Subsea oil and gas production systems are connected electrically and hydraulically. The electric supply that feeds the subsea distribution network comes from a topside electrical power unit (EPU). Subsea networks may be extensive and can be as much as 100km away from the topside facility. The problem operators face is that cable and equipment insulation degrades over time and with it seawater begins to penetrate the system, creating failure points. Just a single ingress point has the potential to drag the complete subsea electrical network down with it, costing thousands in lost production. To give operators warning of this, the status of the subsea infrastructure is monitored by measuring the electrical insulation resistance (IR) which is determined by the level of leakage current in the system, by a topside installed line integrity monitor (LIM).
So, what happens if IR readings drop below an acceptable level and an alarm is triggered on the LIM? Such an alarm informs the operator that there is a risk of failure. The electric current from the topside PSU will increase to compensate for the larger leakage current and subsea systems will continue to operate but, eventually, limits will be reached, cables will reach their maximum current carrying capacity, and the system will fail. To prevent loss of production and ensure continued safe operation, the operator must intervene, locate the fault and schedule a repair.
Subsea electrical fault finding is costly both in terms of vessel day rates and unplanned production outages. “A single low IR reading gives the operator no indication of where in the subsea network the failure has occurred,” Peter Alexander, marketing and business acquisition manager for Viper Innovations, explains. “Without further information, the only option is to start checking each section of the subsea network; disconnecting and reconnecting sections until the fault is isolated.”
Such an operation not only comes at a very high cost, but the process of disconnecting and reconnecting cables can introduce further weaknesses in the system that may result in yet more problems.
In response to this challenge set by five super-major operators including Shell, Chevron, Total and BP to develop a much more cost-effective way of subsea fault-finding, Viper Innovations created the V-IR Network Integrity Monitoring System. This multi-LIM solution improves the visibility of IR by dividing the subsea network into monitored sections. It uses V-SLIM (subsea line integrity monitoring) nodes at strategic points in the network. These can be installed using divers or underwater remotely operated vehicles (ROVs). Once in place, the V-SLIM nodes will communicate via the power cables with a topside V-LIM device by applying a communication signal onto the IR measurement. Importantly the V-SLIM has a failsafe design such that any failure of the monitoring electronics has no effect on the host electrical system.
“The patented design of the V-SLIM nodes means they can tell whether an IR reading is upstream or downstream of their installation point,” Alexander says. “If placed between the major sections of the subsea system, they start to deliver a map of IR readings. Using the V-IR software, the V-LIM can therefore not only give warning of risks to the integrity of the subsea network, but also where in the network the problem area lies.”
The V-IR Network Integrity Monitoring System informs the operator which section of the subsea network has an issue, and repair efforts can be immediately targeted at this area. The more complex the subsea network is, the greater the benefit the solution becomes. “It can save vast amounts in repair costs and return on investment will likely be made as soon as a fault occurs – and data shows that faults will certainly occur sooner or later with the degradation of cable insulation materials being accelerated by the harsh, saltwater conditions in which they lie,” Alexander adds. “Knowing where the fault lies further means there are fewer unnecessary disconnects in the system during the fault-finding process, and so less likelihood of additional areas of weakness being introduced into the network.”
Overall, the clever design of the V-IR Network Integrity Monitoring System offers operators a way to dramatically reduce the cost of maintaining their subsea electrical distribution systems. The increased amount of data and visibility of the status of each part of the network provides advanced warning of not only when but where faults may occur. Armed with this intelligence, the operator can decide when and how to instigate repair.