Studying the sea surface slopes using an array of wave gauge sensors

The deviations of the marine surface slope spectra (measured using an array of wave gauge sensors) from the theoretical estimates obtained using the linear spectral model of the wave field are analyzed. It has been indicated that the average measured full slope spectra (the sum of the slope component spectra in the orthogonal directions) is higher than the theoretical estimates by 6% at frequencies from the surface wave spectral peak (f _m ) to 4.5 f _m . The difference between the measured and theoretical estimates of the full slope spectrum rapidly increases at frequencies of f < f _m . At f _m ≈ 0.75 f _m , the average measured full slope spectrum is higher than the theoretical estimate by a factor of more than 5.     

Stress concentration factors (SCFs) of bulge formed K-joints under balanced axial loads

A novel stiffened joint called bulge formed joint was put forward. Compared with the unstiffened joint, an additional bulge plate is utilized to connect the chord and braces. Based on the finite element (FE) and experimental method, stress concentration factors (SCFs) were investigated for both a bulge formed K-joints and the corresponding unstiffened joint. By the verification of experimental data, the FE models were used to investigate the SCFs of the bulge formed joints. The maximum SCF of the bulge formed K-joint under balanced axial loads is located at the position φ = 105°, which is close to but not exactly the saddle. The SCFs around the intersection weld can decrease remarkably compared with the unstiffened joint by the change of geometrical parameters of the bulge plate. Then 2117 FE analyses were conducted to investigate the geometrical effects on SCFs of the bulge formed joint. These dimensionless geometrical parameters include τ_s, η₁, η₂, θ, β, γ, τ, among which, the first three parameters are typical of the bulge formed joints, while the others are same as the definitions in the unstiffened joints. Finally, a set of SCF parametric formulas were obtained by nonlinear regression analyses.     

A parametric study on seakeeping assessment of fast ships in conceptual design stage

The paper presents an approach to investigate the effects of some parameters on seakeeping assessment of fast ships in conceptual design stage. Hull form parameters have been classified into two groups: main dimensions (L, B and T) and secondary form parameters (LCB and C_P). To demonstrate the approach a fast ship is redesigned as parent hull and alternative hull forms are generated by changing these parameters systematically. Some hull forms are selected related the geometric limits and seakeeping analyzes are here investigated and discussed. The obtained results are satisfactory for seakeeping predictions during the conceptual design stage.     

Local joint flexibility of CHS X-joints reinforced with collar plates in jacket structures subjected to axial load

This paper aims to investigate the local joint flexibility (LJF) of tubular X-joints with collar plates subjected to axial load. A total number of 136 FE models were generated and analyzed to investigate the effect of collar plate size and joint geometry (η, τ_c, β, γ, and τ) on the local joint flexibility factor (f_LJF). Also, the weld profiles and the contact between the chord wall and the collar plates were modeled in the numerical simulations. Results showed that the increase of the collar plate thickness and length results in the decrease of the f_LJF. Despite the difference between the f_LJF of unreinforced and collar plate reinforced X-joints, there has not been any study on the f_LJF of the X-joints reinforced with collar plate. Also, there is no design equation for determining the f_LJF of the collar plate reinforced X-joints. Thus, the results of the parametric study were used to develop a design formula, to determine the f_LJF of X-joints reinforced with collar plates under axial load.     

Late Cretaceous (Late Turonian,Coniacian and Santonian) petroleum source rocks as part of an OAE,Tarfaya Basin,Morocco

Late Turonian, Coniacian and Santonian source rock samples from a recently drilled well (Tafaya Sondage No. 2; 2010) in the Tarfaya Basin were analyzed for quantity, quality, maturity and depositional environment of the organic matter (OM). To our knowledge such a thick sequence of organic matter-rich Turonian to Santonian source rocks was investigated in that great detail for the first time. Organic geochemical and organic petrological investigations were carried out on a large sample set from the 200 m thick sequence. In total 195 core samples were analyzed for total organic carbon (C_org), total inorganic carbon contents and total sulfur (TS) contents. Rock-Eval pyrolysis and vitrinite reflectance measurements were performed on 28 samples chosen on the basis of their C_org content. Non-aromatic hydrocarbons were analyzed on selected samples by way of gas chromatography–flame ionization detection (GC–FID) and GC–mass spectrometry (GC–MS). The organic matter-rich carbonates revealed a high source rock potential, representing type I kerogen and a good preservation of the organic matter, which is mainly of marine (phytoplankton) origin. HI values are high (400–900 mg/g C_org) and in a similar range as those described for more recent upwelling sediments along the continental slope of North Africa. TS/C_org ratios as well as pristane over phytane ratios indicate variable oxygen content during sediment deposition. All samples are clearly immature with respect to petroleum generation which is supported by maturity parameters such as vitrinite reflectance (0.3–0.4%), T_max values (401–423 °C), production indices (S₁/(S₁ + S₂) > 0.1) as well as maturity parameters based on ratios of specific steranes and hopanes.     

Nonlinear meta-models for conceptual seakeeping design of fishing vessels

The scope of this paper is to develop the nonlinear meta-models for seakeeping behaviour, considering the fishing vessels. These models are intended to be inserted either in a multiattribute design selection process or in a comprehensive multiobjective optimization procedure. For this purpose, seakeeping data of fishing vessels in regular head waves are used to develop meta-models of transfer functions of heave, pitch and vertical acceleration by nonlinear analysis. A home-made software considers two databases; the first is composed by the ship dimensions and coefficients of fishing vessels, and the second is their ship motion data obtained by employing a strip-theory calculation. The meta-models are proposed to predict the vertical motion characteristics for given ranges of speed and wave length during the concept design stage. The independent variables are hull size (Δ), main dimensions (L, B, T), and some hydrostatic parameters (C_WP, C_VP, LCB, LCF, etc.). The results estimated by the software show good correspondences with the ones achieved by direct computations. The study provides additional insight on the influence of hull form parameters on seakeeping performance of small vessels having form properties and parametric range corresponding to the investigated vessels.     

Numerical simulation for hydrodynamic characteristics of a bionic flapping hydrofoil

In order to study the propulsion mechanism of the bionic flapping hydrofoil (BFH), a 2-DoF (heave and pitch) motion model is formulated. The hydrodynamic performance of BFH with a series of kinematical parameters is explored via numerical simulation based on FLUENT. The calculated result is compared with the experimental value of MIT and that by the panel method. Moreover, the effect of inlet velocity, the angle of attack, the heave amplitude, the pitch amplitude , the phase difference, the heave biased angle, the pitch biased angle and the oscillating frequency are investigated. The study is useful for guiding the design of bionic underwater vehicle based on flapping propulsion. It is indicated that the optimal parameters combination is v=0.5m/s, θ0=40°.θ0=30°,Ψ=90°,Фbias=0°,θbias=0°and f=0.5Hz .     

A study on the Local Joint Flexibility (LJF) of two-planar tubular DK-joints in jacket structures under in-plane bending loads

In the present paper, results of a parametric study conducted on the Local Joint Flexibility (LJF) of two-planar tubular DK-joints under In-Plane Bending (IPB) loads are presented. DK-joints are among the most common joint types in jacket substructure of Offshore Wind Turbines (OWTs). A total of 324 finite element (FE) analyses were carried out on 81 FE models under four types of IPB loading in order to investigate the effect of the DK-joint’s geometrical parameters on the LJF factor (f_LJF). Based on the results of parametric study, the factors leading to the LJF reduction were introduced. Generated FE models were verified using the existing experimental data, FE results, and parametric equations. The effect of the weld profile was also considered. The f_LJF in two-planar DK- and uniplanar K-joints were compared. Results indicated that the effect of multi-planarity on the LJF is quite significant and consequently the use of the equations already available for uniplanar K-joints to calculate the f_LJF in two-planar DK-joints may lead to highly under-/over-predicting results. To handle this issue, the FE results were used to derive a set of parametric equations for the prediction of the f_LJF in IPB-loaded two-planar DK-joints. The proposed equations were checked against the acceptance criteria recommended by the UK DoE and can be reliably used for the analysis and design of tubular joints in OWTs.     

白令海BR断面海-气CO2通量及其参数特征

通过对2008年夏季白令海大气和海水pCO2连续观测资料,结合BR断面上站位水体垂直采样测量,对白令海不同海区pCO2的分布特征及其与理化参数的关系进行了初步研究,结果表明,将白令海划分为4个具有不同CO2吸收能力的海区,其中陆坡流区碳通量高达-18.72 mmol/(m2·d),是海盆北区的近2倍,比海盆南区高一个量...     

Current-induced scour beneath initially elevated subsea pipelines

When a subsea pipeline is laid on an uneven seabed, certain sections may have an initial elevation with respect to the far-field seabed, e_o, and thus potentially affecting the on-bottom stability of the pipeline. This paper focuses on quantifying the effects of the upstream dimensionless seabed shear stress, θ_∞, and Reynolds number, Re, on: (1) the maximum dimensionless seabed shear stress beneath the pipe, θ_max, to be compared to the critical shear stress in order to determine whether scour would occur and progress towards an equilibrium state; and, (2) the dimensionless equilibrium scour depth beneath the pipe, S_eq/D. Using a 2-D Reynolds averaged Navier-Stokes (RANS) approach along with the k-ω Shear Stress Transport (SST) turbulence model, a parametric study involving 243 computational fluid dynamics (CFD) simulations was conducted. The simulation results were used to develop a closed-form equation for the prediction of θ_max. Subsequently, experimental measurements of S_eq/D have been compiled from published literature, to develop a new closed-form equation for the prediction of S_eq/D with a high correlation to the experimental data. In summary, we present two closed-form equations for the prediction of θ_max and S_eq/D for pipelines with an initial e_o/D, which are applicable for both clear-water and live-bed conditions. The effects of θ_∞ and Re have been included, albeit Re having a small influence as compared to the other parameters.     

喷射式挖沟机机体的稳定性分析

挖沟机是铺设海底管线的必备设备。论文对海洋石油工程股份有限公司设计的挖沟机的受力状态进行了分析,根据整体稳定分析计算了挖沟机在不同工作形态所需要的牵引力。详细计算了(1)海流运动产生的作用力,包括拖曳力和举升力,海流的流动方向包括顺流、逆流和横流;(2)海底对挖沟机滑靴的阻力;(3)高压空气和高压水流对挖沟机机体的作用力;(4)牵引力;(5)海底土壤对机体的阻力(在挖沟机牵引速度大于挖泥速度时,引起的泥土对喷射管的阻力);(6)挖沟机的自重以及浮力。计算结果为挖沟机强度的校核及操作方法提供了依据。     

Prediction of significant wave height using regressive support vector machines

Wave parameters prediction is an important issue in coastal and offshore engineering. In this literature, several models and methods are introduced. In the recent years, the well-known soft computing approaches, such as artificial neural networks, fuzzy and adaptive neuro-fuzzy inference systems and etc., have been known as novel methods to form intelligent systems, these approaches has also been used to predict wave parameters, as well. It is not a long time that support vector machine (SVM) is introduced as a strong machine learning and data mining tool. In this paper, it is used to predict significant wave height (H_s). The data set used in this study comprises wave wind data gathered from deep water locations in Lake Michigan. Current wind speed (u) and those belonging up to six previous hours are given as input variables, while the significant wave height is the output parameter. The SVM results are compared with those of artificial neural networks, multi-layer perceptron (MLP) and radial basis function (RBF) models. The results show that SVM can be successfully used for prediction of H_s. Furthermore, comparisons indicate that the error statistics of SVM model marginally outperforms ANN even with much less computational time required.     

Prediction of offshore bar-shape parameters resulted by cross-shore sediment transport using neural network

In order to understand the features of coastal zone and to utilize the coastal areas, it is necessary to determine the sediment movement and the resulting transport. Waves, topographic features, and material properties are known as the most important factors affecting the sediment movement and coastal profiles. In this study, by taking into consideration of wave height and period (H₀, T), bed slope (m) and sediment diameter (d₅₀), cross-shore sediment movement was studied in a physical model and various bar-shape parameters of the resultant erosion type profile were determined. Using 80 experimental data which are obtained from physical model studies, a neural network (NN) has been calibrated to predict bar-shape parameters of beach profiles. A sensitivity analysis was firstly carried out to decide data of training and test sets. Four different models, in which the rates of their training and testing set data were 80% and 20%, 70% and 30%, 60% and 40%, 50% and 50% were constituted and their performances were compared. It was determined that the model, in which the rate of its training and testing set data was 80% and 20%, respectively, has the best results. Therefore, a total of 64 experimental data were used as training set and the remainders of the experimental data were used as a testing set for the model. The performance of the NN model was compared with the regression equations developed in a previous study and the equations cited in literature indicating better performance over the equations.     

Hydrodynamic shape optimization by high fidelity CFD solver and Gaussian process based response surface method

A computational framework for hydrodynamic shape optimization of complex ship hull form is proposed and applied to improve the calm water performance of the KRISO Container Ship (KCS). The framework relies on three key features: a novel shape morphing method based on a combination of subdivision surfaces and free form deformations, a robust three dimensional viscous computational fluid dynamic solver based on the openFOAM open-source libraries and a Gaussian process-response surface method (GP-RSM) based on ordinary Kriging model which has been created to speed-up the evaluation of the quantity of interest (QoI) of the design process.The accuracy of the hydrodynamic solver is proven by comparing the obtained results against available experimental measurements. A preliminary sensitivity analysis on the mesh size has been carried out aiming at reducing the computational burden required by the CFD predictions. Three GP-RSMs have been trained relying on increasing number of hull designs. Each surrogate model has been cross-validated by both leave-one-out and k-fold techniques. The behaviours of these multi-dimensional surfaces have been analyzed in details by sampling the investigated design space with 10⁷ points according to a Full-Factorial algorithm, highlighting the regions of maximum deviation with respect to the resistance of the reference hull. The three optimum designs provided by the corresponding GP-RSM models have been verified by using high-fidelity CFD simulations with a refined mesh configuration. Calm water resistance, wave patterns and pressure distributions over the selected hull surfaces have been discussed in the light of the generated shape variations.     

On the interest of using field primary production data to calibrate phytoplankton rate processes in ecosystem models

In many ecosystem models based on empirical formulations, parameters generally are calibrated in order to achieve the best fit between measured and simulated chlorophyll a standing stocks. An accurate calibration of rate processes as primary production rarely is taken into account. In this paper, we test the usefulness of calibration of phytoplankton photosynthetic processes in an ecosystem model using field primary production data. We used 18 months of photosynthetic process data from the Baie des Veys ecosystem (Normandy, France). Five empirical formulations of photosynthesis–irradiance curve models amongst the most widely used were tested. In each formulation, the variability of photosynthetic parameters (i.e. the light-saturated rate of photosynthesis (P_max^B) and the initial slope of the photosynthesis–light curve (α^B)) was considered depending on environmental factors (temperature and nutrient availability). The fit of the five equations as well as the calibration of parameters on field measurements (i.e. the light-saturated rate of photosynthesis (P_ref^B), the initial slope of the photosynthesis–light curve (α_ref^B), the half-saturation constant for nitrogen (K_N) and silicates uptake (K_Si), and the coefficient in the exponential thermal effect (K_T)) was performed using the whole available data set of P vs. E curves (n = 143, P vs. E curves). Then, the Smith formulation allowing the best simulation of the Baie des Veys primary production and corresponding parameters were introduced in an ecosystem box model. This formulation led directly to a satisfactory representation of the Baie des Veys phytoplankton dynamics without additional calibration. Results obtained were compared with a more classical approach in which ecosystem models were calibrated using published values of parameters. This comparison showed that for the two years studied, annual primary production estimated through the ecosystem model was 13% and 26% higher with our approach than with the more classical approach. This work emphasizes the importance of accurately representing rate processes in ecosystem models in order to adequately simulate production as well as standing stocks.     

Comparison of marine gas hydrates in sediments of an active and passive continental margin

Two sites of the Deep Sea Drilling Project in contrasting geologic settings provide a basis for comparison of the geochemical conditions associated with marine gas hydrates in continental margin sediments. Site 533 is located at 3191 m water depth on a spit-like extension of the continental rise on a passive margin in the Atlantic Ocean. Site 568, at 2031 m water depth, is in upper slope sediment of an active accretionary margin in the Pacific Ocean. Both sites are characterized by high rates of sedimentation, and the organic carbon contents of these sediments generally exceed 0.5%. Anomalous seismic reflections that transgress sedimentary structures and parallel the seafloor, suggested the presence of gas hydrates at both sites, and, during coring, small samples of gas hydrate were recovered at subbottom depths of 238m (Site 533) and 404 m (Site 568). The principal gaseous components of the gas hydrates wer methane, ethane, and CO₂. Residual methane in sediments at both sites usually exceeded 10 mll^?1 of wet sediment. Carbon isotopic compositions of methane, CO₂, and ΣCO₂ followed parallel trends with depth, suggesting that methane formed mainly as a result of biological reduction of oxidized carbon. Salinity of pore waters decreased with depth, a likely result of gas hydrate formation. These geochemical characteristics define some of the conditions associated with the occurrence of gas hydrates formed by in situ processes in continental margin sediments.     

Defining the concept of stratigraphic grade and applying it to stratal (reservoir) architecture and evolution of the slope-to-basin profile: An outcrop perspective

Stratigraphic grade is the similarity of the morphology of successive slope-to-basin profiles in a genetically related depositional system. In this article we use data collected from regional cross-sections of six depositional systems, stratal architecture derived from outcrops of the Lewis Shale (Wyoming, USA), and the Ross Sandstone (Ireland), and supplementary outcrop and subsurface data from other depositional systems to determine how stratigraphic grade relates to stratal (reservoir) architecture in deepwater systems.Four methods are developed that collectively define stratigraphic grade: (1) regional stacking patterns of fourth-order stratigraphic surfaces, (2) the relationship between the trajectory of the shelf edge (T_se) and the trajectory of the depocenter (T_d) for fourth-order stratigraphic units, (3) morphology of the slope-to-basin profiles of fourth-order stratigraphic surfaces, and (4) the similarity of the morphologies of slope-to-basin profiles of fourth-order surfaces in a system (σ_s, σ_r). Several characteristics of stratigraphic (reservoir) architecture of fourth-order stratigraphic cycles are related to stratigraphic grade: (1) longitudinal distribution of sandstone in fourth-order cycles, (2) location of maximum sandstone relative to the depocenter of fourth-order cycles, (3) lengths of fourth-order submarine fans, and (4) longitudinal and vertical distribution of architectural elements. Stratigraphic grade is thus a predictor of reservoir architecture and can thereby be used to reduce the uncertainty in the interpretation of subsurface data.The concept of stratigraphic grade is useful in understanding the stratigraphic evolution of deepwater systems. Most deepwater systems analyzed in this study initiated as out-of-grade and temporally evolved to graded systems over a time span of millions of years. Systems rarely evolve from graded to out-of-grade. First-order controls on stratigraphic grade are determined to be angle of slope, tectonically forced changes in angle of slope during deposition, and sediment supply.     

Generation mechanism of an alongshore bar on a sandy beach slope

A model explaining the mechanism of alongshore bar formation from the point of view of the sediment balance in the surf zone is considered. A cloud of suspended matter that appears during wave breaking is transported shoreward and simultaneously sediments forming a vertical material flux directed to the bottom (S). Simultaneously, an undertow generates a horizontal offshore flux of suspended matter q _x . Under these conditions, the sediment balance is determined by the equality of the flux -S and the gradient dq _x /dx. The bottom profile satisfying the balance equation is a bar profile with the crest at the point of the flux maximum -S. The model predicts a concave profile of the seaside slope and a concave-convex profile of the slope in the trough. A conclusion is reached on the basis of the calibration and verification of the model based on the field data that the suggested mechanism manifests itself differently in the outer and inner zones of the coastal zone. In the inner zone, the horizontal size of the bar is determined by the length of short wind waves, while, in the outer one, it is determined by the length of the infragravity waves related to the groups of short waves. It is shown that the model can be applied to estimate the parameters of the largest bar in the inner part of the coastal zone.