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Savante's SLV-80 and DLS-550 subsea laser scanners have been used to measure buoyancy block separations and create 3D reconstructions of riser catenaries.

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Subsea Laser Measurement of Riser Buoyancy Module Separation

 

Buoyancy Module Movement Triggers Laser Inspection

In 2016, Savante was asked to support an analysis of the dynamic condition of a production steel catenary riser (SCR) using data generated by our SLV-50 subsea laser profiler.  A previous IRM campaign had detected movement in the position of subsea buoyancy modules used to support and define the shape of a pair of steepwave catenary production risers.  Whilst the buoyancy modules were initially installed as separated units at specific locations along the length of the riser from seabed to a hog-bend and up to an FPSO; general visual inspection revealed general module migration and groups of up to 3 buoyancy modules had slipped and clumped together at a number of points.  (example of three buoyancy modules clumping shown in image below)

Further module slippage had been suspected through the interpretation of screen-grabs taken from further GVI ROV video footage over the winter period.  Buoyancy separation measurements were estimated using image scaling; dimensions such as riser and buoyancy module diameter were used to estimate the separation between consecutive modules observed in the field of view of an ROV inspection camera. As a result, the distribution of buoyancy and tension forces along the catenary length were no longer fit for purpose; the riser shape had changed. Numerical modelling of predicted dynamic response to simulated 100 year wave conditions, FPSO motion and environmental loading identified a potential danger to the asset.  Previously, this triggered an expensive subsea campaign using ROVS to clean both the buoyancy modules and production riser, before repositioning and re-securing the modules to new design locations.

3D Subsea Laser - But why not photogrammetry?

Photogrammetry is an exceptionally powerful 3D modelling tool and Savante is a world leader and pioneer in the application of subsea image metrology technology. The main issue with photogrammetry is that it relies on the presence of distinct optical reference markers on the asset for the tool to self localise and map (SLAM). These markers can be provided by the object itself, corrosion and discoloration features are often excellent marker choices.

HOWEVER, Under dynamic conditions in this specific instance the object being scanned (riser, buoyancy module and a thick coating of soft marine growth) actually changes shape. Under heaving, dynamic conditions, the centration and orientation of the riser through the centre of the buoyancy module varies and the shape of marine growth changes.

This can break down the principals under which 3D photogrammetry, SLAM, video odometry and structure through motion operate; at the best it degrades the resolution and precision of the measurement - at worst it can cause issues in “closing the loop” creating an almost unwrapped, warped model of the subject.

For this reason, Savante recommended the application of laser profiling for the fastest direct measurement of buoyancy module separation.

Overcoming Parallax and Camera Distortion

The client had previously intuitively recognised the effects of camera lens distortion and parallax error on the accuracy of measurements created through image scaling and instigated a methodology for controlling the collection of suitable ROV video footage; pilots were required to position themselves uniformly between consecutive buoyancy modules and "square up" the camera before capturing images.  Datums, such as the visible buoyancy module edge, needed to be located at specific locations within the video footage frame.  This was known to be burdensome and still subject to further measurement errors.

 

Measurement Errors Eliminated Using Subsea Laser Vernier

Savante's SLV-50 subsea laser vernier was tasked with confirming the separation measurements between consecutive buoyancy modules.  A manipulator bracket was mounted onto the beam of an SLV-50 and mounted in the arm of a seven function manipulator arm on a workclass ROV of opportunity.  A black-box data recording system was mobilised to the vessel, avoiding the requirement for Savante technicians to be present during the two-week offshore phase; minimising the cost of survey but importantly enabling the client to execute the inspection using a platform supply vessel instead of a high cost ROV support ship.

Subsea Laser Blackbox Reduces Measurement Time

During the inspection, ROV pilots were required to simply align the swath of the laser profiler between two consecutive buoyancy modules; measurements were acquired in realtime.  The client provided Savante with 12hr notice of the riser measurement workscope; telephone support was provided to the vessel during the early morning execution.  The client's offshore representative transferred datasets to Savante technicans at 0300hrs, with QA/QC results presented later that day.