First of all, majors’ approvals or rather their absence negatively affect tradability of the vessel, i.e. vessels which do not have an approval simply cannot carry products of that oil major.Secondly, an approval is a matter of status,...
Per Lord Justice Longmore in Transpetrol Maritime Services Ltd v SJB (Marine Energy) BV "Rowan"  EWCA Civ 198 at para 18:
That leaves open the question of the meaning of "TBOOK". It can be said with some force that it is not much of a promise for an owner to say at the time of the agreement that his vessel is approved by specified oil companies "to the best of his knowledge". He must, after all, know perfectly well at that time whether his vessel has been approved or not. For this reason it seems to me that the promise must have a limited degree of futurity, albeit related to the knowledge of the owner at the time the promise is made. The force of "TBOOK" (in a voyage charter at any rate) must be first, that the owner has, to the best of his knowledge, at the date of the charter, procured approvals from the named oil companies and secondly that, at the date of the charter, he knows of no facts which would cause the vessel to lose the approval of those oil companies in the course of the duration of the charter.