Oil-to-sea interfaces are a key concept in marine lubrication, and an important area of focus for both compliance and environmental performance.
Under the 2013 US Vessel General Permit (VGP), an oil-to-sea interface refers to any lubricated equipment where oil comes into contact with seawater and where there is a reasonable potential for discharge during normal operation.
More specifically, the EPA identifies several types of equipment as oil-to-sea interfaces because they are fully or partially submerged and rely on oil lubrication. These include:
Stern tubes
Controllable pitch propellers (CPP)
Thrusters (including tunnel and azimuth thrusters)
Rudder bearings
Stabiliser bearings
Propulsion pods / azipods
Paddle wheel propulsion
Thruster bearings
Wire rope mechanisms subject to immersion
Other mechanical equipment subject to immersion
In November 2024, the US Environmental Protection Agency (EPA) finalized the Vessel Incidental Discharge Act (VIDA) rules, further clarifying and expanding this definition. In addition to submerged equipment, VIDA also brings further clarity by explicitly including certain systems that may not be continuously submerged but still present a risk of discharge.
Examples emphasized under VIDA include:
Hydraulic deck cranes that extend overboard
Over-side ramps
Winches or moving equipment mounted outside the hull
Exterior hydraulic systems with potential oil loss to water
While these new rules are not yet enforceable, the VGP remains in effect, and vessel operators must continue to comply with its requirements.
Why does this matter? Because equipment at the oil-to-sea interface requires the use of Environmentally Acceptable Lubricants (EALs). Selecting the right lubricant is therefore essential, not only to meet regulatory requirements, but also to ensure equipment performance and reliability over time.
As regulations evolve, understanding oil-to-sea interfaces is becoming increasingly important for operators managing both compliance and operational efficiency.
To learn more about Lubmarine’s Environmentally Acceptable Lubricants and how they support your operations, visit our dedicated page.
