Offshore concrete structure

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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.

Contents

Introduction

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 ]

Fixed structures

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 Type

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 ]

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 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.

McAlpine/Sea Tank

This design is quite similar to the Condeep type.[ citation needed ]

ANDOC 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 (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.

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.

Floating structures

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

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 ]

Tension Leg Platform (TLP)

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.

Barge/Ship Design

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.

Major offshore concrete structures

Following table summarizes the major[ according to whom? ] existing offshore concrete structures.

No.Year InstalledOperatorField/UnitStructure TypeDepthLocationDesign byConstruction byStatus
11973 Phillips Ekofisk Tank - DORIS71 mNorth Sea (N)DORISAIP
21974 Atlantic Richfield Ardjuna FieldLPG Barge43 mIndonesiaBerger/ABAM
31975 Mobil Beryl A Condeep 3 shafts118 mNorth Sea (UK)NC/Olav Olsen
41975 Shell Brent B Condeep 3 shafts140 mNorth Sea (UK)NC/Olav OlsenCondeep GroupAIP
51975 Elf Frigg CDP1CGS 1 shaft, Jarlan Wall104 mNorth Sea (UK)DORISAIP 2009
61976 Shell Brent D Condeep 3 shafts140 mNorth Sea (UK)NC/Olav OlsenCondeep Group
71976 Elf Frigg TP1CGS 2 shafts104 mNorth Sea (UK)Sea TankAIP 2009
81976 Elf Frigg MCP-01CGS 1 shaft, Jarlan Wall94 mNorth Sea (N)DORISAIP 2009
91977 Shell Dunlin ACGS 4 shafts153 mNorth Sea (UK)ANDOC
101977 Elf Frigg TCP2 Condeep 3 shafts104 mNorth Sea (N)NC/Olav OlsenAIP 2009
111977 Mobil Statfjord A Condeep 3 shafts145 mNorth Sea (N)NC/Olav OlsenNC
121977 Petrobras Ubarana-Pub 3CGS caisson15 mBrazil ?
131978 Petrobras Ubarana-Pub 2CGS caisson15 mBrazil ?
141978 Petrobras Ubarana-Pag 2CGS caisson15 mBrazil ?
151978TAQA BrataniCormorant ACGS 4 shafts149 mNorth Sea (UK)Sea Tank
161978 Chevron Ninian Central CGS 1 shaft, Jarlan Wall136 mNorth Sea (UK)DORIS
171978 Shell Brent CCGS 4 shafts141 mNorth Sea (UK)Sea Tank
181981 Mobil Statfjord B Condeep 4 shafts145 mNorth Sea (N)NC/olav OlsenNC
191981 Amoco Canada Tarsiut Island4 hollow caissons16 mBeaufort Sea ?Removed
201982 Phillips Maureen ALCConcrete base artic. LC92 mNorth Sea (UK) ?Removed
211983 Texaco Schwedeneck A*CGS Monotower25 mNorth Sea (D)DORIS/IMSRemoved
221983 Texaco Schwedeneck B*CGS Monotower16 mNorth Sea (D)DORIS/IMSRemoved
231984 Mobil Statfjord C Condeep 4 shafts145 mNorth Sea (N)NC/Olac OlsenNC
241984Global MarineSuper CIDSCGS caisson, Island16 mBeaufort Sea ?Removed
251986 Statoil Gullfaks A Condeep 4 shafts135 mNorth Sea (N)NC/Olav Olsen
261987 Statoil Gullfaks B Condeep 3 shafts141 mNorth Sea (N)NC/Olav OlsenNC
271988Norsk Hydro]Oseberg A Condeep 4 shafts109 mNorth Sea (N)NC/Olav OlsenNC
281989 Statoil Gullfaks C Condeep 4 shafts216 mNorth Sea (N)NC/olav OlsenNC
291989Hamilton BrosN. RavenspurnCGS 3 shafts42 mNorth Sea (UK) Arup
301989 Phillips Ekofisk P.B CGS Protection Ring75 mNorth Sea (N)DORISAIP
311996Elf CongoN'KossaConcrete Barge170 mCongoBOS/Bouygues
321993 Shell NAM F3-FBCGS 3 shafts43 mNorth Sea (NL)Hollandske Bet.
331992 Saga Snorre Concrete Foundation Templates (CFT)3 cells suction anchores310 mNorth Sea (N)NC/Olav OlsenNC
341993 Statoil Sleipner A Condeep 4 shafts82 mNorth Sea (N)NC/Olav OlsenNC
351993 Shell Draugen Condeep Monotower251 mNorth Sea (N)NC/Olav OlsenNC
361994 Conoco HeidrunConcrete TLP350 mNorth Sea (N)NC/Olav OlsenNC
371996 BP Harding CGS109 mNorth Sea (UK)Taylor Wood Eng.
381995 Shell Troll A Condeep 4 shafts303 mNorth Sea (N)NC/Olav OlsenNC
391995 Conoco Heidrun TLPConcrete TLP350 mNorth Sea (N)NC/Olav OlsenNC
401995 Norsk Hydro Troll B Semisub325 mNorth Sea (N)DORISKCC
411996 Esso West TunaCGS 3 shafts61 mAustraliaKinhill/DORIS
421996 Esso Bream BCGS 1 shaft61 mAustraliaKinhill/DORIS
431996AmpolexWandooCGS 4 shafts54 mAustralia Arup
441997 Mobil Hibernia CGS 4 shafts80 mCanadaDORIS
451999 Amerada Hess South ArneCGS 1 shaft60 mNorth Sea (DK)Taylor Woodrow
462000 Shell MalampayaCGS 4 shafts43 mPhilippines Arup
472005Sakhalin Energy (SEIC)Lunskoye ACGS 4 shafts48 mSakhalin (R)AK/GMAO
482005Sakhalin Energy (SEIC)Sakhalin PA-BCGS 4 shafts30 mSakhalin (R)AK/GMAO
492008 ExxonMobil Adriatic LNG LNG terminal29 mAdriatic Sea (I)AK/GMAO
502008MPU Heavy Lifter (Not completed)Heavy Lift VesselLWAn/anaOlav OlsenDemolished
512012Exxon Neftegas Limited (ENL)Sakhalin-1 Arkutun Dagi (Golden Eagle)GBS 4 shafts33 mSakhalin-1 (R)AK/GMAO
522017ExxonMobil Canada PropertiesHebronGBS Monotower109 mCanadaKKC/GMAOKKC
5320??Husky EnergyWest White RoseGBS Monotower118 mCanadaArup

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<span class="mw-page-title-main">Offshore construction</span> Installation of structures and facilities in a marine environment

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.

Troll A platform

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.

Semi-submersible platform

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<span class="mw-page-title-main">Troll gas field</span>

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Fixed platform

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References

  1. "Patent number 135909". Norwegian Industrial Property Office.
  2. "Troll Gas fact page". Statoil. Retrieved 2 April 2013.

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