This technology assists with preventing costly equipment failures while increasing production reliability and personnel safety.
by Richard J. Zelm

ExxonMobil’s Hebron platform off the coast of Newfoundland and Labrador, Canada, in the North Atlantic is a massive structure. At 230 meters tall, Hebron operates in 93 meters of water and is attached to the ocean floor using a stand-alone concrete gravity-based structure (see Figure 1).

The structure contains more than 40 million cubic feet of concrete—more than what was used in New York’s Empire State Building. It is designed to store some 120 million barrels of crude oil and support 52 well slots.

Offshore Oil Platform DiagramFigure 1. The diagram shows an offshore oil platform attached to the ocean floor using a stand-alone concrete gravity-based structure. (Images and graphics courtesy of Meg-Alert Inc.)

The platform has the capacity to produce 150,000 barrels per day (bpd) of heavy crude oil, with water volume of 200,000 to 350,000 bpd. It has a water injection capability of 270,000 to 470,000 bpd, with gas-handling capacity of 215,000 to 300,000 standard cubic feet per day (mscf/d). The platform is also capable of treating 190,000 to 280,000 bpd of produced water for disposal overboard and has two offshore loading stations with a total crude-oil-handling capability of 5,400 cubic meters an hour.

The platform’s topside facility is equipped with several large, medium-voltage (MV) electrical pumps and compressors that are used during daily operations. Some of these pumps include 2,500-horsepower (hp) crude oil export pumps, 10,000-hp water injection pumps, 2,200-hp fire pumps, 1,150-hp cement pumps and 2,500-hp gas compressors.

ExxonMobil and WorleyParsons, the design engineering company assigned to Hebron’s topside facilities project, understand the daily challenges of operating critical electrical equipment on an offshore platform that is exposed to high waves, ice, excessive moisture, changing temperatures, dense salt-fog and even icebergs. The salt-fog is one of the worst problems because the salt, combined with the moisture, can cause severe damage to the electrical insulation inside the critical motors and generators needed to operate the platform.

The most efficient method to combat these types of problems is to use internal space heaters along with automatic insulation resistance tester (AIRT) technology. The space heaters help to prevent condensation from forming on the stator coils and cables. The AIRT systems are used to constantly test and monitor the electrical insulation resistance (IR) levels to ground in the motors and generators when they are offline, detecting the first signs of insulation breakdown.

Traditionally, this type of preventative maintenance testing was done with a handheld megohm tester device, which was a time-consuming and dangerous chore for maintenance personnel.

Also, human error is a factor in manual IR testing, which requires maintenance personnel to access the inside of the control cabinets where they could be exposed to dangerous arc flash and electrical accidents.

ExxonMobil had implemented AIRT systems successfully on other offshore platforms, and the company enlisted WorleyParsons to design the same technology into the specifications for Hebron’s MV generators, the low-voltage (LV) standby generators and the critical MV pump and compressor motors. Powell Industries provided the switchgear, MV motor control centers (MCC) and system integration for the topside project. Powell had also been installing AIRT systems on offshore platforms for many years.

The AIRT manufacturer was chosen because its systems have the ability to perform the IR testing using a direct current (DC) test voltage that meets the IEEE, ABS and ASTM International standards for proper testing voltage. This DC testing voltage requirement can detect more problems in the earliest stages in order to prevent catastrophic failures.

AIRT Testing UnitImage 1. The AIRT testing unit is located inside the high-voltage compartment of the motor control center.

AIRT Installation

The AIRT system consists of a testing unit, a megohm meter display, a remote-control alarm assembly, a high-voltage (HV) test lead and an HV fuse assembly.

The testing unit is located inside the high-voltage compartment of the MCC or switchgear and directly connects to the motor or generator windings (see Image 1).

The testing unit is connected using an HV test lead with a built-in current-limiting resistor that limits the maximum testing current to a safe level for the equipment as well as the personnel.

The current-limiting resistor is located at the end of the HV test lead where it terminates to the motor or generator leads. The HV fuse assembly is then installed into the HV test lead near the current-limiting resistor, between the resistor and the testing unit, to protect the system in the event of a fault.

The megohm meter and the remote-control alarm assembly are located outside of the MCC or switchgear, normally on the LV door panel. The megohm meter indicates the real-time IR levels of the motor or generator windings and cables.

The remote-control alarm assembly allows maintenance personnel to view the alarm indicators and operate the testing unit without accessing the inside of the MCC or switchgear cabinet.

Some AIRT systems have a lock-out, tag-out power switch located on the remote-control alarm assembly in order to lock out the input power to the testing unit when hands-on maintenance work is being performed on the motors or generators (see Image 2).