ROB is “the sum of liquid volume and non-liquid volume in cargo tanks just after completion of discharge, excluding clingage, hydrocarbon vapours and the contents of associated pipelines and pumps”.
Claims for short out turn arise due to excessive ROB on board after completion of the discharge. Vessel must discharge all ‘pumpable’ cargo that it has in the cargo tanks. Three main factors which contribute to ROB claims:
Nature of the cargo.
Heating of the cargo during carriage and at discharge.
Unpumpable cargo: sediment/sludge.
Nature of cargo: Traditionally, a crude oil wash (COW) is carried out at the completion of discharge. Purpose of carrying out COW is to reduce remove maximum residues and to reduce the ROB. However, not all cargoes are suited for COW and certain cargo might in fact increase the discharge losses such as;
- Viscous/waxy cargoes – COW may lead to high ROB; and
- Volatile cargoes – COW causes excessive gas evolution.
Heating of the cargo during carriage and at discharge: Certain cargoes are required to be carried and discharged at a specific temperature. Any significant variation in the temperature may give rise to solidification resulting in excessive ROB.
Unpumpable cargo: Pumpability is frequently in dispute:
- the total quantity of cargo (i.e., liquid and non-liquid) remaining on board;
- the quantity of liquid cargo remaining on board in comparison with non-liquid cargo;
- whether or not the liquid cargo is pumpable and in fact can be reached by the vessel’s pumps; and
- if the liquid cargo can not be reached, whether or not this is due to any fault, neglect or technical condition for which the carrier is liable.
Upon the completion of discharge ROB inspection is carried out by an independent surveyor and an ROB certificate is issues. ROB certificate is the evidence for any ROB quantity.
Determining ROB quantity on a large vessel could be a difficult task. The tanks are usually very large and single dip measures the quantity located exactly under the dipping point only.
As a minimum requirement the survey should be performed in accordance with the Manual of Petroleum Measurement Standards, Chapter 17 – Marine Measurements, published on behalf of the American Petroleum Institute, or similar guidelines.
It is required that the ROB inspection is carried out from at least four positions in each tank. One of these four dipping points shall be the reference gauging position as per vessel’s tank capacity table. In addition, MARPOL Annex I, Regulation 13 (b) (2) requires vessels to have suitable arrangements for hand-dipping at the aftermost portion of a cargo tank and in three other suitable locations. If four positions are not available for dipping, then the vessel does not comply with the MARPOL regulations.