Foundation engineering for offshore gravity structures

This is the third in a series of three state‐of‐the‐art papers concerning the geotechnical engineering aspects of the installation of gravity platforms in the North Sea. Andresen et al. (1979) discuss the planning and execution of offshore site investigations and the procedures used to obtain soil engineering parameters. Eide et al. (1979) review the details of the different platform designs and the installation procedures for the thirteen gravity structures that so far have been placed in the North Sea in 70 m to 153 m of water. In this paper, the foundation engineering for gravity structures installed in the North Sea is reviewed including recommended analyses of permanent and cyclic foundation deformations, dynamic behavior, foundation stability, skirt penetration resistance and base contact stresses, potential piping and erosion problems, and stresses in oil wells and risers. Field performance observations are reported, and it is concluded that gravity structures may safely be installed in offshore areas with less favorable platform foundation conditions than so far encountered in the North Sea.     

孔压探杆贯入及潮汐作用下超孔压响应规律研究

Excess pore water pressure is an important parameter that can be used to analyze certain physical characteristics of sediment. In this paper, the excess pore water pressure of subseafloor sediment and its variation with tidal movement was measured following the installation of a wharf in Qingdao, China by using a fiber Bragg grating(FBG) piezometer. The results indicated that this FBG piezometer is effective in the field. The measured variation of excess pore water pressure after installation is largely explained by the dissipation of excess pore water pressure. The dissipation rate can be used to estimate the horizontal consolidation coefficient, which ranged from1.3×10~(–6) m~2/s to 8.1×10~(–6) m~2/s. The measured values during tidal phases are associated with the variability of tidal pressure on the seafloor and can be used to estimate the compressibility and the permeability of the sediment during tidal movement. The volume compression coefficient estimated from tidal oscillation was approximately2.0×10~(–11) Pa~(–1), which was consistent with the data from the laboratory test. The findings of this paper can provide useful information for in situ investigations of subseafloor sediment.     

Pore pressure within dipping reservoirs in overpressured basins

To predict reservoir pore pressure, we present a one-dimensional flow model that captures complicated two- and three-dimensional flow present in a dipping permeable reservoir encased in overpressured mudrock. The model incorporates the variation of mudrock permeability with effective stress and includes the effect of reservoir geometry. We find that reservoir pressure is lower when stress-dependent mudrock permeability is assumed relative to the case of constant mudrock permeability. Increased structural relief further reduces the reservoir pressure relative to the far-field pressure and increased effective stress (pore pressure is lower relative to the overburden) results in increased reservoir pressure relative to the far-field pressure. If a large fraction of the reservoir area is in deeper areas where the mudrocks are more overpressured, then the relative pressure is higher than cases where the reservoir area remains constant with depth. The model results compare favorably both to pressures predicted by a more complex numerical model that simulates basin evolution and to field observations in the Bullwinkle Basin (Green Canyon 65, Gulf of Mexico). Our model provides a quick workflow to predict excess pressures in dipping reservoirs encased in mudrock within mechanically-compacted basins. It can be used to analyze trap integrity, understand hydrocarbon migration, and improve drilling safety.     

Needs for oceanographic data to determine loads on coastal and offshore structures

The theoretical design concept as it exists today depends upon the reliable estimation of the environmental factors interacting with structure. This paper deals with the requirements for the oceanographical data as best we can determine them. It is concluded that the requirements for many data are not met at present. In view of the high cost of the offshore and coastal data collection, the coordinated efforts of oceanic data gathering and analysis should be undertaken.     

沉积物孔隙毛细管压力与甲烷水合物饱和度关系研究

To better understand the relationship between the pore capillary pressure and hydrate saturation in sediments, a new method was proposed. First, the phase equilibria of methane hydrate in fine-grained silica sands were measured. As to the equilibrium data, the pore capillary pressure and saturation of methane hydrate were calculated. The results showed that the phase equilibria of methane hydrates in fine-grained silica sands changed due to the depressed activity of pore water caused by the surface group and negatively charged characteristic of silica particles as well as the capillary pressure in small pores together. The capillary pressure increased with the increase of methane hydrate saturation due to the decrease of the available pore space. However, the capillary-saturation relationship could not yet be described quantitatively because of the stochastic habit of hydrate growth.     

Pore pressure response of seabed in standing waves and its mechanism

Experiments were carried out in a newly-developed rectangular flume to investigate the pore pressure response of silty seabed in standing waves, and to emphatically discuss the physical mechanism of wave-induced pore pressure. We first analyzed the features of wave-induced stress, laid out the experimental setup, and particularly monitored the temporal and spatial variation of pore pressure. Then, we summarized the physical essence of wave-induced pore pressure, discussed the applicability of the existing models, and proposed several key points about developing new models. Results indicate that regular standing waves were obtained in the rectangular flume. The oscillating pore pressure (OPP) and the residual pore pressure (RPP) were observed simultaneously in the seabed under wave nodes or antinodes, and were found with coupling effect. The OPP and RPP were related to the elastic and plastic volumetric strains, respectively. Both elastic and plastic volumetric strains may be caused by the wave-induced cyclic spherical stress or deviatoric stress. The elastic model could only simulate the OPP and the pore pressure development mode could only simulate the RPP. The yielding under isotropic compression and the shear expansion of geotechnical materials, as well as the wave-induced stress paths, should be considered in the elastoplastic model to simulate the wave-induced pore pressure accurately.     

The impact of wave loads and pore-water pressure generation on initiation of sediment transport

The build-up of pore-water pressure by waves can lead to sediment liquefaction and subsequent transport by traction currents. This process was investigated by measuring pore-water pressures both in a field experiment and laboratory wave tank tests. Liquefaction was observed in the wave tank tests. The results suggest that sand is less susceptible than silts to wave-induced liquefaction because of the tendency to partially dissipate pore-water pressures. However, previous studies have determined that pore-water pressures must approach liquefaction before current velocities necessary to initiate transport are reduced. Once liquefaction has occurred more sediment can be transported.     

A method to determine the horizontal ice loads on the vertical steel structures which adfreeze to the ice level

In this work the estimation of the load from the ice level on freeze into the ice structure is presented. Nowadays a big uncertainty exists when estimating a freeze-in effect. Considerable inexactness in the ice loads estimation could lead to great differences in the cost of the structures. Proper estimation of ice loads will reduce the cost of constructions. If the structure is freezing-in during no change of sea level — ice collars are occurring. Ice collars could lead to increase of the horizontal ice load from the ice level on the structure. The overview of the existing recommendations is presented. The thermo-dynamical task for calculating ice collar profile was investigated during the present work. The numerical methods were used to calculate an increase of the load when freeze-in effect occurs. Physical experiments were conducted to verify the models. Results of the work provide provisional method for estimation of the ice loads on the freeze into the ice structures.     

Wave-in-deck loads on exposed jetties

This paper presents results from research on the hydraulic loadings of exposed (unsheltered) jetties (open pile piers with decks and beams). The work presented here focuses on results from physical model tests on wave-induced loads on deck and beam elements of exposed jetties and similar structures. These tests investigated the physics of the loading process, and provided new guidance on wave-in-deck loads to be used in design. Wave forces and pressures were measured on a 1:25 scale model of a jetty head with projecting elements. Structure geometry and wave conditions tested were selected after an extensive literature review (summarised in the paper) and consultation with the project steering group. Different configurations were tested to separate 2-d and 3-d effects, and to identify the effects of inundation and of down-standing beams.     

Hydrodynamic loads on subsea robots

Free floating subsea robots are often subjected to complex hydrodynamic loads due to waves/currents and wakes from nearby structures. This makes control extremely difficult. Good hydrodynamics data might counteract this. This paper presents hydrodynamics data for two configurations undergoing various combinations of steady translation and approximately steady rotation. The two configurations are: a spherical body with a single fixed link arm and a two link arm without a body. For both configurations neural networks were found to provide a reasonable fit to the data. We believe that such fits could be used to improve control.     

Basement-involved faults and deep structures in the West Philippine Basin: constrains from gravity field

To reveal the basement-involved faults and deep structures of the West Philippine Basin (WPB), the gravitational responses caused by these faults are observed and analyzed based on the latest spherical gravity model: WGM2012 Model. By mapping the free-air and Bouguer gravity anomalies, several main faults and some other linear structures are located and observed in the WPB. Then, by conducting a 2D discrete multi-scale wavelet decomposition, the Bouguer anomalies are decomposed into the first- to eighth-order detail and approximation fields (the first- to eighth-order Details and Approximations). The first- to third-order Details reflect detailed and localized geological information of the crust at different depths, and of which the higher-order reflects gravity field of the deeper depth. The first- to fourth-order Approximations represent the regional gravity fields at different depths of the crust, respectively. The fourth-order Approximation represents the regional gravity fluctuation caused by the density inhomogeneity of Moho interface. Therefore, taking the fourth-order Approximation as input, and adopting Parker-Oldenburg interactive inversion, We calculated the depth of Moho interface in the WPB. Results show that the Moho interface depth in the WPB ranges approximately from 8 to 12 km, indicating that there is typical oceanic crust in the basin. In the Urdaneta Plateau and the Benham Rise, the Moho interface depths are about 14 and 16 km, respectively, which provides a piece of evidence to support that the Banham Rise could be a transitional crust caused by a large igneous province. The second-order vertical derivative and the horizontal derivatives in direction 0° and 90° are computed based on the data of the third-order Detail, and most of the basement-involved faults and structures in the WPB, such as the Central Basin Fault Zone, the Gagua Ridge, the Luzon-Okinawa Fault Zone, and the Mindanao Fault Zone are interpreted by the gravity derivatives.     

On the estimation of surface gravity waves from subsurface pressure records for estuarine basins

According to small-amplitude theory, the surface gravity-wave spectrum can be estimated from a subsurface pressure-fluctuation spectrum by applying a factor (K) that compensates for the attenuation of surface-wave amplitude as the depth below the water surface and the wave frequency increase.There are a number of factors, however, that cause K to be inaccurate over a large portion of the spectrum's frequency range. Numerous attempts have been made to derive an empirical correction factor (n) that could be applied to K to provide a better estimate of the surface-wave spectrum. This paper evaluates some of these empirical factors, specifically for use in an estuarine environment, and recommends Graces' (1978) equation for n as a function of the non-dimensional frequency parameter kh (where $\text{k =}\text{2π}\text{L}$ is the local wavenumber, h the local depth and L the wavelength).The paper also evaluates the maximum limit (K_max) on the magnitude of K suggested by Esteva & Harris (1970), where relative depth $\text{d}\text{h}$ (d is the pressure transducer height above the bottom) and k_oh (a parameter directly related for large values of kh to wave frequency by the dispersion relation) are the independent variables. The choice of K_max may be made unimportant if d is selected beforehand using an equation (Knowles, 1981a) for the minimum $\text{d}\text{h}$ limit affected by the choice of K_max.     

Pore pressure assessment from well data and overpressure mechanism: Case study in Eastern Tunisia basins

Abstract

Independent and complementary methods were used for pore pressure assessment in the eastern Tunisian basins. Drilling data and surveys allow settling the pore pressure profile in these basins. The main used parameters are mud weights, formation pressure surveys, drilling parameters, well logs, fluids exchange with formation and borehole issues. In the eastern Tunisia platform, the pore pressure profiles show changes in overpressure magnitude in all the three dimensions of the basin (location and depth/stratigraphy). We highlighted two overpressure intervals form bottom to top: The late Cretaceous in the North-eastern part, and the Tertiary overpressure interval hosted in the Palaeocene to Miocene series. The structural analysis of overpressure location shows that the Tertiary interval is likely to have originated in a disequilibrium compaction in Cenozoic grabens. Pore pressure cross sections and maps confirm the link between active normal faults that segmented the basin to grabens and highs and pore pressure anomalous area. In the Senonian interval, we noted mature source-rocks that can explain the overpressure in the late Cretaceous interval. In addition, the recent to active compressive tectonics may have contributed to both pore pressure anomaly generations. The fluid overpressures characterization in the eastern Tunisian sedimentary basins helps in hydrocarbons exploration. Indeed, the overpressure interval in the reservoir levels stimulates and improves the production in the oilfields and contributes to hydrocarbon trapping. Moreover, the adequate prediction of pore pressure profile contributes to reduce drilling cost and enhance the drilling operations safety.     

A study on the Local Joint Flexibility (LJF) of two-planar tubular DK-joints in jacket structures under in-plane bending loads

In the present paper, results of a parametric study conducted on the Local Joint Flexibility (LJF) of two-planar tubular DK-joints under In-Plane Bending (IPB) loads are presented. DK-joints are among the most common joint types in jacket substructure of Offshore Wind Turbines (OWTs). A total of 324 finite element (FE) analyses were carried out on 81 FE models under four types of IPB loading in order to investigate the effect of the DK-joint’s geometrical parameters on the LJF factor (f_LJF). Based on the results of parametric study, the factors leading to the LJF reduction were introduced. Generated FE models were verified using the existing experimental data, FE results, and parametric equations. The effect of the weld profile was also considered. The f_LJF in two-planar DK- and uniplanar K-joints were compared. Results indicated that the effect of multi-planarity on the LJF is quite significant and consequently the use of the equations already available for uniplanar K-joints to calculate the f_LJF in two-planar DK-joints may lead to highly under-/over-predicting results. To handle this issue, the FE results were used to derive a set of parametric equations for the prediction of the f_LJF in IPB-loaded two-planar DK-joints. The proposed equations were checked against the acceptance criteria recommended by the UK DoE and can be reliably used for the analysis and design of tubular joints in OWTs.     

A survey on semisubmersible wind loads

The methods and assumptions of calculating wind drags and overturning moments on semisubmersible offshore drilling rigs are investigated. The empirical methods and the wind tunnel testing are compared.     

Conditions of the propagation of internal gravity waves through wind structures from the troposphere to the ionosphere

We consider the propagation of large-scale internal gravity waves (IGWs) from the troposphere through vertically inhomogeneous wind structures to the ionosphere heights on the basis of an analysis of beam trajectories. We selected different versions of the initial parameters and numerically studied the specific peculiarities of the behavior of beam trajectories related to the location of layers with horizontal and vertical reflection and a critical layer on the pathway of the waves. It was shown that, for sufficiently large-scale IGWs, their propagation from the troposphere to ionosphere heights is possible. We found the characteristic times when the waves generated in the troposphere reach ionosphere heights. We revealed strong variations in these times when the initial parameters of the problem changed. We determined the typical values of the horizontal displacement (relative to the generation source) of the wave packets when they reach the ionosphere.