Vessels are supplied different plans in accordance with various Conventions.

It's not all LPG/Carriers that have have a SOPEP Manual and this because some vessels from their Certificate of Fitness are permitted to transport ‘Noxious Liquid Substances’. Due the ability to carry ‘NLS’ cargoes these vessels shall be supplied a SMPEP Manual.

As not all LPG vessels can carry ‘NLS’ cargoes there is a minority of Eitzen Gas vessels which are fitted with a SOPEP Manual, while the majority have been fitted with a SMPEP Manual.
From the MARPOL Convention: 
Regulation 26 of Annex I of MARPOL 73/78 requires that oil tankers of 150 tons gross tonnage or more and all ships of 400 tons gross tonnage or more carry an approved shipboard oil pollution plan (SOPEP). The International Convention on Oil Pollution Preparedness, Response and Co-operation, 1990, also requires such a plan for certain ships.
Regulation 16 of Annex II of MARPOL 73/78 makes similar stipulations for all ships of 150 tons gross tonnage and above carrying noxious liquid substances in bulk: they are required to carry on board an approved marine pollution emergency plan for noxious liquid substances.
The VRP – Vessel Response Plan is a plan required for vessels trading to/from/in U.S.A.
The Oil Pollution Act of 1990 (OPA-90) and the international treaty, MARPOL 73/78, require owners/operators of certain vessels to prepare Vessel Response Plans (VRP) and/or Shipboard Oil Pollution Emergency Plans (SOPEP) approved by the U.S. Coast Guard. (OPA-90 is an US regulation)
In addition, for certain vessels carrying noxious liquid substances, MARPOL 73/78 requires owner/operators to prepare and submit Shipboard Marine Pollution Emergency Plans (SMPEP), effective January 1, 2003.