Maritime Trade#
Methods#
To estimate the trade volume, we refer to the ship’s dimension and reported changes in draught during port calls. From Arslanalp, et. al (2021), vessels that contribute to trade in the PICs are from container ships, cargo carriers, and bulk carriers.
where:
\(D_{reported}\) = draught of the ship at report time
\(D_{design}\) = design draught or maximum possible draught, constant per ship
\(DWT\) = deadweight tonnage or total load capacity, constant per ship
\(L\) = length of the ship
- \[ \begin{align}\begin{aligned}W$ = width of the ship - $\rho$ = density of salt water - $c_D$ = block coefficient at draught D\\ The $c_{design}$ is taken from a technical report that publishes the block coefficient of ships per design classification, but the classification does not easily map with the categories in Ship Registry. However, from the $Disp$ equation, by using the design displacement, design draught, length and width which are available in the Ship Registry, we can derive the $c_{design}$ as follows:\end{aligned}\end{align} \]
c_{Ddesign} = \frac {Disp_{design}}{L \times W \times D_{design} \times \rho} $$
Fuel tankers are excluded as they are mostly involved in re-exportation or transshipment. In addition, vessels in transit, which are defined as those spending less than 5 hours in the port and without any changes in draft between the current and next port, are also excluded.
To determine the trade flow, the difference in the cargo volume on arrival and on departure was calculated. If there was no difference in volume, it was assumed that the draught value was not accurately reported. In such cases, the trade flow was replaced by the historical average net cargo volume of the vessel for that specific port. Finally, imports were identified as those where trade flow is less than 0, indicating that the departure cargo volume is lower than the arrival cargo volume, and exports were those where trade flow is higher than 0.