Little Room for Error in Lifting These Huge Vessels
West-central Canada is the locale of massive oil sands deposits, but it isn’t practical to move the extracted bitumen out of the area without first doing some processing, if not full refining. A facility expansion project in Alberta involved fabrication of six huge coker vessels used in processing.
The finished size and weight of the vessels, 142 ft long, 35 ft diameter, and approximately 650 tons, precluded fabrication anywhere but on-site, since highways and bridges could not accommodate such a load (Photo 1, right).
Fabrication proceeded in a relatively straightforward manner. The vessels were built in approximately eight-foot segments, each of which consists of three 120-degree curved plates. Rollers allowed for rotation (Photo 2, right).
Eventually, fabrication reached a point where a partially (about one-third) completed vessel had to be lifted off its rollers. A bridge crane was not available, not to mention the size and capacity that would be required. Hydraulic jacks were an obvious answer.
Cradles with provisions for jacks were fabricated. (A cradle and jack are visible at the lower-center of Photo 2.) The fabrication contractor first attempted to operate the jacks via hand pumps, but “That was abandoned because it took a gazillion strokes due to the large cylinder size of the 150-ton jacks employed,” says Lyle McGowan, Territory Manager for Enerpac, manufacturer of the jacks.
An electric pump was brought in to replace the hand pump, but that revealed a new challenge. Operating manual control valves to lift the load in a sufficiently uniform manner to prevent dangerous load shifting proved extremely difficult.
Digitally controlled hydraulics solved the problem. The solution lay in a combination of traditional hydraulics with a digital controller, sensors, and actuators—“synchronized lifting.” In the Enerpac Integrated Solutions sync-lift system, a PLC controller monitors load position at each lift point to within a millimeter (0.040 in.) by means of a cylinder displacement sensor associated with each jack. Two of the four digitally controlled 150-ton jacks used to lift a one-third completed vessel are visible in Photo 3, below.
The position sensors, with mechanical construction analogous to that of a tape measure, unwind a fine wire from a spool to a fixed reference point as shown in the highlighted area of the photo at right, above. Rotation of the spool is monitored with high angular resolution, and the resulting signal goes to the digital controller, which operates solenoid valves controlling each jack as needed. All components other than jacks and hoses are mounted on a custom-built cart, shown below.
The controller’s touch screen gave the operator position readouts. The digitally controlled hydraulics afforded significant advantages:
- Increased safety—The digital controller eliminated the possibilities for confusion and error inherent in a manual control approach using multiple personnel. This is particularly significant if an unexpected event occurs, such as a load shift, and the controller also provided warning and stop capabilities
- Increased productivity—Considerably less staffing is required than for a manually controlled lift. The actual raising process can also proceed more rapidly because the status of every lift point is constantly displayed.
- Less chance of property damage—With all lift points constantly monitored and automatically controlled, concerns about overstressing some part of the vessel or encountering an unexpected load shift due to uneven lifting were eliminated.
The contractor had selected 12-inch stroke cylinders as the most versatile considering other, future applications for the sync-lift system. When total lift heights greater than a foot were necessary, an incremental lift-and-crib method worked fine. With the hardware and factory-provided training for synchronized lifting in hand, the contractor expects to be using the system regularly.
The Canadian Oil Sands In Canada, oil sands are not just a “possible energy source.” Deposits in the province of Alberta are said to contain at least 85 percent of the world’s total reserves of natural bitumen. In 2007, 44 percent of Canada’s substantial oil production came from oil sands. (Canada is the largest supplier of oil to the U.S.)
The coker vessels described in this article are for expanded mining and processing operations expected to produce 120,000 barrels of bitumen per day for an estimated 40 years.