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For many years the offshore industry has relied on the trailing wire survey method to perform close interval surveys on offshore pipelines, the main reason for the popularity of this method is economic. Many operators have reconized the lack of quality data from such methods, but have had little in the way of cost effective alternatives. For this reason, Deepwater Corrosion began to develop the CP Snake system, now only offered through SubSpection Ltd.
The table below compares the critical elements of the two methods:
Direct comparison of cost of the two methods depends on a number of variables, but the cost of the CP Snake method will usually be within +/- 10% of the trailing wire technique, assuming that the same boat and number of operators is used, and the same requirements for system calibration are specified.
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Some operators
have started to use a remote electrode, but
the problem is keeping the tow fish "close
electrode" anywhere near the pipeline.
Environmental concerns over disposal of
copper wire into the ocean.
Survey route is usually a zig-zag over
pipeline route.
V1 - Tow Fish Electrode vs Trailing Wire
V2 - Remote Electrode vs Tow Fish Electrode
(Not used by all operators)
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CP Snake is bottom following, and has redundant systems to indicate when pipe is crossed. Survey pattern is designed lateral passes - no trailing wire is needed.
The system passes laterally over the pipeline
V1 - Snake Electrode 1 vs Remote
V2 - Snake Electrode 1 vs Snake Electrode 2
Fine Field Gradient
V3 - Snake Electrode 1 vs Snake Electrode 3
Coarse Field Gradient
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Deepwater has been in the offshore pipeline survey business since the mid-eighties. In that time, every request to perform a trailing wire / tow fish was declined. The company never felt comfortable with the accuracy of the data, given that it was hard for us to verify that we had our measurement system on the pipe, or at least on the seabed immediately above the pipe. We therefore confined our activities to more costly ROV / Diver assisted surveys, many times to the detriment of our survey department's bottom-line.
In 2000 we were commissioned by a customer to develop a suitable survey method for a pipeline in Cook Inlet Alaska. After much deliberation, even considering trailing wire methods, we arrived at the solution. The CP Snake has now been proven in many areas of the world as the newest low cost survey method. Deepwater Corrosion no longer offers pipeline survey services, but the system is available from Subspection Ltd in the UK.
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