What is insulation resistance?

Insulation Resistance is the ability of the insulation material to prevent leakage currents flowing between live conductors, or from the conductors to ground. When the insulation resistance is high, the integrity of the cable is good. However, if the insulation resistance breaks down, it can eventually cause electrical failure of the subsea equipment and lost production.

In subsea applications, equipment downtime and replacement of umbilicals or the Electrical Flying Leads (EFLs) between equipment will inevitably be expensive. It is crucial that testing and monitoring is carried out. Insulation degradation tends to occur over time, so having complete and continuous information allows developing issues to be monitored, mitigated against, and resolved quickly.

What are the causes of low insulation resistance? 

Understanding the causes of low insulation resistance is crucial for maintaining system reliability and performance. Low insulation resistance can become problematic as insulation ages, and several factors can accelerate this degradation. These factors are typically categorised as either intrinsic or extrinsic.

Intrinsic factors are faults native to the system, such as those that occur during manufacture or assembly, incorrect design or material quality issues.

Extrinsic factors include mechanical stress of the cable or connectors during installation or handling. This can damage the insulation, as can collision damage from vessel anchors or ROVs. Scouring damage from the seabed is also a known source of degradation.  

As insulation deteriorates, it can develop cracks or holes, allowing moisture and other contaminants to infiltrate. This intrusion lowers the insulation’s resistance, worsening over time and potentially leading to significant performance issues.

What is insulation resistance testing?

Insulation resistance testing checks insulation resistance integrity. The theory behind insulation resistance testing is derived from Ohm’s Law – apply a known voltage between conductors and earth before measuring the leakage current drawn. A figure can then be calculated, with the result expressed in Ohms (Ω). 

Continuous monitoring with an insulation resistance test machine, such as an insulation monitoring device (IMD), will typically use an AC waveform because DC measurements are susceptible to interference from noise or stray currents. 

Read more on the theory of insulation resistance testing

How to test insulation resistance

There are two ways to test insulation resistance. These are:   

1. Installing a Line Insulation Monitor (LIM) allows continuous surveillance of the system while in service. Automatic alarms can be generated if the insulation resistance parameters exceed predefined limits, especially useful when integrated with a SCADA system. LIMs typically apply a low frequency AC voltage. 
2. Periodic or on-demand testing can also be carried out on an energised system using an insulation resistance tester that applies a high DC voltage for a set period. The results can be interpreted to identify the approximate nature as well as the presence of a fault. 

What are the types of Insulation Resistance Testing?

There are different types of insulation resistance testing namely; spot reading test, time resistance test, polarisation index test and step voltage test. 

• Spot reading test – also known as spot testing, is a short test that usually takes 60 seconds to get a reading. Despite the quick result, spot testing only provides an approximate indication of insulation integrity.
    
• Time resistance test – this monitors resistance and how it changes over a certain time, such as 10 minutes. This approach provides a short time series of data that can indicate insulation resistance trends. This, in turn, indicates good or poor condition. 
   
• Polarisation index test – this test divides resistance changes over a 10-minute period by the resistance changes over a 60-second period, expressing this as a ratio called the polarisation index. Depending on what is being tested and the ratio, it can give an indication of the insulation health. Less than one is generally poor, while greater than four is good.   
• Step voltage test – this applies increasing voltage levels in steps, taking a current reading at the end of each stepped voltage increase. The key reading is the current which, if following a straight line, means the insulation is healthy. Variations in current readings indicate deteriorating insulation resistance.  

Read about the importance of insulation resistance testing

The Benefits of Insulation Resistance Monitoring with V-LIM

IEC 60364 covers the insulation monitoring requirements of IT (Isolation Terra, unearthed) systems such as those used on subsea equipment. However, where an insulation fault allows seawater ingress, it is known that LIM devices can exacerbate damage to the cable by promoting an electrochemical reaction that degrades the copper itself.

The V-LIM system from Viper Innovations can help to mitigate against this by regulating the applied voltage used during operation, minimising the copper loss while continuing to accurately monitor the insulation resistance. When compared to alternate systems lacking this capability, the time available to rectify the issue before eventual failure is greatly increased.

There are many benefits of using V-LIM for insulation resistance monitoring. For example, it enables continuous monitoring and gathers data in real time. Real-time data and continuous monitoring are ideal for subsea environments, where interventions are difficult and costly. Continuous monitoring can identify degrading insultation resistance before it become serious, allowing the umbilical operator to take early action. V-LIM also supports: 

• Non-intrusive testing, which minimises the risk of damage to subsea equipment 
• Data logging, including historical data for trend analysis and predictive maintenance 
• Automation, meaning that less manual intervention is required. This improves the safety and efficiency of offshore operations.  

The common causes of insulation resistance failures are well known. Without monitoring and prompt action, its effects can be catastrophic to the operation of a subsea field. IEC 60364 requires that an IT earthed system is equipped with a monitoring or protection device, and a LIM is the best option for keeping the system in service in a hostile subsea environment.  

Get in touch using our contact form or email enquiries@viperinnovations.com to learn more about preventative insulation resistance testing, and how to optimise the service life of your subsea cables and equipment.