Location

Arlington, Virginia

Session Start Date

8-11-2008

Session End Date

8-16-2008

Abstract

The paper focuses on the geotechnical design issues facing the design team responsible for the provision of an offshore Gravity Base Structure (GBS) to act as a clump weight for a Power Buoy located in the Lyell field of the northern North Sea, United Kingdom. The structure is to be positioned on the seabed, where ground conditions are considered variable but in general comprise of a surface layer of loose to very dense silty sand, underlain by a thick sequence of firm to very stiff sandy clay. Geotechnical data specific to the location of the GBS is limited; problems encountered during testing allowed only three cone penetration tests to be carried out to maximum depth of 12m and sampling for subsequent laboratory testing was not possible. However, correlation between the results of these tests and CPT results for other areas in the North Sea, allowed geotechnical properties to be inferred and to be used as the basis for the geotechnical design of the foundation. Assessment of the bearing capacity of the structure under hydrodynamic loading, as well as the resistance to sliding indicated that there maybe a risk of instability. As a consequence, a perimeter skirt was specified in order to reduce this risk. A discussion of the geotechnical issues considered during the design process is presented. Other design issues such as cyclic loading and penetration resistance in relation to a perimeter skirt, were considered and are commented on within this paper. The case history highlights some of the design problems faced by geotechnical engineers when designing structures for the offshore environment, and emphasizes the necessity for a comprehensive and, site specific, ground investigation, in order to facilitate the design process.

Department(s)

Civil, Architectural and Environmental Engineering

Appears In

International Conference on Case Histories in Geotechnical Engineering

Meeting Name

Sixth Conference

Publisher

Missouri University of Science and Technology

Publication Date

8-11-2008

Document Version

Final Version

Rights

© 2008 Missouri University of Science and Technology, All rights reserved.

Document Type

Article - Conference proceedings

File Type

text

Language

English

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Geotechnical Design of an Offshore Gravity Base Structure

Arlington, Virginia

The paper focuses on the geotechnical design issues facing the design team responsible for the provision of an offshore Gravity Base Structure (GBS) to act as a clump weight for a Power Buoy located in the Lyell field of the northern North Sea, United Kingdom. The structure is to be positioned on the seabed, where ground conditions are considered variable but in general comprise of a surface layer of loose to very dense silty sand, underlain by a thick sequence of firm to very stiff sandy clay. Geotechnical data specific to the location of the GBS is limited; problems encountered during testing allowed only three cone penetration tests to be carried out to maximum depth of 12m and sampling for subsequent laboratory testing was not possible. However, correlation between the results of these tests and CPT results for other areas in the North Sea, allowed geotechnical properties to be inferred and to be used as the basis for the geotechnical design of the foundation. Assessment of the bearing capacity of the structure under hydrodynamic loading, as well as the resistance to sliding indicated that there maybe a risk of instability. As a consequence, a perimeter skirt was specified in order to reduce this risk. A discussion of the geotechnical issues considered during the design process is presented. Other design issues such as cyclic loading and penetration resistance in relation to a perimeter skirt, were considered and are commented on within this paper. The case history highlights some of the design problems faced by geotechnical engineers when designing structures for the offshore environment, and emphasizes the necessity for a comprehensive and, site specific, ground investigation, in order to facilitate the design process.