The first cold front of the season swept through the Gulf Coast last week, trailing rolling thunderstorms and ushering in a warning of a familiar challenge for shippers and terminal operators: draft restrictions and negative water levels.
At the Port of Houston for example, draft limits are set using a “zero tide” reference based on Mean Lower Low Water (MLLW) (essentially, the long-term average of the lowest daily tide over a 19-year cycle.) When storms push through, water levels may dip below this reference point, falling to 'negative water level', limiting vessel maneuverability and reducing under keel clearance (UKC). Operators account for such risk by building extra cushion into load plans and closely watching PETSS (Probabilistic Extra-Tropical Storm Surge) water-level forecasts.
Failure to account for these changes can quickly escalate into costly disruptions - from scheduling delays and demurrage charges to reduced cargo volumes and hull damage if run aground. Additionally, interference with seaports such as the Port of Houston, where an average of 1.2 MMb/d of crude is exported daily, is not ideal. Although last week's water level warning issued by the Houston Pilots may have been mild, the real test will come as winter tightens its grip in the months ahead.
