Cable Depth Research Articles

Study on the burial depth calculation method for AC submarine cable based on the surface optical fiber monitoring temperature

AbstractThe partial exposure of submarine cable may cause damage to the metal sheath, which will affect its safe and stable operation. Existing detection methods for burial depth of submarine cables are difficult to implement and have a long detection period. In this contribution, a method is proposed to calculate the burial depth of alternating current (AC) submarine cable based on surface optical fiber monitoring temperature. According to the functional relationship between the transient temperature rise of submarine cable outer sheath above the ambient temperature and the burial depth (in the IEC‐60853 standard), the burial depth of submarine cables is calculated. The optical fiber temperature, electric load and meteorological temperature data are used for 30 consecutive days in September 2020 to calculate the change of burial depth of AC 500 kV oil‐filled submarine cable. The calculation results are basically consistent with the detection conclusion of side‐scan sonar, which shows that the method can accurately calculate the burial depth of submarine cables and locate the exposed position. This provides a theoretical basis for the intelligent operation and maintenance of AC submarine cables.

Prediction Model for Trends in Submarine Cable Burial Depth Variation Considering Dynamic Thermal Resistance Characteristics

Fault problems associated with submarine cables caused by variations in their burial depth are becoming increasingly prominent. To address the difficulty of detecting the burial depth of submarine cables and trends in its variation, a prediction model for submarine cable burial depth was proposed which considers the dynamic characteristics of thermal resistance. First, a parallel thermal circuit model of a three-core submarine cable was established, and a formula for calculating the submarine cable’s burial depth was derived based on a formula for calculating the submarine cable’s core temperature. Then, the calculation result was corrected by considering the dynamic characteristics of the thermal resistance of the submarine cable’s structural materials. On this basis, feature vectors associated with the seabed cable burial depth calculation data and time nodes were mined by a convolutional neural network and used as the input parameters of a long short-term memory network for optimization and training, and a prediction model for trends in seabed cable burial depth variation was obtained. Finally, an example analysis was carried out based on the actual electrical parameter data of submarine cables buried by an offshore oil and gas platform. The results showed that the prediction model for trends in variations in the burial depth of submarine cables based on the CNN-LSTM neural network can achieve high prediction accuracy and prediction efficiency.

Wave-seabed observations and a combined approach to determine required burial depth of offshore power cables under large storm waves and currents

The interaction of wave and currents with the seafloor and seabed can result in the erosion, liquefaction and failure of near surface sediments. The recent exposure of power cables for an offshore wind project in the United States has highlighted the need for more robust assessments of seafloor and seabed instability. Pipelines and cables can then become exposed or experience unacceptable and damaging deformations. This paper examines these processes. Experimental results have identified two different types of liquefaction. The first type results from the cumulative rise in pore pressures caused by repeated cyclic loading from waves. This type of liquefaction is most likely to occur in freshly deposited sediments that remain undrained or partially drained during wave loading events. The second type of liquefaction occurs from the combination of low initial stresses in the near seafloor sediments and the changing effective stresses within the seabed from wave action. This liquefaction will be more temporary and will depend on the time during each wave cycle when principal stresses are near zero or slightly negative. In addition to the experimental results two types of advanced numerical approaches are presented. These approaches greatly enhance the ability to make accurate predictions of seafloor and seabed instability. The first approach uses finite elements to examine stress changes within the seabed. Validation of the approach is presented in the paper by comparison with a previous analytical solution. A Computational Fluid Dynamics approach was used to determine the fluid velocities from waves and currents within the bottom boundary layer which can then be used to estimate the bottom seafloor shear stresses. These shear stresses can cause erosion with the potential to expose pipelines and cables. In addition to erosional and liquefiable zones, the paper also identifies a zone where the soil may yield and produce unacceptable deformations for pipelines and cables. The main recommendation from this study is that scour and erosion need to be evaluated by considering both fluid and soil mechanics aspects of the problem.

Opportunities to Improve Marine Power Cable Ratings with Ocean Bottom Temperature Models

Determining reliable cable ampacities for marine High Voltage Cables is currently the subject of significant industry and academic reassessment in order to optimize (maximizing load while maintaining safe operating temperatures) design and reduce costs. Ampacity models can be elaborate, and inaccuracies are increasingly predicated on the uncertainty in environmental inputs. A stark example is the role of ambient temperature at cable depth, which, due to the scale of cables and the inaccessibility of the seafloor, is commonly estimated at 15 °C. Oceanographic models incorporating ocean bottom temperature are increasingly available, and they achieve coverage and spatiotemporal resolutions for cable applications without the requirement for project specific measurements. Here, a rudimental validation of the AMM15 and AMM7 mean monthly ocean bottom temperature models for the NW European Shelf indicates encouraging accuracies (MBE ≤ 1.48 °C; RMSE ≤ 2.2 °C). A series of cable case studies are used to demonstrate that cable ratings can change between −4.1% and +7.8% relative to ratings based on a common static (15 °C) ambient temperature value. Consideration of such variations can result in both significant ratings (and hence capital expenditure and operating costs) gains and/or the avoidance of cable overheating. Consequently, validated modelled ocean bottom temperatures are deemed sufficiently accurate, providing incomparable coverage and spatiotemporal resolutions of the whole annual temperature signal, thereby facilitating much more robust ambient temperatures and drastically improving ampacity estimates.

Numerical Analysis for Estimating Penetration Depth by Ship-dragged Fluke Anchor

Anchor penetration by ship anchoring is emerging as a major risk factor for submarine cable. A numerical analysis method was established to predict the anchor penetration depth by ship-dragged fluke anchor in relation to setting the optimal burial depth of submarine cable. The soil resisting forces (bearing force, friction force) generated during the anchor penetration process were calculated for each of the fluke, shank and mooring chain of the anchor. The results of the numerical analysis were compared with the existing chart values of Vryhof (2015). It was found that it is desirable to use the force coefficients of strip foundation for the fluke and shank, and the force coefficients of a single vertical pile for mooring chain. The numerical analysis well reproduced the penetration trajectory in which the inclination angle of the lower surface of fluke gradually converged to zero.

Analytical model for reinforcement effect and load transfer of pre-stressed anchor cable with bore deviation

Anchors have been widely used in slope engineering, mining engineering and foundation pit engineering, but their stabilities are significantly affected by anchor cable bending which could be contributed by complicated geological conditions and technical limitations. Theoretical analysis in this paper leads to the establishment of the stress model for the bending anchor cable, and the mathematical expression of the relationship between the ultimate pull-out force and the curvature of the bending anchor cable. Numerical simulation is used to evaluate the effect of the anchor cable curvature on the reinforcement effect. The results suggest that the stress distribution and reinforcement effect are affected by different curvatures, the bigger curvature corresponds to the smaller deformation and the larger pull-out force. The rate of decrease in the anchor cable displacement with the burial depth is linearly related to the pull-out force. In addition, the anchor cable curvature affects the response of the surrounding rock mass to stress and strain: The greater the curvature, the larger the area of the responsive rock mass and the higher mobilization of the strength of the rock mass itself. The anchor cable curvature also influences the reinforcement effect of multiple anchor cables. The results are applied to foundation pit engineering, and it is found as follows: The appropriately bending anchor cable (00-0.50 of curvature) with the same length can make wall stress and deformation more reasonable, and better control foundation deformation; however, with the increase in curvature, the effective anchoring length and the anchor cable depth decrease, and the reinforcement effect declines. The study results can provide reference for anchorage engineering.

Fluid–Structure Interaction Analysis and Verification Test for Soil Penetration to Determine the Burial Depth of Subsea HVDC Cable

Recently, there have been frequent reports of subsea cable breakage accidents caused by the drop of an anchor pile for aquaculture works involving subsea cables for high-voltage direct current (HVDC) transmission embedded in the southwest sea of Korea. To determine the burial depth that can ensure the safety of subsea HVDC cables embedded under the seabed from the drop of anchor files, the soil penetration characteristics of anchor piles should be reasonably estimated. In the present study, the penetration characteristics of anchor piles into the soil under which subsea HVDC cables are embedded were evaluated using numerical simulations and field verification tests. The numerical simulation for the soil penetration phenomena of anchor piles was carried out using the fluid–structure interaction analysis method using the general purpose nonlinear finite element analysis code based on explicit time integration. Regarding the soil into which anchor piles penetrate, three types of soil—a clay layer, a sand layer, and a clay–sand mixed layer—were considered, which are the representative soil types in the southwest sea of Korea, where many subsea HVDC cables have been embedded. The result of fluid–structure interaction analysis showed that the maximum penetration into the clay layer was higher than that into the sand layer and the clay–sand mixed layer by 86.3% and 36.4% or more, respectively. The error rates of the field verification test and the fluid–structure interaction analysis were found to be 9.8%, 2.4%, and 2.4% in the clay layer, the sand layer, and the clay–sand mixed layer, respectively, which were found to be reasonable levels when considering that it was the numerical simulation for the soil penetration of an anchor pile resulting from drop impacts. The penetration depths of anchor piles were found to be the deepest in the clay layer, showing values of 3.9 to 4.1 m, and those in the sand layer were the shallowest, showing values of 1.9 to 2.1 m.

A Case Study of Completely Buried Wind-Power Cable Detection Using 3D Acoustic Imaging

As offshore wind power is renewable energy produced through the installation and operation of large-scale offshore infrastructure, risk management is crucial in power platforms. Safety accidents caused by external factors during the operation of submarine power cables can lead to enormous costs, thus necessitating the monitoring of burial depth and route information of cables. In this study, we developed a 3D acoustic imaging method that obtains information on the route and depth of completely buried power cables. An acoustic source-based engineering ocean seismic 3D (EOS3D) system has been used to detect buried objects in the subsurface because conventional sonars, such as multi-beam echo sounder (MBES) and side-scan sonar (SSS), which are used to analyze seafloor characteristics, have limitations in detecting completely buried cables in the subsurface. Field data were obtained as 8-channel data using a chirp source (2–8 kHz) designed to obtain a 25 × 25 cm horizontal spatial resolution from real-time kinematic (RTK) positioning. The image stack method was proposed to effectively detect buried cables, with the vertical gradient analyzed using signals decomposed into representative bin sizes and low-mid-high-frequency components. The acoustic anomalies of buried objects, identified as export cables and protectors, were processed into images using the proposed image stack method and gradient analysis. This case study of buried wind power cables using 3D acoustic imaging could be utilized in burial assessment survey (BAS)-data acquisition, processing/analysis processes, and operation and management of buried cables.

Deep Learning Swell Noise Estimation

Towed-streamer data are often contaminated by low-frequency, high-amplitude noise caused by the streamer motion, cable depth controllers, or sea surface waves. So-called swell-noise usually affects a number of neighbouring traces and appears on the shot record as vertical striping. Attenuating this strong noise over a weak reflection signal can be a significant challenge. In this work, we describe a deep learning approach for estimating and subtracting such noise from the recorded data. Inspired by the ideas of the residual network and the generative adversarial network, we have developed a conditional generative adversarial network to estimate swell noise which could also be applicable to other types of noise found in seismic data. We demonstrate the effectiveness of the proposed network by estimating a high-quality swell noise model on a field data example.

Study on Buried Depth Protection Index of Submarine Cable Based on Physical and Numerical Modeling

The buried depth of submarine cables is very important to avoid damage on the cable from dropping and dragging anchors. This study focused on the actual engineering needs of submarine power cable protection and laying construction. In order to investigate the buried depth protection index of submarine cable, physical model tests, theory analysis, and numerical simulations were conducted in this study. The effects of the bottoming velocity, dropping energy, and anchor mass on the anchor penetration depth were analyzed and investigated. The analytical model based on the impact and drag mechanism is presented to analyze the forces and energy on the anchor. The accuracy and reliable of the model test results are verified by the theory analysis and numerical simulation, indicating that the buried depth protection index of the submarine cable in the research area is recommended to be 3 m. The research results can provide guidance for operation of the submarine cable laying machine and submarine cable protection.

Research on Operation Safety of Offshore Wind Farms

The operation of offshore wind farms is characterized by a complicated operational environment, long project cycle, and complex vessel traffic, which lead to safety hazards. To identify the key factors affecting the operational safety of offshore wind farms, the risk characteristics of offshore wind farm operations are analyzed based on comprehensive identification of hazards and risk assessment theory. A systematic fault tree analysis of the offshore wind farm operation is established. The assessment shows that the key risk factors that induce offshore wind power collapse, corrosion, fire, lightning strikes, blade failure, personal injury, ship collision, and submarine cable damage accidents are gale, untimely overhauling, improper fire stopping methods, high average number of thunderstorm days, the loose internal structure of fan, working at height, collision avoidance failure, and insufficient buried depth of cables.

Electric Field and Temperature Simulations of High-Voltage Direct Current Cables Considering the Soil Environment

For long distance electric power transport, high-voltage direct current (HVDC) cable systems are a commonly used solution. Space charges accumulate in the HVDC cable insulations due to the applied voltage and the nonlinear electric conductivity of the insulation material. The resulting electric field depends on the material parameters of the surrounding soil environment that may differ locally and have an influence on the temperature distribution in the cable and the environment. To use the radial symmetry of the cable geometry, typical electric field simulations neglect the influence of the surrounding soil, due to different dimensions of the cable and the environment and the resulting high computational effort. Here, the environment and its effect on the resulting electric field is considered and the assumption of a possible radial symmetric temperature within the insulation is analyzed. To reduce the computation time, weakly coupled simulations are performed to compute the temperature and the electric field inside the cable insulation, neglecting insulation losses. The results of a weakly coupled simulation are compared against those of a full transient simulation, considering the insulation losses for two common cable insulations with different maximum operation temperatures. Due to the buried depth of HV cables, an approximately radial symmetric temperature distribution within the insulation is obtained for a single cable and cable pairs when, considering a metallic sheath. Furthermore, the simulations show a temperature increase of the earth–air interface above the buried cable that needs to be considered when computing the cable conductor temperature, using the IEC standards.

Transient Thermal Performance of Power Cable Ascertained Using Finite Element Analysis

This paper presents the computation of the cable ampacity and the temperature distribution through long duration based on the equivalent thermal circuit based on IEC 60287 standard and the Finite element method using COMSOL (Multiphysics environment, version 5.5). This study investigated the cable ampacity and the temperature rise of the cable core and sheath at steady state and emergency conditions. The cable ampacity was investigated at different conditions such as the variation of cable depth, soil properties, and soil temperature. The results confirmed the adaptation between the thermal circuit results and the COMSOL results as well as the effectiveness of using the numerical method to compute the cable ampacity. Using the COMSOL-based thermal properties evaluations, the transient performance of the cable is ascertained. The transient study is performed for different cable loading currents and dry zone sizes.

Dynamic analysis of deep-towed seismic array based on relative-velocity-element-frame

Before analyzing the distribution of submarine minerals, the positions of hydrophones installed in seismic array cables must be known. Generally, the hydrophone positions are computed through inversion calculation based on assumptions that conflict with the facts, and some assumed parameters are adjusted repeatedly to reduce the spatial aliasing in seismic reflection profiling. Furthermore, the inversion calculation cannot study the relations between the cable shape and working conditions. Dynamic analysis is proposed to predict the hydrophone positions and investigate these relations herein. The array cable is modeled by the lumped mass method with respect to the relative-velocity-element-frame. The results of the dynamic analysis and inversion calculation are verified using the recorded depth data. Although the cable shape by inversion calculation is the same as that by dynamic analysis, the cable depth by inversion calculation demonstrates an obvious shift in comparison with the depth data; meanwhile, the results obtained via dynamic analysis match well with the depth data. Additionally, three virtual maneuvers—diving and rising, depth change, and lateral current—are designed to study the relation between the cable shape and working conditions. The obtained results may help guide exploration and promote the development of exploration equipment.

Research and application of finite-element time-domain method for transient response of buried cable excited by an electromagnetic wave

To analyze electromagnetic (EM) transient responses of a buried cable, an accurate and reliable time-domain method is proposed in this paper. The approach is based on the transmission line (TL) second-order telegrapher’s current equation, and the expressions of ground impedance and admittance at higher frequencies are used to obtain the discrete finite-element time-domain (FETD) equations for the cable. Finally, transient responses at termination loads of the buried cable are obtained based on the global FETD equation, which is assembled using discrete FETD equations on all intervals. The accuracy and effectiveness of the proposed FETD method are demonstrated by comparing numerical results to those of the finite-difference time-domain (FDTD) method. The proposed method is also employed to investigate the effects of numerous parameters on transient currents of the buried cable, including the soil conductivity, buried cable depth, and incident and polarization angles of the external EM wave.

Estimation of Submarine Cable Burial Depth using CBRA Model

Submarine cable plays an important role in land-island power supply, and domestic and overseas communication. However, damages to the submarine cable have been caused by various dangerous factors such as ship anchors and trawlers. Even though the annual cost for recovering submarine cables is enormous, there is no definite method for protecting submarine cables. The present study investigates the applicability of Cable Burial Risk Assessment (CBRA) model to the estimation of the burial depth of submarine cables. This technique employs a statistical approach to the marine environment of the target area and the navigation data of nearby passing ships. Two computer programs have been developed to facilitate the calculations of anchor penetration depths in clay and sand, respectively. An example application of CBRA model was demonstrated here taking a tentative cable route to Haenam - Jeju sea area. The cable burial depth was calculated to vary from a minimum of 0.6m to a maximum of 0.74m for a 2500 year return period and a minimum of 0.6m to a maximum of 1.40m for a 25,000 year return period. This study also determined the target ship weight in the CBRA model through the statistical expectation. The expected cable burial depth appeared from a minimum of 0.6m to a maximum of 1.26m.

The Study of Burial Depth and Risk Assessment of Submarine Power Cable

In this paper, the determination of submarine cable burial depth and risk assessment method are studied. The main objective of the method is to optimize the burial depth of submarine cable in terms of specifying an economically feasible buried depth which ensures a satisfactory level of protection and by acceptable risk. The risk factors influencing the burial depth of submarine cables were analyzed. The primary risk to which submarine cables are exposed is anchoring, while the environmental factor and the cable itself have relatively smaller impact and cannot be mitigated by burial. Then the calculation method of probability and depth of penetration of anchor deployment near a submarine cable were studied. The method can reduce the protection cost while providing the cable with better protection and to guide the specification of the burial depth of submarine power cables. Compared with traditional methods which relying on operational experience, the probabilistic analysis determines the burial depth based on the water depth and seabed conditions of different sections as well as the vessel traffic of the cable route which is an accurate and quantitative approach.

System Design of Intelligent Buried Cable Path Detection Device

For underground cables, if the design data is not clear enough, it is very difficult to find out the path, not only to waste a lot of manpower, material and time, but also to cause inestimable loss. Therefore, finding a fast and accurate cable path detection system and shortening detection time has become the common goal of scientific and technological personnel both at home and abroad. This paper focuses on the problem of underground cable path detection technology, and designs and develops the underground cable path detection system. The functions of the system include: the peak mode, the relative position of the cable and operator, and the direction of the operator to move; in the valley value mode, the compass display function can be used to judge the direction of the operator and cable, prompting the operator to correct his direction in time until the cable is found. The buried position, the cable depth and the current display function, can show the depth and current size of the buried cable in real time. The development of the underground cable path detection system makes the field personnel more fast, convenient and accurate to detect the important information such as the buried location and depth of underground cable. It can save the cost of inspection and accelerate the maintenance and maintenance progress, which has significant practical significance.

Dynamic analysis of umbilical cable under interference with riser

ABSTRACTIn consideration of the bend moment and torque, the lumped mass method was derived. Based on the lumped mass method, the umbilical cable was discretised into a lumped mass model and the interference between the umbilical cable and the riser was taken into account in the dynamic analysis. According to the specific parameters of the umbilical cable in real work, an umbilical cable dynamic model was established to study the dynamic process of the umbilical cable under certain interference. The simulation results show that the depth of the sea, the wave directions and the length of the pipelines have different effects on the occurrence of interference: the increase in depth and length of the umbilical cable and riser changes the flexibility of the pipeline, the change of the wave direction changes the system’s motion boundary and the form of collision. The conclusions have a certain guiding significance for the study of the umbilical cable during operation.

Analysis of Tow Cables

Hydrodynamic analysis of tow cable has an important role in the design of towed system. Cable catenary, cable depth attained for a given cable length, cable tension and angle of cable with the body are the parameters that affects the towed system related to tow cable. This paper deals with analysis of a set of commercially available cables of different properties using Orcaflex software. The cables were analysed for different payouts at different speeds. A comparative study was performed on the results obtained relating the cable properties (diameter and buoyancy). This provides a rough idea on the catenary profiles that reflects the depth and trail achieved by the cable given payout and speed.