Abstract Offshore shallow foundations may be subjected to uplift due to overturning or buoyancy loading. Peripheral (and often internal) skirts can enable transient tension loads to be resisted because of negative excess pore pressures developed between the underside of the foundation top cap and the soil plug confined by the skirts. Uncertainty exists with regard to the duration over which these negative excess pore pressures can be maintained and the effect of a gap forming along the skirt-soil interface on the transient and sustained holding capacity. This paper presents results from drum centrifuge tests carried out on a shallow skirted foundation subject to transient and sustained uplift in a lightly overconsolidated clay. Results from baseline tests with an intact skirt-soil interface are compared with tests in which a gap was created along the skirt-soil interface prior to transient and sustained uplift. The results are promising, showing for example that uplift loads of 40% of the peak undrained capacity were maintained for up to two years without significant foundation displacement when an intact foundation-soil interface was maintained. However, they also reveal that the presence of a gap may halve the time to reach similar displacements. Introduction Shallow skirted foundations are widely used offshore to support small platforms, seabed protection structures, storage tanks, and subsea frames for oil wells and pipelines, as well as for larger fixedbottom and floating structures (e.g. Støve et al., 1992; Tjelta, 1994; Bye et al., 1995; Watson and Humpheson, 2007). Skirted foundations are also an attractive option for mooring or supporting current meters and wind turbines offshore. Skirted foundations may comprise a top plate a peripheral skirt, and sometimes internal skirts or a cluster of individual skirted units connected together. Regardless of configuration, the foundation penetrates the seabed, confining a soil plug inside the structural members.

Abstract Premature refusal can pose a threat to driven pile installation. One contingency measure that has been applied is pre-drilling ahead of the pile tip to ease driving. While some practitioners may assume that carefully conducted drilling will not give any adverse effect, little evidence is currently available to assess the possible impact on long-term axial capacity. This paper describes a standard gravity (1g) small-scale model laboratory study in which 18 small closed-ended piles were jacked into constrained 300mm cubes of stiff high overconsolidation ratio (OCR) natural London clay, with and without pre-drilling. Four control piles were also installed without any boring. Pile loading tests performed after appropriate equalisation periods showed that pile capacity reduced systematically as the ratio of the drill hole-to-pile solid area ratio increased. The model experiments showed comparable reductions in capacity to those in earlier model and field studies on soft Mexico City clays. An effective stress analysis is suggested to help the application of the model tests to other soils types and pile geometries. Capacity reductions are concluded to be likely when pre-drilling is performed in highly overconsolidated offshore clays. The results have implications for both new piling operations in exceptionally hard clays and the re-analysis of existing installations in a broader spread of soil types, where refusal may have resulted from pile hammer capacity limitations. 1. Introduction 1.1 Use of pre-drilling to ease pile installation Driving refusal has been encountered frequently during offshore foundation pile installation. One field contingency measure applied to achieve target penetration with tubular piles has been pre-drilling ahead of the tip to ease driving. The development of more powerful hammers and improved driveability predictions has reduced the incidence of refusal events, but consideration still has to be given to this possibility when dealing with unusually hard or dense materials.

ABSTRACT Traditional techniques for assessing the quality of offshore soil samples, such as measuring the volume change during reconsolidation back to in situ stress (or recently the normalised void ratio change) are time consuming, often requiring one or several days to perform. This paper assesses a rapid alternative technique for independently evaluating sample quality in soft clay, involving measurements of shear wave velocity, V s using bender elements and soil suction, u r . It is found that the quality of samples indicated by V s and u r . measurements are independently similar to those inferred from anisotropically consolidated undrained triaxial (CAUC) and oedometer tests. Sample quality may therefore be rapidly assessed in the field, leading to important economic and technical implications for the offshore industry. INTRODUCTION The characterisation of soils for design and construction is strongly dependent on the removal of sufficiently high quality samples from the ground. Disturbed samples may result in poor estimates of strength and stiffness parameters from triaxial and oedometer tests, which may in turn lead to serious and costly design errors. Considerable attention has been devoted to the subject of sample disturbance over recent years, ranging from improvements in sampling techniques and equipment, to correcting for sample disturbance effects on disturbed samples. Although these improvements may decrease the effects of sampling disturbance, evaluation of sample quality remains essential if design parameters are to be considered reliable. A number of different techniques may be used to evaluate sample quality, such as the measurement of volumetric strain 1 and the normalised change in void ratio,¿e/e 0 2 . However, most of these techniques require reconsolidation back to in situ stresses before measurement, a process that may require a number of days testing. This assessment inefficiency is a particular problem for offshore sampling, where rapid assessment of sample quality may result in major economic saving. This paper discusses the use of shear wave velocity, V s and soil suction, u p for rapidly evaluating sample quality. The proposed L vs - L u criterion of Donohue and Long 3 is assessed by the addition of supplementary data from two Norwegian research sites at Onsøy and Nybakk-Kløfta. Samples of varying quality extracted using different sampling techniques were acquired from the two sites. The sampling techniques used at Onsøy were standard (steel), 75mm samples taken using the standard Japanese piston sampler and a prototype 100mm-diameter continuous sampler. Block samples were taken using the high quality Sherbrooke block sampling technique at the Nybakk site. All of the samples were tested by the same operators and the same techniques were used at the laboratories of University College Dublin. V s and u r Shear wave velocity, V s The seismic geophysical parameter - shear wave velocity, V s , - has been recently used by a number of authors to assess sample disturbance. It has been observed that V s and corresponding small strain shear modulus (equation 1), when measured in the laboratory after reconsolidation to in situ stresses, are generally lower than the in situ equivalent 4, 5, 6 . This reduction in stiffness has been ascribed to sampling disturbance.

ABSTRACT The Yolla A Platform incorporates a new foundation concept, defined as an un-ballasted raft foundation, and was successfully installed in the Bass Strait of Australia in 2004. Soil conditions at the site comprise a complex sequence of normally consolidated calcareous silt, calcareous sand and calcareous clay, and required extensive soil interpretation to support the design process. The foundation is subject to extreme combinations of environmental loading and was designed using a combination of analytical and numerical design tools. Soil preloading using underbase suction was incorporated in the design to facilitate increases in foundation capacity and to limit post installation long term foundation displacements. Skirt penetration and subsequent suction preloading were issues requiring close attention during detailed design, as well as extensive monitoring and interpretation both during and after installation. This paper presents a general overview of the design and installation processes, and includes a summary of observations made during foundation installation. INTRODUCTION The Yolla A Platform was installed in the central Bass Strait (Figure 1a) of Australia in March 2004, as part of the BassGas Project for client/operator Origin Energy and venture partners AWE, CalEnergy and Wandoo (note that Wandoo has since sold its share in the platform to a new venture partner). The main contractor for the construction and installation of offshore platform was Clough Engineering Ltd, and the substructure design was undertaken by Arup Energy. The Yolla A Platform is a self-installing steel gravity base platform comprising three primary components: the foundation base, the jacket and the deck (Figure 1c). The platform uses the buoyancy of the deck to provide floating stability for the entire platform during installation, with the deck and base initially clamped together during wet tow to the platform site (see Figure 1b, which shows the deck on top of base prior to departing the fabrication yard in Batam, Indonesia). Once on site, a jacking system is used to lower and install the base on to the seabed and raise the deck clear of the water after the base is installed. Once the deck is in the final position, it is welded to the jacket. The Yolla A Platform base comprises a 50m x 50m skirted foundation, with skirts penetrating 5.4m into a seabed comprising primarily soft to very soft calcareous silt. The skirts subdivide the base into 12 individual compartments, and installation is achieved using ‘suction’ within the skirt compartments. This paper outlines two separate aspects of the project, namely foundation design and foundation installation. Foundation Design Water depth at the Yolla A Platform site is approximately 80m. The foundation design is novel in many aspects, including: The Yolla A foundation is believed to be the first foundation of this type (with deep skirts and no ballast) to be installed in calcareous soil Consolidation was incorporated in the design to ensure adequate foundation stability Underbase suction was used to for base installation, which had not previously been undertaken at this scale in calcareous soils

Introduction Aasgard Project The Aasgard is an oil, gas and condensate development offshore mid Norway in 300 m water depth. The field is developed with early oil production from a floating production, storge, and offloading vessel(Aasgard A), gas producton by semi submersible unit(Aasgard B), condensate export through a floating storage unit (Aasgard C) and gas export through a trunk pipeline4 to northern Germany as shown in Fig 1.(Fig. 1 is available in full paper. Soil conditions Soil Conditions at the three locations are well documented through extensive site investigations comprising CPT and sampling borehole at altogether more than 680 locations. The sediments at Aasgard consist of low plasticity clay deposited close to the ice sheet which covered all of Scandinavia up to approximately 10 000 years ago. Due to variable depositional conditions and ice loading after deposition, the undrained shear strength of the clay have much greater variation than usually encountered in marine clays. The undrained shear strength typically increase from 5 to 25 kPa at sea bottom to 40 to 800 kPa at 9 m penetration with large variations in strength at all depths. There are also quite large variations in strength at locations no further than 100 m apart. The ranges in strength for Aasgard A, B and C locations are presented in Fig. 2.(Fig. 2 is available in full paper) The undrarned shear strength at Aasgard also have significant variation with depth at each location No location are consistently stronger than then all other locations, but contain some low and some high strength area with each layer The upper bound strengths are consequently not defined as an envelope to the highest strength at all locations, but as the highest overall strength likely to be found at one location in the area The lower bound strengths are closer to an envelope of the lowest strengths identified as some locations have consistently low strengths and it is likely to find a location with strength close to the lower envelope. Other characteristics of the clayey soil conditions at Aasgard are a relatively high content of gravel, stones and boulders, low plasticity and low water content as shown in Table 1.(table 1 is available in full paper) The number and size of boulders are estimated based on cone tip resistance and registration from drillers logs. Geophysical surveys have been carried out with instrumentation carried by ROV's providing high quality data. Sub seabed boulders have been difficult to identify, unless being very large (one to two metre in diameter), as the sediments consist of all grain sizes giving a chaotic reflection. Boulders at seabed have been identified down to sizes of 0.2 m diameter, and areas with larger seabed boulders have been avoided. The geophysical surveys have identified boulders of several meters in diameter at the sea bottom in other areas of the Aasgard field, supporting the design assumption of accounting for penetrating anchors past boulders up to approximately 0.5 m diameter and relocating anchors if even larger boulders are met.