Downhole Drilling Rigs Connectors in Harsh Environments

The environment in downhole drilling rigs is harsh. It begins with extremely high pressures and temperatures (HPHT). The connectors in these applications must survive HPHT and corrosive environments and still deliver outstanding electrical and mechanical performance.

They must be compact to fit into restricted spaces, handle electrical power, and provide signal interconnects for sensors and other devices. In some cases, they deliver RF energy to reduce the oil’s viscosity, simplifying its transportation to the surface.

Because of the harsh operating conditions, connectors for downhole applications are designed differently and use different materials than those used in more benign conditions. Downhole connectors that support standard power and signal connections, fiber connections, and coaxial connections are available.

Handling the pressure

Connectors in a deep downhole application can encounter pressures of up to 35,000 pounds per square inch (psi). This demands using superalloys for bulkhead connections to protect the electronics packages in drilling and logging tools.

MP35N, vacuum induction melted, and vacuum arc re-melted superalloy with cobalt, nickel, chromium, and molybdenum as its primary alloying elements are options. Other choices are precipitation-strengthened superalloys like Inconel 718, composed of nickel, chromium, iron, and niobium, or Inconel X750, composed of nickel and chromium.

Both Inconel alloys are widely used in aerospace engine applications. However, because Inconel 718 has better corrosion resistance, it is more widely used in oil, gas, and petroleum processing areas.

In addition, bulkhead connector designers like those in Figure 1 use specialized glass formulations compatible with these superalloys and create robust glass-to-metal seal (GTMS) pressure barriers. Examples include borosilicate glasses and alkali borosilicate glasses, which offer excellent thermal shock resistance, high mechanical strength, and good chemical durability.

Figure 1. Bulkhead connector with GTMS for downhole applications. (Image: *Ametek*)

Bulkhead connectors for downhole applications often employ “compression seal” designs. In these designs, the glass has a significantly different coefficient of thermal expansion from the metal. This allows the metal to grip the glass tightly during cooling, creating an especially strong seal.

PEEK is an option

Polyether ether ketone (PEEK) and PEK, a higher-temperature variant, are 20 to 40 times more impact-resistant than glasses. While glass seals are common in bulkhead connectors, PEEK is used for inline connections in the wellbore. PEEK can also be used in the connector body for seals and insulation. In one case, a rotatable connector designed using PEEK can handle temperatures up to 200 °C and pressures of 20,000+ psi shown in Figure 2.

Figure 2. The rotatable HPHT connector was designed for temperatures of 200 °C and pressures of 20,000+ psi (1380 bar). (Image: *Victrex*)

Pressure-balanced oil-filled tech

Pressure-balanced oil-filled (PBOF) connector technology is another option for downhole applications and undersea systems like remotely operated vehicles (ROVs). It uses oil-filled chambers to equalize the pressure of the surrounding water, ensuring reliable mating and unmating of connectors even at extreme depths. The pressure equalization also reduces the stresses on the connector.

In one case, a 15.8 mm cylindrical BPOF connector was designed to support three circuits and can be wet mated up to 15,000 psi in temperatures up to 150 °C as shown in Figure 3. When mated, each electrical band is protected within its own secondary oil-filled chamber. Each half of the connector is individually sealed, and when mated, the whole assembly is pressure-balanced, minimizing stresses within the sealing areas. To ensure good connectivity, the contact bands are wiped before making connections.

Figure 3. Example of a PBOF wet mate connector. (Image: *Ametek*)

45 kW of RF at 50 bar

Figure 4. This IP67-rated coaxial cable assembly can handle 45 kW of RF energy at 100°C 50 bar pressure. (Image: Winchester Interconnect)

Coaxial cables have been developed to reduce the oil’s viscosity with an average RF power handling specification of 45 kW at 100°C in a high-pressure environment of 50 bar (725 psi). The cable has a polytetrafluoroethylene (PTFE) dielectric, also known as Teflon.

It used flexible coated steel conduit armor to meet the flexibility requirements of the curved travel path, extreme crush forces, and corrosive environment in the well. The entire cable/connector assembly was IP67-rated for protection against fluids, as shown in Figure 4.

Summary

Connectors used in downhole drilling rigs are available for power and signal, fiber optic, and high-frequency/RF applications. They must be able to handle challenging HPHT conditions and are often made using materials different from those used in connectors for general electronic applications. Some use PBOF technology that is unique to undersea and downhole applications.