Knut H. Andersen

Database of Friction Angles of Sand and Consolidation Characteristics of Sand, Silt, and Clay

AbstractThe paper presents diagrams of the (1) drained and undrained effective stress friction angles of sand; (2) dilatancy angle of sand; (3) parameters in a nonlinear constrained modulus expression for virgin loading, unloading, and reloading on sand and silt; and (4) diagrams with the coefficient of permeability for sand, silt, and clay. The database parameters are suitable for preliminary assessments or applications when assumed parameters are acceptable. The database can also be helpful in planning and interpreting site-specific tests and in reducing the number of site-specific tests. For important projects the parameters should be determined by site-specific tests.

Finite Element Analyses Applied in Design of Foundations and Anchors for Offshore Structures

Offshore structures for oil and gas exploitation are designed to accommodate severe environments with large cyclic loads. These structures are either founded directly on the seabed, or they are moored to anchors installed in the seabed soil. The permanent and cyclic loading, the foundation or anchor geometry, and the nonlinear soil behavior may be very complex, and many interrelated aspects must be considered in the geotechnical design of the foundations. Finite-element analyses (FEAs) are used increasingly to deal with these complexities and offer the potential to increase accuracy, efficiency, and reliability and reduce the uncertainty of the design process. This paper presents the major geotechnical aspects in the design of foundations for offshore structures and examples from finite-element analyses carried out at the Norwegian Geotechnical Institute (NGI) to deal with these aspects. A brief review of the procedure used at NGI to obtain soil stress-strain-strength relationships from cyclic laboratory ...

Effect of Skirt-Tip Geometry on Set-Up Outside Suction Anchors in Soft Clay

An important part of suction anchor design is the determination of the shear strength along the outside skirt wall. Previous work has suggested that when a suction anchor in clay is installed by applying underpressure inside the anchor, the external skin friction may be reduced compared to the skin friction expected for driven piles. The primary reason for this reduction is that the movement of soil at and beneath the caisson tip during installation will be influenced by whether the anchor is penetrated by weight or by underpressure. To further investigate the impact of installation by underpressure, additional finite element analyses have been performed where the skirt installation process has been better followed than in the previous analyses. The movement of soil around the caisson wall was studied for both a flat caisson tip and a tip with a tapered edge of 45° towards the outside of the anchor. The tapering was made to see if it would cause more of the displaced soil to move outside the anchor and thereby increase the mean total stresses and the shear strength along the outside anchor wall. The analyses were made with two separate wall roughness factors for a typical anchor in soft clay.Copyright

Thixotropy of Marine Clays

A database on thixotropy strength ratio is established for marine clays in this paper. The database includes clay data from 15 offshore sites. The study indicates that the thixotropy strength ratio of marine clays has correlations with the following parameters: activity, plasticity index, liquidity index, sensitivity, and water content. Trend lines are given for the correlations between thixotropy strength ratio and liquidity index, and between thixotropy strength ratio and water content.

Nonlinear Foundation Spring and Calibration Using Measured Dynamic Response of Structure

Some offshore platforms need modifications or addition of new modules that require re-assessment of their design. An economical way of allowing for changes without major strengthening of the platform is by reducing the safety margin used in the original design by use of measured data during the operation. This paper presents application of this concept to Troll A platform in which the measured changes in the platform’s natural periods due to the nonlinear soil behavior under different storm conditions were used to calibrate the soil-foundation spring adopted in the design.Copyright

Reliability of suction caissons for deep water floating facilities

With the extension of the offshore industry into deeper water the types of offshore structures changed from gravity base structures and fixed platforms supported by driven piles to floating vessels supported with vertical tendons or catenary mooring lines. The anchors for these new systems also changed as increased water depths presented problems for under water pile driving. Suction caissons then became the preferred foundation option for these facilities. A number of significant differences exist between how suction caissons and piles provide resistance and how the foundation system for a floating vessel performs compared to a fixed platform. One of the primary challenges with this new foundation system was the determination of the overall system reliability. An initial study for suction caissons by Clukey et al. (2000) showed that the overall system reliability for suction caissons was not as robust as for a fixed platform. The impact of a single suction caisson failing was found to be more severe and could more easily lead to a system failure. Based on this initial generic study, it was recommended that for extreme loading events the factor of safety (FoS) for Gulf of Mexico (GoM) 100-yr hurricane conditions should be increased from 1.5 (for piles) to 2.0 for suction caissons. This FoS is currently being used in API-RP2SK for vertically dominated loading. However, since that initial study significantly more reliability work has been done at the University of Texas, Det Norske Veritas and the Norwegian Geotechnical Institute. These studies considered a wide variety of cases including different floating vessel types, soil conditions, water depths and loading conditions. The results from this more recent work are compared in this paper. These results show that the current design codes are robust and conservative. Based on the studies opportunities exist to make the design codes less conservative while still maintaining desired levels of reliability. In addition the historical development of suction caissons is presented INTRODUCTON & BACKGROUND Since the early 1990s suction caissons have been used to anchor a variety of deep water facilities. These foundations are cylindrical members but with much larger diameters (4 to 8 m) and smaller depth/diameter ratios than offshore piles. They are installed through deadweight and pumping water from inside the caisson which lowers the pressure inside the caisson (referred to as suction) resulting in an additional downward force. If properly sealed the final uplift resistance is derived from deadweight, external skin friction and reverse end bearing at the bottom of the caisson. A list of installed suction anchors is given in Andersen et al. (2005) showing more than 485 suction installations by 2004 at more than 50 locations in water depths to nearly 2000 m. One early application was the foundation for the Snorre Tension Leg Platform (TLP) installed in the North Sea(NS) in 1991 (Stove et al., 1992). This facility had four foundations, one

Effect of sample disturbance on cyclic shear strength of normally to lightly OC clays

Abstract Tests on Onsøy clay (I p = 24–27%) were performed to study the effect of sample disturbance on undrained static and cyclic shear strength. Samples with different degree of disturbance from sampling were used and parallel sets of monotonic and cyclic undrained laboratory tests were performed on block and 54 mm composite piston samples. A silty Drammen clay (I p = 19%) which was studied earlier was included in the study. Test results indicated that there are significant effects of sample disturbance on both static and cyclic undrained shear strengths for both Onsøy and silty Drammen clays.

Discussion of “Database of Friction Angles of Sand and Consolidation Characteristics of Sand, Silt, and Clay” by Knut H. Andersen and Knut Schjetne

The authors are commended for offering diagrams showing (1) the drained and undrained friction angles of sand; (2) the dilatancy angle of sand; (3) the parameters in a nonlinear constrained modulus expression for virgin loading, unloading, and reloading on sand and silt; and (4) the coefficient of permeability for sand, silt, and clay. The most interesting presentations are those of the friction angle and the dilatancy angle of sand. Because of the definition of themean effective stress ppf 9 at failure, it may have been inappropriate for the authors to have used the relative dilatancy index proposed byBolton (1986) to depict the variation of the peak friction anglewith regard to the critical-state friction and dilatancy angles. The ratio of the initial confining pressure to the stress at failure (i.e.,Rpf 5 p0 9=ppf 9 ) was assumed by the authors to be a constant of 60% with a friction angle between 35 and 40 , as shown in Figs. 3 and 8 in the original work. The stress in the relative dilatancy index proposed by Bolton (1986) is at the peak failure state (Chakraborty and Salgado 2010; Salgado et al. 2000; Yang and Li 2004). However, as demonstrated in this discussion, from the viewpoint of the definition analysis, the ratio of the initial confining pressure to the stress at failure is smaller than 60%, and is also dependent on the density and pressure based on the results of tests of granular soils. In this discussion, the ratio of the initial confining pressure to the stress at failure for granular soils is first investigated based on the definition analysis. Second, the ratio of the initial confining pressure to the stress at failure is found to be dependent on the initial void ratio and the initial consolidated pressure based on the test results of Fuji River sand (Tatsuoka 1972), Sacramento River sand (Lee and Seed 1967), Toyoura sand (Verdugo and Ishihara 1996), and Tachengsha rockfill material (Jiang 2009).