Study on key performance parameters of hydro-pneumatic tensioner for top tensioned riser

In this paper an improved mathematical model of a hydro-pneumatic tensioner (HPT) system for top tensioned riser (TTR) is derived by consideration of friction, mass of piston and piston rod, tension loss in hydraulic oil piping, and compressibility of hydraulic oil. The vertical motion of the riser string is also considered. Subsequently, the proposed detailed model and the conventional simplified model are comparatively studied. Finally, the tension characteristic and performance parameters of the HPT are analyzed based on the proposed model. Results show that the conventional simplified model indeed overestimates the tension of tensioner as it neglects some performance parameters resulting in tension loss. The diameters of piston and piston rod and the initial pressure of high pressure (HP) gas have the most significant influence on the tension of the tensioner. The initial volume of HP gas and the initial pressure of low pressure (LP) gas also have some impact. The influence of the initial volume of LP gas and the inner diameter and length of the piping is relatively small. The research has some reference value for HPT and TTR design.     

On natural frequencies and modal shapes in two-dimensional asymmetric and symmetric moonpools in finite water depth

In this paper theoretical models are proposed for computing the natural frequencies and modal shapes of two-dimensional asymmetric and symmetric moonpools in the finite water depth. The boundary value problem is solved by using a domain decomposition approach. On the outer vertical boundary bounded by the beam of the two bodies, linearized velocity potential is assumed to be nil. Eigenvalue problem is formulated by matching the velocity potential and fluid flux on the common boundaries to obtain the natural frequencies and modal shapes of the free surface elevation. In the symmetric moonpool cases, so-called single mode approximations (SMA) have been derived and can be adopted for rapid estimation of the natural frequencies for both piston and sloshing modes. The present results have been extensively compared with the solutions using the two-dimensional infinite water depth model developed by Molin [1], the numerical solutions and experimental data by Faltinsen et al. [2]. It is found that the solutions have been improved from the infinite water depth model. It is demonstrated that the proposed models can well predict the resonance frequencies and modal shapes for the two-dimensional asymmetric and symmetric moonpools.     

Gas hydrates in the western deep-water Ulleung Basin, East Sea of Korea

Geophysical surveys and geological studies of gas hydrates in the western deep-water Ulleung Basin of the East Sea off the east coast of Korea have been carried out by the Korea Institute of Geoscience and Mineral Resources (KIGAM) since 2000. The work included a grid of 4782 km of 2D multi-channel seismic reflection lines and 11 piston cores 5–8 m long. In the piston cores, cracks generally parallel to bedding suggest significant in-situ gas. The cores showed high amounts of total organic carbon (TOC), and from the southern study area showed high residual hydrocarbon gas concentrations. The lack of higher hydrocarbons and the carbon isotope ratios indicate that the methane is primarily biogenic. The seismic data show areas of bottom-simulating reflectors (BSRs) that are associated with gas hydrates and underlying free gas. An important observation is the numerous seismic blanking zones up to 2 km across that probably reflect widespread fluid and gas venting and that are inferred to contain substantial gas hydrate. Some of the important results are: (1) BSRs are widespread, although most have low amplitudes; (2) increased P-wave velocities above some BSRs suggest distributed low to moderate concentration gas hydrate whereas a velocity decrease below the BSR suggests free gas; (3) the blanking zones are often associated with upbowing of sedimentary bedding reflectors in time sections that has been interpreted at least in part due to velocity pull-up produced by high-velocity gas hydrate. High gas hydrate concentrations are also inferred in several examples where high interval velocities are resolved within the blanking zones. Recently, gas hydrate recoveries by the piston coring and deep-drilling in 2007 support the interpretation of substantial gas hydrate in many of these structures.     

深海碳酸盐岩心天然热释光变化特征

东北印度洋赤道90°E海岭和孟加拉扇MD81349,MD77181岩心记录了30万a来气候变化的历史.分析发现浮游有孔虫壳体(方解石晶体)的天然热释光变化与δ18O反映的冰川旋回存在着良好的一致性:天然热释光强度的增大与间冰期对应,降低与冰期对应.对MD81349岩心天然热释光时间序列的频谱分析结果显示热释光的显著周期为99,46,23,20和11ka,其中99,46,23,20ka分别与天文学周期中的地球轨道偏心率周期100ka,地轴倾斜度(黄赤交角)周期54,41ka,太阳辐射年度差异的岁差周期23,19ka接近,这表明地球冷暖交替与太阳辐射强度周期性变化密切相关,在不同海水温度条件下结晶形成的方解石晶体的热释光强度差异显著,因而有孔虫壳体积存的热释光能够反映地球气候的变化.     

海洋立管复模态动力特性分析

考虑阻尼的影响,研究海洋立管的动力特性。通过分析管内流体及管外海洋环境荷载的共同作用,建立海洋立管涡激振动偏微分方程,进而得到立管动力特性方程,用复模态分析法求解动力特性方程得到立管考虑阻尼的自振频率。算例计算表明:考虑阻尼的立管自振频率略小于不考虑阻尼的立管自振频率;立管的自振频率随着内流流速的增加而减小,但内流流速不大时,影响较小;管道长度对立管的自振频率影响较大。     

A new box corer design for the investigation of manganese-nodule distribution in a sediment column

A new, inexpensive and easy-to-handle box corer (47 × 47 × 114 cm³) is described. It is found to be extremely handy when operated from ships equipped with piston coring devices. The box corer uses the same piston core-head (～1000 kg) and the same trigger mechanism as used in piston coring. The whole operation is similar to that of piston coring. At three sites in the nodule-rich abyssal-hill province in the northwest Atlantic, the vertical distributions of the manganese nodules were obtained from the box cores. More than 75% of the nodules by weight is present in the top 10 cm of the sediment; 16–24% is present in the 10–20 cm interval and about 1% in the 20–30 cm interval. Very few nodules are found below 30 cm.     

Acoustic radiation from a pulsating spherical cap set on a spherical baffle near a hard/soft flat surface

Radiation of sound from a spherical piston, set in the side of a rigid sphere, undergoing harmonic radial surface vibrations in an acoustic halfspace is analyzed in an exact fashion using the classical method of separation of variables. The method of images in combination with the translational addition theorems for spherical wave functions is employed to take the presence of the flat boundary into account. The analytical results are illustrated with numerical examples in which the piston is pulsating near the rigid/compliant boundary of a water-filled halfspace. Subsequently, the basic acoustic field quantities such as the acoustic radiation impedance load and the radiation intensity distribution are evaluated for representative values of the parameters characterizing the system. Numerical results reveal the important effects of excitation frequency, source position, and cap angle on the acoustic radiation impedance load and the radiation intensity distribution. The presented work can lead to a better understanding of dynamic response of near-surface underwater transducers.     

Experimental studies of a damaged ship section in forced heave motion

This work documents a detailed series of experiments performed in a wave flume on a thin walled prismatic hull form. The model consists of a rectangular opening located on the side. The length of the model is slightly smaller than the flume breadth to achieve two-dimensional (2D) behavior in the experiments. Forced oscillatory heave tests in calm water have been carried out by varying the model-motion parameters and examining both intact and damaged conditions. Video recordings, measurements of the wave elevation inside the damaged compartment and of the force on the model were performed in all the experiments. The effect of damage opening in the model on hydrodynamic loads is examined by comparing with an intact section. A theoretical analysis is used to explain the behavior of added mass and damping coefficients in heave for a 2D damaged section. The presented results demonstrate occurrence of sloshing and piston mode resonances in the tests and their influence on the hydrodynamics loads of a damaged ship. Detailed physical investigations are presented at these resonance frequencies for the damaged section. Effect of filling level in the damage compartment, damage-opening length and air compressibility in the airtight compartment is examined. Nonlinear effects are documented and appear dominant, especially, for lowest filling level where we have shallow-water depth conditions in the damaged compartment. Resonance phenomena that can lead to significant local loads are identified for the shallow water condition. Air compressibility in the airtight compartment and floodwater act as a coupled system and influence inflow/outflow of floodwater in the compartment. It has a significant effect on local floodwater behavior in the damaged compartment.     

Experimental investigation of gas-water relative permeability for gas-hydrate-bearing sediments from the Mount Elbert Gas Hydrate Stratigraphic Test Well, Alaska North Slope

Permeability of hydrate reservoirs found in nature is likely to be heavily influenced by the percent of the pore volume occupied by hydrates. The quantification of how hydrate saturation affects permeability is of key interest for reservoir engineering studies. In this study, an experimental setup was modified to test permeability characteristics of unconsolidated core samples containing various saturations of methane hydrates. Hydrates were formed in the unconsolidated samples using a refrigerated core holder connected to a brine and methane injection system. Studies of this type conducted to date have rarely been performed on core samples recovered from actual hydrate-bearing sedimentary sections from natural hydrate intervals. Samples from the Mount Elbert site on the Alaska North Slope (ANS) were used for this study.Relative permeability measurements using hydrate constituent components (e.g. water and methane) are not very desirable due to difficulties in preventing additional hydrate formation during displacement experiments. Relative permeability measurements performed with hydrate constituent components (e.g. water and nitrogen) can help to significantly mitigate issues with additional hydrate formation. However, unsteady state relative permeability experiments produce piston like displacement results suggesting that steady state experiments might be preferable.It was observed that as in previous work using consolidated core samples, permeability of both brine and gases was reduced in unconsolidated hydrate-bearing core samples. Experimental results show that low to moderate hydrate saturations (1.5 to 36%) can significantly reduce permeability of porous media. These saturations, in fact, are lower than hydrate saturations observed in the natural hydrate systems at Mount Elbert.     

Tracing Method of Natural Sediments by Neutron Activation and Its Application

A new method of Neutron Activation Analysis (NAA) for sediment movement study is presented in this paper. Through analyzing the natural sediment, the information about natural tracing agents has been obtained, which can show the direction of sediment movement and relative discharge of sediment transportation. Test results in some areas, such as Shantou Harbour and Hangzhou Bay, are in good agreement with those from normal hydrologic measurements and some results are not obtainable with the latter. The NAA is economical, practical and reliable, and should be widely used in engineering practice.     

基于双缸耦合原理的阀控式能量回收系统原理设计与仿真分析

针对小规模海水淡化需要解决能量回收装置的能回效率与空间问题,将能量回收装置与压力提升泵进行集成一体化设计,研究了一种双缸耦合阀控式能量回收系统。系统两液压缸内活塞各由一个电动推杆推动,叠加高压浓海水压力实现压力交换,同时进行活塞推力的耦合控制,避免了压力交换过程中活塞的压力脉冲波动,保障了反渗透膜工作压力稳定;基于小型海水淡化装置,根据压力能流反渗透工艺节点压力流进行了计算分析,得出电动推杆推力压力0.5 MPa左右的补偿压力曲线。通过AMESim进行液压仿真分析,结果表明在系统回收率为30%,高压浓海水进口压力为4.8 MPa,增压海水压力4.6 MPa的情况下,通过压力补充增压海水压力可增压到5.0 MPa,反渗透膜压力与流量波动较小,满足反渗透海水淡化的压力需求。     

南黄海太阳沙西侧潮流脊槽的沉积物分布特征和沉积环境演变

对南黄海太阳沙西侧潮流脊槽区进行了底质调查,采用密集站位的振动活塞柱状沉积物采样器取样。根据粒度分析结果,比较精细地研究了厚度约2～3 m浅表层沉积物的空间分布特征。结果表明:表层沉积物中,细砂主要分布在潮流脊和岸滩上,其它较细的沉积类型主要分布于潮流槽内,呈显“脊砂槽泥”的沉积地貌基本特征。同时,沙脊上的表层细砂的分布范围明显与沙脊至海岸的距离有关,除了潮流的影响外,主要与外侧沙脊的消浪作用有关。另外,根据表层沉积物分布的沉积地貌特征,探讨了潮流脊槽的沉积环境演变。     

Holocene environmental change and the accumulation–erosion balance of sheltered river-mouth sediments, Göteborg, SW Sweden

Holocene sedimentological change was investigated in 15 sediment piston cores (250 cm long) from the mouth of the Göta älv River, outside of Göteborg, which is sheltered by an archipelago. The main objective was to interpret recent, natural and human-induced influences upon the accumulation–erosion balance and the sedimentary environment from Holocene sediment units. The five main units are: consolidated glaciomarine clay (Facies gC) lowermost, consolidated estuarine clay (Facies dC), soft sub-recent sediment (Facies omC and Facies C) uppermost and dumped sediment (Facies D) in some parts of the study area. The sediment facies are divided by two major hiatuses: (a) the early Holocene hiatus between Facies gC and dC involves a time gap of at least 7000 years, ending at ca. 4000 BP and (b) the late Holocene hiatus between the consolidated clays (Facies gC and dC) and Facies omC spans 1000 years and ends ca. 50 BP (i.e. 100 years ago). Both hiatuses probably relate to the effects of isostatic land uplift. Surface sediments consist mainly of an organic-matter-rich clay facies, suggesting changes in the trophic state of the estuary during the last 100 years. Together with human-induced increased river erosion and loss of accommodation space in the estuary, this caused the onset of sedimentation in the estuary. The recent sediments are contaminated with heavy metals (Cu, Zn, Pb, Hg), with highest concentrations in the lower part of Facies omC.     

Evaluation of methods to remove inertial force from measured model wave impact force signals

The subject of wave impact on offshore structures and their components is important to vessel designers and operators for many reasons. They are often required to quantify these impact loads. Standard methods for wave load prediction will underestimate the forces on these structures due to intermittent loading. This necessitates the use of physical model tests to establish wave impact loads. The model measurement systems are designed to have high stiffness. This ensures that the natural frequency of the structure is above the wave frequency. However, it is widely believed that impacting waves contain high-frequency energy components that cause the structure to vibrate at its modal frequencies. This impact-induced vibration is recorded by the measuring system as a force (inertial force), and corrupts the actual applied force measurement. Before scaling can occur, the inertial force must be removed from the measured signal.A number of techniques for removing inertial force from measured signals are described in the published literature. Three methods are discussed, implemented and compared in this paper. The algorithms and procedures are presented. Each technique contains inherent and unique problems, while some are common to all methods.Neither of the techniques produced results that are fully satisfactory. The main problem is unwanted high-frequency content after the application of the methods. While neither method offers the perfect solution, the use of digital filtering techniques is recommended based on their relative performance and ease of implementation.     

Forced vibration analysis of an offshore tower carrying an eccentric tip mass with rotary inertia due to support excitation

Dynamic responses of structures due to earthquake excitation are the important problems in engineering, thus, the information concerned is plenty. However, most of the literature is relating to the discrete methods, particularly to the finite element method (FEM), and the one relating to the method combining both the “continuous” and “discrete” models is rare. The objective of this paper is to provide some information in this respect. First, the analytical solution for the natural frequencies and normal mode shapes of a “continuous” tower, without contacting water (or “dry” tower), carrying an eccentric tip mass possessing rotary inertia is determined. Next, the partial differential equation of motion for the forced vibration of the tower, contacting water (or “wet” tower), subjected to support excitation is transformed into a matrix equation by using the last natural frequencies and normal modes shape of the freely vibrating dry tower. Finally, the numerical integration method is used to solve the matrix equation to yield the seismic response of the wet tower. In theory, the mode superposition method is correct only if the total number of modes considered approaches infinity, however, numerical results of this paper reveal that superposition of only the lowest six modes will yield excellent results to be very close to the corresponding ones obtained from the conventional FEM. For this reason, the CPU time required by the presented approach is less than 5% of that required by the conventional FEM.     

重力活塞取样器贯入深度研究

重力活塞取样器是一种重要的海底连续沉积物采集仪器,贯入深度和样品质量对海洋地质研究有着重要影响。通过对重力活塞取样器贯入过程建立能量守恒方程,得出贯入深度控制方程。已知取样器参数和土质类型条件,可计算取样管贯入深度,深度受取样器重量、体积、形状、取样管直径和沉积物类型等因素共同影响。已知取样器参数和贯入深度,可推测海底沉积物类型。     

深海钻机盘式储芯机构动力学分析

为确保储芯机构设计和工作的可靠性,对储芯机构的动力学特性进行了分析。首先,将转盘油缸的活塞杆等效为一滑块,两端的铰接耳等效为转动副,构建了活塞杆驱动储芯机构的动力学模型;其次,采用Simulink建立了活塞杆驱动储芯机构的仿真模型,并对该机构进行了运动学和动力学分析。仿真分析了储芯机构的运动规律,以及不同驱动速度下,铰接点和油缸受力的变化规律,给深海底钻机储芯机构运动参数的确定提供了指导,也为储芯机构关键部件的设计提供了理论依据。     

Laboratory experiment on heaving body with hydraulic power take-off and latching control

A heaving axisymmetric floating body is tested with sinusoidal incident waves in a wave channel. It is connected to the piston of a pump, and it may be latched by an electromagnetic mechanism. Experimental results are compared with a linear mathematical model, for heave response, hydrodynamic parameters, absorbed wave power and converted hydraulic power. Heave resonance occurs at 1.1 Hz. For sub-resonant frequencies, latching control results in a significant increase in heave response and in absorbed, as well as converted, power. Hydraulic energy capture increases by a factor of 2.8 or 4.3 for frequency 0.75 or 0.5 Hz, respectively.     

Pressure Tight Piston Corer ——A New Approach on Gas Hydrate Investigation

A pressure tight sediment samphng technology, which can be introduced into the modification of the piston corer to accommedate the investigation of gas hydrate, is put forward. In this paper, the three basic rules of the technology are analyzed in detail: specimen transferring rule, seal self-tightening rule and semi-active pressure holding rule. Based on these, the structure of the pressure tight piston corer is put forward and its working principle is analyzed. Finally, a pressure tight sediment sampler, to which the same technology is apphed, is researched through experiments. Results show that the sampler based on the above-mentioned theory has a good ability in samphng and in -situ pressure holding.     

南海东北部海底沉积物波的形态、粒度特征及物源、成因分析

在南海东北部广泛发育沉积物波。通过高分辨率多波束数据、地震剖面以及重力柱状样,对沉积物波的形态特征、粒度特征、物源以及形成机制进行了分析。研究表明大致以台湾浅滩南海底峡谷为界,北侧为近北东向展布,南侧为近南北向展布。对其分布规律、地貌和形态特征及重力柱状样粒度分析表明这些沉积物波为浊流成因。沉积物波的发育与新生代晚期研究区的构造活动密切相关,自距今6.5 Ma以来台湾造山运动使台湾岛强烈抬升剥蚀,这些剥蚀物为研究区提供了大量的陆源物质,而在南海东北部陆坡区大量发育的峡谷-冲沟系统为陆缘物质向下陆坡的输送提供了良好的通道。研究区西侧的东沙隆起长期处于抬升剥蚀状态,这种抬升剥蚀也为研究区沉积物波的发育提供了部分物源。随着坡度的减缓,浊流沉积物开始堆积,在台湾浅滩南海底峡谷的北侧形成了展布方向与冲沟垂直的沉积物波,而在南侧由于台湾浅滩南海底峡谷发生转向,浊流从水道中漫溢出来,沉积物堆积下来,形成了与原先水道近于垂直的近南北向的沉积物波。