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ABSTRACT A new geotechnical investigation tool called the CPT Stinger has recently been developed to acquire geotechnical information cost effectively at deepwater sites. Combining jumbo piston core (JPC) sampling with the CPT Stinger (deployed using the same rigging as the JPC) provides a full geotechnical profile at a given location, down to a depth of 35m below mudline. Cone penetration test (CPT) data are collected during initial free-fall insertion of the CPT Stinger apparatus. These data could be used to provide a continuous stratigraphic profile from mudline down to the depth where conventional CPT data (push rate of 2cm/s) are acquired, if a reliable and repeatable method for correcting the free-fall insertion data could be developed. Several methods have previously been proposed to correct soil shear strength for rate of loading effects. These methods are evaluated in this paper by comparing their results to data acquired in 2011 at a deepwater site in the Gulf of Mexico. The paper gives an operator''s perspective on the potential of different methodologies in correcting free-fall CPT Stinger data for deepwater site investigations. 1. Introduction Deepwater geotechnical acquisition is a cost intensive activity that causes operators to evaluate tradeoffs between project economics and the relative value of the information being gathered. Operators are continuously evaluating more economical options for acquiring high quality data that can be used for design. A new system has been recently developed called the CPT Stinger, and it proposes to reduce geotechnical data acquisition costs by combining the traditional jumbo piston core (JPC) sampling technique with a CPT system that is deployed with the same equipment. The CPT Stinger essentially replaces the JPC core barrel and liner with the CPT cone, data logger, rod assembly and power and control modules (Young et al., 2011).
ABSTRACT To investigate whether chemical additives could be effective at accelerating the strength regain of a clay slurry, a series of laboratory tests were undertaken at the University of Glasgow using a range of additives. Bothkennar clay was used in the test programme, with a small addition of bentonite to replicate the plasticity of the Witch Ground clays found in an area of the North Sea. To produce the slurry, water was added to the samples to increase the water content to 1.5 and 2.0 times the liquid limit. The results of the test programme found that calcium hydroxide can significantly improve the strength of the soil over 1-day, 3-day, 7-day, 30- day and 1-year time periods, for a range of dosing concentrations of 1โ5%. This paper presents the laboratory test results and highlights the potential use of calcium hydroxide offshore. 1. Introduction Clay slurries can loosely be defined as cohesive soils with moisture contents in excess of their liquid limit (LL). The use of additives to increase the strength of such weak slurries has received relatively little attention, although there is an increasing commercial imperative, associated with offshore developments, for pursuing this ground-strengthening technique. For example, clay slurry is produced offshore during jet trenching in clay soils where near rectangular trenches are formed, which generally are partially filled with slurry. The slurry provides little resistance to the uplift movement of trenched pipelines and is typically removed and replaced with either rock dump, or more competent in situ material obtained by destabilising the side walls of the trench. This paper presents the results from laboratory tests undertaken at the University of Glasgow to investigate the influence of a range of chemical additives on accelerating the strength improvement of clay slurries.
- Africa (0.28)
- Europe > United Kingdom > North Sea (0.25)
- Europe > Norway > North Sea (0.25)
- (3 more...)
ABSTRACT In the Norwegian Sea, sea-bottom temperatures can be as low as โ1.9ยฐC in water depths greater than 1000m. In other deepwater areas, such as the Gulf of Guinea and the Gulf of Mexico, sea-bottom temperatures can be as low as 5ยฐC. However, the standard practice up to now has been to carry out laboratory tests at room temperature (i.e. 20ยฐC). Previous studies have indicated that testing at room temperature can result in laboratory measured strengths 10โ20% lower than tests at in situ temperatures. Results from extensive parallel laboratory testing (at room temperature and at in situ temperature) on eight different types of soft clay are presented here, covering intact and remoulded specimens with the range of plasticity of 16โ120%. This study quantifies the temperature effects on testing and storing of deepwater samples. The investigated soil parameters concentrate on the undrained shear strength (su) and preconsolidation stress (p'c). All the parallel tests showed that su increased on average of 2% to 40% when tested at cold temperature. With one exception, where a similar increase in measured p'c of 9% to 38% was observed. Recommendations are given for procedures for testing, transporting and storing deepwater samples to arrive at the closest possible representative soil design parameters for in situ conditions. 1. Introduction In the Norwegian Sea, sea-bottom temperatures can be as low as โ1.9ยฐC in water depths greater than 1000m. Due to the salt content of the pore water (typically about 30g/l), the soil does not freeze. In other deepwater areas, such as the Gulf of Guinea and the Gulf of Mexico, sea-bottom temperatures can be as low as 5ยฐC. In connection with field developments in deep water, the use of low values of su and p'c can lead to foundation solutions that are unnecessarily conservative and costly.
Skirted Spudcans And Perforation Drilling For Installation of Spudcans Close to Existing Footprints
Hossain, M.S. (Centre for Offshore Foundation Systems, University of Western Australia) | Dong, D. (Centre for Offshore Foundation Systems, University of Western Australia) | Gaudin, C. (Centre for Offshore Foundation Systems, University of Western Australia) | Kong, V.W. (Centre for Offshore Foundation Systems, University of Western Australia)
Abstract The large footprints that remain on the seabed after offshore mobile jack-up platforms have completed operations present hazardous conditions for any future jack-up installation at that site. The slope of the footprint causes detrimental horizontal and moment loads to be induced on the spudcan during the preloading process where only vertical loads are expected. This paper reports model tests exploring the efficiency of innovative spudcan foundations and perforation drilling to mitigate this issue. The soil conditions tested simulate soft seabed strength profiles close to the mudline, varying the undrained shear strength. The most critical reinstallation location and existing footprint depth were investigated. In all of the experiments, an initial footprint was created. Skirted, innovative and generic spudcan models were then offset and reinstalled on nonperforated and perforated sites, with the vertical and moment loads on the spudcan recorded using a highly instrumented shaft. The innovative spudcan and the removal of soil inside the spudcan perimeter, with an area of 9%perforated, reduced the induced moment significantly. 1. Introduction 1.1 Mobile jack-up rig and spudcan foundation Most offshore drilling in water depths up to 150m is performed from self-elevating jack-up rigs because of their proven flexibility, mobility and costeffectiveness. These jack-ups typically consist of three independent truss legs, each attached to a large 10โ20m diameter inverted conical footing, colloquially known as โspudcansโ. Unlike a fixed platform or gravity structure, a jack-up unit is mobile in nature. The changing of drilling locations necessitates retrieval of the unit at each site. 1.2 Spudcan footprint geometry On removal of the jack-up unit, the legs are retracted from the seabed leaving depressions (referred to as a crater, or โfootprintโ) at the site. The depth and configuration of a footprint are a function of several factors.
- Europe (0.48)
- Oceania > Australia (0.28)
- North America > United States (0.28)
Abstract Flowlines and pipelines installed in deep-sea waters are submitted to axial and lateral loads due to the effects of flow stoppages and starts, thermal influences and internal pressure. To study the phenomenon of soilpipeline interaction, a physical model was used and special emphasis was given to the application of large horizontal loads coupled with vertical loads. This paper focuses on the experimental results of two sideswipetype tests with a pipeline model in a very soft soil with undrained shear strength of ~3kPa. Experimental yield envelopes are also included. The experimental tests revealed that for very shallow pipe embedments the maximum horizontal load is obtained for a value of V/Vmax = 0.5, and that for larger embedments this value is in the order of 0.2. 1. Introduction The design of pipelines installed in deep-sea waters is still a challenge for offshore geotechnical engineering. Flowlines and pipelines can be submitted to combined vertical and horizontal loads, thermal expansion and internal pressure, among other loadings. The pipeline laying installation is not a guarantee of their penetration embedment and, consequently, their stability. The problem of untrenched pipelines has been studied and reported in literature (Murf et al., 1989; Brennodden and Stokkeland, 1992; Cassidy, 2004; Fontaine et al., 2004; Cathie et al., 2005; Zhang and Erbrich, 2005; Cheuk and Bolton, 2006; Dendani and Jaeck, 2007; Bruton et al., 2008; Tian and Cassidy, 2008; among others). A physical model was used to understand the mechanisms of interaction between pipe and soil through a lateral visualisation, and then to simulate the pipe response under combined vertical and horizontal loads. This paper concentrates on the results obtained using sideswipe tests with large and short horizontal displacements, in order to obtain an experimental yielding envelope of the pipe.
- Europe (0.70)
- North America > Mexico (0.29)
Abstract The Skarv floating production storage and offloading (FPSO) vessel is the key feature of the new Skarv field in the Norwegian Sea, located in the Haltenbanken area. The field includes 16 wells across 5 subsea templates, with reserves of 16.8 million Sm3 of oil and 48.3 billion Sm3 of gas. The vessel is moored using 15 suction anchors, placed in 3 clusters of 5. Due to the complex geological history, soil conditions vary across the site, with hard glaciomarine sediments and till overlain by soft marine clay. The suction anchor design approach was robust, with a limited number of geometries to allow flexibility during the installation phase. Finite element modelling of each anchor's holding capacity, as well as careful prediction of the required installation under-pressures, allowed the anchor geometries to be grouped in this way. The successful installation of the suction anchors was completed early 2011 to allow the hook-up of the FPSO as planned, with production due to start in 2012. This paper describes details of the anchor design and installation process, and outlines how this anchor type can be used successfully in challenging soil conditions. 1. Introduction 1.1 Background on Skarv field The Skarv field development is located in the Norwegian Sea close to the Arctic Circle, 210km off the coast of Norway. The development consists of one large floating production, storage and offloading (FPSO) vessel and five subsea templates (see Figure 1). The water depth at the FPSO location is about 360m, and it is due to start production in 2012. Reserves are around 17 million Sm3 of oil and condensate, and 48 billion Sm3 of rich gas. The vessel is 300m long and 50m wide, with a capacity of 15 million Sm3 of gas and 80 000 barrels of oil per day.
- Europe > Norway > Norwegian Sea > Halten Terrace > Revefallet Fault Complex > Skarv Unit > Block 6507/6 > Skarv-Idun Field > Skarv Field > Tilje Formation (0.99)
- Europe > Norway > Norwegian Sea > Halten Terrace > Revefallet Fault Complex > Skarv Unit > Block 6507/6 > Skarv-Idun Field > Skarv Field > Ile Formation (0.99)
- Europe > Norway > Norwegian Sea > Halten Terrace > Revefallet Fault Complex > Skarv Unit > Block 6507/6 > Skarv-Idun Field > Skarv Field > Garn Formation (0.99)
- (24 more...)
Laboratory Measurement of the Remoulded Shear Strength of Clays With Application to Design of Offshore Infrastructure
DeGroot, D.J. (University of Massachusetts Amherst) | Lunne, T. (Norwegian Geotechnical Institute) | Andersen, K.H. (Norwegian Geotechnical Institute) | Boscardin, A.G. (University of Massachusetts Amherst)
ABSTRACT Remoulded shear strength is an important design parameter for several offshore geotechnical engineering applications, including design of skirted anchor and foundation systems, and for submarine slope stability analysis. However, there are significant differences in the equipment and test procedures used in practice to measure the remoulded shear strength. The paper gives results from tests conducted on a variety of clays with very soft to stiff consistencies, using various methods of remoulding and laboratory devices. The results show that different remoulded shear strength values were obtained depending on the method of remoulding and test device used. In some cases the difference was significant, which could have important consequences for design. Based on the results of this study, the paper recommends remoulding procedures and laboratory test devices for measuring the remoulded shear strength of clays. 1. Introduction The remoulded undrained shear strength (sur) of fine grained soils is an important parameter in the design of offshore infrastructure and for analysis of submarine landslides. For example, in the design of skirted (suction) anchors and foundations, it is a key parameter for calculation of the penetration resistance and the under pressure required for installation. The sur also influences the side shear resistance after penetration is completed (i.e. setup), and thereby the holding capacity of an anchor and the bearing capacity and settlement of a skirted foundation. The geotechnical engineering literature shows that there is a large variety of laboratory and in situ equipment being used to measure sur. Laboratory measurement of sur has the significant advantage over measurement of the intact undrained shear strength (su) because sample disturbance for the former is generally not an issue. However, it remains a relatively complex parameter to evaluate, as results can vary greatly depending on the degree of remoulding and the measurement method used.
- Europe (0.70)
- North America > United States (0.68)
- Asia (0.68)
- Research Report > New Finding (0.54)
- Research Report > Experimental Study (0.34)
Effect of Gapping On the Uplift Resistance of a Shallow Skirted Foundation
Mana, D.S.K. (Centre for Offshore Foundation Systems, University of Western Australia) | Gourvenec, S. (Centre for Offshore Foundation Systems, University of Western Australia) | Randolph, M.F. (Centre for Offshore Foundation Systems, University of Western Australia) | Hossain, M.S. (Centre for Offshore Foundation Systems, University of Western Australia)
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. Stรธve 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.
Numerical Analysis of Spudcan Penetration Into Sand Over Uniform Clay
Hu, Pan (Centre for Offshore Foundation Systems, the University of Western Australia) | Wang, Dong (Centre for Offshore Foundation Systems, the University of Western Australia) | Cassidy, Mark (Centre for Offshore Foundation Systems, the University of Western Australia) | Yang, Qing (Dalian University of Technology)
Abstract The installation of mobile jack-up platforms remains hazardous because of the potential for uncontrolled punch-through failure. Seabeds of sand overlying clay, or stiff clay over soft clay, are problematic because the embedding spudcan pushes the strong layer of soil into the underlying softer layer. Both experimental and numerical approaches have been adopted in previous studies to assess punch-through potential and to understand the factors affecting the failure mechanism. In this paper, the entire penetrating process of a spudcan is simulated using a large-deformation finite element approach - the Coupled Eulerian-Lagrangian method in the commercial package Abaqus. The numerical approach is first compared with centrifuge model test data for uniform clay, uniform dense sand and loose sand over uniform clay. Results from a series of parametric studies of spudcan penetration into sand over uniform clay sites are then described. The dependence of the penetration behaviour on the friction angle and dilation behaviour of sand, the undrained shear strength of the underlying clay and the normalised sand layer thickness are also discussed. 1. Introduction A jack-up rig is typically used at more than one site during its service life and may encounter different soil types and environmental load conditions. Thus, the suitability of the unit for different sites, especially those consisting of sand overlying clay or stiff clay overlying soft clay, must be assessed. This is because in these conditions the spudcan can push the stronger overlying layer into the softer underlying soil. Known as a punch-through, the subsequent reduction of vertical load can cause the jack-up leg to uncontrollably penetrate. Several platform failures have been reported to be due to spudcan punchthroughs (e.g. the discussion in Hossain, 2008). This is because the soil around the spudcan undergoes significant translations and rotations during the penetration process.
ABSTRACT In 1998, a gas emanation occurred in the Gulf of Mexico in an area with shallow gas accumulations, so soil properties were evaluated through two site investigations carried out in 1998 and 2002. The 2002 study was a site investigation that explored both the geophysical and geotechnical aspects of the effects of the gas emanation on the foundation soil properties for four boreholes. Around the site, geotechnical studies in 1978, 2008 and 2011 were conducted for foundation design of platforms. First, it was found that the water content and unit weight of the clay and sand decreased with the presence of gas. Second, the shear strength of the clay could decrease or increase with the presence of gas, similar to what was discovered in Nava (2010), while in sand a decrease could occur. 1. Introduction One of the most complex phenomena that occur in the marine sediments is the sudden or gradual expulsion of gas from shallow accumulations. It becomes problematic when it occurs next to a platform and alters the properties of the foundation soils, as it has been shown to occur by several researchers (Whelan et al., 1977; Nageswaran, 1983; Wheeler, 1986; Rad et al., 1994; Nava, 2010). This results in the bearing capacity of the foundation soils being affected. In the investigation area, a gas emanation occurred in 1998 next to Platform A. As a result, special marine investigations were performed in 1998 and 2002 to evaluate how the foundation soil properties changed after the gas emanation (Nava and Barrera, 2000; Koh, 2005; Nava et al., 2011). This paper compares the results from the 1998 and 2002 studies, with the results from geotechnical investigations performed in the investigation area before the gas emanation occurred and after the 2002 study.
- North America > United States (1.00)
- North America > Mexico (1.00)