The Top Four Causes of Electrical Failure in Subsea Cables
January 31, 2023
Subsea electrical infrastructure underpins the operation of offshore oil and gas fields. When electrical distribution systems fail, the costs are significant, and not just in terms of the cost of intervention and the repair and replacement of faulty or damaged equipment. The opportunity cost of lost oil and gas production can run into millions of pounds a day.
Powering this infrastructure is via the umbilical, a single point of failure that can bring the whole field’s electrical system down if it stops working. In this article, we will discuss the main causes of failure in subsea electrical cables, and the importance of monitoring insulation resistance (IR) to prevent or mitigate the impact of failure.
Why do subsea electrical systems fail?
The causes of wider electrical system failure fall into two broad categories:
- Intrinsic factors
- Extrinsic factors
Intrinsic failures can include faults relating to design, materials or hardware assembly. Extrinsic failures can include installation, handling and mechanical stressing during operation, and environmental conditions.
Measuring insulation resistance (IR) determines the integrity of subsea electrical cables and connectors and is the key to preventing or mitigating the impact of complete electrical system failure. On testing, a low or falling IR indicates that the insulating material surrounding electrical conductors has lost integrity or is degrading. This means that seawater has entered the electrical distribution system, penetrating insulating materials. It is highly probable that electrical breakdown and failure will follow.
What are the top four causes of subsea umbilical cable failure?
Submerged cables are a critical element of offshore infrastructure, with our operational experience of over 15 years showing that the root causes of failure of subsea electrical cables within umbilicals fall into four areas. Several of which are, not surprisingly, aligned with wider electrical system failure modes highlighted above. So, what are the four top causes of subsea umbilical cable failure?
1. Incorrect installation and handling of subsea umbilicals and cables
Around half of umbilical and cable failures result from physical damage caused by incorrect installation – either subsea or onshore during system integration. For example, lateral mechanical stresses placed on connectors when cables are installed on structures are a common failure mode. Damage from trawler activity and anchors as well as poorly piloted remotely operated vehicles (ROVs) are also included in this category. The physical damage to cables and other components means they cannot function properly. Seawater enters the lines through sites of physical damage, causing the cable to fail immediately, or over time.
2. Manufacturing and material faults in submerged umbilical cables
Manufacturing and materials quality issues account for approximately 20% of umbilical failures. Manufacturing defects in system components and subassemblies are typically caused by materials defects, elastomeric failure, and sub-standard assembly, or manufacturing and test process issues.
3. Incorrect operation of umbilical cables
Root cause analysis shows physical damage and disconnection during operation to be one in ten of the contributing factors to electrical failure in umbilicals, mainly because they lead to the seawater ingress that ultimately causes system failure. Poor ballasting and incorrect loading cause stresses that damage cables and their insulating materials.
The use of an IMD is mandatory for the monitoring of unearthed power systems, such as those used in subsea production controls, and research shows that the act of taking the measurements may have some detrimental effect on the conductors due to the way IMDs operate. Research shows that under certain conditions, when there is an insulation-related fault in the system, an IMD can accelerate damage to the copper conductors with the rate of acceleration depending on the type of IMD and its measurement technique employed.
4. Failure of subsea cables through environmental factors
This group of failure modes accounts for around one in ten of all subsea cable failures. These result from environmental conditions, such as the effect of seabed scouring, causing cable spans and cable motion, or extreme motion of dynamic cable sections.
Monitoring insulation resistance (IR) alerts operators of potential faults
Our operational data shows that around 70% of electrical faults are caused by water ingress into umbilicals, cables and connectors. Seawater in cables decreases the dielectric strength of the cable insulating material and increases leakage currents, which eventually leads to failure. Since the predominant failure modes of subsea cables lead to seawater ingress, monitoring IR can alert operators that a fault is imminent or has already occurred.
Learn how you can use IR monitoring to maximise the operational life of your controls and electrical distribution equipment, optimising the uptime of subsea umbilical cables. Email our expert team at email@example.com or get in touch via our contact form.