- Offshore concrete structure
Offshore concrete structures have been in use successfully for about 30 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, whereby 21 of the 47 concrete substructures have been designed (concept and detail designs ) by Dr. techn. Olav Olsen.
- 1 Introduction
- 2 Fixed structures
- 3 Floating structures
- 4 Major offshore concrete structures
- 5 References
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 will 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.
Concrete offshore structures show an excellent performance. 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 ad 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.
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.
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:
- Condeep (with one, two, three or four columns)
- ANDOC (with four columns)
- Sea Tank (with two or four columns)
- C G Doris
- Arup Concrete Gravity Substructure (CGS)
Condeep refers to a make of gravity base structure for oil platforms developed and fabricated by Norwegian Contractors in Norway. A 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 is operated by Norske Shell. 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 or most recent projects are:
- Sakhalin-II platforms (Molikpaq (Piltun-Astokhskoye A; PA-A) platform, Piltun-Astokhskoye B (PA-B) platform and Lunskoye (LUN-A) platform)
C G DORIS Type
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 Platform and the Schwedeneck platforms. Thedesign 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 desigin is quite similar to the Condeep type.
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 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.
Arup Concrete Gravity Substructure (CGS)
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 against salt water and keep maintenance costs low, floating concrete structures have become rather attractive to the oil and gas industry within the last two decades. Temporarily floating structures like e.g. the Condeep platforms float during construction and tow-out but will finally be placed on the ground. Permanently floating concrete structures are used in the discovery of oil and gas deposits, for oil and gas production, as storage and offloading units and also for 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 is Troll B.
Tension Leg Platform (TLP)
A TLP 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 “weather vaning” 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.
Major offshore concrete structures
Following table summarizes the major eziating offshore concrete structures.
No. Year Installed Operator Field/Unit Structure Type Depth Location Design by 1 1973 Phillips Ekofisk Tank - DORIS 71 m North Sea (N) DORIS 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 5 1975 Elf Frigg CDP1 CGS 1 shaft, Jarlan Wall 104 m North Sea (UK) DORIS 6 1976 Shell Brent D Condeep 3 shafts 140 m North Sea (UK) NC/Olav Olsen 7 1976 Elf Frigg TP1 CGS 2 shafts 104 m North Sea (UK) Sea Tank 8 1976 Elf Frigg MCP-01 CGS 1 shaft, Jarlan Wall 94 m North Sea (N) DORIS 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 11 1977 Mobil Statfjord A Condeep 3 shafts 145 m North Sea (N) NC/Olav Olsen 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 Shell 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 19 1981 Amoco Canada Tarsiut Island 4 hollow caissons 16 m Beaufort Sea ? 20 1982 Phillips Maureen ALC Concrete base artic. LC 92 m North Sea (UK) ? 21 1983 Texaco Schwedeneck A* CGS Monotower 25 m North Sea (D) DORIS/IMS 22 1983 Texaco Schwedeneck B* CGS Monotower 16 m North Sea (D) DORIS/IMS 23 1984 Mobil Statfjord C Condeep 4 shafts 145 m North Sea (N) NC/Olac Olsen 24 1984 Global Marine Super CIDS CGS caisson, Island 16 m Beaufort Sea ? 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 27 1988 Norsk Hydro] Oseberg A Condeep 4 shafts 109 m North Sea (N) NC/Olav Olsen 28 1989 Statoil Gullfaks C Condeep 4 shafts 216 m North Sea (N) NC/olav Olsen 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 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 34 1993 Statoil Sleipner A Condeep 4 shafts 82 m North Sea (N) NC/Olav Olsen 35 1993 Shell Draugen Condeep Monotower 251 m North Sea (N) NC/Olav Olsen 36 1994 Conoco Heidrun Condeep 350 m North Sea (N) NC/Olav Olsen 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 39 1995 Conoco Heidrun TLP Concrete TLP 350 m North Sea (N) NC/Olav Olsen 40 1995 Norsk Hydro Troll B Semisub 325 m North Sea (N) DORIS 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 Investment Company Ltd. (SEIC) Lunskoye A CGS 4 shafts 48 m Sakhalin (R) AK/GMAO 48 2005 Sakhalin Energy Investment Company Ltd. (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 (MPU filed for bankruptcy before completion, it was thereafter demolished) Heavy Lift Vessel LWA n/a na Olav Olsen
Clauss, Günther; E. Lehmann and C. Österfaard (1992). Offshore Structures Volume I: Conceptual Design and Hydromechanics. pringer-Verlag Berlin Heidelberg New York. ISBN 978-3-540-19709-6.
Clauss, Günther; E. Lehmann and C. Österfaard (1993). Offshore Structures Volume II: Strength and Safety for Structural Design. Springer Verlag Berlin Heidelberg New York. ISBN 978-3-540-19770-6.
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