Wyobrażam sobie, że kupujący może nie życzyć sobie przepływania przez wąski kanał w ciemności poniewaz zrobienie tego w dzień może być bezpieczniejsze.

Ewentualnie podobnie na bezpieczeństwo może wpływać awaria oświetlenia portu.

Pozdrawiam

English term (edited): lightening

If a port is nominated which cannot accommodate the Vessel with the quantity of cargo carried, Charterers undertake to discharge sufficient cargo at a previous port or place, or into vessels or lighters, to enable the Vessel to enter and lie at such nominated port or place. Freight shall be paid in accordance with Clause 6 and lighterage shall be at the expense of Charterers. A place of lightening at sea shall not constitute a discharge port or place under Clause 19, but all time used for a lightening operation (excluding any time lost or spent by reason of any of the causes stipulated in Clauses 20 and 21) shall count against the number of running hours stipulated in Clause 18 for the purpose of calculating Charterers' liability, if any, for demurrage as provided In Clause 22. For the purpose of this Clause the lightening operation shall be deemed to commence when the Vessel is properly tied up and moored alongside the lightening vessel and to end when unmooring has been completed. Subject to the preceding paragraph of this Clause, any additional steaming and/or waiting time used solely by reason of Charterers' orders to lighten at sea shall count as laytlme or, if the Vessel is on demurrage, as demurrage.
(...)
20. Time shall not count against laytlme or, if the Vessel is on demurrage, for demurrage when spent or lost:-
(a) on an inward passage, including awaiting daylight, tide, opening of locks, pilot, or tugs and moving from anchorage, even if lightening has taken place at the anchorage, until the Vessel is securely moored at the berth or other loading or discharging place specified by Charterers;
http://www.intertanko.com/pdf/legal/bpvoy3.pdf

Time consumed due to any of the following shall not count as laytime or, if vessel is on demurrage, as time on demurrage: on an inward passage moving from anchorage or other waiting place, including but not limited to the vessel’s awaiting daylight, tide, tugs or pilot, even if lightening has taken place at the anchorage orother waiting place.
(...) [teraz fragment podobny do oryginału]
...delay in or suspension of loading directed by SELLER because of BUYER’s failure to comply with the requirements of this Contract relating to payment...
http://tinyurl.com/dftluv