Signum Instruments saw a need in the market to develop a line of state-of-the-art slim logging tools that would be available to service providers. These logging tools allow service providers to obtain high quality data in real time or memory mode with their advanced conveyance system.
In the development of the true 2 ¼” OD tool suite, Signum Instruments began working with Alpha Slip Rings to develop a slip ring that would fit the size and environmental requirements for the downhole swivel tool.
A slip ring is a continuity device which transfers electrical current through a continuous rotation from a stationary member to a rotating member. There are two distinct parts of a slip ring. One part is the conductor ring “core” stack and mounting tube or shaft. The rings that make up the core are made of electrically conductive metal and are mounted, but insulated from, a center shaft. The other part is the brushes and housing. The brush maintains a continuous electrical connection as one portion of the assembly rotates. As the ring turns, the electric current is conducted through the brush to the ring making a connection. The wire leads from the brushes and from the rings are connected to electrical circuits.
The slip ring in the downhole swivel allows the top section of a swivel tool to rotate against the lower section which can prevent the wireline cable to become damaged from being twisted. Because the application and size of the Signum project was very specialized, Alpha began development on a custom slip ring solution for the Signum downhole swivel.
Several challenges needed to be addressed in the design of the slip ring. Because of the size of Signum’s tools, the slip ring needed be a very compact design. Temperature also needed to be considered so that the slip ring could properly function in the harsh environment. Signum required a slip ring that could allow oil to flow through the entire unit, and this was addressed in the housing design. Alpha also needed to consider shock, vibration and torque requirements for the application.
The Alpha slip ring design needed to be less than 1 ¼ inch diameter. In addition, the slip ring needed to be a length of less than 6 inches. To meet the size requirements, Alpha modified the standard brush rods and brushes to accommodate the small diameter. The brush rods were replaced with a new unibody design.
High Temperature Requirements
The slip ring needed to operate at a temperature of -40°C to 180°C. High temperature materials were used for the components and during the assembly process to ensure that the slip ring could survive the harsh environment. This included using plastics and solder rated for a high temperature application. Alpha also built the slip ring for a high reliability of the electric connection between contacts during rotation at elevated temperatures.
The slip ring needed to easily fill with oil and allow oil flow freely into, so holes were added to the cover. The system also is designed to have oil flow through and clean the slip ring, so openings were needed at either end of the slip ring to allow oil to flow through the slip ring and carry away any debris.
The unit was constructed with a stainless-steel housing to prevent corrosion. The cover needed to be removable to allow for inspection of the brushes. Typically, the cover in Alpha’s slip ring designs is used to support the structure and act as a sealing system. Many slip ring designs need to be sealed to protect the slip ring components from any debris in the environment. In this case, the unit did not need the sealing capability, and the design needed to allow the unit to support itself. The unibody design achieved the support application, and with the removal of four screws at the back of the unit, the cover slides off easily.
Shock and Vibration Requirements
To withstand the harsh environment conditions in the oilfield, Alpha designed the slip ring with high vibration and shock ratings. Signum’s application required a high shock rating at 500 G’s for 2 milliseconds. The slip ring unit needed to survive that 500 G impact level without losing contact between brushes and rings. The vibration requirement was 10 G rms at 400 Hz.
The value of shock and vibration testing, just like the value of any environmental testing, is to verify that a product can function properly for the purpose it was designed. A slip ring assembly can have potentially hundreds of individual parts, which are assembled together to create a working product. Each of these parts has a purpose and without even the simplest part, the whole assembly can fail. Therefore, the best way to ensure the assembly will work is through product testing.
To ensure that the slip ring unit met the needed shock requirements, Alpha performed drop testing from a height of one foot onto a hard, wooden surface (to approximate 500 Gs). The rings of the slip ring were connected in sequence, and the electrical continuity through all rings was monitored during the drop using an oscilloscope. Continuity was uninterrupted throughout the drop, so the unit passed Alpha’s internal test, ultimately meet the design requirements.
The unit needed to have a minimized starting torque. One of the challenges with the downhole swivel tool design is minimizing the torque required to rotate in a high-pressure environment. The specification required the slip ring operate at a torque less than 16-inch ounces.
In addition to low torque, there were countervailing design objectives, such as maintaining low contact resistance between brushes and rings both in static and rotating conditions. Alpha also wanted to ensure that the slip ring had the longest possible operating life from the slip ring’s brushes. All of these features needed to fit within the 1 ¼” slip ring design.