Offshore drilling

Drilling platforms

Offshore drilling is usually done from platforms generically known as mobile offshore drilling units (MODU), which can be of one of several formats, depending on the water depth:

• Jackup rig
• Submersible drilling rig
• Semi-submersible platform
• Drillship

Beyond the MODUs mentioned above, offshore developments also employ a range of fixed and floating drilling platforms that also support drilling, production, or both. Key types include:

1. Fixed platforms - These are rigid structures whose legs are anchored directly into the seabed, supporting a deck which holds drilling rigs, production facilities, and even living quarters. These platforms are generally constructed from steel, creating “jacket” platforms, or concrete, creating “gravity-based structures.” Steel-jacket platforms consist of tubular steel members piled into the seabed, while concrete caisson platforms may incorporate large concrete bases and internal buoyancy tanks. Fixed platforms are cost-effective for shallow to moderate water depths, as their practical depth limit is usually a few hundred meters.
2. Compliant Towers - Compliant Towers or CTs are tall, slender, and flexible tower structures anchored to the seabed by piles, but designed to withstand significant lateral deflection under environmental loads like wind, waves, and currents. Their flexibility allows them to operate in much deeper waters than conventional fixed jackets, commonly in the range of several hundred to about 900m.
3. Tension-Leg Platforms - This type of platform is a floating structure stabilized by vertical, tensioned tendons, also known as tethers, that anchor it to the seabed. Because the tendons are very stiff vertically, they eliminate most vertical motion, enabling wellheads and production equipment to be located on the deck rather than subsea. TLPs are well suited for deep-water environments, often hundreds of thousands of meters, and “mini-TLP” variants like Seastar platforms have been developed for smaller fields.
4. Spar Platforms - Spar platforms consist of a large-diameter, vertical cylindrical hull or hard tank that provides buoyancy, with a deck on top, and is moored to the seabed using conventional mooring lines like through chains or wires. There are several types of this platform, notably classic, truss, and cell spars, which vary in hull configuration and internal ballast arrangements. The deep-draft design of spar platforms gives them excellent stability in very deep waters, making them suitable for ultra-deep production and drilling.
5. Floating Production Systems (FPS) - FPS are floating platforms like semi-submersibles that are equipped for both production and, occasionally, drilling. These units are anchored by either mooring lines, usually cables and chains, or maintained in position by dynamic positioning systems. FPS units can operate in ultra-deep waters, which is around thousands of feet.
6. Floating Production, Storage and Offloading (FPSO) Units - An FPSO is a ship-shaped vessel that processes hydrocarbons from subsea wells and stores its oil in its hull. It is moored to the seabed, often via a turret mooring system, allowing it to “weathervane” in response to environmental forces. Periodically, shuttle tankers offload stored oil, which is advantageous in remote or frontier deepwater areas where pipeline infrastructure is limited or absent.

Main offshore fields

Notable offshore fields include:

• the North Sea
• the Gulf of Mexico (offshore Texas, Louisiana, Mississippi, and Alabama)
• California (in the Los Angeles Basin and Santa Barbara Channel, part of the Ventura Basin)
• the Caspian Sea (notably some major fields offshore Azerbaijan)
• the Campos and Santos Basins off the coasts of Brazil
• Newfoundland and Nova Scotia (Atlantic Canada)
• several fields off West Africa most notably west of Nigeria and Angola
• offshore fields in South East Asia and Sakhalin, Russia
• major offshore oil fields are located in the Persian Gulf such as Safaniya, Manifa and Marjan which belong to Saudi Arabia and are developed by Saudi Aramco.
• fields in India (Mumbai High, K G Basin-East Coast Of India, Tapti Field, Gujarat, India)
• the Taranaki Basin in New Zealand
• the Kara Sea north of Siberia
• the Arctic Ocean off the coasts of Alaska and Canada's Northwest Territories

Challenges

Offshore oil and gas production is more challenging than land-based installations due to the remote and harsher environment. Much of the innovation in the offshore petroleum sector concerns overcoming these challenges, including the need to provide very large production facilities. Production and drilling facilities may be very large and a large investment, such as the Troll A platform standing on a depth of 300 meters (980 ft).

Another type of offshore platform may float with a mooring system to maintain it on location. While a floating system may be lower cost in deeper waters than a fixed platform, the dynamic nature of the platforms introduces many challenges for the drilling and production facilities.

The ocean can add several thousand meters or more to the fluid column. The addition increases the equivalent circulating density and downhole pressures in drilling wells, as well as the energy needed to lift produced fluids for separation on the platform.

The trend today is to conduct more of the production operations subsea, by separating water from oil and re-injecting it rather than pumping it up to a platform, or by flowing to onshore, with no installations visible above the sea. Subsea installations help to exploit resources at progressively deeper waters—locations which had been inaccessible—and overcome challenges posed by sea ice such as in the Barents Sea. One such challenge in shallower environments is seabed gouging by drifting ice features (means of protecting offshore installations against ice action includes burial in the seabed).

Offshore manned facilities also present logistics and human resources challenges. An offshore oil platform is a small community in itself with cafeteria, sleeping quarters, management and other support functions. In the North Sea, staff members are transported by helicopter for a two-week shift. They usually receive higher salary than onshore workers do. Supplies and waste are transported by ship, and the supply deliveries need to be carefully planned because storage space on the platform is limited. Today, much effort goes into relocating as many of the personnel as possible onshore, where management and technical experts are in touch with the platform by video conferencing. An onshore job is also more attractive for the aging workforce in the petroleum industry, at least in the western world. These efforts among others are contained in the established term integrated operations. The increased use of subsea facilities helps achieve the objective of keeping more workers onshore. Subsea facilities are also easier to expand, with new separators or different modules for different oil types, and are not limited by the fixed floor space of an above-water installation.

Effects on the environment

Offshore oil production involves environmental risks, most notably oil spills from oil tankers or pipelines transporting oil from the platform to onshore facilities, and from leaks and accidents on the platform (e.g. Deepwater Horizon oil spill and Ixtoc I oil spill). Produced water is also generated, which is water brought to the surface along with the oil and gas; it is usually highly saline and may include dissolved or unseparated hydrocarbons.