Seattle, WA —Engineers designing a new gangway for the Port of Seattle faced a formidable challenge. They wanted to make sure that passengers had as smooth a time boarding and disembarking the cruise ships as they did on their seagoing vacations.
Their solution is an impressive unit that stands 26×110×45 ft, weighs 190,000 lbs, and also:
Adjusts itself to the height of the ship's door in face of water levels that vary by 16 ft plus an additional two feet of travel to accommodate different ship designs;
Keeps the slope of the gangway less than or equal to that required by the Americans with Disabilities Act (a slope of 1 ft rise for 12 ft length); and
Is portable for back and forth movement on the dock and for winter storage.
The completed gangway features two moveable platforms that travel vertically to keep level with the ship's door as the tides cycle. Key element in the design is a set of jackscrews from Joyce/Dayton Corp. (Dayton, OH) that control the height of the platforms. The motion does not have to be fast—typical tide changes run about 10 ft in a six hour period—but it has to be failsafe and accurate.
Designers considered two other alternatives before deciding on the jackscrews—a cable configuration and a hydraulic system. "We rejected the cable approach because it did not meet our safety requirements," says project engineer Tony Holt with the design firm Jesse Engineering. Hydraulic systems can be designed to incorporate adequate safety features, but he dropped that approach as too expensive.
The third option, jackscrews, comes in two major categories. The first is a ball bearing design, which offers a low coefficient of friction and will backdrive or backup in the face of excessive loads.
The second type, inherently safe and chosen by the designers, features an acme thread, which has a square profile and a high coefficient of friction. As a result, the device will not backdrive in the event of heavy loads or loss of power.
In the first platform set, the units are 42 ft long with 28 ft of travel and 30-tons capacity. The second set, also rated at 30 tons each, stand 28 ft long with 14.5 ft of travel.
Jackscrews also represent a direct drive approach and therefore provide better synchronization than would a simple hydraulic system. In a hydraulic solution, separate cylinders provide the driving force for each corner of the platform, and with a simple system, the potential exists for misalignment if the platform is unevenly loaded. A more sophisticated hydraulic design could compensate, but would be more expensive.
In the jackscrew design, a single motor drives the gears and shafts that move the screws, so that they are all directly linked and move in unison. The jackscrews also operate with low levels of noise and vibration—important in this application since people are walking on the platform as it adjusts.
Also key to the successful solution was the choice of materials and finishes that provide protection in the corrosive marine environment. The jackscrews themselves are made of high-grade 316 stainless steel and are supported by galvanized steel legs.
Ultrasonic sensors from Flowline, NEMA 4X-rated for corrosion protection, feed position information to a programmable logic controller to keep the gangway in position. The sensors determine the relative positions of the ship and gangway to within 0.125 inch and signal the drive motor to move accordingly. The programmable units have a sensing range of 18 ft.
The gangway uses utility power for operation but the system also features a 100 kW diesel generator to provide backup power in case of a local power failure. As an interesting sidelight to last year's Y2K preparations, the company had a hard time finding available generators and had to pay a premium to get a quality unit in the required time frame.