Five Facts You Should Know About Insulation Resistance Testing
March 2, 2023
Subsea technology plays a major part in the commercial viability of offshore oil and gas projects. If a new discovery can be tied-back to an existing development, capital expenditure and return-on-investment time is slashed. Projects that might otherwise have not been economically viable can deliver value.
Subsea control equipment draws electrical power from the surface located (topside) equipment of an attached installation using purpose-made cables known as umbilicals. These may contain multiple live copper conductors and it is vital that these are kept electrically isolated from:
- Other conductors
- Earthed cable protection, such as steel armour
- The surrounding seawater
This is the job of the insulation material surrounding the conductors. A good insulator resists the flow of electrical current. If it is compromised or damaged, harmful leakage currents will develop and lead to faults and eventual failure.
Insulation Resistance (IR) testing can be carried out to identify a drop in resistance caused by current leakage, enabling operators to take remedial action before serious faults or even failure occurs. Here are five useful facts about IR theory and practice.
Five facts you should know about insulation resistance (IR) testing:
1. What insulation resistance is and why you should test it
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 IR is high, the integrity of the cable is good. If the IR 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 therefore crucial that IR testing and monitoring is carried out. Insulation degradation tends to occur gradually, so having complete and continuous information allows developing issues to be monitored, mitigated against, and eventually resolved in the most economical manner.
2. Why insulation resistance degrades
Factors that can cause IR degradation can be classified as either Intrinsic or Extrinsic.
Intrinsic factors are faults native to the system, i.e. those occurring during manufacture or assembly, incorrect design or material quality issues.
Mechanical stress of the cable or connectors during installation or handling 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. These are examples of Extrinsic factors.
3. What insulation resistance testing does and how it is performed
The theory behind IR testing is derived from Ohm’s Law. Apply a known voltage between conductors and earth then measure the leakage current drawn. From this result a figure for IR can be calculated with the result expressed in Ohms.
Testing of IR integrity can take two forms, and both have relative merits.
Installing a Line Insulation Monitor (LIM) allows continuous surveillance of the system while in service. Automatic alarms can be generated if the IR parameters exceed pre-defined limits, especially useful when integrated with a SCADA system. LIMs typically apply a low-frequency AC voltage.
Periodic or on-demand testing can also be carried out on a de-energised system using an IR tester that applies a high DC voltage for a set period of time. The results can be interpreted to identify the approximate nature as well as the presence of a fault.
4. Mandatory insulation resistance testing may cause copper loss, but it can be mitigated
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 IR. When compared to alternate systems lacking this capability, the time available to rectify the issue before eventual failure is greatly increased.
5. Insulation resistance faults can be recovered
Replacing umbilicals or EFLs with insulation faults involves production shutdown and a costly work-over campaign. However, there is a corrective method that requires no subsea intervention.
Any subsea system already equipped with V-LIM can be augmented with the installation of V-LIFE. Implementing this unique feature, via a simple software configuration change, introduces an electrical passivation signal which promotes an electrokinetic and electrochemical reaction to occur at the site (or sites) of the seawater ingress.
This actively heals damaged insulation material, preventing further water ingress and restoring the integrity of the cable, at a fraction of the cost of replacement.
Get in touch using our contact form or email firstname.lastname@example.org to learn more about preventative IR testing, and how to optimise the service life of your subsea cables and equipment.