Operation of an Underwater Tracked Vehicle for Buried Pipeline Inspection in Shallow Water
Authors: Weinhoffer D.T. , White D.G. 
Published: October 2008

This paper presents experience to date with operating the subsea crawler under such adverse conditions as shallow water and surf zones and specifically with respect to shallow buried pipeline inspection in the Gulf of Mexico. Included in the paper is a description of the systems characteristics and operating requirements with emphasis on the data collection and processing problems encountered operating off-shore.

INTRODUCTION

Shallow Buried Pipeline Inspection is now required by the Federal Government from the beach out to approximately 15 feet of water. (MLW) Mean Low Water. Precise determination of pipeline burial depth and pipeline position is required. Sophisticated post-plot maps are provided that display this information.

CONCEPT OF OPERATION

The vehicle is designed to operate in shallow waters and within the surfzone under moderate weather conditions. The vehicle is capable of following a pipeline from its shoreline position to an ocean depth of 200 feet. The vehicle moves at a speed of approximately 1.5 knots and is maneuvered by a surface support operator who is either locating the pipeline or is following a track that is defined by the vehicle's Innovatum Passive Active Locator (PAL) system once the pipeline has been found. See Figure 1.

The precise geographical position of the vehicle is determined using a differential GPS system that is integrated with a radio telemetry system that is capable of continuously transmitting the latitude and longitudinal position of the vehicle from a towed buoy or mast head mounted transceiver to the surface support ship.

The surface support operator uses a video display image produced by the Innovatum system to determine the pipeline location and vehicle heading so that he or she may be able to control the movement of the vehicle via a 1500 foot umbilical. Because of the short umbilical length, the surface support operator will be capable of retaining visual contact of the buoy or mast during the entire pipeline survey. While controlling the movement of the vehicle, the surface support operator will also monitor a video display of the ocean bottom and an obstacle avoidance sonar to avert any damage to the vehicle or pipeline.

During the survey, information such as the time, date, latitude, longitude, water depth and burial depth of the pipe is collected, displayed and recorded for post processing. During post processing, position information is corrected and then plotted to produce a two-dimensional pipeline location map and a pipeline profile. The position accuracy is improved during post processing by collecting GPS correction factors and applying them to the raw data collected offshore. Accuracy within a few meters can be obtained utilizing this unique approach.

DETAILED DESCRIPTION

VEHICLE

The crawler vehicle, shown in Figure 2, is approximately 5 feet high, 6 feet wide, and 6 feet long and weighs approximately 1900 lbs. in air and 700 lbs. underwater. The vehicle has been designed to straddle pipelines or obstacles of up to 20 inches in diameter. The vehicle is constructed of mostly non-magnetic materials so not to interfere with the pipeline tracking system.