Self-Weight Penetration Characteristics of the Suction Foundation with Different Diameters in Sand

During the self-weight penetration process of the suction foundation on the dense sand seabed, due to the shallow penetration depth, the excess seepage seawater from the outside to the inside of the foundation may cause the negative pressure penetration process failure. Increasing the self-weight penetration depth has become an important problem for the safe construction of the suction foundation. The new suction anchor foundation has been proposed, and the self-weight penetration characteristics of the traditional suction foundation and the new suction anchor foundation are studied and compared through laboratory experiments and analysis. For the above two foundation types, by considering five foundation diameters and two bottom shapes, 20 models are tested with the same penetration energy. The effects of different foundation diameters on the penetration depth, the soil plug characteristics, and the surrounding sand layer are studied. The results show that the penetration depth of the new suction foundation is smaller than that of the traditional suction foundation. With the same penetration energy, the penetration depth of the suction foundation becomes shallower as the diameter increases. The smaller the diameter of the suction foundation, the more likely it is to be fully plugged, and the smaller the height of the soil plug will be. In the stage of self-weight penetration, the impact cavity appears around the foundation, which may affect the stability of the suction foundation.

     

筒型基础负压沉贯后筒内水膜形成及筒内灌浆效果的试验设计

筒型基础负压沉贯就位后,筒内表面土的沉降会使土层上部产生一层水膜。严重的水膜现象会影响到筒基平台的正常使用,为此专门设计本项试验来模拟筒内水膜形成的机理,并且提出了向筒内灌浆的方法解决水膜问题的方案。该试验通过配土、设计模型筒基、负压沉贯和筒内灌浆等操作过程,设置土槽、安装孔压传感器等实验手段,从土体破坏的角度,寻求负压沉贯过程中沉贯负压、沉贯阻力及孔隙水压力等因素对水膜形成的影响,试图探询水膜形成的规律,并且通过不同泥浆的灌浆试验来寻找有效解决水膜现象的方法。     

筒型基础安装过程中筒内土塞生成的可视化模拟计算

筒型基础安装过程中，筒内的负压作用会导致土塞现象的出现。以试验模型为基础建立可变形离散元计算模型，通过VC＋＋编制的程序SPSAⅡ对筒型基础负压沉贯中土塞的生成过程进行可视化模拟计算。考虑渗流力、筒壁内外摩擦阻力和筒内负压吸力作为土塞产生的主要外荷载，分别模拟渤海地区3种典型土（黏土、粉土和砂土）的情况。其中，粉土情况下。程序的收敛速度最快，黏土情况下，土塞的计算高度最接近试验测量值，数值计算结果表明该计算方法作为筒型基础施工前筒内土塞高度的预测方法是可行的。     

Study on Unloading Creep Characteristics of the Soil and Application of the Stress-Dependent Creep Model in Suction Caisson Foundation

As the anchoring foundation of the tension leg platform(TLP), suction caisson foundation is subjected to the longterm vertical pullout loads. But there are few studies on the mechanism of the unloading creep of soft clay and longterm uplift bearing capacity of suction caisson foundations. To address this problem, unloading creep tests of soft clay were carried out to analyze the strain development with time under different confining pressures. The test results show that the creep curve rapidly d...     

Pullout behavior of suction anchors in soft marine clays

In the field of ocean engineering, a beginning has been made in the use of large‐sized suction anchors for safe anchoring of large compliant structures. Suction anchors derive most of their uplift resistance from passive suction developed during the pullout movement. This article describes a set of laboratory tests on model suction anchors of three different embedment ratios to estimate the pullout behavior of suction anchors in soft clays typical of Indian marine clays. Tests were conducted on model anchors installed in soil beds prepared at four different consistencies in a test tank. This study shows the influence of soil consistency and embedment ratio (L/D) on the pullout behavior of suction anchors and on the variation of suction pressure at the top of the soil plug. The test results reveal that the behavior of suction anchors is much better than the behavior of open‐ended anchors from the considerations of both capacity and deformation. The consistent development of suction inside the anchor top confirms the plug formation and significant breakout resistance in the form of suction‐induced reversed end bearing. The results are further analyzed in terms of suction breakout factors. Further, the effect of burial depth of suction anchor on pullout behavior is shown.     

Numerical Simulation of Installation Process and Uplift Resistance for An Integrated Suction Foundation in Deep Ocean

A concept design, named integrated suction foundation, is proposed for a tension leg platform (TLP) in deep ocean. The most important improvement in comparing with the traditional one is that a pressure-resistant storage module is designed. It utilizes the high hydrostatic pressure in deep ocean to drive water into the module to generate negative pressure for bucket suction. This work aims to further approve the feasibility of the concept design in the aspect of penetration installation and the uplift force in-place. Seepage is generated during suction penetration, and can have both positive and negative effects on penetration process. To study the effect of seepage on the penetration process of the integrated suction foundation, finite element analysis (FEA) is carried out in this work. In particular, an improved methodology to calculate the penetration resistance is proposed for the integrated suction foundation with respect to the reduction factor of penetration resistance. The maximum allowable negative pressure during suction penetration is calculated with the critical hydraulic gradient method through FEA. The simulation results of the penetration process show that the integrated suction foundation can be installed safely. Moreover, the uplift resistance of the integrated suction foundation is calculated and the feasibility of the integrated suction foundation working on-site is verified. In all, the analysis in this work further approves the feasibility of the integrated suction foundation for TLPs in deep ocean applications.     

A new look at the phenomenon of offshore pile plugging

Abstract

Open‐pipe piles are widely used for offshore structures. During the initial stage of installation, soil enters the pile at a rate equal to the pile penetration. As penetration continues, the inner soil cylinder may develop sufficient frictional resistance to prevent further soil intrusion, causing the pile to become plugged. The open‐ended pile then assumes the penetration characteristics of a closed‐ended pile. The mode of pile penetration significantly alters the soil‐pile interaction during and after installation. This affects the ultimate static bearing capacity (mainly in granular materials), the time‐dependent pile capacity (in clays), and the dynamic behavior and analysis of the piles.

Following a summary demonstrating the effects of pile plugging, a review of the common view of offshore pile plugging is undertaken. The interpretation of plugging by referring to the average plug length has led to the erroneous conclusion that in most piles significant plugging action does not occur.

Establishment of an analogy between soil samplers and open‐ended piles enabled correct identification of plugging by referring to the incremental changes in plug length. Examination of case histories of plugging of offshore piles revealed that beyond a certain penetration depth‐to‐diameter ratio, most piles are plugged.     

砂土中吸力式导管架基础沉贯试验研究

吸力式导管架基础具备高承载力、高施工效率、高环境友好度、低造价等特点,使用其作为海上风电底部支撑结构有利于产业的规模化发展,实现上述愿景的前提在于使筒型基础顺利安装以满足设计要求。基于此,在不同初始施加泵压下,对吸力式导管架的沉贯效率特征值、基础内外围渗流变化、筒裙端部土压力特点进行探究。通过抱桩器使吸力式导管架在吸力安装过程中仅具备竖向自由度,从而进行纯沉贯试验。结果表明：初始泵压2 kPa时沉贯效率特征值最高;沉贯过程中基础外围渗流水压小于基础内围渗流水压;筒裙外侧所受土压要大于筒壁内侧所受土压力。     

Response Analysis of Tension-Based Tension Leg Platform Under Irregular Waves

Tension Leg Platform (TLP) is a hybrid structure used as oil drilling and production facility within water depths of 1200 m. The extension of this TLP concept to deeper waters is a challenge and warrants for some innovative design concepts. In this paper, a relatively new concept of TLP which is christened as Tension-Based Tension Leg Platform (TBTLP) and patented by Srinivasan (1998) has been chosen for study. Response analysis of TLP with one tension base under irregular waves for three different sea states has been performed using hydrodynamic tool ANSYS? AQWA?. Results are reported in terms of RAOs, response spectrums for surge, heave and pitch degrees of freedom from which spectral statistics have been obtained. The statistics of TBTLP have been compared with TLPs (without tension base) for two different water depths to highlight the features of the new concept. The effect of viscous damping and loading effects on the RAOs are also investigated.     

Response behaviour of triangular tension leg platforms under impact loading

Excellent station keeping characteristics and relative insensitivity with increasing water depth make triangular tension leg platforms (TLPs) a proven concept in deep water oil exploration. TLPs are often subjected to less probable forces which arise due to collision of ships, icebergs or any other huge sea creature. Dynamic analysis of two triangular TLP models at water depths 1200 and 527.8 m is performed under regular waves along with impulse load acting at an angle of 45 degrees at the TLP column. Hydrodynamic forces on these TLPs are evaluated using modified Morison equation, based on water particle kinematics arrived at using Stokes’ fifth order wave theory. Based on numerical studies conducted, it is seen that impulse loading acting on corner column of TLP significantly affect its response while that acting on pontoons dose not affect TLPs behaviour.     

Response analysis of tension-based tension leg platform under irregular waves

Tension Leg Platform(TLP) is a hybrid structure used as oil drilling and production facility within water depths of 1200 m. The extension of this TLP concept to deeper waters is a challenge and warrants for some innovative design concepts. In this paper, a relatively new concept of TLP which is christened as Tension-Based Tension Leg Platform(TBTLP) and patented by Srinivasan(1998) has been chosen for study. Response analysis of TLP with one tension base under irregular waves for three different sea states has been performed using hydrodynamic tool ANSYS? AQWA?. Results are reported in terms of RAOs, response spectrums for surge, heave and pitch degrees of freedom from which spectral statistics have been obtained. The statistics of TBTLP have been compared with TLPs(without tension base) for two different water depths to highlight the features of the new concept. The effect of viscous damping and loading effects on the RAOs are also investigated.     

Pit bearing capacity effect on status of soil plug during pile driving in ocean engineering

Foundation piles of the offshore oil platforms in the Bohai Bay are usually longer than 100 m with a diameter larger than 2 m.Driving such long and large-sized piles into the ground is a difficult task.It needs a comprehensive consider ation of the pile dimensions,soil properties and the hammer energy.Thoughtful drivability analysis has to be performed in the design stage.It has been shown that judging whether the soil column inside the pile is fully plugged,which makes the pile behave as close-ended,strongly influences the accuracy of drivability analysis.Engineering practice repeatedly indicates that the current methods widely used for soil plug judgment often give incorrect results,leading the designers to make a wrong decision.It has been found that this problem is caused by the ignorance of the bearing capacity provided by the soil surrounding the pile.Based on the Terzaghi’s bearing capacity calculation method for deep foundation,a new approach for judging soil plug status is put forward,in which the surcharge effect has been considered and the dynamic effect coefficient is included.This approach has been applied to some practical engineering projects successfully,which may give more reasonable results than the currently used method does.     

Analytical and experimental studies on installation of a suction caisson with tampered tip in clay

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.     

Large deformation finite element analysis of the installation of suction caisson in clay

The suction caisson (or called suction anchor) which is considered as a relatively new type of foundation of offshore structures, has been extensively studied and applied for offshore wind turbines and oil platforms. The installation of the suction caisson is of great importance in the design and construction because it can bring about several issues and further influence the performance of holding capacity in safety service. In this paper, large deformation finite element (FE) analyses are performed to model the installation of suction caisson (SC) by suction and jacking in normally consolidated clay. The penetration of the suction caisson is modeled using an axisymmetric FE approach with the help of the Arbitrary Lagrangian–Eulerian (ALE) formulation which can satisfactorily solve the large deformation problem. The undrained shear strength of the clay and elastic modulus are varied with depth of soil through the subroutine VUFIELD. The numerical results allow quantification of the penetration resistance and its dependence on the installation method. The centrifuge test and theoretical solution are used for the FE model validation. After the validation, the penetration resistance, the soil plug heave, and the caisson wall friction have been examined through the FE model. Based on the numerical results, it is shown that the ALE technique can simulate the entire suction caisson penetration without mesh distortion problem. The installation method can play an important role on the penetration resistance, namely, the suction installation reduces the penetration resistance significantly compared to the purely jacked installation. With a further study on the suction case, it is found that as the final applied suction pressure increases, the soil plug heave increases, while the penetration resistance reduces with increase of the final suction pressure. The effect of the friction of internal caisson walls has been also investigated and a conclusion is drawn that internal wall friction has a significant contribution to the penetration resistance and it can be implicitly represented by varying coefficient of internal wall friction. As for the penetration resistance, both jacked and suction installation have great dependency on the internal wall friction.     

基于能耗最优的TLP张力腿水下无线监测组网方法研究

张力腿作为张力腿平台(tension leg platform,简称TLP)的重要组成部分,服役水深大,动力学行为复杂多样。为了保障张力腿的结构安全,开展张力腿的水下全方位监测十分必要。文中以某TLP为依托,构建一套能耗最优的张力腿监测系统。首先,利用附加质量法对张力腿进行湿模态分析,结合有效独立法与模态置信度准则确定张力腿监测节点的具体布设方案;其次,以能耗最低为优化目标,基于遗传算法建立了水下无线组网监测方案,同时讨论了监测节点存在失效时监测数据传输路径改变方案;最终,得到一套能耗最优的TLP张力腿水下无线组网监测方法,以降低水下监测系统维护成本。     

In Memoriam

Abstract

The engineering properties of deep continental margin sediments were determined on a worldwide basis. Deep Sea Drilling Project (DSDP) core data and material were utilized from over 900 cores obtained from 89 sites, primarily on the continental margins. Cores were recovered from penetrations to 200 m in water depths averaging 3000 m. Supplementary laboratory testing on selected cores was directed toward determining index properties and shear strength properties of the sediments. The study included a literature review of deep‐sea soil properties, the results of which are to be used by DSDP to evaluate foundation conditions for reentry cones with long casing strings. The results will also be used for a feasibility study of an ultradeepwater marine riser and well‐control system. The marine sediments examined can be divided into three main types: clay, calcareous ooze, and siliceous ooze. Sediment distribution consisted of 48% calcareous ooze, 43% clay, 6% siliceous ooze, and 3% volcanic ash. Because of the sample disturbance inherent in the deepwater coring operation, emphasis was placed on analyzing sediment properties not significantly affected by changes in in situ stresses and structure caused by sampling. Averages and ranges in values of water content and unit weight are presented for the three main sediment types. Plasticity and strength characteristics are discussed in detail and the elastic and compression properties are outlined. The geotechnical properties of deep continental margin soils are summarized.     

Application of Tension Leg Platform Combined with Other Systems in the Development of China Deepwater Oil Fields

1 .IntroductionAfter enteringthe twenty-first century,offshore oil field development in China is rapidly expand-inginto deepwater (Zeng,2005 ;Liu and Pan,2002) .The conventional fixed platformmodels cannotsatisfythe requirement of newfield development .Floatingsystems ,suchastensionleg platform(TLP)or Spar ,will be neededfor oil field development .FPSOhas been usedfor offshore oil field development in China since 1980’s (Xu and Li ,2002 ;Chen and Qian,1999 ; Chen,2003) .Since early 19…     

Evaluation of Inner Soil Pressure Acting on Opened Bottom Cylindrical Structure

1 .IntroductionAnopenedbottomcylinderisathinshellstructureplacedonarubblebaseorembeddedinasoilfoundation .Itiswidelyusedintheconstructionofwharves ,breakwatersandotheroceanprojects.Thestructureisusuallyfilledwithgranularmaterialstoensurethestabilityofthestructurealongwiththeweightofthecylinder.Hence ,itisconsideredasagravitytypeofstructure .However,thesettlementofthecylinderisdifferentfromthatofthetraditionalgravitytypeofstructureandsodoesthedistribu tionofinnersoilpressureovertheopenedbottom…     

Finite element modeling of the tensile capacity of suction caissons in cohesionless soil

When suction caissons are used as foundations for jacket structures, the caissons are exposed to significant vertical loads. If the self-weight of the structure is relatively low, a large horizontal load may lead to tension on the foundation on the incoming side. For steady loads, such as the wind load during production, the soil response will be drained. This paper presents the results from a series of finite element analysis (FEA) on suction caissons in cohesionless soil. The analyses are performed on suction caissons with different dimensions and different soil conditions. For normalization, dimensional analyses of the calculated results are performed to create dimensionless groups. The dimensionless groups are used to establish a relation between the normalized tensile capacity and the interface strength. This relation is used to establish two formulations of the drained tensile capacity for suction caissons in cohesionless soils. One for associated plasticity and one for non-associated plasticity with the dilatation angle equal zero.     

Calculation of Hydro-Dynamic Stability of the Soil Inside Bucket in the Process of Bucket Foundation Penetration

—The offshore platform with bucket foundations is a new type of offshore platform that distin-guishes from traditional template platforms by replacing driven piles with bucket foundations.The suctionpenentration of bucket foundation is a complicated hydro-dynamic process.The key of this process is theseepage field caused by the difference of pressure applied on purpose inside and outside the bucket.The ap-pearance and developement of seepage field has a decisive influence on the suction penetration process.Inthis study,the finite element analysis method is applied to the dynamic simulation of the seepage field ofsuction penetration of bucket foundation.A criterion is suggested to distinguish the hydro-dynamic stabili-ty of the soil inside the bucket according to the critical hydraulic gradient method.The reliability of themodel and its applicability to engineering practice have been proved through comparison between the re-sults of model test and finite element calculation.