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Conductor Integrity Failures detected using Savante’s SLV-50 Subsea Laser Profiler

 

 

 

 

 

 

 

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Are you fretting about your conductors?

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Digital stills image of the marine-growth covered conductor within funnel of guide bucket.

Savante’s general-purpose SLV-50 subsea laser vernier caliper was used to support an enhanced close-visual inspection of the condition of a number of damaged conductors on a prominent platform in the North Sea, UKCS.

A preceeding annual ROV inspection indicated a number of potential anomalies within the annular gap of guide-frames used to constrain the lateral conductor motion; it was believed that the conductor wall may have become damaged as a result through fretting and progressive wear.

The use of divers had been attempted and subsequently ruled out on safety grounds; the potential for limb entrapment during caliper measurements was too high and it would have been too difficult to read the gauge accurately.

 

Savante’s subsea laser calliper; a safer alternative for diver measurements.

Savante was asked to create a solution that could safely scan the conductor surfaces from a safe distance, outside the conductor guides, and provide a quantitative assessment from a remotely operated vehicle. Two surveys were scheduled, pre and post winter to monitor effects of huge north sea swells and heave condtions.

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Why did we use a Subsea Laser Vernier?

The SLV-50 comprises a scanning green-blue laser range-finder and collects range, position and intensity of returned energy simultaneously from multiple discrete points along a fanbeam swath (multiplexed detection) using simple triangulation. 


Spatial precision is typically better than 0.1mm within a range of 1m.  With an acquisition time of microseconds, the system records complete surface profiles with a refresh rate up to 100Hz. Each profile can contain up to 4000 individual data points. 

Laser or Photogrammetry?

We provide both - giving you a goal-focussed solution. Stop worrying about spec-sheets.

Savante provides both subsea laser scanning and photogrammetry solutions; as a result we are impartial to any specific measurement methodology.

Savante selected subsea laser profiling in this application because;

  • Fastest measurement.

    • 60 profiles/sec in real time was essential for collecting data under high heave environment. The conductor was unconstrained and violently thrashing within the guide bucket. Some of the conductor Talon connectors had separated.

  • Smallest measurement package.

    • Our laser system required a clear gap of only 5mm in order to achieve a succesfull measurement. Most digital photogrammetry systems are at least 60mm diameter housings and require strobed lighting solutions.

  • Immune to eutrophic ambient light

    • Working in the eutrophic zone - lots of ambient sunlight - can degrade the capabilities of photogrammetry techniques.

  • Subsea Laser works in restricted access

    • photogrammetry solutions require multiple views of an object from a range of angles in order for the geometry calculation to work efficiently. We determined that the best results would be achieved from a single view point.

    • The principle of photogrammetry relies on the detection of mathematical “corners” or “features” in an image. At high angles of incidence (the camera is practically looking along the length of the conductor) the error margin can build up and exceed that of laser triangulation.

    • Eliminated the requirement for strobed lighting modules.

Fast sampling is critical in the inspection of dynamic objects using a non-stationary platform. 

The SLV-50 collects cross-section profiles with refresh rates of up to 400Hz; almost 15 times faster than the human eye and more than capable of capturing measurements in this application.

In this specific project, it was reduced to 60Hz due to bandwidth limitations on the ROV.

A bespoke frame-work was fabricated to aide the positioning of the laser tool under high heave conditions. The front of the framework was profiled to conform with the external surface of the conductor; directing the laser swath vertically along the conductor within the annular gap. All the pilot needed to do was to vert upwards and drive forwards into the conductor to maximise the measurement opportunity.

During the measurements, a heave of over 5m was noted in the splash zone. Despite this, measurements wre successfully collected over all 20 conductors.

 

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Visual Record and Laser Topographic Profile Data Analysis

An onboard video camera, synchronised to the laser profiler, was used to provide a real-time video feed ensuring that each individual laser profile could be assigned to a single corresponding digital stills image.  An exemplar image from a PAL-standard colour camera is shown in figure 2. This would prove invaluable in the identification of damage.

Figure 2: Digital stills image showing fretting, surface distortion and cracking of 28" conductor casing.

Figure 2: Digital stills image showing fretting, surface distortion and cracking of 28" conductor casing.

Figure 3 3D graph showing topographic profile over area shown in figure 1

Figure 3 3D graph showing topographic profile over area shown in figure 1

Combining topographic profiles with photography has proven valuable in providing context to surface measurements; areas of wall loss, cracks and conductor connector failure can be distinguished accordingly.


The output from the SLV-50 profiler is a package of coordinates from a reference datum point on the instrument (it lies roughly within the optical window of the laser projector).  Figure 3 presents a topographic map corresponding to the location shown in figure 2.

 

Once a laser topographic profile had been captured, it was possible to perform point to point distance measurements or to apply curve-fitting methods for modelling and post-processing purposes. .

 

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