Cuttings dryers are sometimes referred to as secondary drying of cuttings.Cuttings dryer is the most widely used technology for safely handling drilling waste. Drilled cuttings with associated fluid from the rig solids-control equipment have been passed over a second drying shaker for a number of years. The recovery of oil-based drilling fluid coupled with a 10 – 25% reduction in disposal volume is usually easily justified and has become standard procedure in areas where so-called pitless drilling or closed loop systems are the norm. Generally, the secondary drying shaker is a four-panel screening device running at 7.0 – 7.3 G’s at the screen surface. Drilled cuttings from a drying shaker typically test between 8 and 12% base oil (NAF) by wet weight. This retention-oncuttings (ROC) figure is significantly higher than the current minimum allowed for offshore discharge in the United States and, increasingly, elsewhere around the world. Accordingly, there has been increased interest in lowering the ROC figure by the use of different types of drying equipment.
Figure 1. shows an example schematic for one typical horizontal dryer. Cuttings removed in the primary solids control section enter in the feed inlet. Processed cuttings are removed after the dryer removes oil (or NAF). The oil or NAF is recovered and returned to the drilling fluid system.
The quest for improved technology to reduce the ROC figures is ongoing with a number of competing machines and technologies. At this time the market is composed of the following commercial products:
- Horizontal and vertical basket centrifuges.
- Basket centrifuges that move cuttings by vibration。
- Basket centrifuges that move cuttings with a scroll。
- Basket centrifuges that move cuttings with a so-called pusher rod。
- Basket centrifuges that move cuttings with vibration and a pusher rod.
- Perforated rotating vacuum cylinders.
Of these competing technologies, basket centrifuges (vertical and horizontal) using centrifugal force and a flighted scroll to effect separation make up approximately 80% of the units in use today. Typical rotation speeds from 300 to 870 rpm provide a g force between 48 and 375 g’s. Retention time of the drilled solids within the screen basket, screen geometry, and basket rpm vary widely among units. Generally speaking, more rpm means more g force, drier drilled cuttings, and more capacity.
Legislation has been the driving force behind the use of cuttings dryers, coupled with the use of synthetic 16-18 IObase fluids. The basket centrifuge is thus far the only piece of equipment on the market that can reliably reduce ROC figures below 4% while handling the volume and variety of solids presented to it during the drilling process. Fast drilling generally produces large cuttings with small surface area and thus a lower ROC value. Slow drilling with very fine drilled cuttings will have a large surface area that is proportionately more difficult to bring to the desired level of dryness.
Figure 2. shows cuttings prior to being processed by a cuttings dryer. Figure 3. shows cuttings after processing. Visually, the cuttings appear drier and test results confirm a significant reduction in retention on cuttings.
By bringing the average ROC to below 4%, a number of issues are much more easily dealt with. For example, the pneumatic transfer/ conveyance of cuttings is much more reliable. As a prelude to thermal desorption, the process rate through a thermal plant can be increased. As a prelude to land farming or composting, the amount of amendments and the time required to reduce total petroleum hydrocarbons below 1% is significantly reduced. On land, the volume of liquids attached to drilled cuttings to be disposed of offsite can be reduced by between 25 and 45%.
Figure 4. shows the reduction in retention on cuttings when centrifuge cuttings are processed by a cuttings dryer. The volume reduction indicated can be as high as almost 50%.
The vast majority of offshore rigs today were designed before secondary drying was mandated. Frequently, waste management, including solids control, is treated as an afterthought. Accordingly, few offshore rigs are amenable to the installation of equipment required for secondary drying.
One of the issues related to secondary drying is the amount of fine solids that accompany the recovered drilling fluid. The effluent from dryer operations is always laden with low-gravity solids that are passed either over a fine screen or through one or more centrifuges. Passing the recovered drilling fluid through one or more high-speed centrifuges should allow the cleaned drilling fluid being returned to the active system to have a content of low-gravity solids below the target in the operator’s drilling program.
Figure 5. shows a schematic of fluid removed by a cuttings dryer being further processed by a centrifuge. The intent is to remove fines from the fluid prior to returning the fluid to the drilling system. By removing LGS cleaner fluid is returned, but some fluid is lost again.
Figure 6. shows a picture of an installation of a cuttings dryer with associated processing equipment and tanks. An idea of the size of the processing area can be envisioned in this picture. Another benefit of any secondary drying operation is the ability to catch all drilling fluid that may flow off the end of the shaker due to screen blinding, surges in circulation, etc. The potential for excess fluid, particularly on ultra-deepwater operations, should be evaluated and planned for. For instance, ‘‘boosting the riser’’ frequently will overwhelm the screening capacity of a deepwater drilling rig. The single most difficult aspect of operating a reliable cuttings dryer operation is getting the volume and variety of drilled cuttings with associated drilling fluid to the dryer in a continuous stream. The second most problematic aspect of dryer operations is dealing
with the discharge ‘‘on stream,’’ that is, balancing all equipment, fluid levels, and feed streams. Dryers and their associated centrifuges or fines units are fed with a variety of equipment types. Without delving into the various feed methodologies, suffice it to say that simplicity reigns.
The simpler systems are generally more reliable and more easily brought back online when something untoward occurs.
Any secondary drying installation for an offshore rig must provide the capability to easily switch between any of the following modes:
- Bypass all drilled solids, cement, and displacement fluids overboard.
- Zero discharge—everything from the well bore is diverted to either cuttings boxes, cuttings tanks, or cuttings barges for further processing. In some instances, cuttings are ground and injected down the annulus on-stream, or as created.
- Normal dryer operations in which all well-bore discharges are passed through a cuttings dryer prior to final disposal.