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Background

In advance of a drilling campaign, our client required a last-minute inspection of 23 slot guides at four conductor guide levels (splash zone, intermediate levels and drilling template.). The purpose of the inspection was to confirm guide bucket dimensions and that the guide slot alignment was per the as-built documentation. Specifically, this included distances between adjacent guide flares and between tubular elements.

In addition, one of the slots (no. 3) was believed to have been damaged and significantly out of roundness. This slot was to be surveyed across 8 cardinal points to look for deformations.

We eliminated the need to go offshore using 3D SLAM Mapping and archived video.

Our client managed to locate HD video footage from a number of earlier GVI campaigns. We recieved this and used 3D SLAM mapping to render a full 3D model of the conductor guides at each elevation. Some examples of conductor guide modelling is shown below.

3D Subsea Scanned Models of a conductor guide slot

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Determination of a scale factor

Whilst it was extremely useful to have a great resource of video archived inspection data, ultimately the problem of measurement using monocular 3D mapping is that it requires a scaling factor to convert the dimensions of the model into a real-world answer.

The presence of particularly thick marine growth obscured almost all features on the asset.

We noticed that each conductor had a region of exposed steel; marine growth had been mechanically cleared from the conductor [ figure 1]. We created a 3D model of the cleared region [figure 2], then fit a cylindrical surface to the model [figure 3]. We compared this data with as-built information on the conductor to create an approximate scaling factor. (We accepted measurement error that might arise from loss of material from the conductor wall, but it was particularly low compared to the conductor diameter.)

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Figure 1 - Digital stills image of cleared conductor surface

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Figure 2 - 3D digital model of the region shown in figure 1.

Figure 3 - Region of cleaned conductor surface with nearly all marine growth removed numerically.  We then curve fitted this with a cylindrical model.

Figure 3 - Region of cleaned conductor surface with nearly all marine growth removed numerically. We then curve fitted this with a cylindrical model.