Submersible Platforms Research Articles

Dynamic responses and mooring line failure analysis of the fully submersible platform for floating wind turbine under typhoon

The mooring line of the floating platform may fail due to various reasons in long-term service conditions, such as overloading, accidental conditions, and performance deterioration. Combining the advantages of three types of floating offshore wind turbine (FOWT) platforms, i.e., spar type, semi-submersible platform type, and tension-leg platform (TLP) type, a novel fully submersible platform (FSP) with vertical CFRP tendons and circumferential catenary chains was developed. The commercial hydrodynamic software ANSYS-AQWA is used to analyze the dynamic responses and mooring forces of the FSP under different wind speeds. It is assumed that a certain mooring line is disconnected from the platform when its safety factor is smaller than the threshold value. The influence of the mooring line failure on the motion characteristics of the platform and the states of the remaining mooring lines are analyzed. Simulated results show that the failure of mooring lines mainly affects the responses of sway, roll, and yaw, which is closely related to the stiffness and symmetry of the mooring system. Once a mooring line fails abruptly, the equilibrium of the mooring is disturbed and the mooring forces are redistributed, leading to a continuous failure of the same type of mooring on the same side, but the entire structure remains stable without overturning, indicating that the mooring system has good redundancy under extreme conditions.

Learning-imitation strategy-assisted alpine skiing optimization for the boom of offshore drilling platform

The boom of a semi submersible platform has large moment of inertia and high operating cost. How to effectively achieve the optimal solution of the boom to minimize the design and operation cost is a dynamic problem. In this study, a learning-imitation strategy-assisted alpine skiing optimization (LISASO) is proposed to find the optimal solution of the semi submersible platform boom. Firstly, the optimization model of the boom of the semi submersible platform is established. Secondly, the learning-imitation strategy (LIS) is implemented to improve the performance of the alpine skiing optimization (ASO). In LIS, the learning ability of individuals and the imitation of competitions are introduced to strengthen the association between individuals and the first individual. The performance of the LISASO is verified by three truss examples. The statistical results demonstrate that the LISASO is more competitive compared with other state-of-the-art optimization algorithms. Finally, the LISASO is applied to solve the optimal structural parameters of the boom. Results show that the energy consumption is reduced by 18.32% compared with the initial design.

Transient analysis of a novel full submersible floating offshore wind turbine with CFRP tendons

Floating offshore wind turbines (FOWTs) to exploit steady and abundant wind resources have attracted a great deal of attention in recent decades. The deep sea is rich in wind energy, but wind farms in these areas face some extreme environmental conditions, e.g., typhoons. A conceptual anti-typhoon full submersible platform (FSP) with carbon-fiber-reinforced polymer (CFRP) tendons, combining the advantages of existing various types of floating wind turbine foundations is proposed. The stability and tendon stresses of the proposed FSP were assessed through a combination of ANSYS-AQWA and FAST, and a comparison with other traditional platforms was made to verify the performance superiority of the proposed FSP. Additionally, the effects of mooring system type on the stability of the FSP under extreme wind speed conditions were investigated. Numerical results showed that the displacements of the proposed FSP in the surge and heave directions were significantly reduced, but in the pitch direction, they were not much different from those of the tension-leg platform. A hybrid mooring system can significantly reduce the surge displacements of the platform, which is essential for a high power output from a FOWT. In general, the proposed FSP is a promising and competitive offshore wind structure.

A Prediction Method of Internal Solitary Wave Loads on the Semi- Submersible Platform

A Prediction Method of Internal Solitary Wave Loads on the Semi- Submersible Platform

Advances in Underwater Oil Plume Detection Capabilities

Historically, visual observation is an emergency responder's first ‘tool’ in identifying spilled oil. Optical detection has since expanded to include a myriad of signals from space, aircraft, drone, vessel and submersible platforms that can provide critical information for decision-making during spill response efforts. Spill monitoring efforts below the air-water interface have been vastly improved by advances with in situ optical sensors and vehicle platform technology. Optical techniques using fluorescence, scattering, and holography offer a means to determine dissolved versus droplet fractions, provide oil concentration estimates and serve as proxies for dispersion efficiency. For subsurface spills over large space and time scales, Autonomous Underwater Vehicles (AUVs) can be used to provide subsurface plume footprints and estimate oil concentrations. For smaller, more frequent spills, tethered compact Remotely Operated Vehicles (ROVs) may be more appropriate as they are easy to deploy for rapid detection. Two underwater oil detection technologies have been developed: (1) A Remote Environmental Monitoring UnitS (REMUS-600) AUV equipped with fluorescence and backscatter SeaOWL UV-A (Oil-in-Water Locator; Sea-Bird Scientific WET Labs Inc.), holographic imager (HoloCam; SeaScan, Inc), hydrographic information, video camera, CTD and a water/oil sampler. (2) A tethered ROV system (DTG2, Deep Trekker Inc.) equipped with video camera, UviLux (Chelsea Technologies Group, Inc) fluorometer, a CTD and water/oil sampler. Calibration and validation tests of the sensor suite were conducted at the Coastal Response Research Center flume tank (NH, USA). Oil concentration estimates were verified by chemical analysis of hydrocarbons and particle size analysis (LISST 200X, Sequoia, Inc). Operational performance of the ROV platform and sensors was evaluated at the Ohmsett wave tank (NJ, USA). Field performance of the REMUS and sensor suite was evaluated at natural seeps near Santa Barbara, CA. This research demonstrates the forensic value of in situ optical data for improved understanding of the behavior and transport of spilled oil below the air-sea interface.

Innovative Benthic Lander for Macroalgae Monitoring in Shallow-Water Environments

The colonization of underwater environments by exotic seaweeds is causing major ecological problems around the world. This project, referred to AMALIA, aims to transform this current ocean threat into an opportunity by adding value to the macroalgae present off the northwest of the Iberian Peninsula. To do so and to observe the presence of seaweeds in situ, an ocean modular submersible platform was developed. This platform was designed to be capable of detecting and surveying surges of invasive seaweeds while withstanding sea conditions. Conceptual designs followed by a screening process were performed, taking into consideration criteria such as operational range and modularity. An open-frame lander was considered and further developed using buckling criteria. In parallel, a state-of-the-art monitoring system was created using spectral imaging, allowing for the future creation of a macroalgae identification system. In addition, sensorial systems for characterizing growth conditions were introduced. Laboratory trials were executed to assess the capability of the system, and sea trials are currently being performed. Numerical simulations and laboratory trials indicate that the structure is fully capable of being deployed for shallow-water environments with a state-of-the-art invasive seaweed monitoring system while maintaining a high degree of modularity.

Influence of Payloads on the Hydrodynamic Response of Semi Submersible Platform

The hydrodynamic analysis of the semi submersible platform was carried out for height. with and without mooring condition by changing the metacentric A physical model of scale 1 in 100 was tested in 1m width and 30 m long regular wave flume at CWR (Centre for Water Resource) Laboratory. The model has four rectangular columns and two rectangular pontoons along with mooring. The natural heave, pitch and roll periods of the semi-submersible have greatest impact on the downtime of platform operation.This paper focus on the influence of payloads on the hydrodynamic behavior semisubmersible platform. On the basis of present work, the configuration of Semi-submersible having four rectangular columns and two rectangular pontoons has been arrived at for the proposed desalination plant.

Discussion of assumptions behind the external dynamic models in ship collisions and groundings

ABSTRACTIn the past decades, a prevailing way for the assessment of responses in ship collisions and groundings has been to decouple the problem into two parts: external dynamics and internal mechanics. The external dynamics deals with global motions of the two interacting bodies prior to, during and after the collision. The main outcome of an external dynamics assessment is the energy loss during the collision, which will be dissipated by structural deformations in the assessment of internal mechanics. The decoupled method works well for right-angle collisions in general, but the accuracy can be much less in many scenarios, e.g. skew collisions with small collision angles and collisions with long durations. This is mainly because the assumptions in the decoupled method are violated in the studied cases. This paper reviews the assumptions and simplifications behind the external dynamic models and discusses validity of the assumptions by comparing with coupled simulation results. Decoupled and coupled models in both planar 3DOF and full 6DOF are addressed. Various collision scenarios are studied, including colliding with oblique plates, grounding on a sloping sea floor, crushing into rigid plates with normal vectors misaligned with coordinate axes, and collision with a submersible platform. In some scenarios, cases with different attack angles and impact velocities are simulated. The outcome will help understand potential limitations of the decoupled method, which should be used with care.

Failure analysis of a Cu 12Mn mechanical fastener in marine environment

Abstract The current work presents a failure analysis study conducted on a fractured mechanical fastener made of Cu 12Mn alloy. The fastener was obtained from a flange of a large-diameter steel tube operating in a sea submersible platform. Macrofractography and Microfractography, Standard Optical Metallography, Scanning Electron Microscopy (SEM), X-ray Fluorescence (XRF) and Energy-Dispersive X-ray Spectroscopy techniques as well as Numerical Modeling were employed to assist the investigation. Fracture was eventually attributed to the synergistic action of fatigue and selective leaching (corrosion-assisted fatigue).

Design and Development of Redeployable Underwater Data Communication Link for Defence Application

<p class="p1">Testing of underwater system for defence application is carried out from submersible platform at specified depth. The underwater platform houses the article under test, equipment and instruments required to conduct the test and to control the platform. During development phase of defence systems, large numbers of sensors are mounted on the test article and data is collected to validate the design. Data acquisition system, video camera and high speed video cameras are positioned on the platform to record physical parameters and observe visually the performance of the article under evaluation. Since the data and video signals are parallely recorded on ship, the bandwidth demand for real-time data communication from underwater platform to control station is very high. The existing technology presently used for underwater communication has limitation of bandwidth and not suitable for defence application. This paper describes in detail the design and development of a re-deployable data communication link by laying a specially designed negatively buoyant fibre optic cable in high-sea from ship to underwater platform to provide higher bandwidth required for defence application. The link has been successfully used for sea state less than one during evaluation of underwater defence system. The availability of bandwidth from underwater platform to control system can be increased significantly by laying fibre optic cable in high sea. It also provides unlimited bandwidth for the above requirement.<span class="Apple-converted-space"> </span></p>

Autonomous Underwater Vehicle untuk Survei dan Pemantauan Laut

AUV is an unmanned submersible platform to accomplish a mission. Side-scan sonar, Conductivity Temperature Depth (CTD), and underwater video camera are usually attached on AUV. These sensors were used for identifying seawater and seabed condition. Data acquired from a survey with an AUV in Kepulauan Riau processed by Neptus software. Side-scan sonar (SSS) visualization is compared to the video image. SSS signal visualization has a unique pattern that can be identified within the video image. Different substrate structure caused different signal visualization. The relation between the video image and SSS visualization can be used for identifying habitat benthic profile.

Design and Development of Power Management System of Unmanned Underwater Platform for Defence Application

<p style="margin-bottom: 0.14in; line-height: 115%; page-break-before: always;" align="JUSTIFY"><span style="font-family: Times New Roman,serif;"><span style="font-size: medium;">Development of underwater weapon systems for defence application are carried out on unmanned underwater platform. Extensive testing and measurement of physical parameters are carried out during development phase trials. After several trials, system is qualified and installed on the actual platform. Mostly battery bank is used to power the unmanned platform. Design and development of power management system is an important issue of underwater platform to increase the underwater time of the platform. During development trials some additional equipments are also included for which power requirement was not considered during design of battery bank. This also becomes an issue to manage the power requirement of additional equipment beyond the design capability of the battery bank. Unmanned submersible platforms at a depth of 50 m to 70 m are used during development of underwater missile. Platform houses the missile to be tested, equipment required for missile launch and control of the platform. A power management system (PMS) with redundancy and fault tolerant features achieving reliability figure of 99 per cent has been designed, developed and tested successfully for underwater phase of missile launch. The system allows remote operation of individual or group of equipment, current monitoring, and isolation of faulty equipment. Power requirement for additional equipment on platform is also included in PMS without affecting the reliability of the system. Power control of equipment on requirement basis to save battery power, to increase underwater time of platform and to manage additional power requirement beyond the designed capacity of the battery bank is described in detail.</span></span></p>

Comparison of Simulation of Torpedo-shaped Anchor whit and without fins for submersible offshore platforms under tensile force

Comparison of Simulation of Torpedo-shaped Anchor whit and without fins for submersible offshore platforms under tensile force

Implementation of Linear Potential-Flow Theory in the 6DOF Coupled Simulation of Ship Collision and Grounding Accidents

Ship collisions and groundings are highly nonlinear and transient, coupled dynamic processes involving large structural deformations and fluid structure interactions. It has long been difficult to include all effects in one simulation. By taking advantage of the user-defined load subroutine and the user common variable, this article implements a model of hydrodynamic loads based on linear potential-flow theory into the nonlinear finite element code LS-DYNA, facilitating a fully coupled six degrees of freedom (6DOF) dynamic simulation of ship collision and grounding accidents. Potential-flow theory both with and without considering the forward speed effect is implemented for studying the speed influence. With the proposed model, transient effects of the fluid, global ship motions, impact forces, and structural damage can all be predicted with high accuracy. To the authors' knowledge, this is the first time the fully coupled 6DOF collision and grounding simulations are carried out with linear hydrodynamic loads for transient conditions but without simplification of collision forces. The proposed method is applied to calculations of an offshore supply vessel colliding with a rigid plate and with a submersible platform. The results are compared with a decoupled method and discussed with emphasis on the influence of different initial velocities. The proposed method is capable of predicting both the 6DOF ship motions and structural damage simultaneously with good efficiency and accuracy; hence, it will be a very promising tool in the application to ship collision and grounding analysis.

A new approach for coupling external dynamics and internal mechanics in ship collisions

This paper proposes a new model, which efficiently couples the external dynamics and internal mechanics in ship collision accidents. This method is especially useful for design purposes since the detailed ship hull profile is not needed.Ship motions in the horizontal plane are considered. The collision forces are calculated using the explicit nonlinear finite element code LS-DYNA. The hydrodynamic forces in surge, sway and yaw are calculated using a traditional ship maneuvering model with a series of nondimensional coefficients determined from experiments. These forces are applied to the center of gravity (COG) of the ship with the user defined load subroutine in LS-DYNA and are solved together with structural response analysis during simulation.The proposed 3DOF coupled method was applied to calculations of an offshore supply vessel colliding with a rigid plate and a submersible platform. The results were compared with those predicted by a decoupled method. Ship motions with the proposed method compared reasonably with SIMO simulations. The advantages and limitations of the method were further discussed in the view of the equivalent added masses.

Transient Numerical Simulations for Hydrodynamic Derivatives Predictions of an Axisymmetric Submersible Vehicle

In this study, a method for obtaining of hydrodynamic derivatives by numerically replicating the Planar Motion Mechanism (PMM) tests of an axisymmetric submersible model is demonstrated. The numerical simulations of PMM tests are regarded as transient due to the movement of the model in the discretized computational domain thus causing mesh deformation. To accommodate the sway and yaw oscillation motions of the model, the entire computational domain is divided into three zones namely rotating, inner and outer zone. Multi-block structured grid is generated with finer resolution in the proximity of the model to capture the boundary-layer flows. Non-conformal fluid interfaces are used to connect the three zones. Commercial CFD Solver FLUENT is used to simulate the flow characteristics while the dynamic mesh capability included in the software is applied to handle the mesh deformation during the movement of the model. In order to verify the CFD method, 6:1 prolate spheroid is used as it can be idealized as an axisymmetric submersible model. The CFD results of added mass derivatives of the model show very close agreement when compared with the theoretical values. The present study is an attempt towards developing an economical CFD method for evaluating the hydrodynamic derivatives of submersible platforms such as submarines, torpedoes and autonomous underwater vehicles during early design stages.

A new video survey method of microtopographic laser scanning (MiLS) to measure small‐scale seafloor bottom roughness

A novel video survey method measures small‐scale seafloor bottom roughness in fragile and deep‐sea habitats called microtopographic laser scanning (MiLS). Using a controlled submersible platform, an attached downward‐ facing video camera with a single optical laser can return imagery to detail the bottom profile at a resolution of ~1–2 cm. The method compares the position of the underlying substratum and laser dot between successive video frames to determine distance traveled in the forward direction and substratum height. The video imagery is processed using photogrammetry to calculate small‐scale topography (horizontal and vertical axes). MiLS is adaptable for most aquatic habitats as it can be executed using any platform that can move forward with a constant slope over the desired transect. Traditional techniques of measuring small‐scale roughness are largely restricted to easily accessible habitats and often yield measurements that are relative and not comparable among different habitats and studies. Quantifying roughness in ways that permit comparisons is critical to understanding effects of bottom roughness and would benefit many fields of aquatic science. With its versatility, ability to access remote locations and output of quantified measurements, MiLS has the potential to fill this need. It is also likely this method will be useful in subaerial habitats such as wetlands. Here, we describe the MiLS equipment, theory, and method in detail, and then demonstrate its application in a lab trial and in a field study in a deep‐sea (≤450 m depth) sponge and coral habitat where its high resolution, accuracy, and precision is made evident.

Visual motion ambiguities of a plane in 2-D FS sonar motion sequences

Sonar is the most common imaging modality in underwater, and high-resolution high data rate 2-D video systems have been emerging in recent years. As for visually guided terrestrial robot navigation and target-based positioning, the estimation of 3-D motion by tracking features in recorded 2-D sonar images is also a highly desirable capability for submersible platforms. Additionally, theoretical results dealing with robustness and multiplicity of solution constitute important fundamental findings due to nature of sonar data, namely, high noise level, narrow field of view coverage, scarcity of robust features, and incorrect matches.This paper explores the inherent ambiguities of 3-D motion and scene structure interpretation from 2-D forward-scan sonar image sequences. Analyzing the sonar image motion transformation model, which depends on the affine components of the projective transformation (or homography) of two plane views, we show that two interpretations are commonly inferred. The true and spurious planes form mirror images relative to the zero-elevation plane of the sonar reference frame. Even under each of pure rotation or translation, a spurious motion exists comprising both translational and rotational components. In some cases, the two solutions share certain motion components, where the imaged surface becomes parallel to a plane defined by two of the sonar coordinate axes. A unique solution exists under the very special condition where the sonar motion aligns the imaged plane with the zero-elevation planes. We also derive the relationship between the two interpretations, thus allowing closed-form computation of both solutions.

3-D motion estimation by integrating visual cues in 2-D multi-modal opti-acoustic stereo sequences

Object reconstruction and target-based positioning are among critical capabilities in deploying submersible platforms for a range of underwater applications, e.g., search and inspection missions. Optical cameras provide high-resolution and target details, but their utility becomes constrained by the visibility range. In comparison, high-frequency (MHz) 2-D sonar imaging systems introduced to the commercial market in recent years can image targets at distances of tens of meters in highly turbid waters. Where fair visibility permits optical imaging at reasonable quality, the integration with 2-D sonar data can enable better performance compared to deploying either system alone, and thus enabling automated operation in a wider range of conditions. We investigate the estimation of 3-D motion by exploiting the visual cues in optical and sonar video for vision-based navigation and 3-D positioning of submersible platforms. The application of structure from motion paradigm in this multi-modal imaging scenario also enables the 3-D reconstruction of scene features. Our method does not require establishing multi-modal association between corresponding optical and sonar features, but rather the tracking of features in the sonar and optical motion sequences independently. In addition to improving the motion estimation accuracy, another advantage of the proposed method includes overcoming the inherent ambiguities of monocular vision, e.g., the scale-factor ambiguity and dual interpretation of motion relative to planar scenes. We discuss how our solution can also provide an effective strategy to address the complex opti-acoustic stereo matching problem. Experiment with synthetic and real data demonstrate the advantages of our technical contribution.

Epipolar Geometry of Opti-Acoustic Stereo Imaging

Optical and acoustic cameras are suitable imaging systems to inspect underwater structures, both in regular maintenance and security operations. Despite high resolution, optical systems have limited visibility range when deployed in turbid waters. In contrast, the new generation of high-frequency (MHz) acoustic cameras can provide images with enhanced target details in highly turbid waters, though their range is reduced by one to two orders of magnitude compared to traditional low-/midfrequency (10s-100s KHz) sonar systems. It is conceivable that an effective inspection strategy is the deployment of both optical and acoustic cameras on a submersible platform, to enable target imaging in a range of turbidity conditions. Under this scenario and where visibility allows, registration of the images from both cameras arranged in binocular stereo configuration provides valuable scene information that cannot be readily recovered from each sensor alone. We explore and derive the constraint equations for the epipolar geometry and stereo triangulation in utilizing these two sensing modalities with different projection models. Theoretical results supported by computer simulations show that an opti-acoustic stereo imaging system outperforms a traditional binocular vision with optical cameras, particularly for increasing target distance and (or) turbidity.