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In-hawse mooring inspection project

The Operation

Following an inaugural project with a North Sea-based operator in Q2 of 2018, a further approach was made to Viewport3 to perform underwater 3D scanning of several mooring line locations at an FPSO in the Central North Sea.

Additional inspections of certain mooring links were required to satisfy ‘Class Inspection’ regulations. In this case, the inspection requirement and location nuances meant the scanning had to take place within the hawse pipe (a short sleeve within the splash zone that encompasses the mooring line at a certain depth). Atypical damage to the links in this location had been identified previously, but efforts to measure the anomalies had proved unsuccessful.

Following consultation with the customer, modifications were made to ‘off-the-shelf’ camera systems to enable them not only to fit within an extremely tight space restriction, but also to provide lighting and collect high quality data sets. We were awarded the contract to design and supply the camera systems, provide project-specific training to the dive-team and manage retrieval and verification of the data from the work-site. Following 3D reconstruction, we also provided the reverse engineering and CAI (computer aided inspection) services to the customer, enabling ‘Class’ approval.

The customer further invigorated the project discussions by telling us that a total of four companies had tried, and subsequently failed to retrieve such data in the past.

The Challenge

The industry has often faced challenges in trying to inspect mooring line links which reside within a hawse-pipe. As well as the space restriction, there are also limits in how far divers are permitted to reach inside hawse-pipes. In some circumstances, ‘paying-out’ some of the mooring line to make the target link more visible, is not an option.

As a result, we needed to design a capture system which applied the correct level of lighting and photographic integrity, whilst maintaining full compliance with the safety limitations and negating any need for the divers to ‘reach into’ the hawse pipe.

Due to the shallow depths of the mooring link anomalies, it was necessary to complete a full sub-millimetric reconstruction of the links identified and report on the remaining diameter across the bar-stock of the links in question.

During project conception, we also identified a need to perform the 3D scan on the entire mooring link, in order that bar-stock diameters from undamaged sections could be measured and used as ‘norms’ for comparison purposes.

The Solution

In hardware terms, two sets of capture equipment were designed and provided for the project:

Firstly, a modern compact digital camera with additional external lighting was configured to suit shallow-water diver operation. With resolution and photographic capabilities rarely seen underwater, this arrangement was used to efficiently and accurately capture areas of the mooring link which were not obscured by the hawse-pipe.

Secondly, an arrangement aimed at overcoming the space restrictions within the hawse was designed. It not only complied with the limits of diver reach in these areas, but also provided the lighting for correct camera operation, which was vital as the divers were only able to operate the camera remotely.

In an example of great collaboration with the subsea team, each diver on the project was made available on the vessel to undergo photography instruction and project specific training sessions with Viewport3. These sessions proved highly valuable, resulting in a data-set which fully satisfied the end-user of the data.

The Result

The project was completed successfully and met the needs outlined by the customer at project definition. Viewport3 developed several bespoke work-flows in order to deliver the best results to the customer, as well as performing reverse engineering and dimensional reporting as part of the project close-out.

Without any records available showing that this task has been completed in the past, it is difficult to provide any comparative savings indications. We believe this scanning activity to be something of a world first.

It is essential that the geometry of mooring lines is fully understood, so that remediation activities can be actioned at the most appropriate time, ensuring that the customer did not replace the lines too early, or indeed allow them to degrade past the point of usability.

The nature of the 3D data also allows the customer to interrogate the data further at a future date, without having to mobilise for subsea operations. For example, the data can be analysed in an alternative fashion, or indeed measurements taken from alternate parts of the mooring link in question.

With the North Sea being home to many extended life assets, it is very important to the operator or licence holder that IRM costs are predictably low. Such assurances provide a reduced risk profile for planned campaigns, and give the customer the opportunity to improve the value level and data quality of brown-field operations as a whole.

It is testament to the success of the project that Viewport3 have been invited to discuss the 2019 campaign, using the previous year’s data as basis for more advanced deviation analysis techniques. Deviation analysis will show the customer and related stakeholders how the geometry has changed in the time between the two scanning activities. Such data will give the customer a deeper understanding of the time-line of geometry change, which they can use to better inform future IRM operations.