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Vacuum preloading combined with prefabricated vertical drains (PVDs) has the potential to improve the soft sediments under water, however, its development is partly limited by the unclear understanding of the mechanism. This paper aims to extend the comprehension of the influential mechanism of overlapping water in the scenario of underwater vacuum preloading with PVDs. The systematic investigations were conducted by small strain finite element drained analyses, with the separated analysis schemes considering suction-induced consolidation, seepage and their combination. The development of settlement in the improved soil region and the evolution of seepage flow from the overlapping water through the non-improved soil region into improved zone are examined in terms of the build-up of excess pore pressure. Based on the results of numerical analyses, a theoretical approach was set out. It was capable to estimate the time-dependent non-uniform settlement along the improved soil surface in response to the combined effects of suction-induced consolidation and seepage. The difference of underwater and onshore vacuum preloading with PVDs is discussed with some practical implication and suggestion provided.

To study the stress-strain characteristics of reinforced cushion with multi-layer geogrids, a new calculation method was proposed. Based on hyperbolic model, the reinforced cushion was regard as a nonlinear elastomer. The effect of reinforcing geogrids was considered as an equivalent additional pressure and included into the gravel skeleton. Considering the friction effect between geogrids and crushed gravel, the calculation method of stress-strain relationship and shear strength of the reinforced crushed gravel was deduced through definite integral. The proposed method was verified by triaxial tests and the calculated results were in good agreement with the measured data. Furthermore, the key parameters influencing the shear strength, such as the total number of geogrids, the geogrids spacing and the corresponding position, were quantified through sensitivity analysis. The effects of the key parameters on the stress-strain relationship of reinforced cushion were determined. It is shown that the ability of reinforced body to resist bulging deformation is great improved by geogrids. The key parameters are the confining pressure and the total number of geogrids. With the increase of the total number and the confining pressure, the shear strength of the reinforced body increases; but the shear strength is less affected by its location. The research results can provide reference for practical projects.

Biogrouting, which is a new method for soil improvement, was used in an attempt to cement a type of hydraulic fill fine sands (called black sands) in reclamation projects in Tianjin, China, to form a working layer for mechanical equipment. Several factors influencing biogrouting with regard to cementing solution, including injection frequency, reaction time, concentration, and flow rate, were controlled to prepare black sand columns. This paper reports on an investigation of bacterial fixation, calcium ion utilization, and calcium carbonate distributions of biogrouted sand specimens. At the end of the tests, the geotechnical performances of the sand specimens were determined. The results showed that the biogrouting method effectively solidified black sands, by increasing the unconfined compressive strength of a sand column to 1.91 MPa and reducing the permeability coefficient by three orders of magnitude. A relationship between the unconfined compressive strengths and calcium carbonate contents wa...

The super-long and large-diameter steel pipe piles are often adopted for the construction of offshore oil platforms in deep sea. One constructability issue related to driving heavy pipe piles is th...

Industrial and construction wastes are often stored and heaped up in a large-scale mound on the storage yard. The load generated to the foundation soil tends to be wide in area, long term in duration and high in intensity which requires higher bearing capacity of foundation soil. The step loaded surcharge preloading method is often adopted to consolidate the foundation soils and thus improve their engineering properties. To reasonably evaluate the stability of the foundation soils, the consolidation effect is therefore required to be considered when estimating the increased bearing capacity of the foundation soil. The stress path method is adopted in this paper to calculate the strength development of the foundation soil during the consolidation process. An analytical method was proposed to estimate the bearing capacity of the foundation with respect to consolidation effect. FEM analysis was used to verify the reasonability and validity of the proposed method. Two case studies were also conducted ...

Offshore platforms in deep seas require extremely long large diameter pipe piles (LDPPs) to support the loads generated from the structure itself, wind and waves. One of the construction issues related to the installation of a LDPP into a seabed is pile running. Unexpected pile running during the LDPP driving process may break the steel wires connected to the heavy hammer or may cause the loss of the hammer into the sea. An analytical method to predict the occurrence of pile running is proposed in this paper. The framework for determining the calculation parameters from a laboratory or field test are established. The dynamic resistance of the surrounding soil during the pile driving process is considered through analysis of the soil sensitivity and the induced excess pore water pressure. Three case studies were conducted to verify the accuracy of the proposed method. Good agreement indicated that the proposed method is capable of predicting pile running during the driving process of LDPPs in layered soil deposits.

Piles used for oil platform foundation in Bohai Gulf, China, are usually over 100m long and 2m in diameter.Such long piles have to be manufactured in segments before installation because of transformation and hoistingdifficulties. The segments of the pile are assembled during installation by welding one to another, whichoften takes one or more days for two pile segments. After pile segment assembling, it proves to be difficult torestart the successive penetration. In this paper the mechanism of pile refusal is discussed and the process ofdriving a segmented pile is simulated using a finite element program. The calculated results show that whenthe stick-up stability is satisfied, increasing the length of the last segment of the pile and assembling the pilesegments when the pile tip stays in a clay layer can greatly ease the difficulty of restarting pile driving.

Pipeline walking is a global axial movement caused by the asymmetric distribution of axial force along the pipeline during the repetitive operational loading cycle. The pipeline walking rate, increasing with the loading cycles, may lead to high stress thereby the failure of connectors at PLETs or PLEMs. Current research studies and design guidelines focused on free-end pipelines and neglected changes in the pipeline walking rate and axial resistance with effects of pipeline end structures. However, in practice, several pipelines are adjacent to PLETs or PLEMs, which potentially affect the pipeline walking and expansion behaviour thereby walking rate and axial displacement. This study analyses the walking phenomenon considering the effects of mobilisation of the resistance at the pipeline end structures and develops new analytical solutions for the present problem, in terms of different walking mechanism, maximum axial resistance Pmax, and positions of pipeline end structures. The analytical solutions are validated through numerical simulation results. The case study results indicate the axial resistance generated by PLETs/PLEMs can effectively mitigate pipeline walking rate compared with free-end pipeline walking. In particular, the influence of PLETs/PLEMs is investigated, highlighting the sensitivity of the pipeline walking to the end boundary conditions covering different design scenarios.

Concrete suction caissons have been successfully used as breakwaters or seawalls in recent years. The relative large wall thickness-to-diameter ratio of a concrete caisson can lead to the formation of a full soil heave plug that may cause difficulties in the installation of concrete caisson in clay. One way to overcome this limitation is to use a tampered tip for the caisson wall. An analytical method is proposed in this article to calculate the minimum suction pressure required to penetrate a caisson and the maximum allowable suction pressure that can be applied to avoid too much soil heave plug during the installation of the suction caisson. Four model tests were conducted in normally consolidated clay to study the installation process of a concrete suction caisson with tampered tip and to verify the proposed analytical method. The height of the soil heave plug in the caisson with a tampered tip is observed to be about half of that in the caisson with a flat tip.

With the development and utilization of underground spaces, subways and tunnels are often located in poor underground environments subject to problems with water and mud bursts. To reduce project losses, areas of gushing water must be isolated as soon as possible to enable repairs. To accomplish this isolation, the installation of a rubber airbag in a tunnel as a temporary retaining structure is proposed. An airbag is a membrane structure, which differs from a conventional structure. As a result, the force and deformation under the external pressure directly affect an airbag’s slip failure mode. In this study, by combining the boundary conditions and stress conditions of an airbag in a tunnel, the rules governing the deformation of the airbag under uniform loading and the control conditions of the failure limit state are obtained. Model tests are conducted using airbags of two different diameters to block the external pressure, and the test results agree well with the theory. To improve the static friction resistance of the airbag and enable convenient and easy construction, a double airbag system is proposed. The deformation characteristics and failure mechanism of the double-airbag system are obtained from a force analysis. The double-airbag system blocks external pressure better and each of the airbags weighs less and easy to handling.

In ocean engineering, the large-diameter and super-long pile can be seen as a frictional pile because of its long length, indicating that the lateral friction plays a significant role in the bearing capacity of the pile. According to in situ measured data, the lateral friction of the pile tends to be lower than the friction obtained by calculating the parameters of soils in the penetration process of super-long piles. This is caused by large deformation shearing during pile driving and might lead to a reduction of the pile-bearing capacity, which may render engineering projects unstable and unsafe. To study the lateral friction of piles in ocean engineering, large-scale direct shear tests and ring-shear tests were performed to investigate the interfacial shearing behavior of pile-soil during the pile-driving procedure. Three kinds of soil samples, including sand, silty clay and clay, were used in the tests to study the interfacial shearing behavior of soils of various textures. A reduction coefficient was proposed to determine the interfacial strength by referring to the soil strength. Based on the test results and previous works by other researchers, a discussion of the analysis of the factors that influence the residual strength of the soil is provided.. Advice for engineering projects is put forward in accordance with the analytic conclusions in this paper.

The vacuum preloading method combined PVDs has been widely used to improve the engineering properties of dredged marine clay. Comparing with the surcharge preloading method, the vacuum preloading method has no stability problems. However, it is found in the field test that the improved soil still has further settlement after unloading the vacuum pressure. To investigate the mechanism of the vacuum preloading process, series of triaxial tests were conducted to simulate the vacuum preloading process for dredged clay. The compressibility of the clay under different stress paths was measured. It is found in this paper that the deformation of dredged clay after unloading of the vacuum pressure includes two parts: one is the elastic deformation caused by the unloaded instant soil, and the other is the plastic shear deformation produced by the soil under the partial stress state. When partial stress keeps constant, the larger stress ratio induces larger deformation.

An analytical method is proposed in this paper to calculate the maximum embedded depth of a dragged Hall anchor when passing through rock berm and to thus define a minimum buried depth of pipelines in rock berm to prevent pipelines from being damaged by dragging anchors. The movement of a Hall anchor in rock berm is interpreted based on the equilibrium conditions for resisting and driving moments acting on the anchor. To verify the accuracy of the proposed analytical method, model tests were carried out by using three scaled Hall anchor models and dragging them through rock berm. The comparisons between the two studies show that the average value of their differences for the stable embedded depth of a Hall anchor in sand and in rock berm are only 1.7% and 2.7%, respectively. The good agreements indicate that the proposed method is accurate enough to calculate the minimum buried depth of pipeline in rock berm during pipeline design. Accepted version

With the development of submarine oil and gas resources, research on global pipeline buckling caused by high temperature and pressure in the process of oil and gas transport is becoming an important issue. Owing to human factors or to uneven seabed conditions, local bends are produced in the process of pipeline manufacturing and laying. Those bends are called geometric initial imperfections whose deformation forms can be divided into a single arch symmetric deformation and a double arch antisymmetric deformation. In this paper, the energy method is introduced to calculate the analytical solution of pipeline lateral buckling with a single arch initial imperfection, and the cause of snap buckling phenomenon is discussed. A FEA model is established, and the difference between the analytical solution and numerical solution is also presented.

When geosynthetic tubes are placed on soft ground, the ground settlement can be large enough to influence the design and performance of geosynthetic tubes. The existing method to model the ground deformation for the design of geosynthetic tubes is to use the Winkler model. In this paper, an analytical solution is proposed to calculate the impermeable geosynthetic tubes resting on deformable foundation soil. The proposed analytical method adopts the 1-D consolidation relationship (the e − log p curve) to describe approximately the stress–strain behavior of the soil. The vertical surcharge pressure distribution within the soil mass is calculated using the Boussinesq solution. The numerical analyses are also carried out using FLAC to evaluate the accuracy of the proposed method. The results from the analytical method agree well with those from the numerical analysis.

Submarine pipelines are the primary component of an offshore oil transportation system. Under operating conditions, a pipeline is subjected to high temperatures and pressures to improve oil mobility. As a result, additional stress accumulates in pipeline sections, which causes global buckling. For an exposed deep-water pipeline, lateral buckling is the major form of this global buckling. Large lateral displacement causes a very high bending moment which may lead to a local buckling failure in the pipe cross-section. This paper proposes a lateral global buckling failure envelope for deep-water HT/HP pipelines using a numerical simulation analysis. It analyzes the factors influencing the envelope, including the thickness t, diameter D, soil resistance coefficient μ, calculating length Lf, imperfection length L and imperfection amplitude V. Equations to calculate the failure envelope are established to make future post-buckling pipeline failure assessment more convenient. The results show that (1) the limit pressure difference pmax (the failure pressure difference for a post-buckling pipeline when it suffers no difference in temperature) is usually below the burst pressure difference pb (which is the largest pressure difference a pipeline can bear and is determined from the strength and sectional dimensions of the pipeline) and is approximately 0.62–0.75 times the value of pb and (2) thickness t has little influence on the normalized envelopes, but affects pmax. The diameter D, soil resistance coefficient μ, and calculating length Lf influence the maximum failure temperature difference Tmax (the failure temperature difference for a pipeline suffering no pressure difference). The diameter D also significantly affects the form of the normalized envelope.

Yan, S.; Jia, Z.; Liu, W., and Zhang, X., 2015. Case history: Large diameter and supper long pile-running under self-weight in Liwan Oil Field. The pile-running frequently occurs because of the great weights of both the pile and the hammer. Thepile-running mechanism is super long and large-diameter piles are commonly used for constructing ocean platform foundations. Pile-running discussed in the current study through case analyses. A predicting procedure is developed based on the limit equilibrium of pile weight and soil resistance. Reduction factors for soil strength used to calculate the skin friction are also suggested in the current study. The Berezantsev approach is recommended for calculating the end bearing resistance to the running pile, which can improve the precision of the estimation compared with that of commonly used methods. The suggested method is used to predict the pile-running case in a practical project, and the calculated results are consistent with the driving records.

Liu, R.; Guo, S., and Yan, S., 2015. Study on the lateral soil resistance acting on the buried pipeline. Pipe-soil interaction is a crucial problem in the global buckling design of submarine pipeline under thermal stresses. Soil resistance acting on the pipelines determines the deformation shape and amplitude as the global buckling occurs. The fine sand and soft clay are distributed widely in the upper soil layer of Bohai Gulf. Therefore, these two kinds of soil were chosen as medium in the pipe-soil interaction laboratory tests. Three different diameter pipe segments were adopted to simulate the pipeline. The pipe segments lateral displacement and the corresponding suffered soil resistance were measured during the tests. The tests were carried out separately for the pipe segments with different diameters and different depth to diameter ratios. The test data showed that the lateral soil resistance acting on the pipe depends on the soil properties and pipe diameter and depth-to-diameter ratio. A n...

Yan, S.; Guo, B.; Sun, L., and Lei, Z., 2015. Theoretical analysis and model test of rock berm protection. Submarine pipelines are exposed to the risk of damage from dragged anchors of ships that lose control during severe storms. Rock armor berms are commonly employed to protect the pipelines by deflecting the dragged anchors. Theoretical work and model tests are implemented in this study to understand the protection mechanism provided by rock berms. A limit equilibrium method is proposed to address the interaction between the dragged Hall anchor and the developed soil wedges. The total drag force on the anchor is obtained as the sum of the drag force on the anchor crown and that on the anchor fluke. Model tests are performed with three model anchors with different linear scales. The drag forces measured in the model tests agree well with those calculated by the theoretical approach. Results show that when the anchor is dragged into the rock armor berm from the soil foundation, an unbalanced mom...

Sun, L.; Huo, Z., and Yan, S., 2015. Design of hydraulic telescopic grouted clamp device and analysis of its closing collision. Bucket foundations represent a new type of foundation for offshore wind turbines. They are large cylindrical structures that open at the base and close at the top. Compared with traditional offshore pile foundations, bucket foundations exhibit the advantages of shorter installation periods, lower material costs, and more rational bearing patterns. The complex interaction between a bucket foundation and soil defines the loading mechanism of this foundation type, and its failure mode has yet to be clearly defined. A simplified limit equilibrium method or other oil-platform design codes (e.g., API) is typically adopted to design bucket foundations. However, the simplified limit equilibrium method does not reflect the working characteristics of the soil in the foundation, thus resulting in a relatively considerable deviation. A numerical analysis is performed to simulate the...

Sun, L.; Jia, T.; Zhuo, R.; Yan, S., and Guo, B., 2015. Numerical solutions for consolidation of under-consolidated dredger fill under vacuum preloading. This paper presents the numerical solutions for consolidation of the under-consolidated dredger fills. By a field test the delivery rules of negative vacuum pressure in the PVDs are studied, the relationship between permeability and effective stress of the high water content soil is also studied by an experiment, then a plain strain FEM program was coded for the dredger fills consolidation analysis in which the nonlinearity constitutive relation Duncan-Chang's model is applied into the Biot's consolidation theory, and the nonlinear permeability law is also employed in the Biot's consolidation theory. As an under-consolidated soil, the self-weight that is not consolidated before the enforcement and negative vacuum pressure is included as the consolidation load in this paper. At last, two project cases were calculated using this program, the resul...

The scale of fluctuation is one of the vital parameters for the application of random field theory to the reliability analysis of geotechnical engineering. In the present study, the fluctuation function method and weighted curve fitting method were presented to make the calculation more simple and accurate. The vertical scales of fluctuation of typical layers of Tianjin Port were calculated based on a number of engineering geotechnical investigation data, which can be guidance to other projects in this area. Meanwhile, the influences of sample interval and type of soil index on the scale of fluctuation were analyzed, according to which, the principle of determining the scale of fluctuation when the sample interval changes was defined. It can be obtained that the scale of fluctuation is the basic attribute reflecting spatial variability of soil, therefore, the scales of fluctuation calculated according to different soil indexes should be basically the same. The non-correlation distance method was improved, and the principle of determining the variance reduction function was also discussed.

A membraneless vacuum preloading method is proposed in this paper for soft soil improvement. The method offers several advantages over the conventional vacuum preloading in which membrane is used to create the airtight condition and sand blanket layer to distribute vacuum. To assess the effectiveness of the proposed method, a pilot test was conducted at a land reclamation site in Tianjin, China. The ground settlement and the pore water pressure (PWP) at different elevations in soil were measured. After vacuum preloading, the average water content of the soft soils reduced by approximately 12% and the undrained shear strength increased twofold. The average degree of consolidation at the end of the vacuum preloading achieved 85.1% based on the settlement data and 84.5% based on the PWP data. The pilot test data have shown that the proposed method exhibits similar efficiencies to the conventional vacuum preloading method. Accepted version

A simplified method for the design of impermeable geosynthetic tubes inflated using liquid is proposed in this paper. Adopting a computer program for an existing theoretical model, relationships between pumping pressure and geometric parameters for geosynthetic tubes can be established. A set of simplified dimensionless design equations are then derived using the Chapman–Richard curve fitting method. The validity of this simplified method was verified using other established methods and laboratory model tests. The proposed simplified method can thus be used for routine or preliminary design. Accepted version

A review of the recent developments in soft soil improvement through consolidation or preloading is presented in this paper. The topics covered range from fundamental analysis to methods of implementation. Various methods and processes related to vertical drains, vacuum preloading or combined vacuum and fill surcharge, and dynamic consolidation with enhanced drainage or vacuum are compared and discussed. Factors affecting the design and analyses for the methods discussed are also elaborated.

It is very common that a submarine pipeline has to pass through a ship mooring area near a harbor zone in the Bohai Bay, China. The risk assessment of accidental events induced by the potential anchoring ships is carried out, which will lead to external interference with the pipeline. A procedure to calculate the probability for the anchoring activity in the ship mooring area to damage the underlying pipeline is proposed. The adopted methodology is based on the recommendations suggested by the DNV Codes. The same philosophy is also applied to estimate the damage probability that is concerned with sinking ships.

Subsea pipelines buckle globally because of their movement relative to surrounding soil. Global buckling is often triggered by high operational temperature of the oil in pipelines, initial imperfections in the pipeline, and/or a combination of both. Pipeline global buckling is a failure mode that must be considered in the design and in-service assessment of submarine pipelines because it can jeopardize the structural integrity of the pipelines. Global buckling is increasingly difficult to control as temperature and pressure increase. Therefore, location prediction and buckling control are critical to pipeline design. Finite element analysis (FEA) is often used to analyze the behavior of pipelines subject to extreme pressures and temperatures. Four numerical simulation methods based on the finite element method (FEM) program ABAQUS, i.e., the 2D implicit, 2D explicit, 3D implicit, and 3D explicit methods, are used to simulate pipeline global buckling under different temperatures. The analysis results of the four typical methods were then compared with classical analytical solutions. The comparison indicates that the results obtained using the 2D implicit and 2D explicit methods are similar and the results obtained using the 2D implicit method are closer to those obtained using traditional analytical solutions. The analysis shows that the results of the 3D implicit and 3D explicit methods are similar, but the results obtained using the 3D methods are significantly different from those obtained using the analytical solution. A novel method to introduce initial pipeline imperfections into the FEA model in global buckling analysis is also presented in this paper.

Subsea pipelines are commonly employed in the offshore oil and gas industry to transport high-pressure and high-temperature (HPHT) hydrocarbons. The phenomenon of pipeline walking is a topic that has drawn a great deal of attention, and is related to the on-bottom stability of the pipeline, such as directional accumulation with respect to axial movement, which can threaten the security of the entire pipeline system. An accurate assessment of pipeline walking is therefore necessary for offshore pipeline design. This paper reports a comprehensive suite of numerical analyses investigating the performance of pipeline walking, with a focus on the effect of increasing axial soil resistance on walking rates. Three walking-driven modes (steel catenary riser (SCR) tension, downslope, and thermal transient) are considered, covering a wide range of influential parameters. The variation in walking rate with respect to the effect of increased soil friction is well reflected in the development of the effective axial force (EAF) profile. A method based on the previous analytical solution is proposed for predicting the accumulated walking rates throughout the entire service life, where the concept of equivalent soil friction is adopted.

Since the early seventies, pipelines have become one of the main means of transporting oil and gas offshore in many parts of the world. In-service hydrocarbons must be transported at high temperature and pressure to ease the flow and prevent solidification of the wax fraction. The buckling of subsea pipelines occur because of the introduction of axial compressive forces caused by the constrained expansions set up by thermal and internal pressure actions. Such compressive forces can lead to either lateral buckling in the plane of the seabed or buckling in a vertical plane. To ensure minimal interference with other marine activities, the pipelines can be buried in a trench. In such cases, the lateral soil restraint exceeds the vertical uplift restraint created by the pipe’s submerged weight. Therefore, vertical buckling is of particular interest with respect to entrenched subsea pipelines. The possibility of upheaval buckling in subsea pipelines has been appreciated in the pipeline industry for over...

A new application of geosynthetic tube for offshore dike construction is geosynthetic mattress in which its horizontal dimension is much greater than the vertical one. This makes it more stable than the sausage shaped geosynthetic tube. However, the existing analytical or numerical methods developed for the sausage shaped geosynthetic tube may not be applicable directly to geosynthetic mattresses as the solution may not converge. In this paper, a closed-form solution was derived for a direct determination of the cross-section of geosynthetic mattress. Laboratory large scale model tests were also carried out to verify the proposed analytical method. The cross-sections of the geosynthetic mattresses obtained from the closed-form solutions agreed well with the data obtained from model tests. The tensile forces measured from the model tests were rather scattered but showed a consistent thread as those from the closed-form solutions.

Under high thermal stress, buckling of submarine pipelines may occur to release the stresses accumulated in the walls of the pipelines. The likelihood that buckling occurs is largely determined by the capability of the soil to resist pipeline movements. Therefore, it is very important to determine the soil resistance in upheaval buckling when designing submarine pipelines. A series of large-scale model tests were carried out with soft clay chosen as the supporting medium in view of Bohai Sea geotechnical conditions. Pipe segments with diameters of 30, 50, and 80 mm were used. The pipes were buried in different depth-to-diameter ratios between 1 and 9. The uplift, lateral, and axial resistances were recorded during the tests. The test results have shown that the soil resistance depends on the pipe’s diameter and cover depth. According to the uplift test results, the load-displacement relationships with smaller depth of cover differ greatly from those with larger depth of cover. The results of the ...

In-service hydrocarbons must be transported at high temperature and high pressure to ease the flow and prevent the solidification of the wax fraction. The pipeline containing hot oil will expand longitudinally due to the rise in temperature. If such expansion is resisted, for example by frictional effects over a kilometer or so of pipeline, compressive axial stress will be built up in the pipe-wall. The compressive forces are often so large that they induce vertical buckling of buried pipelines, which can jeopardize the structural integrity of the pipeline. A typical initial imperfection named continuous support mode of submarine pipeline was studied. Based on this type of initial imperfection, the analytical solution of vertical thermal buckling was introduced and an elastic-plasticity finite element analysis (FEA) was developed. Both the analytical and the finite element methodology were applied to analyze a practice in Bohai Gulf, China. The analyzing results show that upheaval buckling is most likely to build up from the initial imperfection of the pipeline and the buckling temperature depends on the amplitude of initial imperfection. With the same amplitude of initial imperfection, the triggering temperature difference of upheaval buckling increases with covered depth of the pipeline, the soil strength and the friction between the pipeline and subsoil.