ABS 246-2017 GUIDANCE NOTES ON GEOTECHNICAL PERFORMANCE OF SPUDCAN FOUNDATIONS.pdf

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1、 Guidance Notes on Geotechnical Performance of Spudcan Foundations GUIDANCE NOTES ON GEOTECHNICAL PERFORMANCE OF SPUDCAN FOUNDATIONS JANUARY 2017 American Bureau of Shipping Incorporated by Act of Legislature of the State of New York 1862 2016 American Bureau of Shipping. All rights reserved. ABS Pl

2、aza 16855 Northchase Drive Houston, TX 77060 USA Foreword Foreword For the last decade, ABS has been involved in numerous joint industry projects on jackup spudcan foundations. ABS has outlined this knowledge herein to provide guidance covering the design and installation of jackup spudcan foundatio

3、ns addressing the following topics: Site assessment Spudcan penetration prediction in a single layer of soil, such as clay or sandy soil Punch-through prediction and mitigation Foundation stability assessment Foundation fixity Spudcan-jacket pile interaction Spudcan footprint interaction These Guida

4、nce Notes become effective on the first day of the month of publication. Users are advised to check periodically on the ABS website www.eagle.org to verify that this version of these Guidance Notes is the most current. We welcome your feedback. Comments or suggestions can be sent electronically by e

5、mail to rsdeagle.org. Terms of Use The information presented herein is intended solely to assist the reader in the methodologies and/or techniques discussed. These Guidance Notes do not and cannot replace the analysis and/or advice of a qualified professional. It is the responsibility of the reader

6、to perform their own assessment and obtain professional advice. Information contained herein is considered to be pertinent at the time of publication, but may be invalidated as a result of subsequent legislations, regulations, standards, methods, and/or more updated information and the reader assume

7、s full responsibility for compliance. This publication may not be copied or redistributed in part or in whole without prior written consent from ABS. ii ABSGUIDANCE NOTES ON GEOTECHNICAL PERFORMANCE OF SPUDCAN FOUNDATIONS .2017 Table of Contents GUIDANCE NOTES ON GEOTECHNICAL PERFORMANCE OF SPUDCAN

8、FOUNDATIONS CONTENTS SECTION 1 Introduction 1 1 General Comments . 1 3 Scope and Application 1 5 Terms and Definitions . 1 7 Symbols and Abbreviations 2 7.1 Symbols . 2 7.3 Abbreviations . 5 SECTION 2 Site Assessment 6 1 General . 6 3 Geotechnical Site Investigation 6 5 Sampling and Field Testing 7

9、7 Laboratory Testing 8 9 Soil Engineering Parameter Selection 9 9.1 Identification of Soil Type and Layers 9 9.3 Clay . 10 9.5 Sand 10 9.7 Derivation of Soil Strength Profile 12 TABLE 1 Relative Reliability of Tests Measuring the Strength of Clay Soils 8 TABLE 2 Index Properties for Jackup Foundatio

10、n Site Specific Assessment . 9 TABLE 3 Values of cvand QcrushingDerived from Triaxial Compression Tests . 12 SECTION 3 Spudcan Penetration in Single Layer Soil . 14 1 Background . 14 3 Numerical Simulations 14 5 Simplified Prediction Methods 15 5.1 General 15 5.3 Spudcan Penetration in Clay . 15 5.5

11、 Spudcan Penetration in Sand 18 5.7 Lattice Leg Effect on Cavity Depth and Spudcan Penetration . 20 ABSGUIDANCE NOTES ON GEOTECHNICAL PERFORMANCE OF SPUDCAN FOUNDATIONS .2017 iii 7 Alternative Approach: Direct Correlation with Penetrometer Tests . 22 9 Consequences for Observation 23 TABLE 1 Bearing

12、 Capacity Factors Ncfor Conical Shaped Footings 18 TABLE 2 Bearing Capacity factors N 20 FIGURE 1 Embedded Spudcan with Open Soil Cavity 16 FIGURE 2 Definition of Equivalent Cone 17 FIGURE 3 Backflow in Sand . 19 FIGURE 4 Lattice Leg View 21 FIGURE 5 Top Mounted Skirt on Spudcan 22 SECTION 4 Punch-t

13、hrough in Strong Soil Overlying Soft Soil . 24 1 General . 24 3 Prediction Methods . 24 3.1 Sand over Clay 24 3.3 Strong Clay over Soft Clay 27 5 Mitigation Method Perforation Drilling 28 7 Remedial Action during Punch-through 29 FIGURE 1 Nomenclature for Spudcan Penetration in Sand over Clay 24 FIG

14、URE 2 Values for Spudcan Bearing Capacity Calculation when h hlayerRef.18 26 FIGURE 3 Nomenclature of Spudcan Penetration in Sand over Clay when h hlayer27 FIGURE 4 Nomenclature of Spudcan Penetration in Strong Clay over Soft Clay 28 SECTION 5 Foundation Stability Assessment . 30 1 Approach . 30 3 A

15、cceptance Criteria . 30 FIGURE 1 Foundation Stability Checks . 31 SECTION 6 Foundation Fixity 32 1 Introduction . 32 3 Foundation Fixity . 32 3.1 Initial Stiffness . 32 3.3 Yield Surface under Combined Loadings 35 3.5 Calculation Procedures 36 5 Consolidation Effect 37 7 Lattice Leg Effect on Founda

16、tion Fixity . 37 iv ABSGUIDANCE NOTES ON GEOTECHNICAL PERFORMANCE OF SPUDCAN FOUNDATIONS .2017 FIGURE 1 Spudcan Soil Rotational Stiffness . 32 FIGURE 2 Depth Factors kd1for Vertical Spring Stiffness 34 FIGURE 3 Depth Factors kd2for Horizontal Spring Stiffness 34 FIGURE 4 Depth Factors kd1for Rotatio

17、nal Spring Stiffness 34 FIGURE 5 Yield Envelopes for Conical Footings . 36 SECTION 7 Spudcan-Pile Interaction . 38 1 Introduction . 38 3 Soil Flow Mechanism for Spudcan in Soft Clay 40 3.1 Spudcan Penetration . 40 3.3 Spudcan Operation and Extraction 40 5 Dimensionless Charts . 41 FIGURE 1 Potential

18、 Soil Loading Effects on Jacket Platform (after Ref.29) . 39 FIGURE 2 Spudcan-Pile Interaction . 39 FIGURE 3 Incremental Soil Movement Trajectories at Different Distances from Spudcan . 40 FIGURE 4 Nomenclature for a Socketed Pile 42 SECTION 8 Spudcan-Footprint Interaction . 43 1 Introduction . 43 3

19、 Footprint Characteristics and Its Influence on Spudcan-Footprint Interaction . 44 5 Mitigation Methods 46 FIGURE 1 Bathymetry of an Established Site 43 FIGURE 2 Generalized Soil Condition of a Footprint (for 180 kPa Piezocone 1 2 2 2 4-5 T-bar 2 High to moderate reliability; 3 Moderate reliability;

20、 4 Moderate to low reliability; 5 Low reliability. 2 The test result reliability is dependent on the sample quality (or degree of sample disturbance) and soil homogeneity. 8 ABSGUIDANCE NOTES ON GEOTECHNICAL PERFORMANCE OF SPUDCAN FOUNDATIONS .2017 Section 2 Site Assessment 9 Soil Engineering Parame

21、ter Selection Section 2, Table 2 lists the basic soil properties which should be obtained for jackup foundation assessment purposes for homogeneous clay or sand. For silty material, the properties defined for clay may be applicable but with consideration of partial drainage characteristics. For a hi

22、ghly variable and complex ground where advanced foundation performance modeling is to be conducted more soil properties may be obtained. TABLE 2 Index Properties for Jackup Foundation Site Specific Assessment Soil Type Strength Properties Index Properties and Additional Parameters Clay Undrained she

23、ar strength Su Remoulded shear strength and soil sensitivity St Water content Plastic limit Liquid limit Submerged unit weight of clay Coefficient of consolidation Over consolidated ratio Sand Critical state angle of friction Crushing strength Particle size distribution curve Relative density Submer

24、ged unit weight of sand Over consolidated ratio 9.1 Identification of Soil Type and Layers In order to determine the soil stratification, the desk study, geophysical site survey report and geotechnical testing results should be viewed in a holistic and integrated manner. As part of this process, var

25、iation of the subsoil conditions across the planned jackup spudcan locations should be assessed. Soil classification charts are empirical correlations which should be verified against the site geotechnical data (i.e., borehole logs and laboratory test data). For offshore conditions, pore pressure da

26、ta may be generally considered more reliable than friction sleeve data for soil classification. Continuous penetrometer profiling is suggested in order to provide a full profile for site characterization. Where intermittent piezocone, penetrometer and soil sampling is used in an alternating manner,

27、data gaps are introduced and precise identification of layer boundaries may not be possible. Furthermore, where intermittent penetrometer profiling is used, portions of test data may be of reduced quality and reliability due to soil disturbance from the drilling and soil sampling operations. Discrep

28、ancies in the penetrometer resistance profiles between two sequential penetrometer test strokes may be difficult to resolve, and this may introduce uncertainty in the soil layer interpretation. If intermittent profiling is unavoidable, the interval between strokes should be kept as small as practica

29、ble and drilling disturbance should be minimized. It is also suggested that penetrometer tests be conducted adjacent to continuous sampling borehole(s) with a separation on the order of 5 m to avoid interference. Also obtaining pore pressure can aid in the identification of soil type and layering. T

30、he following paragraphs consider strength measurement in: Clay, where undrained conditions are assumed for both testing and spudcan penetration; Sand, where drained conditions are assumed for both testing and spudcan penetration. ABSGUIDANCE NOTES ON GEOTECHNICAL PERFORMANCE OF SPUDCAN FOUNDATIONS .

31、2017 9 Section 2 Site Assessment 9.3 Clay The undrained shear strength, Su, can be established from a number of commonly conducted strength tests as follows. In-situ testing: field penetrometers (cone), vane shear High quality laboratory testing: CAUC, CAUE, DSS Simple laboratory testing: unconsolid

32、ated undrained triaxial test (UU), miniature vane (MinV), motor vane (MV), torvane (TV), pocket penetrometer (PP) The reliability of the interpreted strength parameters relies heavily on the quality of test and sample conditions. The simple laboratory tests typically should be conducted immediately

33、after the samples are extruded in the laboratory. However, the test results may tend to show significant variability both within a given type of test and between different tests, and are operator dependent. Tests of this type may be sufficient where reasonable knowledge of the ground conditions and

34、knowledge of successful jackup operations in the locality are available; the data may also be used to extend or interpolate between results from previous high quality laboratory tests. As samples are not reconsolidated prior to testing, the reliability of these tests suffers from unquantifiable samp

35、le quality. The knowledge of remoulded shear strength and sensitivity is necessary for consideration of strain softening effects, particularly for highly sensitive clays. Strain softening reduces the average operational strength during spudcan penetration, thus increasing the penetration. Partial re

36、moulding of the soil will also affect the magnitude of strength recovery or enhancement with time and this is relevant for spudcan breakout force assessment and bearing capacity evaluation for jackup revisits. In-situ cyclic full-flow penetrometer tests (using T-bar or Ball) with 10 cycles of penetr

37、ation and extraction, will generally show a well-defined remoulded penetration resistance. The ratio by which the penetration resistance decreases between initial and post-cyclic penetration resistance will be less than the actual sensitivity at the elemental test level, largely due to the partial r

38、emoulding that occurs during initial penetration. Such tests do, however, provide an appropriate measure of remoulded shear strength that is directly applicable to spudcan performance. Where the sediments may be susceptible to significant strength loss during cyclic loading under environmental or se

39、ismic loading conditions, cyclic shearing tests should be undertaken in the laboratory in addition to monotonic tests. In the depth range of expected spudcan penetration, sufficient cyclic tests should be undertaken to establish a “cyclic fatigue” curve, showing how the normalized shear stress to ca

40、use a given magnitude of strain varies with the number of cycles. This information may then be used to assess an appropriate cyclic shear strength for use in quantifying the jackups performance during installation and under ultimate cyclic loading conditions. 9.5 Sand The most common laboratory test

41、s for determining effective strength parameters in sand are CID and DS tests, which represents triaxial and plane strain condition, respectively. Sample disturbance is inevitable when sampling cohesionless material from the seabed. The samples are reconstituted to their approximate in-situ state, wi

42、th the relative density generally estimated from the cone resistance. Appropriate effective stresses are then applied before the shearing stage. During shearing, it is important to verify that the shearing rate applied is slow enough to prevent the development of excess pore pressure. Theoretically,

43、 under a given stress level, it is expected that the friction angle from DS tests will be greater than that from CID tests. To better account for the stress level effect on , the design value may be estimated from the value of the relative density, ID, and the critical state friction angle, cv, usin

44、g an appropriate strength-dilatancy relationship that takes account of the mean effective stress p during bearing failure. Since the value of critical state friction angle cvlies within a small range, at least for silica sand, it is possible to estimate the in-situ directly from the cone resistance,

45、 qc. The following expression has been applied widely to sandy sites in the North Sea by Ref.2. 10 ABSGUIDANCE NOTES ON GEOTECHNICAL PERFORMANCE OF SPUDCAN FOUNDATIONS .2017 Section 2 Site Assessment ID= a1lnrcpq a2lnrvpk3)21(00+ a3where a1= 0.336 a2= 0.154 a3= 1.91 0v = effective overburden stress,

46、 in kPa (kgf/mm2, lbf/in2) qc= cone resistance, in kPa (kgf/mm2, lbf/in2) pr= 100 kPa (0.010197 kgf/mm2, 14.5 lbf/in2) k0= coefficient of earth pressure at-rest, normally taken as 0.5 and 1.0 in order to generate upper and lower bounds of ID, respectively For a given ID, the design value of can be d

47、etermined using the general strength-dilatancy framework established by Ref.3 which makes allowance for different sand types and loading conditions as follows. = cv+ mIRDIRD= IDQcrushing ln(p) 1 0 IRD 4 where m = constant = 3 for failure under triaxial or general loading conditions = 5 under plane-s

48、train conditions IRD= relative dilatancy Qcrushing= particle crushing strength on a natural log scale, in kPa (kgf/mm2, lbf/in2) p = mean effective stress which in turn depends on the value of , in kPa (kgf/mm2, lbf/in2). As an approximation, the recommendation is to treat p as the maximum preload p

49、ressure. The value of cvmay be obtained from direct shear tests on disturbed sand, from the “steady state” friction angle in the later stages of the test. Some of the reported values for cvand Qcrushingby Ref.4 are given in Section 2, Table 3. The above procedures provide an estimate of the peak angle of friction, including the effect of the average stress level in the soil. It is important to note, however, that as a spudcan continuously penetrates the soil, the peak strength is not mobilized simultaneously t