Severe Marine Environment Research Articles

Oscillating water column supporting structures: A review

Abstract Numerous studies on the potential for wave energy in Indonesian waters have been conducted using Wave Energy Converter (WEC). One of the most extensively studied and developed wave energy conversion technologies is the Oscillating Water Column (OWC). Although OWC technology has the good potential, there are still several difficulties and problems that prevent its widespread development and use. One is the difficulty of keeping the systems working in hostile marine conditions. The supporting structure is an integral part of the design and operation of an OWC system. An OWC system’s structural design must guarantee lifespan, stability, and dependability in a severe marine environment. More study is required to create durable and dependable systems, mainly supporting structures functioning in these conditions. This study reviews existing literature to analyse the structural strength of OWC systems, aiming to provide insights into methods for enhancing their durability and reliability, particularly in harsh marine conditions. Key findings include the classification of various supporting structure technologies used in OWC projects, providing insights into their effectiveness and applicability in different environmental settings. Moreover, the study emphasizes ongoing efforts to address obstacles limiting the widespread adoption of OWC technology, underscoring the need for further research and development.

Novel insights on the setting process, hardened properties, and durability of sustainable ultra-high-performance seawater sea sand concrete

Using seawater and sea sand to prepare concrete could effectively alleviate the shortage of freshwater and river sand resources and promote the sustainable development of concrete. However, the current research on the setting process, hardened properties, and sustainability of ultra-high-performance seawater sea sand concrete (UHPSSC) remain considerably limited. In this paper, the UHPSSC was designed to mitigate the autogenous shrinkage and enhance the sustainability, and the effect of seawater, untreated and desalinated sea sand on the setting process, mechanical properties, durability, and sustainability was explored by various tests. Results show that the abundant ions (e.g., Cl- and SO42-) in seawater and untreated sea sand promote the dissolution and nucleation of cement, accelerating the hydration process, and seawater provides a high pH hydration environment, which enhances the pozzolanic reaction activity of silica fume and fly ash. This leads to the enhanced flexural and compressive strengths at early age, reduced flowability, and increased autogenous shrinkage. Although the seawater and untreated sea sand considerably alleviate the natural raw materials consumption, the mechanical performance at 28 d and durability are reduced, thereby decreasing the sustainability index by 7.8 %. The desalination treatment of sea sand decelerates the hydration process of UHPSSC with untreated sea sand due to the lower ion content in desalinated sea sand. The application of desalinated sea sand mitigates the adverse impact of untreated sea sand on the workability, mechanical properties, and durability. However, this desalination process requires more energy and freshwater consumption, thereby decreasing sustainability index. The fly ash incorporation could improve the workability of UHPSSC, and decrease the autogenous shrinkage value by 17.9 %. Moreover, it mitigates the environmental effect without compromising the mechanical properties and durability, leading to an impressive 42.8 % improvement in sustainability. Durability assessment reveals that the prepared UHPSSC specimen maintains outstanding mechanical properties even after 360 d of exposure to severe marine environment, indicating its excellent durability. This paper provides novel insights for achieving a cleaner and more sustainable concrete production.

Atmospheric Corrosion of Al-Zn Coated Steel Sheets Exposed under Marine Environments in Kenya

Corrosion contributes to multibillion USA dollar loses due to deterioration, weakening and ultimate failure of engineering infrastructure globally necessitating further research to build more corrosion resistant materials. The aim of this study was to investigate atmospheric corrosion of aluminium-zinc coated steel roofing sheets exposed to outdoor marine environment in Kenya at Diani and Mariakani sites, representing severe marine and urban marine environments respectively. Al-Zn coated sheets measuring 130 mm x 190 mm coated with masses ranging from 60-150 gm2 were exposed for 2 years. The specimens were collected at periodic intervals following ASTM G1-90 standard method and corrosion products analyzed by Raman spectroscopy, while corrosion rate was determined by weight loss method. Corrosion rate ranged from 0.03 to 0.05 mmpy and increased with decreasing masses coating materials from 150 to 60 (gm2). Diani site recorded higher corrosion due to severe marine conditions characterized by high chlorides, RH of 81%, UV of 12mW/cm2 and a temperature of 27 ℃. Dominant rust constituents were goethite (a-FeOOH) and lepidocrocite (g-FeOOH), while ferrihydrite and maghemite (a-Fe2O3) were measured at low levels suggesting the influence of suphur dioxide and chloride pollution on corrosion rate at both sites. Corossion rates ˂ 0.15 mmpy at both sites suggest excellent protection by the formulated coating media. The results of this contribute to improving the quality of the AZ metal coating media under marine environments.

Numerical Study of the Hydrodynamic Response of Biodegradable Drifting Fish Aggregating Devices in Regular Waves

Fish-aggregating devices play a significant role in tuna purse fisheries. The severe marine environment and the large number of non-biodegradable fish-aggregating devices impact structural safety and cause marine litter. Therefore, hydrodynamic performance and the use of biodegradable materials are crucial issues for ensuring the sustainability of fish-aggregating devices. In this study, a type of virtual biodegradable drifting fish-aggregating device (Bio-DFAD) was designed. Numerical simulations were conducted to investigate the motion responses and relative velocities of Bio-DFADs in regular waves (first- and fifth-order waves). The numerical model was applied based on unsteady Reynolds-averaged Navier–Stokes equations with the realizable k–ε model. For different scenarios of modeling, various conditions were modeled, including the relative length, wave steepness, and diameter of the balsa wood, to analyze their effects on the hydrodynamic response of the Bio-DFADs. The results indicated that the increased relative length, wave steepness, and diameter of balsa wood had a significant influence on the motion response amplitude operators (RAOs) and relative velocity of Bio-DFADs. The results suggested that a relative length (LF/B = 1.5) and smaller diameter (DF = 30 mm) were recommended for fewer motion responses and relative velocity. The obtained results provide insight for practical engineering applications of the hydrodynamic design of Bio-DFADs.

Diffusion-reaction models for concrete exposed to chloride-sulfate attack based on porosity and water saturation

Reinforced concrete structures in an unsaturated state experiences significant degradation when exposed to combined chloride and sulfate attack in marine environments. For evaluating durability and predicting service life of reinforced structures, it is crucial to better understand ions diffusion, porosity variation and water saturation of concrete. Considering effect of porosity and water saturation degree on chloride diffusivity, a new diffusion-reaction model was established to investigate how concrete degrades exposed to the severe marine environment. The model was validated using experimental results from both hydrophobic and plain concrete that had undergone a 14-month exposure test at the Qingdao Wheat Island exposure station. The simulation results of the multi-ions model showed that both plain concrete and hydrophobic concrete experienced maximum reduction of chloride concentration of about 68% when compared to concrete exposed to the unsaturated condition.

Time series prediction based on LSTM neural network for top tension response of umbilical cables

The umbilical cable is an essential component of offshore oil and gas extraction systems. The severe marine environment poses a high challenge to the safety of the umbilical cable structure during operation. The analysis of an umbilical cable requires complex and resource-demanding finite element time-domain simulations to obtain their nonlinear dynamic response. Therefore, in order to solve the problem of structural safety monitoring and real-time assessment of remaining life of umbilical cables under extreme sea states, there is a great need to predict the dynamic response of umbilical cables quickly and accurately during operation, for ease of making fast decisions for system operation and maintenance before the arrival of extreme sea states. Given the strong nonlinear function-approximation ability of the neural network, this study proposes an efficient method for the prediction of the time series of umbilical cable top tension response based on LSTM (long short-term memory) neural network. We use LSTM neural network and ARIMA (autoregressive integrated moving average) model in a real engineering case for time series prediction of the top tension response of the umbilical cable, and the results of the two models are analyzed and compared, and the efficiency and accuracy of the LSTM neural network model are verified. Furthermore, the hyperparameter, dataset and generalization ability of LSTM model are discussed. The results indicate that feasibility of the tension response prediction of umbilical cables under dynamic load in complex marine environments.

Coral Bleaching on the Great Barrier Reef and Causes

With the increasing problem of global warming, the Earth's environment has been seriously damaged in recent decades, especially the Marine environment. Covering 71 percent of the Earth, the oceans are an important part of the planet, but such a beautiful and rich resource is now in decline. This paper mainly expounds the embodiment of environmental problems in coral reefs (take the Great Barrier Reef as an example). Large numbers of coral reefs are bleaching and dying because of climate change. Nowadays, the problem of severe Marine environment is getting worse and worse, which has aroused people's great attention in the world. The main causes of coral bleaching are rising temperatures, eating crown-of-thorns starfish and acidification. Finally, this paper expounds the damage to the ecological environment and the disasters faced by human beings if coral reefs are lost. In the future, we should do our best to protect coral reefs and prohibit vandalism, pollution and overfishing. And can artificially cultivate, transplant coral reefs, remove coral pests and so on. This article aims to point out the causes of coral bleaching, so that more people can realize the crisis we are facing, and call on people to take small steps to protect our only planet.

Investigation on Corrosion Protection Systems and Wear Problems for a Ship Unloader

In the harbour of Port Louis, a wheat ship unloader was facing serious corrosion problems due to the severe marine environment. Therefore, the aim of this study is to propose an effective corrosion protection system for the wheat ship unloader. In addition, wear was observed in its conveyor system. Solutions were proposed to reduce this. Different coating systems were selected, based on ISO 12944-5, including the coating presently being used on the equipment. Outdoor atmospheric exposures and lab tests were performed. The latter consisted of immersion test, water vapour transmission rate of coatings, wet scrub abrasion test, scratch test, pull-out test and flexibility test. For investigating the wear in the conveyor system, flow simulations were performed on Soildworks using different materials. From the investigation performed, it was found that the combination of modified vinyl epoxy primer and acryline polyurethane acrylic top coat was the most suitable for the ship unloader. For the wear problem, aluminium 6061was found to be a more wear resistant material.

Novel intelligent self-responsive function fillers to enhance the durable anticorrosion performance of epoxy coating

Traditional coatings struggle to match the maritime industry's long-term anti-corrosion needs due to the severe marine environment. As a result, integrating functional fillers into coatings is a viable solution to this issue. The sericite (SC) was modified with polyethyleneimine (PEI) as a “binder” and zinc molybdate (ZM) as an “additive” in this study. Following that, a new SC/PEI@ZM filler with self-responsive performance was synthesized effectively. The experimental results demonstrated that the |Z|0.01Hz value of the SC/PEI@ZM coating remained above 8.39 × 1010 Ω·cm2 after being immersed in 3.5 wt% NaCl solution at atmospheric pressure for 60 days. In particular, the coating still maintained favorable corrosion resistance after 7 days immersion under 3 MPa pure O2. The higher corrosion resistance of SC/PEI@ZM coating is mainly attributed to the barrier effect and the complex protective film formed by Zn2+/MoO42− on the surface of the substrate.

A global-local approach for dynamic stress evaluation of lazy wave flexible risers subjected to random wave and vessel motion

ABSTRACT With the increasingly severe marine environment, the safety reliability of unbonded flexible risers under service conditions is closely related to its dynamic stress response. In this paper, a systematic approach for riser dynamic stress evaluation subjected to random wave and vessel motion is developed to explore the influencing factors. This methodology proposes a global model to acquire the riser global dynamic response under random wave and vessel motion, considering the effects of nonlinear bend stiffener (BS) constraint and nonlinear bending behaviour of flexible pipes. Then, an axisymmetric load model and bending load model are employed to convert the forces and curvatures obtained from the global model into the dynamic stresses of the tensile armour layers. The findings show that BS and riser top angle can affect the dynamic curvature for the riser top-end region, however, has little influence on the effective tension and bending curvature for the other regions.

Corrosion of Cu-doped Ni–Mo low-alloy steel in a severe marine environment

The corrosion performance of Cu-doped Ni–Mo steel has been comprehensively studied in a simulated severe marine environment. Results have indicated that the addition of Cu accelerates the generation of corrosion products and increases the corrosion current density during the initial corrosion process, which can be attributed to the increased proportion of high-angle grain boundaries. Cu improves the long-term protection of a rust layer by forming CuFeO2, and stabilizes substances such as NiFe2O4 and compounds containing Mo4+ and Mo6+. Rust layers optimized by Cu display improved electrochemical properties in 3.5 wt% NaCl solution, indicating the beneficial effect of Cu on their protective properties.

Kriging Model for Reliability Analysis of the Offshore Steel Trestle Subjected to Wave and Current Loads

Offshore steel trestles (OSTs) are exposed to severe marine environments with stochastic wave and current loads, making structural safety assessment challenging and difficult. Reliability analysis is a suitable way to consider both wave and current loading intensity uncertainties, but the implicit and complex limit state functions of the reliability analysis usually imply huge computational costs. This paper proposes an efficient reliability analysis framework for OST using the kriging model of optimal linear unbiased estimation. The surrogate model is built with stochastic waves, current parameters, and the corresponding load factors. The framework is then used to evaluate the reliability of an example OST subjected to wave and current loads at three limit states of OST, including first yield (FY), full plastic (FP), and collapse initiation (CI). Three different distributions are used for comparison of the results of failure probability and reliability index. The results and the computational cost by the proposed framework are compared with that from the Monte Carlo sampling (MCS) and Latin hypercube sampling (LHS) method. The influences of sample number on the prediction accuracy and reliability index are investigated. The influence of marine growth on the reliability analysis of the OST is discussed using MCS and the kriging model. The results show that the reliability analysis based on the kriging model can obtain the reliability index for the OST efficiently with less calculation time but similar results compared with MCS and LHS. With the increase of the number of samples, the prediction accuracy of the kriging model increases, and the corresponding failure probability fluctuates greatly at first and then tends to be stable. The reliability of the example OST is reduced with the increase of marine growth, regardless of the limit state.

Experimental study on the durability and microstructure of marine concrete covered with barnacles

This paper characterizes the effect of barnacle settlement on the durability and microstructure of concrete. Twenty-seven concrete samples with different coverage areas of barnacles were extracted from a breakwater in the tidal zone of Huangdao district in the Yellow Sea. Durability properties, including the water absorption, chloride ion resistance, chloride ion content, and pH profiles of the concrete were determined, and the microstructure and pore structure of the concrete were explored to elucidate these durability properties. Results confirm that the settlement of the barnacles could enhance the resistance of the concrete to water absorption, chloride ion penetration and neutralization, and this positive effect is increased as the coverage area of the barnacles expands. The barnacle adhesive at the interface between the concrete and the barnacles has a dense structure and significantly improves the pore structure of the concrete. The shells of the barnacles, barnacle adhesive, and the penetration of the adhesive collectively act as three lines of defense, aid in enhancing the durability of the concrete and then protecting the concrete from the severe marine environment.

Durability assessment and behavior under axial load of circular GFRP-RC piles conditioned in severe simulated marine environment

This study concerns the feasibility of using innovative glass fiber-reinforced polymer (GFRP) bars, spirals, and hoops in constructing sustainable marine reinforced concrete structures. The experimental investigation aimed at assessing the durability and structural performance of GFRP-reinforced concrete (RC) circular piles exposed to two simulated marine environments in the laboratory. A total of 15 GFRP-RC circular piles measuring 304 mm in diameter and 1000 mm in height were longitudinally reinforced with GFRP bars and transversely with GFRP spirals or hoops. Five specimens were fabricated as references (unconditioned). The remaining 10 specimens were continuously immersed in simulated marine environments (conditioned) for 12 months: five were kept in seawater at room temperature (22 °C), while the other five were stored in a large chamber that produced hot waves at high temperature (60 °C). After conditioning, all the specimens were tested under axial compression loads. Two structural variables were considered: the transverse reinforcement configuration (spirals versus hoops) and the confinement level (spiral spacing). The durability assessment included microstructure examinations (i.e., differential scanning calorimetry (DSC) and scanning electron microscopy (SEM)) were performed on GFRP bars and spirals taken from the conditioned piles. Furthermore, the bond between concrete and bars or spirals was checked by optical microscopy (OM). The microanalysis observations indicate no visible damage in the contact surface between concrete and GFRP bars or individual fibers and the resin matrix. The compressive test results pointed out that the GFRP-RC piles subjected to severe marine environments experienced 19% enhancement in their axial capacities compared to their reference counterparts. The GFRP spirals and hoops performed well in severe marine environments. The nominal axial capacities of the columns were predicted with the available design equations and are presented and discussed herein.

Experimental investigation of monopile in overconsolidated marine clay subjected to multiple cyclic lateral loading events

Monopile foundations installed at an offshore site must withstand severe marine environments during their service life. Consequently, their long-term behaviors require careful estimation to ensure adequate safety. This paper details a program involving 1-g model tests on the monotonic and cyclic lateral behavior of a monopile installed in overconsolidated marine clay. Using a self-manufactured loading device, attempts were made to simulate the possible response of the monopile during the serviceability limit state. Experimental results indicate that the deformational response of the monopile, which behaves as shakedown when applied to higher load levels, heavily depends on the load level. However, despite the dominant influence of the load level, the variation in loading frequency could extend the accumulation process of the pile-head deformation sustained from the first hundred to over 1000 loading cycles. In addition, the cyclic regimes of the foundation secant stiffness are affected by the loading frequencies, exhibiting either pure softening or combined softening/stiffening. The feasibility of the available design criteria and empirical models to estimate deformation accumulation is also evaluated based on the proposed test results, indicating that reassessment of the available design methodology is needed to promote applications of the monopile foundation.

Formation and Properties of Superhydrophobic Al Coatings on Steel.

Thermal sprayed aluminum coatings are widely scalable to corrosion protection of the offshore steel structure. However, the corrosion rate of the Al coating increases considerably due to the severe marine environment. It has remained a challenge to improve the corrosion resistance and protective ability of Al coatings. The superhydrophobic surface provides a potential way to improve the corrosion resistance of metal materials. Hence, the development of superhydrophobic Al coatings with superior corrosion resistance is of great interest. In this work, the feasibility of the preparation of superhydrophobic Al coatings on a steel substrate was explored. First, Al coatings were prepared onto the steel substrate by the arc-spraying process, followed by ultrasonic etching with 0.1 M NaOH solution, and afterward passivated using 1% fluorosilanes. The effects of the etching time on morphology, contact angle, and corrosion resistance of the Al coatings were evaluated. The schematic model of the fluorosilane passivation process on the Al coating surface was provided. The micro/nanoscale surface structure of the low-surface-energy fluorosilanes promotes the wetting angle of 153.4° and a rolling angle to 6.6°, denoting the superhydrophobic properties. The superhydrophobic Al coating surface displays excellent self-cleaning performance due to its weak adhesion to water droplets. The corrosion current density of the superhydrophobic Al coating (1.36 × 10–8 A cm–2) is 2 orders of magnitude lower than that of the as-sprayed Al coating (1.18 × 10–6 A cm–2). Similarly, the charge-transfer resistance is found to be 12 times larger for the superhydrophobic Al coating and the corresponding corrosion inhibition efficiency reaches 98.9%. The superhydrophobic Al coating displays superior corrosion resistance and promising applications in a marine corrosion environment.

Marine Atmospheric Corrosion of Additively Manufactured Stainless Steels

Additively manufactured (AM) stainless steels (SSs) exhibit numerous microstructural differences compared to their wrought counterparts, such as Cr-enriched dislocation cell structures. The influence these unique features have on a SSs corrosion resistance are still under investigation with most current works limited to laboratory experiments. The work herein shows the first documented study of AM 304L and 316L exposed to a severe marine environment on the eastern coast of Florida with comparisons made to wrought counterparts. Coupons were exposed for 21 months and resulted in significant pitting corrosion to initiate after 1 month of exposure for all conditions. At all times, the AM coupons exhibited lower average and maximum pit depths than their wrought counterparts. After 21 months, pits on average were 4 μm deep for AM 316L specimen and 8 μm deep for wrought specimen. Pits on the wrought samples tended to be nearly hemispherical and polished with some pits showing crystallographic attack while pits on AM coupons exhibited preferential attack at melt pool boundaries and the cellular microstructure.

Joint User-Subcarrier Pairing and Power Allocation for Uplink ACO-OFDM-NOMA Underwater Visible Light Communication Systems

Underwater visible light communication (UVLC) is a promising technique to provide high-speed data transmission in the water. Since it is quite difficult to supply power to the underwater devices in the complex and severe marine environment, energy consumption would be a main consideration in the practical deployment of UVLC system. In this article, we investigate an asymmetric clipped optical orthogonal frequency division multiplexing and non-orthogonal multiple access (ACO-OFDM-NOMA) scheme for the uplink UVLC system to realize energy saving and support massive device connectivity. A mixed integer nonlinear programming problem of jointly optimizing the user-subcarrier pairing and power allocation is formulated to maximize the system utility. To tackle this problem, we propose a low-complexity iterative algorithm to obtain an optimal solution. Simulation results verify that the proposed algorithm converges quickly, and the uplink ACO-OFDM-NOMA system using the proposed algorithm has better utility compared to the uplink ACO-OFDM-NOMA system using the greedy algorithm and the traditional uplink ACO-OFDM access (ACO-OFDMA) system.

Static and dynamic loading behavior of a hybrid foundation for offshore wind turbines

Considering the deficiencies of the traditional monopile foundation for offshore wind turbines (OWTs) in severe marine environments, an innovative hybrid foundation is developed in the present study. The hybrid foundation consists of a traditional monopile and a wide–shallow bucket. A series of numerical analyses are conducted to investigate its behavior under the static and dynamic loading, considering various loading eccentricities. A traditional monopile with the same steel volume is used as a benchmark. Although the monopile outperforms the hybrid foundation in terms of the ultimate moment capacity under each loading eccentricity, the latter can achieve superior or the same performance with nearly half of the pile length in the design loading range. Moreover, the horizontal load and moment are mainly resisted by the bucket and the single pile in the hybrid foundation respectively. The failure mechanism of both the hybrid foundation and the monopile is excessive rotation. In the rotation angle of 0.05 rad, the rotation center is located at the depth of approximately 0.6–0.75 times and 0.65–0.75 times the pile length for the hybrid foundation and the monopile respectively. The increasing loading eccentricities can lead to increasing moment bearing capacity, increasing initial stiffness and upward movement of the rotation center of the two foundations, while decreasing load sharing ratio of the single pile in the hybrid foundation. Three scenarios are considered in investigating the dynamic loading behavior of the hybrid foundation. Dynamic response results reveal that addition of the bucket to the foundation can restrain the rotation and lateral displacement effectively. The superiority of the hybrid foundation is more obvious under the combined wave and current loading.

Experimental determination and numerical validation of the chloride penetration in cracked hydraulic concrete structures exposed to severe marine environment

Concrete is one of the most broadly materials that used in hydraulic structures. Both in new and service, and for various reasons, concrete structures are oftentimes cracked. The existence of cracks plays as a catalyst role of reduction the target service life. One of the main reasons of the impairment of the durability of these structures is chloride-motivated corrosion of the iron reinforcement. This paper aims to find, for the first time, the effect of cracks, experimentally and numerically, on chloride penetration in Hydraulic Concrete Structures (HCS) exposed to varied heads of seawater. The experimental results were acquired with Darcy law, by using new laboratory method called Hydraulic Pressure Tank (HPT) with artificial cracked specimens, whereas the chloride penetration profiles were obtained with development Electrical Conductivity Technique (ECT). The numerical simulation results were obtained using microstructure finite element models. A good acceptance between experimental and the numerical chloride penetration results was gotten.