5.4.3 Crosswind Scaling
The resistance to crossflow is obviously greater for wide decks and less for tall decks. The scaling arguments in Appendix D conclude that the pen air turnover induced solely by
crosswind is proportional to HW
1.5
where H is deck height and W is deck width. At very low crosswinds, the effect of crosswind on PAT cannot be seen, as the flow is dominated by
the natural convection forces. At more significant crosswinds, Figure 5.2 and Figure 5.4 gave rise to the following correlations for the ‘standard’ 24m wide sheep and cattle decks.
Cattle Decks:
Crosswind PAT = 250 x V1.5 – 234 Sheep Decks:
Crosswind PAT = 60 x V2.0 + 10 Where V is the crosswind velocity in ms and the units of PAT are mhr.
These correlation equations are simply the equations describing the straight line tangents to the 24m deck data in Figure 5.2 cattle and Figure 5.4 sheep. The correlations take effect
in the last step of the flowcharts in Figure 5.7 and Figure 5.8.
5.4.4 Minimum Required Crosswind
The flowsheets in Figure 5.7 and Figure 5.8 show the process by which the minimum crosswind required for open decks is estimated. The steps in the process are explained as
follows: Step 1 – Required Pen Air Turnover
As mentioned earlier, the lack of useful wind statistics has led to the risk management for open decks being very different to that for closed decks. Rather than assessing risk for
given PAT, the method for open decks first adopts a risk figure, and then looks for the crosswind to give the necessary PAT.
Step 1 in HS adopts the current risk guideline limit 2 chance of 5 or greater mortality and calculates the required PAT for the livestock loading, seasonal weather variation and
sailing route Gulf or Red Sea. The weather statistics and animal factors are as described earlier. No account is yet made of the actual weather during sailing.
Step 2 – Is Natural Convection Enough? In this step, the required PAT is compared to that which would be generated by natural
convection, even taking account of the reingestion. If natural effects are sufficient, not only is there no crosswind requirement, there is also no requirement for mechanical air supply.
Step 3 – Is the Mechanical PAT Sufficient? Still accounting for reingestion, the mechanical PAT may be sufficient to meet the required
PAT. This is likely to be the case for very well ventilated ships, or more widely at cooler times of the year. In this case, no crosswind is necessary.
Step 4 – How Much Crosswind is Required? By reaching Step 4 in the decision flowchart, we know that some crosswind is required.
There are two calculations here. The first calculates the required crosswind using the correlations and scaling developed as above. It may be that the answer is very low and
insufficient to eliminate reingestion. Any significant reingestion reduces PAT locally in some areas to the zero-crosswind levels and so we apply a second calculation to ensure that the
crosswind is sufficient to prevent reingestion. The flowsheets of Figure 5.7 and Figure 5.8 are embodied in Version 2.0 and later of the HS
software.
Project: LIVE.116 – Development of a Heat Stress Risk Management Model Revision F
Maunsell Australia Pty Ltd Page 47 of 129
Final Report December 2003
6 Closed Deck Risk Estimate Calculation
The probability of heat stress or mortality for particular lines of stock being transported on a particular ship depends on:
The type, breed, coat, condition, acclimatization and weight of livestock The particular ventilation rate PAT deck of the ship they are being transported on
The time of year of the voyage The voyage route, destination port and duration of transit or stay in critical zones.
The statistical treatment of the above data has been as follows: The wet bulb temperature data are fitted to a Normal distribution Section 2.3.
The survival rate of livestock will be assumed to conform to a beta distribution Sections 3.2 and 3.3.
The calculation procedure by which the above information is used to estimate risk is detailed in Section 6.2.
6.1 Deck Wet Bulb Temperature Rise
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