Scanning Old IronsidesIn October 2004, Meridian Associates Inc., a planning, civil engineering, land surveying and landscape architecture firm in Danvers, Mass., laser scanned the spar deck (the upper deck) and the deck's 33 supporting beams of the USS Constitution. This ship, the oldest commissioned warship afloat in the world, is fondly known as "Old Ironsides." The scanning of the Constitution is a story about how 21st century technology is being used to preserve an 18th century icon of naval authority.
Launched in 1797 from Boston with a mission to stave off Barbary Coast pirates, the Constitution won engagements with the British in the War of 1812, hunted slave ships off the west coast of Africa, and served as an active unit in the U.S. Navy until 1881. Today, the warship's enemy is rainwater that accumulates on the ship's spar deck, then drips down the ship's structure. Fresh water combined with wood results in rot, which spells death for most of yesterday's ships. As wooden ships age, they sag and experience an arching of the ship called "hogging." For 1992-1996, the Constitution was in dry dock and most of the 141â2 inches of accumulated hogging was removed. However, the camber (convex curve) in the spar deck was flattened out, resulting in water accumulation on the deck.
To prevent further damage from fresh water leaking into the ship's structure, the Naval Historical Center Detachment Boston (NHC DET Boston) decided to replace the spar deck and restore the deck camber. This requires first locating the actual centerline in order to establish the new camber line. New beams will then be fabricated from laminated white oak which, when installed, will support the new spar deck with the desired camber. Meridian Associates is contracted to deliver dimensional information to support design, fabrication and construction for this project.
Meridian Associates digitally scanned the spar deck with a Leica Geosystems HDS (San Ramon, Calif.) HDS2500 instrument; the company compiled a total of 31 scans conducted over two days. The deck beams supporting the spar deck were scanned using a Visi Image (Houston, Texas) 3Dguru scanner (21 scans in all). Survey control was established using a Leica Geosystems (Atlanta, Ga.) reflectorless TCRA1105 total station. The instrument's measurements were made by laying out a base line from stem to stern on the spar deck. A base line coinciding with the spar deck was then established on the gun deck immediately beneath the spar deck. Connection of the base lines allowed scan data from both instruments to be unified.
Meridian Associates' partner Don Bowen explains that the Leica time-of-flight scanner was preferred for the spar deck work because of its longer reach-the Constitution is 204 feet stem to stern. The Visi Image amplitude-modulated continuous wave scanner was preferred below deck because of its speed and dense image capture. Bowen explains that congested conditions on the spar deck from cannon and rigging deemed it an ideal application for scanning. Capturing the data with a total station alone would have entailed up to two weeks of field data collection, which was not practical.
Contracted deliverables for the scanning work included a 3D AutoCAD model of the deck from which the ship's centerline will be determined as well as drawings of the deck beams and dimensional detailing. A preliminary model was developed in Leica HDS Cyclone 5.1 3D point cloud processing software.
Using the original drawings to design replacement deck beams was not an option. According to Richard Whelan, director of the NHC DET Boston, most of the original drawings were destroyed in a fire in 1812. As-built drawings prepared in 1927 show conditions several decades ago, but these have been altered significantly by time, weather and subsequent structural modifications undertaken to preserve the vessel.
Whelan expects the spar deck replacement project to be finished by October 2009, in time for the bicentennial commemoration of the War of 1812. Why did he elect to use laser scanning on the Constitution? "We take care of her using the best methods available," he said. "We're adopting technology to make the ship last another 200 years."
Outlook 2005Last year 3D laser scanning had its best year ever. Prelimi-nary results from Spar Point Research LLC's annual survey of the laser scanning market indicate that more scanners were sold last year than ever before. The anecdotal evidence also says the existing fleet of scanners is working harder than ever. Some users are so busy with their instruments that they're purchasing second and third scanners. Service providers that previously rented machines are now buying their own.
Laser scanning is a technology that really shines when labor is expensive, capital is scarce, schedules are tight and appetites for project risk are low. Five trends are predicted for 2005:
1. Stable prices for hardware
It is doubtful that scanner prices will come down significantly in the upcoming year. Service providers who own their own scanners don't want to see price erosion-that would only make it easier for new competitors to enter the market. Scanner providers want to sell all the six-figure scanners they can. Consumer surplus is what economists call the difference between the maximum a customer is willing to pay for a product and the amount actually paid; there is little evidence that scanner providers are in a big hurry to expand this surplus. Most have significant development costs to recoup.
2. Increased involvement from asset owners
A small but growing number of production asset owners who understand the values that can be delivered by 3D laser scanning have started to insist that laser scanning be used on their capital projects, particularly revamp projects where rework is paramount. At stake is not just the capital cost of fixing flawed or incomplete design in the field, but also outage or delayed production cost. More and more owners from a variety of industries and markets-civil infrastructure, refining, nuclear and fossil fuel power generation, chemical, pharmaceutical, offshore construction, consumer products and automobile manufacturing-specify the use of laser scanning for projects, or acquire the technology themselves. Convincing EPC (engineering/procurement/construction) contractors to use the technology can be challenging, particularly when the contractor's business model is based on cost reimbursement. It appears that the scale is tipping and that contractors will have to adopt the technology or lose the business. Predictions are that asset owner awareness of the value of laser scanning will snowball in 2005.
3. More construction applications
The business case for using laser scanning gets even more compelling when it boosts construction efficiency. Reducing surveying and engineering costs is certainly an important value, but a 10 percent reduction in engineering costs pales in comparison to reducing construction costs by the same amount. Now 3D laser scanning is beginning to be used as a tool for better construction sequencing and pre-job simulation. In some instances, scanning is useful for monitoring and auditing construction progress. Some companies are also beginning to appreciate that scanning brings a new level of accountability to three groups that have to work together to execute projects: engineering, construction and fabrication. Last year several cases surfaced where laser scanning measurements revealed discrepancies between invoiced materials and delivered materials; these measurements are the basis of subsequent claims litigation.
4. Better integration with CAD
Last year saw some design software companies advance integration of point cloud data with CAD geometry in meaningful ways. Bentley (Exton, Pa.) has had this capability for years. Intergraph (Huntsville, Ala.) started shipping SmartPlant Review 5.1, which integrates with BitWyse Solutions' (Salem, Mass.) LASERGen and Leica Geosystems HDS (San Ramon, Calif.) Cyclone Point Cloud Engine software. AVEVA (Cambridge, UK) says it will deliver a product with this capability at the SPAR 2005 conference in May. Some Autodesk (San Rafael, Calif.) customers, as voiced at the recent Autodesk University conference, are starting to see the value of scanning integration. More solutions from third parties are expected this year, making it easier for customers to use point cloud data in the future, either directly or with their partners.
5. Data skirmishes
Who owns the data? How accessible is it? Asset owners, engineering contractors and service providers are beginning to ask themselves how much they value access to the raw data produced by laser scanning devices. Some users want to decouple the decision about which post-processing application to use from the decision about which data acquisition tool to use. Some scanner vendors see their performance advantage strongly linked to their database know-how and are consequently unwilling to expose this to their competitors. Other vendors see their openness as competitive differentiation. Some service providers that want to add value beyond data acquisition are less than enchanted by the prospect of completely open data access. Others depend on open data formats to get the results their customers demand. We think it very unlikely that any new neutral data formats with enriched functionality much beyond today's ASCII formats will emerge any time soon. Any new neutral data formats with limited lowest common denominator capabilities would find only marginal support.