With 32 military bases in Okinawa, Japan, and the need for fuel for military ships and aircraft, the U.S. Defense Logistics Agency (DLA), Government of Japan (GOJ) and Pond are working together to provide a solution that will save costs, time, allow larger ships to come through port, and allow locals to reclaim fishing areas in the region.
The History of Mooring The history of mooring in the oil and gas industry started in the late fifties with loading/unloading terminals using single buoy mooring systems. The Single Point Mooring System (SPMS) is a floating buoy anchored offshore. The buoy facilities loading/unloading of liquid cargo such as petroleum products to/from tanker ships. A boat landing space on the buoy deck provides access to the buoy for setting up connections and securing ships. Single point moorings are capable of handling extra-large vessels in a better capacity than certain loading docks and anchor systems due to their ability to rotate and move with the ship as the wind or sea currents turn the vessel. A ship can simply be connected to the buoy using a series of lines and connectors. This allows the vessel to pull up to the mooring to load/unload materials and doesn’t require ships to come to the port and thus save fuel and time.
Less Prone To Fishtailing A SPMS consists of a single integrated mooring and fuel transfer buoy. The SPMS buoy is secured to the seafloor with one to twelve anchor chains and anchors. The ship moors to the buoy using a hawser (large nylon rope). The vessel weathervanes under the action of forces such as wind or sea currents, which reduces the mooring load. This type of mooring requires much less room than a ship at anchor because the pivot point is much closer to the vessel. A vessel at a mooring buoy is much less prone to fishtailing (oscillating at the ship anchor connection point) than a ship at anchor. The design of this new SPMS includes six chains and anchors, and will be an upgrade to the existing SPMS that is equipped with four anchors chains and anchors.
The Vessels and The Buoy Tanker ships moor at SPMS buoys by connections made at the bow of the tanker. Fuel is transferred between the tanker ships and the SPM buoy through floating flexible hose strings connected between the vessels and the buoy. A typical SPMS includes two floating hose strings. SPMS buoys include a mooring swivel and a fuel transfer swivel. The mooring swivel allows the upper section of the buoy, connected to the tanker ship, to rotate around a stationary base. The mooring swivel also allows the tanker ship to swing freely around the SPMS
buoy in response to changes in the environment.
Typhoons and Tsunamis: The Potential Wrath of Mother Nature As you can imagine, there is a lot to consider when designing a system that deals with large ships, water and the potential wrath of Mother Nature. In Japan there is also concern for typhoons and tsunamis. The rules set by the American Bureau of Shipping (ABS) require design for adverse weather conditions up to a 100-year recurrence interval. To comply with this requirement additional wave and hydrodynamic modeling efforts to compensate for the lack of available historic water level or wave data at the SPMS buoy location inside Kin Bay, Okinawa where conducted. 100-year typhoon wind speeds were estimated for Kin Bay based on the IBTrACS database. All typhoons passing within 100 km of the site were included in an extreme value analysis to obtain a conservative estimate of the 100-year wind speed. A 100-year significant wave height offshore of Okinawa was obtained from a recent scientific publication. A peak wave period of 12 seconds was assumed for this wave height, a relatively long period for waves that are actively being generated in a typhoon. A dynamic analysis of the system was conducted and included evaluations against adverse weather events such storms, wind gusts and tsunamis. The system design loads were developed to accommodate loadings from all of these types of events.
Anchor Chains and Underwater Hoses The Government of Japan is participating in the project by supplying and installing the anchor chains and the underwater hoses which will be used with the SPMS buoy. Therefore, it is imperative that Pond’s designs and specifications include guidelines and equipment that are compatible with what the GOJ is installing. Coordination with GOJ and DLA is integral to ensure that this project is organized effectively.
Overall, the benefits of adding this new SPMS will allow larger vessels to anchor away from the shore more frequently, allowing them to save time by not docking near land. This new SPMS will also allow the decommissioning of an existing three-legged mooring that is closer to shore and occupies a valuable fishing area that can be returned to the GOJ. This new SPMS will allow the return of desired fishing waters back to the Government of Japan while allowing the U.S. to maintain its mission capabilities.