Transport Efficiency Gains and Capacity Conversion in Closed-Loop Sand Supply by Self-Unloading Sand Carriers Under Unloading-Point Acceptance Constraints

Self-unloading sand carriers dredge, load, transport, and belt-unload fill sand in a closed supply cycle. Shorter transport raises the return frequency, but if the shared unloading point does not recover its acceptance capacity accordingly, part of the added potential throughput is absorbed by waiting. A layered model was developed linking the capacity release at the unloading point, an equivalent acceptance state, and a finite-fleet closed cycle under a non-preemptive rule, in which the acceptance state controls only the start of the next unloading. Using 295 complete voyages of three carriers in a land reclamation project, equivalent acceptance states were identified from the waiting occurrence rate, mean waiting time, and 90th-percentile waiting time. The observed capacity transfer ratio of eight operating periods ranged from 0.564 to 0.951; two periods with two carriers each reached 0.817 and 0.564. Explaining the non-acceptance by capacity release alone would require transfer-rate ratios of at most 59.1%, 22.7%, and 13.4% of the design value in the lightly, moderately, and heavily constrained states. A 10% reduction in the entire outer-cycle time, including dredging, loading, and transport, converted 94.4%, 71.2%, and 27.7% of the added theoretical capacity into output. Under heavy constraint, raising the long-run acceptance capacity of the unloading point should take priority over faster transport.