拖曳锚在海床中运动特性的大变形有限元分析

拖曳锚由于其承载性能和深水中便于安装被广泛应用于海洋工程系泊系统中,如:适用于悬链式系泊系统的传统拖曳锚和适用于绷紧式系泊系统的法向承力锚。拖曳锚安装过程中涉及诸多运动特性:锚板运动方向、系缆点处拖曳力和拖曳角及运动轨迹。基于大变形有限元分析技术耦合的欧拉-拉格朗日法,并引入缆绳方程,建立起锚-缆绳-海床土耦合作用的有限元分析模型;模拟了拖曳锚在均质和线性强度黏土中的嵌入安装过程,研究了锚板运动方向、系缆点处拖曳力和拖曳角及运动轨迹等运动特性;通过与已有的有限元分析方法及理论方法进行对比,验证了该分析模型的有效性;与已有的有限元分析方法相比,提出的分析模型有效地提高了计算效率。     

土层抗浮锚杆承载力关键影响因素现场试验研究

工程实践及理论研究均表明,锚土界面特性和锚杆的几何形状是影响抗浮锚杆承载力的2个关键因素。采用不同的施工工艺进行现场试验施工,得到了具有不同锚土界面特性和锚杆几何形状的抗浮锚杆。通过现场锚杆抗拔试验得到锚杆的应力应变关系及极限抗拔承载力。研究结果表明,改善锚土界面特性和采用变截面的锚固体可显著提高锚杆的抗拔承载力。同时,提出了1种经济高效的抗浮锚杆施工工法。     

Spar平台吸力式基础极限承载特性数值分析

以国外某深海Spar平台工程为背景,针对其所采用的细长型吸力式基础的抗拔承载特性进行三维有限元数值分析.分析中充分考虑土体强度、加载位置和加载角度对吸力式基础极限抗拔承载力的影响,本构模型中钢筒基础采用弹塑性模型.分析结果表明,吸力式基础的极限抗拨承载力随着土体强度的增大而增大,倾斜加载时在基础插入土体部分中点左右加载可取得最大的极限承载力,极限抗拔承载力还随着加载角度的增大而增大.吸力式基础存在倾斜加载时桶基础与桶内外土体的共同塑性屈服破坏和垂直加载时桶外土体的局部剪切破坏等两种不同的破坏模式.     

海洋工程固结锚的结构与特性初探

介绍了一种拥有自主知识产权的海工固结锚技术,描述了该新型锚的内部结构和工作原理;并在室内对不同设计参数和使用工况的锚开展了垂向上拔试验,初探了其抗拔能力。初步试验表明:该新型锚具有超高的抓重比;其次生固结体显著增加了锚体的剪切面积,从而大大提升了锚体的抗拔力;锚体结构上宜具有多个喷管且喷管管径较粗,安装过程中对固化剂的推进速度应较缓。该新型锚应具有良好的应用前景,但需对此进一步深入研究,以满足其设计和工程应用的要求。     

深水半潜式平台张拉锚泊系统失效分析

针对深水半潜式平台在极端工况下张拉锚泊系统失效情况,基于板锚失效上拔过程中位移与承载力的关系曲线,提出在AQWA软件中用非线性锚缆模拟板锚走锚过程的方法,对深水半潜式平台张拉锚泊系统在百年一遇工况下断锚、走锚情况的平台响应、锚缆拉锚力及最大缆张力进行分析。研究表明:断锚时各锚链拉力瞬变,平台迅速发生大距离偏移;走锚时随着板锚的拔出各锚链上的拉力逐渐增大,平台位置逐渐偏移,最后达到平衡状态。单缆断锚时,迎浪向5号锚缆拉力最大;单缆断锚且单锚走锚时,3、6号锚缆拉锚力最大。失效缆超过一根时,平台偏移和单缆最大缆张力均超出规范要求,同时其它锚缆最大拉锚力极有可能出现超过板锚极限承载力的情况,最终造成整个锚泊系统破坏。     

新型铰接式重力锚的设计及其性能研究

重力锚锚固是一种常见的锚固形式。为了能够提供足够的水平承载力,传统型式的重力锚普遍比较笨重,在上拔回收时会产生较大的竖向吸附力,不利于重复利用。针对此问题,设计了一种新型铰接式重力锚,并阐述了其铺设与回收方案。其次采用有限元方法对其整体强度进行了校核,结果均符合规范。最后基于模型试验,对铰接式重力锚在黏土中的运动过程进行了研究,进而确定了其在黏土中的水平承载力和回收时的上拔力。结果表明：相较于传统重力锚,新型铰接式重力锚在确保水平承载性能的基础上,能够大幅减小上拔力,从而有效地降低铺设和回收作业的难度,且可适应多种海底土质,但该锚型仅适用于悬链线式系泊系统。相关研究结果可为实际工程中铰接式重力锚的设计提供参考。     

新型伞状锚拉拔破坏机制与承载力数值分析

锚泊基础的承载性能直接影响着海洋浮式结构物的稳定性,因而研究新型有效的锚泊基础已成为海洋工程结构设计中的关键问题之一。文中提出了一种基于海洋软土液化特性的伞状锚,充分利用桩端土体增强抗拔承载能力。应用二维颗粒流分析程序,对该新型伞状锚的安装、抗拔承载能力进行了数值模拟,并与普通锚桩进行比较分析,验证其有效性。针对伞状锚与普通锚桩在拉拔过程中的土体破坏机制,从细观角度分析了其抗拔承载能力的提高机制。研究结果表明,对于相同抗拔锚泊设计竖向承载要求,伞状锚所需材料可大为减少,安装难度明显减低,是值得推广应用的新型锚泊基础形式。     

在潮汐流作用下25万吨级油轮系缆力的模型试验研究

通过２５万吨级油轮泊系福建炼油厂码头的室内系缆力模型试验,研究不同潮汐和流速下油轮泊系时的系缆力,讨论了一些主要因素的影响和计算方法,指出首缆力、尾缆力和倒缆力是最主要的系缆力,流速是对系缆力影响最大的因素,提出了系缆方式和安全措施。     

载人潜水器圆柱形耐压壳体的优化设计研究

采用分枝定界法和序列二次规划方法,对载人潜水器圆柱形耐压壳体的重量最小化进行了研究.设计变量是壳板的厚度、肋骨的型号、间距和数量,讨论了下潜深度、材料几何参数对重量以及其它特征量的影响.算例计算表明,下潜深度越大,屈服极限越高的材料重量减轻越明显.对于大深度而言,选用高屈服极限材料,可以使得材料能够充分利用.文中还对结构重量占排水量比例随深度变化的情况进行了研究.     

带横隔板局部开孔沉箱在斜向波作用下的受力研究

基于线性势流假定,对斜向波作用下带横隔板局部开孔沉箱防波堤的水平波浪力进行了理论研究。给出了开孔沉箱法向水平力和横隔板受力的理论计算方法,在极限情况下波浪力的计算结果与文献中的已有结果一致。利用数值算例分析了开孔沉箱总水平力的主要影响因素。开孔沉箱法向总水平力的减小主要集中于结构上半部分波浪影响范围以内。增加单个开孔沉箱的长度有利于减小结构所受总水平波浪力。当波浪入射角或沉箱前开孔墙孔隙影响系数幅值较大时,开孔沉箱横隔板上总水平力的最大值要超过相应的沉箱法向总水平力,此时要注意横隔板的强度问题。     

Numerical study of effect of fluke inclination on behavior of drag anchor in clay with linearly increasing shear strength under unidirectional and combined loading

Drag anchor is a widely used anchor type in offshore engineering for the mooring system. The prediction of the anchor trajectory installation and the final position is important for anchor selection in design. The existing method using yield envelope method for trajectory prediction ignored the shallow anchor behavior but applied the deep yield envelope from a deeply embedded horizontal fluke in uniform clay for the whole drag-in installation process. However, the anchor fluke embedment depth and inclination angle change continually during installation in clay with linearly increasing shear strength soil profile in practice. Studies on the effect of fluke inclination angle on the anchor behavior in clay with such non-uniform soil profile under unidirectional and combined loading are important and necessary for the improvement of the yield envelope method to ensure a reasonable prediction. The current 2D finite element studies investigate the anchor behavior for inclined fluke in clay with linearly increasing shear strength under unidirectional vertical, horizontal and rotational loadings first. Then the effects of the fluke inclination angle, soil non-homogeneity and embedment depth ratio on the shallow yield envelopes are investigated. It is found that the effect of fluke inclination angle on the vertical capacity factors for anchor in clay with non-uniform and uniform soil profile is largely different. The resultant large impact on the yield envelopes shown here illustrates the importance of considering the fluke inclination angle and soil non-homogeneity in the prediction of anchor trajectory using yield envelope method.     

Experimental investigation of the keying process of OMNI-Max anchor

ABSTRACT

The OMNI-Max anchors are newly developed dynamically installed anchors for deep water mooring systems. After installation, the anchor is keyed to a new orientation and position by tensing the attached mooring chain, which is known as the “keying process”. This study conducted 1g model tests to study the trajectories and capacity developments of OMNI-Max anchors in homogeneous and lightly overconsolidated (LOC) clays. A testing arrangement was designed to simulate the anchor keying process with a constant pullout angle at the mudline. A half model anchor which could move against the box glass was used to determine the anchor trajectory in the soil. The effects of padeye offset angle, uplift angle at the mudline, anchor fluke thickness, anchor initial embedment depth, and soil strength on the anchor trajectory and capacity were systematically investigated. Moreover, the critical uplift angle at the padeye and the anchor critical initial embedment depth were discussed. The results indicate that the anchor can dive both in homogeneous and LOC clays under certain conditions. A padeye offset angle of 24–30° is recommended for the OMNI-Max anchor to maintain high capacity and diving trend simultaneously. Besides, the anchor diving trend can be improved with small uplift angles at the mudline and with thick anchor flukes. A critical initial embedment depth of 1.3 times the anchor length is recommended to preclude the anchor from being pulled out.     

Influence of Kinematic Analysis Parameters of Drag Anchor Trajectory Prediction Using Yield Envelope Method

Drag anchor is widely applied in offshore engineering for offshore mooring systems. The prediction of the invisible trajectory during its drag-in installation is challenging for anchor design in determining the anchor final position for ensuring sufficient holding capacity. The yield envelope method based on deep anchor failure for kinematic analysis was proposed as a promising trajectory prediction method for drag anchor. However, there is a lack of analysis on the effects of the parameters applied in the kinematic analysis. The current work studies the effects of the yield envelope parameters, anchor line bearing capacity factor and the anchor/soil interface friction. It is found that the accuracy of the yield envelope parameters has large impact on the prediction results based on deep yield envelopes.Analyses of cases with smooth fluke predict deeper embedment depth than that from analyses of cases with rough fluke. The decrease of the capacity factor results in the increase of the anchor embedment depth, the anchor line load,the anchor chain angle and the stable value of the normalized horizontal load component for the same drag length,while the stable value of the normalized vertical load component decreases when the capacity factor decreases. This illustrates the importance in applying reasonable parameters and improving the method for more reliable prediction of the anchor trajectory.     

An efficient approach to incorporate anchor line effects into the coupled Eulerian–Lagrangian analysis of comprehensive anchor behaviors

With the application of innovative anchor concepts and advanced technologies in deepwater moorings, anchor behaviors in the seabed are becoming more complicated and significantly affected by the anchor line. Based on the coupled Eulerian–Lagrangian (CEL) method, a numerical approach incorporating anchor line effects is developed to investigate comprehensive anchor behaviors in the soil, including penetration of drag anchors, keying of suction embedded plate anchors and diving of gravity installed anchors. Compared to the method directly incorporating the anchor line into the CEL analysis, the proposed method is computationally efficient. To examine the robustness and accuracy of the proposed method, numerical probe tests and then comparative studies are carried out. It is found that the penetration (or diving) and keying behaviors of anchors can be well simulated. A parametric study is also undertaken to quantify the effects of various factors on the behavior of OMNI-Max anchors, whose mechanisms are not yet fully understood. The maximum embedment loss of OMNI-Max anchors during keying is not influenced by the initial anchor embedment depth, whereas significantly increases with increasing drag angle at the embedment point. With decreasing initial anchor embedment depth or increasing soil strength gradient, drag angle at the embedment point and diameter of the anchor line, the behavior of OMNI-Max anchors could change from diving to pullout, which is undesirable in offshore engineering practice. If the drag angle increases over a certain limit, the anchor will fail similar to a suction anchor.     

半潜式支持平台系泊系统的设计方法及应用

系泊系统是半潜式支持平台抵抗恶劣海洋环境作用、限制平台偏移、实现海上定位的重要设备，辐射状多点系泊是常用的布置方式。针对该平台系泊系统开展了设计方法和分析流程研究，阐述了系泊系统配置设计，包括系泊缆的数量、抛出长度、单根系泊缆的刚度、直径、破断负荷、定位锚的型式、最大抓力等；系泊系统的布置设计，包括系泊缆之间的水平夹角、系泊绞车、导缆器、定位锚的位置等；同时归纳风载荷、流载荷以及波浪慢漂载荷的常用估算方法；总结适用于该类平台系泊系统设计的规范要求。将上述方法和流程应用于某型半潜式支持平台系泊系统的开发和设计，采用系泊定位分析程序MIMOSA对该系统的定位能力进行分析，研究了系泊缆形状、夹角等参数随张力的变化特征，同时系泊缆按照船级社的规范要求进行衡准，反复调整和优化系泊系统的配置和布置方式，直至系泊系统满足要求，最终设计出较合适的系泊系统。相关方法、流程和结论为实际工程项目提供重要的设计思路。     

Numerical implementation of the installation/mooring line and application to analyzing comprehensive anchor behaviors

With the application of innovative anchor concepts and advanced technologies in deepwater moorings, anchor behaviors in the seabed are becoming more complicated and pose a great challenge to the analytical methods. In the present work, a large deformation finite element (FE) analysis employing the coupled Eulerian–Lagrangian technique is performed to simulate the installation/mooring line, and then is applied to analyzing comprehensive anchor behaviors in the seabed. By connecting cylindrical units with each other using connector elements, the installation/mooring line is constructed. With the constructed installation/mooring line, FE simulations are carried out to investigate comprehensive anchor behaviors in the seabed, including long-distance penetration of drag anchors, keying of suction embedded plate anchors and non-catastrophic behavior of gravity-installed anchors. Through comparative studies, the accuracy of the proposed method is well examined. A parametric study is also undertaken to quantify the effects of the frictional coefficient, initial embedment depth, and soil weight on the profile of the embedded anchor line and the shackle load. The present work demonstrates that the proposed FE model, which incorporates the installation/mooring line and the anchor, is effective in analyzing the comprehensive anchor behaviors in the seabed.     

Toward a quick evaluation of the performance of gravity installed anchors in clay: Penetration and keying

Gravity installed anchors (GIAs) are the most recent generation of anchoring solutions to moor floating facilities for deepwater oil and gas developments. Challenges associated with GIAs include predicting the initial embedment depth and evaluating the keying performance of the anchor. The former involves high soil strain rate due to large anchor penetration velocity, while the later influences the subsequent behavior and pullout capacity of the anchor. With the coupled Eulerian–Lagrangian method, three-dimensional large deformation finite element models are established to investigate the penetration and keying of GIAs in non-homogeneous clay. In the penetration model, a modified Tresca soil model is adopted to allow the effects of soil strain rate and strain softening, and user-defined hydrodynamic drag force and frictional resistance are introduced via concentrated forces. In the keying model, the anchor line effects are incorporated through a chain equation, and the keying, diving and pulling out behaviors of the anchor can all be replicated. Parametric studies are undertaken at first to quantify the effects of various factors on the performance of GIAs, especially on the penetration and keying behaviors. Based on the results of parametric studies, fitted formulae are proposed to give a quick evaluation of the anchor embedment depth after the installation, and the shackle horizontal displacement, shackle embedment loss and anchor inclination at the end of the keying. Comparative studies are also performed to verify the effectiveness of the fitted formulae.     

An Improved Method of Predicting Drag Anchor Trajectory Based on the Finite Element Analyses of Holding Capacity

Drag anchor is one of the most commonly used anchorage foundation types. The prediction of embedded trajectory in the process of drag anchor installation is of great importance to the safety design of mooring system. In this paper, the ultimate anchor holding capacity in the seabed soil is calculated through the established finite element model, and then the embedded motion trajectory is predicted applying the incremental calculation method. Firstly, the drag anchor initial embedded depth and inclination angle are assumed, which are regarded as the start embedded point. Secondly, in each incremental step, the incremental displacement of drag anchor is added along the parallel direction of anchor plate, so the displacement increment of drag anchor in the horizontal and vertical directions can be calculated. Thirdly, the finite element model of anchor is established considering the seabed soil and anchor interaction, and the ultimate drag anchor holding capacity at new position can be obtained. Fourthly, the angle between inverse catenary mooring line and horizontal plane at the attachment point at this increment step can be calculated through the inverse catenary equation. Finally, the incremental step is ended until the angle of drag anchor and seabed soil is zero as the ultimate embedded state condition, thus, the whole embedded trajectory of drag anchor is obtained. Meanwhile, the influences of initial parameter changes on the embedded trajectory are considered. Based on the proposed method, the prediction of drag anchor trajectory and the holding capacity of mooring position system can be provided.     

Behavior of drag anchor in clay with linearly increasing shear strength under unidirectional and combined loading

Drag anchor is a widely used economical anchor option for offshore floating structures. The anchor behavior under unidirectional loading and combined loading is important for anchor selection. The anchor behavior under combined loading, characterized by the yield envelope, can also be used for the prediction of anchor installation, which is still an issue in anchor design. However, most existing studies on anchor capacity are for plate anchors which focused only on the anchor pullout capacity in soil with uniform shear strength. The behavior of drag anchor under unidirectional and combined loading in soil with linearly increasing shear strength profile is seldom investigated. The current 2D finite element studies investigate the anchor behavior for a horizontal anchor fluke in clay with linearly increasing shear strength under unidirectional vertical, horizontal and rotational loadings first. Then based on the results of anchor unidirectional loading behavior, the yield envelopes for anchor under combined loading for both shallow and deep embedded flukes are studied. The effect of anchor embedment depth, soil non- homogeneity, soil overburden pressure and the soil/anchor interface breakaway conditions are studied to provide insight for drag anchor design.