What is an Offshore Support Vessel, or OSV?
Updated: Nov 28, 2020
Offshore Support Vessels, also known as Offshore Supply Vessels, are specialty ships designed for operating on the ocean, serving multiple purposes. They can serve as platform support, anchor handling, construction, maintenance, and more. Today we’ll take a closer look at these versatile ships and how they work.
What’s the purpose of an OSV?
OSVs provide support offshore - anything from bringing equipment to rigs to repairing offshore wind turbines. They are integral to getting supplies and materials where they need to go, as well as building and repairing offshore equipment. Their versatility means they can be built for just about any type of project. According to Clarksons, there are more than 5000 OSVs in service with more than 600 on the order book. Some of the main types of OSVs are:
Seismic survey ships
Platform Supply Vessels, or PSVs
Anchor Handling Tug Supply Vessels (AHTS)
Construction Support Vessels
Diving Support Vessels
Inspection, Maintenance, and Repair Vessels
ROV Support Vessels
Offshore Supply Vessel. Image Credit: Safety4Sea
What do offshore support vessels actually do?
Some of the more common types of OSVs are offshore supply vessels. In simple terms, they transport supplies, materials, and people from land to offshore rigs and ships. The subset of Platform Supply Vessels are commonly used in drilling to bring equipment, stores, parts, food, and anything else an offshore city like an oil rig might need. According to Wärtsilä, some of the other drilling consumables are: “cement, baryte and bentonite transported as dry powders; drill water; oil or waterbased liquid mud, methanol and chemicals for specialized operations.”
The PSV’s transport journey starts when they load up at shore. Equipment, drill pipes, and other large items are carried on the deck. Dry cargo is put in special pneumatic pressure tanks and liquid cargo is put in double bottom tanks for storage. Powder and liquid cargos are pumped to the rig while deck cargo is moved by rig crane. They spend over half their time loading and unloading - according to Wärtsilä, “A typical PSV operating profile shows the vessel spending about 25% of the time in harbour loading and unloading, 40% sailing at a service in the 14-16 knot range and 35 % loading or discharging at sea, often in strong winds, high seas and strong currents.” The job of working on a PSV can be quite challenging and dangerous due to sea conditions and heavy machinery. In the next section, we’ll look at how PSVs, and OSVs in general stay steady during these operations using dynamic positioning.
Offshore Supply Vessel. Image Credit: Offshore Mag
Anchor Handling Tug Supply Vessels, or AHTS have multiple functions, but are capable of assisting drilling rigs with the handling of mooring chains and anchors, towing rigs, and supply platforms. Due to this work, they need a high bollard pull, reflected in the propeller size, power requirements, and hull shape. Stability is paramount and there must be space for chains and equipment. A stern roller is used to help with the passage of wires and anchors over the stern during operations.
Construction Support Vessels, also called Offshore Construction Vessels, have a lot of open deck area, with plenty of space for accommodations. They can remain on site for long periods of time and frequently work on larger projects. They typically have heavy lift crane capacity and support surface and subsea (underwater) installations and IRM work, or inspection, repair, and maintenance. IRM includes well stimulation and maintenance, with many of those vessels performing ROV (Remotely Operated Vehicles) ops and supply duties. “ROV Support Vessels are equipped with computer-controlled, precision, position-keeping capabilities with added redundancy features, such as multiple computers, thrusters and reference systems. Such vessels have additional cabins, mess room facilities and Client offices, to comfortably accommodate the Client’s ROV support crews,” according to Wärtsilä.
Damen Offshore Construction Vessel. Image Credit: Offshore Mag
Other subsets of these include vessels specifically designed to install and service wind farms, which are now being specially built for turbines that are growing ever larger. You can check out our article on offshore wind turbine installation to learn more about turbines and the vessels that install them. Cable-lay vessels are also in this category, they are designed to hold thousands of kilometers of undersea cable.
The last type of Construction Support Vessel is the Diving Support Vessel. These ships are designed for underwater maintenance and inspection of platforms, wells, pipelines, etc. They have one or more holes in the middle of the vessel called a moonpool, where divers, ROVs, and equipment are lowered into the water. All CSVs need to stay in position, but staying still is especially important with DSVs, they have a dynamic positioning system to stay in place during operations in all types of conditions. If you’re curious as to what happens when a vessel does not stay in position during diving operations, check out the movie and documentary Last Breath about the failure of a DP system causing a vessel to drift off course and trap a diver underwater with only 5 minutes of oxygen.
Diving vessel moonpool. Image Credit: Stanford
OSVs can also provide service to disabled vessels in an emergency, and are sometimes called escort vessels. They can also act as oil spill recovery vessels, recovering oil from the water in the event of an oil spill. Fast Supply Intervention Vessels (FSIV) are smaller, faster versions of OSVs that provide emergency response and urgent delivery of needed items. Stand-by/Rescue vessels provide security, patrolling an area in case of a sea fall, evacuation, or the need to fight fires.
Seismic Survey Vessels, or simply Seismic vessels, are purpose-built for seismic surveys in the ocean. These unique ships help to locate the best area to drill for oil, as well as to minimize the disturbance of marine life. They shoot seismic waves and measure their return to determine if an area is good for drilling. These vessels are specially designed and as you can see from the photo below, can look quite strange compared to a standard ship or OSV. They also study geology of the oceans and seas like rocks, trenches, and other underwater structures. They are fitted with a lot of sensitive equipment specifically for this purpose.
Ramform Atlas Seismic Survey Ship. Image Credit: PGS
We mentioned that OSVs are equipped with varying dynamic positioning systems, which are rated DP1, DP2, and DP3. Dynamic positioning systems keep a vessel in place on the water through the use of computer-controlled thrusters and propellers. The computer takes readings from current, waves, and wind to continuously monitor vessel position and keep it steady. Kongsberg describes the ratings as:
DP1 - no redundancy, a single fault could take the DP system offline.
DP2 - has redundancy so no single fault will cause a system failure.
DP3 - additional redundancy with “At least two independent computer systems with a separate back-up system separated by A60 class division”.
Dynamic Positioning diagram. Image Credit: Offshore Engineering
Most Offshore Support Vessels will have a DP2 or DP3 system for safety, depending on the type of work they do. OneStep Power supports DP2 and DP3 vessels with electrical system testing to prove they can survive a fault and continue operating. This verification allows them to save fuel, lower emissions, and reduce maintenance costs while running in closed bus operation safely. In the future, we may cover some of the more famous OSVs and DP ships, until then, happy Fun Fact Friday!