Caisson Anchors Research Articles

Seismic performance of shared suction caisson anchors installed in liquefiable sand for floating offshore wind turbines

Shared anchors will reduce the levelized cost of energy for floating offshore wind (FOW) arrays currently under active development in various seismically active regions. This study investigates the seismic performance of a shared suction caisson anchor installed in liquefiable sand for a FOW array using a nonlinear dynamic 3D finite element analysis. The advanced UBC3D-PLM constitutive model was calibrated against cyclic element tests then validated with centrifuge experimental data. Strong ground motions with orthogonal horizontal and vertical components were applied to the model with varying frequency content, peak ground acceleration (PGA), and orientation. The model captured the stress–strain response of the soil and evolution of excess pore pressure to the onset of liquefaction, which occurred after application of a PGA of 0.2 g and above. Despite significant liquefaction caused by a PGA of 0.8 g, displacement of the anchor remained within around 10% of anchor diameter. The results suggest some resilience to pull-out during seismic loading in liquefiable soils, even with vertical tensile loading on the anchor developed by the inverse catenary. Further investigation could be carried out under specific site conditions and serviceability criteria to optimize the size of the caisson and reduce costs while ensuring anchor performance during seismic loading.

Inclined loading capacity of caisson anchor in South China Sea carbonate sand considering the seabed soil loss

Seabed soil losses, e.g., seabed trench and general scour, greatly affect the bearing capacity and failure mode of caisson anchors. In this paper, a series of centrifuge model tests have been conducted to investigate the soil loss influence on the anchor capacity in carbonate sand. The semi-model anchor was adopted, and the anchor movement and soil displacement were captured through the particle image velocimetry (PIV) analyses. It is found that the soil loss changes the anchor failure mode, and the anchor capacity decreases accordingly. For the intact seabed, the caisson anchor firstly rotates backward and then forward. With the increase of trench depth, the anchor rotation mode changes from backward to forward. Two types of soil regions are found during the test, i.e., the shear zone and wedge-type region. When the anchor vertical movement is dominant, the shear zone will form near the anchor wall. In other cases, the wedge-type region will be more apparent. Based on the centrifuge test data, the influence of seabed soil loss on the anchor failure mode is assessed, and a modified framework is proposed for predicting the inclined loading capacity of caisson anchor considering seabed soil loss.

Insight into the Behavior of a Caisson Anchor under Cyclic Loading in Calcareous Silt

This paper provides insight into the behavior of a stiffened caisson anchor under inclined cyclic loading in calcareous silt. A series of tests was conducted in a beam centrifuge. A monotonic test was first performed, quantifying the pure monotonic capacity, and then four cyclic loading tests varying the mean load, amplitude, and number of cycles. Cyclic soil characterization T-bar tests and caisson tests were linked. Undrained cyclic T-bar tests led to generate excess pore pressure, resulting in degradation of soil strength and stiffness. For partially drained cyclic caisson tests, the excess pore pressure generated during initial undrained monotonic loading experienced partial dissipation. Healing due to consolidation outweighed the damage due to initial pore pressure generation. Postcyclic monotonic capacity was found to be up to 35% higher compared with the pure monotonic capacity unless the anchor failed during cyclic loading. Measured rotation indicated the evolution of anchor failure mechanism. Caisson capacity under inclined loading was presented as a failure envelope, with the effect of cyclic loading accentuated. The contribution of the soil–chain interaction on the caisson capacity was minimal. No trenching was apparent on the soil surface, and no gap was formed around the anchor in the considered centrifuge testing conditions.

Integration of wind and tidal turbines using spar buoy floating foundations

<abstract> <p>Floating platforms are complex structures used in deep water and high wind speeds. However, a methodology should be defined to have a stable offshore structure and not fail dynamically in severe environmental conditions. This paper aims to provide a method for estimating failure load or ultimate load on the anchors of floating systems in integrating wind and tidal turbines in New Zealand. Using either wind or tidal turbines in areas with harsh water currents is not cost-effective. Also, tidal energy, as a predictable source of energy, can be an alternative for wind energy when cut-in speed is not enough to generate wind power. The most expensive component after the turbine is the foundation. Using the same foundation for wind and tidal turbines may reduce the cost of electricity. Different environment scenarios as load cases have been set up to test the proposed system's performance, capacity and efficiency. Available tidal records from the national institute of Water and Atmospheric Research (NIWA) have been used to find the region suitable for offshore energy generation and to conduct simulation model runs. Based on the scenarios, Terawhiti in Cook Strait with 110 m water height was found as the optimized site. It can be seen that the proposed floating hybrid system is stable in the presence of severe environmental conditions of wind and wave loadings in Cook Strait and gives a procedure for sizing suction caisson anchors.</p> </abstract>

Failure envelope of suction caisson anchors subjected to combined loadings in sand

This paper reports the behaviour of caisson anchors under vertical-horizontal (V-H) loadings in medium dense to dense sand. The three dimensional finite element (3D-FE) analyses are carried out using a modified Mohr-Coulomb (MMC) soil model to capture the stress dependent hardening – softening behaviour of sands. The results are validated against centrifuge test data prior to undertaking a detailed parametric study, exploring the relevant range of parameters in terms of caisson aspect ratio, padeye location, mooring angle at the padeye and sand relative density. It is found that the caisson anchor failure is governed by the sand relative density, mooring angle and the normalised padeye position. The padeye position not only affects the anchor capacity, but also influences the rotation angle of the caisson at failure. An optimal padeye position inducing minimal rotation at caisson failure is found to lie within the range of 0.6-0.7L, where L is the caisson length. To assess capacity under inclined loading, normalised failure envelopes are presented in the V-H space. A design framework is established to estimate the optimal padeye location and caisson capacity for sands with strain hardening – strain softening characteristics.

Cyclic response of a stiffened suction caisson anchor in calcareous silt

The offshore industry attempts to extend the application of suction caisson anchors in calcareous seabed sediments. However, very little data exists on the performance of caisson in calcareous soils. The aim of this paper is to provide insight into the behaviour of a stiffened caisson anchor under cyclic operational loading in lightly overconsolidated calcareous silt. A series of model tests were carried out in a beam centrifuge, varying the mudline load inclination at 0°, 20°, 40°. In each cyclic loading test, three episodes of loading were applied, varying mean load (average load in proportion to the corresponding monotonic capacity) as 30%, 50% and 70%, while the amplitude (defined as the maximum variation from the average load) and number of cycles for each episode were kept constant at 20% and 50 cycles, respectively, and then loaded monotonically. The accumulated displacement and negative excess pore pressure at the trailing side of the skirt tip increased, particularly from the second episode of cyclic loading, with decreasing mudline load inclination angle. The ratio of post-cyclic monotonic capacity to pure monotonic capacity reduced as 1.20, 1.01, and 0.90, with changes of excess pore pressure and effective stress dominating the behaviour for high load inclination angle and cyclic degradation for low load inclination angle. Comparing the results to the performance of other foundations and anchors, it was found that post-cyclic monotonic capacity depends heavily on the embedment depth of the object along with load/mooring inclination, cyclic load amplitude, and seabed material.

Multiline Ring Anchor system for floating offshore wind turbines

The trends in the offshore wind industry have shown floating offshore wind turbines (FOWTs) technology and installation in deep waters due to stronger, more consistent wind resources and mitigation of aesthetic issues. Nevertheless, the high capital cost of the support systems remains a primary obstacle. Thus, this study presents a novel, integrated, and networked multiline ring anchor (MRA) system for FOWTs that can provide significant capital cost savings. Other attractive features of the anchor include versatility in regard to the range of soil conditions in which it can be deployed, relatively high geotechnical efficiency, capability for precise positioning and a deep embedment which provides for robust performance under unintended torsional loading. A limited comparative study of the MRA to a suction caisson anchor in soft clay shows the MRA to be clearly more efficient under horizontal loading, but only a marginally better under vertical loading. Incorporating keying flaps in the design has promise for eliminating the latter performance issue.

Load capacity of caisson anchors exposed to seabed trenching

Floating structures are often secured in position with a taut mooring system and suction caissons. Large seabed trenches have been observed adjacent to some suction caisson anchors with taut-line mooring systems. The trenches may jeopardise the geotechnical capacity of the caissons and in turn the stationkeeping of the floating structures. Finite-element method is employed to examine the geotechnical capacity of suction caissons in a trenching seabed. The results show that the reduction in the geotechnical capacity becomes more significant with increasing trench width due to the loss of soil support and a change in failure mechanism as the caisson rotates into the trench. For a given trench width, the reduction in capacity becomes more significant as the load inclination angle to the horizontal decreases. However, the shape of the normalised failure envelopes for combined vertical and horizontal load is insensitive to trench width. A strategy to design for inevitable trenching by moving the padeye shallower to reduce the depth of trench formation is not straightforward. The gain from a shallower trench may often be outweighed by the reduction in capacity from rotation of the caisson at failure for loading angles typical of taut moorings.

Behavior of soil heave inside stiffened caissons being installed in clay

The length of suction caisson anchors has been increasing to support increasing dimensions and weight of floating facilities, which necessitates employing horizontal ring stiffeners at intervals along the inner wall of the thin skirt of caissons to ensure structural integrity. The addition of these stiffeners has created significant uncertainties regarding soil flow mechanisms, in particular soil heave inside the caisson, which may reduce the caisson final penetration depth and influence the process of installation due to the need to avoid inside soil suction in the pumping equipment. This paper reports results of large-deformation finite element (LDFE) analyses investigating soil heave inside stiffened caissons during installation in nonhomogeneous clay deposits, with the corresponding evolution of soil flow mechanisms and penetration resistance profiles reported by Zhou et al. in 2016. The LDFE analyses have simulated continuous penetration of stiffened caissons from the seabed surface. A detailed parametric study has been undertaken, exploring the relevant range of soil strength nonhomogeneity and normalized strength, stiffened caisson geometry, soil effective unit weight, and caisson roughness. Of particular interest is the influence of stiffeners on soil heave and potential penetration refusal. The results have been validated against previously published centrifuge test data in terms of soil heave and penetration resistance profile, with good agreement obtained. It is shown that the soil normalized strength at the mudline and its nonhomogeneity, caisson diameter relative to the sum of skirt thickness and stiffener width, and caisson penetration depth have significant influence on the inner soil heave and its profile across the caisson radius. An expression, based on the LDFE results is proposed to predict the maximum inner soil heave during installation of stiffened caissons in the field.

Simplified load estimation and sizing of suction anchors for spar buoy type floating offshore wind turbines

Floating offshore wind turbines are complex dynamic structures, and detailed analysis of their loads require coupled aero-servo-hydro-elasto-dynamic simulations. However, time domain approach used for such analysis is slow, computationally expensive and requires detailed data about the wind turbine. Therefore, simplified approaches are necessary for feasibility studies, front-end engineering design (FEED) and the early phases of detailed design. This paper aims to provide a methodology with which the designer of the anchors can easily and quickly assess the expected ultimate loads on the foundations. For this purpose, a combination of a quasi-static wind load analysis and Morison's equation for wave load estimation using Airy waves is employed. Dynamic amplification is also considered and design load cases are established for ultimate limit state (ULS) design. A simple procedure is also presented for sizing suction caisson anchors. All steps are demonstrated through an example problem and the Hywind case study is considered for such purpose.

Fuzzy modeling of holding capacity of offshore suction caisson anchors

SummaryA fuzzy algorithm, the Takagi–Sugeno model, is implemented to develop a fuzzy inference system for predicting the holding capacity of suction caisson foundations for offshore platforms. The premise parameters of the fuzzy model are optimized by using a subtractive clustering algorithm. The consequent parameters are optimally determined via a weighted least square estimation. The input variables used for training the fuzzy model include the aspect ratio of the caisson, the undrained shear strength of the clay, and the angle that the chain force forms with the horizontal. The output of the proposed fuzzy model is the capacity of the suction caisson anchor. To demonstrate the effectiveness of the fuzzy modeling framework, the results of extensive finite element analyses are investigated. Comparisons of the trained fuzzy model with the data demonstrate that the proposed modeling framework is an effective method to estimate the holding capacity of offshore suction caisson systems. Moreover, the performance of the fuzzy model is robust against higher levels of input data uncertainties. Copyright © 2017 John Wiley & Sons, Ltd.

Installation and Monotonic Pullout of a Suction Caisson Anchor in Calcareous Silt

AbstractThis paper reports results from a series of model tests undertaken to provide insight into the behavior of a stiffened caisson anchor during installation and monotonic pullout in lightly overconsolidated calcareous silt. The tests were carried out in a beam centrifuge, varying the installation process (jacked in or jacked in followed by suction-assisted installation), and the consolidation period before anchor pullout. The mudline load inclination was also varied to encompass various mooring configurations. The centrifuge data were used to calibrate analytical installation and vertical pullout capacity models, based on conventional bearing and frictional resistance factors but with strain rate-dependent and strain-softened undrained shear strength for the soil. Piezocone-based direct design approaches were also proposed, deriving end-bearing and frictional resistance from cone tip resistance and sleeve friction, respectively. Holding capacity of suction caisson under inclined loading was presented...

Finite Element Analysis of the Holding Capacity of Shallow Suction Caisson Anchors

The foundations of offshore structures have been actively studied to provide design guides for oil platforms. Recently, techniques developed for the oil industry have found new applications in the design of the foundations for offshore wind turbines. The analytical solutions for the holding capacity of suction anchors have been developed based on limit equilibrium and limit analysis theorems and have often been compared with the results of sophisticated approaches such as finite element analysis. However, some solutions show discrepancies with finite element analysis results under specific conditions, (e.g., certain values of the ratio of length to diameter). The aim of this study was to provide a guide for estimating the location of the optimal loading point and the holding capacity of shallow suction caisson anchors, under an inclined load for various profiles of undrained shear strength. A set of graphs and formulae proposed may be used for preliminary determination of the anchor capacity and verification of other solutions. The effect of soil disturbance is also discussed.

단일벽 및 이중벽 모형앵커를 이용한 석션케이슨 앵커의 거동분석

풍력발전은 신재생 에너지 중 가장 유망하다고 판단되는 에너지 중 하나이다. 심해에서는 부유식 풍력발전기가 가장 경제적인 것으로 평가받고 있으며, 이에 따라 부유식 해상풍력발전기에 대한 연구가 세계적으로 활발하게 진행되고 있다. 해상풍력발전기의 개발에는 부유체를 해양지반에 고정하는 앵커의 개발이 필수적이며, 석션케이슨 앵커는 현재까지 가장 널리 쓰이는 해양앵커 중 하나이다. 본 논문은 모형실험을 통해 카올리나이트 점토지반에 관입된 석션케이슨 앵커의 수직인발력을 분석하고, 수직인발력으로부터 점토와 앵커 사이의 마찰계수와 앵커의 선단 역지지계수를 산정한다. 산정된 마찰계수와 역지지계수는 기존의 문헌들의 결과와는 유사한 경향을 보이나, 현재 설계에 적용되는 값들보다는 다소 작은 값을 나타낸다. 따라서, 본 모형실험장비는 해양앵커의 정성적 거동을 해석하는데 더 적절한 것으로 판단된다. 향후, 본 논문의 결과를 바탕으로 반복하중 하 석션케이슨 앵커의 거동 계측을 통해 해양환경에서 앵커의 영구변형을 산정하기 위한 연구를 이어 진행할 예정이다. Wind power presently reflects a very promising source of renewable energy. The optimal type of foundations for offshore wind turbines, in very deep water, is the floating type; the research on floating wind turbines is very active worldwide. It is essential to develop anchoring system for floating structures, and the suction caisson anchor is one of most widerly used in deep oceans. This paper addresses the anchor holding capacity of suction caisson anchors under vertical pull-out, and friction and reverse end bearing coefficients based on mode tests. The values of friction and reverse end bearing coefficients put in similar ranges with literature values, but are still lower than those suggested in design guides. It is, therefore, suggested the current model tests may predict qualitative trends of suction caission anchors' behaviors. As subsequential works, based on current results, it is planned to conduct cyclic tests to estimate the permanent behavior of anchors under oceanic environments.

Holding capacity of suction caisson anchors embedded in cohesive soils based on finite element analysis

SUMMARYThe foundations of offshore structures have been intensively studied to provide design guidelines for oil platforms. Recently, techniques developed for the oil industry have found new applications in the design of foundations for offshore wind turbines. There are simplified analytical solutions for the holding capacity of suction anchors, which are based on postulated failure mechanisms and are often compared with the results of finite element analyses. The merit of the results of a finite element analysis over the simplified solutions is that the finite element analysis satisfies all the basic equations in mechanics and does not require the assumption of reasonable failure mechanisms. The aim of this study is, based on the results of extensive finite element analyses, to provide a set of formulae for the estimation of the holding capacities of optimally loaded suction caisson anchors embedded in cohesive soils with a linear strength distribution. The bearing capacity factor and the depth of the optimal loading point are also estimated and reported. Copyright © 2014 John Wiley & Sons, Ltd.

Uplift Capacity of Suction Caisson in Clay Using Artificial Intelligence Techniques

Caisson anchor is the most common structure for the offshore structures. In this study artificial intelligence techniques artificial neural network (ANN), genetic programming (GP), support vector machine (SVM), and relevance vector machine (RVM) have been used to predict the uplift capacity (Q) of suction caisson in clay. The model inputs included the L/d (L is the embedded length of the caisson and d is the diameter of caisson), undrained shear strength of soil at the depth of the caisson tip (Su), D/L (D is the depth of the load application point from the soil surface), inclined angle(θ) and load rate parameter (Tk). A comparative analysis is made with the results of the above listed artificial intelligence techniques based on different statistical performance criteria like correlation coefficient (R), root mean square error (RMSE), Nash-Sutcliff coefficient of efficiency (E), log normal distribution of ratio of predicted to observed load capacity. Model equations based on the above techniques are discussed and a sensitivity analysis is made to identify the important parameters contributing to the uplift load capacity of caisson.

균일한 점성토 지반에 시공된 석션 케이슨 앵커의 인발력

심해 부유식 구조물에 대한 연구는 주로 석유 시추용 플랫폼에 대한 적용을 위해 발전해 왔으며, 최근에는 신재생에너지에 대한 사회적 요구로 인해 해상풍력발전기에 대한 적용도 그 대상이 되고 있다. 심해에서 경제적인 것으로 알려져 있는 부유식 해상풍력발전기 계류체의 장력은 앵커에 전달이 되며, 앵커의 자중과 부유체에 가해지는 바람, 파도, 해류 등의 영향으로 앵커에는 수직, 수평성분을 모두 가진 경사진 하중이 작용하게 된다. 본 연구에서는 유한요소해석을 통하여 앵커의 형상 및 앵커의 인발각에 따른 석션 케이슨 앵커의 인발력을 산정한다. 균일한 지반에 관입된 앵커의 경우에, 앵커의 인발력과 관입깊이의 관계는 선형에 가까운 형태를 보였으며, 파괴포락선의 형상은 앵커의 관입깊이가 커질수록 더 각이 진 타원의 형태로 변화한다. 최적 하중재하점은 앵커 관입깊이의 0.57배에 해당하는 위치에 존재하는 것을 확인할 수 있었다. Floating offshore structures have been actively studied mainly for the application of oil productions in the past. Presently, this asset from oil industry finds its way for the application to floating wind turbine systems. Tensile forces in mooring systems pull the anchoring systems at the connection, and due to the combination of anchor weight, wind, wave, and current, the anchor may experience inclined forces. The main scope of this paper is to analyze the ultimate holding capacity of suction caisson anchors according to the anchor dimensions and anchor translation angle based on finite element analysis. The holding capacity is linearly proportional to the embedment depth of anchor when installed in uniform soil. The shape of failure envelope becomes more angular as the anchor gets slenderer. The depth of optimal loading point is found to be at 0.57 times the length.

유한요소해석을 통한 얕은 석션 케이슨 앵커의 인발력

해양구조물 기초에 대한 연구는 석유 시추를 위한 기술과 발전을 같이해 왔으며, 여기에서 얻어진 기술은 최근 사회적 요구로 인해 급성장하고 있는 분야인 해상풍력발전기 기초에 대한 설계 및 연구에도 적용되고 있다. 현재 상용화된 해상풍력발전기는 주로 수심이 얕은 해양을 대상으로 하지만, 나아가 수심이 깊은 곳의 풍력발전기에 대한 연구도 계속되고 있다. 수심이 깊어지면 발전기 플랫폼의 형태가 부유식이 되는 것이 경제적인 것으로 알려져 있다. 부유식 발전기는 해상지반에 앵커에 의해 지지되며, 앵커의 자중 및 부유식 구조물에 가해지는 바람, 해류 등의 영향으로 앵커에는 수직, 수평의 조합하중이 전달된다. 본 연구에서는 유한요소해석을 통해서 길이-직경비가 2인 석션앵커의 파괴포락선을 산정하고, 기존의 앵커 인발력 산정식과 비교한다. 또한 앵커의 관입시 발생되는 지반의 교란정도에 따른 인발력 감소와 최적의 하중 재하 위치를 검토한다. Foundations of offshore structures have been actively studied to provide design guides for oil platforms in the past. Presently, this asset from oil industry now finds its application in the design of the foundations for offshore wind turbines. The analytical solutions for the hold capacity of suction anchors have been developed based on limit equilibrium and limit analysis theorems and often compared to the results of novel approaches such as finite element (FE) analysis. However, some solutions show discrepancy with the FE analysis results under specific conditions, say under certain value of the ratio of length to diameter. The main scope of this paper is to analyze the failure envelope of suction caisson anchors, especially with an aspect ratio of 2 based on FE analysis. The results will provide a basis to enhance the existing plasticity solutions for anchor hold capacity. The effect of soil disturbance and location of optimal loading point are also investigated.

Effects of Electrode Configuration on Electrokinetic Stabilization for Caisson Anchors in Calcareous Sand

The study is on the electrokinetic strengthening of caisson anchors embedded in offshore calcareous sand. The effects of electrode configuration on the effectiveness of electrokinetic treatment are investigated based on electric field analysis and are verified by results from a series of large scale laboratory tests on caisson models of 200 mm diam and 400 mm height, embedded in calcareous sand submerged under seawater. The electrokinetic treatment generates cementation of soil solids as well as bonding between soil and caisson shafts, which leads to increases in the side resistance and overall pullout resistance. The effectiveness of electrokinetic treatment is directly related to the electric field intensity. A linear relationship is observed between the increase in the side resistance and energy consumption. The study shows that the effectiveness of electrokinetic treatments can be maximized by the optimization of the electric field distribution through the electrode configuration.

Simplified limit solutions for the capacity of suction anchors under undrained conditions

Upper bound plastic limit analyses (PLA) can provide a useful framework for estimating the load capacity of suction caisson anchors in purely cohesive soils. Since arbitrary assumptions regarding the soil stress state are not required in the PLA formulation, it may be used with greater consistency compared to other simplified approaches such as limit equilibrium methods. While PLA methods do not attempt to include all of the complexities of anchor behavior, they can provide a relatively simple framework for visualizing anchor kinematics leading to an understanding of the relative importance of various parameters on suction anchor load capacity. The most rigorous PLA formulations involve postulating a three-dimensional anchor-soil failure mechanism and deriving expressions for internal energy dissipation throughout the mechanism. This approach can involve extensive numerical integrations and a relatively complex scheme for optimizing the failure mechanism to obtain a least upper bound collapse load. Considerable simplification is possible if the problem is formulated in terms of ultimate unit resistances (lateral, axial, and their interaction) that can be exerted by the soil on the caisson. In this case, the caisson failure mechanism can be characterized in terms of one or two optimization variables. Simple expressions for the ultimate unit resistances acting on the caisson can be obtained from several sources including rigorous PLA solutions, finite element techniques, or experimental measurements. General expressions are possible by limiting consideration to common, idealized strength profiles such as uniform or constant gradient. Such simplified formulations are particularly valuable for providing an analysis tool accessible to practicing engineers. Suction caisson anchors can be subjected to a variety of load orientations including nearly vertical uplift forces imposed by the vertical tendons of tension leg platforms, horizontal loads imposed by catenary mooring systems, and inclined loads imposed by taut moorings. Recently, PLA methods have been applied to the analysis of suction caissons subjected to this range of loading conditions. This paper reviews the formulation of these analyses and summarizes the most significant findings.