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Spool Metrology with 2G Robotics’ ULS-500

20 May 2015

Background

As part of a major North Sea installation contract, Subsea 7 selected 2G Robotics’ ULS-500 high precision underwater laser scanning system for a hybrid LBL/laser scanning solution to perform subsea spool metrology. The deployment was conducted in the North Sea at a depth of 90m.

Spool Metrology with 2G Robotics' ULS-500 Underwater Laser Scanner

3D point cloud model of a spool with flange

 

Objectives

The customer’s objective was to design five spool pieces to connect a newly installed bundle towhead to five previously laid spool pieces. The mattresses covering the spool pieces were peeled back to reveal approximately 6 metres of each spool. To obtain towhead and spool piece positions and headings, acoustic LBL was used. Survey specifications, however, dictated that a 6 metre long acoustic baseline to derive at an acoustic heading was insufficient. Additional survey control was required to satisfy measurement accuracy. An underwater laser scanning method was used to obtain the required accuracy for executing a dimensional control survey of the towhead structure with flanges and quantifying the positional and heading relationship between the parallel spools.

“The end results were very impressive and the detail within the point cloud is fantastic. The operational time savings will be motivation to use this system again.”

-Pieter Jansen, Survey and Subsea Data Manager

Methodology

To save time and reduce costs, an underwater laser scanning approach was implemented. The center flange was positioned within the long baseline (LBL) acoustic array by deploying two saddle mounted transponders with control spheres onto the largest of the five spools. 2G Robotics’ ULS-500 completed 360° scans at various locations around the spools. The acquired data was then used to determine the orientation and position of the four other flanges relative to the center flange in order to accurately position each flange within the LBL acoustic array. The traditional LBL survey approach would have required an entire day of operation to install, remove, and relocate the transponders for each of the five pipes. By using laser scanning, The metrology took less than a quarter of the time of the traditional approach. In only eight scans and 3.5 hours the ULS-500 captured highly detailed 3D visualizations of the flanges, bundle towhead, beacons, control spheres, and surrounding environment. Whereas the traditional approach would have yielded only one point per beacon, the laser scanning approach generated a complete 3D visualization comprised of 14.6 million points which provided the accuracy needed to effectively design the spools.

 

 

Operation

A work class ROV (WROV) was used to attach the crane hooks to the protective concrete mats in order to remove the mats and expose the flanges and spool pieces. The saddles, beacons, transponders, and control spheres were then positioned by the WROV. Using the WROV, the ULS-500 with rotary actuator and tripod was deployed. A gripping frame was added to the standard mount location of the ULS-500 so that the WROV’s manipulators could grab the scanner to position it on the seabed. The ULS-500 was tethered to the WROV and the WROV’s fibre optic umbilical cable was used to communicate with topside operators for real-time data acquisition.

 

Results

Compared to a pure LBL approach, using 2G Robotics’ ULS-500 provided time savings and cost savings as a result of reduced vessel time. The data was captured in real-time and processed while offshore, which enabled preliminary measurements of the flange orientation and position to be obtained. The control spheres and natural features of the subsea environment were used to register the six scans to produce the full 3D model. This highly accurate 3D model provided a more complete understanding of the subsea flanges in relation to their environment and enabled critical measurements to be easily captured using a single system. Pieter Jansen, Survey and Subsea Data Manager, stated: “The end results were very impressive and the detail within the point cloud is fantastic. The operational time savings will be motivation to use this system again. There will no doubt be many other subsea situations for which highly accurate dimensional detail is required where this laser system would fit in perfectly.”

 

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Spool Metrology with 2G Robotics' ULS-500 Underwater Laser Scanner

3D point cloud model captured using 2G Robotics’ ULS-500