This is the UT745 Far Swan with a fill deck of cuttings skips - photo taken some years ago.

Now in 2016 it is probable that “skip and ship” has become the normal technique for disposing of drill cuttings, but it was not always so as this article written in 2001 shows. Actually several years before that I was employed by a major operator to investigate a variety of means of cuttings disposal, and in order to produce a realistic report I was given the data from the previous year in terms of the total quantity of cuttings produced, and the locations. I then determined what techniques could be used, which included the use of a Canadian self-unloading system and big open topped containers on the ships. I then had to decide what sort and how many ships would be required and to schedule them and cost the whole exercise.

During 1999, the last year for which the UK Oil Operator's association has published figures, 67,000 tonnes of drill cuttings were produced. This is no more than could be carried as a single cargo in a medium sized bulk carrier, or which might be represented by a single slag heap in the valleys to the west of Merthyr Tydfil. So what's all the fuss about?

The trouble is not so much the drill cuttings but the drilling fluid with which they are coated. Roughly speaking the 67,000 tonnes of cuttings were coated with 6,000 tonnes of drilling fluid of which 90% was synthetic oil based mud and 10% was real oil based mud. 

But enough of these statistics!

During the 1990s increasingly prescriptive legislation has been put in place both in UK and Norway to prevent the disposal of oil contaminated drill cuttings to the sea. In 1997 regulations came into force which limited the amount of oil on drill cuttings discharged to the sea to 10 grammes per kilogramme, so the operators were required to dispose of the cuttings in some other way, to clean them up or to use no oil based drilling fluid.

At that time the way forward was seen as being the use of synthetic or "pseudo" oil based muds which would be as effective as the real stuff, but which would not limit the discharge of the cuttings to the sea, although even then there was an awareness that things would change. Indeed from later this year something else must be done with drill cuttings contaminated with synthetic drilling fluid.

Back in the early 1990s some effort was made to develop plant for removing the oil from the drill cuttings offshore, but the prototype systems often involved extremely high temperatures, which had the effect of destroying the facility rather than cleaning the cuttings. As a result  the operators have all opted for injecting the cuttings back into the substrata or carrying them ashore for cleaning and disposal.

Re-injection would appear to be the most suitable means of disposal. It is after all where the stuff came from in the first place and the bonus for the operators is that each well is capable of receiving much more than the volume extracted.  Some re-injection systems can return the residues from up to twenty wells into a single hole and this capability means that platform based drilling operations can carry on almost unimpeded by the new legislation, although at some cost.

It is almost ironic that exploration and development offshore is moving steadily into deeper water and  as a result more and more exploration and development work is being carried out from semi-submersibles. These are the least suitable drilling units for the provision of the re-injection systems.

For the semi-submersibles everywhere, the operators have opted for cuttings transportation by supply vessel, described most simply as "skip and ship". In Norway where the legislation offers more options, the shipping may be done between one installation and another, suggesting that eventually one or more platforms in the Norwegian sector may be used purely for cuttings disposal.

In the UK sector oil based mud contaminated drill cuttings must either be re-injected at the point of production, or else transported to the shore for disposal and so a number of technologies are emerging to carry out that operation. Naturally they all require the use of supply vessels which might be contributing to the day rates now being achieved by platform ships. The day rates have in turn set people thinking about some way of reducing the total numbers of ships employed, which involves increasing the payload, reducing voyage time or reducing turn round time.

Amusingly for those who have been involved in the business since the nineteen seventies, the concept of the offshore island has re-emerged as a means of reducing voyage time. This object looks something like an aircraft carrier and for this purpose contains all the plant necessary to remove the oil from the cuttings which can then be legally returned to the sea. If such an object was strategically positioned voyage times for the supply vessels could be halved and all the peripheral port costs would be reduced. There are downsides to such a scheme, even without considering the cost. The interface between the ship and the point of loading or discharge would be dependent on good weather at both ends of the voyage, and in order to comply with the legislation, the cuttings from every location must be segregated from all other cuttings so that they can be totalled up and reconciled with the quantity removed from the hole.

The UK operators have therefore resigned themselves to the "Skip and Ship"  technique for the time being which seems to require a dedicated ship for each semi-submersible. Also required are  large numbers of small skips since there must be some on the rig being filled with cuttings, some on the ship either on their way to the shore full or on their way back empty, and some in the cleaning plant. In addition the skips are sized for a rock with a high specific gravity but usually the product is much lighter. This means that the skips could be bigger – some of the time. But of course the can't be bigger, just in case.

This is a logistic nightmare for the oil industry who, when it comes to materials handling, could learn a lot from MFI, but as always when faced with a real problem they are bracing themselves and throwing money at it. This is sometimes known as "research", and has already resulted in an alternative technique for getting the stuff ashore,  which is to pipe it as a slurry into tanks on the deck of the supply vessel and then pump it ashore to a suitable facility. There is apparently at least one PSV which has had its underdeck mud tanks converted to carry slurry. This of course leaves the deck clear for normal cargo, but could probably leave it with permanent ballast if the mixture was a bit too stiff.

The pumping technique has its own drawbacks. A large quantity of drilling fluid must be added to the cuttings to ensure that they remain fluid. This would seem to be almost counter productive since it means adding fluid to the cuttings on the rig, rather than removing it, and so the latest line of research is directed towards the use of compressed air to blow dry cuttings in the appropriate direction.  One can only guess at the amount of energy which may be required for this part of the operation, but when one adds on the energy consumed by the ship and that required to process the cuttings ashore it seems possible that the remedy may be a greater pollutant than the problem.

 
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