Evaluation of fibre rope properties for offshore mooring

An examination has been conducted of fibre rope mechanical properties measured in laboratory tests on materials, constructions and terminations intended for deepwater mooring applications. This data, combined with field experience, provide a better understanding of rope behaviour and enhances the knowledge required to engineer fibre ropes for these demanding applications.This has also led to improved laboratory test techniques for measurement of mechanical properties relevant to mooring system response analyses required to meet regulatory requirements. Preliminary evaluation of the data also indicates that a post-test evaluation method to determine retained properties of insert strops removed from mooring lines may be possible.     

Undrained strength behaviour of a cemented marine clay under monotonic and cyclic loading

The paper presents the results of a series of monotonic and cyclic triaxial shear tests carried out to study the influence of the strain effect and load cycles on the undrained shear strength of a cemented marine clay from the East coast of India. The undrained shear strength of Indian coastal marine clay has been established from a detailed shear testing carried out in three phases. Undisturbed soil samples taken out from the seabed were used in the test. In the first part, a comprehensive monotonic shear testing has been carried out under both stress-controlled and strain rate-controlled conditions. In the second phase on identical soil specimen, undrained cyclic shear tests were carried out at various cyclic stress ratios (CSR) and these stress levels are chosen in such a way so that no failure occurred during testing. In the final phase post cyclic monotonic shear testing was conducted to qualitatively evaluate the damage caused by cyclic loading. The monotonic shear test results bring out the influence of cementation that can be detected by the stress-controlled test. The cyclic stress results are analysed in terms of CSR. Further, the results are correlated in terms of stress path.     

A creep–rupture model of synthetic fiber ropes for deepwater moorings based on thermodynamics

The synthetic fiber ropes such as polyester, aramid and high modulus polyethylene (HMPE) are increasing applied to deepwater mooring systems for oil and gas exploitation. Due to that mooring ropes generally bear tensions for a long period, synthetic fiber ropes that are composed of the viscoelastic material would present creep behaviors and even the creep rupture, which is the failure mode of greatest concern especially for HMPE ropes and on which still less study can be found. A creep damage analysis of synthetic fiber ropes is of necessity to ensure the safe and economic operation of mooring systems. Therefore further investigation on the creep–rupture behavior is beneficial to fully establishing confidence in the viability of synthetic fiber ropes for deepwater moorings. In the present study, a creep–rupture model is proposed within the framework of thermodynamics to investigate the creep and damage behaviors of synthetic fiber ropes. Methods for identifying the model parameters are also proposed in detail, which apply to any component of fiber ropes such as the fiber, yarn, strand and rope. Experimental data of aramid yarns available from the literature are utilized to validate the constitutive model. Creep and creep–rupture tests of HMPE strands at different loading levels are specially performed to further examine the present model. The present work demonstrates that the proposed model can effectively describe the viscoelastic property and damage evolution of synthetic fiber ropes at different loading levels.     

Time-domain numerical simulation of ocean cable structures

This paper describes the numerical features of WHOI Cable, a computer program for analyzing the statics and dynamics of oceanographic cable structures. The governing equations include the effects of geometric and material nonlinearities, bending stiffness for seamless modeling of slack cables, and a model for the interaction of cable segments with the sea floor. The program uses the generalized-α time integration algorithm, adaptive time stepping, and adaptive spatial gridding to produce accurate, stable solutions for dynamic problems. The nonlinear solver uses adaptive relaxation to improve robustness for both static and dynamic problems. The program solves surface and subsurface single-point mooring problems, multi-leg and branched array systems, and towing and drifting problems. User specified forcing can include waves, currents, wind, and ship speed.     

Long-term response analysis of FPSO mooring systems

The design of mooring systems for floating production units usually considers extreme environmental conditions as a primary design parameter. However, in the case of FPSO (Floating, Production, Storage and Offloading) units, the worst response for the mooring system may be associated with other sea state conditions due to the fact that its extreme response may be associated with a resonant period instead of an extreme wave height. The best way to deal with this problem is by performing long-term analysis in order to obtain extreme response estimates. This procedure is computationally very demanding, since many short-term environmental conditions, and their associated stochastic nonlinear time domain numerical simulations of the mooring lines, are required to obtain such estimates. A simplified approach for the long-term analysis is the environmental contour-line design approach. In this paper a Monte Carlo-based integration procedure combined with an interpolation scheme to obtain the parameters of the short-term response distribution is employed to hasten the long-term analysis. Numerical simulations are carried out for an FPSO at three different locations considering a North Sea joint probability distribution for the environmental parameters. The long-term analysis results are compared against those obtained using extreme environmental conditions and environmental contour-line methodology. These results represent the characteristic load effect for the design of mooring systems of floating units using the reliability analysis for mooring line. The results show that the long-term results are usually more critical than those obtained with the other approaches and even different mooring lines can be identified as the critical ones.     

Reliability assessment of mooring lines for floating structures considering statistical parameter uncertainties

This work focuses on the effect of parameter uncertainty on the reliability index and on the predictive reliability of mooring lines for floating structures under loading from extreme sea-states. A first-order analytic formulation is developed which takes into consideration uncertain parameters in the statistical models of the maximum dynamic tension and the breaking resistance of the mooring lines. Expressions are derived for the mean and the variance of the reliability index in terms of the mean values and the covariance matrix of the uncertain parameters, and of the sensitivity of the reliability index to the uncertain parameters. The predictive reliability index is expressed in terms of the mean and the variance of the reliability index. The formulation is applied to case studies of catenary and taut-leg mooring lines of an offshore structure and the relative effects of the sources of statistical uncertainty are assessed. The case studies demonstrated the applicability and capability of the formulation to capture and represent the relative contributions of the different sources of uncertainty on the predictive reliability and predictive failure probability, as well as on the statistics of the uncertain reliability index.     

A methodology for assessing the reliability of taut and slack mooring systems against instability

In the present study, a methodology for reliability assessment of slack and taut mooring systems against instability has been presented. For this purpose, first, stability analysis of slack and taut mooring systems has been carried out and instable regions are obtained using procedure available in the literature. Having known the instable region(s), methodology for reliability assessment has been proposed which is based on Monte Carlo Simulation technique. After using the proposed methodology, probabilities of failure and reliability indices has been obtained for the above systems. Some parametric studies, such as, effect of lower and upper limits of instability and effect of frequency range of generations are also included to obtain the results of practical interest.     

Experimental investigation on the dynamic response of a moored wave energy device under regular sea waves

This paper attempts to provide a better understanding of the hydrodynamic behaviour of a floating multi-resonant oscillating column wave energy device which combines the concept of a floating breakwater and a wave energy device. Experiments were conducted on a 1:20 scale model of the floating wave energy device moored by six mooring lines to study the dynamics of the device under regular waves for various scopes. The effect of non-dimensionalized wave frequency parameter on the motion response and mooring force are reported and discussed in detail in this paper.     

水平荷载作用下裙式吸力基础承载性能研究

传统吸力基础是一个单桶结构,被广泛作为海洋平台、漂浮结构的基础,近年来也被推广到海上风电塔架。作为风电塔架基础,要充分提高其水平承载能力。为此,提出一种改进的基础形式—裙式吸力基础。采用Z_SOIL有限元软件,针对砂土地基,从水平单调加载和循环加载两个方面,对传统单桶吸力基础和裙式吸力基础进行了承载性能对比研究,得到了相应的荷载-位移曲线。研究结果表明,裙式吸力基础由于设置了"裙"结构,显著提高了其抵抗水平静载和循环水平动力荷载的能力,并能有效控制基础的水平位移,是值得推广应用的一种新型海洋工程基础形式。     

Coupled dynamic analysis of a mini TLP: Comparison with measurements

The dynamically coupled interaction between the hull of a floating platform and its risers and tendons plays an important role in the global motions of the platform and the tension loads in the tendons and risers. This is an especially critical design issue in the frequency ranges outside the wave frequencies of significant energy content. This study examines the importance of this coupled dynamic interaction and the effectiveness of different approaches for their prediction. A numerical code, named COUPLE, has been developed for computing the motions and tensions pertaining to a moored floating structure positioned and restrained by its mooring/tendon and riser systems. In this study the experimentally measured motions of a mini-TLP are compared with those computed using COUPLE and alternative predictions based upon quasi-static analysis. The comparisons confirm that COUPLE is able to predict the dynamic interaction between the hull and its tendon and riser systems while the related quasi-static analysis fails. The comparisons also show that wave loads on the mini-TLP can be accurately predicted using the Morison equation provided that the wavelength of incident waves is much longer than the diameters of the columns and pontoons and that the wave kinematics used are sufficiently accurate. Although these findings are based upon the case of a mini-TLP, they are expected to be relevant to a wide range of floating or compliant deepwater structures.     

单向荷载作用下海上风机多桶基础承载特性数值分析

作为一种新型的深水海上基础型式,桶形基础在海上风机设施设计与建设中逐步得到了发展和应用。以三桶基础为例,采用大型有限元软件ABAQUS对海上风机多桶基础的承载特性进行了三维有限元数值分析。根据海上风机的荷载受力特点,分别探讨了三桶基础在竖向荷载、水平荷载和力矩作用下的极限承载力,得出荷载作用方向及桶间距对极限承载力的影响程度,研究成果为复合加载模式下海上风机多桶基础的承载特性分析奠定了基础。     

Modelling the drained response of bucket foundations for offshore wind turbines under general monotonic and cyclic loading

The response of bucket foundations on sand subjected to planar monotonic and cyclic loading is investigated in the paper. Thirteen monotonic and cyclic laboratory tests on a skirted footing model having a 0.3 m diameter and embedment ratio equal to 1 are presented. The loading regime reproduces the typical conditions of offshore wind turbines: very large cyclic overturning moment, large cyclic horizontal load and comparatively little, self-weight induced, vertical load. The experimental soil-foundation response is interpreted within the macro-element approach, using an existing analytical model, suitably modified to accommodate the footing embedment and the application of cyclic load. Details of the proposed model are provided together with evidences of its ability to reproduce the essential features of the experimentally observed behaviour. The results of the study aim at increasing the confidence in the use of the macro-element approach to predict the response of bucket foundations for offshore wind turbines, notably as the long-term accumulated displacements are concerned.     

船舶受冲击力作用时的水动力试验研究

根据冲量相似的要求进行船舶在冲击作用下的模型试验，并将模型试验结果与理论计算及实际试验进行了对比和分析。     

水平循环荷载作用下单桩动力特性的数值模拟

采用修正的p-y曲线方法，分析了桩基础振动过程中桩土之间由于土体塑性变形导致的桩土脱开现象，给出了计算桩土脱开现象的数值模拟方法，并据此分析计算了中国渤海湾JZ20－2MUQ平台的桩基础动力特性，得到了桩基础振动过程中桩土脱开的区域，分析了桩土之间的脱开对桩基础动力响应的影响等问题。     

Undrained behavior of natural marine clay under cyclic loading

To study the undrained behavior of natural marine clay under cyclic loading, two kinds of stress-controlled cyclic triaxial tests were conducted on natural K₀-consolidated Wenzhou clay. In the Series I tests, samples were cyclically sheared until failure, and the accumulative behavior was studied; based on the results, a suitable cyclic failure criterion is suggested for natural clays. The effect of loading frequency was also investigated, and it was observed that the loading duration t is a key factor in controlling the undrained cyclic behavior. In the Series II tests, cyclic undrained tests followed by strain-controlled monotonic compression tests were carried out, and special attention was given to changes in the undrained strength after cyclic loading. The degradation of the post-cyclic peak strength was affected by the accumulative behavior during cyclic shearing, but the deviatoric stresses at the critical state were nearly constant. Finally, the accumulative behavior of natural clays was simulated using a proposed anisotropic elastic viscoplastic model with a pseudo-static method of equivalent undrained creep, and the results indicate that this equivalent creep simplification is suitable in practice. By taking the apparent overconsolidation after cyclic loading into account, the post-cyclic strength degradation can also be explained by this model.     

潜器在冲击载荷下的运动和控制研究

潜器在水下发射火箭时会产生复杂的运动变化,对其研究很有实际意义。参照格特勒运动方程,建立潜器运动的非线性数学模型。导入实验模型的参数,对其六自由度基本运动进行了仿真,并与水池实验结果相比较,验证了仿真模型的有效性。导入发射火箭时潜器受到的完整载荷,计算分析了不同航速下潜器的运动响应和运动控制。结果表明,发射载荷对潜器运动将产生较长时间的显著影响,包括速度损失、升沉运动和纵倾角振荡。航速越小,运动变化越大,恢复所需时间越长,控制越困难。     

Structural properties of fractured and faulted Cretaceous platform carbonates,Murge Plateau (southern Italy)

The Upper Cretaceous carbonates cropping out in the Murge Plateau are good analogues of the fractured and faulted carbonate oil reservoirs of southern Italy. For this reason, a detailed field analysis focused on structural architecture of fault and fracture networks has been carried out in the Murge Plateau. The well-bedded carbonates exposed there are crosscut by a set of bed-parallel stylolites and two sets of bed-perpendicular cross-orthogonal joints/veins. These structural elements were likely formed under vertical loading during burial diagenesis and flexure of the Apulian foreland of the Southern Apennines fold-and-thrust belt. Bed-parallel stylolites and bed-perpendicular cross-orthogonal joints/veins represent the background deformation that was overprinted by the fault-related localized deformation. The fault sets documented in the study area are arranged in two kinematically-compatible fault networks. The first one is made up of WNW-ESE and NNW-SSE oriented strike-slip faults, right- and left-lateral, respectively, and NW–SE oriented normal faults. The second fault network consists of WNW-ESE oriented left-lateral strike-slip faults, and NE–SW oriented normal faults.First, both architecture and dimensional parameters of the fault and fracture networks have been characterized and computed by means of statistical analysis. Then, the permeability structures associated to the aforementioned networks have been assessed in order to determine the role exerted by fault architecture and dissolution/cementation processes on the fluid storage and migration pathways within the studied platform carbonates. Network 1 faults show a quite variable fluid behavior, in which the fluid flow is strongly affected by inherited structural elements and karst dissolution, whereas network 2 faults show a more uniform, fluid conduit behavior.     

冲击荷载作用下新型延展式张力腿平台时域分析

延展式张力腿平台是一种适用于深水油气开采的海洋平台结构。提出一种新型平台由四个立柱和一个环形浮箱连接而成,环形浮箱是由四个箱型梁焊接而成。根据给出的南海海域的海况条件和相应的规范计算结构的环境载荷,运用SESAM软件Deep C模块,考虑风、浪、流等环境荷载的作用和张力腿系统的非线性影响,进行了时域耦合分析,得到了平台的运动位移时程、张力腿系统张力时程和张力极值等。模拟波浪破碎作为冲击载荷,得到张力腿平台在冲击荷载作用下的运动位移时程、张力腿系统的张力时程。研究结果表明:该新型平台具有良好的运动响应,张力筋腱应力满足安全要求;在冲击载荷作用下新型平台的运动响应也满足要求。     

浮桥结构在移动载荷作用下的振动响应

浮桥的载荷具有移动性，并有可能在同时遭受风、浪、流联合作用的恶劣环境中使用。为了使浮桥的设计做到既经济又安全，必须对浮桥的载荷和强度有一个比较精确的评估方法。推导了适合模型计算的用户单元的特性矩阵，并将该用户单元导入现有大型有限元分析程序ABAQUS，利用其功能强大的求解器，求解了静水中自由浮式均匀梁在移动载荷作用下的振动响应，为后续研究奠定了基础。     

两型半潜式平台波浪作用下载荷敏感性研究

为探究半潜式平台在波浪作用下的载荷敏感特性,根据频域计算得到的水动力衍射/辐射结果,将动水压力作为输入项,利用ANSYS软件进行两型半潜式平台对波浪载荷的敏感性分析。以此对比两型平台响应特性,确定危险工况,并为平台整体结构设计提供参考。其中A型平台为柱稳式半潜平台,B型平台以A型平台为母型进行了水动力性能优化,以适用于北海海域作业环境。研究发现:两型平台的应变均在弹性形变范围内;两型平台会在相似位置处出现较大的等效弹性应力和应变分布,但相比于母型平台A,B型平台在立柱与浮体连接处也存在明显的高应力区;各波浪入射角度下,两型平台的入射波浪最危险周期不同,且存在B型平台响应大于A型平台响应的工况,因此基于水动力性能改进的B型平台,结构上可能存在不足。