Single Point Mooring Research Articles

Design optimization of mooring system: An application to a vessel-shaped offshore fish farm

Design optimization of mooring systems of offshore floating structures is a challenging task, partly because of the large number of design variables, complicated design constraints, nonlinear system behavior, and time-consuming numerical simulations. For engineering designs, efficient yet accurate approaches are needed. This paper proposes an integrated optimization methodology for design of mooring systems. The methodology integrates the design of experiments, screening analysis, time-domain simulations, and a metamodel-based optimization procedure. To demonstrate the methodology, the mooring system of a vessel-shaped offshore fish farm was designed considering the ultimate limit state. The fully-coupled numerical model includes a floater, flexible fish cages and a single-point mooring system. The Kriging metamodels were applied as surrogates for the responses of time-domain simulations. The optimal solutions were found by exploring the design space using a gradient-based search algorithm. Validations were performed on the metamodels and the global optimal solutions. The proposed methodology is also applicable to design optimization of other marine structures.

KAJIAN LOADING-UNLOADING CRUDE OIL DARI PLATFORM KE FSO ABERKHA

FSO (Floating Storage & Offloading), is a floating / ship building that is used as a place to store crude oil. This crude oil is usually from FPS or FPU which is transferred to the FSO through a submerged turret loading (STL) system, then exported/offloaded to the tanker shelter through flexible hoses on the back of the FSO. The existence of FSO greatly benefits the oil industry, since it can be placed as close to the drilling area which is generally offshore. The method of loading ships that arrive can be done in various ways, including STS (ship to ship), Tandem (stern mooring), and SPM (single point mooring). A ship used to store oil is only referred to as a floating storage and offloading vessel (FSO). The results of the analysis show that the efficiency of pump usage has an effect on the fluid flow rate, it’s evident from the use of pumps to move crude oil in a pipeline resulting in an image with the highest stress value reaching 89 Pa at an efficiency value of 98%. criteria for safety factors that are allowed.

Design and Analysis of an Innovative Concept for Submerging Open-Sea Aquaculture System

This paper presents an innovative design of a system of cages for an open-sea aquaculture. The concept is based on an existing successful concept of a system, which is flexible, submersible, and has a single point mooring. The new design, besides being much larger, presents a new concept of stabilizing the system at the submerged state. This new system is preliminarily designed by a simplified method, and then analyzed by the AQUASIM finite elements software system. The results of the nonlinear hydroelastic analysis confirm that the preliminary design by simplified calculations is adequate to the design conditions. The new concept of stabilizing the system at the submerged state is studied by simulations of four configurations, where the optimal is selected. The simulations show that the cages always retain above 70% of the volume. The paper presents valuable information of a practical design, such as the method of the analysis, the environmental design criteria, the scantlings of the structure, and the obtained usage factors at extreme sea conditions. The system was recently being constructed, launched, and anchored at open-sea offshore Ashdod. It is now tested at the sea before cultivation of fishes.

Drift stability of HyStOH semi-submersible supported by airfoil shaped structures

This study presents the results of the German research project HyStOH funded by German Federal Ministry of Economic Affairs and Energy (BMWi). The project consortium of German universities, wind turbine designers, wind farm developers and certification bodies design a novel semi-submersible steel structure with a single point mooring and self-aligning capabilities. The tower, which carries a 6 MW two-bladed downwind turbine, has an airfoil shaped cover, which supports the self-alignment of the full structure towards the main wind direction. The downwind operating wind turbine in combination with the lift force generating tower enables a passive yaw system acting against hydrodynamical impacts from waves and currents. Simulations in time domain applying the fully coupled aero-hydro-servo-elastic code Bladed_4.8 demonstrate the drift sensitivity and the self-aligning capabilities of the HyStOH design. The hydrodynamic coefficients for the simulations have been adjusted by calculations with an 1:1 model of the panel code panMARE, developed by Technical University of Hamburg-Harburg. The motivation of this analysis is to capture the complex coupled motions of an innovative FOWT by numerical simulation tools [1]. The analysis presents dynamic simulations of the HyStOH design operating in turbulent wind and irregular sea state with special focus on the yaw drift behaviour of the FOWT. The sensitivity study of yaw drift is based on numerous variations of wind-wave-current misalignments during normal operation. The results of the simulations demonstrate the weathervane capabilities of the airfoil shaped structures.

The Investigation of Internal Turret Single Point Mooring Slip Ring Structure Design

Research the structure of turret mooring system slip ring abroad, for the first time in China presents a slip ring structure design, contact fatigue, finite element analysis of the key components of the brush, and through the research on the feasibility of assembly precision test equipment, main circuit resistance and withstand voltage, power frequency withstand voltage and dynamic continuous high current test of static electric slip ring in the working process and rationality.

Numerical Study on the Heading Misalignment and Current Velocity Reduction of a Vessel-Shaped Offshore Fish Farm

Recently, the concept of a vessel-shaped fish farm was proposed for open sea applications. The fish farm comprises a vessel-shaped floater, five fish cages, and a single-point mooring system. Such a system weathervanes, and this feature increases the spread area of fish waste. Still, the downstream cages may experience decreased exchange of water flow when the vessel heading is aligned with the current direction, and fish welfare may be jeopardized. To ameliorate the flow conditions, a dynamic positioning (DP) system may be required, and its power consumption should relate to the heading misalignment. This paper proposes an integrated method for predicting the heading misalignment between the vessel-shaped fish farm and the currents under combined waves and currents. A numerical model is first established for the fish farm system with flexible nets. Current reduction factors are included to address the reduction in flow velocity between net panels. The vessel heading is obtained by finding the equilibrium condition of the whole system under each combined wave and current condition. Then, the Kriging metamodel is applied to capture the relation between the misalignment angle and environmental variables, and the probability distribution of this misalignment angle is estimated for a reference site. Finally, the requirement for the DP system to improve the flow condition in the fish cages is discussed.

Multibody dynamical modeling of the FPSO soft yoke mooring system and prototype validation

The Soft Yoke Mooring System (SYMS) is a single point mooring system for shallow water. It is composed of a mooring framework, mooring legs, yoke, and single point, and is located at the Floating Production Storage and Offloading (FPSO) through 13 hinge joints, such as universal joints and thrust bearings. Mooring restoring force, motions and postures of mooring components, and mechanical behaviors of hinge joints are major criteria for the structural design of the SYMS. Aiming at the difficulties of the multibody dynamics in traditional design of the SYMS, a multi-body dynamic mathematical modeling with seven independent degrees of freedom (DOFs) which is applicable to prototype field engineering was developed in this study. The proposed mathematical modeling of the SYMS multibody dynamic system has several advantages: 1. Internal tribological behaviors in hinge joints are considered within the presented multibody dynamics model to illustrate the good dynamic effects of the SYMS. 2. The multibody dynamic model can be applied in field service. Correctness and feasibility of the proposed multibody dynamic simulation method for describing motions and postures of hinges and single-body were validated by the prototype monitoring data. 3. The horizontal restoring force of the SYMS was calculated according to field measurement data. The motion state of each single body and internal stress distributions at each hinge joint in the SYMS are given. 4. The multibody dynamics calculation program can be directly used for the real-time monitoring of mechanical behaviors of the SYMS under the service state. The simulated results can provide real-time guarantee for safety alarming of the system. The vulnerability of the mooring system in service was evaluated based on long-term monitoring data analysis.

Marine Archaeology Assessment in Support of the Bluewater SPM Project, Nueces and Aransas Counties, Texas and Adjoining Federal Waters

Marine Archaeology Assessment in Support of the Bluewater SPM Project, Nueces and Aransas Counties, Texas and Adjoining Federal Waters

Evaluation of the structural strength and failure for floating collar of a single-point mooring fish cage based on finite element method

Deep-water fish cage floating collar also referred to as float, bears a load in the form of waves, fishing nets, and moorings, in which the longitudinal wave force is the main type of loading. The long-term continuous effects caused by wave force could cause reduction in strength, or vibration failure in the floating collar. In this study, the peak responses of a single-point mooring (SPM) cage floating collar, due to either static or vibration loadings, were calculated by the finite element method (FEM) based on an elastic model. In this model, the float exhibited macro-plastic deformation when the stress of 26.1 MPa, generated under 100 kN load, was greater than the yield limit (25 MPa). With the increase in order modes from 1 to 18 (1.2–7.9 Hz), the stress increased. Moreover, the harmonic stress corresponding to 8 and 0.1 Hz was clearly higher than that for 0 Hz under 100 kN loads, due to structural resonance. The range of stress values due to the random vibration was 16.0–63.7 MPa, when the angular velocities (k) were in the range of 0.01–16 rad s―1. Moreover, the transient response (34.4 MPa) was maximum when the angular velocity was 10 rad s―1 (range 1–100 rad s―1). The reliability rate of the entire body was 53.1% and 97.0% after 108 loading cycles for 100 kN and 10 kN loads, respectively. Furthermore, the fatigue modes of the key components were determined from the outer triangular tops and inner hexagonal intersections. In short, static analysis, vibration analysis, and fatigue analysis of FEM could be used as reliable ways to evaluate structural strength and failure for floating system. Finally, the floating parts could be strengthened with partially double pipes or optimized by increasing the stiffness, and appropriate damping could be widely used.

Dynamic Analysis of Embedded Chains in Mooring Line for Fish Cage System

Abstract Investigation of the embedded chains in soil starts to play an important role in understanding the structural performance of mooring system, when the embedded anchors will be employed to sustain large loads with the gradually growth of installation depth of offshore aquaculture farm. The aim of this study is to investigate the dynamic response of mooring line considering the influence of embedded chains in clay soil for net cage system. Lumped-mass method is used to establish the numerical model for evaluating the performance of mooring line with embedded chains. To validate the numerical model, comparisons of numerical results with the analytical formulas and the experimental data are conducted. A good agreement of the profile and the tension response is obtained. Then, the effect of embedded chains on the static and dynamic response of mooring line is evaluated, and the dynamic behavior of mooring system considering embedded chains for net cage system is investigated. The results indicate that the soil resistance on embedded chains should be included to predict the mooring line development and the load on the embedded anchors in the numerical simulations. An appropriate safety factor should be included if employing the simplified model Case C at the initial design phase. And the effect of embedded chains on the holding capacity of embedded anchors in single-point mooring system for single net cage cannot be negligible during the design and operation phases. Consequently, it is profound to take into account the interaction of embedded chains and soil for accurately predicting the reliability of mooring system for fish cage.

Response Characteristics of the DeepCwind Floating Wind Turbine Moored by a Single-Point Mooring System

In recent years, the SPM (Single-Point Mooring) concept has been widely employed in several branches of the naval architecture and marine engineering field, such as FPSOs (Floating Production, Storage and Offloading units), offshore oil rigs, etc., but not yet popular in the offshore wind energy. To investigate the response characteristics of an SPM-moored FWT (Floating Wind Turbine), in the present work, we perform a numerical study on the DeepCwind semisubmersible wind turbine, using the state-of-the-art open-source tool FAST. The free-decay test results show that the SPM layout affects the natural periods of the wind turbine in rotational modes, as well as the mooring stiffness of the diagonal rotational and crossing rotational-translational terms, especially in relation to the yaw mode. Comparisons of the RAOs (Response Amplitude Operators) elucidate that the presence of wind influences significantly the sway, roll and yaw motions of the SPM layout. Finally, the weathervane test shows that an asymmetry exists in the free-yaw motion response when the semisubmersible wind turbine is moored by an SPM system.

Time-Domain Analysis of the Relative Motions Between Side-by-Side FLNG and LNGC Under Oblique Waves

Strong hydrodynamic interactions during the side-by-side offloading operation between floating liquefied natural gas (FLNG) and liquefied natural gas carrier (LNGC) can induce high risks of collision. The weather vane effect of a single-point mooring system normally results in the satisfactory hydrodynamic performance of the side-by-side configuration in head seas. Nevertheless, the changes in wave directions in real sea conditions can significantly influence the relative motions. This article studies the relative motions of the side-by-side system by using the theoretical analysis method and the numerical calculation method. Based on the three-dimensional potential theory modified by artificial damping-lid method, the frequency-domain hydrodynamic coefficients can be improved to calculate the retardation functions for the multi-body problem. An in-house code is then developed to perform the time-domain simulation of two vessels, through which the relative motions are subsequently obtained. A range of oblique waves are chosen for the extensive calculation of relative motions between the two vessels, which are further analyzed in terms of the phase shift of motion responses induced by specific resonant wave patterns. Investigation results show that wave directions have a significant influence on the relative sway, roll, and yaw motions. Under the circumstance that the absolute phase shift between the roll motions of two vessels approaches 180°, stronger relative motions are induced when LNGC is on the weather side. Moreover, the gap water resonances at high frequencies tend to cause the dangerous opposed oscillation of two vessels in the sway and yaw modes, whereas FLNG reduces the gap water resonances and relative motions when located on the weather side.

The research of soft yoke single point mooring tower system damage identification based on long-term monitoring data

In this research, to identify the damage of the nonlinearity system under ambient loads, an intelligent damage identification method based on long-term monitoring data is proposed. The random decrement technique and the autocorrelation function algorithm are used to extract free decay of the structure from long-term monitoring data. The random decrement signatures, autocorrelation function, the frequency of free response and the peak points of the frequency spectrum are used as the features of the structure. These features are then input into the Support Vector Machine (SVM) to classify the current state of the system and their identification accuracy is compared. The simulation experiments results show the extracted features are capable of representing the changes of the system inherent characteristics. Finally, the proposed method is applied to the data analysis of the soft yoke single point mooring (SPM) tower system, and provide the reference for the damage identification of the soft yoke SPM tower system.

Design of Single-Point Mooring System with Moored Buoy

The single-point mooring system is currently under a wide range of applications in marine engineering, marine observation, marine farming and other engineering fields. So this paper intends to investigate into the singlepoint mooring system with moored buoy that is made up of a buoy, four steel pipes, a steel drum, a heavy ball, an anchor chain and an anchor. The aim here is to design a single-point mooring system mentioned above, which can be used off shore. Namely, the following parameter values have to be determined: the type and length of chain, the mass of compact ball, such that do not only both the buoy draught and the moving range, but also the vertical angle of steel drum reach as small as possible. For the above purpose, firstly, analyse each component of this system by means of the static force method. Secondly, establish a mathematical model to calculate under the different wind speeds all the important parameters, including the vertical tilt angle of the steel drum, the horizontal tilt angle of each steel pipe, the buoy draught, the buoy moving range and the chain shape. Thirdly, on the basis of the above work, a multi-objective programming model is established to obtain the design schemes in the diverse environmental conditions involving wind speed, seawater velocity and sea depth. Finally, by simulation it has turned out that these design schemes obtained above are reasonable and applicable. Therefore, they have definite theoretical significance and practical values.

Numerical Analysis of a Vessel-Shaped Offshore Fish Farm

The aquaculture industry is aiming to move fish farms from nearshore areas to open seas because of many attractive advantages in the open water. However, one major challenge is to design the structure to withstand the environmental loads due to wind, waves, and currents. The purpose of this paper is to study a vessel-shaped fish farm concept for open sea applications. The structure includes a vessel-shaped hull, a mooring system, and fish cages. The shape of the hull minimizes the wave loads coming from the bow, and the single-point mooring system is connected to the turret at the vessel bow. Such a system allows the whole fish farm to rotate freely about the turret, reduces the environmental loads on the structure and increases the spread area of fish wastes. A basic geometry of the vessel hull was considered and the hydrodynamic properties were obtained from the frequency-domain (FD) analysis. A mooring system with six mooring lines was designed to avoid possible interactions with the fish cages. Time-domain (TD) simulations were performed by coupling the hull with the mooring system. A simplified rigid model of the fish cages was considered. The global responses of the system and the mooring line loads were compared under various wave and current conditions. The effects due to misalignment of wave and current directions on the responses were discussed. Finally, the responses using flexible and rigid net models were compared under steady current conditions.

Reliability Analysis of Single Point Mooring (SPM) System under Different Environmental Conditions

Reliability Analysis of Single Point Mooring (SPM) System under Different Environmental Conditions

Optimization Design and Calculation of Mooring System Parameters based on Genetic Algorithm

Based on the static equilibrium model of mooring system parameters, the multi - objective optimization model is established and the genetic algorithm is introduced as the optimization algorithm, which improves the crossover operator. Taking the example of a single point mooring system, concerned optimization design and calculation has been done. Compared with the general genetic algorithm, the result of this algorithm is superior, significantly improving the stability of the mooring system.

Research and development about the mechanisms of a single point mooring system for offshore wind turbines

A new type of floating wind turbine, in which the floater itself can rotate from the wind is proposed in this paper. A new single point mooring system is necessary for the development of this new wind turbine. In this paper, the single point mooring system, which includes component such as catenary chains, the slip ring for the electric cable and the control cable were researched. This work will examine two different configurations of the single point mooring system that we developed. One uses a thrust bearing at the connecting points between the mooring parts and the floater parts. The other one uses not only a thrust bearing, but also an aligning bearing. Weather vane tests about the offshore wind turbine with each mooring system were conducted in an ocean basin. After performing wind tests of both configurations of the single point mooring system, it was concluded that the latter one functioned superior to the former one. This work discusses and clarifies the reason based on a physics model which focuses on the relation between the restoring yaw moment caused by the mooring chains and the static friction moment due to the bearing unit. We also performed a wind test at a real sea and verified that the bearing unit with not only the thrust bearing but also the aligning bearing could function properly.

Delta-type VLFS – hydrodynamic aspects

ABSTRACTA Very Large Floating Structure (VLFS) is a promising concept for the utilisation of the sea. Currently, VLFSs are classified into two broad categories: the pontoon-type, limited to sheltered water, and the semi-submersible type, for open-ocean. We present a new concept, the delta-type, an important feature of which is the formation of a sheltered basin, providing accessibility in most weather conditions.Initiating the development of this concept, we study its hydrodynamic aspects: basin efficiency, motion, comfort and mooring.We present a case study, for which we compare the hydrodynamic aspects for four configurations of equal utilisation areas.The agitation in the basin and the motion are solved linearly for regular and irregular seas. A single-point mooring system is designed and solved in the time-domain, considering the second-order slow-drift motion that is critical to the mooring loads.We obtained promising results that justify the next stage of the study – structural aspects.

Experimental investigations into the current-induced motion of a lifeboat at a single point mooring

This paper presents a series of model experiments on the current-induced motions of a 1:40 scale lifeboat at a single point mooring (SPM). The influence upon vessel and buoy motion of the mooring configuration factors of (a) three mooring line (hawser) lengths, (b) four buoy shapes and (c) two buoy sizes have been investigated. A motion tracking algorithm was successfully employed and validated against data from an inertial measuring unit allowing small scale testing without the influence of instrument cabling. The results show that the dominant translational motion, of the model lifeboat at a SPM, is sway and the rotational motion is yaw, with double pendulum-like fishtailing behaviour prevalent. Increasing the hawser length, when no buoy was present, resulted in an increase in the vessel's sway velocity. No significant effects on vessel motion were observed from changes in the shape of the 1:40 and 1:20 scale buoys. However, the presence and increasing size of the buoy was found to increase the sway velocity of the buoy and reduce the motions of the model lifeboat. These results suggest that changes in buoy size influence the motions of the model lifeboat which may enable mooring efficacy to be improved.