Predicting the Changing Soil Response for Vertical Pipe-Seabed Interaction Accounting for Remoulding, Reconsolidation and Maintained Load

2017 ◽  
Author(s):  
Zefeng Zhou ◽  
David J. White ◽  
Conleth D. O’Loughlin
Keyword(s):  
Cyclic Loading ◽  
Back Analysis ◽  
Analytical Framework ◽  
Soil Strength ◽  
Soil Resistance ◽  
Vertical Pipe ◽  
Soil Response ◽  
Term Operation ◽  
Seabed Interaction

Steel catenary risers (SCRs) are subjected to fatigue in the touchdown zone (TDZ) where the pipe interacts with the seabed. In this zone the seabed is subjected to intermittent episodes of cyclic loading and reconsolidation during long-term operation. Cyclic loading, reconsolidation and maintained load can cause variations in the soil strength and stiffness, which has a significant influence on the fatigue life of the riser in the TDZ. The weakening effect of cyclic loading on soil strength is well recognized throughout design practice, and methodologies for determining the cyclic ‘fatigue’ of clay during undrained cyclic loading are well established (e.g. Andersen et al. 1988; Andersen 2015). However, traditional undrained assessments neglect the effects of drainage and consolidation that inevitably occur in pipe-seabed interaction during long-term operational stages, and can lead to changes in stiffness by a factor of up to 5 or 10. This overlooked effect of consolidation on soil resistance and stiffness can be very important for SCR fatigue analysis. In this paper, a new analytical framework considering these effects has been used to analyze vertical pipe-seabed interaction. This framework is developed using a critical-state concept with effective stresses, and by discretizing the soil domain as a one-dimensional column of soil elements. The model can accurately capture the changing soil resistance and stiffness to account for the effects of remoulding, reconsolidation and maintained load. The framework is used to back-analysis the pipe-soil interaction response during small and large amplitude vertical cycles. The simulation prediction compares well with the measured results from the laboratory (Aubeny et al., 2008), and can accurately capture the observed changes in stiffness of up to a factor of 5.

scholarly journals Structural and mechanical defects of materials of offshore and onshore main gas pipelines after long-term operation

2015 ◽  
Vol 5 (1) ◽  
Author(s):  
Pavlo Maruschak ◽  
Sergey Panin ◽  
Iryna Danyliuk ◽  
Lyubomyr Poberezhnyi ◽  
Taras Pyrig ◽  
...  
Keyword(s):  
Fatigue Crack ◽  
Fatigue Life ◽  
Quantitative Analysis ◽  
Stress Concentration ◽  
Cyclic Loading ◽  
Fatigue Cracks ◽  
Preliminary Deformation ◽  
Term Operation ◽  
Steel Pipes

AbstractThe study has established the main regularities of a fatigue failure of offshore gas steel pipes installed using S-lay and J-lay methods.We have numerically analyzed the influence of preliminary deformation on the fatigue life of 09Mn2Si steel at different amplitudes of cyclic loading. The results have revealed the regularities of formation and development of a fatigue crack in 17Mn1Si steel after 40 years of underground operation. The quantitative analysis describes the regularities of occurrence and growth of fatigue cracks in the presence of a stress concentration.

scholarly journals The changing strength of carbonate silt: parallel penetrometer and foundation tests with cyclic loading and reconsolidation periods

2020 ◽  
Vol 57 (11) ◽  
pp. 1664-1683 ◽  
Author(s):  
Z. Zhou ◽  
D.J. White ◽  
C.D. O’Loughlin
Keyword(s):  
Cyclic Loading ◽  
Soil Strength ◽  
Life Changes ◽  
Test Procedures ◽  
Coefficient Of Consolidation ◽  
Model Foundation ◽  
Order Of Magnitude ◽  
Potential Link ◽  
Surface Foundation

This paper describes a centrifuge study using novel penetrometer tests (T-bar and piezoball) and model foundation tests to explore through-life changes in the strength of a reconstituted natural carbonate silt. The test procedures include episodic cyclic loading, which involves intervals of pore pressure dissipation between cyclic packets. These loads and the associated remoulding and reconsolidation cause significant changes in the soil strength and foundation capacity. Soil strength changes from penetrometer tests differed by a factor of 15 from the fully remoulded strength to a limiting upper value after long-term cyclic loading and reconsolidation. For the model foundation tests, the foundation capacity of a surface foundation and a deep-embedded plate were studied. The soil strength interpreted from the measured foundation capacity varied by a factor of up to three due to episodes of loading and consolidation, with an associated order of magnitude increase in the coefficient of consolidation. The results show a remarkable rise in soil strength over the loading events and provide a potential link between changes in soil strength observed in penetrometer tests and the capacity of foundations, allowing the effects of cyclic loading and consolidation to be predicted.

Crack propagation in a pipe steel subjected to cyclic loading after long-term operation

2006 ◽  
Vol 2006 (4) ◽  
pp. 329-336
Author(s):  
L. R. Botvina ◽  
V. G. Budueva ◽  
I. B. Oparina ◽  
B. R. Pavlovskiĭ ◽  
V. N. Tikhomirov ◽  
...  
Keyword(s):  
Cyclic Loading ◽  
Crack Propagation ◽  
Pipe Steel ◽  
Term Operation

scholarly journals Influence of stress history on undrained cyclic shear strength evolution

2021 ◽  
Author(s):  
Fauzan Sahdi ◽  
Joe Tom ◽  
Zhechen Hou ◽  
Mark Fraser Bransby ◽  
Christophe Gaudin ◽  
...  
Keyword(s):  
Experimental Data ◽  
Cyclic Loading ◽  
Large Amplitude ◽  
Soil Strength ◽  
Excess Pore Pressure ◽  
Stress History ◽  
Cyclic Shear ◽  
Initial Strength ◽  
Operational Life

Offshore infrastructure often interacts cyclically with the seabed over the operational life of a project. Previous research on the evolution of soil’s undrained strength under long term, large-amplitude cyclic loading has focused on contractile clays and demonstrated that this cyclic interaction can lead to the initial generation and later dissipation of positive excess pore pressure in the soil. This process generally leads to an initial strength reduction, with subsequent densification and soil strength gains that can have consequences on the performance of seabed infrastructure during its design life. In this paper, new experimental data from T-bar penetrometer testing in reconstituted kaolin and Gulf of Mexico clays is presented. The data illustrate how the stress history, quantified via the overconsolidation ratio, affects soil strength changes during large-amplitude cyclic loading. The experiments explore both long-term continuous loading cycles and episodic loading with packets of undrained cycles followed by quiescent consolidation periods. A critical state-based framework is used to interpret the experimental data and provide predictions of the long-term steady-state strength of both soils as a function of the initial in situ state of the soil.

Centrifuge Modelling of Riser-Soil Stiffness Degradation in the Touchdown Zone of a Steel Catenary Riser

2008 ◽  
Author(s):  
M. S. Hodder ◽  
D. J. White ◽  
M. J. Cassidy
Keyword(s):  
Fatigue Life ◽  
Soil Surface ◽  
Soil Strength ◽  
Soil Resistance ◽  
Vertical Pipe ◽  
Vertical Movements ◽  
Riser Pipe ◽  
Seabed Stiffness ◽  
Water Entrainment ◽  
Cyclic Degradation

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.

Sign in / Sign up

Export Citation Format

Share Document