Why does the bunker specification matter, how do you draw a representative sample, and how do you keep it acceptable in a dispute?

flowchart LR
  A["Order to ISO 8217 and the trading pattern: 0.50% sulphur, ECA limit inside an ECA"] --> B["Continuous-drip sampler at the receiving ship's manifold for the whole transfer"]
  B --> C["Mixed and divided in front of both parties; sealed and signed by the supplier and the officer in charge"]
  C --> D["Seal numbers on the signed bunker delivery note"]
  D --> E["MARPOL sample retained under the ship's control for not less than twelve months; BDN for three years"]
  E --> F["Stem quarantined until the laboratory result; irregularities protested at the manifold"]

• Specification: the ISO 8217 grade on the order and the BDN; the statutory floor is MARPOL Annex VI: sulphur not over 0.50% m/m, the ECA limit inside an ECA (regulation 14); petroleum hydrocarbons, no inorganic acid, no harmful additives (18.3).
• Why: non-compliant fuel is an offence read off the BDN; off-specification fuel defeats the purifier and damages injection equipment; non-availability is notified (18.2.4) with evidence (18.2.1).
• Representative sample (18.8.1): continuous drip at the receiving ship’s manifold over the entire transfer, attended, mixed and divided before both parties, sealed and signed by the supplier’s representative and the officer in charge, seal numbers on the BDN.
• Custody: retained under the ship’s control until substantially consumed and not less than twelve months, in a locked locker with a register; the BDN kept three years (18.6).
• In a dispute: jointly witnessed, sealed and signed; seal numbers on the BDN; an unbroken, recorded chain of custody; analysis by the appendix VI procedure (18.8.2); the stem quarantined until the result; irregularities protested before the barge disconnects.