Offshore is quite wide segment. It in compasses various fields including oil and gas industries (exploration, exploratory drilling, construction and installation of stationary platforms, installation of underwater infrastructure, laying of underwater pipelines, supply and maintenance of oil and gas jack-up rigs), wind energy production (installation and service of wind turbines), dredging operations, cable laying and maintenance, and others.

If we compare merchant and offshore fleet: Merchant fleet (container ships, universal vessels, gas carriers, tankers, etc.) has a single purpose, which is to transporting various cargoes from point A to point B; Offshore fleet, on the other hand, consists of specialized vessels, designed and deployed for particular operations.

Here are some examples of offshore vessels:

Anchor Handling Tug Supply Vessels (AHTS)

Anchor-Handling Tug (AHT) vessels are used for handlight, removing up and moving of the anchors.  AHT are also employed to transport drilling vessels, oil-drilling platform vessels, lifters and similar service vessels. 

Anchor-handling Tug/Supply combines the capabilities of a supply vessel and anchor handling tug vessel.  In its capacity as a supply vessel, AHTS is used to deliver equipment, supplies and personnel to offshore vessels.  In certain cases, they can serve as an Emvergency Rescue and Recovery Vessel (ERRV) or as Standby Rescue Vessel (SRV) in the oil-drilling regions.

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FPSO (Floating Production, storage, and offloading vessel) 

FPSO vessels receive and process crude oil from subsea template or oil platforms, and store it in their tankers until it can be offloaded to a shuttle-tanker or to oil barges for the subsequent transportation to shore.  In addtion to FPSO vessel, it is common to see FSO vessels being employed (Floating Storage and Offloading).  FSO vessels do not have the oil processing capability and is essentially a simplified FPSO.  FSO vessels are employed in oil and gas production in the same regions and for the same purposes as FPSO vessels.

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Shuttle tankers, also called «Floating Pipelines» are used for offloading crude oil from oil rigs or FPSO type vessels (Floating Production Storage and Offloading vessel) and the subsequent transportation of the cargo to oil refineries and storage facilities. 

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PSV (Platform Supply/Support Vessel) - is a service vessel (mostly 20 to 110 meters long) that is used for delivering equipment and supplies to and from oil rigs. Typically these vessesl are employed for transportation of personnel, supplies, fresh water, fuel, refrigerated containers, various lubricants and chemicals, dry and liquid cargoes that are necessary for the normal functioning of the oil platforms. 

The photos below PSV vessel type designed by Ulstein Design AS type X-Bow for Bourbon Offshore Norway.

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Drillships is a vessel used for exploratory offshore drilling.  Most of the time drillships work autonomously (without using supply vessels) in remote regions.  Modern drill ships are 150 – 255 m long, 24-42 m wide, and 15,000 – 95,000 dwt. Drillships have the capacity to accomodate up to 6500 cu.m. of drill  extractions and to store both driling and casing. Working speed of drillsips varies from 11 to 15 knots.  As a rule, these vessels are also equipped with a helicopter landing pad. The superstructure of a drill ship can accomodate anywhere from 110 to 230 people at a time. Drillships work at various water depths, ranging from 400 to 3600 m. Total (combined) depth of drilling reaches up to 11,400m.

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Jack up Rig/Barge – is a self-elevating unit (platform/barge) – that consists of a buoyant hull fitted with a number of movable legs (support columns), which allow the rig to maintain a steady position on the seabed. The most popular types are the ones with 3 or 4 legs.  Hydraulic cylinders, used in the construction of the self-elevating system (jacking system) enable for the platform to be raised and lowered relatively fast. Support columns are equipped with a sensor system that allows for balancing of the hull. 
Modern jack rigs are used in in depths up to 120 meters however rigs of premium and ultra-premium jack up class can be used on depths ranging from 150 to -190 meters.  Generally Jack up rigs are not self-propelled, in which case a vessel will be transported with the assistance of a tug or a Heavy Lift. Typically Jack up rigs are used as exploratory drilling platforms (Jack Up Drilling Rigs).Total drilling depth may reach up to 12 km. These vessels are also used as service/supply vessels for the construction and installation of wind farm platforms. 

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Multi Role Service/Support Vessesl (MRSV) are universal supply/service vessels that are mainly used for facilitation and undertaking of underwater, construction and installation work, using specialised cranes and ROV (Remotely Operated Vehicles) equipment.

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