Technical Engineer Alaska Department of Transportation and Public Facilities Juneau, Alaska, United States
Abstract: One of three facilities operated by the Alaska Marine Highway System (AMHS) in Ketchikan, Alaska is a stern-loading berth oriented at a right angle to the channel. On approach the ferry passes just beyond the berth, turns approximately 45 degrees and then backs astern leading with the vessel quarter. Just before making contact, the ferry is rotated in an attempt to land parallel to the breasting face.
The designers were challenged to construct a fender system that absorbed energy without hindering the vessel’s backward travel. Their solution was a concrete float with steel superstructure supporting a series of individual panels, rigidly fixed to form an unyielding breasting face. Energy absorption was placed on the opposite side of the float, between the superstructure and two 4-pile restraint dolphins.
Soon after completion, reoccurring damage was observed in the fender framework. The built-up steel fender panels deformed, wales supporting the panels yielded and column webs crushed at the junction with the wales. The ferries are equipped with sponsons, or belting, at the level of the car deck and these are capable of transmitting large forces over a narrow width. Even though the energy absorbing units were properly sized using the conventional balance of energy approach, it became apparent the berthing forces were underestimated.
The impulse momentum principle gives insight into the magnitude of the actual berthing force. The principle states the change in momentum of a moving object is equal to the sum of force applied for the duration of the collision. The final vessel velocity is zero so this can be restated as the vessel momentum is equal to the area under the pulse waveform. If the duration of the collision is short, the peak force is large for a given impulse. Using this principle the berthing force was calculated to be several times larger than that derived from the balance of energy method.
The remedy is to lengthen the duration of the vessel collision by relocating the energy-absorbing elements just behind the breasting face. Yet the same challenge existed as before; how to transfer the berthing energy to the fender system while allowing the ferry to back into the berth unimpeded?
The new design concept employed a series of three contiguous fender panels interconnected by hinges. As the ferry contacts and compresses a panel, the adjacent panels deflect similarly so as not to obstruct the ferry’s travel astern. Each panel is supported by a “four-bar mechanism” suspended from the existing steel superstructure. The panels remain plumb as they are compressed by the sponson and do not rotate outward, potentially damaging the hull. Forcing the panels to remain vertical also simplified the hinge design. The result is a successful system with adequate energy absorption and low reaction force that reduced the impact on the guide structure and vessel alike.
How does this project / topic contribute to the advancement of the industry and profession?: Understanding why things go wrong is the first step to getting them right.
By describing the structural damage of a facility designed using the balance energy approach, other marine/structural engineers are alerted to those situations where this standard of practice is inadequate by itself. The fender and float system is analyzed with the impulse momentum approach using a synthesized impact waveform. The results demonstrate the magnitude of the peak force and substantiate the results from a forensic analysis of the damage.
Does the project / topic implement new and innovative techniques, materials, technologies, and delivery methods?: The project demonstrates a novel solution using a simple four-bar linkage to restrict the fender to parallel motion.
The fixed link is attached overhead to the steel frame and with links of equal length, the coupler moves in a horizontal, rocking motion. The fender panels are supported by the coupler and remain plumb throughout their travel. Enforcing parallel motion eliminated twisting of the panels when force is applied off-center and reduced the degrees of freedom required of the hinge connections. Most importantly, parallel motion prevented outward rotation of the panel and accidental contact with the vessel hull.
What was the most challenging aspect of your project / topic and how did you handle it to ensure success?: The greatest challenge was devising a 100 ft. long fender face that did not tip outward or block the vessel’s sternward travel when impacted at any level along its 8 ft. height.
The solution limited the fender panels to parallel motion using a four-bar linkage that rocks in a vertical plane perpendicular to the fender line. The fender panels are supported perpendicular to the bottom link; therefore, the panels are always plumb regardless of their deflection. A pair of back-to-back cone fenders connected to the mechanism provide high energy absorption at a low reaction force.
Who is the target audience for this paper?: Waterfront and structural engineers who are responsible for planning terminal configurations, quantifying berthing loads and developing fender structural systems are the intended audience. This paper demonstrates the risks of ignoring the dynamic response of the complete fender system. The description of one such design may alert others of the pitfalls and prevent them from doing the same.
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