Project at a glance
- Client: Saipem, with as-built models submitted to ConocoPhillips
- Sector: Oil and gas, subsea fabrication and quality assurance
- Location: Fabrication at the Wilton site, UK. Development in the Southern North Sea, approximately 14km north east of the Kelvin platform location
- Services: 3D laser scanning, as-built modelling, QA dimensional quality assurance of fabricated subsea structures and spools
The challenge
When fabricating parts for subsea installations, accuracy is paramount. The reason is unforgiving. A manifold and its connecting spools are built in a fabrication hall and then mated together underwater, by vessel, in the North Sea. There is no adjusting on site. If a spool run does not present its flanges where the manifold expects them, the error is discovered at the most expensive possible moment.
Spools are the specific risk. A spool is a connecting pipe section, and its whole function is to absorb the real distances and angles between two structures. Get the bends wrong and the piece is scrap. The tolerance that matters is not any single dimension but the cumulative geometry of the whole run.
The conventional answer is a dimensional control survey. In the past, verification would have been carried out that way. However, these surveys only verify a limited number of points. A handful of measured targets can all be correct while the shape between them is not.
Our approach
We scanned the fabricated items in the hall, before shipment. Laser scanning provides peace of mind by verifying the build tolerance before the item is shipped to site.
Multiple laser scans are carried out across the work site from a range of vantage points, and at least 30% overlap from each scan is accommodated. That overlap is what allows the scans to be joined. Scans are registered together into a single dataset using a technique called cloud-to-cloud registration, in which software optimises and aligns the scans. On average, 40 to 50 scans can be registered by one technician per day. Once scans have been registered, the data is optimised through a process called unification, which can take 6 to 24 hours depending on the size of the dataset.
The result is a full 3D replica of the item rather than a list of coordinates. Laser scanning creates a complete measured surface, which makes it very easy to check dimensions at any point, or even to clash the point cloud against the design model. Clash detection is the process of testing one piece of geometry against another to find where they physically conflict. It also reduces the need for costly re-surveys, as everything is captured first time.
For the manifolds, the registered data was converted into as-built models for submission to ConocoPhillips. For the spools, the scan data was converted to 3D models specifically to identify the correct bends within the pipe. That is a shape problem, not a dimension problem, and it is the case where full surface capture earns its keep.
Our close range and terrestrial capability sits behind this kind of work. Within a 25m radius each scan point is accurate to plus or minus 2mm, scan resolution can be as detailed as one point of data every 3mm, and a typical scan can take just 3.5 minutes. Fabrication halls are busy, and short occupancy at each setup matters.
What we found
This service identified a number of critical errors which had not been picked up from a previous traditional dimensional control survey.
That single sentence is the whole argument for the method. The items had already been through a conventional dimensional control survey. The scan data, covering the complete surface rather than a limited set of points, found errors that the point-based survey had passed. The errors were critical, and they were found in the hall.
The arithmetic follows. A spool with a wrong bend, identified at Wilton, is rework in a fabrication shop. The same spool identified subsea is vessel time, crane time and a delayed tie-in on a live development.
Deliverables
Saipem and ConocoPhillips received accurate 3D laser scans of both subsea manifolds, as-built 3D models of those structures for client submission, and 3D models of the connecting spools showing the correct bends within the pipe.
Models are supplied in a generic format, 3D .dwg, unless otherwise specified. Alongside the models we issue the registered point cloud itself, in raw Leica .imp, .e57 or FARO .fls, so the client keeps the measured evidence and not only the interpretation of it. From every laser scan we create, a 360 spherical image is also generated at that spot, which can be used to build a very high resolution virtual tour of the fabrication yard as it stood.
The outcome
The Katy Field work established as-built scanning as a fabrication QA verification step rather than a record-keeping exercise. Scanning does not simply document what was built. It tests it, against the design model, across the whole surface, while the item is still on the shop floor and still cheap to correct.
It also settled a comparison. Traditional dimensional control had already been applied to these parts. Scanning found what it had missed. For a new offshore gas development where every fabricated item has to fit first time, that difference is the value.
This service identified a number of critical errors which had not been picked up from a previous traditional dimensional control survey.