Powering Progress: Offshore Vessels Explained

As a lay investor, does the plethora of vessel types in offshore oil & gas production confuse you? Do you know the difference between a MOPU and an FSO? Or what terms like Aframax and Panamax mean? What are support vessels and what do they do?

Here’s a breakdown of the more common vessel types, and the distinct role each plays in offshore energy operations.

Common Vessel Types

Panamax¹ vessels are medium-sized ships specially designed to pass through the Panama Canal. Built for efficiency, they can carry up to 120,000 deadweight tonnes and transport around 5,000 twenty-foot equivalent units (TEU). These versatile vessels transport containerised goods and bulk commodities such as coal, grain, oil, and iron ore, playing a vital role in global trade.

Aframax² vessels are medium-sized, with a capacity of up to 120,000 deadweight tonnes and able to carry around 750,000 barrels of crude oil. Their size allows them to enter ports that larger vessels cannot, making them ideal for short- and medium-haul routes. Aframax vessels are used for routes between South America and the U.S. Gulf, North Africa to Southern Europe, and Black Sea ports to Northern Europe³.

Mobile Offshore Production Units (MOPU)⁴ are portable structures used when procuring oil and gas from subsea reservoirs. Equipped with self-elevating legs for stability, they can be relocated and reused, making them ideal for short-term or marginal fields. MOPUs include jack-ups, semi-submersibles, spar platforms, and production barges, typically deployed in deep waters.

Floating Storage and Offloading Units (FSOs)⁵ are vessels designed to store and offload oil and gas extracted from subsea wells. FSOs do not process oil or gas and instead act as a floating storage tank. FSOs hold crude oil until it is transferred to shuttle tankers or pipelines for transport to onshore facilities.

Offshore Support Vessels (OSVs)⁶, while not directly engaged in the drilling and production process, provide essential support services throughout offshore operations. Examples include Platform Supply Vessels that deliver supplies and equipment; Anchor Handling Tug Supply vessels used to tow rigs and manage anchors, and Multi-Purpose Support Vessels that perform a variety of duties from subsea construction to installation tasks.

Dry Docking

Similar to how a car is sent to the garage for regular service and maintenance, dry docking⁷ is essential for vessels. It involves taking a vessel out of the water to inspect, maintain and repair parts that are normally underwater, such as the hull, propellers, rudders, sea valves and other critical systems. The process involves docking the vessel, performing maintenance, testing systems and updating certifications. All vessels are required to undergo dry docking regularly, usually every five years for major inspections, and with smaller checks in between. Proactive maintenance during dry docking prevents corrosion, improves fuel efficiency, and extends the vessel’s lifespan. Timely dry docking ensures the vessel remains compliant with regulatory standards.

Conclusion

Offshore oil and gas operations rely on a diverse fleet of vessels, each critical to transporting, producing and maintaining production operations. At Rex International Holding, MOPUs and FSOs are specifically deployed for our offshore assets in Oman (operated by Masirah Oil Ltd) and Benin (operated by Akrake Petroleum Benin S.A.), where they are cost-effective and well-suited for smaller or marginal fields. Careful selection and management of these vessels enhance safety and efficiency in our operations.

¹Inbound Logistics, Panamax Ship: History, Advantages, and Capacity, 1 September 2023 | ²Clarksons, A guide to bulk vessel sizes, 30 October 2025 |

³Marine Insight, Panamax and Aframax Tankers: Oil Tankers with a Difference, 20 August 2021 |

⁴Oil & Gas IQ, What are MOPUs and FPSOs?, 12 January 2025 |

⁵London Maritime Academy, What is Floating Production Storage and Offloading (FPSO) and How Does It Work?, 27 January 2025 |

⁶Crewlinker, An In-Depth Guide to Offshore Vessels: Types, Careers, and More, 2 October 2023 |

⁷Clarksons, What is dry docking?, 30 October 2025

LinkedIn