exactly this more than once  during the hours before the disaster.

So with all these means of detection and prevention, and an ongoing and increasing level of experience at the disposal of the crew of the rig how could things go wrong? Buy a copy of "A Catalogue of Disasters" to find out.


Copyright © 2018 Ships and Oil. All Right Reserved.