Reliability-based design optimization of an FPSO riser support using moving least squares response surface meta-models

The paper deals with the reliability-based design optimization (RBDO) of a riser support installed on a floating production storage and offloading (FPSO) unit under operation, extreme, damaged, and one line failure cases and installation loading conditions. The optimization problem is formulated such that probabilistic thickness variables described with random characteristics are determined by minimizing the weight of the riser support structure subjected to stress constraints for the given target reliability. The initial design model is generated based on actual FPSO riser support specifications. The finite element analysis is conducted using NASTRAN, and the probabilistic optimal solutions are obtained via the moving least squares method in the context of RBDO using a response surface meta-model. For the meta-modeling of the inequality constraint functions of stresses, a constraint-feasible moving least squares method (CF-MLSM) is adopted in the present study. The CF-MLSM has been shown to ensure constraint feasibility regardless of the nonlinearity of the constraint function, the feasible bounds, and the random characteristics during the meta-model-based RBDO process. The solution results from the proposed RBDO strategy present improved design performances under various riser operating conditions.     

Computation of calibration parameters for multibeam echo sounders using the least squares method

A new method for field calibration of multibeam echo sounders is presented. The method applies least-squares adjustment computation of static offsets in parameters such as roll, pitch, heading and the horizontal shift of the transducer relative to the reference point of the positioning system. All the parameters are derived simultaneously. An investigation demonstrates the precision and the flexibility of the method. Flexibility is here related to the design of calibration lines, i.e., their number, their direction and length. The strength of the method is its ability to utilize all the depth measurements in an optimal way. The variance of the parameters can be derived directly from least-squares adjustment, but limited knowledge of the stochastic properties of the observations can lead to optimistic precision estimates. An alternative method to compute quality estimates is proposed and demonstrated.     

基于小波分解和最小二乘支持向量机的ENSO集成预测

用小波分解和最小二乘支持向量机相结合的方法,建立了ENSO的集成预报模型。该方法将复杂海温系统分解为相对简单的带通分量信号,然后建立分量信号的独立预报模型,最后对预报结果进行集成。试验结果表明,模型在保留预报对象主要特征的前提下,有效地降低了预报难度,集成预报准确率和预报时效均较传统方法有明显的改进和提高。     

Comparison of Kalman and least squares filters for locatingautonomous very low frequency acoustic sensors

The Marine Physical Laboratory has designed, fabricated, and taken to sea self-contained, freely drifting acoustic sensors which can measure signal propagation and ambient ocean noise in the 1-20-Hz band for up to 25-hour periods. The deployment of several freely drifting floats forms an array of sensors whose outputs can be combined after the experiment with a beamformer. A Kalman filter and a least-squares estimator have been developed to estimate float positions from travel-time measurements. Computer simulation is used to compare filter performance-under several deployment scenarios. Results show that the Kalman filter performs better than the least-squares filter when the floats are subjected to small-magnitude accelerations between measurements. Neither filter was sensitive to relatively major changes in deployment geometry as long as the sound-speed profile is known exactly     

Global ocean circulation patterns based on seasat altimeter data and the GEML2 gravity field

The Seasat altimeter data has been completely adjusted by a crossing arc technique to reduce the crossover discrepancies to approximately ±30 cm in five regional adjustments. This data was then used to create sea surface heights at 1° intersections in the ocean areas with respect to the GRS80 ellipsoid. These heights excluded the direct tidal effects but included the induced permanent deformation. A geoid corresponding to these sea surface heights was computed, based on the potential coefficients of the GEML2 gravity field up to degree 6, augmented by Rapp's coefficients up to degree 180. The differences between sea surface heights and the geoid were computed to give approximate estimates of sea surface topography. These estimates are dominated by errors in both sea surface heights and geoid undulations. To optimally determine sea surface topography a spherical harmonic analysis of raw estimates was carried out and the series was further truncated at degree 6, giving estimates with minimum wavelengths on the order of 6000 km. The direction of current flow can be computed on a global basis using the spherical harmonic expansion of the sea surface topography. Ths has been done, not only for Seasat/GEML2 estimates, but also using the recent dynamic topography estimates of Levitus. The results of the two solutions are very similar and agree well with the major circulation features of the oceans.     

Tidal estimation in the pacific with application to SEASAT altimetry

Abstract

The techniques for computing the eigenfunctions of the velocity potential (Proudman functions) set out in Sanchez et al. (1986) in relation to the Atlantic‐Indian Ocean are here applied to the Pacific Ocean, using a 6° × 6° grid of 510 points (455 points for the associated stream functions). Normal modes are computed from the first 150 Proudman functions and have natural periods from 43.9 hours downwards. Tidal syntheses are derived from these modes by direct application of the (frictionless) dynamic equations and by least‐squares fitting of Proudman functions to the dynamically interpolated tide‐gauge data of Schwiderski (1983). The modes contributing most energy to the principal harmonic tidal constituents are different in the two computations; their natural periods are typically in the range of 9–16 hours for semidiurnal, 14–43 hours for diurnal tides. The RMS of fit for Proudman functions is in all cases better than the corresponding value for the same number of spherical harmonics.

Before fitting the Proudman functions to the altimetry from the 3‐day repeat cycle of Seasat, the data are processed by novel methods. The geoid component is eliminated by taking collinear differences at a fixed time‐lag of two repeat cycles. Orbit errors are reduced by extracting the 1 rev^?1 component at every ascending node; this component varies slowly and nonlinearly in time. The spatial fitting process includes M₂ and O₁ frequencies, both of which emerge with significant and realistic tidal mapping, but residual noise in the data limits the number of Proudman functions to about 50–60 before showing signs of “over‐fitting.”; Fitting the same data by spherical harmonics gives marginally lower predicted variance for the same number of parameters.     

A method of simultaneous analysis of wind and temperature profile data over the water surface by use of the method of least squares

Priestley’s 1959 discussion concerning the estimations of momentum flux and of heat flux from profile observations of wind and temperature, stating that the closeness of the curve fitting is deceptive, is first critically re-examined. Then a method is proposed to estimate vertical flux of momentum and heat over the water surface, from the combined data of of wind speed and temperature are assumed to have similar log-linear profiles, and the most probable profiles are determined by applying the method of least squares simultaneously to wind and temperature data. Consequently, the most probable values of vertical flux of momentum and heat may be estimated to satisfy as much as possible observed data of both wind and temperature simultaneously. The coefficient of the linear term of the log-linear profile, which is treated as an indeterminate coefficient in this method, may be determined from each observed data as a function of stability length. By tentatively applying the method to Rider’s 1954 data it is found that the coefficient shows a characteristic behavior with the stability length.     

Applications of satellite altimetry to oceanography and geophysics

Satellite-borne altimeters have had a profound impact on geodesy, geophysics, and physical oceanography. To first order approximation, profiles of sea surface height are equivalent to the geoid and are highly correlated with seafloor topography for wavelengths less than 1000 km. Using all available Geos-3 and Seasat altimeter data, mean sea surfaces and geoid gradient maps have been computed for the Bering Sea and the South Pacific. When enhanced using hill-shading techniques, these images reveal in graphic detail the surface expression of seamounts, ridges, trenches, and fracture zones. Such maps are invaluable in oceanic regions where bathymetric data are sparse. Superimposed on the static geoid topography is dynamic topography due to ocean circulation. Temporal variability of dynamic height due to oceanic eddies can be determined from time series of repeated altimeter profiles. Maps of sea height variability and eddy kinetic energy derived from Geos-3 and Seasat altimetry in some cases represent improvements over those derived from standard oceanographic observations. Measurement of absolute dynamic height imposes stringent requirements on geoid and orbit accuracies, although existing models and data have been used to derive surprisingly realistic global circulation solutions. Further improvement will only be made when advances are made in geoid modeling and precision orbit determination. In contrast, it appears that use of altimeter data to correct satellite orbits will enable observation of basin-scale sea level variations of the type associated with climatic phenomena.     

黄海、渤海TOPEX/Poseidon高度计资料潮汐伴随同化

首先将大约10a的TOPEX/Poseidon(T/P)高度计资料沿星下轨迹点做潮汐调和分析,提取得到各分潮的调和常数,利用伴随同化方法,同化到二维非线性潮汐数值模式中,模拟了黄海、渤海区域M₂,S₂,O₁,K₁等4个潮汐分潮,并根据计算结果给出了各分潮的同潮图.将计算值与观测值的进行偏差统计,结果表明计算值与验潮站资料符合良好.研究过程中做了两类试验:一类试验是针对不同的参数进行优化,一类试验是针对不同的资料进行同化.第一类试验表明:将开边界条件和底摩擦系数同时作为模型优化的控制参数,其结果明显优于单独优化开边界条件;第二类试验表明:同时同化高度计资料与验潮站资料,比单独同化其中任一种资料,对模式计算结果都有较好的改进.研究结果表明,采用伴随同化方法,利用T/P高度计资料和验潮站资料作为同化数据能有效改进模拟结果,用来反演黄海、渤海的潮波系统是可行的.     

Collocation resolution of the oceanic geoid based on satellite altimetry

The approach presented is directed toward a specific adaptation of the least‐squares collocation with noise, yielding smooth predictions of geophysical quantities. The smoothing corresponds here to a truncated gravity field equivalent to an (n’, n') spherical‐harmonic expansion. This is reflected in the truncation, at the degree n‘, of the pertinent covariance and cross‐covariance functions in most (but not all) instances. The smooth predictions of geophysical quantities, made in an equilateral grid corresponding to the truncation degree n‘, serve in constructing contour maps after having been densified for the needs of a contour routine. Such a densification is carried out efficiently via errorless collocation with the degree truncation n‘ throughout. Consistent with this procedure, “residuals” at observation points (i.e., discrepancies between the contour map and the data) are computed using the same algorithm. The complete collocation approach is utilized for a 2° resolution of the earth's gravity field with emphasis on the oceanic geoid, based on the residuals from a global spherical‐harmonic adjustment of SEASAT altimetry. The presented results include contour maps of geoid undulations and gravity anomalies. They are compared to the results of a point‐mass adjustment, another technique based on the spherical‐harmonic adjustment. The agreement between these two techniques is found to be excellent.     

基于多源融合卫星高度计观测数据的南海海浪有效波高的季节变化研究

The seasonal variability of the significant wave height(SWH) in the South China Sea(SCS) is investigated using the most up-to-date gridded daily altimeter data for the period of September 2009 to August 2015. The results indicate that the SWH shows a uniform seasonal variation in the whole SCS, with its maxima occurring in December/January and minima in May. Throughout the year, the SWH in the SCS is the largest around Luzon Strait(LS) and then gradually decreases southward across the basin. The surface wind speed has a similar seasonal variation, but with different spatial distributions in most months of the year. Further analysis indicates that the observed SWH variations are dominated by swell. The wind sea height, however, is much smaller. It is the the largest in two regions southwest of Taiwan Island and southeast of Vietnam Coast during the northeasterly monsoon, while the largest in the central/southern SCS during the southwesterly monsoon. The extreme wave condition also experiences a significant seasonal variation. In most regions of the northern and central SCS, the maxima of the 99 th percentile SWH that are larger than the SWH theoretically calculated with the wind speed for the fully developed seas mainly appear in August–November, closely related to strong tropical cyclone activities.Compared with previous studies, it is also implied that the wave climate in the Pacific Ocean plays an important role in the wave climate variations in the SCS.     

几种卫星测高海洋重力场模型精度比较分析

以中国南海部分海域作为试验区,依据测区船载重力测量结果分别对S&S V28.1、S&S V29.1、DTU15和DTU17卫星测高海洋重力场模型开展了精度评估与分析工作。首先利用EIGEN6C4超高阶位系数模型计算得到的参考重力异常,使用多项式拟合技术对船载重力异常进行了长波误差改正;然后借鉴3σ准则进一步控制船载重力测量结果质量;最后以RMS作为精度主要评价指标,完成了对4种海洋重力场模型精度评估。结果表明,4种海洋重力场模型检核精度整体相当,其中S&S V29.1检核精度略高于S&S V28.1,4种重力异常模型在试验海域精度大致在1.5~6mGal之间。试验结论对于后续重力场模型应用具有一定参考价值。     

Reconstruction of mean dynamic topography of the Black Sea for altimetry measurements

In this study a new approach for reconstructing the Mean Dynamic Topography of the Black Sea is applied. Constant observations (SVP measurements), drifters, and data of vertical sounding of the temperature and salt content together with measurements of sea level anomalies received from Topex/Poseidon mission satellite data were used. The absolute sea level received by altimetry data using the mean dynamic topography received during work was compared to the dynamic level received according to independent marine surveys. The comparison showed that the method represented in the study permits one to define more exactly the dynamic topography of the Black Sea when compared with the studies of previous authors. The results of this study will be useful to reconstruct the areas of the geostrophic currents according to satellite altimetry.     

M2 model of the global ocean tide derived from SEASAT altimetry

Abstract

The recovery of ocean tides from satellite altimetry, an attractive alternative to the hydrodynamical‐numerical approach, is investigated to create a global model of the M2 tide. From the outline of the difficulties faced in altimetry interpretation, we bring out general guidelines to extract the tidal information from a short span of measurements. In particular, we discuss the choice of a reference surface and the effect of the orbit error and tidal aliasing on the recovery. From space‐time harmonic analyses of twenty‐four days of SEASAT altimetry, we derive M2 solutions expanded into series of surface spherical harmonics for the Indian, Pacific, and Atlantic Oceans separately and for the world ocean. The M2 cotidal maps we obtain feature qualitatively realistic tidal patterns and are consistent with the deep sea gages data. We then cast the bases to estimate the error budget of the altimeter tide solutions. The M2 fundamental harmonics involved in tidal energetics are evaluated from a spectral convolution of the global solutions with the ocean function and are used to test and discuss our results.

The present tidal recoveries must still be considered as preliminary trials because they are strongly dependent on the limits of the SEASAT mission and subject to improvements via an updating of our analysis procedure. But the altimeter approach of the open ocean tide modelling proves to be efficient, and the objective—to produce highly reliable models with the support of the next generation of satellite altimeters—is reasonably optimistic.     

Evaluation of tidal error in altimetry data in the Asian marginal seas

The magnitude and geographical distribution of the error in the Archiving, Validation and Interpretation of Satellite Oceanographic data (AVISO) altimetry data associated with tidal correction around Asian marginal seas has been revealed. The errors were evaluated by harmonic analysis of the AVISO corrected sea surface heights data (CorSSH). Errors of more than 15 cm of tidal correction were recognized in the western and northern parts of the Yellow Sea, Celebes Sea, Kuril Islands, and the northwestern part of the Okhotsk Sea. It was found that the CorSSH and sea level anomaly (SLA) data downloaded from the AVISO are not available for direct use in those marginal seas. To reduce the tidal correction error, the harmonic constants calculated from the latest tide model and regional tide model were applied as the tidal correction of the Altimetry data. The tidal errors in the Yellow Sea and the northwestern part of the Okhotsk Sea were reduced by approximately 20 cm and 10 cm, respectively. Root mean square differences between the harmonic constants derived from tide models and those derived from altimetry data were calculated. The root mean square differences were large in the Yellow and the Okhotsk Seas. Root sum squares for four principal tidal constituents in the Yellow and East China Seas and Okhotsk Sea were 7.72 cm and 8.36 cm, respectively.     

Seasonal variability of geostrophic currents in the Atlantic Ocean according to the altimetry data

The monthly average values of the anomalies of the ocean level (according to the satellite data for 1992–2002) and the annual average dynamic heights (hydrological data) are used to compute the seasonal cycle of geostrophic currents on the surface of the Atlantic Ocean. It is shown that the west and east currents are intensified with a phase difference of several months. At the same time, their latitudinal displacements are quasisynchronous. A delay of the seasonal signal in the east-west direction of about 2–3 months (on the average) is typical of currents in the tropical zone of the Northern Hemisphere. On the contrary, in the South Atlantic, the seasonal signal propagates in the west-east direction and its phase delay can be as large as almost six months. __________ Translated from Morskoi Gidrofizicheskii Zhurnal, No. 3, pp. 60–71, May–June, 2006.     

南大洋太平洋扇区中尺度涡旋的统计特性及其变化

中尺度涡旋在南大洋海洋动力学中具有重要地位,其对气候变化的响应表现也引起了海洋学家与气候学家的广泛关注。本文利用涡动动能与涡旋自动探测技术两种方法对南大洋太平洋扇区的涡旋特性及其变化进行了分析。与前人结果相一致的是,高值的涡动动能主要集中在南极极锋海区,并且自西向东逐渐减弱。在过去的20年里,涡动动能在太平洋扇区的显著增强也集中在中西部海域,这里也是南极绕极流斜压性较强的海域。涡旋统计特性揭示了涡动动能的空间分布及其年际变化主要归因于涡旋振幅与旋转速度,而并非涡旋个数或者涡旋半径。这些结果进一步确认了对应于南半球环状模正位相的绕极西风异常改变了南大洋的涡旋特性,从而表现出涡旋活跃性增强。     

Effect of skewness and kurtosis of sea-surface elevations on the accuracy of altimetry surface level measurements

We assess the influence of changes in the skewness and kurtosis of the distribution of elevations on a rough sea surface that were observed in field conditions on the accuracy of the recovery of the sea-surface level along the satellite trace. For different values of a significant height of surface waves, we obtain numerical estimates for the bias in relief recovery due to the simultaneous variation in skewness and kurtosis. It is shown that, with a decrease in kurtosis, the effect of skewness changes on the accuracy of altimetry measurements of the level increases. We note the limitations of the Gram-Charlier model in simulating the radar waveforms return from the sea surface.     

Spectral characteristics of Rossby waves in the Northwestern Pacific based on satellite altimetry

Using satellite altimetry measurement data for 1993–2013, we study the spectral characteristics of Rossby waves in the Northwestern Pacific (25°–50° N, 140°–180° E). For each latitude degree, we draw integral plots of spectral power density calculated with a two-dimensional Fourier transform (2D-FFT). We compare the dispersion equations of Rossby waves calculated from the WKB-approximation and an approximation of a two-layer ocean model with the empirical velocities determined by the slope of isopleths by the Radon method; also, we compare the dispersion equations with the spectral distributions of level variations. It is shown that the main energy of Rossby waves in the Northwestern Pacific corresponds to the first baroclinic mode. At almost all latitudes, there is good agreement between the empirical phase velocities calculated by isopleths by the Radon method and the theoretical values; also, the spectral peaks correspond to graphs of the dispersion equations for the first baroclinic mode Rossby waves, except for the Kuroshio region, where some peaks correspond to the second mode.