The Large Vessel Interface Lift On/Lift Off (LVI Li/Lo) system has been under development by the US Office of Naval Research (ONR) and contractor Oceaneering Advanced Technology for the past five years. The project was developed as part of the US Navy’s Sea base Concept, which envisages the capacity to establish ‘Sea Bases’ or offshore floating ports that would allow the Navy to ensure logistics flow between ships without securing a deep harbour.
A demonstrator version of the system was installed on the SS Flickertail State in 2009. This month, the Flickertail State performed tests of the system in the Gulf of Mexico, safely transferring 128 containers from one ship to another, with waves up to 1m in height. The crane’s operators were able to lift and place unobstructed containers, to lift containers obstructed on multiple sides, and to place containers into obstructed holes in a container pile.
The ONR’s specification for LVI Li/Lo calls for it to be able to maintain optimal cargo throughput rates through Sea State 4 (when waves can reach from 1.25–2.5m). It should be able to transfer cargo between two ships directly alongside each other at zero forward speed or underway at slow speed in the open ocean. Motion sensing and compensation for the ships and/or the cranes should allow safe and efficient transfer of cargo.
An ONR guide to the system explains, “The system is comprised of a crane architecture supported by a sensor suite to detect crane, payload and ship positions/motions, and a control system to automate motion compensation and optimize operator demands. The crane architecture has two main subsystems. The ‘macro crane’ consists of an eccentric arm that attaches to the ship and has the main crane housing at the end of the arm. A boom extends outward from this crane housing. The gross relative motions are removed by controlling the movements of the eccentric arm and the boom. The ‘micro crane’ is an eight-wire ‘inverted stewart platform’ hanging from a wrist at the end of the boom. Each pair of wires is attached to one corner of a spreader bar that has twist-locks to attach to the top of a standard 20ft ISO shipping container. The micro crane removes the remainder of the relative motion and matches the motion of the spreader bar to the top of the container and locks the container to the spreader bar for transfer.
“When the container is attached to the spreader bar, it can be controlled in all six degrees of freedom and does not pendulate due to the natural anti-pendulation properties of the inverted stewart platform combined with the system’s active motion control.”
Oceaneering said its OTECH division’s effort had concentrated on the science and technologies required for a crane to move large containers between ships while they are underway. The control system was proven through the use of a 1/20th scale version of the crane that included all electrical sensor hardware interfaces, including motion sensors. The Large-Scale Demonstrator reaches 85 feet and can lift a 20,000 lb container in Sea State 4, and 40,000lb in Sea State 3.
