There are three main circumstances where a PSCO might find that a ship is exempted from a convention requirement:
Exemptions
SOLAS 74 Convention / Ch I / Reg. 4
single international voyage and novel design
SOLAS 74/Ch III, Reg 35(a)(i)
near-neighbouring countries
SOLAS 06 Amend / Ch II-1 / Reg. 1
SOLAS 99/00 Amend / Ch II-2 / Reg. 1.4
SOLAS 83 Amend / Ch III / Reg. 2
voyages in sheltered waters and passenger ship on special trade
Note: MSC Circ 606 “Port State Concurrence with SOLAS Exemptions” recommends that if a flag Administration exempts a ship based on "short international” (defined in SOLAS ChIII/3) or “sheltered” voyages it should communicate in advance with the administration of the other States to which the ship may sail.
SOLAS 01-03 Amend / Ch IV / Reg. 3
certain radio provisions
SOLAS 99/00 Amend / Ch V / Reg. 3
navigational requirements for ships on restricted voyages
MARPOL 04 Amend / Annex I / Reg. 3
craft of certain construction and other waivers
MARPOL 04 Amend / Annex II / Reg. 4
amendments to carriage requirements due to the upgrading of the categorization of a substance
ILL 1988 Protocol Annex A Art.6
single international voyage. voyages in sheltered waters. novel design
COLREG/rule 1 (e)
vessels of special construction
COLREG/rule 38
lights and sound signal appliances
Equivalents
SOLAS 74 Convention /Ch I / Reg. 5
MARPOL 06 04 Amend / Annex I / Reg. 5
MARPOL 06 04 Amend/Annex I/ Reg 14.5
MARPOL 06 04 Amend / Annex II / Reg. 5
MARPOL 06 92/97 Amend / Annex VI / Reg. 4
ILL 1988 Protocol Annex A Art.8
STCW IX
IBC 04 amend/1/1.4
IGC 83/90 amend/1/1.4
HSC 2000/1/1.11
use of another fitting, material, appliance or apparatus or any other provision
Ships must meet the requirements of the conventions, and the flag State has overall responsibility to ensure this. The flag (or RO if delegated) may issue an exemption if this is provided for by the convention as in Para 2 above, a Flag State Exemption.
However a ship not meeting the requirements of the conventions for any other reason is deficient and may be detained.
If a ship is detained, the principle is that all deficiencies are rectified. There may be cases when it is not practicable to rectify a deficiency(s) according to the owner/master/flag/RO. It is up to the port State to agree an alternative arrangement with some form of equivalence (eg replacement of lifeboat by liferaft or emergency fire pump by portable fire pump) based on the duration, sailing area and nature of the voyage.
When the flag/RO and port State come to an acceptable agreement, this can be formalized in a document issued by the flag or the delegated RO on behalf of the flag, including when the situation has to be rectified.