Drilling fluid (mud) is an essential component of modern drilling processes: it lubricates and cools the drill bit and conveys drilled cuttings away from the borehole. This fluid is a mixture of expensive and environmentally sensitive
chemicals in a water- or oil-based solution. To reduce drilling operational costs and existing environmental concerns, shale shakers are used to mechanically filter cuttings and solids, enabling the drilling fluid to be recycled (Figure 1).
The three main shale shaker components are the hopper, the screen basket and the vibrator (Figure 2). The hopper, also known as the shaker base, serves as a collection pan for screened fluid, also known as underflow. The screen basket holds the fluid-sifting screens securely in place. The vibrator applies the vibratory force profile to the screen basket. The vibrator is generally a specialized set of electric motors connected to eccentric weights whose centrifugal forces are coupled to generate vibration profiles.
Historically, the progression of shale shaker design has been the introduction of finer mesh screens and more sophisticated screen vibration profiles. The design evolution comprises four distinct vibration profiles, as illustrated in Figure 3:
The unbalanced elliptical motion machines have a single rotating vibrator located above the screen basket’s centre of gravity. The resulting motion is elliptical at the ends of the deck and circular below the vibrator, as shown in Figure 3(a). These types of shakers usually have a downward slope that allows transportation of cuttings across the screen and off the discharge end. However, the downward slope reduces fluid retention time and limits machine capacity. The next generation of shale shakers, introduced in the late 1960s and early 1970s, produces a balanced circular motion, as illustrated in Figure 3(b). This type of motion can be achieved by placing a single rotating vibrator at the screen basket’s centre of gravity. The consistent, circular vibration allows adequate transport of solids with the screen basket in a horizontal orientation. Figure 3(c) illustrates a relatively new design that uses a pair of eccentric shafts rotating in opposite directions to produce linear screen basket motion. When placed at an angle to the screen basket, as shown in Figure 3(d), the eccentric shafts produce balanced elliptical motion. Linear and balanced elliptical motions provide superior separation and conveyance of cuttings, enabling inclined screens to provide improved liquid retention.
Shale shakers are traditionally designed and built for specific anticipated operating conditions. Factors that influence the associated vibration profile include the expected cutting types and mud flow rates. Because no one profile works efficiently across all drilling conditions, the shale shaker’s operating performance is fully specified at the design stage. Once the shale shaker has been built, its vibration profile is neither tunable nor adaptable; any field operation that deviates from anticipated conditions results in suboptimal shale shaker performance.
To address the existing shale shaker design limitations and enable adaptable and optimal performance, a prescribed vibration system (PVS) is under consideration. This system comprises:
- Actuators, also referred to as circular force generators—these are motorized eccentric rotors
implemented in proximal pairs that enable the production of controllable rotating forces;
- Accelerometers affixed to the shale shaker structure for measuring the vibration profiles;
- A controller that monitors these sensors and regulates actuator force magnitudes and phases to
achieve and maintain a prescribed vibration profile.
Figure 4 shows a schematic of the PVS with three actuators mounted to the shale shaker. With appropriate actuator placement and sufficient force output, this system can be controlled to achieve all four of the primary vibration profiles of Figure 3. In addition, progressive elliptical shape motion, illustrated in Figure 4, can be attained. This vibration profile enables superior conveyance of cuttings and a staged processing of cuttings at the shale shaker entrance, middle and exit.
The increasing popularity of active vibration control strategies for low- to mid-frequency vibration problems has identified the need to optimize actuator layouts. Implementing an active vibration control solution for a well-posed problem has become practical, if not yet routine, given recent advances in control algorithms and microprocessors. However, the performance bottleneck for most systems involves poor compromise between performance and efficiency as a function of actuator placement. For simple systems, the designer can rely on intuition and experience
in placing actuators, but more complex systems require multiple actuators whose optimal placement may defy intuition, and optimization of such systems by trial and error can be costly and time consuming.
The presence of multiple attributes in an optimization problem, in principle, gives rise to a set of optimal solutions (known as Pareto-optimal solutions), instead of a single optimal solution. In the absence of any further information, none of these Pareto-optimal solutions can be said to be better than the others. This demands a user to find as many Pareto-optimal solutions as possible. The Pareto-optimal solutions to a multi-attribute optimization problem are often distributed very regularly in both the decision space and the objective space. A problem that arises, however, is how to normalize, prioritize and weight the contributions of the various objectives in arriving at a suitable measure. In addition, these objectives can interact or conflict with each other in nonlinear ways. The present research seeks a more general method for optimizing the locations of multiple actuators in a shale shaker system with conflicting attributes.