Sea Test Research Articles

Design and experiment of a new mode of mechanized harvesting of raft cultured kelp

The conventional kelp culture rafts remains in extensive use into the present day. However, such rafts rely on manual harvesting and are associated with a number of problems: a lack of uniform quality standards and dimensions, inconsistent length and spacing between kelp ropes due to the mixed use of old and new ropes, the bending and warping of rafts, and difficulty in controlling harvesting vessels due to the large flow of water during the harvesting process. All of these limit the consistent use of mechanized harvesting practices. In order to enable the systemic mechanized harvesting of kelp, this study thoroughly reviewed a range of interrelated culture rafts and harvesting equipment, and a new, optimized mechanized kelp harvesting model was created, involving ropes connected in series along with harvesting cutter systems. Conventional rafts using kelp ropes connected in parallel were transformed and upgraded into serial-connected ones maintaining the original frame width and spacing between kelp ropes and connecting kelp ropes in a "▪" shape, while the harvesting equipment installed on the vessel is designed to lift the kelp ropes, cut kelp stipes and handholds, transport kelp, reel in and store kelp ropes, etc. Kelp ropes are designed to be quickly run through the hole in the cutting device, after which the stipes and holdfasts are cut off at high speed and separated from the kelp ropes. Sea tests showed that, after a single culture cycle, the average yield of kelp cultivated in series-connection rope rafts was 99.77 % compared to conventional parallel connection rope rafts. The cutting and harvesting equipment for kelp culture in series connection rope rafts had a yield of 12–15 t/h, and required 4 operators with a harvest yield of 3–3.75 t/h/person. However, the conventional manual harvesting method for kelp cultures grown on parallel-connected rope rafts had a yield of 8 − 10 t/h, and generally required a single harvesting team consisting of 16 operators. The yield of the mechanized harvesting system was 1.5 times that of manual harvesting, saving 3/4 of labor, while the per capita yield of mechanized harvesting was 6–7 times that of the manual harvesting system. Tests therefore showed that mechanized harvesting has greatly improved efficiency and significantly reduced labor intensity.

Hydrate-bearing sediment of the South China Sea: Microstructure and mechanical characteristics

The successful offshore hydrate production tests in the South China Sea proved the high productivity of clayey-silty hydrate reservoirs. However, the mechanical characteristics of these clayey-silty sediments have not been comprehensively investigated since the concerns about the productivity of such low-permeability reservoirs. In this study, based on the sediment matrix from the South China Sea, specimens were remolded with a porosity of 41% and hydrate saturation of 30% to perform a comprehensive mechanical investigation from micro to macro. Microcosmically, the hydrate in the clayey-silty sediment presents patchy distribution, being particle-displacive and particle-wrapped segregated hydrate morphology. Macroscopically, the compression and swelling indices of hydrate-bearing sediments in the South China Sea are obtained for the first time, which are larger than that of the hydrate reservoir in Krishna-Godavari Basin. The increasing effective confining pressure can slightly reduce the strain-softening and dilation characteristics of the hydrate-bearing sediments in the South China Sea. What's more, the failure strength and stiffness increase with an increasing effective confining pressure, while the failure volumetric strain firstly increases and subsequently decreases with an increasing effective confining pressure. These results are essential input parameters to establish a constitutive model and numerical simulations.

Geodetic SAR for Height System Unification and Sea Level Research—Results in the Baltic Sea Test Network

Coastal sea level is observed at tide gauge stations, which usually also serve as height reference stations for national networks. One of the main issues with using tide gauge data for sea level research is that only a few stations are connected to permanent GNSS stations needed to correct for vertical land motion. As a new observation technique, absolute positioning by SAR using off the shelf active radar transponders can be installed instead. SAR data for the year 2020 are collected at 12 stations in the Baltic Sea area, which are co-located to tide gauges or permanent GNSS stations. From the SAR data, 3D coordinates are estimated and jointly analyzed with GNSS data, tide gauge records and regional geoid height estimates. The obtained results are promising but also exhibit some problems related to the electronic transponders and their performance. At co-located GNSS stations, the estimated ellipsoidal heights agree in a range between about 2 and 50 cm for both observation systems. From the results, it can be identified that, most likely, variable systematic electronic instrument delays are the main reason, and that each transponder instrument needs to be calibrated individually. Nevertheless, the project provides a valuable data set, which offers the possibility of enhancing methods and procedures in order to develop a geodetic SAR positioning technique towards operability.

High-efficient built-in wave energy harvesting technology: From laboratory to open ocean test

The ocean contains a huge amount of renewable energy. There is a tremendous need to develop new ocean energy harvesting technology for long-term and self-sustained global ocean observation. Herein, an omnidirectional and high-efficient built-in wave energy harvesting (WEH) system fully integrated in ocean observing platform is introduced, realizing energy harvesting and self-powered ocean-wave sensing, simultaneously. Based on the chaotic pendulum design and high-efficient electromagnetic coupling effect, the high output power of 520 mW and power density of 0.66 mW/cm3 have been achieved under ultra-low-frequency wave excitation with wave height of 20 cm and period of 1 s, which is extremely higher than most of the reported ones. More significantly, the built-in WEH system was fully integrated with the buoy and successfully completed the offshore test in the Yellow Sea for one month and open ocean test in the Kuroshio Extension (KE) region of Northwestern Pacific for four months. During the offshore test, the working time of an autonomous positioning sensor of the buoy was significant extended from 10 to 25 days, which is 2.5 times longer. During the open ocean test, the built-in WEH system was able to long-term survive in the Kuroshio Extension, which is one of the most dynamically-complex regions in the global ocean. The maximum and averaged output power of 210 and 24.5 mW, respectively, have been achieved, under the ocean wave heights and periods varying from 0.4 to 2.2 m and from 4.2 to 7.2 s. Meanwhile, the generated voltage data can be transmitted via Iridium Satellite and utilized for evaluating and sensing the real-time wave conditions, i.e., wave height and period. It comes to an encouraging conclusion that the built-in WEH system cannot only harvest sufficient energy to extend the service life of ocean observing platform, but also as a self-powered wave sensor to assist ocean monitoring. This work shows a promising milestone in WEH technology from laboratory prototype to practical open ocean application.

Submarine Optical Fiber Sensing System for the Real-Time Monitoring of Depth, Vibration, and Temperature

Ocean observation becomes increasingly important as the ocean climate changes diversely and the marine disasters (such as tsunamis, typhoon, and earthquakes) occur frequently, which typically requires widespread and reliable monitoring techniques. In such a scenario, this paper presents a submarine optical fiber sensing system to realize real-time monitoring of the environmental parameters. The system consists of an undersea optical interrogation module together with multiple fiber Bragg grating (FBG)-based sensors, particularly for the measurement of depth, vibration, and temperature. The experimentally demonstrated sensitivities of the pressure, temperature, and vibration sensors are -1.993 nm/MPa, 0.08 nm/°C, and 0.139 nm/g (g = 9.8 m/s2), corresponding to the resolutions of 0.25 kPa, 0.006°C, and 0.004 g, respectively, based on the interrogation resolution of ~0.5 pm. To verify the feasibility and reliability of the proposed submarine sensing system, a prototype was developed and a proof test under the sea was conducted in an area close to Pearl River Estuary in China. The achieved results from the sea test show promising accuracy that is comparable to the commercially available electric-based sensors. Good characteristics of the surface water wave were observed by conducting the fast Fourier transform of the measured depth change, which shows a dominant frequency of ~0.25 Hz. The system provides the flexibility of replacing various optical fiber sensors easily and the capability of real-time monitoring in a remote way. The demonstrated submarine sensing system could find potential applications in real-time monitoring of the undersea ecosystem and the environmental evolution where multiparameter sensing is in demand.

Long-term numerical simulation of a joint production of gas hydrate and underlying shallow gas through dual horizontal wells in the South China Sea

Recent field tests to recover natural gas from marine gas hydrate reservoirs in Japan and China have exhibited worldwide attention to this strategic energy form; significant challenges still remain in improving the accumulative gas yield for a better economic efficiency. According to the geological survey in marine hydrate reservoirs, there exists a concomitant free gas layer underlying the gas hydrate reservoir. Consequently, here we propose a new scheme to jointly produce the gases from hydrate layer and its underlying shallow gas layer. It was found that a dual horizontal well deployment respectively in the corresponding two layers could contribute to a 4.1 times cumulative gas yield comparing that of a single vertical well scenario. This implies a remarkable enhancement of gas productivity via making full use of the gases in the shallow gas layer. Notably, a potential risk of interlayer failure could occur upon the great interbedded pressure difference (maximum 10.7 MPa) arising from the varying behavior of pressure propagation in the layers. This can be effectively alleviated by separately controlling the depressurization scheme in different layers; a mild step-wise depressurization was suggested in the more permeable shallow gas layer while the hydrate layer with a lower permeability should experience a sharper pressure drop. This was found beneficial reducing the interbedded pressure difference by about 70% without intervening the gas production. An energy return on investment (EROI) analysis showed a promising positive energy harvest of our optimized layer-dependent pressure scheme. It could be therefore a potential method to be applied in the field tests in the South China Sea to improve the economic efficiency without disturbing the reservoir stability.

A tracking method of equivalent constraint boundary for offshore monopile wind turbines from measured accelerations

For offshore wind turbines, the foundation constraint boundary is important for safe operation yet difficult to determine due to the invalidity of the fixed point assumption and influences from soil variability, liquidation, scour, etc. In this paper, a tracking method of equivalent constraint boundary is proposed on the basis of a limited numbers of measured responses, which are entirely different from the results by fixed constraints. The most theoretical achievement of the proposed method is that the time series of so-called first-order displacements can be accurately retrieved from the measured accelerations, leading to the significant difference from the first-order mode shape of conventional modal identification methods. The obvious advantage of the proposed method is that the change in the equivalent constraint boundary can be easily tracked just from the assembled accelerometers, which means that the difficulty and uncertainty of considering soil and water can be avoided. To verify the performance of the proposed method, two numerical examples and a beam experiment are conducted, and measured sea test data from a monopile wind turbine is processed. In detail, the first example is a cantilever beam with a theoretical fixed constraint position, and the results show that the fixed boundary position of the cantilever beam can be accurately estimated with a maximum relative error of 5.3% when six sensors and a fifth-order polynomial are used. The second example is a physical experiment of a cantilever beam and the change of constraint is considered by removing segments one by one. The experimental results show that the predicted constraint position is consistent with the cut length, with a maximum error of 0.009 m at 1.9 m. Furthermore, a 4 MW offshore monopile wind turbine evaluated by commercial software Ansys considering pile–soil interaction is analyzed, and the changes of the predicted equivalent constraint position are in good agreement with the preset values. Finally, the proposed method is applied to the field test data of a 6 MW monopile wind turbine, and a stable estimation of the equivalent constraint position can be obtained. The good accuracy of the proposed method demonstrates its capability in monitoring structural health for offshore wind turbines.

Hovering control of UUV through underwater object detection based on deep learning

It is difficult to find a target object in water and hover an UUV at a relative position from the object to work on it. To solve this problem in this study, we used the geometric principles of camera image mapping to find the real-time position of an underwater object, based on which we investigated the hovering control of an UUV. For underwater object recognition, we used YOLOv2, a deep learning-based object detection algorithm, which has excellent real-time performance. To recognize objects in various underwater environments, the training was conducted on data obtained under different underwater environment conditions, such as illuminance, distance, and obstacles, and the performance of underwater object recognition was increased by using UUV to acquire training data in this study. An UUV was designed and fabricated to implement the proposed algorithm, and a hovering control algorithm was developed for the UUV to recognize a star-shaped object and control its relative distance and direction. The learning data were built, and the object recognition rate was increased using many real sea tests. Moreover, the camera mapping algorithm and the UUV control algorithm proposed in this paper were applied together and tested at sea, to achieve a stable control of the UUV.

Maximum Power Control Algorithm for Power Take-Off System Based on Hydraulic System for Floating Wave Energy Converters

In this study, a hydraulic system generator power converter was modeled to verify the performance of a hydraulic-based power take-off (PTO) system. Moreover, the characteristics and output performance of the PTO system were analyzed with various load control algorithms applied for maximum power control. The simulation performance was verified through a comparison with actual sea test results. Unlike previous studies on hydraulic-based PTO system control for input power performance, the performance of a hydraulic-based PTO system was analyzed through electrical load control in this study. The electrical load control was analyzed by applying a speed control algorithm based on the perturb and observe algorithm and an optimal torque control algorithm. A load control algorithm suitable for maximum power control of the PTO system was proposed by analyzing the characteristics and power generation performance of the system according to the control variables of each algorithm. The proposed optimal torque control algorithm proved to be suitable for maximum power control of the considered PTO system.

Suitability and Sensitivity of Golden Grey Mullet Chelon auratus (Risso, 1810) as a Reference Fish Species for Ecotoxicity Tests in the Black Sea

In recent years, hydrocarbon exploration and production operations have intensified in the Black Sea. Alongside growth in exploration and production activities, the influence of chemical usage across multiple industrial sectors within the Black Sea environment has become increasingly interesting. The aim of this research was to define a protocol for determining the acute toxicity of chemicals using the golden grey mullet, Chelon auratus (Risso, 1810), a native pelagic fish species of the Black Sea. Juvenile golden grey mullets were exposed for 96 h, under semi-static conditions, to dilutions of the reference toxicant 3,5-Dichlorophenol. Results from three reference toxicity tests (LC50 = 1.25 mg/L, 1.739 mg/L, and 1.409 mg/L) indicated that C. auratus is of moderate sensitivity when compared to literature values from EPAs Ecotox database. The protocol described within is intended to ensure Black Sea native organisms are represented by standard hazard assessment practices.

Numerical investigation on the long-term production behavior of horizontal well at the gas hydrate production site in South China Sea

The gas hydrates-bearing shaly silt reservoirs have the characteristics of high reserves and low permeability. The trial production test in South China Sea in 2020 shows that horizontal well has broad prospects for exploiting this kind of hydrate reservoir. However, due to the short trial production time, the long-term gas production behavior is still unknown and needs further investigation. Based on the trial production data, the geological model of multi-layer hydrate reservoir and multi-physical field coupling mathematical model are established. The influence of optimization of depressurization scheme and optimization of reservoir selection on the improvement of gas production efficiency and the influence of compaction effect on attenuation of gas production rate and reservoir stability are investigated. The results show that the pressure drop gradient and constant pressure duration are the key factors affecting step-wise depressurization. The constant pressure duration of single-stage depressurization is more appropriate between 5 and 10 days. When the pressure drops below the equilibrium pressure of hydrate phase, the pressure drop gradient of 0.2 MPa/d is the best for long-term stable gas production. On the other hand, the low permeability of HBL inhibits pressure drop propagation and heat transfer in this sublayer, keeping the gas production rate at a low level in the later stage of production. Further simulation shows that HBL with hydrate saturation of 19% ∼ 25% should be selected for the target reservoir in order to improve gas production efficiency. In addition, the compaction effect in the hydrate decomposition zone is obvious. Compared with the 30 days, the near-well effective permeability drops by more than 30% at 720 days, which is an important reason for the decrease of gas production rate in the later stage of production. Finally, the formation subsidence increases with the expansion of hydrate decomposition radius. The maximum formation subsidence is 0.31 m after 720 days of exploitation.

A switchable zwitterionic ester and capsaicin copolymer for multifunctional marine antibiofouling coating

In this work, a multifunctional marine antibiofouling coating was prepared by covalently grafting switchable zwitterionic ester and capsaicin copolymers onto the polydimethylsiloxane (PDMS) network in a one-pot reaction. Zwitterionic esters could kill bacteria via their positive charge and release dead bacterial cells after hydrolysis to antifouling zwitterionic moieties. Meanwhile, chemically conjugated capsaicin molecules could also be sustainedly released via hydrolyzation to repel marine organisms. Compared with the pristine PDMS matrix, the hydrolyzed coating reduced protein adhesion by 88.5%, bacterial adhesion by 99.0%, and diatom adhesion by 99.5%. The adhesion force of pseudo barnacles was reduced by 72.4%. Marine field tests in the Yellow Sea further confirmed that the coating possessed excellent antibiofouling ability for at least 261 days. This study presents a promising strategy for the development of eco-friendly high-efficiency marine antibiofouling coatings.

Alignment Error Modelling, Analysis and Experiment of the Deep-Water Bolt Flange Automatic Connection Tool

A deep-water bolt flange automatic connection tool plays a very important role in the process of offshore oil exploitation and transportation. In the connection process, the alignment error of bolts and nuts is the key factor to ensure the connection process is successful. Using the kinematics modeling method, this paper created the alignment error model of the deep-water bolt flange automatic connection tool and analyzed the influence of manufacturing accuracy on the alignment error of bolts and nuts through computer simulation software. Based on the error matching design method, the manufacturing accuracy of parts were optimized with a part-size-based priority sequence to ensure the bolt–nut alignment error was within the allowable limits. The land tests, the pool tests and the sea test were carried out. The test results showed that the bolt and nut can be connected in the subsea environment reliably.

Fast and Accurate Underwater Acoustic Horizontal Ranging Algorithm for an Arbitrary Sound-Speed Profile in the Deep Sea

Pairwise ranging between nodes plays a crucial role in Internet-of-Underwater-Things (IoUT) networks, and it typically impacts the overall performance of such networks. As the sound speed depends on several parameters, time-of-flight-based techniques cannot work well under varying sound speeds in actual underwater conditions. Pairwise ranging algorithms that consider stratification should be studied to improve ranging accuracy. However, there is a tradeoff between underwater acoustic ranging accuracy and number of calculations. This makes it challenging to implement underwater acoustic ranging algorithms in IoUT networks. In this article, we propose an underwater acoustic horizontal ranging algorithm that rapidly and accurately estimates the horizontal range from a sender to a receiver under an arbitrary sound-speed profile in the deep sea. Simulation results show that the proposed algorithm accurately calculates the horizontal range with a low computational complexity. We validate the performance of the proposed algorithm using the data collected during an ultrashort baseline precision test in the South China Sea.

Stopping Macroplastic and Microplastic Pollution at Source by Installing Novel Technologies in River Estuaries and Waste Water Treatment Plants: The CLAIM Project

Marine pollution from debris is a major issue nowadays, since every year large amounts of litter enter into the sea. Under the Horizon 2020 framework and within the Cleaning Litter by developing and Applying Innovative Methods in European Seas (CLAIM) project, innovative devices were designed, developed, tested and applied in laboratory and in the field. These consisted of a system named CLEAN TRASH for the prevention of macrolitter in river estuaries before entering the Sea and a filtering system for microplastics (MPs), to be placed at waste water treatment plants (WWTP). Laboratory experiments showed that macrolitters were blocked by 90% by the CLEAN TRASH system, while during the sea testing period at the Kifissos river estuary, a significant source of terrestrial based litter for the Saronikos Gulf, a total amount of 1,175 kg of litter was collected in 38 days before entering the sea, of which the 708 kg (60%) were plastic debris of various sizes and another 164 kg (14%) of styrofoam parts. The lab scale prototype of the filtering system for MPs had an efficiency of about 95%. The upscaled device was tested at the Megara WWTP and was able to withhold a significant amount of MPs. The theoretical contribution of such devices toward the reduction of plastic pollution in the Saronikos Gulf area and the Natura conservation areas therein, was also studied with the use of a 3-D coupled Hydrodynamic-Lagrangian litter tracking model. In all experiments performed, the installation of the above devices for a period of 2 years, resulted in a microplastics reduction by about 87% and a macroplastics reduction ranging from 13 to 43%, depending on the sources.

THE FLOODING AFTER DAMAGE OF A WARSHIP WITH COMPLEX INTERNAL COMPARTMENTS – EXPERIMENTS ON A FULLY CONSTRAINED MODEL IN CALM WATER AND REGULAR BEAM SEAS

In order to provide data to assist in developing and validating a numerical code to simulate the flooding immediately following damage scale model experiments were conducted on a fully constrained model to investigate the progressive flooding through a complex series of internal compartments within a generic destroyer type hull form.
 A 3.268 metre long model of a generic destroyer hull form with a simplified, typical internal arrangement was constructed to cover the configuration of greatest interest. A very rapid damage opening scenario was simulated by rupturing a taut membrane covering an opening. The model was instrumented to measure the levels of water and the air pressures in various compartments. In addition, video footage was obtained of the flooding process from both internally and externally of the model.
 Previous work presented by Macfarlane et al. (2010) showed the results for the unconstrained model. This paper reports on the outcomes from the experimental program where the model was fully constrained in all six degrees of freedom. Firstly, tests were conducted in calm water with damage opening extents ranging from 50% to 100%. When the damage opening was only 50% the rate of rise of water in each of the compartments was only marginally slower than for the 100% damage extent case.
 Secondly, the test results in calm water were compared against results from tests in regular beam seas. A ‘set-up’ of water inside each of the compartments on the 2nd Deck was found during the wave tests. The result of this is that the mean equilibrium water level in each compartment in the regular beam sea cases is noticeably higher than the equivalent calm water case, particularly for the two compartments on the port side, away from the damage. Finally, analysis of the data from further calm water and beam sea tests suggests that a similar result also occurs when the model is fixed at various non-zero heel angles.

Nanoparticles in marine antifouling coatings: a case study

The problem of fouling of hydraulic structures, marine structures, ships and vessels has been acute since the beginning of the era of navigation. The formation of a biofilm of fouling reduces the speed of the vessel, worsens its controllability, increases fuel consumption, shortens the service life of hydraulic structures and increases the load on them. Many methods have been proposed to control fouling. One of the most promising coatings is considered to be based on the use of nanoparticles of biologically active metals and their oxides. The paper discusses various strategies for using nanoparticles to combat biofouling. The paper also presents preliminary results of a study of the antifouling efficiency of coatings modified with nanoparticles of metal oxides Fe-ZnO, ZnO, CuO. The study was carried out by exposing plates with experimental compositions at the sea test site in the Sevastopol Bay for a year, starting from August 2020. The species composition of a separate group of microperiphyton - microalgae - on plates with experimental coatings in the first two months of exposure, as well as the result of photographic fixation of the state of the surface of the plates from the point of view of macro-fouling, was investigated.

The Development History and Latest Progress of Deep-Sea Polymetallic Nodule Mining Technology

Deep-sea polymetallic nodules are a mineral resource with potential for commercial development. Due to the unique deep-sea environment in which they are found, specialized technology and equipment are required for their extraction. In this paper, firstly, the development of deep-sea polymetallic nodule mining technology is classified into three stages, and its characteristics are summarized. Moreover, the results from research into deep-sea polymetallic nodule mining technology are analyzed, including proposals for mining systems, research into key technologies, basic scientific problems, and proof of technical feasibility from sea tests. Secondly, the testing of the collector prototype and the environmental impact assessment study of Global Sea Mineral Resources NV, as well as the progress of the deep-sea polymetallic nodule mining test project in China, are introduced. On this basis, the opportunities and challenges brought by the fast-growing demand for electric vehicles to the development of deep-sea polymetallic mining technology is analyzed, and a possible technical scheme for a mining system and the trends in its development towards high reliability and high standards of environmental protection according to the requirements of commercial exploitation are explored. This provides a reference for the research and development of high-efficiency technology and equipment for the mining of deep-sea polymetallic nodules.

PENGARUH LAMA PENGUJIAN TERHADAP SERANGAN PENGGEREK KAYU DI LAUT PADA KAYU MALAPOGA (Toona ciliata) DAN KAYU TEA (Artocarpus elasticus Reinw. Ex Blume)

Timber in brackish waters and at sea is often damaged by marine wood borer (marine borers). For some fishing communities and people living in coastal areas, the use of boats is very important for them as a support for their livelihood as a means of transportation. Until now, the use of wood as raw material for boats is still frequently used. The wood used in boat building will of course often come in contact with sea water so that it does not rule out the attack of marine borers that will reduce the strength of the wood especially if it is used for a long time. The research objective was to determine the effect of the length of testing at sea on the strength (MOE and MOR) and the natural durability of Malapoga wood (Toona ciliata) and Tea wood (Artocarpus elasticus Reinw. Ex Blume). This research was conducted for 3 months at the Material Testing Laboratory, Faculty of Engineering and Agrotechnology Laboratory, Faculty of Agriculture, Tadulako University, while testing for marine borers was carried out in the waters of Talise Village, East Palu District, Palu City, Central Sulawesi. The test uses the Malapoga wood and Tea wood test samples with sizes2 cm x 2 cm x 30 cm. Meanwhile, at sea testing and data analysis using a completely randomized design (CRD) factorial pattern with 2 factors was repeated 5 times so that there were 30 samples with 10 test samples (control) without immersion in the sea. The results showed that the length of the test had a very significant effect on the strength of wood (MOE and MOR) and the natural durability of wood (weight loss percentage) and the types of Malapoga wood and Tea wood were classified in the strong class V (five).

Iterative turbo receiver design for superposition coding based downlink underwater acoustic OFDM communication system

Superposition coding (SC) with successive interference cancellation (SIC) is considered a promising non-orthogonal multiple access (NOMA) technique for improving spectral efficiency recently. In contrast, the residual interference after SIC at the receiver severely degrades the BER performance when applied in underwater acoustic (UWA) channels. In this paper, we propose a receiver that combined SIC and turbo equalization for superposition coding based downlink UWA orthogonal frequency-division multiplexing (OFDM) system, where a near-far user pair is mainly considered. We design an SC signal detector based on the minimum mean square error (MMSE) algorithm and eliminate the far user information with high power after detection. After SIC, the turbo equalization composed of an MMSE equalizer and LDPC decoder is adopted for the near user symbols with residual interference from the SIC process. Simulation results and recorded data from the Yellow Sea, China, show that the proposed receiver has a performance gain of at least 1 dB under different modulation pairs compared with traditional SIC receiver based on maximum a posteriori probability principle (MAP SIC). The error-free two-user downlink communication result shows a maximum sum-rate of 8kbps over the horizontal range of downlink 3.55km has been achieved in the Yellow Sea test.