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Offshore vessel types


Seismic vessel
is a ship that is used at the initial  (exploration) stage. The main task is to carry out the seismic surveys at designated locations. 


Seismic survey is an exploration method in which strong low-frequency sound waves are generated on the surface or in the water to find subsurface rock structures that may contain hydrocarbons. the sound waves travel through the layers of the earth’s crust; however, at formation boundaries some of the waves are reflected back to the surface where sensitive detectors pick them up. reflections from shallow formations arrive at the surface sooner than reflections from deep formations, and since the reflections are recorded, a record of the depth and configuration of the various formations can be generated. interpretation of the record can reveal possible hydrocarbon-bearing formations.


Anchor Handling Tug Supply (AHTS) vessels are vessels which supply oil rigs, tow them to location, anchor them up and, in a some cases, serve as an (ERRV)
Emergency Rescue and Recovery Vessel.


AHTS vessels are fitted with winches for towing and anchor handling, having an open stern to allow the decking of anchors, and having more power to increase the bollard pull. The machinery is specifically designed for anchor handling operations. They also have arrangements for quick anchor release, which is operable from the bridge or other normally manned location in direct communication with the bridge. The reference load used in the design and testing of the towing winch is twice the static bollard pull.


Cable Layer or Cable Ship deep-sea vessel designed and used to lay  and repair underwater cables for telecommunications, electricity, and other industries.



Cable ships are distinguished by large cable sheaves for guiding cable over bow or stern or both. Bow sheaves, some very large, are characteristic of all cable ships. Newer ships are tending toward pure stern layers with stern sheaves only. The names of cable ships are often preceded by "C.S." as in “C.S. IT Interceptor”


Diving Support Vessel (DSV) is a ship that is used as a floating base for professional diving projects



The key components of the diving support vessel are:
Dynamic Positioning (DP) - Controlled by a computer with input from position reference systems, it will maintain the ships position over a dive site by using multi-directional thrusters,
Saturation diving system - For diving operations below 50m, a mixture of helium and oxygen (heliox) is required to eliminate the narcotic effect of nitrogen under pressure. There are a number of support systems for the saturation system on a Diving Support Vessel, usually including a Remotely Operated Vehicle (ROV) and heavy lifting equipment.


Drillship is a maritime vessel that has been fitted with drilling apparatus. It is most often used for exploratory offshore drilling of new oil or gas wells in deep water or for scientific drilling.



The drillship can also be used as a platform to carry out well maintenance or completion work such as casing and tubing installation or subsea tree installations.
The greatest advantages these modern drillships have is their ability to drill in water depths of more than 2500 meters and the time saved sailing between oilfields worldwide what makes them independent.
 
In order to drill, a marine riser is lowered from the drillship to the seabed with a blowout preventer (BOP) at the bottom that connects to the wellhead.
 

Floating Production, Storage and Offloading (FPSO) unit is a floating vessel used by the offshore industry for the processing of hydrocarbons and for storage of oil.


A FPSO vessel is designed to receive hydrocarbons produced from nearby platforms or subsea template, process them, and store oil until it can be offloaded onto a tanker or transported through a pipeline. FPSOs are preferred in frontier offshore regions as they are easy to install, and do not require a local pipeline infrastructure to export oil. FPSOs can be a conversion of an oil tanker or can be a vessel built specially for the application. A vessel used only to store oil (without processing it) is referred to as a Floating Storage and Offloading vessel (FSO).


The Emergency Response Rescue Vessel (ERRV)
is permanently located at the offshore installations where it provides service and is ready for 24 hours a day, 7 days a week. Vessels can work in all kinds of weather.


Emergency Response and Rescue Vessels have a special survival equipment and designated medical areas on board. Qualified crew members (Paramedics) present on board can carry out basic medical operations  and treat the injured. The wards (medical recovery areas) on some vessels have the capacity for over 150 survivors.


Flotel refers to the installation of living quarters on top of rafts or semi-submersible platforms. Accommodation vessels can also be considered as flotes.



Accommodation vessels/rigs have traditionally been used wherever there is a need for additional accommodation, engineering, construction or storage capacity offshore. Typically, employed for installing and commissioning new facilities, upgrading or maintaining existing installations, hooking-up satellite fields to existing infrastructure, and removing installations.
 
The units are positioned alongside the host installation and are connected by means of a telescopic gangway, or personnel can be transported to and from the unit by boat or helicopter.
 

Heavy Lift ship is a vessel designed to move very large loads that cannot be handled by normally equipped ships. 


They are of two types:
•Semi-submerging (more common) - which are capable of lifting another ship out of the water and transporting it. These vessels have a long and low well deck between a forward pilot house and an aft machinery space. In superficial appearance  it is somewhat similar to a dry bulk carrier or some forms of oil tanker.
•Vessels that augment unloading facilities at inadequately equipped ports.


Jack-up Rig is a type of mobile platform that is able to stand still on the sea floor, resting on a number of supporting legs. The most popular designs use 3 independent legs, although some jackups have 4 legs or more. On "mat-type" jackups the legs are connected to a submerged hull.

A jackup is a floating barge fitted with long support legs that can be raised or lowered. The jackup is maneuvered (self-propelled or by towing) into location with its legs up and the hull floating on the water. After preloading, the jacking system is used to raise the entire barge above the water to a predetermined height or "air gap", so that wave, tidal and current loading acts only on the relatively slender legs and not on the barge hull. Modern jacking systems use a rack and pinion gear arrangement where the pinion gears are driven by hydraulic or electric motors and the rack is affixed to the legs. Jackup rigs can only be placed in relatively shallow waters, generally less than 400 feet (120 m) of water.
 

Pipelaying ship is a maritime vessel used in the construction of subsea infrastructure. It serves to connect oil production platforms with refineries on shore.

To accomplish this goal a typical pipelaying vessel carries a heavy lift crane, used to install pumps and valves, and equipment to lay pipe between subsea structures.



Lay methods consist of S-lay and J-lay and can be reel-lay or welded length by length. Pipe-laying ships make use of dynamic positioning systems or anchor spreads to maintain the correct position and speed while laying pipe. Recent advances have been made, with pipe being laid in water depths of more than 2,500 meters. The term "pipelaying vessel" or "pipelayer" refers to all vessels capable of laying pipe on the ocean floor. It can also refer to "dual activity" ships. These vessels are capable of laying pipe on the ocean floor in addition to their primary job.


Platform Supply Vessel (PSV) is a ship specially designed to supply offshore oil platforms. These ships range from 20 to 100 meters in length and accomplish a variety of tasks.


The primary function for most of these vessels is transportation of goods and personnel to and from offshore oil platforms and other offshore structures.
Cargo tanks for drilling mud, pulverized cement, diesel fuel, potable and non-potable water, and chemicals used in the drilling process comprise the bulk of the cargo spaces. Common and specialty tools are carried on the large decks of these vessels. Most carry a combination of deck cargoes and bulk cargo in tanks below deck. Many ships are constructed (or re-fitted) to accomplish a particular job. Some of these vessels are equipped with a fire fighting capability and fire monitors for fighting platform fires.
 
Multi Purpose Support Vessel (MPSV)



This type of vessel was designed for world-wide, deep water operations e.g.:
•Subsea Intervention services
•ROV operations
•Deep-water precision lifting and installation
•Transportation of liquid products, deck cargo and other equipment for drilling
•and production support
•Oil recover operations
•Guidewire based module replacement operations via optional moon pool
 

Offshore Construction Vessel (OCV)
These  subsea construction support ships are designed with focus on good station-keeping and excellent manoeuvrability.


The design allows for flexible configuration with respect to the different operations this kind of vessels may be outfitted and arranged for, typically:
 
•Subsea construction and installation
•Inspection, maintenance and repair (IMR Vessels)
•Flexible pipe laying
•Well intervention
•Diving support
 

Well Stimulation / Intervention vessels executes everything from small matrix treatments to large hydraulic fracturing operations. Features include superior handling and control, high stability, and an air monitoring and conditioning system.


Types of well work:
•Pumping (Oil well)
•Wellhead tree maintenance (Well integrity)
•Slick-line / Braided line (Wire cabling)
•Coiled tubing
•Snubbing
•Work-over
•Subsea well intervention
 

Semi Submersible Drill Rigs make stable platforms for drilling for offshore oil and gas. They can be towed into position by a tugboat and anchored, or moved by and kept in position by their own azipod propellers with dynamic positioning.


Mobile offshore  drilling units (MODU) is used exclusively to drill offshore exploration and development wells and that floats upon the surface of the water when being moved from one drill site to another. It may or may not float once drilling begins. Two basic types of mobile offshore drilling units are used to drill most offshore wildcat wells: bottom-supported drilling rigs and floating drilling rigs.



Offshore vessel types

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