Answer
If heavy rain begins during a bunkering operation, the primary hazard changes from a standard oil spill to deck flooding, oil-water overflow, and structural loading.
Immediate Actions as Chief Engineer:
Deploy Continuous Deck Monitoring: Assign a specific watchkeeper to monitor the scupper plugs.
The "Controlled Scupper" Strategy: When heavy rain hits, water rapidly accumulates inside the coaming/deck area because all scuppers are plugged. If the water level rises higher than the fishplate/coaming, it will spill oil-stained water directly overboard, causing an automatic MARPOL violation.
The Action: Instruct the crew to manually open a single, designated downwind/aft scupper under strict personal supervision. A watchkeeper must stand directly over this open scupper with a portable VHF radio and an oil-absorbent pad. They will slowly bleed off the clean rainwater accumulation. If any trace or rainbow sheen of oil is spotted on the deck water, the scupper plug must be hammered shut instantly.
Slow Down or Suspend Transfer: If the rain reduces visibility to the point that the barge or deck manifolds cannot be safely monitored, or if the rain threatens to contaminate fuel sample drip-bottles, immediately suspend the bunkering operation, close manifold valves, and drain the lines back into the barge.