Authors: Ernest D Casey – StarTrak Pigging Systems
             Ian Casey

Pre-amble:
U.S. Government regulations require Pipelines crossing inland navigable waterways 100-ft wide or wider maintains a minimum cover of 48”. The requirement is for crossings to be inspected at least once every five years. In most cases, the major operating companies conduct surveys every three years or even at greater frequency for fast flowing rivers, crossings where erosion is either a known or suspected factor or during years where flooded conditions have been experienced.

Conventional Type Surveys:
It has, in the past, been the acceptable practice to utilize diver labor in order to probe the pipelines that traverse rivers. This is carried out utilizing a dive boat, two divers and one tender, for large river crossings. Smaller crossings may be carried out using less sophisticated equipment and fewer field personnel to operate out of a smaller craft.
A diver, utilizing a marinized pipeline locator, enters the water equipped with a bar in order to probe until he feels that he has hit the line. The diver measures the depth of cover by utilizing his known anatomy measurements against the bar, either arm or his leg, then transmits the information to the surface crew who take approximate position by Differential Global Positioning System. There are often instances when the extent of cover is too great for the probe bar, or where the diver has not hit the line but feels that the extent of cover is adequate to satisfy the conditions of the inspection.

Most pipeline technicians know that it is not uncommon to miss a small diameter pipeline during land surveys, marine surveys are many times more difficult. At this juncture we would like to provide a simple illustration of a particular problem encountered in a South Texas System. The pipeline was incorrectly installed and had a huge bend up-stream of the designed crossing. This line was caught by currents during the installation process and was removed from its ditch. Divers had given this crossing a good bill of health for the previous ten years prior to the survey being carried out by the ”One-Pass” system. The profile shows the line has no cover and it was later found that barges had been scraping the pipe. The line was packed with Ethanol at a pressure of 750psi.

“One-Pass” Electromagnetic Technology:
 The “One-Pass” system was developed in response to the industry need for a system to locate and profile pipelines crossing under navigable waterways. Development started in 1979 with a crude but effective system and continues today as a total pipe survey package.

 The River Crossing System is designed to locate and profile pipelines crossing beneath rivers and produce an engineering drawing that shows both plan and profile of the crossing. The system consists of several operations, which combine to provide the user with detailed information of the pipeline crossing, including plan & profile drawings, river bottom contours and wrapping evaluation.

The system utilizes an electromagnetic receiver to read a signal radiating from the pipe wall. As with any electromagnetic signal the amplitude of the signal degrades as the distance between the pipe and the receiving antenna increases. This serves as a basis for calculating the pipe position and depth of burial, which are handled by a sophisticated. Software package. The system utilizes two distinct methods of calculating the depth of pipe, therefore allowing little chance of error. All readings are taken from the inside of the boat thus making it easy to acquire pipe depth readings without dragging any underwater equipment or towing fish along the river bottom. Differential Global Positioning System coordinates together with water depths from the same plane are utilized to obtain river depth information at one pulse per second, .in order to capture contour information.

The software program used to collect and compile all field data is named “Ariver” copyright@1998 Ian D Casey.
 

 The basic system is comprised of six elements:

1. Low frequency electromagnetic transmitter
2. Tuned receiver.
3. System console containing electronics for analysis and recording, “Ariver” package. The console is complete with 10.4” LCD screen, disk drive and unit function controls.
4. Receiving antenna
5. Digital depth (sonic) sounder
6. Differential Global Positioning System

The pipeline’s position and all data is displayed on screen as taken, also an icon referencing the position of the boat is shown.
 

OFFICE:
 All data collected during the field operation is analyzed. Utilizing the calibration factors, the data is processed in order to obtain accurate depth of pipe information together with the additional survey data also any other features which need to be included in the final drawings. The results are compiled into DXF format for insertion into the drawing. The nodes derived from the DXF file are used in order to prepare the final drawing together with the other pertinent features. The presentation is prepared in AutoCAD release 13 or 14.

FIELD OPERATIONS:
 The system requires a low frequency electromagnetic field be applied to a loop created by the pipeline and a cable laid parallel as per fig.1.
 

 The transmitter is run from a 12vdc-power supply, which can output 25-watt signal onto the pipeline. Once the signal has been applied, the receiving unit can be calibrated on a known point, which has been probed in order to verify the actual depth.
After the calibration process has been completed, the receiver is connected directly into the console unit on board boat.

The program “Ariver” once set-up will be enabled to continuously monitor and record all data from the DGPS and Water depth (sounder) in addition to the electromagnetic receiver.

 During operations, the boat will traverse the pipeline at regular intervals as shown in fig.1 until the entire pipeline has been profiled. The traverses are normally taken at 25-ft or 8-10 meter intervals. To ensure accurate results, after the first run a second run will be made after re-calibration. On a river, which may be approximately 3,000-ft wide it is normal to take up to 120 readings, but as each traverse normally only takes 45-60 seconds it does not take very long to complete.
 

REPORTING:
The resultant information provided to the client in the form of a “D” sized reproducible drawing complete with full report of findings together with a 3.5” diskette or CD. Video or still pictures can also be included on a CD.