Page 209 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

198 CHAPTER 8 Cables and Connectors
The optical fibers can have either an individual buffer on each fiber optic, or they can have a common housing for all the optical fibers. The user will need to weigh the trade-off with the differ- ent approaches for the specific application because there is no single design that is correct for all applications.
Several standard cable design specification sheets follow throughout this section and are repre- sentative of state-of-the-art cables provided to the offshore industry by Falmat. A successful innovator in the ROV cable industry, they have been designing, manufacturing, and testing such cables for over 25 years.
8.8.4 Strength requirements
The strength member provides the mechanical link to the ROV. It usually has to support the cable weight, the ROV, and any additional payload and handle any dynamic loads. Also, the cable size can influence the load on the cable due to drag. Therefore, there are many variables to consider when choosing the cable strength.
Mechanical strength of a cable must consider:
• (Anticipated) working load
• Maximum peak dynamic load
• Minimum bend radius/diameter
• Expected cycle-life performance
The cable design must also consider the handling system (see Chapter 9), including:
• Deployment/retrieval scheme
• Drums, sheaves, and level wind, which may have restrictive bend radii
• Heave/motion compensation
Steel is the most common strength member material for umbilical cables. This material is usu- ally a carbon steel wire with a galvanizing coating on the outside to protect the steel from corro- sion. This material’s tensile strength, modulus, and abrasion resistance protect the cable from damage in service. Typically, an ROV umbilical will be double helix wrapped (in opposing direc- tions) to balance torque under load, thereby reducing the umbilical’s tendency to rotate as it is payed out and taken up.
Synthetic fibers, such as Kevlart from DuPont and Spectrat (UHMWPE or ultra-high- molecular-weight polyethylene) from Honeywell can reduce weight. Synthetic fibers are frequently necessary in tether cables and also in umbilical cables for deepwater systems. Synthetic fiber strength members usually require an overmolded or woven outer jacket, such as Dacront, for abra- sion resistance. A synthetic strength member is generally more expensive than steel, but the weight difference is significant. In many cases, this is the only way to get to the necessary depth. For very deepwater applications, the in-water weight of a steel umbilical will be beyond the steel’s tensile strength required to support its own weight—much less the weight of the vehicleTMS combination.
There are reasons to consider both strength member materials for different applications; these issues should be discussed with your cable manufacturer.