Segregated ballast tanks (SBT) are ballast tanks entirely separate from a vessel’s cargo and fuel oil systems — with completely independent piping, no shared pumps or lines — ensuring ballast water never comes into contact with oil cargo or fuel.
Why Segregation Matters
Before SBT requirements, some vessels used cargo tanks for ballast when empty of cargo, then discharged that oil-contaminated ballast water at sea — a major source of operational oil pollution. SBT eliminates this risk entirely by design, since segregated tanks have no piping connection through which contamination could occur.
MARPOL Requirement
MARPOL Annex I mandates SBT on tankers above specified size thresholds, as part of the broader move toward reducing operational (not just accidental) oil pollution from tanker operations.
SBT and Double Hull Design
On modern double-hull tankers, the space between the inner and outer hull is often used specifically as segregated ballast tank space — combining the pollution-prevention benefit of segregation with the collision-protection benefit of double-hull construction in the same structural space.
Protective Location
Beyond simple segregation, SBT tanks on tankers are often positioned to provide “protective location” — placed where they’re most likely to absorb collision or grounding damage before it reaches cargo tanks, adding a safety benefit alongside the pollution-prevention purpose.
Exam Relevance
Segregated ballast tank requirements and their role in MARPOL Annex I compliance are core Ship Knowledge topics in deck and engine officer competency exams, particularly for tanker-focused training.