HVUN UHKS-OMk ruwc mranMI-iu»
Typical fiber optic cables common to UCT operations (continued).
exact location. The probe may be used underwater, on the beach, or from a vessel. Depending on the strength of the tracking tone impressed in the conductors or the cable, the probe may be used to locate cables in up to 100-foot water depths from the water surface (100-milliamp signal at 25 hertz).
Detection and tracking of a cable with the probe is accomplished by interpretation of either a meter display (mounted on the back end of the probe) or audible output from a diver ear phone. The primary detection technique used in tracking a cable is called the "null" method (Figure 5-4a). As the probe is brought within range of the cable (with induced signal), the meter deflections (or audible output) begin to increase. When the probe is oriented with its axis direcdy pointing at the cable, the meter deflections reduce to a "null." Because the "null" is very sharp, this technique can be used to identify cables that are laying within 2 to 3 feet of each other.
The depth of buried cable can be established using the null mode, as shown in Figure 5-4b. A visual marker is placed direcdy above the cable (this is the position of the vertical null reading). The probe is then rotated at a 45-degree angle to the seabed and backed away from the marker until the probe indicates another null reading. By knowing the horizontal distance between the point of vertical null reading and the point of 45-degree null reading, the depth of the cable can be estimated.
In order to locate or track the cable, an appropriate magnetic field must be impressed onto the cable conductors by either the dry signal generator or the submersible signal injector. The dry signal is generally used to hard-wire either a 25- or 1,024-hertz signal onto a conductor of the cable. Access to the shore end of the cable is required when using the dry signal generator. The submersible signal injector shown in Figure 5-5 is used to inductively couple a tracking tone onto all of die conductors in the cable through a split torrodial core that is hinged together. This system operates at 1,024 hertz only.
Generally, the signal strength required for tracking with the cable can vary between 20 milliamps (when tracking within 10 feet of the cable) to 100 milliamps (for tracking from 70 to 100 feet away from the cable). If possible, it is preferred to operate at 25 hertz since lower frequency signals do not attenuate as fast as higher frequency signals (hence providing increased tracking range with the diver probe).
Once the cable is located, it should be followed to determine the location and cause of the fault. This can be done visually by divers or by using ROVs. The general location of a fault can also be determined by testing with a cable fault locator instrument, such as a Time Domain Reflectometer (TDR) (available in the OCEI).
It is important for the diver inspector to document the extent and position of all damage and potential situations which may pro-
/ Probe / orientation Cj 'and range^
Vertical null reading
Figure 5-4a. Null technique for tracking with the diver probe. Note that the probe is held vertically.
Vertical null reading
Mark position /
Was this article helpful?