Double-cycling strategies for container ships and their effect on ship loading and unloading operations

Loading ships as they are unloaded (double cycling) can improve the efficiency of a quay crane and container port. This paper describes the double-cycling problem, and presents solution algorithms and simple formulae to determine reductions in the number of operations and operating time using the technique. We focus on reducing the number of operations necessary to turn around a row of a ship. The problem is first formulated as a scheduling problem, which can be solved optimally. A simple lower bound for all strategies is then developed. We also present a greedy algorithm that yields a simple and tight upper bound. The gap between the upper and lower bounds is so small that the formula for either bound is an accurate predictor of crane performance. The analysis is then extended to double cycling when ships have deck hatches. Results are presented for many simulated vessels, and compared to empirical data from a real-world trial. The research demonstrates that double cycling can create significant efficiency gains in crane productivity, typically reducing the number of cycles by about 20% and the operational time by about 10% when double cycling only below deck.