5. Available techniques

There are a number of possible options for reducing the effect of waste drill cuttings on the environment, falling in three main categories.

1. The ideal solution would be to use a drilling fluid which is genuinely non - harmful to the environment such that cuttings might be discharged with impunity. The synthetic muds were produced with this aim but are now seen to have fallen short of their goal. Irrespective of environmental performance, the application of ester muds is restricted by their temperature limit of 320 ºF. Mud companies are continuing to work on new high performance drilling fluids and as long as their environmental acceptability can be positively demonstrated and is consistent with new legislation this work should be encouraged. Currently WBM is the only environmentally acceptable option allowing the discharge of untreated cuttings. The disadvantage of using WBM is that it has inferior drilling performance to OBM in a number of respects, principally maintaining formation stability and providing good lubrication for high deviation or extended reach wells.

2. The second option is to use OBM and to implement a total containment system allowing disposal of drill cuttings in an environmentally acceptable manner. Currently available methods are either transportation to shore for treatment and disposal or the grinding of cuttings to a slurry which is then pumped back into the ground either in a dedicated disposal well or into the annulus of a standard well. The main advantage of this approach is that it allows continued use of the highest performance and relatively low cost OBMs. The disadvantage is that the costs associated with containment and disposal are substantial.

3. The last option is to continue to use OBM but rather than contain the waste, to clean drill cuttings prior to discharge overboard. A number of approaches to cleaning drill cuttings have been pursued and several have demonstrated technology capable of the required quality of cleaning. None of the existing systems have yet been developed to a commercial stage for offshore applications, however, although the availability of shore based facilities is important in option two above. There is a considerable ongoing effort to develop such technology for offshore use. The advantage of on site cleaning is in eliminating the expensive process of transporting waste to shore and the associated vulnerability to weather related downtime.

The approach of incinerating rather than cleaning the waste offshore would be unlikely to receive government approval and would itself be a source of polluting emissions inconsistent with improved environmental performance. On a practical level, proper incineration of oily cuttings requires a high temperature furnace and heavy fuel consumption and such plant would not be desirable for a rig site location.

Although the 1% target is currently unattainable at the rig site, where limited discharge of contaminated cuttings does continue there will be an increased emphasis on using the best solids control equipment and effective operational controls to minimise the quantity of mud discharged with cuttings.

The final option for reducing the impact of any discharges is to reduce cuttings to a fine particle slurry prior to discharge. This has the benefit that the suspension time of solid matter before reaching the sea bed is very much greater resulting in a much wider dispersion area. It is also likely that the increased surface area of the solids, combined with the greater suspension time, will lead to a much more rapid breakdown of any associated drilling fluids. Therefore, although the area over which the cuttings spread is much greater, the thinner layer will have a much reduced impact on the sea bed.

The application of the above techniques involves consideration of a number of factors which will vary on a case by case basis as explained below. It is therefore impossible to attribute simple figures for the cost of widespread application of discharge reduction projects.

Table 1. Available techniques