System requirements VS Cable requirements – how are they connected and what impact do they have?
To get the ROV right cable it is absolutely vital to determine the system requirements. After all, the ROV system requirements largely impact the cable requirements and fit-for-purpose design.
The system requirements with the biggest impact on the umbilical design are:
On this page, we will explain how these system requirements impact the following cable requirements:
With Function, Deployment, Depth, Downtime, Power, Life and Budget there are 7 ROV System Requirements that can impact your Cable Design. The link between system requirements, cable requirements and cable design are crucial for a successful subsea project.
On this page, we discuss these requirements briefly. In our blog, you’ll find more information about the system requirements with the greatest impact on cable requirements and the cable design.Read more
Supplying power, sending control instructions and sending/receiving data are common functions of an ROV cable. The function determines the type, size and number of components and how they are integrated: the geometry of the cable.
Another aspect impacted by the function is the so-called ‘safe workload’. Safe workload is typically used to define the maximum weight load for a lifting device. It represents the weight that can be applied to the end of the cable. At DeRegt, we prefer to speak about the maximum working load of the cable: we define this as the load necessary to stretch the cable by 0,6%. Higher stretching can cause permanent deformation of copper conductors and quickly result in failure.
The deployment/retrieval also has a high impact on cable requirements. The diameter influences the minimum bend radius, which directly impacts the size of the handling system. Flexibility impacts the ease of handling; a cable can be made more flexible by increasing the helix angle – shortening the helix length. Crush resistance is also often a key design criterion. Because steel has a superior crush resistance compared to synthetic fibers, it is often preferred for lift cables for work class ROVs. For free hanging cables in combination with a high payload, the logical choice is a synthetic fiber like aramid.
The handling system can have a large impact on the design of the cable. That’s why we ask our customers for insight on how the cable will be launched and deployed.
In deeper waters, the cable will come under great pressure. The required length and diameter of the cable are directly related to the operating depth. There is also a limitation on length when using steel armoring and the deeper you go, the greater the voltage drop. How do you design an ROV cable that can deal with these challenges of depth?
One challenge when working at great depths is void filling. Working at depths deeper than 6 km requires the cable to be completely air-void-free. Hydrostatic pressure can also have an impact on the cable’s weight in water. Due to the pressure, the volume of the cable can be reduced. This causes the density to increase. Foam may be used to reduce the cable’s weight in water, but pressure can change the properties of the foam.
Developments to better deal with deepsea challenges are ongoing. Of course, we will keep you informed of the latest developments at DeRegt.
Power is a big driver for the diameter, weight and geometry of the cable. It has a direct impact on the size of the conductors and the thickness of your insulation. The size of the conductors is either determined by current or voltage drop.
For current, heat is often the limiting factor. As the cable is usually water-cooled, this is no problem. Deployment in shallow water, when lots of layers are being kept on the winch, could be a problem though. Thermal analyses help us to prevent heat issues from happening.
The voltage drop can also be a driving factor for the size of the conductors. For the electrical circuit of the system, it could become problematic if the voltage drop is too high. Normally, the maximum allowed voltage drop is 10%.
A third important consideration is electrical stress. DeRegt designs cables that keep the voltage stress below 2.5 kilovolts per mm. Otherwise, air can ionize and generate discharges. This will eventually burn up your plastic insulation. Air voids might increase electrical stress, which is another reason why it is important to fill the void in your cables.
Compared to other structures on board, the cable is quite a fragile component. It is crucial to make targeted choices in the design phase, to avoid downtime as much as possible.
One way to do this is by using extra optical fibers in the cable. This way, there are always spare fibers present. Secondly, we generally place fragile components towards the inner core of the cable, to avoid crushing.
Another important aspect is the fatigue life of steel. By making the steel armored cable strong enough to stay under its fatigue limit can be considered. Aramid cables work differently: they fail because of internal abrasion, which happens especially at a low bend radii. Fatigue is an important consideration in the cable’s design phase.
It may be necessary to repair a cable in the field. Steel armored cables are much easier to re-terminate than aramid cables, which require special tooling. Our field service engineers are of course happy to help, but taking this into consideration might save complications later.Read more
Another important factor for the cable is of course the required cable life. How long should the cable last?
One way of looking at fatigue life is the life factor. The life factor is the safety factor on load, multiplied by the D/d ratio. The D/d ratio is the diameter of the sheave wheel divided by the diameter of the cable. According to the theory, the life factor must be constant. This means when the diameter of the sheave wheel is two times bigger, the load can be twice as big without reducing the fatigue life. Note: the life factor theory only holds up within a certain range and doesn’t apply to aramid cables.
We consider the so-called fatigue life of the cable by designing steel armored cables in a way that the strength member fails first. Strength can be seen as direct strength, but you also have to look at the stresses in bending. By making the cable more flexible, you can reduce bend stress. Flexibility is a function of the helix length. When we increase the helix length, the cable will become stronger in tension but less flexible and vice versa. The balance between those two is crucial.
With a fatigue model based on fatigue test data, we can determine the cable life for different operating scenarios – and answer the most important question: does the strength member meet the requirements on the design life?
Please note that it is very important to determine early on exactly how the cable will be used. Most problems occur when cables are used for other functions or on other devices. Using the cable differently on the ship has a direct impact on downtime and life expectancy.
Last but not least, there is the budget. How much money is available for the project? Downtime can be reduced by not buying the cheapest cable, but you might have a tight budget. A large part of the cable cost is due to the number of required lay-up operations.
Twisting all components up at the same time is called unilay. This is the most compact construction, with often the smallest diameter achievable. Because everything is twisted together in one go, production costs are lower. A disadvantage of unilay is the build-up of torsion by laying up the components in one pass..
Another construction with an impact on costs is concentric lay. This means you need to lay-up each layer separately, making it more expensive. The advantage is that the components in each layer can be laid up without generating torsion.
A cable can also be composed by laying up a set of subcables, with each subcable composed of its individual laid-up components. The so-called group lay construction that is created like this is often the most flexible construction but also the most expensive.
With all these different lay-up operations, there are plenty of options to minimize the cable cost. However, costs should never be leading in a subsea project. The quality of the cable and the suitability for your specific application should always be the most important consideration.
How do all of these cable requirements affect your cable design? And how do you make sure the cable suits your specific application as well as possible? In our exclusive Webinar On-Demand, our R&D Manager gives away more tips and tricks about cable design. Re-watch the most interesting parts any time you like with the recording!