Deep-water Structure Research Articles

Target depth estimation by deep neural network based on acoustic interference structure in deep water

Automatic and robust target depth estimation is an important issue of an active detection system working in the reliable acoustic path (RAP) environment. In this paper, the target depth-sensitive acoustic interference structure is used as input, and a deep neural network (DNN) method is proposed to realize automatic depth estimation. Furthermore, to improve the robustness of the network, both the sound speed profile (SSP) uncertainty and the target trajectory uncertainty are considered in the training dataset. The former is modelled as a random weighted sum of the empirical orthogonal function (EOF), and the latter is modelled by the partial-data-lacking of the network input. The method was evaluated through simulated data and compared with the conventional method of matching the measured interference structure with the replicas. The presented method exhibits a higher accuracy of depth estimate in uncertain environments when the signal-to-noise ratio (SNR) is higher than −6 dB, but its performance deteriorates rapidly as the SNR decreases further. The method is also verified by semi-simulation with experimental data in deep water.

Research on Risk Evaluation of Flooding in Immersed Tunnels During Operation

As more and more immersed tunnels are built in China, due to the characteristics of closed space, long-term structure in deep water, and complicated surrounding environment, if an accident occurs during the operation period, it will cause immeasurable losses. The safety risks faced during the period must also be taken seriously. This paper analyzes the causes of flooding risks and identifies the sources of flooding risks for the flooding risks faced during the operation of the immersed tunnel. Based on the scenario design and simulation analysis, the impact on the tunnel structure at different depths of flooding and the consequences of flooding on immersed tunnels are proposed. Based on this, anti-flooding measures and related suggestions for immersed tunnels during operation are proposed.

Practical evaluation of flows due to arbitrary singularity distributions in the 3D theory of ship motions in regular waves at [formula omitted

The 3D frequency-domain analysis of the flow around a ship advancing in regular waves, in deep water, via the Green-function and boundary-integral method requires a practical and reliable way of evaluating the flow due to an arbitrary distribution of singularities (sources and dipoles) over the (flat or curved, quadrilateral or triangular) panels that are commonly used to approximate a ship hull-surface. This crucial element of the Green-function method, also widely called panel method, in a 3D analysis of ship motions is classically performed via a two-step procedure that involves evaluation of the Green function G and its gradient ∇G as a first step, and their subsequent integration over hull-surface panels. This usual direct method, widely studied and applied in the literature, involves notorious well-documented mathematical and numerical complexities. A novel alternative method, based on the Fourier–Kochin approach, is expounded in the study. In this method, evaluation of G and ∇G is bypassed, i.e. G and ∇G are not evaluated, and the flow due to a distribution of singularities is evaluated directly, in the manner already expounded for the particular cases of steady flow around a ship advancing in calm water and diffraction-radiation of regular waves by an offshore structure or a ship at zero forward speed. The method expounded in these previous studies and extended here is based on an analytical representation of the flow due to a distribution of singularities (including the special case of a Green function and its gradient) that yields a formal decomposition into a wave component, given by single Fourier integrals along the dispersion curves associated with the dispersion relation, and a non-oscillatory local-flow component given by a double integral that has a smooth integrand mostly dominant within a compact region of the Fourier plane. An important aspect of this analytical flow decomposition is that the wave and local-flow components are smooth, unlike the wave and local-flow components in the expressions for G and ∇G given in the literature. Moreover, the analytical representation of the flow due to a distribution of singularities given in the study provides a practical mathematical basis that is well suited for accurate and efficient numerical evaluation, as is demonstrated via the illustrative applications reported previously for steady flow around a ship advancing in calm deep water and diffraction-radiation of regular waves by an offshore structure in deep water, and reported here for the more general case of a ship advancing in deep water at a constant speed V through regular waves of frequency ω in the regime τ≡Vω∕g<1∕4 where g denotes the acceleration of gravity.

Newly discovered hydrate-bearing structure in Lake Baikal

A new deep-water hydrate-bearing structure, named after MSU, has been discovered the 29th of July 2018 during the geological-geophysical research cruise of the Class@Baikal project. The structure lies on a fault scarp at the bottom of the central basin of Lake Baikal at a water depth of 1380 m (coordinates N 52°52’ / E 107°07’). The “MSU” structure is sub-isometric in planar view, 500 m in diameter, and reveals a multi-summit morphology. Results of initial investigations show an active fluid seepage and gas-hydrate formation within the discovered structure. Laboratory analyses of the sampled gases indicate a significant concentration of thermogenic hydrocarbons.

Effects of Permeable Cylinder on the Flow Structure in Deep Water

Flow behaviors around permeable cylinders were investigated using Particle Image Velocimetry technique in deep water. The height of deep water and free stream velocity were kept constant as hw = 340 mm and U = 156 mm/s. To find out the effect of the permeable cylinders on the flow structure, eight different porosities (β = 0.4, 0.5, 0.6, 0.65, 0.7, 0.75, 0.8, and 0.85) were used. The results have indicated that the permeable cylinders are effective on the control of large-scale vortical structures downstream of the permeable cylinder. As the porosity increases, turbulent kinetic energy and Reynolds shear stress decrease. This means that the fluctuations in the wake region are significantly weakened by permeable cylinders. The permeable cylinders having the porosity higher than 0.6 do not pose an obstacle in the flow. Furthermore, for all diameter values of permeable cylinders, it can be concluded that the flow structures downstream of the permeable cylinder show similar trend with each other.

Two simplified methods for fatigue crack growth prediction under compression-compression cyclic loading

Submersibles and submarines are subject to cyclic compressive sea loading during their service life, which is quite different from the ships and platforms under tensile and bending loading. It is important to study the fatigue crack propagation under cyclic compression in order to assess the fatigue life of the submersible and submarine. An extended McEvily model is proposed for the fatigue crack growth prediction under cyclic compression-compression loading. First, Finite Element Method (FEM) simulation and a simplified stress estimation method are proposed for efficient fatigue crack growth analysis. In the Finite Element Method simulation, the crack opening loads of different crack lengths are calculated individually instead of plastic wake calculation behind crack tip with node releasing technique, which will save a lot of calculating time. Afterwards the fatigue life under cyclic compression is predicted based on new fatigue crack growth rate curve model for cyclic compression. Meanwhile the analytical method is an engineering estimation for fatigue crack growth under constant amplitude compression. In this method, the crack opening load is estimated based on the symmetry of compression and tension. Then the fatigue life under cyclic compression is also predicted based on a new model for cyclic compression. Finally, fatigue life prediction of a double edged specimen under cyclic compressive loading is taken for example to illustrate the analysis procedure of two simplified methods. By comparing the predicted results with the test data, it is found that the a-N curve and final crack length by two methods are in good agreement with the test data, so the Finite Element Method and analytical method are reasonable and feasible for fatigue crack growth prediction of deepwater structure under cyclic compression.

Advances in deepwater structure installation technologies

Advances in deepwater structure installation technologies

Spatial correlation of the high intensity zone in deep-water acoustic field**Project supported by the National Natural Science Foundation of China (Grant Nos. 11434012 and 41561144006).

The spatial correlations of acoustic field have important implications for underwater target detection and other applications in deep water. In this paper, the spatial correlations of the high intensity zone in the deep-water acoustic field are investigated by using the experimental data obtained in the South China Sea. The experimental results show that the structures of the spatial correlation coefficient at different ranges and depths are similar to the transmission loss structure in deep water. The main reason for this phenomenon is analyzed by combining the normal mode theory with the ray theory. It is shown that the received signals in the high intensity zone mainly include one or two main pulses which are contributed by the interference of a group of waterborne modes with similar phases. The horizontal-longitudinal correlations at the same receiver depth but in different high intensity zones are analyzed. At some positions, more pulses are received in the arrival structure of the signal due to bottom reflection and the horizontal-longitudinal correlation coefficient decreases accordingly. The multi-path arrival structure of receiving signal becomes more complex with increasing receiver depth.

The effects of the sloping bottom environment on sound propagation in deep water

Variation of bathymetry has large effects on sound propagation. An acoustic propagation experiment was conducted in the South China Sea. Some different propagation phenomena are observed for two different tracks with the flat bottom and the sloping bottom environments when the source depth is 131 m. Numerical analysis based on the ray methods is carried out to explain the causes of the differences. The experimental and numerical results show that when a sea hill with height of 320 m, which is less than 1/10 of water depth, exists at the range of sound beams with the grazing angles from 10° to 20° incident on bottom first time, an inverted-triangle shadow zone appears at the reflection-blockage area of the sea hill. Compared with the TLs results in the flat bottom environment, TLs increase up to about 8 dB in the corresponding area of the first shadow zone, and the abnormal TLs effects can reach to maximal depth of 1300 m. The first convergence zone caused by refractions of sound beams with small grazing angles within 10° from the water above the axis of sound channel is lost. There are only the sound beams refracted back from water below the axis of sound channel. It’s quite different from the convergence-zone structure in deep water with flat bottom. Thus, the accuracy of bathymetry is meaningful for sound propagation and target detection in deep water.

Risk Assessment of Operation Period Structural Stability for Long and Large Immersed Tube Tunnel

Immersed tube tunnel has been widely used in virtue of its characteristics of shallow burial, good waterproof performance, strong stratum adaptability, etc., however, it is also characterized by enclosed space, prolonged submersion of structure in deep water, complicated surrounding environment, etc., particularly, in case of accident of tunnel structure, inestimable losses will be caused, therefore, the control of structural stability risk in operation period is especially important. This article carries out risk source analysis and identification, scenario design and simulation analysis, risk loss judgment standards and weight research for structural stability of long and large immersed tube tunnel in operation period on basis of extra-large immersed tube tunnel of the fourth lane of Shenzhen-Zhongshan Thoroughfare, based on which and in combination with the relevant domestic and foreign risk assessment and expert research results, the structural stability risk levels and risk scores of structural stability in operation period are obtained by quantitative risk calculation method and the relevant suggestions for risk control are proposed.

Inertial Coefficient Adopted in Pile-Pier Structure under Deep Water

In order to study the effect of hydrodynamic pressure acted on pile-pier structure in deep water, an investigation on the inertial coefficient is developed. Based on the Morison equation, the usual suggested value is analyzed, especially the noncircular slender cylinder is considered. Besides, the effect of scale of structure is studied, including accurate algorithm and approximation algorithm. Study results indicate that the Morison equation is also applied to noncircular slender cylinder, and the value of inertial coefficient is influenced by the scale of structure. Moreover, the calculated results of accurate algorithm and approximation algorithm are consistent within a certain range, so the choice of calculation method should be governed by the required precision.

An active truncation method for simulating deep-water riser installation

An active truncation hybrid model test is regarded as the most effective way for deep-water flexible structure test by The International Towing Tank Conference (ITTC). Owing to environmental loads and the movement of vessel, the dynamic boundary conditions at the truncated position related to the active truncation method are hard to be controlled. It has little relevant research which is reported on this testing technology. In this paper, an active truncation method is performed to simulate the process of deep-water steel catenary riser installation. The mechanical model of steel catenary riser (SCR) installation process is developed and corresponding equations for the developed model are derived. At the truncated position, device control principles are fitted from the solutions of the derived equations. 6-DOF (degrees of freedom) platform is used to simulate the motions of vessel. And the software and hardware of control system for this active truncation method are developed. The concept is first proposed, of which, that deep-water steel catenary riser installation test is conducted onshore. The test results show that this active truncation method is capable to simulate the process of steel catenary riser installation well. The test method has also provided the foundation for future works of other deep-water projects.

NON-LINEAR DYNAMIC ANALYSIS OF COUPLED SPAR PLATFORM

Spar platforms are treated as cost-effective and resourceful type of offshore structure in deep water. With increasing depth there are significant changes in its structural behaviour due to coupling of spar hull-mooring line along with radical influence of mooring line damping. So these phenomena should be precisely counted for accurate motion analysis of spar mooring system. In present study, spar platform are configured as a single fully coupled integrated model in ABAQUS/AQUA. Non-linear dynamic analysis in time domain is performed adopting Newmark-β automatic time incrementation technique. Non-linearities due to geometric, loading and boundary conditions are duly considered. Displacement and rotational responses of spar and mooring tensions are obtained during long-duration storm. spar responses get significantly modified and mean position of oscillations gets shifted after longer wave loading. The surge, heave and pitch responses are predominantly excited respectively. The energy contents of PSDs of these responses reduce considerably after long wave loading. Mooring tension responses are significantly different reflecting the damping effect of mooring lines. The pitch response is fairly sensitive to the wave loading duration. After long duration of storm the wave frequency response increases. However, low frequency and wave frequency responses may simultaneously occur due to synchronising sea states.

Evaluation of the carbon cycle components in the Norwegian Earth System Model (NorESM)

Abstract. The recently developed Norwegian Earth System Model (NorESM) is employed for simulations contributing to the CMIP5 (Coupled Model Intercomparison Project phase 5) experiments and the fifth assessment report of the Intergovernmental Panel on Climate Change (IPCC-AR5). In this manuscript, we focus on evaluating the ocean and land carbon cycle components of the NorESM, based on the preindustrial control and historical simulations. Many of the observed large scale ocean biogeochemical features are reproduced satisfactorily by the NorESM. When compared to the climatological estimates from the World Ocean Atlas (WOA), the model simulated temperature, salinity, oxygen, and phosphate distributions agree reasonably well in both the surface layer and deep water structure. However, the model simulates a relatively strong overturning circulation strength that leads to noticeable model-data bias, especially within the North Atlantic Deep Water (NADW). This strong overturning circulation slightly distorts the structure of the biogeochemical tracers at depth. Advancements in simulating the oceanic mixed layer depth with respect to the previous generation model particularly improve the surface tracer distribution as well as the upper ocean biogeochemical processes, particularly in the Southern Ocean. Consequently, near-surface ocean processes such as biological production and air–sea gas exchange, are in good agreement with climatological observations. The NorESM adopts the same terrestrial model as the Community Earth System Model (CESM1). It reproduces the general pattern of land-vegetation gross primary productivity (GPP) when compared to the observationally based values derived from the FLUXNET network of eddy covariance towers. While the model simulates well the vegetation carbon pool, the soil carbon pool is smaller by a factor of three relative to the observational based estimates. The simulated annual mean terrestrial GPP and total respiration are slightly larger than observed, but the difference between the global GPP and respiration is comparable. Model-data bias in GPP is mainly simulated in the tropics (overestimation) and in high latitudes (underestimation). Within the NorESM framework, both the ocean and terrestrial carbon cycle models simulate a steady increase in carbon uptake from the preindustrial period to the present-day. The land carbon uptake is noticeably smaller than the observations, which is attributed to the strong nitrogen limitation formulated by the land model.

DUCTILE STRUCTURES IN GRAVITATIONAL DECOLLEMENT STRUCTURE SYSTEM AT PASSIVE CONTINENTAL MARGIN OF WEST AFRICAN

This article deals with the ductile structures in the deep-water gravitational decollement structure system at the Passive Continental Margin of West African.It is found that ductile structures are widely distributed along the whole passive continental margin,and the ductile salt rocks are particularly developed in the southwest and northwest within the transitional structural layer(J-K).In the Niger delta at the middle part of the passive margin of West Africa,ductile bed is mainly consisting of mudstone of Tertiary.According to the structural characteristics of the gravitational detachment structure system,the ductile structures could be divided into three types: the gravitational decollement extensional structures at the upper part characterized by normal fault and ductile;the gravitational decollement diapir structures at the middle part;and the gravitational decollement thrust structures at the lower part.According to the time of deformation of the ductile structure,the ductile structures could be grouped into two types of epigenetic and syngenetic structures.Deformation of ductile beds is mainly affected by the basement tilting and deposition rate of the overlying strata.The control of structures over oil and gas accumulation decreases in an order of diapir,fold,thrust and ductile welding structure.

Finite Element Modeling for Cathodic Protection of Pipelines under Simulating Thermocline Environment in Deep Water Using a Dynamic Boundary Condition

A finite element model (FEM) was established to calculate the cathodic protection (CP) potential distribution on pipelines, under the environment of the thermocline structure in deep water. Supplemental boundary conditions were applied on the interfaces of two different mediums in the thermocline model. To solve the problem of time-dependent polarization curves due to calcareous deposits formed on the steel surface during CP in seawater, a dynamic boundary condition based on Ohm's law was proposed, instead of polarization curves or the Butler-Volmer equation. A good agreement was obtained between the calculated and experimental potential distributions, which proved that the model was practical and feasible. The anode placement studies revealed that the anode could supply a more efficient protection when it was placed at the thermocline zone rather than the surface zone or the deep water zone.

Effect of time elapse after wave hitting on coupled Spar platform

Oil and gas exploration has moved from shallow to much deeper water now-a-days. Offshore compliant floating structure like Spar platforms in this region are competent deep water platform. As water depth increase operational Spar platforms experience more influence of mooring lines suggestively. So the motion analysis of platforms in deep waters requires counting the mooring upshot with spar hull. The most common approach for solving the dynamics of Spar platform is to employ a decoupled quasi- static method, which ignores all or part of the interaction effects between the platform and mooring lines. Coupled analysis in the present study includes the mooring lines and platform in a single model. This model can cope with the coupling conduct and match the forces, displacement, velocities and acceleration at the fairlead position along with all possible significant nonlinearities. The output from such analyses is essentially platform motions as well as a mooring line response. In actual field problems hydrodynamic loads due to wave and currents act simultaneously on Spar platform and mooring lines. In this finite element model, the entire structure acts as coupled in consistent manner. This offshore Spar platform model has been analyzed in regular wave of ocean environment. Surge, heave and pitch motion responses of coupled Spar platform is obtained after 1000 and 6000 s of storm. The behaviors of dynamic responses have been evaluated as noteworthy even for time elapse after wave hitting. Key words: Deep-water structure, oil and gas exploration, offshore compliant floating, Spar platform, nonlinearities, ocean environment, mooring lines, coupling , time elapse, motion responses, wave hitting.

Snapshot observation of the physical structure and stratification in deep-water of the South Caspian Sea (western part)

Abstract. The physical parameters structures and different water masses using CTD measurements in southwestern part of the Caspian Sea (CS) adjacent to Anzali Port (AP) are investigated. CTD profiles were conducted along a transect perpendicular to the coastline on 13 stations from the coast down to 720 m on 22 January 2008. According to the results the continental shelf waters are located in the surface mixed layer. The surface mixed layer extends itself down to almost 100 m in outer areas of the continental shelf with a weak seasonal thermocline layer between 80 to 140 m Freshwaters inflow of local rivers is clearly seen outside the continental shelf at the surface layers. Investigating the dissolved oxygen reveals that winter convection is traceable down to 500 m in the lateral waters over the shelf break. Among the deeper stations that are located in continental rise and abyssal plain, 300 m seems to be a threshold for penetration of seasonal changes; therefore deeper waters tend to be impermeable against seasonal variances. Despite the small variations, stability is positive in the study area and temperature plays an important role in static stability and in triggering the lateral mixing. In view of both temperature-salinity and temperature-oxygen distributions in the southwestern part of the CS, two different water masses are separable in cold phase. Snapshot observations of physical properties in the early winter 2008, to some extent revealed that a mixing was triggered at least in the lateral waters of the study area.

Factors controlling the evolution of the Perdido Fold Belt, northwestern Gulf of Mexico, determined from numerical models

The Perdido Fold Belt (PFB) is a prominent salt‐cored deep water structure in the northwestern Gulf of Mexico. It is characterized by symmetric, kink‐banded folds of a ∼4.5 km thick prekinematic layer and its vicinity to the extensive Sigsbee Salt Canopy. We use 2‐D finite element numerical models to study the evolution of the PFB as a gravity‐driven fold belt both in a local context and in the context of the larger‐scale passive margin, influenced by adjacent allochthonous salt structures. We show that parameters such as overburden strength, salt geometry, or salt viscosity determine timing, extent, and location of the modeled fold belt. Simplified models of the Gulf of Mexico show that toe‐of‐slope folding is a viable mechanism to develop diapirs in the deep salt basin and to delay folding of the distal overburden. In this scenario, the PFB likely represents the terminal folding of a much larger, diachronously formed fold belt system.

Methodical aspects of the application of acoustic Doppler current profilers in the Black Sea

For the first time, an acoustic Doppler current profiler (ADCP) produced by the RDI is used in the water area of the Black Sea in the lowered mode. This direction in the application of acoustic Doppler current profilers is now in the stage of development and verification of various procedures of measurements with subsequent data processing and enables one to get the distributions of current velocities down to depths of 1000–6000 m depending on the modification of the instrument. We describe the procedures of measurements performed with the help of the ADCP in the lowered mode in the course of an expedition and the stages of processing of the primary data based on the experience of application of similar acoustic current profilers at the Marine Hydrophysical Institute of the Ukrainian National Academy of Sciences accumulated in the 1980s. We generalize the experience of application of ADCP under the hydrological conditions of the Black Sea, propose the algorithms of data processing, present the profiles of absolute current velocity at several stations, compare these profiles with the geostrophic velocities, and determine the deep-water structure of the field of currents in a shelf-open-sea section made along 31.17°E.