Does wind have a significant influence on the loading of fruit ships at Table Bay Harbour?

An interesting project, which GPB was asked to carry out a few years ago, involved a study of the influence of wind on the export of fruit through Table Bay Harbour.

Table Bay Harbour is the biggest deciduous fruit export harbour in South Africa. When high winds blow, loading of perishable fruit into ships at the harbour cannot take place.

The biggest delays due to wind occur in December to February i.e. the period during which stone fruit and grapes are being loaded for export. These fruits have a short shelf life and relatively short marketing (sales) period. Delays during these times therefore have major consequences for the exporters.

It is the longer delays (more than 1 day) which start to incur significant costs which could include:

  • ship demurrage cost
  • labour cost at port - stevedoring
  • labour cost at port - shoreside
  • transfer of fruit to and between inland cold stores
  • market opportunity sales lost.

It was found that an average wind speed of more than 18 m/s (e.g. 35 knots or 65 km/h) causes a delay in ship loading at the harbour. Comparing the wind speeds measured at Cape Town International Airport with that of speeds measured at Table Bay Harbour during 1976-1996 it was found that a South Easterly wind of 10 m/s at Cape Town International correlates to the 18 m/s at Table Bay Harbour Fruit Terminal.

The graph below shows the number of ships which were delayed for more than 6,12, 24 and 48 hours due to wind for the period 1986 to 1996.

The question one can ask is why loading of a ship ceases when high winds are experienced?

Big cranes on the quayside are used to load pallets of cartoned fruit from the quay into the ship's hold. It appears that the maximum operation wind capability of these existing cranes is approximately 70 km/h. The reasons for this are that even though the cranes can theoretically operate at higher wind speeds:

  • the crane operators are unable to control swinging loads safely above 70 km/h
  • the crane operators themselves begin to feel uncomfortable and unsafe inside the cabins at these high wind speeds

Other operational reasons are:

  • The loading activities take place in the open and become difficult to carry out effectively, productively and safely at high wind speed.
  • Sea spray is forced onto the quay under certain South Easterly wind conditions. Operating under such conditions is not possible if the fruit is in open-top or other unprotected cartons.

As mentioned it is not the short periods of wind that cause the main problems but rather the shipping delays that stretch over 2-days and more.

Therefore the wind data at the terminal was analysed to see what the risk of a 2-day delay to shipping due to wind would be. It was shown that the risk is fairly low except for January and early February. As a result high capital expenditive solutions are not cost effective.

A theoretical value for the fruit at risk was determined and a number of recommendations were made on how to manage the risk. Further information my be obtained from Gerhard Potgieter at GPB Consulting.

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