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Autonomous Rope and Riser Inspection System for Floating Production Platforms
Crawler Vehicle Concept Will Provide Less Expensive Option For Performing Frequent Inspections of Subsea Riser or Rope

Bob Christ
SeaTrepid International LLC
Robert, Louisiana

Ian Crowther
Senior Vice President and General Manager
WFS Technologies Subsea Division
Edinburgh, Scotland

Art J. Schroeder Jr.
Energy Valley Inc.
Houston, Texas

When risers break, production revenues flow out of containment and into the open ocean—a situation sure to be followed by a visit from angry regulators. When mooring ropes break, the safety of lives and facilities are immediately at risk. If a means of close, constant and cost-effective monitoring of the integrity of the ropes and risers becomes possible, the life of a project can be extended based upon sound engineering data. Furthermore, centralized decision making based upon automated real-time information is critical for managing modern field operations.

Currently, a boat-deployed, free-flying ROV equipped with a video camera is predominantly used to inspect the moorings and risers of deepwater moored production facilities. This method is costly, inefficient and substantially weather-limited. Further, the use of a noncontacting vehicle introduces inaccuracies into the inspection process due to inconsistent vehicle standoff, thus producing unreliable inspection results.

The effectiveness of a contacting- or mechanically-crawling vehicle over a free-flying ROV is obvious. This method allows for the delivery of various nondestructive testing devices, high-tolerance gauging of rope thickness and diameter, and full-coverage, acoustic-optic-radiographic imaging sensors. This technique also allows for operation from the production platform, saving significantly on deployment costs. This test method, which involves the concept of a 'sleep-at-the-bottom' inspection vehicle, controlled and powered from the platform, can be quickly and inexpensively fielded based upon current technologies.

The installation and docking process of the crawler vehicle.

The business case for the autonomous rope and riser inspection system is to provide better information at a lower cost. With inexpensive robots profiling and monitoring floating production platforms' risers and mooring ropes, any compromise in a facility's structural integrity can quickly be identified and managed before the fault becomes a threat to operational and personnel safety. With a structured monitoring approach, the facility's life can be extended, and the environmental impact of any potential release can be mitigated or avoided through early detection and repair.

In June 2010, a tethered version of a rope and riser crawler was deployed in the Gulf of Mexico, with sponsorship from Chevron Energy Technology Co. (San Ramon, California), in order to prove the concept. The test proved the vehicle's ability to swim to a rope and crawl the rope's length while making close visual and laser inspection of rope surface. The lessons from that trial will be incorporated into the latest design.

The crawler vehicle performs inspection and then redocks with the docking station.

Early Versions of the Inspection Vehicle
Propulsion and Attachment System. The vehicle houses a means of mechanically propelling itself along the mooring or riser; an adjustable latching mechanism; and a spring-loaded clasp for attaching or detaching to a structure.

The Phase 1 prototype crawler featured two radial rings of rollers arranged around an axis, with each roller placed at the 4, 8 and 12 o'clock positions. The 4 o'clock and 8 o'clock rollers are simple idlers, while the 12 o'clock roller has a rotary actuator (for propelling the vehicle along the rope) and a second roller with a linear counter (for measuring distance travelled).

Cameras. Cameras allow the characterization of the rope's internal and external structural integrity while also allowing the outer diameter to be measured through pixel counting. For the prototype of the crawler vehicle, SeaTrepid International LLC arranged four cameras into a ring formation, enabling close imaging of the rope's outer jacket. With a 35/8-inch rope, the field of view was sufficient for the entire rope. For larger ropes, the number of cameras will need to increase.

Laser. Each rope is scanned from five sides, creating a high-resolution point cloud representation of the rope. 2G Robotics Inc. (Waterloo, Canada) software allowed the integration of these five scans for a single model.

Acoustic Attenuation. SeaTrepid partnered with Imagenex Technology Corp. (Port Coquitlam, Canada) to determine a way to acoustically measure the rope. Eventually an acoustic frequency of 50 kilohertz was found to result in a consistent acoustic penetration through a rope diameter of up to 10 inches.

Navigation. The navigation package consists of a three-axis inclinometer, a magnetometer and an accelerometer (all for sensing vehicle orientation), a pressure-sensing depth gauge, an optional acoustic positioning system and a linear distance encoder for measuring distance traveled along the surface of structure. Also, a 'bump' sensor (i.e., a limit switch) is included for direction change sensing upon hitting any obstruction.

Backup Power. If power or communication is lost, the vehicle is uncoupled to or from the structure with an ROV using manipulators, a purpose-built docking mechanism or both. Batteries are stored internally.

Communication. Communication with the surface is via a wireless subsea radio frequency transducer and transformer for both communication with the docking station and for recharging the vehicle batteries. To continue this article please click here.

Bob Christ is president and founder of SeaTrepid International LLC. He co-authored 'The ROV Manual' with Robert Wernli Sr. and is an alumnus of Oceaneering's ROV program. Christ is also co-founder of VideoRay LLC.

Ian Crowther leads the subsea division of WFS Technologies. Educated at Cambridge University, he has 15 years of experience in international business development.

Art J. Schroeder Jr. is the CEO of Energy Valley Inc. Schroeder received his master of business administration with a major in finance from the University of Houston and a bachelor's and master's in chemical engineering from Georgia Institute of Technology.

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