Digital Surveys have just returned from Kristiansund, Norway after a scanning commission which formed part of the prestigious Wave Hub Development.
The Wave Hub is a groundbreaking renewable energy project to create the UK’s first offshore facility to demonstrate the operation of arrays of wave energy generation devices.
Many different devices are being developed in the UK and elsewhere to generate electricity from the power of the waves. After the devices have been tested as prototypes elsewhere, the Wave Hub provides an area of sea with power grid connection and planning consent where arrays of devices can be operated over several years.
CTC marine of Darlington (part of the Trico Group) were awarded the contract to provide installation and trenching services for the Wave Hub. The project involves using CTC Marine’s 2000tn carousel lay system and T2 trencher to install the 25km subsea power cable, the wave hub and its 300m connection tails. The subsea power cable is then connected to the ‘wave hub’ which allows developers of wave energy converters (WECs) to plug into the national grid and so conduct full scale trials of their devices.
From icebreaker to cable layer
CTC marine operate their own fleet of vessels but due to a high workload they chartered the Nordica from the Finnish Government for this project. The Nordica is an Ice breaking ship in winter but also functions as an offshore support vessel during the summer months.
To equip the Nordica for the project, the CTC Marine carousel cable laying system needed to be attached to the ship’s deck. The carousel, which weighs approx 50 tonnes and is capable of carrying 2000 tons of cable, needed to be precisely positioned on the ships deck on a fabricated reinforced grillage.
Digital Surveys, who have worked with CTC on numerous similar projects, were asked to carry out an initial as-built laser scan survey of the ship which would allow CTC engineers to design the grillage shims and enable precise positioning of the grillage to the deck. Traditionally this survey would have been carried out with a digital level and would consist of basic points taken where the grillage would sit. It was decided that far more detail could be captured using Digital Surveys laser scanner.
Digital Surveys opted to use their ScanStation 2 for the project. They have since invested in a phase-based solution and, although ScanStation is slower, it produces very clean data and is a very versatile instrument.
The logistics of flying the equipment out to Norway were the hardest part of the project. Even though protected by a heavy duty flight case, it did not instill much confidence for the survey team to watch overzealous baggage handlers throwing bags off the plane. There was a collective sigh of relief all around when the instrument was opened undamaged and fired up on ship!
The team had hoped that the deck of the ship would be clean of obstructions, but unfortunately a lot of welding and maintenance working going on, which had to be worked around. It was decided to scan the centre of the deck where the grillage would sit first and then if time allowed pick up any additional detail later. Scanning was carried out at a density of 10mm at 25m. As the ship was in the water it was decided to switch off the Scan Station’s dual axis compensator. The dual axis compensator is used to adjust for any small movements, making it a very accurate survey tool. If it is moved too far off level it kills the scanning process. As the ship was sitting in water and had a large crane operating on deck, it was obvious there would be some motion. By scanning without the compensator on the instrument would move with the ship and also have the added bonus of scanning at a faster speed.
As there would be plenty of overlap between scans, it was decided to use cloud to cloud registration, thus speeding up the data capture process. In all six 360 degree scans with colour images were taken in approximately five hours onboard, giving a very good coverage of the whole deck.
Point Cloud
The scan data was cleaned and registered using Leica Cyclone 7 software. The main deliverable required by the client was a grid of levels across the ship’s deck.
To achieve this, the point cloud was first opened up in AutoCAD using Leica Cloudworx 4.1 plug-in. The deck needed to be levelled first to take into account the declivity of the ship sitting in the water. This was done by drawing a horizontal and vertical lines across the deck intersecting on the centre line of the ship.
The as built plans were then imported and overlaid and aligned so that the origin of the scan data could be set to the ship’s datum and the intersection of the poly lines that had been drawn.
The two intersecting poly lines created in AutoCAD were then exported using the COE plug-in back to Cyclone. These poly lines could now be used to create a new co- ordinate system in Cyclone setting the origin in Cyclone to the datum of the ship.
Once the co-ordinate system was set, all the unwanted information on the deck was stripped away giving a relatively smooth surface. Cyclone’s mesh option was then used to create a TIN mesh of the deck. From this a grid of vertices was generated at 0.8m, the same spacing as the deck markings.
TIN MESH
This grid of points could then be exported as a .dxf back into AutoCAD and overlaid on the existing design plans. The Design plans were also updated to more accurately reflect the as-built conditions of the ship, as a number of vents and hatches were not shown in their actual locations.
Digital were keen to present the results of the project to CTC and show them some of the additional deliverables that had been generated from the laser scan data.
One of the real benefits of laser scanning to surveyors, is the additional information that can be up sold to the client bringing added value to a project.
As 360 degree images and scans had been taken at each setup, in made sense to generate a TruView of the project. TruView is the Leica collaborative, online system for viewing, measuring and marking up scan data. Requiring just a simple download for Internet explorer, it puts the user in the scanner location and allows full view of all the captured information. The advantage of this is that you don’t need an expensive plug-in (it is free for users) and you don’t have to be an expert in CAD to view the results.
Previously CTC would take photos to document a new vessel. Although a valuable reference they don’t give the immersive feeling of actually being on the ship and seeing areas in relationship to each other, which TruView provides.
If there was an area of the deck that was not documented, a surveyor would have to be flown out to the vessel, which could be anywhere in the world, just to check a few measurements. Using TruView, CTC were able to virtually inspect the deck without the need for any additional trips back to ship. Using this system they were able to identify a number of penetrations and protrusions on the deck that they hadn’t known about. If these had not been identified in the design phase it could have significantly slowed down the mobilisation of the vessel.
Sam Taylor engineer at CTC commented: “It is the first time that CTC Marine has used a 3D scan for this type of work, opting for traditional survey methods in the past, but because of the amount of useful data retrieved, CTC is considering scanning more vessels using 3D laser scanning to create a library of information that will aid future projects.”
The point cloud data was also made available to CTC and setup with a demonstration of CloudWorx. With time, hopefully this will gain more confidence and introduce laser scanning into their current work flows.
The offshore renewable energy sector is certainly looking like a growth area in the future for dimensional control using 3D laser scanning. Digital Surveys have recently carried out scans of wind farm platforms in Denmark and have been commissioned for similar projects in Holland and Sweden. From providing accurate as built plans and models to the final QA in fabrication, many engineers and contractors are finally embracing laser scanning technology and the benefits which it brings. Digital Surveys are now using a Faro 120 phased based scanner to survey complex manifold assemblies similar to the Choke manifold recently used to close off the leaking oil well in the Gulf of Mexico – but that’s another story !!
