Marpol Annex 1, Regulation 30.4.2  requires every oil tanker delievered after 1 June 1982, required to be provided with segregated ballast tanks or fitted with a crude oil washing system, shall comply with the following requirements

• it shall be equipped with oil piping so designed and installed that oil retention in the lines is minimized; and
• means shall be provided to drain all the cargo pumps and all oil lines at the completion of cargo discharge, where necessary by connection to the stripping device. The line and pump draining shall be capable of being discharged both ashore and to a cargo tank or a slop tank. For discharge ashore, a special small diameter line shall be provided and shall be connected outboard of the ship's manifold valves

The small diameter line mentioned here is also called Marpol Line.

Marpol under unified interpretations (no 57) has mentioned the size of this small diamter line. As per the interpretation,

For the purpose of application of regulation 30.4.2, the cross sectional area of the small diameter lineshould not exceed:

• 10% of that of a main cargo discharge line for oil tankers delievered after 1 June 1982; or
• 25% of that of a main cargo discharge line for oil tankers delivered on or before 1 June 1982

Considering there are hardly any oil tankers in operation that were delivered on or before 1 june 1982, we can consider the size of the marpol line not to exceed to 10% of the size of a main cargo discharge line.