Charting a New Course
August 31, 2011
What’s the best way to measure and model a ship at sea? Such projects once were the purview of highly trained specialists using total stations and other traditional surveying equipment. Today, laser scanning technology is opening the field to others with an interest in marine applications while providing much faster and more comprehensive results to marine vessel owners and operators.
One example is the offshore dredging company Tideway, which needed to determine whether it would be technically and economically feasible to convert one of its ships--a stone dumper--into a cable layer. Blue Offshore, a marine installation contractor and cable-laying specialist, had already conducted a feasibility study on Tideway’s behalf. The converted ship, aptly named Tideway Rollingstone, would be deployed at the wind park on Thorntonbank, 30 km (19 mi) off the Belgian coast from Zeebrugge, to interconnect to the transformer station at sea. Forty-eight new turbines would be installed, each capable of producing six megawatts of peak power. When complete, the Thorntonbank would supply 325 megawatts of green electricity to the Belgian grid.
However, before the conversion could take place, a comprehensive model of the Rollingstone was required. The challenge was that accurate line plans of certain parts of the ship were no longer available, and re-measurement was required. On top of that, the ship was deployed on a major offshore project.
Tideway turned to the Dutch firm Stemar Engineering bv, a service engineering firm focusing on the process, steel and offshore industry. In April 2010, the firm had added laser scanning to its areas of expertise with the purchase of a Trimble FX Scanner. That investment proved to be crucial for the Tidway project. “When we were hired, the stone dumper was active 24 hours a day, seven days a week in the construction of the Nord Stream pipeline in the Baltic Sea,” says Mark Rood, managing partner for Stemar. “Since the daily rate of that type of stone dumper is very high, there was no possibility of the ship being held at port for us, even for half a day. We were lucky to have a Trimble FX Scanner.”
In August 2010, Stemar flew a team to Finland to begin the project. At the time, the ship was traveling between the port of Kotka, where stones were dumped in the hold, and out to sea, where the cargo was dumped. The hold could only be measured when the empty vessel was returning to port.
At 10 p.m. on August 25, Rood and his team were picked up at their hotel near Kotka, about 140 kilometers (87 miles) east of Helsinki on Finland’s southern coast. They took an hour and a half ride to the wharf, where they waited another hour until the pilot boat arrived to take them to the Rollingstone.
“It was just before 5 o’clock in the morning when the pilot boat pulled alongside the stone-dumper,” Rood says. “The entire cargo was poured into the sea, and a ladder was thrown down to let us climb onboard with 25 kilograms (55 pounds) of equipment. We had already calculated that six scans would be needed to measure the entire hold, so we got to work right away. Unfortunately, our stand had not been flown over, and we first had to find one on the ship, which luckily proved not to be difficult.”
Rood and his team scanned the cargo area nonstop from the moment they climbed onboard until they returned to port. The scanner’s 360-degree-wide and 270-degree-high field of view captured every detail of the ship, and its data capture rate of 216,000 points per second allowed the team to complete the project quickly. “By the time we finished the last scan, the ship was docked again,” Rood says. “It was 7 o’clock in the morning, and the conveyor belts were pushed above our heads to dump a new load of stones into the cargo hold.”
While the stones were being dumped in the hold, Stemar’s team took an additional eight scans of the sides and stern of the ship. In the process they made an outflanking movement and covered a U-shaped track with a path length of 80 meters (262 feet). The entire job was complete before the ship was loaded again, allowing the Rollingstone to maintain its tight schedule.
The Trimble FX Scanner does not use compensators, and this turned out to be an advantage on the ship. “Devices with dual-axis compensators can consume extra electricity,” Rood says, noting that the scanner is set up on the ship itself, so the rolling and pitching do not affect the results. “A stabilization system for our application was totally unnecessary, and we were able to use the battery for scanning alone.”
Back at Stemar’s office in Alkmaar, the Netherlands, the team processed and recorded the data using Trimble RealWorks Software. The registration involved the seamless connection of the different scans using targets that had been set at the beginning of the measurements on the ship. Stemar also used the software to view the data and convert the point cloud into 3D CAD models or surfaces that allowed designers to work in the latest CAD programs. In parallel with this effort, Stemar used the Trimble LASERGen CAD plug-in to utilize the point clouds inside AutoCAD. The CAD plug-in can be used for designing and checking to ensure that the new model fits the existing conditions, which provides added confidence and lowers the risk of costly rework.
The 3D model of the ship compiled from the 14 scans was both very clean and highly accurate. “We used the thickness of the steel hulls to check this, and according to our scans, the steel panels were 17 millimeters thick, while the actual thickness was 20 millimeters,” Rood says. “Our measurements were therefore accurate to 3 to 5 millimeters. Given the circumstances, that is pretty impressive.”
The planned conversion of the stone dumper to a cable layer was fairly substantial. At the place where the panel and the crane are located, a 4,000-ton carousel was to be fitted with electrical cables. To unwind this carousel evenly, a tensioning device also had to be provided. Space was to be made for a diving robot, which meant that Stemar’s brief visit was not limited to measuring. “Blue Offshore asked us to make a new deck layout and provide all the associated modifications to the ship,” Rood says.
Stemar’s design plans served as the basis for a detailed cost estimate. Although developments in the market led Tideway to abandon its plans for the Rollingstone, the project served as a valuable model for future open-sea measurement applications. “The Trimble FX Scanner is perfect for these kinds of measurements,” Rood says. “Everything in the field of view is measured with the same accuracy. Sometimes details that seem irrelevant during the scan prove to be very important in the design phase. With the Trimble FX Scanner, you can be assured that you will never be short of measurements.”