Part 1 of this article was originally published in edition 34 of Port Technology International and is available for download at https://184.108.40.206 under journal archives.
Part 1 introduced us to the challenge of improving ship unloading turnaround time and focused on single lift cranes and what can be done with them to keep up productivity as ship sizes continue to increase. A study was carried out at the Port of Virginia incorporating two tools to analyse crane compatibility and performance, a crane simulation programme and a crane monitoring programme. Part 2 now continues with the evaluation of the actual data from the study.
Evaluation of actual data
The next challenge was to record and evaluate real operational data under varying conditions to see if the simulation results could be supported. The simulation used a homogeneous bay plan, something rarely found in common practice at multi-user terminals such as Virginia International Terminals (VIT) operates. Also, a variety of delays to the operation are encountered and operator proficiency varies.
However, if a sufficiently large sample of data is taken and, as I mentioned previously, this data could be broken down to exclude certain delays, a reasonable comparison of the simulated versus actual measured cycle time could be made. Over the course of nine months this data was recorded on the Newton handheld touch screen computer, then downloaded into a Microsoft Excel file for further analysis.
Only the delays for IBCs and hatch covers, along with the single container loads and discharges, were considered in order to closely compare this data with the simulation analysis. A histogram of cycle time frequencies is presented in Figure 1. The actual data shows a difference in the mode or most common cycle time equal to the equivalent of 4.0 containers per hour. This correlates very nicely with a difference of 3.2 containers per hour for the computer simulation for the Panamax vessels, supporting the results which indicate that, at least for the 5th generation crane, we have not reached the point of diminishing returns for operator productivity.
Conclusions can be reached by taking data and doing simulations, but feedback from operators who spend many waking hours at the crane controls is critical to the equation. I interviewed several of VIT’s most proficient operators querying them on the differences they’ve seen in their own productivity as the evolution of crane size and speed has progressed. The opinion was unanimous: that although the cab was farther from the spreader, and the wires longer from trolley to spreader, the faster speeds and system responsiveness made up for any parallax or sway period differences. Responsiveness of controls seemed to be a major factor in their favorable opinion. From an engineer’s perspective, this faster response is a combination of increased accelerations and the ability of modern solid state PLCs to scan the programme permissives faster, resulting in quicker drive commands to the motors.
The operators felt that they would not be concerned about further size and speed increases beyond the 5th generation machine. They also commented that anti-sway systems designed to reduce the damping time in the spotting maneuvers were only marginally effective at reducing cycle time and that there was no substitute for experience and the ‘feel’ of the crane.