Deep-water riser fatigue monitoring systems based on acoustic telemetry

Marine risers play a key role in the deep and ultra-deep water oil and gas production. The vortex-induced vibration (VIV) of marine risers constitutes an important problem in deep water oil exploration and production. VIV will result in high rates of structural failure of marine riser due to fatigue damage accumulation and diminishes the riser fatigue life. In-service monitoring or full scale testing is essential to improve our understanding of VIV response and enhance our ability to predict fatigue damage. One marine riser fatigue acoustic telemetry scheme is proposed and an engineering prototype machine has been developed to monitor deep and ultra-deep water risers’ fatigue and failure that can diminish the riser fatigue life and lead to economic losses and eco-catastrophe. Many breakthroughs and innovation have been achieved in the process of developing an engineering prototype machine. Sea trials were done on the 6th generation deep-water drilling platform HYSY-981 in the South China Sea. The inclination monitoring results show that the marine riser fatigue acoustic telemetry scheme is feasible and reliable and the engineering prototype machine meets the design criterion and can match the requirements of deep and ultra-deep water riser fatigue monitoring. The rich experience and field data gained in the sea trial which provide much technical support for optimization in the engineering prototype machine in the future.     

ROV based underwater blurred image restoration

1　Introduction　TheROV (RemoteOperatedVehicle)isakindofsystemthatcanbeusedforunderwatermeasurementanddetection (Caimi,1996 ;KevernandLeGall,1991) .Inthispaper ,theirradianceofthelightre flectedbythetargetthroughwaterbodiesindifferentconditionsissimulatedbyacomputer .Underdiffer entwaterconditions,therelationbetweentheirradi anceandthedistanceispresented .Thenthemaxi mumdetectiondistanceofthedetectorcanbeob tained .WealsorestoretheunderwaterblurryimagesusingtheWienerfilterbasedonthesimula…     

Analysis of the Dynamic System of Wave Glider with a Towed Body

Mobile observation platforms are widely used in oceanographic and marine resource exploration and other applications. Wave Glider is a mobile platform that can transform wave energy into locomotion power and overcome the bottleneck of low energy supply. Wave Glider has recently been applied to tow underwater sensors fulfilling observation tasks. In this paper, the dynamic system of Wave Glider with a towed body is studied by applying multibody mechanics, and the relevant motion conditions of the system are investigated. Dynamic models of Wave Glider with a towed body and tether are first developed individually and then integrated into a whole model. The numerical method is used to obtain the dynamic responses and assess performance of the towed body pulled by the submerged glider of Wave Glider. The effects of sea state, mass of the towed body, and length of the towed cable are investigated on the basis of simulation results. This work can be used for the design and analysis of Wave Glider-towed body systems.     

Underwater topography detection of Taiwan Shoal with SAR images

Under suitable conditions of tidal current and wind, underwater topography can be detected by synthetic aperture radar (SAR) indirectly. Underwater topography SAR imaging includes three physical processes: radar ocean surface backscattering, the modulation of sea surface short wave spectrum by the variations in sea surface currents, and the modulation of sea surface currents by the underwater topography. The first process is described usually by Bragg scattering theory because the incident angle of SAR is always between 20°–70°. The second process is described by the action balance equation. The third process is described by an ocean hydrodynamic model. Based on the SAR imaging mechanism for underwater topography, an underwater topography SAR detection model and a simplified method for its calculation are introduced. In the detection model, a two-dimensional hydrodynamic model — the shallow water model is used to describe the motion of tidal current. Due to the difficulty of determining the expression of SAR backscattering cross section in which some terms can not be determined, the backscattering cross section of SAR image used in the underwater topography SAR detection is pro-processed by the simulated SAR image of the coarse-grid water depth to simplify the calculation. Taiwan Shoal, located at the southwest outlet of Taiwan Strait, is selected as an evaluation area for this technique due to the occurrence of hundreds of sand waves. The underwater topography of Taiwan Shoal was detected by two scenes of ERS-2 SAR images which were acquired on 9 January 2000 and 6 June 2004. The detection results are compared with in situ measured water depths for three profiles. The average absolute and relative errors of the best detection result are 2.23 m and 7.5 %, respectively. These show that the detection model and the simplified method introduced in the paper is feasible.     

Elimination of the impact of vessels on ocean wave height inversion with X-band wave monitoring radar

Directional wave spectra and integrated wave parameters can be derived from X-band radar sea surface images.A vessel on the sea surface has a significant influence on wave parameter inversions that can be seen as intensive backscatter speckles in X-band wave monitoring radar sea surface images.A novel algorithm to eliminate the interference of vessels in ocean wave height inversions from X-band wave monitoring radar is proposed.This algorithm is based on the characteristics of the interference.The principal components(PCs) of a sea surface image sequence are extracted using empirical orthogonal function(EOF)analysis.The standard deviation of the PCs is then used to identify vessel interference within the image sequence.To mitigate the interference,a suppression method based on a frequency domain geometric model is applied.The algorithm framework has been applied to OSMAR-X,a wave monitoring system developed by Wuhan University,based on nautical X-band radar.Several sea surface images captured on vessels by OSMAR-X are processed using the method proposed in this paper.Inversion schemes are validated by comparisons with data from in situ wave buoys.The root-mean-square error between the significant wave heights(SWH) retrieved from original interference radar images and those measured by the buoy is reduced by 0.25 m.The determinations of surface gravity wave parameters,in particular SWH,confirm the applicability of the proposed method.     

The Coupling VIV Analysis of SCRs with Rigid Swing

With the development of deepwater oil and gas exploration, Steel Catenary Risers(SCRs) become preferred risers for resource production, import and export. Vortex induced vibration(VIV) is the key problem encountered in the design of SCRs. In this study, a new model, the rigid swing model, is proposed based on the consideration of large curvature of SCRs. The sag bend of SCRs is assumed as a rigid swing system around the axis from the hanging point to the touch down point(TDP) in the model. The torque, produced by the lift force and the swing vector, provides the driving torque for the swing system, and the weight of SCRs provides the restoring torque. The simulated response of rigid swing is coupled with bending vibration, and then the coupling VIV model of SCRs is studied in consideration of bending vibration and rigid motion. The calculated results indicate that the rigid swing has a magnitude equal to that of bending vibration, and the rigid motion affects the dynamic response of SCRs and can not be neglected in the VIV analysis.     

Progressive modelling of the gravity-induced landslide using the local dynamic strength reduction method

The failure of slope is a progressive process, and the whole sliding surface is caused by the gradual softening of soil strength of the potential sliding surface. From this viewpoint, a local dynamic strength reduction method is proposed to capture the progressive failure of slope. This method can calculate the warning deformation of landslide in this study. Only strength parameters of the yielded zone of landslide will be reduced by using the method. Through continuous local reduction of the strength parameters of the yielded zone, the potential sliding surface developed gradually and evolved to breakthrough finally. The result shows that the proposed method can simulate the progressive failure of slope truly. The yielded zone and deformation of landslide obtained by the method are smaller than those of overall strength reduction method. The warning deformation of landslide can be obtained by using the local dynamic strength reduction method which is based on the softening characteristics of the sliding surface.     

Seasonal-to-interannual variability of chlorophyll in central western South China Sea extracted from SeaWiFS

Using 10-year (January 1998–October 2007) dataset of Sea-viewing Wide Field-of-view Sensor (SeaWiFS), we extracted the dominant spatial patterns and temporal variations of the chlorophyll distribution in the central western South China Sea (SCS) through Empirical Orthogonal Function (EOF) analysis. The results show that the first EOF mode is characterized by a high chlorophyll concentration zone along the Vietnam coast. We found two peaks in summer (July–August) and in winter (December), respectively, in no...     

用前期海温预报四川夏季气温的EOF迭代方案

应用ＥＯＦ（经验正交函数）迭代方案，考虑多时次历史资料，在全球海区海温与四川盆地气温非同步联系的基础上，以海温为预报因子进行了夏季气温的长期预报。结果表明：西太平洋高温区等关键海区海温的异常对未来四川盆地夏季气温变化有重要影响，由此建立的引入多时次海温的ＥＯＦ迭代长期温度预报方法，具有较强的预报能力     

Combined Bearing Capacity of Spudcans on a Double Layer Deposit of Strong-Over-Weak Clays

An extreme sea storm process can lead to a jack-up rig under the combined loading condition of vertical load(V), horizontal load(H), and moment(M) to have stability problems. This paper presents the analysis of combined bearing capacities of a circular spudcan on layered clays with a strong layer overlying a comparatively weaker layer. Numerical models combined with displacement-based load tests, swipe tests, and constant ratio displacement probe tests are adopted to calculate the uniaxial bearing capacities, failure envelopes in combined V-H, V-M planes, and failure envelopes in a combined V-H-M load space, respectively. A parametric study on the effects of vertical load level V, the layer strength ratio s_(u,t)/s_(u,b), and the hard layer thickness t_1 on the bearing capacities is then performed. Results show that the vertical load level is a key factor that influences the values of H and M and the size of the H-M failure envelope. The existence of the underlying weak clay decreases the bearing capacities in all directions, and the vertical capacity Vult is affected more than the horizontal(H_(ult)) and moment(M_(ult)) capacities based on a single uniform deposit. The influence of the underlying weak clay on H-M failure envelope is mainly shown where H and M are coupled in the same direction. In contrast, little difference is observed when H and M are coupled in opposite directions.     

Predictability of Chinese Summer Extreme Rainfall Based on Arctic Sea Ice and Tropical Sea Surface Temperature

Chinese summer extreme rainfall often brings huge economic losses, so the prediction of summer extreme rainfall is necessary. This study focuses on the predictability of the leading mode of Chinese summer extreme rainfall from empirical orthogonal function(EOF) analysis. The predictors used in this study are Arctic sea ice concentration(ASIC) and regional sea surface temperature(SST) in selected optimal time periods. The most important role that Arctic sea ice(ASI) plays in the appearance of EOF1 may be strengthening the high pressure over North China, thereby preventing water vapor from going north. The contribution of SST is mainly at low latitudes and characterized by a significant cyclone anomaly over South China. The forecast models using predictor ASIC(PA), SST(PS), and the two together(PAS) are established by using data from 1980 to 2004. An independent forecast is made for the last 11 years(2005-2015). The correlation coefficient(COR) skills between the observed and cross-validation reforecast principal components(PC) of the PA, PS, and PAS models are 0.47, 0.66, and 0.76, respectively. These values indicate that SST is a major cause of Chinese summer extreme rainfall during 1980-2004. The COR skill of the PA model during the independent forecast period of 2004-2015 is 0.7, which is significantly higher than those of the PS and PAS models. Thus, the main factor influencing Chinese summer extreme rainfall in recent years has changed from low latitudes to high latitudes. The impact of ASI on Chinese summer extreme rainfall is becoming increasingly significant.     

Study on dynamic characteristics of coupled model for deep-water lifting system

The underwater installation of marine equipment in deep-water development requires safe lifting and accurate positioning. The heave compensation system is an important technology to ensure normal operation and improve work accuracy. To provide a theoretical basis for the heave compensation system, in this paper, the continuous modeling method is employed to build up a coupled model of deep-water lifting systems in vertical direction. The response characteristics of dynamic movement are investigated. The simulation results show that the resonance problem appears in the process of the whole releasing load, the lifting system generates resonance and the displacement response of the lifting load is maximal when the sinking depth is about 2000 m. This paper also analyzes the main influencing factors on the dynamic response of load including cable stiffness, damping coefficient of the lifting system, mass and added mass of lifting load, among which cable stiffness and damping coefficient of the lifting system have the greatest influence on dynamic response of lifting load when installation load is determined. So the vertical dynamic movement response of the load is reduced by installing a damper on the lifting cable and selecting the appropriate cable stiffness.     

MEAN SEA LEVEL AND SEA SURFACE VARIABILITY OF NQRTHWEST PACIFIC OCEAN AND EASTERN CHINA SEAS FROM GEOSAT ALTIMETRY

Collinear analysis technique is widely used for determining sea surface variability with Geosat altimeterdata from its Exact Repeat Mission(ERM).But most of the researches have been only on global scaleor in oceans deeper than 2000 m.In shallow shelf waters this method is hampered by the inaccuracy ofocean tide data supplied with Geosat Geophysical Data Records(GDRs).This work uses a modified collinearanalysis technique characterized by simultaneous separation of mean sea level and ocean tide with theleast squares method,to compute sea surface variability in the Northwest Pacific Ocean and eastern ChinaSeas.The mean sea level map obtained contains not ouly bathymetric but also dynamic features such asamphidromes,indicating considerable improvement over previous works.Our sea surface variability mapsshow clearly the main current system,the well-known Zhejiang coastal upwelling,and a northern East Chi-na Sea meso-scale eddy in good agreement with satellite sea surface temperature(SST)observation and his-to     

Dominant modes of geopotential height in the northern hemisphere in summer on interdecadal timescales

Empirical orthogonal function(EOF) analysis is performed on the field of the northern hemisphere geopotential height at 200-hPa using a 54-year(1958-2011) record of summer data on an interdecadal time scale.The first dominant mode,which shows smooth semi-hemispheric variation with maximum action centers in the western hemisphere in the mid-latitudes over the eastern Pacific,North America,and the North Atlantic,is related to global warming.The second mode,which has a pronounced tropical-extratropical alternating pattern with active centers located over the eastern hemisphere from Western Europe across East Asia to the western Pacific,has a close relationship with the Arctic Oscillation.Further analysis results indicate that the two dominant modes show good correlation with the Arctic sea ice concentration(SIC),with correlation coefficients between these two modes and the first two EOF modes of the Arctic SIC reaching 0.88 and 0.86,respectively.     

Analysis of typhoon wind hazard in Shenzhen City by Monte-Carlo Simulation

As one of the most serious natural disasters, many typhoons affect southeastern China every year. Taking Shenzhen, a coastal city in southeast China as an example, we employed a Monte-Carlo simulation to generate a large number of virtual typhoons for wind hazard analysis. By analyzing 67-year historical typhoons data from 1949 to 2015 using the Best Track Dataset for Tropical Cyclones over the Western North Pacific recorded by the Shanghai Typhoon Institute, China Meteorological Administration(CMASTI), typhoon characteristic parameters were extracted and optimal statistical distributions established for the parameters in relation to Shenzhen. We employed the Monte-Carlo method to sample each distribution to generate the characteristic parameters of virtual typhoons. In addition, the Yan Meng(YM) wind field model was introduced, and the sensitivity of the YM model to several parameters discussed. Using the YM wind field model, extreme wind speeds were extracted from the virtual typhoons. The extreme wind speeds for different return periods were predicted and compared with the current structural code to provide improved wind load information for wind-resistant structural design.     

不同时间尺度的西北太平洋海洋表面温度距平模态的比较

本文利用经验正交分解法(Empirical Orthogonal Function,EOF)对不同月平均和10天平均的西北太平洋海洋表面温度距平数据进行分解,得到月平均和10天平均的西北太平洋海洋表面温度距平分布模态,并对分布模态进行比较。发现尽管数据源相同,但是不同时间分辨率的EOF结果出现正好相反的情况,文章最后从数学的角度解释了月平均和10天平均的海洋表面温度距平模态及其时间系数发生反向的原因。     

Numerical Investigation of Vortex-induced Vibrations for Flow past a Circular Cylinder

The objective of the present investigation is to study the vortex-induced vibrations （VIV） for flow past a circular cylinder. The turbulent flow is simulated by using a 2-D standard k-ε model incorporating the finite volume method （FVM） and the Semi-Implicit Method for the Pressure Linked Equations （SIMPLE） algorithm on non-orthogonal boundary-fitted collocated grids. The wall boundaries are approximated with wall functions. In the numerical cases, the turbulent wake patterns are studied by plotting the streamlines and the turbulent kinetic energy contours. The pressure distributions are investigated. Analyses of the vortex-induced force coefficients and the structural vibrations are carried out. The variations of the Strouhal number with the Reynolds number and of the vortex-induced force coefficients with the reduced velocity are obtained. The results show that this numerical approach is feasible and efficient in investigating the VIV problem for a circular cylinder.     

EQUILIBRIUM COASTAL PROFILES:Ⅱ. EVIDENCE FROM EOF ANALYSIS

IamODUCTIoNInoIdertoestablishtheconceptofeqUilibriumforbeachprofiles,anuInberofproh-lemsmustbesolved,including:(i)pndoftheealstehceofeqUilibrium;(ii)definitionoftheshapeofaneqUilibriumcOaStalprofile;and(iii)evaluationofthePeriodoftimere-qUiredtoreachtheeqUilibritunstate(knownastheresPOnsetime).Forthefirstoftheseprohlems,areviewofpreviousinvestigationsandaSynhesisarepresentedelsewhere(GaoandCollins,1998).Accowhngtosuchtheoreticalconsidera-tions,threetwrsofeqUilibriurnInayoccurinnatllT…     

冬夏季热带太平洋至印度洋次表层海温变化的模态特征

采用美国Scripps海洋研究所的1955—1998年全球海洋上层海水温度月距平资料，对热带太平洋至印度洋各层海温进行经验正交函数分解，分析其主要模态特征。结果表明：热带太平洋至印度洋次表层海温场主要表现出东、西太平洋海温异常反位相变化的特征，异常强度冬季明显强于夏季。冬季赤道东太平洋40m层，东印度洋至西太平洋120m层，夏季赤道东太平洋40m层，东印度洋至西太平洋160m层为海温异常的显著区域。冬季0—60m层第一特征向量表现出厄尔尼诺（拉尼娜）模态特征，第二特征向量表现出海温异常的东西运移模态特征，80—400m层第一特征向量表现出西太平洋暖池模态特征，第二特征向量表现出海温异常的东西运移模态特征。夏季0—60m层特征向量表现出厄尔尼诺（拉尼娜）模态，80—400m层特征向量表现出西太平洋暖池模态特征。     

Interannual to decadal variation of spring sea level anomaly in the western South China Sea

Satellite observations of sea level anomalies(SLA) from January 1993 to December 2012 are used to investigate the interannual to decadal changes of the boreal spring high SLA in the western South China Sea(SCS) using the Empirical Orthogonal Function(EOF) method. We find that the SLA variability has two dominant modes. The Sea Level Changing Mode(SLCM) occurs mainly during La Ni?a years, with high SLA extension from west of Luzon to the eastern coast of Vietnam along the central basin of the SCS, and is likely induced by the increment of the ocean heat content. The Anticyclonic Eddy Mode(AEM) occurs mainly during El Ni?o years and appears to be triggered by the negative wind curl anomalies within the central SCS. In addition, the spring high SLA in the western SCS experienced a quasi-decadal change during 1993–2012; in other words, the AEM predominated during 1993–1998 and 2002–2005, while the La Ni?a-related SLCM prevailed during 1999–2001 and 2006–2012. Moreover, we suggest that the accelerated sea level rise in the SCS during 2005–2012 makes the SLCM the leading mode over the past two decades.