Reliability of mooring lines and piles for a permanently manned vessel in a tropical cyclone environment

The factors of safety for stationkeeping systems in current standards (ISO, API) are not derived or validated using reliability analyses. As the oil and gas exploration and production is breaking new boundaries, deploying new floating systems and moving into regions with harsher environments, it is of paramount importance to understand what level of reliability these new marine structures are achieving. This paper presents a reliability analysis of the mooring system of a Floating LNG (FLNG) vessel permanently moored and permanently manned offshore North West Australia in a tropical cyclone environment. The reliability analysis addresses both the mooring chain and the pile foundation. The analysis accounts for the long term characteristics of the environment, including the short term variability, in response to a given sea state and the variability and uncertainty in strength of the mooring chain and the pile. The stationkeeping system was analysed using detailed time domain simulations, capturing system non-linearities and low frequency oscillations as well as wave frequency responses and, thereby, reducing modelling uncertainties to a minimum.It is found that for the conditions modelled, neither the chain nor the pile meet a target reliability of 10⁻⁴/annum using the factors of safety commonly used in design following current ISO and API standards. New factors of safety are proposed to achieve this target reliability. For the pile design, one complicating factor is that current design standards do not explicitly define the exceedance probability that should be associated with the characteristic value of the undrained shear strength to be used in the design. It is demonstrated that the required factor of safety is crucially dependent on the definition of this characteristic value and on the level and the type of uncertainty in the soil strength profile. A recommendation is made regarding the definition of this characteristic value and the associated factor of safety. Furthermore, it is found that designing the mooring system to an environmental condition with a return period of 10,000 yr (as an Abnormal Limit State event), and setting the factor of safety to unity, meets the target reliability of 10⁻⁴/annum for the pile, if the characteristic undrained shear strength is a lower bound, defined in this paper by the 10th percentile value. For the chain however, this target reliability is not achieved.     

Fatigue reliability analysis of mooring system for fish cage

Fish cages in the open sea are exposed to cycle loads due to irregular wave climate during their service life, and thus the fatigue reliability assessment of mooring system should be conducted to ensure the safe operation. The aim of this study is to evaluate the fatigue failure probability of mooring system for fish cage. Numerical simulation of net cage in random waves is performed and the time dependent approach is applied to conduct the fatigue reliability analysis of shackle chains based on S-N curve method. The sensitivity analysis of fatigue reliability of mooring line to the uncertainty of random variables in the fatigue limit state is conducted. In addition, the system reliability for mooring system is analyzed and the effect of the initial pretension and safety factor on system reliability is investigated. The results indicate that a case without the initial pretension on anchor lines is helpful to decrease the failure probability of mooring system and the safety factor of mooring lines in the current regulation is conservative for the system reliability against fatigue damage.     

半潜式起重平台系泊系统设计与优化研究

近年来,国内兴起了大力发展半潜式起重平台的势头,系泊系统的设计与优化是该类型平台开发中的关键问题之一。采用时域方法对半潜式起重平台系泊系统进行耦合动力分析,研究系泊系统主要参数对其动力响应特性的影响,并给出主要参数的设计优化流程,在此基础上,以能够满足安全校核且性能较优的系泊方案为目标,对该半潜式起重平台的系泊系统进行优化设计,并研究单根系泊缆发生破断对整个系泊系统的影响。结果表明:基于主要参数对系泊性能的影响规律,经优化设计得到的系泊方案,预留的安全欲度适中,能够保障半潜式起重平台在恶劣海况下的生存能力,同时具备较好的经济性。     

超大型海洋浮式储油系统的风险评估

对超大型海洋浮式储油系统的危险灾害识别、失效概率计算、失效后果评估、风险准则评定和风险管理决策等几个主要步骤进行了风险分析,计算结果为在设置防坡堤情况下,储油船漂移的概率为2 217×10-6,沉没的概率为6 778×10-8,结构损坏的概率为6 300×10-7,总的经济损失为70万人民币;在未设置防波堤的情况下,储油船漂移的概率为3 167×10-3,沉没的概率为9 683×10-5,结构损坏的概率为9 000×10-4,总的经济损失为9 91亿人民币。结果表明,采取合理的措施,如在储油船系泊地点设置防波堤,以降低风险水平是十分必要的。     

A Mooring System Deployment Design Methodology for Vessels at Varying Water Depths

In this paper, a methodology for designing mooring system deployment for vessels at varying water depths is proposed. The Non-dominated Sorting Genetic Algorithm-II(NSGA-II) is combined with a self-dependently developed vessel-mooring coupled program to find the optimal mooring system deployment considering both station-keeping requirements and the safety of the mooring system. Two case studies are presented to demonstrate the methodology by designing the mooring system deployments for a very large floating structure(VLFS) module and a semi-submersible platform respectively at three different water depths. It can be concluded from the obtained results that the mooring system can achieve a better station-keeping ability with relatively shorter mooring line when deployed in the shallow water. The safety factor of mooring line is mainly dominated by the maximum instantaneous tension increment in the shallow water, while the pre-tension has a decisive influence on the safety factor of the mooring line in the deep water.     

软刚臂单点系泊系统静力特性分析

软刚臂系泊系统是一种利用压载质量为浮式生产储油装置(FPSO)提供回复力的单点系泊方式。为快速准确地评估多铰接组成的软刚臂系统的受力状态,避免应用高复杂度的有限元软件,进而快速完成优化设计等任务,建立一种静力计算方法十分有必要。因此,针对由刚臂(YOKE)、柔性系泊腿和压载舱组成的刚柔多体系统,以分解组合的方式,提出一种二维静力计算方法,用以研究FPSO软刚臂系泊系统的静力特性。该方法主要依靠软刚臂的几何构型,在有限铰接约束下,形成非线性静力平衡方程,建立了压载舱式水上软刚臂单点系泊系统二维静力平衡计算算法。采用OrcaFlex、SESAM等软件开展了理论和数值对比分析,同时与系泊试验结果开展验证分析。对比结果表明,文中所提方法计算精度较高,可为软刚臂单点系泊系统的研究提供可靠支持。     

半潜平台锚泊辅助动力定位时域模拟研究

随着海洋油气资源开发逐渐走向深海,浮式结构物的定位能力越来越受到人们的重视。锚的泊辅助动力定位系统是将锚泊定位和动力定位相结合的一种新型海上定位系统。它具有安全性高、定位能力强、消耗功率小的特点。本文以某深水半潜式钻井平台为例,通过建立锚泊辅助动力定位时域模拟程序,分析平台在工作海况下的定位精度和功率消耗,并与相应海况下的动力定位进行比较。分析结果表明,该时域模拟程序能较为准确的地反映实际平台的定位情况。与动力定位相比,锚泊辅助动力定位能够取得更好的定位精度和较小的功率消耗,是更理想的定位方式。     

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.     

双体波浪能装置运动及系泊系统特性研究

波浪能是一种清洁、可再生的新型能源,波浪能发电装置在海上作业时会受到变化的风、浪、流载荷作用,需要系泊系统保证其稳性和安全性。以适用于中国南海500 m水深的振荡双浮体式波浪能发电装置为研究对象,运用频域计算与时域计算结合的方法对双浮体及其系泊系统的运动响应和动力载荷进行计算,获取极端海况与工作海况下浮体运动和系泊缆索张力的时历数据。参照BV船级社NR-493规定的海上浮式结构物系泊安全系数规范,对3种系泊方案进行安全校核和对比分析。选定其中一种系泊方案,通过改变系泊系统以及能量转换器（PTO）的参数,探究参数变化对双体波浪能装置运动响应以及系泊系统特性的影响,为类似应用于深水的双体波浪能装置系泊系统的设计提供参考。     

多船旁靠系泊作业关键特性试验研究

旁靠带缆作业是实现海上补给、模块吊运、原油外输等运转的常用方式,针对浮式平台与两艘供应船以旁靠方式作业涉及的关键特征进行水池模型试验研究,阐述了旁靠带缆系统和护舷设备的属性和需遵循的规范衡准,深入分析旁靠系缆的张力、护舷的挤压力、船体之间的相对运动特征随有效波高和波浪入射角度等环境参数的变化规律,确定了适用于多船旁靠作业的海况条件。研究表明:波浪能量随有效波高的增大呈现非线性增大趋势,是影响旁靠作业窗口期的主要外部因素,而波浪入射角度对旁靠作业的影响程度与海况有较大关系;三船在纵荡、横荡以及艏摇方向具有明显的跟随性特点,运动响应的幅值和相位比较接近,浮式平台与供应船之间的相对运动较小,完全可满足作业要求。垂荡、横摇以及纵摇这三个波频方向,各船的运动并不同步,相对运动大,是影响多船旁靠作业窗口期的主要因素;有效波高为2.5 m的海况条件可满足多船旁靠要求。相关结论为实际的工程操作提供参考。     

Effect of Failure Mode of Taut Mooring System on the Dynamic Response of A Semi-Submersible Platform

Mooring system failure can lead to largely different dynamic response of floating structures when compared to the response under the condition of intact mooring system. For a semi-submersible platform with taut mooring system under extreme environmental conditions, the typical mooring system failure includes anchor line breaking failure due to the broken anchor line, and the anchor dragging failure caused by the anchor failure in the seabed soil due to the shortage of the anchor bearing capacity. However, study on the mooring failure caused by anchor failure is rare. The current work investigates the effect of three failure modes of taut mooring system on dynamic response of a semi-submersible platform, including one line breaking failure, two lines breaking failure, and one line breaking with one line attached anchor dragging failure. The nonlinear polynomial mooring line model in AQWA was used with integrating the load and displacement curve from the anchor pulling study to characterize the anchor dragging behavior for mooring system failure caused by the anchor failure. The offsets of the platform and the tension of mooring lines were analyzed for mooring system failure with 100-year return period. It is found that the mooring failure of one line breaking with one line attached anchor dragging is a case between the other two mooring failures. The traditional mooring analysis considering only the damaged condition with one line breaking is not safe enough. And the simple way of mooring analysis of two lines breaking is too conservative for the costly offshore engineering.

     

Time and frequency domain coupled analysis of deepwater floating production systems

The dynamic analysis of a deepwater floating structure is complicated by the fact that there can be significant coupling between the dynamics of the floating vessel and the attached risers and mooring lines. Furthermore, there are significant nonlinear effects, such as geometric nonlinearities, drag forces, and second order (slow drift) forces on the vessel, and for this reason the governing equations of motion are normally solved in the time domain. This approach is computationally intensive, and the aim of the present work is to develop and validate a more efficient linearized frequency domain approach. To this end, both time and frequency domain models of a coupled vessel/riser/mooring system are developed, which each incorporate both first and second order motions. It is shown that the frequency domain approach yields very good predictions of the system response when benchmarked against the time domain analysis, and the reasons for this are discussed. It is found that the linearization scheme employed for the drag forces on the risers and mooring lines yields a very good estimate of the resulting contribution to slow drift damping.     

Mooring system reliability in tropical cyclone and North Sea winter storm environments

The characteristics of waves, winds and currents in a tropical cyclone environment differ significantly from those in a winter storm environment, like the North Sea. This can have a significant effect on the reliability of a mooring system that is designed to satisfy 100 yr conditions with specified Factors of Safety in accordance with ISO19901-7 or API RP 2SK. This paper presents reliability analysis of the mooring system of a permanently connected Floating LNG vessel, placed at two locations: (a) a tropical cyclone environment of the North West Shelf of Australia and (b) a winter storm environment of the North Sea. It is demonstrated that as a result of differences in the long term distribution of environmental parameters (waves, winds) between a North Sea environment and a tropical cyclone environment, the long term distribution of the mooring line response differs significantly in these two locations.This paper shows that a mooring system which is designed in accordance with ISO (or API), in these two environments, will achieve very different reliability levels because of the significant differences in environmental characteristics. In order to achieve the same reliability for the mooring system at these two geographical locations, Factors of Safety for use with 100 yr environmental conditions (Ultimate Limit State) were derived to achieve the same target probability of failure of 10⁻⁴/annum. It was found that for the North Sea environment, a factor of 1.5 is required for both the mooring chain and the pile, while for the tropical cyclone environment the required Factor of Safety has to be increased to 2.1. These differences are very significant and design standards need to be revised to reflect these findings.     

海上油气田工程压裂作业船及装备配置技术探讨

用于海上油气田的压裂作业船具备高效性和安全性,具有优越的操控性能和良好的稳定性,配备了先进的船舶航行系统、动力定位系统和强大的动力系统.全船配备的压裂作业设备包括支撑剂存储装置,酸液和胶液存储装置,搅拌混合系统,各类液体和化学药剂添加系统,压裂作业泵系统,中央控制室以及数据采集系统,完备的质量控制体系,安全的作业管线和快速脱离装置.压裂作业船能连续施工作业,效率高,安全可靠.     

新型网箱浮架系统水动力分析及系泊方式研究

设计了一种新型网箱浮架系统,利用SESAM对其进行了频域水动力分析以及考虑不规则波浪、风、流载荷和系泊共同作用的时域耦合分析,并与传统双浮管网箱浮架系统进行了分析对比,得到方形网箱与圆形网箱的水动力特性的差异以及各自的优缺点,对实际中网箱的开发设计有一定的借鉴意义;通过计算,证明新型网箱浮架系统在工作海况能正常工作,在极端海况下也能满足安全性的要求;最后对4种常见系泊方式进行了时域耦合分析,得到了四种系泊方式的系泊特性,对实际工程中系泊方式的选择有一定的参考意义。     

Dynamic behavior of offshore spar platforms under regular sea waves

Many innovative floating offshore structures have been proposed for cost effectiveness of oil and gas exploration and production in water depths exceeding one thousand meters in recent years. One such type of platform is the offshore floating Spar platform. The Spar platform is modelled as a rigid body with six degrees-of-freedom, connected to the sea floor by multi-component catenary mooring lines, which are attached to the Spar platform at the fairleads. The response dependent stiffness matrix consists of two parts (a) the hydrostatics provide restoring force in heave, roll and pitch, (b) the mooring lines provide the restoring force which are represented here by nonlinear horizontal springs. A unidirectional regular wave model is used for computing the incident wave kinematics by Airy’s wave theory and force by Morison’s equation. The response analysis is performed in time domain to solve the dynamic behavior of the moored Spar platform as an integrated system using the iterative incremental Newmark’s Beta approach. Numerical studies are conducted for sea state conditions with and without coupling of degrees-of-freedom.     

Design of mooring system of vessels inside breakwater

A method of simulation calculation for a mooring system of vessels inside a breakwater is illustrated. Using a simulation program based on the above method, a test design for a mooring system for oil-storage vessels of total 5.6 million kiloliters was conducted.In order to obtain data for the simulation program, various experiments, such as wind tunnel tests to test wind resistance, and wave tank tests to test the waves transmitted through or over the breakwater, and the motions of vessels inside the breakwater due to waves, were conducted.Some proposals concerning design criteria and safety factors are presented.Finally, the results of the test design under estimated sea and weather conditions are shown, and the design is concluded to be both technically and economically feasible.     

Neural networks in the dynamic response analysis of slender marine structures

Risers and anchor lines play important roles in offshore oil exploitation activities nowadays. For this reason the proper analysis and design of such slender structures has been of a paramount interest. The principal characteristics to be accounted for in riser and mooring line analysis are the severe nonlinearities involved and the random dynamic effects associated. The Finite Element Method (FEM) is an essential step to cope with this kind of analysis. But the use of the FEM can be computationally very expensive for the solution of the resultant nonlinear differential equations of motion, because the time-domain integration should produce sufficiently long response time-histories using small time-steps in order to obtain reliable time-series statistics of any structural response parameter, e.g., top tension in an anchor line or stresses occurring at a critical section in a steel catenary riser (SCR). This paper presents a very efficient hybrid Artificial Neural Network (ANN)–Finite Element Method (FEM) procedure to perform a nonlinear mapping of the current and past system excitations (inputs) to produce subsequent system response (output) for the random dynamic analysis of mooring lines and risers. Firstly, a quite short FEM-based time-domain response simulation is generated. Then, an ANN is used to predict the remaining structural response time-history simulation. The hybrid ANN–FEM approach can be very efficient for predicting long response time-histories. It has been observed that a 3 h response time-history can accurately be obtained with approximately the computational cost of a 500 s one, i.e., 20 times faster than a complete simulation using finite element-based solution. Roughly, this can represent a reduction of about a dozen of hours of computer time for a single mooring line analysis and about two dozens of hours (or more) for a single SCR analysis, both belonging to a deep-water floating unit.     

Approximated dynamic analysis of single anchor leg storage system

This paper is concerned with the determination of dynamic load variations in the riser and yoke pivot components of an offshore vessel/mooring oil production and storage system.Forces on the yoke system arise from wave induced inertial and drag forces including current and motion due to vessel surge, heave and pitch.The analysis is carried out in the frequency domain and the results yield the limiting wave height for a specific design of SALS which will give rise to loss of tension in the riser. Also presented are the maximum horizontal and vertical components of load variation at the yoke pivoted connections with the vessel.