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 (OO).

Introduction

Concrete offshore strucutres 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.

Generally, offshore concrete structures can be 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 hold 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, almost maintenance-free material, suitable for harsh environment (like ice and seismic regions), can carry heavy topsides, often offer storage capacities, suitable for soft grounds, very economic for water depths larger than 150m. Most gravitity-type platforms need no dadditional fixing because of their large foundation dimensions and extremly high weight.

Construction types

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 seperate 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
• Malampaya
• Wandoo
• Molikpaq platform
• Piltun Astokhskoye (PA-B) platform
• Lunskoye (LUN-A) platform

The Ninian Platform consists of a large volume caisson based on the sea floor merging into a monolithic structure, which is offering the base for the deck.

Floating concrete strucutres ....

References

Printed Literature

• Clauss G., Lehmann E and Oesterfaard C. (1988) Offshore Structures Volume I, Textbook, Springer Verlag Berlin Heidelberg New York. ISBN 3-540-19709-5
• Clauss G., Lehmann E and Oesterfaard C. (1988) Offshore Structures Volume II, Textbook, Springer Verlag Berlin Heidelberg New York. ISBN 3-540-19770-2
• Olsen T.O. (2001) Recycling of offshore concrete structures, Structural Concrete, 2, No.3.

External links

• Dr.techn.Olav Olsen Inc. World leading designer of offshore concrete structures
• Dr.techn.Olav Olsen a.s World leading designer of offshore concrete structures
• Structurae [en]: Troll A Platform (1995)
• The sinking of the Sleipner A offshore platform