A semi-submersible platform is a specialised marine vessel used in offshore roles including as offshore drilling rigs, safety vessels, oil production platforms, and heavy lift cranes. They have good ship stability and seakeeping, better than drillships. [1]
Offshore drilling in water depth greater than around 520 metres (1,710 ft) requires that operations be carried out from a floating vessel, since fixed structures are not practical. Initially in the early 1950s monohull ships such as CUSS I were used, but these were found to have significant heave, pitch and yaw motions in large waves, and the industry needed more stable drilling platforms.
A semi-submersible obtains most of its buoyancy from ballasted, watertight pontoons located below the ocean surface and wave action. Structural columns connect the pontoons and operating deck. [2] The operating deck can be located high above the sea level owing to the good ship stability of the design, and therefore is kept well away from the waves.
With its hull structure submerged at a deep draft, the semi-submersible is less affected by wave loadings than a normal ship. With a small water-plane area, however, the semi-submersible is sensitive to load changes, and therefore must have their onboard mass trimmed to maintain stability. Unlike a submersible, a semi-submersible vessel is not supported by resting on the seabed.
Semi-submersible vessels are able to transform from a deep to a shallow draft by deballasting (removing ballast water from the hull), thereby becoming surface vessels. Usually they are moved from location to location in this configuration. The heavy lift vessels use this capability to submerge the majority of their structure, locate beneath another floating vessel, and then deballast to pick up the other vessel as a cargo.
The semi-submersible design was first developed for oil platform activities in the early 1960s. Bruce Collipp of Shell is regarded as the inventor. [3]
However, Edward Robert Armstrong may have paved the way with his idea of "seadrome" landing strips for airplanes in the late 1920s, since his idea involved the same use of columns on ballast tanks below the surface and anchored to the ocean floor by steel cables. [4]
The first jackup rigs, for shallow waters, was built in 1954. [5]
The first semisubmersible arrived by accident in 1961. Blue Water Drilling Company owned and operated the four column submersible drilling rig Blue Water Rig No.1 in the Gulf of Mexico for Shell Oil Company. As the pontoons were not sufficiently buoyant to support the weight of the rig and its consumables, it was towed between locations at a draught midway between the top of the pontoons and the underside of the deck. It was observed that the motions at this draught were very small, and Blue Water Drilling and Shell jointly decided that the rig could be operated in the floating mode. [2]
The first purpose built drilling semi-submersible Ocean Driller was launched in 1963. [6]
By 1972, there were 30 semi-submersible units.[ citation needed ]
Drilling rig construction has historically occurred in boom periods and therefore "batches" of drilling rigs have been built. Offshore drilling rigs have been loosely classified in nominal "generations" depending upon the year built and water depth capability as follows:[ citation needed ][ original research? ]
| Generation | Water depth | Dates | |
|---|---|---|---|
| First | about 600 ft | 200 m | Early 1960s |
| Second | about 1000 ft | 300 m | 1969–1974 |
| Third | about 1500 ft | 500 m | Early 1980s |
| Fourth | about 3000 ft | 1000 m | 1990s |
| Fifth | about 7500 ft | 2500 m | 1998–2004 |
| Sixth | about 10000 ft | 3000 m | 2005–2010 |
Semi-submersible 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 azimuth thrusters with dynamic positioning.
The International Maritime Organization MODU Code is an accredited design and operational guideline for mobile offshore drilling units of the semi-submersible type. [7]
The advantages of the semi-submersible vessel stability were soon recognized for offshore construction when in 1978 Heerema Marine Contractors constructed the two sister crane vessels called Balder and Hermod. These semi-submersible crane vessels (SSCV) consist of two lower hulls (pontoons), three columns on each pontoon and an upper hull. Shortly after J. Ray McDermott and Saipem also introduced SSCVs, resulting in two new enormous vessels DB-102 (now Thialf) and Saipem 7000 , capable of lifting respectively 14,200 and 14,000 tons.
During transit an SSCV is de-ballasted to a draught where only part of the lower hull is submerged. During lifting operations, the vessel is ballasted down. This way, the lower hull is well submerged. This reduces the effect of waves and swell. High stability is obtained by placing the columns far apart. The high stability allows them to lift extremely high loads safely.
Semi-submersibles are particularly suited to a number of offshore support vessel roles because of their good stability, large deck areas, and variable deck load (VDL).
Notable vessels are as follows:
When oil fields were first developed in offshore locations, drilling semi-submersibles were converted for use as combined drilling and production platforms. These vessels offered very stable and cost effective platforms. The first semi-submersible floating production platform was the Argyll FPF converted from the Transworld 58 drilling semi-submersible in 1975 for the Hamilton Brothers North Sea Argyll oil field.
As the oil industry progressed into deeper water and harsh environments, purpose-built production semi-submersible platforms were designed. The first purpose-built semi-submersible production platform was for the Balmoral field in the UK North Sea in 1986. [8]
Notable platforms are as follows:
Ocean Odyssey was converted from a mobile drilling rig to a launch platform in 1997. In total, 36 rockets were launched from the platform. The last launch happened in 2014, shortly before its owner Sea Launch significantly reduced all operations.
The potential application of converted semi-submersible oil drilling rigs is being explored by Space Exploration Technologies (SpaceX) for launching and landing of Starship. SpaceX has acquired two former offshore oil drilling rigs in 2021, similar to the ENSCO/Valaris 8506 offshore model, [9] but later sold them in 2023 after they were considered unsuitable. [10]
An oil platform is a large structure with facilities to extract and process petroleum and natural gas that lie in rock formations beneath the seabed. Many oil platforms will also have facilities to accommodate the workers, although it is also common to have a separate accommodation platform bridge linked to the production platform. Most commonly, oil platforms engage in activities on the continental shelf, though they can also be used in lakes, inshore waters, and inland seas. Depending on the circumstances, the platform may be fixed to the ocean floor, consist of an artificial island, or float. In some arrangements the main facility may have storage facilities for the processed oil. Remote subsea wells may also be connected to a platform by flow lines and by umbilical connections. These sub-sea facilities may include one or more subsea wells or manifold centres for multiple wells.
Offshore construction is the installation of structures and facilities in a marine environment, usually for the production and transmission of electricity, oil, gas and other resources. It is also called maritime engineering.
Ocean Ranger was a semi-submersible mobile offshore drilling unit that sank in Canadian waters on 15 February 1982. It was drilling an exploration well on the Grand Banks of Newfoundland, 267 kilometres (166 mi) east of St. John's, Newfoundland, for Mobil Oil of Canada, Ltd. (MOCAN) with 84 crew members on board when it sank. There were no survivors.
Mighty Servant 2 was a 29,000-ton semi-submersible, heavy-lift ship operated by Dockwise. The ship drew worldwide attention in 1988 for transporting the mine-damaged USS Samuel B. Roberts from Dubai to Newport, Rhode Island.
The Saipem 7000 is the world's third largest semi-submersible crane vessel, after the SSCV Sleipnir and the SSCV Thialf. It is owned by the oil and gas industry contractor Saipem S.p.A.
A drillship is a merchant vessel designed for use in exploratory offshore drilling of new oil and gas wells or for scientific drilling purposes. In recent years the vessels have been used in deepwater and ultra-deepwater applications, equipped with the latest and most advanced dynamic positioning systems.
A ballast tank is a compartment within a boat, ship or other floating structure that holds water, which is used as ballast to provide hydrostatic stability for a vessel, to reduce or control buoyancy, as in a submarine, to correct trim or list, to provide a more even load distribution along the hull to reduce structural hogging or sagging stresses, or to increase draft, as in a semi-submersible vessel or platform, or a SWATH, to improve seakeeping. Using water in a tank provides easier weight adjustment than the stone or iron ballast used in older vessels, and makes it easy for the crew to reduce a vessel's draft when it enters shallower water, by temporarily pumping out ballast. Airships use ballast tanks mainly to control buoyancy and correct trim.
A moon pool is an equipment deployment and retrieval feature used by marine drilling platforms, drillships, diving support vessels, fishing vessels, marine research and underwater exploration or research vessels, and underwater habitats. It is also known as a wet porch. It is an opening found in the floor or base of the hull, platform, or chamber giving access to the water below. Because of its stable location, it safely allows technicians or researchers to lower tools and instruments into the sea.
The SSCV Thialf is a huge semi-submersible crane vessel operated by Heerema Marine Contractors, and it was the largest crane vessel in the world until the SSCV Sleipnir became the largest in 2019.
A crane vessel, crane ship, crane barge, or floating crane is a ship with a crane specialized in lifting heavy loads, typically exceeding 1,500 t for modern ships. The largest crane vessels are used for offshore construction.
Offshore drilling is a mechanical process where a wellbore is drilled below the seabed. It is typically carried out in order to explore for and subsequently extract petroleum that lies in rock formations beneath the seabed. Most commonly, the term is used to describe drilling activities on the continental shelf, though the term can also be applied to drilling in lakes, inshore waters and inland seas.
Mighty Servant 3 is a 27,000-ton semi-submersible heavy lift ship. Its deck is 40 by 140 m. The vessel was built in 1984 by Oshima Shipbuilding Co. Ltd. in Ōshima, Japan, for Dutch shipping firm Wijsmuller Transport, which merged in 1993 with Dock Express Shipping to become Breda-based offshore heavy lifting group Dockwise Shipping B.V.
A jackup rig or a self-elevating unit is a type of mobile platform that consists of a buoyant hull fitted with a number of movable legs, capable of raising its hull over the surface of the sea. The buoyant hull enables transportation of the unit and all attached machinery to a desired location. Once on location the hull is raised to the required elevation above the sea surface supported by the sea bed. The legs of such units may be designed to penetrate the sea bed, may be fitted with enlarged sections or footings, or may be attached to a bottom mat. Generally jackup rigs are not self-propelled and rely on tugs or heavy lift ships for transportation.
A heavy-lift ship is a vessel designed to move very large loads that cannot be handled by normal ships. They are of two types:
Deepwater drilling, or deep well drilling, is the process of creating holes in the Earth's crust using a drilling rig for oil extraction under the deep sea. There are approximately 3400 deepwater wells in the Gulf of Mexico with depths greater than 150 meters.
Offshore concrete structures, or concrete offshore structures, are structures built from reinforced concrete for use in the offshore marine environment. They serve the same purpose as their steel counterparts in oil and gas production and storage. The first concrete oil platform was installed in the North Sea in the Ekofisk oil field in 1973 by Phillips Petroleum, and they have become a significant part of the marine construction industry. Since then at least 47 major concrete offshore structures have been built.
A submersible drilling rig is a marine vessel design that can be floated to location and lowered onto the sea floor for offshore drilling activities.
SSCV Sleipnir is a semi-submersible crane vessel (SSCV) owned and operated by Heerema Marine Contractors. It is named for Sleipnir, the eight-legged horse ridden by Odin in Norse mythology. The vessel is equipped with two revolving cranes built by Huisman Equipment B.V., each with a capacity of 10,000 t ; the main cranes can be operated in tandem to jointly lift 20,000 t. It was ordered in 2015 and built in Singapore by Sembcorp Marine. After its completion in 2019, SSCV Sleipnir succeeded Heerema's earlier SSCV Thialf as the largest crane vessel in the world.
Ocean development refers to the establishing of human activities at sea and use of the ocean, as well as its governance.
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