Revealing the Fluttering Motions of Mid-Water Trawl Codend Through Sea Trials:Case Study of Antarctic Krill Trawl Codend

The dynamic coupling between the fluttering motions and hydrodynamic characteristics of codend is essential in understanding the trawl selectivity through fish response and the drag force acting on the whole trawl.This study investigated the effect of towing speed,warp length,warp tension,and catch size on the fluttering motions of Antarctic krill trawl codend during net shooting,towing,and hauling by using sea trial data.The time-periodicity of codend oscillation was analyzed by the Morlet wavelet transform method.Results indicated that the period of codend oscillation was between 50 s and 90 s and showed an increasing trend with the warp tension but a decreased value at the towing stage.The coefficient amplitude of codend oscillation was between 0 and 4 at the net shooting and hauling stages,and between 0.2 and 0.6 at the towing stage.The amplitude of codend oscillation increased with the warp tension,towing speed,and catch size,but decreased with the increase of the warp length.In addition,the period of codend oscillation increased with the towing speed at the net shooting and hauling stages,but decreased at the towing stage.These results from codend fluttering motions can improve the understanding of fish behavior and gear shape that modify the hydrodynamic force on the codend instantaneously.

Lateral bearing characteristics of subsea wellhead assembly in the hydrate trial production engineering

Conductor and suction anchor are the key equipment providing bearing capacity in the field of deep-water drilling or offshore engineering,which have the advantages of high operation efficiency and short construction period.In order to drill a horizontal well in the shallow hydrate reservoir in the deep water,the suction anchor wellhead assembly is employed to undertake the main vertical bearing capacity in the second round of hydrate trial production project,so as to reduce the conductor running depth and heighten the kick-off point position.However,the deformation law of the deep-water suction anchor wellhead assembly under the moving load of the riser is not clear,and it is necessary to understand the lateral bearing characteristics to guide the design of its structural scheme.Based on 3D solid finite element method,the solid finite element model of the suction anchor wellhead assembly is established.In the model,the seabed soil is divided into seven layers,the contact between the wellhead assembly and the soil is simulated,and the vertical load and bending moment are applied to the wellhead node to simulate the riser movement when working in the deep water.The lateral bearing stability of conventional wellhead assembly and suction anchor wellhead assembly under the influence of wellhead load is discussed.The analysis results show that the bending moment is the main factor affecting the lateral deformation of the wellhead string;the anti-bending performance from increasing the outer conductor diameter is better than that from increasing the conductor wall thickness;for the subsea wellhead,the suction anchor obviously improves the lateral bearing capacity and reduces the lateral deformation.The conduct of the suction anchor wellhead assembly still needs to be lowered to a certain depth that below the maximum disturbed depth to ensure the lateral bearing stability,Thus,a method for the minimum conductor running depth for the suction anchor wellhead assembly is developed.The field implementations show that compared with the first round of hydrate trial production project,the conductor running depth is increased by 9.42 m,and there is no risk of wellhead overturning during the trial production.The method for determining the minimum conductor running depth in this paper is feasible and will still play an important role in the subsequent hydrate exploration and development.

船舶操纵性海试实验虚拟仿真设计与应用

操纵性试验是船舶海试的重要组成部分,可有效评估船舶投入运营后操纵性能的优劣。船舶操纵性海试准备时间长、成本高、风险大,难以开展实验教学。依托虚拟仿真软硬件设施,充分融合虚拟现实技术、3D成像投影技术与船舶操纵性能分析方法,探索新型实验教学模式,完成船舶操纵性海试虚拟仿真设计;并以集装箱船为例,开展实验教学应用研究。船舶操纵性海试实验虚拟仿真有效弥补了海试的诸多不足,为学生沉浸式体验船舶操纵性海试全过程、掌握相关实验规程与分析方法提供良好的虚拟仿真环境。

基于海试数据的水声信号自适应波束形成方法性能分析

为了更好验证自适应波束形成技术的实际应用,本文研究了基于海试数据的不同自适应波束形成方法的性能,对比分析CBF算法、MVDR算法、MUSIC算法的优缺点,总结了各算法适用条件及实用性。研究结果表明,在声呐的实际应用中,CBF算法更适用于实时性要求较高、目标数量较少的方位探测;MUSIC算法更适用于实时性要求不高、目标数量较多、目标数量已知的方位探测;MVDR算法更适用于实时性要求不高、目标数量较多、目标数量未知的方位探测。

大功率海洋装备发动机链条设计与试验

针对大功率海洋装备发动机传动系统高疲劳性能及高耐磨性能的要求,结合啮合特征及受力分析对滚子链的销轴、套筒零部件进行了结构创新设计,并对热处理工艺进行了优化改进。对改进前后的链条进行硬度、耐磨性及抗疲劳性能测试的结果表明,改进后链条的销轴表面硬度可达800 HV_(0.2)以上、内单节压铆力降低了24.7%、链条经150 h磨损测试后伸长率降低了9%,链条疲劳寿命全部满足要求。最后,将改进后的链条装载到大功率海洋船舶发动机中,进行了2 000 h的航海试验,证明了改进后的链条可以满足大功率船用发动机正时链条的要求。该研究突破了中国海洋装备发动机链传动领域的技术难题,使中国成为全球第三个具备大功率海洋装备发动机传动链条生产能力的国家,并为高端制造业中的技术创新提供了宝贵经验。

波浪滑翔器脐带缆结构设计与海上试验

以“先锋”波浪滑翔器为例,介绍其结构组成和性能,重点对脐带缆结构中的柔性铠装缆、接线盒及缓冲减振结构进行详细设计。以类NACA翼型作为截面的铠装缆比传统扁平缆最大可减少40%的流体阻力。铠装缆2根3.5 mm钢丝绳与接线盒通过固定压板和绕柱的连接方式可提供约9994 N承载拉力。基于组合式碟形弹簧的缓冲减振结构可缓冲吸收约115.6 J冲击能量,保证脐带缆结构可承受8级海况产生的周期性载荷。在南海开展为期69 d的海上试验,航行里程为2383 km,在3级海况下平均航速为1.28 kn,虚拟锚泊精度半径小于80 m,验证了波浪滑翔器的海上航行性能。

大型邮轮船体部分航行试验要点分析

为更准确地测试邮轮的快速性、操纵性、舒适性等性能指标,对未来的航行安全提供重要的参考依据,交船前需进行航行试验。本文基于国产首制大型邮轮的实船试航情况,并参照规范和实船数据,分析了大型邮轮航行试验中,船体试验方面的航速测试、操纵性测试和振动噪声测试的流程和要点,并提出相关改进措施,以达到更好的试验结果,满足规范和船东要求,并可为后续建造的大型邮轮提供参考。

Study on Al-Zn-In Alloy as Sacrificial Anodes in Seawater Environment

Electrochemical properties including conventional electrochemical properties: open circuit potential, polarization poten- tial, electrochemical capacity, morphology of surface dissolution, and the initial polarization properties of Al-Zn-In-Mg-Ti and Al-Zn-In-Si sacrificial anodes are investigated in the sea. The curves of anode output current and work potential with time, the im- pact of area ratio of cathode to anode on anode output current is discussed, and the initial polarization properties are investigated with cyclic voltammetry (CV) finally. The results show that for the two anodes, the current efficiency got in the sea environment is higher than the data of the 4-day-accelerated test in laboratory and the corrosion morphology is more uniform in the sea. With the same exposed area, the current efficiency of Al-Zn-In-Mg-Ti anode is higher than that of Al-Zn-In-Si in the sea. For both anodes, the cur- rent efficiency becomes larger with the increase of the area ratio of cathode to anode. The two anodes can output larger current in strong polarization state. The situation is consistent with the results that small area anode released higher current in the sea trial. The difference rests with that Al-Zn-In-Mg-Ti anode output far greater current than that of the Al-Zn-In-Si under the same polarization potential. The initial polarization property of the Al-Zn-In-Mg-Ti anode is better than Al-Zn-In-Si anode.

The second natural gas hydrate production test in the South China Sea

Clayey silt reservoirs bearing natural gas hydrates(NGH)are considered to be the hydrate-bearing reservoirs that boast the highest reserves but tend to be the most difficult to exploit.They are proved to be exploitable by the first NGH production test conducted in the South China Sea in 2017.Based on the understanding of the first production test,the China Geological Survey determined the optimal target NGH reservoirs for production test and conducted a detailed assessment,numerical and experimental simulation,and onshore testing of the reservoirs.After that,it conducted the second offshore NGH production test in 1225 m deep Shenhu Area,South China Sea(also referred to as the second production test)from October 2019 to April 2020.During the second production test,a series of technical challenges of drilling horizontal wells in shallow soft strata in deep sea were met,including wellhead stability,directional drilling of a horizontal well,reservoir stimulation and sand control,and accurate depressurization.As a result,30 days of continuous gas production was achieved,with a cumulative gas production of 86.14×104 m3.Thus,the average daily gas production is 2.87×10^4 m^3,which is 5.57 times as much as that obtained in the first production test.Therefore,both the cumulative gas production and the daily gas production were highly improved compared to the first production test.As indicated by the monitoring results of the second production test,there was no anomaly in methane content in the seafloor,seawater,and atmosphere throughout the whole production test.This successful production test further indicates that safe and effective NGH exploitation is feasible in clayey silt NGH reservoirs.The industrialization of hydrates consists of five stages in general,namely theoretical research and simulation experiments,exploratory production test,experimental production test,productive production test,and commercial production.The second production test serves as an important step from the exploratory production test to experimental production test.

Novel Gas-tight Multi-sampler for Discrete Deep-sea Water

The issues of how to quickly collect seawater samples and of how to make sure that those samples truly reflect the in-situ information on gas composition and concentration have therefore become a hot but difficult topic in the field of ocean technology.Most conventional seawater samplers only focus on collecting seawater itself,but take little consideration on gas preservation.A set of new oceanographic tools are presented for ocean resource exploration such as hydrothermal sulfide and gas hydrate,and for investigations on the processes and mechanisms of marine physical,chemical and biological evolutions.A gas-tight deep-sea water sampling system（GTWSS） is designed for the collection of deep-sea geochemical samples.This set of tools mainly consists of a conductivity temperature depth profiler（CTD）,release devices and gas-tight deep-sea water samplers（GTWS）.The GTWS is able to hold the gases in deep-sea water samples tightly,providing in-situ information on gas contents in the seawater samples and can be deployed on a routine wire-deployed CTD sampler for multi-layer discrete sampling of gas-tight seawater.Sea trials are performed successfully in 2008 and 2009,on a research vessel named HaiYang Si Hao in South China Sea,with the deepest trial depth 3 930 m.GTWSS is capable of quickly sampling 12 discrete gas-tight seawater samples（8.3 L per sample） during its single deployment.The head space method is employed to separate the gases from the seawater samples immediately after recovery of the seawater samples on the vessel.Field geochemical analysis is carried out by gaseous hydrocarbon sensors and an infrared gas analyzer.Results show that the concentrations of CH4 and CO2 in the seawater sampled by GTWSS are higher than those sampled by general non-gas-tight water samplers,thus confirming the gas tightness of GTWSS.Seawater samples can be collected quickly by using GTWSS,and GTWSS can keep the samples＇ integrity quite well.

Evaluation of clayed silt properties on the behavior of hydrate production in South China Sea

Gas hydrate is one kind of potential energy resources that is buried under deep seafloor or frozen areas.The first trial offshore production from the silty reservoir was conducted in the South China Sea by the China Geological Survey(CGS).During this test,there were many unique characteristics different from the sand reservoir,which was believed to be related to the clayed silt physical properties.In this paper,simulation experiments,facilities analysis,and theoretical calculation were used to confirm the hydrate structure,reservoir thermo-physical property,and bond water movement rule.And the behavior of how they affected production efficiency was analyzed.The results showed that:It was reasonable to use the structure I rather than structure II methane hydrate phase equilibrium data to make the production plan;the dissociation heat absorbed by hydrate was large enough to cause hydrate self-protection or reformation depend on the reservoir thermal transfer and gas supply;clayed silt got better thermal conductivity compared to coarse grain,but poor thermal convection especially with hydrate;clayed silt sediment was easy to bond water,but the irreducible water can be exchanged to free water under high production pressure,and the most obvious pressure range of water increment was 1.9–4.9 MPa.

Application of wellhead suction anchor technology in the second production test of natural gas hydrates in the South China Sea

Traditional suction anchor technology is mainly used in the fields of subsea structure bearing foundations,single-point mooring systems and offshore wind power.It is characterized by providing sufficient lateral and vertical bearing capacities and lateral bending moment.The anchor structure of a traditional suction anchor structure is improved with wellhead suction anchor technology,where a central pipe is added as a channel for drilling and completion operations.To solve the technical problems of a low wellhead bearing capacity,shallow built-up depth,and limited application of conductor jetting in the second production test of natural gas hydrates(NGHs)in the South China Sea(SCS),the China Geological Survey(CGS)took the lead in independently designing and manufacturing a wellhead suction anchor,which fulfilled the requirements of the production test.This novel anchor was successfully implemented in the second production test for the first time,providing a stable wellhead foundation for the success of the second production test of NGHs in the SCS.

Assessment of natural gas hydrate reservoirs at Site GMGS3-W19 in the Shenhu area,South China Sea based on various well logs

To obtain the characteristics of the gas hydrate reservoirs at GMGS3-W19,extensive geophysical logging data and cores were analyzed to assess the reservoir properties.Sediment porosities were estimated from density,neutron,and nuclear magnetic resonance(NMR)logs.Both the resistivity and NMR logs were used to calculate gas hydrate saturations,the Simandoux model was employed to eliminate the effects of high clay content determined based on the ECS and core data.The density porosity was closely in agreement with the core-derived porosity,and the neutron porosity was higher while the NMR porosity was lower than the density porosity of sediments without hydrates.The resistivity log has higher vertical resolution than the NMR log and thus is more favorable for assessing gas hydrate saturation with strong heterogeneity.For the gas hydrate reservoirs at GMGS3-W19,the porosity,gas hydrate saturation and free gas saturation was 52.7%,42.7%and 10%,on average,respectively.The various logs provide different methods for the comprehensive evaluation of hydrate reservoir,which supports the selection of candidate site for gas hydrate production testing.

Stability analysis of seabed strata and casing structure during the natural gas hydrates exploitation by depressurization in horizontal wells in South China Sea

Natural gas hydrates(NGHs)are a new type of clean energy with great development potential.However,it is urgent to achieve safe and economical NGHs development and utilization.This study established a physical model of the study area using the FLAC^(3D) software based on the key parameters of the NGHs production test area in the South China Sea,including the depressurization method,and mechanical parameters of strata,NGHs occurrence characteristics,and the technological characteristics of horizontal wells.Moreover,this study explored the law of influences of the NGHs dissociation range on the stability of the overburden strata and the casing structure of a horizontal well.The results are as follows.With the dissociation of NGHs,the overburden strata of the NGHs dissociation zone subsided and formed funnelshaped zones and then gradually stabilized.However,the upper interface of the NGHs dissociation zone showed significant redistribution and discontinuity of stress.Specifically,distinct stress concentration and corresponding large deformation occurred in the build-up section of the horizontal well,which was thus prone to suffering shear failure.Moreover,apparent end effects occurred at the end of the horizontal well section and might cause the deformation and failure of the casing structure.Therefore,it is necessary to take measures in the build-up section and at the end of the horizontal section of the horizontal well to prevent damage and ensure the wellbore safety in the long-term NGHs exploitation.

Seismic fine imaging and its significance for natural gas hydrate exploration in the Shenhu Test Area, South China Sea

Shenhu area in South China Sea includes extensive collapse and diapir structures,forming high-angle faults and vertical fracture system,which functions as a fluid migration channel for gas hydrate formation.In order to improve the imaging precision of natural gas hydrate in this area,especially for fault and fracture structures,the present work propose a velocity stitching technique that accelerates effectively the convergence of the shallow seafloor,indicating seafloor horizon interpretation and the initial interval velocity for model building.In the depth domain,pre-stack depth migration and residual curvature are built into the model based on high-precision grid-tomography velocity inversion,after several rounds of tomographic iterations,as the residual velocity field converges gradually.Test results of the Shenhu area show that the imaging precision of the fault zone is obviously improved,the fracture structures appear more clearly,the wave group characteristics significantly change for the better and the signal-to-noise ratio and resolution are improved.These improvements provide the necessary basis for the new reservoir model and field drilling risk tips,help optimize the favorable drilling target,and are crucial for the natural gas resource potential evaluation.

Identification of functionally active aerobic methanotrophs and their methane oxidation potential in sediments from the Shenhu Area,South China Sea

Large amounts of gas hydrate are distributed in the northern slope of the South China Sea,which is a potential threat of methane leakage.Aerobic methane oxidation by methanotrophs,significant methane biotransformation that occurs in sediment surface and water column,can effectively reduce atmospheric emission of hydrate-decomposed methane.To identify active aerobic methanotrophs and their methane oxidation potential in sediments from the Shenhu Area in the South China Sea,multi-day enrichment incubations were conducted in this study.The results show that the methane oxidation rates in the studied sediments were 2.03‒2.36μmol/gdw/d,which were higher than those obtained by sediment incubations from other areas in marine ecosystems.Thus the authors suspect that the methane oxidation potential of methanotrophs was relatively higher in sediments from the Shenhu Area.After the incubations family Methylococcaea(type I methanotrophs)mainly consisted of genus Methylobacter and Methylococcaea_Other were predominant with an increased proportion of 70.3%,whereas Methylocaldum decreased simultaneously in the incubated sediments.Collectively,this study may help to gain a better understanding of the methane biotransformation in the Shenhu Area.

Application of frequency division inversion in the prediction of heterogeneous natural gas hydrates reservoirs in the Shenhu Area,South China Sea

Drilling results suggest that the thickness of natural gas hydrates(NGHs)in the Shenhu Area,South China Sea(SCS)are spatially heterogenous,making it difficult to accurately assess the NGHs resources in this area.In the case that free gas exists beneath hydrate deposits,the frequency of the hydrate deposits will be noticeably attenuated,with the attenuation degree mainly affected by pore development and free gas content.Therefore,the frequency can be used as an important attribute to identify hydrate reservoirs.Based on the time-frequency characteristics of deposits,this study predicted the spatial distribution of hydrates in this area using the frequency division inversion method as follows.Firstly,the support vector machine(SVM)method was employed to study the amplitude versus frequency(AVF)response based on seismic and well logging data.Afterward,the AVF response was introduced as independent information to establish the nonlinear relationship between logging data and seismic waveform.Then,the full frequency band information of the seismic data was fully utilized to obtain the results of frequency division inversion.The inversion results can effectively broaden the frequency band,reflect the NGHs distribution,and reveal the NGHs reservoirs of two types,namely the fluid migration pathway type and the in situ self-generation self-storage diffusion type.Moreover,the inversion results well coincide with the drilling results.Therefore,it is feasible to use the frequency division inversion to predict the spatial distribution of heterogeneous NGHs reservoirs,which facilitates the optimization of favorable drilling targets and is crucial to the resource potential assessment of NGHs.

Hydrate phase transition and seepage mechanism during natural gas hydrates production tests in the South China Sea:A review and prospect

Natural gas hydrates(NGHs)are globally recognized as an important type of strategic alternative energy due to their high combustion efficiency,cleanness,and large amounts of resources.The NGHs reservoirs in the South China Sea(SCS)mainly consist of clayey silts.NGHs reservoirs of this type boast the largest distribution range and the highest percentage of resources among NGHs reservoirs in the world.However,they are more difficult to exploit than sandy reservoirs.The China Geological Survey successfully carried out two NGHs production tests in the Shenhu Area in the northern SCS in 2017 and 2020,setting multiple world records,such as the longest gas production time,the highest total gas production,and the highest average daily gas production,as well as achieving a series of innovative theoretical results.As suggested by the in-depth research on the two production tests,key factors that restrict the gas production efficiency of hydrate dissociation include reservoir structure characterization,hydrate phase transition,multiphase seepage and permeability enhancement,and the simulation and regulation of production capacity,among which the hydrate phase transition and seepage mechanism are crucial.Study results reveal that the hydrate phase transition in the SCS is characterized by low dissociation temperature,is prone to produce secondary hydrates in the reservoirs,and is a complex process under the combined effects of the seepage,stress,temperature,and chemical fields.The multiphase seepage is controlled by multiple factors such as the physical properties of unconsolidated reservoirs,the hydrate phase transition,and exploitation methods and is characterized by strong methane adsorption,abrupt changes in absolute permeability,and the weak flow capacity of gas.To ensure the long-term,stable,and efficient NGHs exploitation in the SCS,it is necessary to further enhance the reservoir seepage capacity and increase gas production through secondary reservoir stimulation based on initial reservoir stimulation.With the constant progress in the NGHs industrialization,great efforts should be made to tackle the difficulties,such as determining the micro-change in temperature and pressure,the response mechanisms of material-energy exchange,the methods for efficient NGHs dissociation,and the boundary conditions for the formation of secondary hydrates in the large-scale,long-term gas production.

Velocity-porosity relationships in hydrate-bearing sediments measured from pressure cores,Shenhu Area,South China Sea

Evaluating velocity-porosity relationships of hydrate-bearing marine sediments is essential for characterizing natural gas hydrates below seafloor as either a potential energy resource or geohazards risks.Four sites had cored using pressure and non-pressure methods during the gas hydrates drilling project(GMGS4)expedition at Shenhu Area,north slope of the South China Sea.Sediments were cored above,below,and through the gas-hydrate-bearing zone guided with logging-while-drilling analysis results.Gamma density and P-wave velocity were measured in each pressure core before subsampling.Methane hydrates volumes in total 62 samples were calculated from the moles of excess methane collected during depressurization experiments.The concentration of methane hydrates ranged from 0.3%to 32.3%.The concentrations of pore fluid(25.44%to 68.82%)and sediments(23.63%to 54.28%)were calculated from the gamma density.The regression models of P-wave velocity were derived and compared with a global empirical equation derived from shallow,unconsolidated sediments data.The results were close to the global trend when the fluid concentration is larger than the critical porosity.It is concluded that the dominant factor of P-wave velocity in hydrate-bearing marine sediments is the presence of the hydrate.Methane hydrates can reduce the fluid concentration by discharging the pore fluid and occupying the original pore space of sediments after its formation.

A 3D basin modeling study of the factors controlling gas hydrate accumulation in the Shenhu Area of the South China Sea

Great advancement has been made on natural gas hydrates exploration and test production in the northern South China Sea.However,there remains a lot of key questions yet to be resolved,particularly about the mechanisms and the controls of gas hydrates enrichment.Numerical simulaution would play signficant role in addressing these questions.This study focused on the gas hydrate exploration in the Shenhu Area,Northern South China Sea.Based on the newly obtained borehole and multichannel reflection seismic data,the authors conducted an integrated 3D basin modeling study on gas hydrate.The results indicate that the Shenhu Area has favorable conditions for gas hydrate accumulation,such as temperature,pressure,hydrocarbon source,and tectonic setting.Gas hydrates are most concentrated in the Late Miocene strata,particularly in the structual highs between the Baiyun Sag and the Liwan Sag,and area to the south of it.It also proved the existence of overpressure in the main sag of source rocks,which was subject to compaction disequilibrium and hydrocarbon generation.It also shown that the regional fault activity is not conducive to gas hydrate accumulation due to excess gas seepage.The authors conjecture that fault activity may slightly weaken overpressure for the positive effect of hydrocarbon expulsion and areas lacking regional fault activity have better potential.