Electrical Tester online
August 2014
TDR delivers umbilical success

TDR delivers umbilical success

01 August 2014

Keith Wilson, electrical engineer

A fault in the umbilical cable on a prototype seafloor drill seemed at first as if it would be a difficult and costly problem to rectify. By using a time domain reflectometer (TDR) however, freelance remote operated vehicle (ROV) pilot and technician Ian Jary was able to locate the fault quickly, accurately and conveniently, which meant that the time needed to make the repair was much less than expected, as was the cost of the remedial work.

ROV expert Ian Jary was working for Gregg Marine, a part of US-based Gregg Drilling, on the new and innovative Gregg Marine Seafloor Drill (SFD), which was at the time undergoing trials in the Indian Ocean approximately 120 miles off the west coast of Australia.

The Gregg SFD has been developed to provide an efficient and exceptionally cost-effective alternative to ship-based drilling rigs for carrying out seabed sampling and core penetration testing at depths up to 150 m. The SFD is readily transportable as, when it is disassembled, it fits into three 20-foot sea containers and it can also be used for carrying out seafloor surveys at depths up to 3,000 m.

During the Indian Ocean trials, the SFD was remotely operated from multipurpose platform supply and support vessel Greatship Mamta via a 3,000 m long hybrid fibre optic/copper umbilical. At an early stage of the trails, problems were experienced with the fibre-optic cores of the umbilical, but these were quickly rectified. Later, however, several of the copper conductors showed a dead short.

The usual procedure for location a fault of this type is to use the half-split method, which involves repeatedly cutting sections of the umbilical in half until a short section containing the fault is found. After this the sections, excluding the one with the fault, are spliced back together. This process is slow, costly and often results in an umbilical with an undesirably large number of splices.

In search of a better solution, Ian Jary suggested that it might be possible to locate the fault using a Megger TDR500 time-domain reflectometer (TDR). This compact handheld instrument launches short pulses into copper cables and, if there is a fault on the cable, this reflects some of the energy from the pulses back to the instrument. By measuring the time between the launch of the pulse and the arrival of the reflection, the TDR can accurately calculate the distance to the fault.

Unfortunately, Ian Jary had one small problem. While he had access to a TDR500, he had no operating instructions! An email to the Megger technical support department elicited a quick response in the form of a pdf instruction manual, and TDR testing on the faulty umbilical was soon underway. In a very short time, the instrument indicated that the fault was 270 m from the end of the umbilical and subsequent dissection of the cable at this point confirmed that the measurement was accurate to within less than a metre.

“In the world of marine exploration, time is not just money, it’s a lot of money” said Ian Jary, “so the speed and ease with which we were able to locate the umbilical fault using the TDR translated into savings that certainly run into thousands of dollars, if not tens of thousands. And the prompt response we received when we needed a new copy of the operating instructions was also noteworthy. I have absolutely no reservations in recommending this invaluable little instrument to anyone who works in the offshore sector. By finding just one fault it will pay for itself many times over.”

Tags: cable, fault, Gregg, location, ROV, SFD, TDR, umbilical