Offshore concrete structures have been in use successfully for about 50 years. They serve the same purpose as their steel counterparts in the oil and gas production and storage. The first concrete oil platform has been installed in the North Sea in the Ekofisk field in 1973 by Phillips Petroleum. Since then 47 major concrete offshore structures have been built.
Concrete offshore structures are mostly used in the petroleum industry as drilling, extraction or storage units for crude oil or natural gas. Those large structures house machinery and equipment needed to drill and/or extract oil and gas. But concrete structures are not only limited to applications within the oil and gas industry. Several conceptual studies have shown recently, that concrete support structures for offshore wind turbines are very competitive compared to common steel structures, especially for larger water depths.
Depending on the circumstances, platforms may be attached to the ocean floor, consist of an artificial island, or be floating. Generally, offshore concrete structures are classified into fixed and floating structures. Fixed structures are mostly built as concrete gravity based structures (CGS, also termed as caisson type), where the loads bear down directly on the uppermost layers as soil pressure. The caisson provides buoyancy during construction and towing and acts also as a foundation structure in the operation phase. Furthermore, the caisson could be used as storage volume for oil or other liquids.
Floating units may be held in position by anchored wires or chains in a spread mooring pattern. Because of the low stiffness in those systems, the natural frequency is low and the structure can move in all six degrees of freedom. Floating units serve as productions units, storage and offloading units (FSO) or for crude oil or as terminals for liquefied natural gas (LNG). A more recent development is concrete sub-sea structures.[ citation needed ]
Concrete offshore structures show an excellent performance.[ clarification needed ] They are highly durable, constructed of almost maintenance-free material, suitable for harsh and/or arctic environment (like ice and seismic regions), can carry heavy topsides, often offer storage capacities, are suitable for soft grounds and are very economical for water depths larger than 150m. Most gravity-type platforms need no additional fixing because of their large foundation dimensions and extremely high weight.[ citation needed ]
Since the 1970s, several fixed concrete platform designs have been developed. Most of the designs have in common a base caisson (normally for storage of oil) and shafts penetrating the water surface to carry the topside. In the shafts normally utility systems for offloading, drilling, draw down and ballast are put up.[ citation needed ]
Concrete offshore platforms of the gravity-base type are almost always constructed in their vertical attitude. This allows the inshore installation of deck girders and equipment and the later transport of the whole structure to the installation site.
The most common concrete designs are:[ citation needed ]
Condeep refers to a make of gravity base structure for oil platforms invented by Olav Mo [1] and fabricated by Norwegian Contractors in Norway. Condeep usually consists of a base of concrete oil storage tanks from which one, three or four concrete shafts rise. The original Condeep always rests on the sea floor, and the shafts rise to about 30m above the sea level. The platform deck itself is not a part of the construction. The Condeep Platforms Brent B (1975) and Brent D (1976) were designed for a water depth of 142m in the Brent oilfield operated by Shell. Their main mass is represented by the storage tank (ca. 100m diameter and 56m high, consisting of 19 cylindrical compartments with 20m diameter). Three of the cells are extended into shafts tapering off at the surface and carrying a steel deck. The tanks serve as storage of crude oil in the operation phase. During the installation these tanks have been used as ballast compartment. Among the largest Condeep type platform are the Troll A platform and the Gullfaks C. Troll A was built within four years and deployed in 1995 to produce gas from the Troll oil field which was developed by Norske Shell, since 1996 operated by Statoil. [2] A detailed overview about Condeep platforms is given in a separate article.
Concrete Gravity Base Structures (CGBS) is a further development of the first-generation Condeep drilling/production platforms installed in the North Sea between the late 1970s and mid '90s. The CGBS have no oil storage facilities and the topside installations will be carried out in the field by a float-over mating method. Current[ when? ] or most recent projects are:[ citation needed ]
The first concrete gravity platform in the North Sea was a C G Doris platform, the Ekofisk Tank, in Norwegian waters. The structure has a shape not unlike a marine sea island and is surrounded by a perforated breakwater wall (Jarlan patent). The original proposal of the French group C G DORIS (Compagnie General pour les Developments Operationelles des Richesses Sous-Marines) for a prestressed post-tensioned concrete "island" structure was adopted on cost and operational grounds. DORIS was general contractor responsible for the structural design: the concrete design was prepared and supervised on behalf of DORIS by Europe-Etudes. Further example for the C G DORIS designs are the Frigg platforms, the Ninian Central Platform and the Schwedeneck platforms.[ citation needed ] The design typically consists of a large volume caisson based on the sea floor merging into a monolithic structure, which is offering the base for the deck. The single main leg is surrounded by an outer breaker wall perforated with so called Jarlan holes. This wall is intended to break up waves, thus reducing their forces.
This design is quite similar to the Condeep type.[ citation needed ]
To achieve its goal and extract oil within five years after discovering the Brent reservoir Shell divided up the construction of four offshore platforms. Redpath Dorman Long at Methil in Fife, Scotland getting Brent A, the two concrete Condeeps B and D were to be built in Norway by Norwegian Contractors (NC) of Stavanger, and C (also concrete) was to be built by McAlpine at Ardyne Point on the Clyde (which is known as the ANDOC (Anglo Dutch Offshore Concrete) design). The ANDOC design can be considered as the British construction industry's attempt to compete with Norway in this sector. McAlpine constructed three concrete platforms for the North Sea oil industry at Ardyne Point. The ANDOC type is very similar to the Sea Tank design, but the four concrete legs terminate and steel legs take over to support the deck.
The Arup dry-build Concrete Gravity Substructure (CGS) concept was originally developed by Arup in 1989 for Hamilton Brothers' Ravenspurn North. The Arup CGS are designed to be simple to install, and are fully removable. Simplicity and repetition of concrete structural elements, low reinforcement and pre-stress densities as well as the use of normal density concrete lead to economical construction costs. Typical for the Arup CGS is the inclined installation technique. This technique helps to maximise economy and provide a robust offshore emplacement methodology. Further projects have been the Malampaya project in the Philippines and the Wandoo Full Field Development on the North West Shelf of Western Australia.
Since concrete is quite resistant to corrosion from salt water and keeps maintenance costs low, floating concrete structures have become increasingly attractive to the oil and gas industry in the last two decades. Temporary floating structures such as the Condeep platforms float during construction but are towed out and finally ballasted until they sit on the sea floor. Permanent floating concrete structures have various uses including the discovery of oil and gas deposits, in oil and gas production, as storage and offloading units and in heavy lifting systems.
Common designs for floating concrete structures are the barge or ship design, the platform design (semi-submersible, TLP) as well as the floating terminals e.g. for LNG.
Floating production, storage, and offloading systems (FPSOS) receive crude oil from deep-water wells and store it in their hull tanks until the crude is transferred into tank ships or transport barges. In addition to FPSO’s, there have been a number of ship-shaped Floating Storage and Offloading (FSO) systems (vessels with no production processing equipment) used in these same areas to support oil and gas developments. An FSO is typically used as a storage unit in remote locations far from pipelines or other infrastructures.
Semi-submersible marine structures are typically only movable by towing. Semi-submersible platforms have the principal characteristic of remaining in a substantially stable position, presenting small movements when they experience environmental forces such as the wind, waves and currents. Semi-Submersible platforms have pontoons and columns, typically two parallel spaced apart pontoons with buoyant columns upstanding from those pontoons to support a deck. Some of the semi-submersible vessels only have a single caisson, or column, usually denoted as a buoy while others utilize three or more columns extended upwardly from buoyant pontoons. For activities which require a stable offshore platform, the vessel is then ballasted down so that the pontoons are submerged, and only the buoyant columns pierce the water surface - thus giving the vessel a substantial buoyancy with a small water-plane area. The only concrete semi-submersible in existence[ when? ] is Troll B.[ citation needed ]
A Tension Leg Platform is a buoyant platform, which is held in place by a mooring system. TLP mooring is different to conventional chained or wire mooring systems. The platform is held in place with large steel tendons fastened to the sea floor. Those tendons are held in tension by the buoyancy of the hull. Statoil's Heidrun TLP is the only one with a concrete hull, all other TLPs have steel hulls.
FPSO or FSO systems are typically barge/ship-shaped and store crude oil in tanks located in the hull of the vessel. Their turret structures are designed to anchor the vessel, allow “weathervaning” of the units to accommodate environmental conditions, permit the constant flow of oil and production fluids from vessel to undersea field, all while being a structure capable of quick disconnect in the event of emergency.
The first barge of prestressed concrete has been designed in the early 1970s as an LPG (liquefied petroleum gas) storage barge in the Ardjuna Field (Indonesia). This barge is built of reinforced and prestressed concrete containing cylindrical tanks each having a cross-section perpendicular to its longitudinal axes that comprises a preferably circular curved portion corresponding to the bottom.
Following table summarizes the major[ according to whom? ] existing offshore concrete structures.
No. | Year Installed | Operator | Field/Unit | Structure Type | Depth | Location | Design by | Construction by | Status |
---|---|---|---|---|---|---|---|---|---|
1 | 1973 | Phillips | Ekofisk | Tank - DORIS | 71 m | North Sea (N) | DORIS | AIP | |
2 | 1974 | Atlantic Richfield | Ardjuna Field | LPG Barge | 43 m | Indonesia | Berger/ABAM | ||
3 | 1975 | Mobil | Beryl A | Condeep 3 shafts | 118 m | North Sea (UK) | NC/Olav Olsen | ||
4 | 1975 | Shell | Brent B | Condeep 3 shafts | 140 m | North Sea (UK) | NC/Olav Olsen | Condeep Group | AIP |
5 | 1975 | Elf | Frigg CDP1 | CGS 1 shaft, Jarlan Wall | 104 m | North Sea (UK) | DORIS | AIP 2009 | |
6 | 1976 | Shell | Brent D | Condeep 3 shafts | 140 m | North Sea (UK) | NC/Olav Olsen | Condeep Group | |
7 | 1976 | Elf | Frigg TP1 | CGS 2 shafts | 104 m | North Sea (UK) | Sea Tank | AIP 2009 | |
8 | 1976 | Elf | Frigg MCP-01 | CGS 1 shaft, Jarlan Wall | 94 m | North Sea (N) | DORIS | AIP 2009 | |
9 | 1977 | Shell | Dunlin A | CGS 4 shafts | 153 m | North Sea (UK) | ANDOC | ||
10 | 1977 | Elf | Frigg TCP2 | Condeep 3 shafts | 104 m | North Sea (N) | NC/Olav Olsen | AIP 2009 | |
11 | 1977 | Mobil | Statfjord A | Condeep 3 shafts | 145 m | North Sea (N) | NC/Olav Olsen | NC | |
12 | 1977 | Petrobras | Ubarana-Pub 3 | CGS caisson | 15 m | Brazil | ? | ||
13 | 1978 | Petrobras | Ubarana-Pub 2 | CGS caisson | 15 m | Brazil | ? | ||
14 | 1978 | Petrobras | Ubarana-Pag 2 | CGS caisson | 15 m | Brazil | ? | ||
15 | 1978 | TAQA Bratani | Cormorant A | CGS 4 shafts | 149 m | North Sea (UK) | Sea Tank | ||
16 | 1978 | Chevron | Ninian Central | CGS 1 shaft, Jarlan Wall | 136 m | North Sea (UK) | DORIS | ||
17 | 1978 | Shell | Brent C | CGS 4 shafts | 141 m | North Sea (UK) | Sea Tank | ||
18 | 1981 | Mobil | Statfjord B | Condeep 4 shafts | 145 m | North Sea (N) | NC/olav Olsen | NC | |
19 | 1981 | Amoco Canada | Tarsiut Island | 4 hollow caissons | 16 m | Beaufort Sea | ? | Removed | |
20 | 1982 | Phillips | Maureen ALC | Concrete base artic. LC | 92 m | North Sea (UK) | ? | Removed | |
21 | 1983 | Texaco | Schwedeneck A* | CGS Monotower | 25 m | North Sea (D) | DORIS/IMS | Removed | |
22 | 1983 | Texaco | Schwedeneck B* | CGS Monotower | 16 m | North Sea (D) | DORIS/IMS | Removed | |
23 | 1984 | Mobil | Statfjord C | Condeep 4 shafts | 145 m | North Sea (N) | NC/Olac Olsen | NC | |
24 | 1984 | Global Marine | Super CIDS | CGS caisson, Island | 16 m | Beaufort Sea | ? | Removed | |
25 | 1986 | Statoil | Gullfaks A | Condeep 4 shafts | 135 m | North Sea (N) | NC/Olav Olsen | ||
26 | 1987 | Statoil | Gullfaks B | Condeep 3 shafts | 141 m | North Sea (N) | NC/Olav Olsen | NC | |
27 | 1988 | Norsk Hydro] | Oseberg A | Condeep 4 shafts | 109 m | North Sea (N) | NC/Olav Olsen | NC | |
28 | 1989 | Statoil | Gullfaks C | Condeep 4 shafts | 216 m | North Sea (N) | NC/olav Olsen | NC | |
29 | 1989 | Hamilton Bros | N. Ravenspurn | CGS 3 shafts | 42 m | North Sea (UK) | Arup | ||
30 | 1989 | Phillips | Ekofisk P.B | CGS Protection Ring | 75 m | North Sea (N) | DORIS | AIP | |
31 | 1996 | Elf Congo | N'Kossa | Concrete Barge | 170 m | Congo | BOS/Bouygues | ||
32 | 1993 | Shell | NAM F3-FB | CGS 3 shafts | 43 m | North Sea (NL) | Hollandske Bet. | ||
33 | 1992 | Saga | Snorre Concrete Foundation Templates (CFT) | 3 cells suction anchores | 310 m | North Sea (N) | NC/Olav Olsen | NC | |
34 | 1993 | Statoil | Sleipner A | Condeep 4 shafts | 82 m | North Sea (N) | NC/Olav Olsen | NC | |
35 | 1993 | Shell | Draugen | Condeep Monotower | 251 m | North Sea (N) | NC/Olav Olsen | NC | |
36 | 1994 | Conoco | Heidrun | Concrete TLP | 350 m | North Sea (N) | NC/Olav Olsen | NC | |
37 | 1996 | BP | Harding | CGS | 109 m | North Sea (UK) | Taylor Wood Eng. | ||
38 | 1995 | Shell | Troll A | Condeep 4 shafts | 303 m | North Sea (N) | NC/Olav Olsen | NC | |
39 | 1995 | Conoco | Heidrun TLP | Concrete TLP | 350 m | North Sea (N) | NC/Olav Olsen | NC | |
40 | 1995 | Norsk Hydro | Troll B | Semisub | 325 m | North Sea (N) | DORIS | KCC | |
41 | 1996 | Esso | West Tuna | CGS 3 shafts | 61 m | Australia | Kinhill/DORIS | ||
42 | 1996 | Esso | Bream B | CGS 1 shaft | 61 m | Australia | Kinhill/DORIS | ||
43 | 1996 | Ampolex | Wandoo | CGS 4 shafts | 54 m | Australia | Arup | ||
44 | 1997 | Mobil | Hibernia | CGS 4 shafts | 80 m | Canada | DORIS | ||
45 | 1999 | Amerada Hess | South Arne | CGS 1 shaft | 60 m | North Sea (DK) | Taylor Woodrow | ||
46 | 2000 | Shell | Malampaya | CGS 4 shafts | 43 m | Philippines | Arup | ||
47 | 2005 | Sakhalin Energy (SEIC) | Lunskoye A | CGS 4 shafts | 48 m | Sakhalin (R) | AK/GMAO | ||
48 | 2005 | Sakhalin Energy (SEIC) | Sakhalin PA-B | CGS 4 shafts | 30 m | Sakhalin (R) | AK/GMAO | ||
49 | 2008 | ExxonMobil | Adriatic LNG | LNG terminal | 29 m | Adriatic Sea (I) | AK/GMAO | ||
50 | 2008 | MPU Heavy Lifter (Not completed) | Heavy Lift Vessel | LWA | n/a | na | Olav Olsen | Demolished | |
51 | 2012 | Exxon Neftegas Limited (ENL) | Sakhalin-1 Arkutun Dagi (Golden Eagle) | GBS 4 shafts | 33 m | Sakhalin-1 (R) | AK/GMAO | ||
52 | 2017 | ExxonMobil Canada Properties | Hebron | GBS Monotower | 109 m | Canada | KKC/GMAO | KKC | |
53 | 20?? | Husky Energy | West White Rose | GBS Monotower | 118 m | Canada | Arup | ||
An oil platform, oil rig, offshore platform, or oil and/or gas production 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 contain facilities to accommodate their workforce, although it is also common for there to be 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 solutions may consist of one or more subsea wells or of one or more 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.
The Troll A platform is a Condeep gravity-based structure offshore natural gas platform in the Troll gas field off the west coast of Norway. Built from reinforced concrete, as of 2014, it was the tallest structure that has ever been moved to another position, relative to the surface of the Earth, and is among the largest and most complex engineering projects in history. The platform was a televised sensation when it was towed into the North Sea in 1996, where it is now operated by Equinor. It is often incorrectly referenced as the heaviest object ever moved as well, but it is actually the second heaviest after another Condeep Oil platform the Gullfaks C, which had a displacement approaching 1.5 million tons in comparison with Troll A's 1.2 million.
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.
A floating production storage and offloading (FPSO) unit is a floating vessel used by the offshore oil and gas industry for the production and processing of hydrocarbons, and for the storage of oil. An FPSO vessel is designed to receive hydrocarbons produced by itself or from nearby platforms or subsea template, process them, and store oil until it can be offloaded onto a tanker or, less frequently, 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 is referred to as a floating storage and offloading (FSO) vessel.
The Brent field was an oil and gas field located in the East Shetland Basin of the North Sea, 186 kilometres (116 mi) north-east of Lerwick, Shetland Islands, Scotland, at the water depth of 140 metres (460 ft). The field operated by Shell UK Limited was discovered in 1971 and was once one of the most productive parts of the UK's offshore assets but has reached the stage where production is no longer economically viable. Decommissioning of the Brent field is complete with the exception of Brent C, which is producing from another field. The discovery well 211/26-1 was drilled in 1971 by the semi-submersible drilling rig "Staflo". This was a major surprise at the time as the nearest land in Scotland and Norway is composed of granite and other non reservoir metamorphic rocks.
Hibernia is an oil field in the North Atlantic Ocean, approximately 315 kilometres (196 mi) east-southeast of St. John's, Newfoundland, Canada, in 80 m of water.
Troll is a natural gas and oil field in the Norwegian sector of the North Sea, one of the biggest in the North Sea, holding 40% of Norway’s gas – it also possesses significant quantities of oil, in thin zones under the gas cap, to the west of the field. The field as a whole consists of the main Troll East and Troll West structures in blocks 31/2, 31/3, 31/5 and 31/6, about 65 kilometres (40 mi) west of Kollsnes, near Bergen. Most of the gas lies in Troll East.
Condeep is a make of gravity-based structure for oil platforms invented and patented by engineer Olav Mo in 1972, which were fabricated by Norwegian Contractors in Stavanger, Norway. Condeep is an abbreviation for concrete deep water structure. A Condeep usually consists of a base of concrete oil storage tanks from which one, three or four concrete shafts rise. The Condeep base always rests on the sea floor, and the shafts rise to about 30 meters above the sea level. The platform deck itself is not a part of the construction.
Malaysia Marine and Heavy Engineering Holdings Berhad is a Malaysian owned shipbuilding and heavy engineering industries company. It was formerly known as Malaysia Shipyard and Engineering Sdn Bhd. MHB has been long involved in oil and gas engineering and construction works.
Very large floating structures (VLFSs) or very large floating platforms (VLFPs) are artificial islands, which may be constructed to create floating airports, bridges, breakwaters, piers and docks, storage facilities, wind and solar power plants, for military purposes, to create industrial space, emergency bases, entertainment facilities, recreation parks, mobile offshore structures and even for habitation. Currently, several different concepts have been proposed for building floating cities or huge living complexes. Some units have been constructed and are presently in operation.
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.
"Offshore", when used in relation to hydrocarbons, refers to operations undertaken at, or under the, sea in association with an oil, natural gas or condensate field that is under the seabed, or to activities carried out in relation to such a field. Offshore is part of the upstream sector of the oil and gas industry.
A fixed platform is a type of offshore platform used for the extraction of petroleum or gas. These platforms are built on concrete and/or steel legs anchored directly onto the seabed, supporting a deck with space for drilling rigs, production facilities and crew quarters. Such platforms are, by virtue of their immobility, designed for very long-term use. Various types of structure are used, steel jacket, concrete caisson, floating steel and even floating concrete. Steel jackets are vertical sections made of tubular steel members, and are usually piled into the seabed. Concrete caisson structures, pioneered by the Condeep concept, often have in-built oil storage in tanks below the sea surface and these tanks were often used as a flotation capability, allowing them to be built close to shore and then floated to their final position where they are sunk to the seabed. Fixed platforms are economically feasible for installation in water depths up to about 500 feet ; for deeper depths a floating production system, or a subsea pipeline to land or to shallower water depths for processing, would usually be considered.
The Hutton oil field, located on the UK continental shelf, was the location for the first ever production Tension Leg Platform (TLP).
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.
MODEC Inc. is a global supplier and operator of offshore floating platforms.
Suction caissons are a form of fixed platform anchor in the form of an open bottomed tube embedded in the sediment and sealed at the top while in use so that lifting forces generate a pressure differential that holds the caisson down. They have a number of advantages over conventional offshore foundations, mainly being quicker to install than deep foundation piles and being easier to remove during decommissioning. Suction caissons are now used extensively worldwide for anchoring large offshore installations, like oil platforms, offshore drillings and accommodation platforms to the seafloor at great depths. In recent years, suction caissons have also seen usage for offshore wind turbines in shallower waters.
Sesam is a software suite for structural and hydrodynamic analysis of ships and offshore structures. It is based on the displacement formulation of the Finite Element Method.
The Schredeneck-See oil field was a significant oil field in the German sector of the Baltic Sea. It was the first oil field in the Baltic, and was Germany’s first offshore oil field. lt produced oil from 1984 until 2000.