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Centrifuge modelling of riser-soil stiffness degradation in the touchdown zone of a steel catenary riser

Centrifuge modelling of riser-soil stiffness degradation in the touchdown zone of a steel catenary riser
Centrifuge modelling of riser-soil stiffness degradation in the touchdown zone of a steel catenary riser

Steel catenary risers (SCRs) are economical to assemble and install compared to conventional vertical risers. However, accurate evaluation of the fatigue life of an SCR remains a major challenge due to uncertainty surrounding the interaction forces at the seabed within the touchdown zone (TDZ). Fatigue life predictions are heavily dependant on the assumed stiffness between the riser and the seabed and therefore an accurate assessment of seabed stiffness - or more specifically the nonlinear pipe-soil resistance - is required. During the lifespan of an SCR, vessel motions due to environmental loading cause repeated penetration of the riser into the seabed within the TDZ. This behaviour makes assessment of seabed stiffness difficult due to the gross deformations of the seabed and the resulting soil remoulding and water entrainment. This paper describes a model test in which the movement of a length of riser pipe was simulated within the geotechnical beam centrifuge at the University of Western Australia. The model soil was soft, lightly over-consolidated kaolin clay with a linearly increasing shear strength profile with depth, typical of deepwater conditions. The pipe was cycled over a fixed vertical distance from an invert embedment of 0.5 diameters to above the soil surface. This range represents a typical vertical oscillation range of a section of riser within the TDZ during storm loading. The results indicate a significant degradation in the vertical pipe-soil resistance during cyclic vertical movements. Due to the cyclic degradation in soil strength, the component of the vertical resistance created by buoyancy was significant, particularly due to the influence of heave. A new approach to the interpretation of heave-enhanced buoyancy was used to extract the separate influences of soil strength and buoyancy, allowing the cyclic degradation in strength to be quantified. During cycling, the soil strength reduced by a factor of 7.5 relative to the initial penetration stage. This degradation was more significant than the reduction in soil strength during a cyclic T-bar penetration test. This contrast can be attributed to the breakaway of the pipe from the soil surface which allowed water entrainment. This dramatic loss of strength and therefore secant stiffness, and the significance of the buoyancy term in the total vertical pipe-soil resistance, has implications for the fatigue assessment of SCRs.

241-249
The American Society of Mechanical Engineers
Hodder, M.S.
2c7413ea-6ff3-42ec-b93c-8ac67cede77e
Cassidy, M.J.
095b5237-97db-4ee5-9cb1-0b68d4731497
White, D.J.
a986033d-d26d-4419-a3f3-20dc54efce93
Hodder, M.S.
2c7413ea-6ff3-42ec-b93c-8ac67cede77e
Cassidy, M.J.
095b5237-97db-4ee5-9cb1-0b68d4731497
White, D.J.
a986033d-d26d-4419-a3f3-20dc54efce93

Hodder, M.S., Cassidy, M.J. and White, D.J. (2008) Centrifuge modelling of riser-soil stiffness degradation in the touchdown zone of a steel catenary riser. In ASME 2008 27th International Conference on Offshore Mechanics and Arctic Engineering. vol. 3, The American Society of Mechanical Engineers. pp. 241-249 . (doi:10.1115/OMAE2008-57302).

Record type: Conference or Workshop Item (Paper)

Abstract

Steel catenary risers (SCRs) are economical to assemble and install compared to conventional vertical risers. However, accurate evaluation of the fatigue life of an SCR remains a major challenge due to uncertainty surrounding the interaction forces at the seabed within the touchdown zone (TDZ). Fatigue life predictions are heavily dependant on the assumed stiffness between the riser and the seabed and therefore an accurate assessment of seabed stiffness - or more specifically the nonlinear pipe-soil resistance - is required. During the lifespan of an SCR, vessel motions due to environmental loading cause repeated penetration of the riser into the seabed within the TDZ. This behaviour makes assessment of seabed stiffness difficult due to the gross deformations of the seabed and the resulting soil remoulding and water entrainment. This paper describes a model test in which the movement of a length of riser pipe was simulated within the geotechnical beam centrifuge at the University of Western Australia. The model soil was soft, lightly over-consolidated kaolin clay with a linearly increasing shear strength profile with depth, typical of deepwater conditions. The pipe was cycled over a fixed vertical distance from an invert embedment of 0.5 diameters to above the soil surface. This range represents a typical vertical oscillation range of a section of riser within the TDZ during storm loading. The results indicate a significant degradation in the vertical pipe-soil resistance during cyclic vertical movements. Due to the cyclic degradation in soil strength, the component of the vertical resistance created by buoyancy was significant, particularly due to the influence of heave. A new approach to the interpretation of heave-enhanced buoyancy was used to extract the separate influences of soil strength and buoyancy, allowing the cyclic degradation in strength to be quantified. During cycling, the soil strength reduced by a factor of 7.5 relative to the initial penetration stage. This degradation was more significant than the reduction in soil strength during a cyclic T-bar penetration test. This contrast can be attributed to the breakaway of the pipe from the soil surface which allowed water entrainment. This dramatic loss of strength and therefore secant stiffness, and the significance of the buoyancy term in the total vertical pipe-soil resistance, has implications for the fatigue assessment of SCRs.

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More information

Published date: 2008
Venue - Dates: 27th International Conference on Offshore Mechanics and Arctic Engineering, , Estoril, Portugal, 2008-06-15 - 2008-06-20

Identifiers

Local EPrints ID: 419864
URI: http://eprints.soton.ac.uk/id/eprint/419864
PURE UUID: 84ba0517-6d37-44a2-80ff-fab33474f72d
ORCID for D.J. White: ORCID iD orcid.org/0000-0002-2968-582X

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Date deposited: 23 Apr 2018 16:30
Last modified: 16 Mar 2024 04:32

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Contributors

Author: M.S. Hodder
Author: M.J. Cassidy
Author: D.J. White ORCID iD

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