Numerical and Experimental Research on the Global Performances of Cell-Truss Spar Platform

Spar technology has been applied to the deep-sea oil and gas exploitation for several years.From the first generation of classic spar,the spar platform has developed into the second generation of truss spar and the latest cell spar.Owing to its favorable adaptability to wide range of water depth and benign motion performances,spar has aroused quite a lot of interests from oil companies,universities and research institutes.In the present paper,a new cell-truss spar concept,put forward by the State Key Laboratory of Ocean Engineering(SKLOE)at Shanghai Jiao Tong University,is studied both numerically and experimentally.The numerical simulation was conducted by means of nonlinear time-domain fully coupled analysis,and its results were compared to the experimental data.Whereafter,detailed analysis was carried out to obtain the global performances of the new spar concept.Proposals for the improvement of numerical calculation and experimental technique were tabled meanwhile.     

内孤立波作用下Spar平台动力响应特性

以三类内孤立波理论(Kd V、e Kd V和MCC)的适用性条件为依据,采用Morison和傅汝德-克雷洛夫公式分别计算Spar平台内孤立波水平力和垂向力,结合时域有限位移运动方程,建立了有限深两层流体中内孤立波与带分段式系泊索Spar平台相互作用的理论模型。以东沙群岛某海域实测内孤立波为对象,数值分析了在内孤立波作用下某经典式Spar平台的内孤立波动态载荷、运动响应及其系泊张力的变化特性。研究表明,内孤立波不仅会对Spar平台产生突发性冲击载荷,使其产生大幅度水平漂移运动,而且还会使其系泊张力显著增大。因此,在Spar平台等深海平台的设计应用中,内孤立波的影响不可忽视。     

一种新型的Spar平台及其动力特性

介绍了一种新型的Spar平台。由于Spar平台属柔性连接结构,允许其在一定范围内运动,故二阶漂移力为系统稳定性的控制荷载,而线形波浪力则可导致系泊系统产生疲劳破坏。为此,文中结合分析此新型平台所受水动力特性,比较其相对于现有Spar平台的优势。由于浮筒位于水下一定深度处,新型平台所受线性波浪力和二阶波浪力均大幅度减小,尤其是后者可减至现有Spar平台受力的2%左右,所以大大地改善了平台的稳定性和系泊系统的抗疲劳破坏能力。由于在上部平台和浮筒之间增加了导管架,使得各模块间的连接点大大减小,从而又方便了海上施工作业。     

Parametric Resonance Analyses for Spar Platform in Irregular Waves

The parametric instability of a spar platform in irregular waves is analyzed. Parametric resonance is a phenomenon that may occur when a mechanical system parameter varies over time. When it occurs, a spar platform will have excessive pitch motion and may capsize. Therefore, avoiding parametric resonance is an important design requirement. The traditional methodology includes only a prediction of the Mathieu stability with harmonic excitation in regular waves. However, real sea conditions are irregular, and it has been observed that parametric resonance also occurs in non-harmonic excitations. Thus, it is imperative to predict the parametric resonance of a spar platform in irregular waves. A Hill equation is derived in this work, which can be used to analyze the parametric resonance under multi-frequency excitations. The derived Hill equation for predicting the instability of a spar can include non-harmonic excitation and random phases. The stability charts for multi-frequency excitation in irregular waves are given and compared with that for single frequency excitation in regular waves. Simulations of the pitch dynamic responses are carried out to check the stability. Three-dimensional stability charts with various damping coefficients for irregular waves are also investigated. The results show that the stability property in irregular waves has notable differences compared with that in case of regular waves. In addition, using the Hill equation to obtain the stability chart is an effective method to predict the parametric instability of spar platforms. Moreover, some suggestions for designing spar platforms to avoid parametric resonance are presented, such as increasing the damping coefficient, using an appropriate RAO and increasing the metacentric height.     

Spar平台垂荡-纵摇耦合运动失稳机理

研究参数激励和强迫激励共同作用下Spar平台垂荡-纵摇耦合运动的失稳机理.考虑静稳性和排水体积的变化,推导平台的垂荡回复力和纵摇回复力矩表达式,建立规则波浪中平台垂荡-纵摇耦合的运动方程.以经典Spar平台为例,分析平台垂荡-纵摇耦合运动发生马休类型不稳定运动的条件以及平台运动失稳的形式,给出波高和波浪周期平面上平台因大幅运动失稳的参数域.结果表明,当波高相对较小时,波浪的临界周期接近于垂荡固有周期,平台失稳的形式为马休失稳;当波高相对较大时,波浪的临界周期远离垂荡固有周期,平台由于大幅摇摆运动而失稳.     

Development of an adaptive disturbance rejection system for the rapidly deployable stable platform–part 1: Mathematical modeling and open loop response

A Rapidly Deployable Stable Platform (RDSP) concept was investigated at Florida Atlantic University in response to military and civilian needs for ocean platforms with improved sea-keeping characteristics. The RDSP is designed to have enhanced sea-keeping abilities through the combination of a novel hull and thruster design coupled with active control. The RDSP is comprised of a catamaran that attaches via a hinge to a spar, enabling it to transit like a trimaran and then reconfigure so that the spar lifts the catamaran out of the water, creating a stable spar platform. The focus of this research is the mathematical modeling, simulation, and response characterization of the RDSP to provide a foundation for controller design, testing, and tuning. The mathematical model includes a detailed representation of residual drag, friction drag, added mass, hydrostatic and hydrodynamic pressure, and control actuator dynamics. Validation has been performed by comparing the simulation predicted motions of the RDSP operating in waves to the measured motions of the 1/10th scale prototype measured at sea. Resulting from this paper is an empirical assessment of the response characteristics of the RDSP that quantifies the performance under extreme conditions and provides a solid basis for controller development and testing.     

Experimental Study and System Identification of Hydrodynamic Force Acting on Heave Damping Plate

Although Morison equation is often applied for simulating hydrodynamic force of marine structure, it may give poor results when non-linear behavior is severe or random wave is encountered. This leads to some modifications of Morison equation or other methods for predicting hydrodynamic force. One of them is the system identification technique. In this paper, NARMAX model theory is firstly used to identify the hydrodynamic system of heave damping plates, which are commonly installed on spar platform. Both linear and non-linear models are obtained. The comparisons between the predieted results and measured data indicate that NARMAX model can predict hydrodynamic force of a heave damping plate very well. The measured data for identification originate from forced oscillation tests, which are random records with given spectrum. The forced oscillation forms in experiment also contain simple harmonic, multi-frequency ones.     

一种新型Semi-Spar式海上风机平台系泊系统优化分析

参考英国的Kincardine风机采用的新式的Semi-Spar概念,结合spar式基础和半潜式基础的特点,提出了一种新式海上浮式风机平台模型,并基于三维势流理论,利用AQWA软件进行水动力计算,验证新式平台可靠性。分析了在风、浪、流荷载联合作用下,锚链竖向夹角、系缆数量对风机浮式平台运动性能和系泊张力的影响,对系泊系统进行优化,并验证极端工况下的可靠性。结果证明风机平台水平运动和纵摇运动幅值较小,但垂荡幅值略大,而通过减小锚链竖向夹角可以控制平台运动响应幅值,增加系缆数量可以同时减小系泊张力大小。计算结果证明了新型Semi-Spar式海上风机平台可行性,为浮式风机平台及系泊系统的设计提供参考。     

The effect of nonlinear multi-contact coupling with gap between risers and guide frames on global spar motion analysis

Nonlinear multi-contact coupling between vertical risers and guide frames inside the spar moon-pool is studied. The existing numerical model for hull/mooring/riser coupled dynamics analysis treats riser as an elastic rod truncated at the keel (truncated riser model), and the risers are free to slide in vertical direction with constant tension, while restricted in horizontal direction. The truncated riser model neglects the portion of the riser inside the moon-pool and tends to overestimate the spar pitch motion. In the newly developed model, the risers are extended through the moon-pool with realistic boundary conditions at multiple guide frames, and thus additional contact forces and moments on the spar hull are considered. The gap effects between the riser buoyancy-cans and riser guide frames are also modeled using three different types of gap springs. Their different dynamic characteristics are extensively studied. The new riser model also considers the Coulomb damping between buoyancy-cans and riser guide frames and also allows the impact-like contact force calculation on risers for ensuring fatigue analysis.     

一种新型干树式半潜平台设计

开发了一种新型的深吃水干树式半潜生产平台(TCDD-Semi),该平台可用于中国南海深水区域的油气田开发。它的一个显著特点是立柱的截面形状呈现为渐变形式,而非传统的等截面形式。立柱的底部最大,向上逐渐变小,再结合适当的下浮体设计,能够为平台在服役前期及在位状态时提供足够的浮力以及稳性。最重要的是这种新型立柱的设计能够明显地减小平台在恶劣海况时的垂向运动,从而使在平台上布置具有干式采油树的TTR立管系统成为可能。此外,这种渐变式的立柱设计也有利于减弱平台在流作用下的涡激诱导运动(VIM)。分析结果表明这种渐变式立柱半潜平台可以明显降低平台的垂向运动,也可以在码头组装时提供足够的浮力以及在平台下沉过程中提供足够的稳性。平台优越的垂向运动性能可以使顶部张紧式立管的行程范围限制在10.5 m之内(包括平台偏移、垂荡运动、潮汐影响、海床沉降、热膨胀等因素)。分析结果表明平台的垂荡范围、水平偏移、倾侧角度、上部模块重心处的加速度等可以满足设计要求,证明了这种新型干树式半潜平台的可行性。     

Effect of slowly varying drift forces on the motion characteristics of truss spar platforms

Spar platform has been regarded as a competitive floating structure for deep and ultra deep water oil and gas production. In this paper, an efficient methodology has been developed to determine the slow motion responses of slender floating offshore structures due to wave forces. Based on this methodology, a MATLAB program named ‘TRSPAR’ was developed to predict the dynamic responses in time domain and it was used in this study to obtain the numerical results of a typical truss spar platform connected to sea bed using nine taut mooring lines. The difference frequency forces were calculated using the principles of the extension of Morison equation for an inclined cylinder and the wave kinematics were predicted using hyperbolic extrapolation. Mooring lines were modelled as nonlinear springs and their stiffness was obtained by conducting the static offset simulation. Because of the lack of detailed calculations in literature, most of the equations used were derived and presented in this paper. The effects of the different sources of the second order difference frequency forces were compared for inertia and drag forces in terms of response spectra. To validate the TRSPAR code, its results were compared to results of a typical truss spar model test.     

不同孔距固定气泡幕对黑鲷的阻拦效果

于１９８７年３月－１９８８年１月，在室内水池中观测孔径为０．５ｍｍ，孔距分别为２．５，５．０，７．５，１０．０，２０．０ｃｍ５中固定气泡幕的视觉特征和声学特征；对孔距分别为５．０，１０．０和２０．０ｃｍ３种气泡幕对黑鲷的阻拦作用予以重点分析研究，以期探讨适合阻拦黑鲷的最适孔距。结果表明，这３种气泡幕对黑鲷都具有明显的阻拦作用，平均阻拦率分别为７５．１％，５５．５和５４．５％；其中，以５．０ｃｍ孔距     

Nonlinear coupled dynamics analysis of a truss spar platform

Accurate prediction of the offshore structure motion response and associate mooring line tension is important in both technical applications and scientific research. In our study, a truss spar platform, operated in Gulf of Mexico, is numerically simulated and analyzed by an in-house numerical code ‘COUPLE'. Both the platform motion responses and associated mooring line tension are calculated and investigated through a time domain nonlinear coupled dynamic analysis. Satisfactory agreement between the simulation and corresponding field measurements is in general reached, indicating that the numerical code can be used to conduct the time-domain analysis of a truss spar interacting with its mooring and riser system. Based on the comparison between linear and nonlinear results, the relative importance of nonlinearity in predicting the platform motion response and mooring line tensions is assessed and presented. Through the coupled and quasi-static analysis, the importance of the dynamic coupling effect between the platform hull and the mooring/riser system in predicting the mooring line tension and platform motions is quantified. These results may provide essential information pertaining to facilitate the numerical simulation and design of the large scale offshore structures.     

Truss Spar平台桁架管节点疲劳分析

Truss Spar平台在风、浪、流的作用下,结构受力十分复杂,准确分析其疲劳强度相当困难。通过建立Truss Spar平台三维有限元模型,利用HydroD软件进行载荷预报、波浪搜索,得到产生最大应力幅响应的最不利浪向和相位角。运用Sestra软件进行整体有限元分析,得到的结果作为桁架管节点细化模型的边界条件,再用Nastran软件进行局部分析求得节点的热点应力,修正壁厚影响。最后根据DNV规范的S-N曲线计算了管节点的疲劳寿命。     

Snap Tension in Mooring Lines of Deepwater Platform

Based on the theory of impact dynamics, the motion equations for a mooring line-floating body system before and after impact loading are established with consideration of the viscoelastic property of mooring lines. The factors that influence the taut-slack conditions of a mooring system are analyzed through classifying the taut-slack regions, which are defined by non-dimensional ratios of displacement, frequency, and damping of the system. The mooring system of Jip spar platform is analyzed, and the snap te...     

Energy Anaiysis for TMD-Structure Systems Subjected to Impact Loading

This paper investigates the characteristics of reduction of the lateral vibration by use of a Tuned Mass Damper (TMD) for offshore jacket platforms under impact loading. Unlike traditional analysis, the present analysis focuses on the energy concept of TMD/structure systems. In this study, a time domain is taken. The platform is modeled as a simplified single-degree-of-freedom (SDOF) system by extraction of the first vibration mode of the structure and the excited force is assumed to be impact loading. The energy dissipation and energy transmission of the structure-TMD system are studied. Finally, an optimized TMD design for the modeled platform is demonstrated based on a new type of cost function - maxi-mum dissipated energy by TMD. Results indicate that TMD control is effective in reducing the Standard deviation of the deck motion but less effective in reducing the maximum response under impact loading.     

Design analysis of a truss pontoon semi-submersible concept in deep water

Truss pontoon semi-submersible (TPS) is a new offshore structure concept in industry, where a truss spar is used to create the added mass by the heave plates. In the present paper, the effect of the heave plates on the vertical motion of the floating structure is demonstrated. A TPS is analyzed by utilizing the linear diffraction theory as well as the linear part of the Morison equation. The close agreement of the analysis results with the experimental results suggests that the simplified Morison equation can be used for the present analysis without sacrificing the quality of the results. However, good engineering judgment is required for estimating the values of the hydrodynamic coefficients as well as the amount of damping introduced in the structure. It is also found that the heave plates indeed introduce large added mass and considerable damping in the system motion in the vertical direction such that the resonant oscillation becomes less of a problem. This suggests that the TSP concept may have merits as a heave-controlled floating production structure in the deepwater development.     

An analytical approximation for the horizontal drift force acting on a deep draught spar in regular waves

An analytical solution is presented for the horizontal drift force acting on a uniform circular cylinder that is undergoing surge and pitch motion in regular waves. The solution is exact to second order, for a perturbation expansion in terms of wave steepness, but is limited in a strict sense to the hypothetical case when the draught is equal to the water depth. The formulation is used to provide an analytical approximation for investigating the influence of key parameters on the horizontal drift force acting on a deep draught spar.     

Energy Analysis for TMD-Structure Systems Subjected to Impact Loading

This paper investigates the characteristics of reduction of the lateral vibration by use of a Tuned Mass Damper(TMD) for offshore jacket platforms under impact loading. Unlike traditional analysis, the present analysis focnses on theenergy concept of TMD/structure systems. In this study, a time domain is taken. The platform is modeled as a simplifiedsingle-degree-of-freedom (SDOF) system by extraction of the first vibration mode of the structure and the excited force isassumed to be impact loading. The energy dissipation and energy transmission of the structure-TMD system are studied.Finally, an optimized TMD design for the modeled platform is demonstrated based on a new type of cost function - maxi-mum dissipated energy by TMD. Results indicate that TMD control is effective in reducing the standard deviation of thedeck motion but less effective in reducing the maximum response under impact loading.     

A single point mooring spar buoy for measuring ocean waves

We made and set a spar buoy in September 1975 at Tsuyazaki for the purpose of developing the buoy system to acquire the oceanographical data. Motions of the buoy were also measured in terms of three components of acceleration, roll, and pitch. The buoy was removed from the site in May 1978.A method of eliminating influence of the buoy motion on the measured wave data was invented and examined by using the field data. It was found that the influence of the buoy motion on the wave data was so small that the amount of correcting the motion was negligible. In addition wave data obtained at the buoy were compared with those obtained at the fixed type platform, which was built in 1974 to get the reference data of wind, waves and currents. The agreement was found to be good. The performance of the buoy was as good as intended. Thus, it was shown that the buoy so far developed could be used as the platform for oceanographic research such as measuring wind and waves with higher precision.