Where Oil and Water Could Well Mix: Laser Scanning

Offshore oil production facilities provide tremendous dimensional control challenges throughout their entire lifecycle – but there is a solution.

During construction, tolerances must be closely managed between topsides and hull structures even though they are often built in different facilities.

The laser scanning process can be used to validate the entire jobsite geometry including locating tie-ins and anchor bolt locations and verifying footing dimensions and positions. In the case of damage from a storm, the structure must be rapidly documented to aid in repair or decommissioning.

Laser scanning can be used to provide measurements on land prior to subsea installation which will, when combined with acoustically acquired position data about the installation point, eliminate the need for much deepwater measurement while providing an accurate first-time fit.

The new laser scanners often called terrestrial, long range or “large-scale” laser scanners, work by projecting a laser beam onto an angled rotating mirror that reflects the beam to the object being scanned, while the entire unit rotates around a vertical axis. Thus the unit scans 360° in a horizontal plane and 320° in a vertical plane. The net effect is that everything the scanner can “see” within a sphere of a 76m radius (approximately 250') can be digitized. The laser scanner can take accurate measurements even in the dark.

When the beam hits the object being measured, the beam is reflected back to the scanner. The distance of the object being measured can be determined by either of two different methods. The first, called “Time of Flight” (TOF), measures the time that it takes the laser beam to “fly” out to the object being digitized and then return to the laser emitter and multiplies this by the speed of light. The second method, called “Phase Shift,” involves projecting constant waves of varying length and measuring the difference, or shift, in the phase of the reflected beam in relation to a reference signal kept at the laser scanner.

For example, the long-range scanner that GKS Inspection Services uses splits the laser beam into three component parts operating on three different modulation lengths, 76m, 9.6m, and 1.2m. The distance of the reflecting object from the scanner is determined by identifying the location of the reflection on the 1.2m cycle. The cycle on which the reflection has occurred is identified through its registration on the longer wavelength cycles. Due to subtle differences, phase shift scanners acquire data faster and are slightly more accurate than TOF scanners.

Long-range laser scanners build a 360° data set, or “point cloud,” that records all objects within the range and sight of the laser scanner. Grayscale values are applied to the X, Y, and Z coordinates based on the strength of the returned laser signal. This gives the data the appearance of a black and white photograph. Data registration spheres can be placed within the area to be scanned and the scanner can be repositioned to capture objects out of the original line of sight or out of the original range of the scanner. The individual scans can then be linked together using the registration spheres to complete the 3D point cloud. The software used offers a variety of additional options such as overlaying digital images on the top of the point cloud to provide a color image for enhanced realism.

Laser scanning technology is timesaving, clocking in at 100 times faster than conventional scanners, collecting 8 megapixels of data in less than one minute, so even the largest scan scenario is time-efficient. Non-contact laser scanning means that no detail is omitted or disturbed.

Using the highly accurate 3D scan data generated with long-range scanning, engineers and designers can create CAD models of actual site conditions by processing the scan data into universally useable CAD models.

Long-range laser scanning systems are portable and quick to set up, so scanning can be performed in environments where health and safety issues must be considered.

Depending on the need, the raw point cloud data can be exported into a variety of formats, including ASCII, DXF, DWG, IGES, and STEP. The raw scan data can also be optionally post-processed into many other deliverable data file formats including AutoCAD, SolidWorks, Pro/Engineer, Catia, UG, Ideas, and more.

About GKS

GKS Inspection Services has been a leading provider of 3D laser scanning, dimensional inspection, long range scanning, and CT scanning services for over 25 years.