Davie Garrick, TWMA
The treatment of non-aqueous fluid contaminated cuttings has gone through many phases and generated a number of “solutions” over the years. The driver for innovation and development in the field has been, and to a large extent remains, legislation. Operators are becoming increasingly pro-active in looking for solutions to the issues relating to drilled cuttings disposal and are now often leading the way by putting in place their own policies and controls.
Legislative framework
The substantial differences in cuttings disposal legislation and practice from one country to another creates difficulties. In the UK, for instance, the OSPAR decision 2000/3 established a 1% by weight limit on the quantity of non-aqueous based fluids (NABF), including synthetic fluids, that could be retained on the cuttings discharged to the marine environment. Elsewhere regulation varies from zero discharge to a 10% maximum of oil on cuttings. To a certain extent, legislators can only follow best available technology while at the same time taking note of developments in drilling fluid technology in general and the base oils used in particular. The industry will continue to see changes and developments in both the technology used and the legislative framework in which the industry operates.
Options
The industry has broadly had to rely on a number of onshore disposal options to meet disposal requirements. These have included thermal desorption, fixation, and bio-remediation. Offshore the re-injection of cuttings has been one option. Where legislation is less restrictive, various methods of “mechanical” separation are being applied to meet discharge limits. The driers widely used in the Gulf of Mexico are one example. The onshore options involve transportation (“skip and ship”) of large tonnages of invert emulsion contaminated cuttings from offshore installations to shore-based processing facilities. This practice, widely used in the North Sea and increasingly elsewhere, carries with it considerable cost as well as logistics and HSE implications. Typically, there are at least 11 crane lifts for each cuttings bin shipped to shore. The potential impact on cost and health, safety, and environment (HSE) is significant, and that is before liability issues relating to processed powder disposal are taken into account.
Logistics, HSE
Moving cuttings from offshore installations to shore-based processing facilities is not the perfect solution. An ideal solution would the remove “skip and ship” from the equation, but to do that, a technology for processing cuttings offshore would need to be found.
Since early trials, eight operators working in the UK sector of the North Sea have integrated the TCC-RotoMill and CCDS system to treat and handle cuttings offshore.
Various types of thermal desorption have been used for years to process cuttings onshore. The process uses thermal energy to evaporate the fluid phases of the cuttings before recovering those phases separately as oil and water. In theory, this technology could be modified to meet the more stringent demands of an offshore environment, but certain objectives would have to be met first. Safety issues are clearly paramount. The means of generating the thermal energy is a vital component. Because cuttings treatment has to take place “online” with drilling operations, equipment has to be very reliable and must provide sufficient processing capacity to keep pace with the rate of cuttings generation. Other considerations include footprint, weight, mobility, and cost. Meeting these and other criteria would be a challenge.
Technological development
In the mid 1990s, TWMA partnered with a major oil company to look into the feasibility of applying a specific thermal technology in offshore applications. Key elements addressed during this development program included modularizing of the plant, reducing weight and footprint, meeting zoning specifications, and getting processing throughput high enough to cope with cuttings generation rates.
Individual module weights had to be kept to a minimum to meet crane lift restrictions and footprints reduced to fit within the limited space available on offshore installations. Zoning specifications were designed to meet Zone 2 requirements. Some of the safety features included using enclosed containers with a CO2 purge system, tying the equipment into the rig’s emergency shutdown, and using computers to control operations. Processing rates were increased to keep pace with cuttings generation at typical rates of penetration in 8 1/2-in. (215 mm), 12 1/4-in. (311 mm), and 17 1/2-in. (445 mm) hole sections.
The prototype TCC-RotoMill cuttings treatment plant operated on a continuous basis governed by a computerized control system. A temperature of 260° C (500° F) was generated to flash evaporate the fluid phase (water and oil). Evaporated fluids were condensed and recovered separately. The process released no emissions to the atmosphere and the condensed fluids were suitable for recycling into the drilling fluid. Analysis of recovered base oil during initial onshore trials showed that the process had little or no effect on the oil or its performance in the drilling fluid. The level of retained hydrocarbons in the recovered water was typically <20 parts per million (ppm) and the suspended solids between 5 and 15 mg/liter. The inert processed powder had retained a hydrocarbon content of less than 0.1%.
Field trials
With the initial modifications complete, field trials had to be carried out to prove the concept and the process. The operator made an offshore installation available throughout the development stages for these trials.
- The first system, capable of operating in a Zone 2 area on an offshore installation was commissioned in April 2001
- The first offshore proving trial was initiated in June 2001 on board the rigGlomar Arctic III (now GSF Arctic III)
- Two additional trials were undertaken to allow regulatory bodies such as the Department of Trade and Industry (DTI) and various environmental organizations the opportunity to audit the efficiencies of the TCC-RotoMill technology on the Skene field
- Upon completion of the third trial period the unit was accepted by the DTI as an alternative method for the processing and disposing of drill cuttings from an offshore installation
- The first 950-kW TCC-RotoMill was built and mobilized to theOcean Guardian in early August 2003.
Operations
Eight operators working in the UK sector of the North Sea have used the TCC-RotoMill and complete cuttings collection and storage (CCDS) system to process cuttings.
The complete system is designed to transfer cuttings from the shakers to the point of disposal following treatment. The CCDS system consists of a cuttings transfer pump (CT unit), a duplex pump capable of moving cuttings from the shakers to temporary storage or directly to the TCC-RotoMill, and a cuttings storage tank (CST) unit for buffer storage to take the thermal processing offline from drilling operations. Each unit has a pumping capability in excess of 100 bbl/hr that can keep pace with cuttings generated even in larger hole sections. TCC-RotoMill cuttings processing can operate 24 hrs/day offline from drilling operations.
The system has been used on more than 15 wells, including high-pressure/high-temperature wells, 17 1/2-in. (445-mm) hole sections, and combinations of the two. The processing of 15,000 metric tons (16,535 tons) of cuttings offshore reduced the number of bin mobilizations by 3,000, reduced the number of crane lifts by 33,000, and reduced the tonnage of powder sent to landfill by about 10,000 metric tons (11,023 tons), contributing to improved safety, simplified logistics, and minimized environmental impact.
Case history
In July 2006, the processing and handling system was mobilized to theTransocean 712 semisubmersible working for Oilexco and then installed in readiness for NABF used to drill multiple-leg 8 1/2-in. (215-mm) hole sections as part of the Sheryl appraisal.
In July 2006, the handling system was mobilized to theTransocean 712 semisubmersible.
The system transferred cuttings from the shakers to the TCC-RotoMill. A series of screw conveyors fed cuttings from the two shale shakers on the pipe-deck and from the VSM 100 shakers on a lower deck. The screw conveyors taking cuttings from the scalpers fed into either one of two CST units. The conveyors taking cuttings from the VSMs fed into a CT unit. The CST unit and the CT unit were able to feed directly to the TCC-RotoMill if necessary.
The CST units, which each have a storage capacity of 70 metric tons (77 tons), were used for buffer storage to take cuttings processing offline from drilling operations. This ensured that interruption to processing operations or a cuttings generation rate exceeding processing capacity would not disrupt drilling operations.
A 950 kW TCC-RotoMill was installed on the starboard aft deck adjacent to the CST units. The unit, in four-module format, has a footprint of 64 sq m (689 sq ft). The installation allowed recovered oil to be returned to the mud system. Water and solids could be discharged overboard down the existing cuttings discharge line.
Over the course of drilling multiple legs totaling 10,211 m (33,500 ft), all of the cuttings generated were processed offshore. The oil recovered by the process amounted to an estimated 800 bbl, all of which was suitable for recycling back into the mud system. Processed powder was discharged overboard with an average oil content of no more than 0.1% by weight.
The benefits of offshore processing were considerable. There was no need to mobilize cuttings to shore with all the associated safety and logistical implications. There was no need to use the rig crane for long periods moving cuttings on and off supply vessels and around the rig. And there was no need to dispose of recovered powder.
Currently, 3,240 metric tons (3,571 tons) of cuttings have been processed on theTransocean 712 without incident. A second TCC-RotoMill has been mobilized to the Ocean Guardian, for Oilexco.
