OFFSHORE PLATFORMS

Most offshore drilling vessels/platforms are designed to be moved from location to location. The majority of exploratory wells drilled are failures, so it would not make sense to build a permanent structure at the start of a drilling campaign. We use a variety of styles of temporary/mobile structures to drill exploratory wells (and often to drill the development wells, too). These are called Mobile Offshore Drilling Units (MODUs).

Only once an oil reservoir is 1) found, 2) appraised, and 3) assigned development funding does it make sense to build a permanent, semi-immobile structure over the field. These permanent facilities may or may not have an integral drilling rig onboard, but we don't call them drilling vessels -- we call permanent facilities "production platforms". So drilling equipment is generally mobile, while production equipment is generally stationary.

There are many different types of drilling vessel. Some are self-powered and some are towed. In increasing order of water depth: 

Swamp Barge

Barge rigs are used in extremely shallow water (5-10 ft) which usually makes them suitable for use in swamps and sheltered bays. They are moved to location, and then ballasted down to sit directly on bottom. You see these in south Louisiana a lot -- it's a pretty common sight to be stuck at a drawbridge while a drilling barge floats through.

Jack Up

Jack-up rigs are used on the continental shelf, near land. The defining feature of a jack-up rig is having 3-4 enormous legs. Once floating above the desired location, they use massive hydraulic jacks to lower the legs down to the seafloor. Then, as the jacks continue pushing the legs down, the rig gets jacked up out of the water. Once the legs settle into the seafloor mud and the rig is above the wave zone, everything is stable enough for drilling.

Semi-Submersible

This is a fully-floating rig that is usually used up to 8,000 ft water depth. Semi-subs are usually anchored in place up to about 5,000 ft water depth, but in deeper water they are more often dynamically positioned, using only an array of large directional thrusters on the lower hull section to maintain position  according to various GPS receivers and sonar beacons.
Semi-submersibles are so named because they have a large lower structure that can be filled/drained of water to ballast the rig up and down. When moving from place to place, the rig can be ballasted up out of the water to reduce hull drag. Then upon arriving at the drill site, the lower structure is filled with water to put a large amount of mass beneath the wave zone. This submerged mass stabilizes the rig so that waves have minimal ability to rock and roll the rig. This gets into some pretty complicated naval architecture, but the basic premise is that the farther the center of mass is below the center of buoyancy, the more stable the structure is. Only the legs provide buoyancy and are affected by wave action. Most of the mass is below the legs.

Semi-sub vs Drillship:

Drillship

Drillships are basically regular ship-form hulls, except they have a big hole in the middle (called the moonpool) for the drilling equipment to pass through on its way to the sea floor. They can move around exactly like a normal ship. Almost all drillships are dynamically positioned, using two sets (front and back) of 360-degree rotatable thrusters. You can anchor a drillship in shallower water, but the ability to "weathervane" the rig to keep the bow into the wind/waves/current is extremely important to the stability of the vessel. You always want the smallest cross-section pointed towards the dominant environmental load. Since anchoring usually does not permit weathervaning, it's only feasible in areas with exceedingly mild weather. 

An important consequence of dynamic positioning is that drillships are usually only good for deep water (3,000-10,000 ft). They're next to useless in shallow water, because errors in the positioning system of a few tens of feet cause large bending moments to form in the pipes going to the seafloor. It's basic trigonometry: the deeper the water, the less angular offset you get for a particular position offset. The accuracy of the GPS and sonar systems is usually within a couple feet, but like any system, occasionally there are failures. If the rig suffers a power blackout, it will start drifting off position. 

The drilling riser pipe has flex joints that are usually designed for around 6 degrees of bending at the seafloor. Your blackout recovery or emergency disconnection time needs to be faster than the time it takes to drift to a 6 degree offset. In 8,000 ft of water, that safe drifting time window is 10 times longer than in 800 ft of water. A lot of analysis goes into this sort of thing.
Aside from drilling vessels, there are also many different types of production facilities:

 Fixed Jacket Platform

This type of platform is usually built in three stages. First, massive pilings are driven into the sea floor to provide a stable base. These must secure the structure against large bending/toppling moments from wind/waves/current. Then, the pre-fabbed steel "jacket" is towed into position and set down on the seafloor, where divers secure it to the pilings. Finally, the "topsides" are lifted onto the jacket with a large crane barge and secured in place by welders.
These are built and stay in place until decommissioned many decades later. Usually the topsides are cut off and returned to land for recycling, whereas the jacket can be toppled onto its side and used as an artificial reef. Marine life loves oil rigs.  

Gravity Base Platform

Where the seafloor is extremely hard (like the North Sea), driving deep pilings is not feasible. So the force required to keep the platform from being knocked over by weather is provided by absurdly large concrete or metal weights that simply sit on the seafloor. Sometimes, these platforms are re-floated at the end of the field's productive life, but it's a difficult technical challenge to move such a massive structure after it has settled in place for a few decades.

Truss, Spar

Spars and truss platforms have a lower  section that is floated to place on its side and then ballasted down in what amounts to a controlled sinking process. Imagine the Titanic's end rising up into the air as sequential compartments filled with water, except you're doing it on purpose. Like semi-submersibles, the majority of the mass is far below the waterline to ensure stability in rough conditions. Anchor chains/cables are run to the seafloor to keep the structure in place. Then the topsides are lifted onto the platform and integrated via welding, pipe-fitting, etc.  

Towing a truss on its side:

Lifting a topsides module onto a spar:

The crane semi-sub used to lift this module onto Chevron's Tahiti platform, theSSCV Thialf, is the largest heavy lift vessel in the world. Absolutely huge.
Truss and spar platforms can be decommissioned and moved in the reverse process to their installation.


Tension Leg Platform

Another floating platform style is the TLP. These use high-tension vertical anchor lines attached to large weights or subsea pilings to pull the rig down somewhat below the level where it would normally float. By pulling the rig down, a large buoyant force is created that helps keep the vessel stable. These are basically immune to wave-induced heave, so fixed pipes can be run to the seafloor. (Other types of floating platform require top-tensioning motion compensators or flexible semi-buoyant risers.)
Moving a TLP would require cutting all the production riser pipes, releasing from the piles or picking up the weights, and being towed away.

Semi-Submersible
Semi-submersibles can be used as production platforms, exactly as they're used for drilling rigs. They do tend to be bigger as production  platforms, because they must support the weight of many different  production risers instead of just one drilling riser. Production semi-subs are always anchored because dynamic positioning burns a lot of fuel and is not reliable enough for decades of continuous production. The lack of integral thrusters means semi-sub platforms are either towed or carried to the field, as pictured below. 

Heavy transport ships can be used to pick up fully-built platforms and simply carry them to the proper location. This tends to be done when the platform is crossing an ocean because one large ship-form vessel is faster and safer than a bunch of tugboats towing an irregularly-shaped semi-sub hull, which isdesigned to be hard to move.

So how do they get the entire production platform onto the transport ship? Simple! Partially sink the transport ship, tow the platform over it, and then un-sink the lift ship from beneath the platform. This is an absolutely incredible process. I highly recommend you visit the Dockwise Vanguard website to watch an animation about how these ships can move mega-structures. 

Floating Production/Storage/Offloading

FPSOs are highly-specialized vessels that, unlike most production platforms, have no onboard drilling rig. They tend to be more mobile than other production facilities -- it's fairly common to disconnect an FPSO during bad weather or iceberg danger and move it off location. That can be via tugs like the round FPSO pictured, or via onboard propulsion as a ship-form FPSO pictured below. Pipeline connections generally require a permanent structure, so FPSOs offload oil via tankers.

Turreted FPSO:

FPSO "turrets"allow ship-form vessels to rotate with the weather. This is a big advantage over drillships, in that an FPSO can be anchored in position via the turret and still weathervane to minimize the impact of wind/waves/current. Turrets are highly complex structures with many rotating sealing elements to allow oil to flow up and hydraulic/electrical signals to go down. If the FPSO leaves location, the lower part of the turret will stay behind (often floating just below the surface). Then when the danger passes, the vessel comes back and reconnects to regain control of the field.

I think that's all the basics of how we move offshore oil rigs and platforms into and out of place over the reservoir. Offshore drilling is a very highly specialized pursuit with enormous equipment, so over the years we've had to push the limits of engineering and invent a lot of mind-blowing techniques.

Offshore drilling platform is depending on the water depth and remoteness of the location, these "rigs" may be jack-ups (up to 400 feet of water), or semisubmersibles, or drillships (up to 12,000 feet of water). Jack-ups are bottom-supported units; semisubmersibles and drillships are floating units 

One of the most important pieces of equipment for offshore drilling is the subsea drilling template. Essentially, this piece of equipment connects the underwater well site to the drilling platform on the surface of the water. This device, resembling a cookie cutter, consists of an open steel box with multiple holes in it, dependent on the number of wells to be drilled. This drilling template is placed over the well site, and usually lowered into the exact position required using satellite and GPS technology. A relatively shallow hole is then dug, in which the drilling template is cemented into place. The drilling template, secured to the sea floor and attached to the drilling platform above with cables, allows for accurate drilling to take place, but allows for the movement of the platform, which will inevitably be affected by shifting wind and water currents.
In addition to the drilling template, a blowout preventer is installed on the sea floor. This system, much the same as that used in onshore drilling, prevents any oil or gas from seeping out into the water. Above the blowout preventer, a specialized system known as a ‘marine riser’ extends from the sea floor to the drilling platform above. The marine riser is designed to house the drill bit and drillstring, and yet be flexible enough to deal with the movement of the drilling platform. Strategically placed slip and ball joints in the marine riser allow the subsea well to be unaffected by the pitching and rolling of the drilling platform