A Novel Design of Parallel Motion Fenders and Contiguous Panels Preserves an Alaska Marine Highway Berth in Ketchikan, Alaska

An innovative parallel motion fender system combined with contiguous fender panels reduces berthing impacts and preserves an existing fender float at an Alaska Marine Highway ferry berth. One of the three berths operated by the state’s ferry system in Ketchikan, Alaska, is an end berth oriented at a near right angle to the approach channel, and the ferry is required to back into the berth leading with its quarter. The designers were challenged to construct a guide structure with a fender system that absorbed energy without hindering the vessel’s travel astern. The original solution was a concrete float with a steel superstructure supporting a series of individual panels, rigidly fixed to form an unyielding breasting face. Energy absorption components, designed using conventional standards of practice, were placed on the opposite side of the float between the superstructure and two 4-pile restraint dolphins. Soon after the completion of the structure, repeated damage occurred in the fender panels, wales, and columns, most severely in column webs adjacent to the wales. Even though the fender system and supporting structure were properly designed using the conservation of energy approach, it became apparent the berthing reactions had been underestimated. A dynamic analysis of the float system demonstrates the actual berthing force is more than twice the predicted level and it provides insight into a means to reduce the impact. Through a novel adaptation of the existing framework, the guide structure was retrofitted by replacing the original berthing face with three contiguous panels interconnected by hinges and supported by parallel motion mechanisms. This eliminated snagging risks, simplified hinge design, and prevented double contact with a belted hull. This technique can be applied to new construction as well as retrofitting existing structures.