Seismic and Mooring Retrofit of Existing Mooring Wharf

Abstract: The Naval Base San Diego Mole Pier South has served as a berth for Navy vessels and is presently undergoing retrofitting to accommodate a new 700-foot Floating Dry Dock (FDD). The primary challenge for this project involved upgrading the existing facility to meet new seismic and mooring standards.

Originally built in the early 1980s, the Mooring Wharf is supported by piles and spans approximately 588 feet in length and 53 feet in width, connected to land by three 50-foot piers. The initial seismic resisting system is comprised of a combination of plumb and battered rectangular piles. However, during seismic events, the batter piles were determined to be prone to tension rupture, while the plumb piles lacked the necessary stiffness and strain capacity to restrain the structure from possible collapse.

To address this, the mooring wharf underwent seismic retrofitting utilizing seismic clusters composed of four batter piles, each oriented in a different direction. A Non-Linear Pushover analysis was conducted, modeling batter piles with nonlinear T-Z / Q-W axial soil springs and P-Y lateral soil springs. These springs allowed for the capture of soil non-linear axial movements, facilitating energy dissipation. Moreover, the analysis considered the effects of inelastic vertical displacement and rotational effects on the deck due to differential displacements, while capacity protecting the piles and deck from failure. The batter piles, engineered for pile plunging and pull-out, exhibited a notably stiffer response compared to plumb piles, all while avoiding the typical issue of pile tension rupture.

Another main concern with the seismic retrofit was limiting the seismic load exerted from the FDD. The FDD is connected to the Mooring Wharf via a rigid gripper and mooring beam system. Load determination is contingent on the failure of four seismic nuts at the ultimate fuse strength of the mooring gripper. The gripper fuse is sized to accommodate the maximum displacement of the structure following rupture, limiting the load from vessel-structure interaction.

The introduction of the new seismic cluster designed for pile plunging and pull-out yielded a significantly stiffer response compared to traditional plumb piles and led to a more cost-effective solution, resulting in savings. Given that numerous piers constructed in the 1970s and 1980s share similar structural characteristics, this approach offers a promising cost-saving alternative to conventional plumb pile seismic retrofits.

This paper will discuss utilizing batter piles designed for pile plunging and pull-out, aimed at reducing costs for seismic retrofits. Additionally, there will be discourse on implementing a fuse between the FDD and Mooring Pier to eliminate vessel-structure interaction.

How does this project / topic contribute to the advancement of the industry and profession?: The example project shows a cost effective solution for retrofit of existing structures utilizing pile clusters set within the footprint of the structure that utilize batter piles with limited soil axial capacity. This discussion highlights how batter piles can be used in a manner that provides energy dissipation while not resulting in tension rupture of the piles. Additional discussion is given to design for connections of floating dry docks that allow for limited load transfer.

Does the project / topic implement new and innovative techniques, materials, technologies, and delivery methods?: The use of batter piles with soil limited axial capacity is an unusual approach which is not common for seismic retrofits. The use of a fused connection for seismic loads on floating dry docks is also a topic of interest as floating structures become more common in seismic regions.

What was the most challenging aspect of your project / topic and how did you handle it to ensure success?: The selection of appropriate soil axial properties was critical to ensuring that the capacity of the batter piles in tension was sufficient to ensure that rupture of the structure would not occur prior to pull-out or plunging of the piles. Induced rotations at the deck were also imperative to design for. Design of the fused FDD connection required close coordination with the FDD design to ensure that allowable displacements were satisfied.

Who is the target audience for this paper?: Owners / facility operators, designers, and regulators