3.3 DISCHARGING SEGREGATED BALLAST
The SBT ballast system eliminates many of the concerns of CBT ballast
operations and provides additional freedom to optimise the ballast discharge
according to the trim, stress and freeboard constraints of the cargo
loading plan. SBT systems also eliminate the delay between arriving
at the loading berth and being able to immediately begin loading cargo.
To limit the freeboard of the ship (due to restricted loading-arm operating
envelope, etc.), some SBT ballast may be retained or the rate of ballast
discharge reduced until enough cargo is on board to maintain the required
maximum freeboard. This can only be done if the maximum draft for the
berth is not exceeded and the stress on the hull is properly monitored
A tanker will usually arrive at the loading port in a 'hogged' condition,
with the concentration of buoyancy amidship, relative to the heavy end
structures of the ship. This condition is accentuated if ballast is
discharged from amidship ballast tanks before cargo is loaded to the
midship cargo tanks. With SBT, this exaggerated stress can be avoided
by retaining the amidship ballast until loading of cargo is well under
way. The timing of SBT discharge must be carefully considered at ports
where cargo is loaded at high rates. Some crude oil loading ports can
load cargo at rates exceeding the ship's maximum ballast discharge rate.
At those ports, SBT discharge must begin early and be maintained at
a high enough rate to maintain a safe under-keel clearance. When loading
a deadweight cargo, all ballast draining must be completed before the
final cargo tanks are filled.
Before SBT is discharged in port, each SBT tank must be inspected for
contamination. SBT tank lids must be securely closed while de-ballasting
to avoid drawing in the hydrocarbon and inert gasses being expelled
from the cargo tanks.