15. On site cuttings cleaning technology

The ability to clean cuttings at the rig site to a level better than the 1% oil on cuttings (OOC) limit for unrestricted discharge would increase the range of options to allow the continued use of synthetic mud or OBM. Where Cuttings Re-Injection is possible, i.e. for platform operations, on site cleaning is unlikely to be a competitive option but for MDU drilling subsea wells there are a number of advantages over the alternative of ship to shore. The requirement to stock pile large volumes of cuttings on the rig will be avoided as well as vulnerability to weather related down time.

Whilst the technology is available for this type of processing on shore, however, there are a number of problems associated with transferring it to the rig site. The first difficulty is that processing units are typically large and producing a facility that could realistically be installed on a drilling rig is in itself a major challenge. Secondly, the available technologies all utilise either high temperature processing or the use of volatile solvents for cleaning. These operations clearly have safety implications which will need to addressed in detail before any offshore application could be considered. Finally, the plant in use onshore is highly sophisticated and may be hard to operate and maintain reliably offshore. In an application where any downtime is likely to result in an immediate drilling stoppage this is potentially a major concern. Some concepts are described below:

TORBED

The toroidal bed reactor or TORBED is an established technology with a number of applications, particularly in the food processing industry.

The basic principal of the process is the use of high temperature steam directly in contact with the cuttings to produce highly efficient thermal cleaning of the drilling waste. The process is controlled such that hydrocarbons are recovered without cracking for re-use. A prototype unit has been built and used in a proof of concept trial which successfully demonstrated cleaning performance in excess of the 1% oil on cuttings (OOC) target and a second reliability and versatility trial is in its final planning stage.

The TORBED project has attracted firm interest within the service industry and if the current trials are successful it is expected that commercial development of an onshore facility could be quite rapid. The evolution of the process for offshore application is still at a conceptual stage.

The Star Tech process

Another cuttings cleaning treatment application is the "Star Tech" process. This is also a thermal application, designed to allow recovery of hydrocarbons for re-cycling. The unique feature of this process is that the plant operates under partial vacuum of 21 inches of mercury. This has the effect of increasing the efficiency of the hydrocarbon distillation process at lower temperature. The system also incorporates a condensing and secondary filtration stage to recover and clean hydrocarbons in the vapour phase exhaust.

Ultrasonic cleaning

The field of ultrasonics is an area in which there is potential for cuttings cleaning applications.

Solvent extraction techniques

The cuttings are mixed with a solvent in an agitated reaction vessel in which the oil / water emulsion is broken allowing the oil to dissolve into the solvent. The solvent / cuttings mix is then fed to a centrifuge for separation and the cleaned dried cuttings are discharged overboard. As a result of the hydrophilic nature of the cuttings solids, the water content of the solvent phase is very low. The solvent / oil mix is then passed through an evaporator and condensation column to separate the two, with the oil returned to the mud pit and the solvent to a holding tank for re-use.