Cary, NC -Wind-induced vibration can fatigue the welded joints of above-ground oil and gas pipelines. For nearly a decade, Lord Corp.'s Mechanical Products Div. has helped pipelines brave the aeolian winds above the Arctic Circle and thwart environmental disasters with its tuned vibration absorbers (TVAs).
Lord's original TVA suspends a mass from the bottom of the pipeline. But these dangling dampers got in the way of migrating caribou. So when BP Exploration Inc. began planning a new pipeline from the Endicott and Badami oil fields to the Trans Alaska Pipeline, the company asked Lord for a more compact design.
Lord engineers applied the age-old lever principle to come up with a TVA that mounts on top of the pipeline, instead of hanging below it. By using a cantilevered weight, the new design is smaller, and leaves plenty of clearance for migrating caribou and people on snowmobiles. But it also offers more flexible tuning options—a big plus since each TVA must be tuned to the same natural frequency as the span of pipe it's mounted on.
Vibration results from cyclic lift forces associated with vortex shedding at or near the natural frequency of the pipe, which ultimately depends on the particular geometry of the pipe span and the distance between supports. "The longer the span, the less wind you need to excite it," explains Keith Ptak, senior engineer at Lord Mechanical Products Div. "As wind blows over the pipe, it creates turbulent pockets of air on the opposite side that alternate from top to bottom. When they alternate at or near the pipeline's natural frequency, the pipe tends to vibrate up and down."
Based on either analytical or empirical data, Lord engineers tune each TVA to match the natural frequency of a particular span of pipe. "When the wind excites the pipeline, the TVA kicks in, vibrating at the same frequency, only 180° out of phase," explains Ptak. "So in-stead of seeing the pipeline moving, you only see the weight on the damper moving up and down as it absorbs the wind's energy."
Lord designed a second-generation tuned vibration absorber that mounts on top of the pipeline, instead of hanging below it.
The old TVA design suspended a cast ball from the bottom of the pipeline with a chain composed of elastomer springs connected in series with metal plates. Tuning it required changing the ball size, or adding springs to the chain, which varied both the length and size of the system and the clearance between the TVA weights and the ground. In addition, the cantilevered TVA is a much more flexible design, explains Ptak, "since we can change the location of the weight on the arm, add weights, or change spring properties, without changing the size of the overall system."
For more information about TVAs from Lord Mechanical Products Div.: Enter 540