Dynamic Motions of Piled Floating Pontoons Resulting From Boat Wake and Their Impact on Postural Stability

Abstract

This research focuses on the dynamic motions of piled floating pontoons and their impact on a standing person’s stability. Piled floating pontoons are public access structures that provide a link between land and sea. There is limited useful data on the dynamic motions (acceleration and rotation) of piled floating pontoons to wave excitation. Similarly, there are no design standards specific to floating pontoons specifying suitable motion limits in order to maintain the postural stability of users. This research first proposes a set of Safe Motion Limits (SML) in the form of lateral, vertical and rotational accelerations in order to maintain a standing person’s stability. Both laboratory and prototype testing have been undertaken in order to record the motion response of piled floating pontoons, resulting from boat wake. The motions recorded are compared against the proposed Safe Motions Limits (SML), to ascertain the impact on a standing person’s postural stability. Extensive laboratory-scale physical model experiments were undertaken at UNSW Water Research Laboratory. Two varying width piled floating pontoons of variable draft, subjected to regular boat wake conditions were tested. Five Inertial Measurement Units (IMUs), were positioned on each pontoon and used to record accelerations and rotations. Observed accelerations and roll angles were dependent on beam to wavelength (B/L). Internal mass played a secondary role, with larger mass structures resulting in overall lower accelerations for similar B/L ratios. Increasing draft improved attenuation performance, most notably at a wave period of 3 seconds. As draft increased peak heave acceleration decreased however the percentage exceedance of the lateral SML increased. Prototype testing documenting both pontoon motions and user perceptions of motion was undertaken with motions recorded exceeding the nominated SML and users conveying levels of discomfort. Importantly, results have revealed the complex interaction between the piles and pontoon that result in peak accelerations more than six times the nominated operational SML of 0.1g. Root-mean-square accelerations were observed to be more than three times greater than the nominated comfort limit (0.02g) and angles of rotation more than double what would be perceived as safe/comfortable (6 degrees) for the mild wave conditions tested.