Tanker Cleaning and Cargo Compatibility
in West African Operations

By Calmwaters Maritime Team · Published July 2024

Tanker cleaning and cargo compatibility management are fundamental to successful operations in West African waters, where vessels frequently transition between different cargo types — various crude oil grades, refined products, and specialty chemicals. Proper cleaning procedures ensure cargo quality, prevent contamination, and maintain vessel certification compliance. In West Africa, where some Nigerian crude grades are notoriously difficult to clean after, the planning required before a cargo transition is particularly demanding.

Nigerian Crude Grades — Cleaning Challenges

Nigerian crude oils possess unique characteristics that influence cleaning requirements and cargo compatibility. Bonny Light, the benchmark West African crude, is a relatively sweet low-sulphur grade that cleans relatively well. However, Qua Iboe, Escravos, and Forcados crudes can have waxy residue characteristics that require prolonged hot washing procedures to achieve the cleanliness standards required before loading clean products.

The wax appearance temperature of Nigerian crudes is an important planning parameter: if tanks cool below this temperature after discharge, wax deposits form and adhere to tank surfaces. Heating during washing, and washing immediately after discharge before tanks cool, significantly reduces the cleaning burden.

Cleaning Standards and Cargo Sequences

The required cleaning standard depends on the next cargo. The MARPOL programme for minimising residues, combined with the requirements of specific next cargo owners, defines the standard that must be achieved. Common West African cargo sequences and their cleaning implications include:

Common Cargo Transitions in West Africa

• —Crude → DPP (dirty petroleum products): requires ROB removal and basic washing only
• —Crude → CPP (clean petroleum products): requires comprehensive hot sea water wash and inspection
• —Crude → Chemicals: may require butterworth washing, fresh water rinse, and independent survey
• —CPP → Different CPP grade: depends on cargo compatibility — may need rinsing or full clean
• —Any → Edible oils: highest cleaning standard required — typically food-grade stainless steel or full wash with fresh water and ventilation

MARPOL Compliance During Cleaning

Tank cleaning operations must comply with MARPOL Annex I requirements — specifically the restrictions on overboard discharge of tank washings. At sea, discharge is permitted only outside special areas under specific conditions of water depth, speed, and cargo residue concentration. In Nigerian territorial waters, discharge must be to shore reception facilities or retained on board for later disposal.

Accurate and contemporaneous entries in the Oil Record Book Part I are essential for regulatory compliance. Port state control officers at Nigerian ports have been known to scrutinise ORB entries during inspections, and inconsistencies attract investigation.

Slop Tank Management and Port Reception

Slop tank management is an important cost variable in West African operations. Tank washing residues require either treatment on board (through gravity separation in the slop tank) or transfer to port reception facilities. Lagos, Port Harcourt, and Bonny have reception facilities, though their capacity and operational reliability varies. Pre-arranging slop reception before arrival avoids costly delays.