When do large ships increase cargo bunching at ports? A test of the compound Poisson prediction at 1,264 ports and a decomposition of the rise in cargo per call, 2019–2026
Büyük gemiler limanlarda yük yığılmasını ne zaman artırır? 1.264 limanda bileşik Poisson öngörüsünün sınanması ve uğrak başına yükteki artışın ayrıştırılması, 2019–2026
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Abstract
It is often argued that the growth in ship size increases peak loads at ports, but this claim has not been tested on a global sample of ports and against a theoretical benchmark. The compound Poisson model, in which ships arrive at random intervals and carry cargo of random size, gives a clear prediction: the elasticity of the coefficient of variation of daily cargo volume should be −0.5 with respect to cargo volume and +0.5 with respect to cargo per call. In this study the prediction is tested with daily port records derived from satellite-based ship tracking, in an annual panel of 1,264 ports for 2019–2025 with port and year fixed effects. The estimated elasticities are −0.49 (standard error 0.015) and 0.43 (0.035). The median ratio of the variance to the mean of the daily number of calls is 0.93, so ship arrivals cannot be distinguished from a Poisson process; the excess variance comes from the inequality of call sizes. The counter-intuitive implication of the model is also confirmed: with the number of calls held constant, an increase in call size does not affect bunching (elasticity −0.06); bunching rises only when the same cargo arrives in fewer calls. In the world total, cargo per call rose by 7.2% between 2019 and 2025 and accounted for between 49% and 75% of the growth in cargo volume, depending on the end year; yet only 29% of ports fit the pattern of fewer and larger calls. The growth in call size raised daily bunching by 1.8% at the median port and by more than five per cent at 37% of ports, while in the aggregate it was offset by the smoothing effect of the growth in cargo volume. The peak-load effect of large ships is real, can be computed with a simple rule and is concentrated at ports whose calls grow without an increase in cargo volume.
Keywords
Ship size; Peak load; Compound Poisson; Port calls; Cargo per call; Port planning
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