Development of Roll Stabilization for the Floating Dock at Port of Long Beach Fireboat Station 15

Abstract: At the Port of Long Beach (POLB) Fireboat Station 15, a pile-supported Boat Bay provides a berth for mooring a fireboat and a floating dock to load/unload staff and equipment. The dock is 96-foot-long, 12.5-foot-wide and 5.67-foot-deep concrete pontoon with a draft of 3.67 feet. A steel frame superstructure supports a fiber reinforced plastic deck elevated 3.36 feet above the pontoon approximately matching the fireboat deck elevation. The dock is anchored with guide piles at both ends which allow dock motions in heave, roll and pitch. An aluminum gangway provides access to the dock from the Boat Bay deck.

The Boat Bay is next to POLB main navigation channel and exposed the traffic. While the dock exhibited negligible roll motions in typical wind-wave conditions, it exhibited excessive roll motions when impacted by vessel wakes. That was the case when a US Coast Guard (USCG) patrol boat, in response to a distress call, passed at full speed at approximately 800 feet away and about parallel to the dock generating an estimated 2.5 feet high, 4.5-second wake. The wake propagated towards the dock impacting it broadside and generating excessive roll motions that resulted in the derailing of the gangway toe ramp and wheels. The dock also exhibited large and long-duration roll motions when heeled over by the 108-foot long fireboat berth swaying on a slack mooring. These events prompted the POLB to investigate methods to mitigate the dock’s roll motions. Among the methods considered, the inclusion of a bilge keel was favored resulting in a recommendation for further analysis by means of Computational Fluid Dynamics (CFD) modeling.

To determine the efficiency of the bilge keel, free decay roll simulations of the Existing and Modified docks were performed, and resulting corresponding roll motions, natural periods and damping ratios compared. The Modified dock included a 3-foot deep full dock length bilge keel on one side. The CFD model used was OpenFOAM and interFoam, the solver for floating structures. A 200 x 200-foot domain was setup with a water depth of 33.5 feet. The mesh size was 2 x 2 feet in the horizontal and 1-foot in the vertical, gradually refined to 1/4 x 1/4 x 1/8-foot around the dock to capture large-scale eddy shedding. The k-omega shear stress transport turbulence model was used to assess the energy dissipation and a smooth boundary condition around the dock was assumed.

The simulations showed that the damping ratio of the Existing dock was 0.048 and the natural period was 5.3 seconds, which is very close to the 4.5-second patrol boat wake period, and which explains, with the low damping, the large and long duration roll motions that the Existing dock experienced. For the Modified dock, the natural period was 6.4 seconds, and the damping ratio was 0.121. At 2.5 (= 0.121/0.048) times more damping than the Existing dock, the bilge keel was considered an efficient means to reduce the dock roll motions and the dock modification was implemented successfully.

How does this project / topic contribute to the advancement of the industry and profession?: The use of Computational Fluid Dynamics (CFD) modeling for the estimation of floating vessel motions constitutes a significant advance over traditional methods. At levels of detail consistent with the level of design to be achieved, the vessel geometry and mass properties, fluid properties, and site conditions effects (waves, bathymetry, boundaries, etc.) are represented and modeled.

Does the project / topic implement new and innovative techniques, materials, technologies, and delivery methods?: Yes. While a mature technology, CFD is a novel application in floating dock design, rarely used by fabricators, which reduces the level of uncertainty in dock response potentially reducing the risk of damage and accidents. Furthermore, CFD is a great tool to understand the performance of floating docks when subjected to waves with different periods, thus helping in the preparation of specifications.

What was the most challenging aspect of your project / topic and how did you handle it to ensure success?: The accurate prediction of dock motions, like for any other floating vessel, highly depends on an accurate definition of the mass properties (mass, gyradii, etc.). An accurate mass model of the dock was developed by the dock fabricator which allowed to compute the dock motions with high accuracy.

Who is the target audience for this paper?: Consulting and port engineers, small-craft harbor operators, and floating dock fabricators.