Seabed Soil Research Articles

Numerical simulation of the wave-induced dynamic response of poro-elastoplastic seabed foundations and a composite breakwater

In this study, an integrated numerical model FSSI-CAS 2D (previously known as POROWSSI 2D) is developed for the problem of wave-elasto-plastic seabed-structure interactions, where the Volume Average Reynolds Average Navier–Stokes (VARANS) equation is taken as the governing equation for wave motion and porous flow in porous medium; the dynamic Biot’s equation known as “u-p” is taken as the governing equation for the dynamics of porous seabed soil under wave loading. The Pastor–Zienkiewicz Mark III proposed by Pastor et al. (1990) [45] is used to describe the dynamic behaviour of poro-elasto-plastic seabed under wave loading. This developed integrated numerical model is validated by a centrifuge test conducted by Sassa and Sekiguchi (1999) [30]. The developed integrated numerical model is applied to investigate the wave-induced dynamic response of a composite breakwater and its elasto-plastic seabed foundation. The numerical results indicate that the pore pressure in an elasto-plastic seabed builds up under wave loading, leading to the reduction of the contact effective stresses between soil particles. The residual liquefaction occurs when the effective stresses decrease to a value approaching zero. The wave-induced residual liquefaction in seabed is progressive downward. A parameter considering the cohesion and friction angle of soil is defined to evaluate the residual liquefaction potential. Analysis results illustrate that the friction angle of soil has significant effect on the soil liquefaction; and Nevada dense sand becomes liquefied if the defined parameter exceeded 0.86. Parametric study shows that wave characteristics and soil properties have significant effects on the wave-induced progressive residual liquefaction in loose elasto-plastic seabed foundation.

Trenching of Pipelines for Protection in Ice Environments

Summary Subsea pipelines located in ice environments need to be protected from potential ice gouging (also known as ice scouring) created when a moving ice keel interacts with the seabed. The integrity and operability of the pipeline can be affected by direct contact between the ice keel and the pipeline, or from loading imposed on the pipeline through soil deformation caused by ice gouging. The typical method considered for protecting against the risk of damage caused by ice gouging is through pipeline burial. Conventional methods of pipeline burial use equipment such as ploughs, mechanical trenchers, and jetters, the majority of which have been designed to accomplish a maximum of 2 to 3 m of pipeline burial. Dredges can be used, but they have water depth limitations and limited productivity. Land-based equipment has been used for shore crossings, but is limited to shallow depths where temporary construction berms can be used as a working platform. The capabilities of existing technologies are based on current industry practice developed for pipeline burial in ice-free environments. Pipeline burial requirements have generally been for improved hydrodynamic stability, mechanical protection (e.g., from anchors), controlling pipeline movements (e.g., expansion and buckling) and/or for flow-assurance purposes (reducing heat loss). A 2- to 3-m burial depth has been generally adequate to satisfy these conventional requirements. As developments are proposed for areas that experience relatively deep ice gouging (up to 5 m), burial depth requirements will exceed the capabilities of current pipeline burial technologies. New technologies capable of working in deeper water, achieving greater burial depths, achieving reasonable trenching advance rates, operating in harsh environments, and trenching through variable and difficult seabed soils will be required. This paper highlights the issues and challenges surrounding pipeline trenching and burial in ice-gouge environments. The current state of practice is discussed along with the technology gaps that need to be addressed to prepare for future offshore developments in ice-covered waters where there is the potential for seabed ice gouging.

Centrifuge modeling of steel catenary risers at touchdown zone part I: Development of novel centrifuge experimental apparatus

The critical location for fatigue damage in Steel Catenary Risers (SCRs) is often located within the touchdown zone (TDZ), where cyclic interaction of the riser with the seabed occurs. Riser–fluid–soil interaction at the TDZ is a complex phenomenon with significant room for improving the understanding of the interaction, the trench formation mechanism, and their influence on fatigue life. This paper describes a novel experimental tool developed for SCR testing in a geotechnical centrifuge. It discusses the modeling parameters, scaling considerations, and developed actuation system. Further, it presents the results of a series of trial tests done on an elastic seabed model and compares the measurements to the results of finite element analysis. Finally, it discusses the potential of the tool in modeling SCRs at TDZ using seabed soil and dual frequency Response-Amplitude-Operator (RAO) motions.

Suction Force in the Bottom of Spudcans Resting on Silty Sediments

As the development of exploiting marine resources, many ocean platforms were built. But there is still a problem unsolved completedly. The suction force between seabed soil and ocean platform perplexed jackup technically and is discussed in the article. The comparisons between model experiments and Skempton's Model are much helpful to realize the significance of suction force.

Dynamic Response of the Casing String in Seabed Soil for Deepwater Drilling

The marine riser has transfer dynamic cyclic load to the top of casing string when deepwater drilling operating. Considering with the nonlinear characteristics of seabed soil, the variable string cross section, and the axial load on the top of string, a dynamic differential equation of casing string below mud line and its numerical solution were established. Results illustrate that, the lateral displacement, the bending moment, and the shear of the casing string are not equal when lateral dynamic cyclic loading achieves amplitude due to the soil hysteresis effect. The diameter and wall thickness of conductor have less affect to string vibration frequency, cannot adjust the string natural frequency by adjusting these parameters. The natural frequency of casing string in sand is bigger than it in clay. The undrained shear strength of soil has great effect on the natural frequency of casing string in clay. The lateral displacement amplitude is highest when the vibration frequency of casing string at its natural frequency.

Soils on exposed Sunda Shelf shaped biogeographic patterns in the equatorial forests of Southeast Asia

The marked biogeographic difference between western (Malay Peninsula and Sumatra) and eastern (Borneo) Sundaland is surprising given the long time that these areas have formed a single landmass. A dispersal barrier in the form of a dry savanna corridor during glacial maxima has been proposed to explain this disparity. However, the short duration of these dry savanna conditions make it an unlikely sole cause for the biogeographic pattern. An additional explanation might be related to the coarse sandy soils of central Sundaland. To test these two nonexclusive hypotheses, we performed a floristic cluster analysis based on 111 tree inventories from Peninsular Malaysia, Sumatra, and Borneo. We then identified the indicator genera for clusters that crossed the central Sundaland biogeographic boundary and those that did not cross and tested whether drought and coarse-soil tolerance of the indicator genera differed between them. We found 11 terminal floristic clusters, 10 occurring in Borneo, 5 in Sumatra, and 3 in Peninsular Malaysia. Indicator taxa of clusters that occurred across Sundaland had significantly higher coarse-soil tolerance than did those from clusters that occurred east or west of central Sundaland. For drought tolerance, no such pattern was detected. These results strongly suggest that exposed sandy sea-bed soils acted as a dispersal barrier in central Sundaland. However, we could not confirm the presence of a savanna corridor. This finding makes it clear that proposed biogeographic explanations for plant and animal distributions within Sundaland, including possible migration routes for early humans, need to be reevaluated.

Experimental research on the marine hydrodynamic action on the consolidation process of the sediments in the Yellow River Estuary

Based on the in-situ measurements, the impact of the marine hydrodynamics, such as wave and tide, in the rapidly deposited sediments consolidation process was studied. In the tide flat of Diaokou delta-lobe, one 2 m×1m×1 m test pit was excavated. The seabed soils were dug and dehydrated, and then the powder of the soil was mixed with seawater to be fluid sediments. And an iron plate covered part of the test pit to cut off the effect of the marine hydrodynamics. By field-testing methods, like static cone penetration test (SPT) and vane shear test (VST), the variation of strength is measured as a function of time, and the marine hydrodynamics impact on the consolidation process of the sediments in the Yellow River estuary was studied. It is shown that the self-consolidated sediments’ strength linearly increases with the depth. In the consolidation process, in the initial, marine hydrodynamics play a decisive role, about 1.5 times as much as self-consolidated in raising the strength of the sea-bed soils, and with the extension of the depth the role of the hydrodynamics is reduced. In the continuation of the consolidation process, the trend of the surface sediments increased-strength gradually slows down under the water dynamics, while the sediments below 50 cm are in opposite ways. As a result, the rapidly deposited silt presents a nonuniform consolidation state, and the crust gradually forms. The results have been referenced in studying the role of the hydrodynamics in the soil consolidation process.

Analysis of Soil Strength Degradation during Episodes of Cyclic Loading, Illustrated by the T-Bar Penetration Test

Pipelines and risers form an essential part of the infrastructure associated with offshore oil and gas facilities. During installation and operation, these structures are subjected to repetitive motions which can cause the surrounding seabed soil to be remolded and soften. This disturbance leads to significant changes in the operative shear strength, which must be assessed in design. This paper presents an analytical framework that aims to quantify the degradation in undrained shear strength as a result of gross disturbance—in this case through repeated vertical movement of a cylindrical object embedded in undrained soil. The parameters of the framework were calibrated using data obtained in a geotechnical centrifuge test. In this test a T-bar penetrometer, which is a cylindrical tool used to characterize the strength of soft soil, was cycled vertically in soil with strength characteristics typical of a deep water seabed. Using simple assumptions regarding the spatial distribution of “damage” resulting from movement of the cylinder, and by linking this damage to the changing undrained shear strength via a simple degradation model, the framework is shown to simulate well the behavior observed in a cyclic T-bar test. This framework can potentially be extended to the similar near-surface behavior associated with seabed pipelines and risers.

An experimental study on non-linear progressive wave-induced dynamic stresses in seabed

In this study, the dynamic stresses within the seabed induced by non-linear progressive waves were explored through a series of hydraulic model tests on a movable bed within a wave flume. By comparing Stokes’ 2nd-order wave theory with the theory of wave-induced dynamic stresses within the seabed as proposed by Yamamoto et al. [1978. On the response of a poro-elastic bed to water waves. Journal of Fluid Mechanics 87 (1), 193–206.] and Hsu and Jeng [1994. Wane-induced soil response in an unsaturated anisotropic seabed of finite thickness. International Journal for Numerical and Analytical Methods in Geomechanics 18, 785–807], the experimental results show that the pressure on the seabed surface, the pore water pressure within the seabed as well as the vertical and the horizontal stresses are all smaller than their theoretical values. If we were to obtain the characteristics of seabed soil, the analytical solution of Hsu and Jeng [1994. Wane-induced soil response in an unsaturated anisotropic seabed of finite thickness. International Journal for Numerical and Analytical Methods in Geomechanics 18, 785–807] might agree to the simulation of the wave-induced effective stresses and shear stress in the sandy seabed. A different phase shift exists among all the three soil stresses. Their influences on the three dynamic stresses within seabed soil are important for seabed stability, and can be used in the verification of numerical models. In the whole, the non-linear progressive waves and the naturally deposited seabed are found to have a strong interaction, and the behavior of the induced dynamic stresses within the seabed is very complicated, and should be investigated integrally.

Wave damping analysis in a porous seabed

This study computed numerically wave damping passing over the porous seabed using a boundary element method. The fluid is assumed to be inviscid and incompressible, and the flow is assumed to be irrotational. The analytical domain is formulated mathematically as a non-linear problem in terms of velocity potential. The Darcy’s law is applied to account for the seabed porosity. The pressure and vertical velocity potential in the interface boundary between seabed soil and fluid is assumed to be continuous. The comparison with other results in order to prove the analysis model shows a good agreement in spite of small amplitude wave theory, and also those results obtained by present numerical method show that wave height profiles and horizontal velocities passing over the porous seabed are damped due to porous seabed media. Therefore, numerical analysis method considering porous seabed conditions seems to be more useful.

STEEL JACKET STRUCTURES FOR THE NEW EKOFISK COMPLEX IN THE NORTH SEA.

Keywords IMPERIAL COLLEGE, FUGRO KVAERNER OIL & GAS STRUCTURES, JACKETS, STEEL, EKOFISK, DESIGN, OFFSHORE, OIL, SOILS, SEABED, MOVEMENT, EXTRACTION, SUBSIDENCE, PROJECTS, WAVES, DIFFRACTION, TANKS, CAST, NODES, FABRICATION, PLATFORMS, OILFIELDS, WATER, DEPTH, CONFIGURATION, DRILLING, PILES NORTH SEA, EKOFISK NORWAY, SCANDINAVIA, EUROPE... Show All

GEOTECHNICAL ASPECTS OF SUBSIDENCE RELATED TO THE FOUNDATION DESIGN OF EKOFISK PLATFORMS.

Keywords PHILLIPS PETROLEUM, IMPERIAL COLLEGE, FUGRO FOUNDATIONS, DESIGN, PLATFORMS, EKOFISK, SUBSIDENCE, GEOTECHNICS, STEEL, PILED, JACKETS, FINITE ELEMENTS, ANALYSIS, CRUST, MEASUREMENT, MOVEMENT, PROGRESSIVE, EFFECT, IN SITU, BEHAVIOUR, SOILS, SEABED, STRENGTH, STRESS, STRAIN, MODELS, STRATIGRAPHY, PILES, CAPACITY, STRUCTURES, INTERACTION EKOFISK, NORTH SEA... Show All

Analysis of single pipeline pull-in procedure using small scale models

The structural elastic behaviour of steel pipelines during on-bottom pull-in procedure is investigated experimentally by using small scale models. Despite the lack of a sea-bed soil pipeline interaction effect in the model, it is possible to show that the unevenness of the sea-bed surface and the non-uniform distribution of friction forces can lead to large deflection bending stresses, higher than the numerical results yielded by standard software packages commonly used for design calculations. Due to foreseen difficulties in overcoming the drawbacks in practical and numerical procedures, the off-bottom pull-in operation arises as an attractive alternative even for shallow waters.

Adhesion at Jute Geotextile/Clay Slurry Interface

In the use of jute fabric for the layered clay-jute geotextile-sand method for land reclamation, it is important to determine the interface behavior between the jute geotextile and various clay types commonly found in dredged seabed soils. Since jute is used to separate and support thin sand seams to form a horizontal drainage layer on top of a soft clay slurry, the interface behavior between the jute and the slurry must be examined. The adhesion on the interface between the jute and the clay slurry is measured using the incremental penetration of a suspended end- weighted jute sheet under an incremental load into a prepared clay slurry with various moisture contents, in order to obtain the interface stress value under a quasi-static condition. The effects of sheet width and sheet type were investigated earlier, and found to have a negligible effect on the interface adhesion for the water content range tested. In order to examine the effects of clay-type, several mixtures of Kaolin-Bentonite in fixed proportions were prepared to produce clays of liquid limits ranging from 100% to 400%. The interface adhesion between the jute and four samples of Kaolin-Bentonite mixes at various water contents from two to six times the liquid limits were measured using the vertical sheet test. These mixes included clay types with liquid limits from 100% to 400%, thus allowing the nature of jute/slurry interaction to be studied for a wide range of clay type. When the water contents are normalized by the liquid limits, it was observed that the interface adhesion values fall into a very narrow behaviour band, thus making it possible to estimate the available interface strength between jute and clay slurry based on water contents.

Foundation‐Response Predictions Below Caisson‐Retained Island

The results of two‐dimensional (axisymmetric) finite element analyses of a clay sea bed located in the Canadian Beaufort Sea are described herein. The sea bed is subjected to loads associated with excavation, construction of a sand island, and placement and infilling of a caisson. This construction process is numerically simulated using an”incremental construction approach. The sea bed is characterized using the bounding surface model for isotropic cohesive soils. Values for the parameters associated with the model are determined by matching laboratory data for similar soils. The initial state of the sea‐bed soil and the construction/loading time history employed are, however, obtained directly from field data. The numerical predictions generated agreed quite favorably with in situ measurements. In addition to providing information concerning the response of the sea bed to the construction of a sand island, the present study provides additional evidence that validates the bounding surface model for cohesive soils.

Stability of a purely cohesive sea bed soil under wave loading: technical note: Dormieux, L GeotechniqueV38, N1, March 1988, P121–123

Stability of a purely cohesive sea bed soil under wave loading: technical note: Dormieux, L GeotechniqueV38, N1, March 1988, P121–123

Stability of a purely cohesive sea bed soil under wave loading

Stability of a purely cohesive sea bed soil under wave loading

Trials of Anchors and a Methodology to Evaluate the Maximum Holding Power

In order to realize a reliable and optimum mooring system, it is essential to estimate the behavior of anchors at the mooring site. But, the information about the performance of anchor is very poor in regard to various type of soil. At the same time, it is not uncommon that the mechanical characteristics of sea bed soil are not known over the anchorage.This paper introduces two indexes, the Holding Index and the Anchor Index, to evaluate the maximum holding power of anchor at the mooring site. The former which represents the condition of sea bed soil is measured at the mooring site as the holding power of the standard sounding anchor. The Anchor Index which represents the performance of an anchor is defind as the ratio of its maximum holding power to that of the standard sounding anchor. The maximum holding power of a bower anchor in any dimension can be calculated on the basis of these indexes.Trials of five types of anchor were conducted at three sites in order to determine anchor indexes and to make a map of holding index. It is also shown that the mechanical characteristics of sea bed soil in a bay varies considerably in short range.

アンカーの最大把駐力係数

Full scale trials on three types of anchor were carried out in the sand bed off Matsuho-no-Ura, Hyogo Prefecture. Newly designed absolutely-roll-stable single fluke anchors (ARS-S Anchors: 6, 3, 1ton) proved to be very active and developed the holding power approximately 6 times the weight of anchor. The maximum holding power ratio of stockless bower anchor (Hall's Anchor: 5.6ton) was about 3.5. It should be noted that Hall's Anchor turned over in all trials. It was observed that the fluke often failed to penetrate the sea bed. Stocked Anchor (5.2ton) was stable but developed poor maximum holding power ratio about 3.6. The maximum holding power of anchor depends on the deformations property of the sea bed soil. There are, however, few informations about those of the anchorage. Under existing circumstances, it is not easy for an anchored ship to estimate the capacity of mooring force in order to prevent anchors from dragging. In this paper, the philosophy and the procedure of measurement are discussed for the inspection of the geological characteristics of the sea bed. The concept of "standard inspection anchor" is introduced so as to get the index of the sea bed soil for the advantage of anchoring. With the aid of "standard inspection anchor", it will be possible to fulfil the map of holding power index of sea bed soil over the anchorage.

Design of Hydrostatically Supported Sand Islands for Arctic Drilling

The search for oil and gas has already extended to the Arctic areas of the world. To date conventional sand islands have been used for exploration drilling purposes in water depths of up to 43 ft. In deeper water exploration has only been possible using floating drilling equipment which can only operate during the short summer season of open water. This paper briefly outlines the geotechnical principles and development to date of hydrostatically supported sand islands. This construction technique, which utilizes hydrostatic water pressure to stabilize dredged sand at near vertical underwater slopes, would allow sand islands to be quickly and economically built in water depths of up to 200 ft. The hydrostatically supported sand island is a gravity structure and, therefore, is only suitable for use on competent seabed soils. This paper presents two different designs which are currently proposed for use as drilling structures in such areas as the Beaufort Sea. One design is intended for use as a movable exploration structure and the second for a permanent production island that would remain on location for 30 to 50 yr. The near vertical side slopes of the hydrostatically supported sand islands reduce the amount of sand required to manageable amounts, and allow the construction on location to be safely completed during the short Arctic summers. The sand provides sufficient mass to resist ice pressures.