Rigs

Semi-submersible Drilling Rig
This type of rig is more like a ship than a Jack-up. It's main design principle is built along the lines of a ship. A drilling package is basically built on top of a barge consisting of 6-8 cylindrical legs, with pontoons at the base to aid in the moving of the rig.Ballast tanks are incorporated into the base of the legs and pontoons and are filled with water to 'semi-submerge' the platform to provide stability during drilling operations. In addition to this, Dynamic Positioning or maybe up to 8 anchors would be used to ensure the position of the rig does not alter during drilling operations. When the rig needs to be moved, the anchors are heaved in and the ballast tanks are emptied of water which raised the rig out of the water so that almost the whole rig can be seen.Tug boats or supply vessels are usually attached to move the rig to it's next location, though some of the bigger semi-submersible drilling rigs can move under their own steam with the aid of thrusters and GPS. Once on it's new location, the ballast tanks would be filled with water to 'semi-submerge' the rig into the sea, and the DP would be engaged or anchors set to ensure the rig does not move off location.An oil company, ExxonMobil, Shell, BP etc would hire a drilling rig like this to drill exploratory wells in deeper water to find oil or gas before constructing a production platform on the site after confirming the existence of commercially viable resources.

Jack-up Drilling Rig

This type of rig is basically a hull, usually triangular, supported by 3 legs with a drilling package on top.The legs pass through the hull of the rig and are 'jacked' up and down using very large motors.When the rig is on location the bottom of the legs are touching the seabed. When the rig needs to be moved, the legs are jacked up.Initially, this just causes the hull to move down the legs towards the water. When the hull reaches the water however it floats and the motors then start to raise the legs off the seabed, very slowly and up through the hull and into the air.Tugs are then attached to the rig and it is towed to it's new location. When on it's new location, the legs are jacked down towards the seabed again.Once they touch the seabed and settle the jacking action then starts to raise the hull out of the water and up the legs. Once a safe distance has been achieved between the base of the hull and the water, around 60-70' the rig is stabilised by taking on tons of water into 'preload' tanks and the rig is allowed to settle for a day to maximise stability. Drilling operations may then resume.An oil company, ExxonMobil, Shell, BP etc would hire a drilling rig like this to drill exploratory wells in relatively shallow water to find oil or gas before constructing a production platform on the site after confirming the existence of commercially viable resources.

Drillship

This type of drilling unit is more in line with a conventional ship than any of the others. Designed around the normal hull of a ship, complete with bridge etc, a drilling package is built into the ship, as can be seen by the tall drilling derrick on the Transocean's Discoverer Spirit left, along with all the associated additional equipment, pump rooms etc below decks within the hull.Much more versatile than a Jack-Up or Semi-Sub when it comes to moving to a new location, it retrieves it's own anchors and moves to it's next location under it's own steam, just as a traditional ship.It carries the same drilling crews as a Jack-Up or Semi-Submersible but because it moves around unaided, it carries a larger contingent of traditional marine crew also. Many of the newer built drillships are designed to carry out drilling operations in ultra-deep waters where even most Semi-Submersibles can not drill. These drillships are aided by Dynamic Positioning (DP) systems which use satellites orbiting the earth to fix their position on the sea to within a tiny margin.

Platform

An oil company, ExxonMobil, Shell, BP etc will do geological surveys using specialist vessels to way up the chances of certain areas of seabed containing reservoirs of oil or gas. If the results are favourable, they will hire a Jack-Up or a Semi-Submersible drilling rig, or possibly a drillship to drill some exploration wells. If these wells prove fruitful, the oil company will develop the field with the drilling unit before constructing and placing on site a production platform, or if the reserves are marginal, an FPSO might be used.The platform or FPSO will pump the oil out of the ground into a pipeline to shore, or as is sometimes the case to an 'export' tanker directly which then takes the oil to it's final destination. There are several different types of platform as technology has advanced so rapidly over the years.The Jade platform seen above is a more conventional production platform placed on top of a steel structure (the 'jacket') which is permanently secured deep into the seabed. There is no way it can ever be moved once it is set in place, short of blasting it with dynamite. The main differences between a drilling rig and a production platform are the fact that drilling rigs move from one location to another and float on the surface of the water, while a production platform does not and also the large flare boom present on production platforms which vents off excess gas from the production wells.

FPSO

Traditionally, oil fields have been produced by building a platform on the site after appraisal drilling operations had been carried out.However, advancements in technology has meant that smaller oil fields, where it would not have been economically viable to build a platform, are now able to be developed by the use of a cheaper option, a Floating Production Storage & Offloading vessel, or FPSO for short. The Zafiro Producer, above, is just this type of vessel.Originally a 270,000 ton oil tanker called the M.T. Swift, built in 1973, it was converted to an FPSO and arrived in it's present location offshore Bioko Island, Equatorial Guinea in 1996. It was originally intended for around a 5 year life in the Zafiro field. But more oil discoveries in the field have meant the estimated field life is around 15 years. A life enhancement program was undertaken to improve the FPSO still further and as can be seen in the picture in the oil rig photo gallery, there is now another tanker sited nose-nose with the Zafiro Producer, the Harrier.The Zafiro Producer is now the production part of this partnership and pumps the oil into the hull of the Harrier. The Harrier was replaced by the Magnolia storage unit, another oil tanker on 26th August 2000. The Zafiro Producer is connected to subsea oil wells and pumps the oil out of the ground directly to the hull of the Magnolia.When the hull of the Magnolia is full, it pumps the oil through a hose into another tanker via a Single Point Mooring buoy located approximately 1 mile away. You can see the hose in the picture above floating on the sea surface. FPSOs are a very versatile offshore installation. At the end of a life for an oil field, these vessels are designed to enable them to move to another location and start work on pumping another oilfield dry over a period of years.

DSV

Dive Support Vessels (or DSV's) are just one of a number of many types of support vessels used in the offshore industry. They come in all shapes and sizes. Some are in the form of a conventional ship, like the 'Normand Carrier' pictured on this page or they can be similar in looks, and often mistaken for, a Semi-Sub drilling rig. The hull of these vessels is basically the same as a 'semi' but they wouldn't have the drilling package, derrick etc, onboard. DSV's are used quite extensively within the oil industry to assist with subsea work on pipelines, wellheads, platform maintenance etc.They will have decompression chambers, diving bells etc and a lot of them also carry Remote Operated Vehicles (ROVs) which are controlled from the deck of the ship and have the benefit of being able to carry out work in the water without the need for any divers to be in the water. Even with today's technology though, some jobs can still only be carried out by the human hand. Other types of support vessels include supply boats, tugs for towing rigs, pipelaying ships and barges and many other specialist vessels.