Design of a Dampened Stiff Leg Mooring System, Transfer Bridges, and Foundations at McMurdo Station

Abstract: McMurdo Station is the largest research station on Antarctica. The NSF runs the station and is due south of New Zealand in Winter Quarters Bay (WQB). Over 1,000 people live at the station during summer with over a hundred residents during winter. Resupply occurs twice a year. The station has historically utilized an ice pier to support offload operations. The piers are created by freezing sheets of ice with steel reinforcement, topped with a gravel surface course, and cabled to a series of bollards. Containers are offloaded on the ice pier, cross a WWII Bailey Bridge, and are stored on glacial fast ice, which is dubbed the “ice wharf.” The ice piers are critical to station resupply efforts; no alternative exists for marine offload operations.
Climate change has significantly reduced the lifespan of the ice piers from over fifteen years to three or fewer years. The ice piers have failed multiple times during offload operations, creating immediate life and safety hazards and significantly impacting station resupply. When the ice piers fail, the NSF uses a modular pontoon causeway belonging to USACE. The ice piers are expensive to construct and the cost of turning away a resupply ship at the station is even more costly. A permanent solution is needed to replace the ice piers.
A concept study was performed by NAVFAC recommending a roughly 100-foot by 300-foot floating steel barge with an ice-breaking hull that would remain in water over winter. The barge would be moored by cross-bay mooring cables and new mooring bollards and support two steel transfer bridges with landslide foundations. This new system would be taken to dry dock for maintenance every ten years with a design service life of 45 years. Preliminary mooring analyses showed the barge had radial movements over 40 feet, which the bridges could not accommodate.
There were several technical challenges in selecting a new mooring system. The bottom of WQB has significant contamination and could not be disturbed, prohibiting the installation of in-water piles. The only viable barge location option was by the existing ice wharf which had bedrock below over 70-feet of glacial ice. Hurricane Class 5 wind speeds and extreme ice forces imposed significant loads. Climate change posed challenges in a retreating ice wharf and unknown future design temperatures. Hydrocarbon contamination in the ice wharf reduced the efficacy of the ice as a foundation material. Limited access to emergency repair materials and expertise created operations and maintenance concerns. Numerous configurations were examined and analyzed. Ultimately, a stiff-leg mooring system was selected for the new mooring system. In this system, steel pipe struts with inline load cells are installed connecting the barge to precast foundations on the ice wharf. The load cells are typically used in offshore applications and dissipate energy through shearing rubber bonded to steel sleeves. A series of custom pins at each strut end creates pinned connections. The precast foundations are post tensioned together and attached to slurry freezeback piles installed 70 feet into the ice wharf.

How does this project / topic contribute to the advancement of the industry and profession?: The mooring system may be the only stiff leg system that uses offshore structural dampers.
The foundations include slurried freezeback steel piles in 70+feet of glacial ice. USACE CRREL is unaware of a comparable foundation this deep in pure ice.
The project exists as a direct response to climate changes in the region. Several project design criteria are directly related to climate change.

Does the project / topic implement new and innovative techniques, materials, technologies, and delivery methods?: The mooring system uses an innovative combination of existing technologies to produce a unique product.
USACE CRREL was unable to find another foundation utilizing the same pile foundations this deep in pure ice.
The transfer bridges have unique connections to allow large translations and rotations.

What was the most challenging aspect of your project / topic and how did you handle it to ensure success?: • Challenge: Not allowed to install anything on the sea floor, including piles, due to environmental contamination.
• Solution: Innovative stiff leg mooring system with offshore structural dampers.
• Solution: Install piles in glacial ice.
• Challenge: The bridges must regularly accommodate extreme movements.
• Solution: Bridge connections utilizing a slotted and sleeved connection combined with large disc bearings.

Who is the target audience for this paper?: A target audience is waterfront engineers, particularly those who are interested in cold climates or areas impacted by climate change.