When working with ROVs and ROV cables, it is crucial that your subsea project runs as efficiently as possible. That means minimal downtime and a long life expectancy. However, knowing when your subsea ROV equipment needs to be replaced is also essential.
Continuing to work with equipment that is actually overdue is guaranteed to result in damage and downtime. But how do you know when the lifespan of your subsea ROV equipment has reached its end?
Deployment and retrieval of ROV equipment: impacting the fatigue life
To answer this question directly: subsea equipment’s replacement is mainly related to the number of deployments and retrievals. The number of times the ROV cable is deployed and retrieved determines how long the cable will last.
That’s because an important limiting factor of the ROV cable’s lifespan is the number of bend cycles. When the cable is deployed from the winch and lowered from the sheave wheel and vice versa, the cable goes through tension and bend cycles. The fatigue life of a cable determines how many times this can be done before the cable starts showing defects.
In short, the number of dives determines how often the cable is stretched and bent, which is at the expense of the fatigue life of your ROV equipment. But how many deployments/retrievals can your ROV equipment handle?
Fatigue life of your ROV Cable: D/d ratio as a rule of thumb
As mentioned above, the number of deployments affects the life of your ROV equipment. However, what should not be forgotten is the connection with the load. The higher the weight during launch and retrieval, the lower the lifespan will be.
A rule of thumb for this comes from the steel industry: for steel ropes, they use the life factor to calculate fatigue life. This is partly applicable to steel armored cables as well.
Life factor = (safety factor on load) x (the D/d ratio)
The D/d ratio refers to the diameter of the sheave wheel divided by the diameter of the cable.
When using this rule of thumb, it is important to know that the life factor is approx. constant. This means that if the diameter of the sheave is twice as large, the load can be twice as high without reducing the fatigue life. However, the theory is not universally applicable; it only works for steel armored cables (like main lift cables for work-class ROVs) and not for aramid-reinforced ROV cables (like ROV tether cables) and only applies up to certain limits for the applied load and bend radius.
Repairing or replacing subsea equipment
The fatigue life of your ROV equipment is a good indicator of when your ROV cable needs to be replaced. However, there is another reason to replace a component: damage. Unexpected circumstances can never be fully dealt with. On the open sea, a lot can happen (high impact loads due to a storm for example or a shark bite), but technical electrical issues like short circuits can also occur.
There are three options to find out if your ROV equipment needs to be replaced in such circumstances.
- The repair can be done by the crew, with remote explanation.
- A field engineer comes aboard to make a repair (this is mostly done for more severe damages).
- The ROV cable must return to the factory to be repaired there.
Our ROV equipment experts ask the crew to take photos and certain measurements to determine which of these three options are necessary. It is essential to know if the damage is just visual or if there is also functional damage.
Are the fiber optics still running properly? What is the attenuation in the cable and insertion loss at the connectors? What is the resistance of the electrical components? And what are the values for the insulation resistance between the components? Such data is compared with the original data from the FAT to assess the damage properly.
Design Considerations that determine fatigue life
Whether your subsea ROV equipment needs replacing depends on two factors most of the time. First, the fatigue life can be calculated for some scenarios in advance, but is also often verified by performing a bend over sheave (BOS) fatigue test during the qualification phase and is therefore verifiable. The other factor is damage due to unforeseen circumstances, after which experts determine whether repair or replacement is necessary.
Specific Design Considerations are essential in the development phase to ensure that an ROV Cable meets the desired requirements, like fatigue life. Our R&D Manager will explain in our On-Demand Webinar which ones can be essential for your unique ROV project. Download the free Webinar here:
If you are looking for a suitable ROV cable for your specific project or your existing ROV cable needs replacement, our experts will be happy to create a Custom Cable Design for you. Of course, we can also advise you on the fatigue life of your cable; contact my colleagues or me here.
