Point Mooring Research Articles

Hydrodynamics of submersible aquaculture cage system using numerical model

A numerical model analysis was performed to analyze the motion and mooring tension response of submersible fish cage systems in irregular waves and currents. Two systems were examined: a submersible cage mooring with a single, high tension mooring and the same system, but with an additional three point mooring. Using a Morison equation type model, simulations of the systems were conducted with the cage at the surface and submerged. Irregular waves(JONSWAP spectrum) with and without a co-linear current with a magnitude of 0.5m/s were simulated into the model as input parameters. Surge, heave and pitch dynamic calculations were made, along with tension responses in the mooring lines. Results were analyzed in both the time and frequency domains and linear transfer functions were calculated.

Oil Spill Sensitivity Analysis and Risk Assessment for Gulf of Kachchh, India, using Integrated Modeling

Abstract Oil-related activities such as single point mooring (SPM), installation, navigation, marine transportation, and oil exploration all have the potential to cause oil spills and pose major threats to the coastal ecosystem along the Indian coast. The Gulf of Kachchh, a biologically rich ecosystem composed of mudflats, submergible shoals, islands, sandy beaches, rocky shore, coral reefs, and mangroves, is located along the northwest coast of India. Its proximity to the Persian Gulf has attracted the establishment of facilities for transferring, piping, and loading of crude oil and petroleum products, mainly the Arabian crude. The marine ecosystem of the gulf is always at risk due to accidental and operational oil spills in the gulf. In this paper, an attempt is made to identify the resources that are at risk due to such oil spills. An integrated numerical simulation modeling approach has been applied to generate the probable oil trajectory and fate analysis, and the results have been integrated into g...

Mooring loads analysis of submersible aquaculture cage system using finite element method

The expansion of near shore aquaculture is feasibility of moving aquaculture facilities into the open ocean. Numerical modeling technique using finite element method was used to enable the optimum design and evaluation of submersible aquaculture cage system. The characteristics of mooring loads response in mooring lines under waves and current and their response amplitude operators were calculated for single and three point mooring configuration at the surface condition and submerged one. The static mooring loads without wave and current loading were similar for both the surface and submerged configuration. It was calculated that three point mooring was more adequate than single point mooring for the mooring configuration of submersible aquaculture cage system. The wave induced response amplitude operators for the single point mooring configuration with the influence of currents were identical to those without the influence of currents.

Dynasim—A Time Domain Simulator of Anchored FPSO

The prediction of the dynamic behavior of moored tankers like FPSO system is very important to the mooring systems design. There are several types of mooring systems, such as Turret, Spread Mooring System (SMS) and Single Point Mooring (SPM), for which the specification of number and dimensions of the lines depends on the dynamic of the entire system composed by tanker and mooring and riser lines. Although several moored systems were already installed as a positioning system of FPSO, there are still some unknowns that should be investigated to provide more safety and lower cost systems. The drag/damping terms on the ship hull and the mooring and riser lines damping are one of the most important terms in the dynamics of the moored tanker that should be investigated. The hydrodynamic model due to the interaction between hull and fluid has been very studied due to the complexity of the phenomenon. The authors have studied the models of several researchers and three of them were picked up and implemented in the DYNASIM, a moored ship dynamic simulator. These models will be discussed and compared. In deep water, the total length of the mooring lines and risers becomes large and consequently the forces acting on these elements become more important. However, in many cases these important forces acting on the lines have not been considered. There are some works that point out the magnitude of the damping forces of the lines. Recently, a full scale decay test using a 30kDWT moored tanker was realized in Brazil offshore. The results showed the significance of the line damping, in particular on surge motion. Another important phenomenon that has been discussed is the wave drift damping term. There are several research studies concern’s this force resulting in several estimation methodologies. However, only surge direction has been considered important and there is little research about the damping in others directions. The time simulation methodology will be used to analyze and compare the effects of each term in the dynamics of the system. Finally, the comparison between simulated results and model test data will be performed in order to validate the model proposed. In this study, the experimental data of DICAS system (Differentiated Compliant Anchoring System) will be used.

Stability Analysis Of Interaction Of An ArticulatedTower With Waves

This paper investigates the stability problem of a single point mooring (SPM) system using a buoyant tower. In particular, the stability of an inclined articulated loading tower (ALT) concept is discussed. The interaction of this tower with waves and the resulting Duffing s equation and Matheiu type instability are analysed. A wave tank test of a model of the inclined ALT and corresponding SPM was performed to demonstrate the instability problem. For larger waves the motion of the tower appeared chaotic and the transverse oscillation was observed to be large. The frequency domain analysis applicable to upright articulated tower was generally found to be conservative for ALT as well as the SPM except at the lower periods. The complete time domain solution explained the observed ALT oscillation in waves.

Mitigating the environmental effects of mariculture through single-point moorings (SPMs) and drifting cages

The production of finfish in cages causes a measurable impact on the nearby water and seabed due to faeces production and uneaten feed. The impact depends on the specifics of the culture operation and on the environment in which it is located. The most severe impact has been associated with large, intensive operations in areas with inadequate water circulation, where benthic habitats have been seriously degraded. The current practice of mooring a cage, or a cage array, over a permitted site exacerbates the problems. An alternative approach is to allow cages to move in response to the environment. For example, the use of a single point mooring (SPM) would allow the operation to maintain a “watch circle” where the position of the cage(s) depends on the sum of the environmental forces. By spreading out the accumulation of organic matter, one can prevent the local environment from being overwhelmed. Preliminary analyses of the benefits of SPM indicate a two-fold to 70-fold reduction in deposition of waste on the seabed, depending on mooring geometry and current type. Other advantages are related to reduced anchoring costs, improved accessibility, and the ability of having certain cages in the lead position with respect to currents and good oxygen conditions. The concept of drifting cages is introduced as a further alternative to minimizing impact on the seabed.

A hybrid Boussinesq-panel method for predicting the motion of a moored ship

This paper describes a new computational technique for predicting the wave-induced motion of a restrained floating body in restricted water. A combination of established methods is used in an attempt to account for the most important physical processes involved in this complicated problem, while keeping the computational burden modest. Potential theory is invoked to describe both the wave transformation over the bathymetry of the harbour, and the hydrodynamic interaction between the waves and the floating structure. Modified Boussinesq theory is used to predict the transformation of the waves as they propagate from deep water into to the harbour or bay where the body is moored. This model includes the effects of shoaling, refraction and non-linear wave–wave interaction, and most importantly sub-harmonic generation. This flow is then linearised locally, at the structure, to provide the incident wave forcing term in the equation of motion which is solved in the time-domain. Linear wave radiation and diffraction forces are computed using a constant-strength panel method, while the instantaneous, non-linear, point mooring forces are included exactly. The model is validated for the linear problem, and non-linear calculations are compared with experimental measurements for a ship moored in an L-shaped harbour. Qualitatively, the non-linear features of the dynamical system are successfully captured by the model. Some tuning, in the form of empirically obtained damping coefficients, is, however, necessary in order to get a reasonable prediction of the response amplitudes near resonance, when the linear hydrodynamic damping is very small.

Slow motion dynamics of turret mooring and its approximation as single point mooring

The mathematical model for the nonlinear dynamics of slow motions in the horizontal plane of Turret Mooring Systems (TMS) is presented. It is shown that the TMS model differs from the classical Single Point Mooring (SPM) model, which is used generally to study the TMS dynamics. The friction moment exerted between the turret and the vessel, and the mooring line damping moment resulting from the turret rotation are the sources of difference between TMS and SPM. Qualitative differences in the dynamical behavior between these two mooring systems are identified using nonlinear dynamics and bifurcation theory. In two- and three-dimensional parametric design spaces, the dependence of stability boundaries and singularities of bifurcations for given TMS and SPM configurations is revealed. It is shown that the static loss of stability of a TMS can be located approximately by the SPM static bifurcation. The dynamic loss of stability of TMS and the associated morphogenesis may be affected strongly by the friction/damping moment, and to a lesser extent, by the mooring line damping. Nonlinear time simulations are used to assess the effects of these properties on TMS and compared to SPM systems. The TMS mathematical model consists of the nonlinear horizontal plane fifth-order, large drift, low speed maneuvering equations. Mooring line behavior is modeled quasistatically by submerged catenaries, including nonlinear drag and touchdown. External excitation consists of time independent current, steady wind, and second-order mean drift forces.

A statistical analysis of extreme events in current variations due to internal waves from the Australian North West Shelf

Measurements of current fluctuations due to internal waves obtained from two fixed point moorings on the Australian North West Shelf have been analyzed with the aim of calculating the probability of short‐scale, large‐amplitude internal waves exceeding particular values. Such waves are generated as a result of breaking or hydrodynamical instability of the semidiurnal internal tide. The statistical stationarity of each record has been checked, and it is shown that the records can be used for the calculation of the frequency by which currents exceed a particular value for wave heights (double amplitudes) of less than 60 cm s−1. The theoretical forms for exceedance frequency from the theory of extreme statistics have been checked, and it is shown that for one location a power asymptotic provides a slightly better descriptor of the statistics than an exponential asymptotic and for the other location an exponential asymptotic is found to provide the better fit. The exceedance frequency for internal waves near the Australian North West Shelf is found to be less than 1.3 waves per hour. The exceedance probability for each location has been calculated using Poisson statistics.

A Basic Study on the Wave Induced Responses of a Single Point Mooring System for Medium-Depth Water

The development of ocean oil fields are now going to deeper and smaller oil fields. A FPSO (Floating, Production, Storage and Offloading) system is one of the most economical systems for the production of marginal fields and thus is in use worldwide. For the position keeping of a FPSO system, a single point mooring system is usually employed to economize the operation. However, the conventional single point mooring systems such as a catenary anchor leg mooring (CALM) or a turret mooring (TM) become less economical as the water depth increases because the system becomes heavier in order to sustain the large weight of mooring lines. In this paper, we apply a Counterweight Articulated Mooring (CAM) system, which was originally proposed for a mooring system in ice infested water, as the alternative single point mooring system for a FPSO that may be used in a medium-depth water (500 m).Since a CAM system is mechanically much more complicated than conventional systems, we examine how the behaviours of a CAM system in waves can be predicted theoretically. A computer code is developed based on the constraint matrix method proposed by Langley for the analysis of a floating system composed of a number of structures.Numerical results obtained by the developed code are compared with model test data. And the following conclusions are obtained through the present study.(1) The numerical prediction of wave induced responses of a CAM system gives fairly accurate results.(2) The low frequency responses in irregular waves are governed by the low frequency oscillations of a tanker, which should be predicted while properly accounting for the damping force.(3) The connecting mechanism between a tanker and a yoke should be free rather than be fixed because the stress and the mooring lines' tensions are drastically decreased.

Dynamic Response of Seabed around Anchors of a Double Point Mooring System

control to change the direction of ship for minimizing a long period mooring force. Distributions of stresses and pore water pressure in seabed around anchors are investigated by analyzing a frequency response of cnd bearing anchor due to ship motion. Considering the interaction between anchor and seabed, the feasibility of a double point mooring system are examined, and the effects of both, the stiffness and the mass ratio of anchor plate on the response of anchor plate are numerically investigated.

Strumming of nonlinear cable elements using modal superposition

A wake-oscillator model of cable strumming effects is used in a numerical procedure to predict the strumming response of hydrodynamically loaded cable systems incorporating long curved cables. The finite element method is used in a combined time-domain and frequency-domain solution method. Transverse strumming motions are assumed to be small compared to nonlinear in-line displacements and to occur at much higher frequencies than the in-line frequencies. A modal superposition technique is used to predict strumming response superposed on nonlinear time-domain solutions for in-line motions. Two examples are included, a horizontal cable vibrating in a uniform current, and a three-leg single point mooring of a large buot subject to ocean waves.

一点係留型浮遊式生産システム最適化プログラムの開発

The Floating Production Systems (FPS) or the Floating, Storage and Offloading Systems (FPSO) have been applied to marginal oil fields or early production systems since late `70 in view of less capital and short delivery. There are two types of floating production systems: semi-submersible based spread mooring type and tanker-based single point mooring. This paper describes the analysis method, the technical development of optimization programs and the model experiments for the Single Point Mooring (SPM) systems for FPSO, which have been carried out as a joint research project by Japan National Oil Corporation, Akishima Laboratories Inc. and MODEC Inc. The detailed analysis program is developed based on the constraint matrix method in frequency and time domains and is capable of calculating the motions and constraint forces of SPM in wind, current and random waves. Outline of calculation method is presented.

Analysis Method of Single Point Mooring (Report 3)

The model experiments for three different single point mooring systems of an Articulated Column system, two Articulated Columns system and SALS system were carried out in wind, current and irregular waves.The spectral responses are calculated by constraint matrix method in frequency and time domains and compared with those of the model experiments.Theoretical analysis methods are verified by these comparison and frequency domain analysis is implied to be useful for the determination of design loads.

自動車専用船における係留方法の改善について : Low Point Mooring System

When a moored pure car carrier receives a strong land wind, it may cause an accident of a ship separation from a quay wall, because the ship has a large exposed area and the loading area is flat. As the countermeasures for the problem, a device like a ring is fixed at a low point of the bow and/or the stern, and then the mooring ropes are moored to the bitts on a pier through the ring. We call the system "Low Point Mooring". In this operation, ship's mooring points are lowered, so the mooring forces in the right and left direction are strengthened and the safety of the mooring ship is improved.

An experimental study of the large amplitude fish-tailing instabilities of a tanker at a single point mooring

The fish-tailing instability of a model vessel moored in a laboratory current flume is examined. Under a slowly varying control parameter, the vessel is shown to exhibit an, unstable sub-critical Hopf bifurcation involving a sudden dynamic jump from the equilibrium rest state to a finite amplitude limit cycle. A hysteresis, loop is then observed when the control parameter is subsequently decreased, with a final smaller reversed jump back to the sub-critical equilibrium state at a cyclic fold. The possibility of predicting the onset of this dynamic instability by estimating the effective damping in the active fish-tailing mode is establihsed. It is shown that this effective damping can be satisfactorily evaluated from the monitored fluctuating time series of the experimental vessel, driven by naturally occurring fluctuations in the fluid flow, using a system identification technique based on partial spectral moments. This may offer the possibility of the real-time monitoring of an actual tanker, allowing emergency action if an incipient instability is predicted. The experiments were designed to explore carefully the dynamic instability phenomena of a vessel in a well-controlled laboratory environment without attempting to simulate, a real tanker in an ocean environment. Some qualitative comparisons are however made with some related theoretical simulations of a real tanker at a single point mooring.

A parametric experimental study of low-frequency motions of single point mooring systems in waves

This paper contains results of an extensive model test programme on the behaviour of singlepoint mooring (SPM) systems to investigate the influence on system response, as measured by motions and mooring forces, of variations of certain wave environment and system parameters. The variations of the environment were frequency and height of regular waves; tests were also made using irregular waves and were compared with the regular wave results. Neither current nor wind were included. The influence of the hawser was examined by varying its length and elastic properties. Tests were made to examine the influence of tanker draft and also to compare the measurements for the tanker with those for an elliptical model representation. Most of the experiments were carried out using a fixed mooring point because tests using an articulated column showed that the stiffness of the buoy or column had almost no effect on the low frequency motions. Finally, tests were carried out in a larger wave basin, at the Hydraulics Research Station, Wallingford, UK which provided broad confirmation of the measurements made in the smaller Heriot-Watt basin. The paper presents the main results and overall conclusions which can be drawn from the experimental work.

Estimation of the Mooring Force

In the first report, outline of the basic designing of the single point mooring systems (SPM) was explained. And it was pointed out that the dynamic stability analysis can be applied practically to the primary planning of SPMs. Slow oscillation or weather vane motion of a moored ship sometimes causes excessive mooring force and whether the oscillation occurs or not can be predicted by the dynamic stability analysis.But the maximum mooring force, which is necessary for estimating the capacity of mooring system, has to be predicted by the numerical simulation in the time domain or model test.In this report, taking FPSO (Floating Production, Storage and Offloading system) with a turret mooring system for example, results of numerical simulation of the motion and mooring force of the FPSO in combined external forces of wave, wind and current are presented along with the model test results, which is conducted for the varification of the simulation.Dynamic effect on the mooring line tension in deep water and results of extreme value analysis of the tension are also discussed.

Characteristic of Motion and Constraint Force Response in Frequency Domain

In the previous paper, a mathematical model of multi-bodies was derived by means of the constraint matrix, and it was applied to a conventional SALM buoy without tanker-moored. The numerical results were compared with those of the experiments and the result proved the effectiveness of the mathematical model.In this paper further numerical studies in case of tanker-moored-condition are carried out on the several single point mooring systems, such as CALM, SALM, SALS and AC system.The motion and the constraint force characteristics in frequency domain are studied precisely based on the calculation results.Furtheremore, the calculation model of slowly varing response of constraint force in spectral analysis is shown for CALM system.

Basic Equation and SALM Buoy Motion

Single Point Mooring (SPM) Systems are in use worldwide as an FS, FPS or FPSO.The SPM System is of multi-component structure normally composed of a vessel, yoke or hawser, buoy and mooring chains. It is designed on the basis of model test results, because no suitable mathematical method is available for its motion and structural analysis under the combined loads condition.In this paper, we have derived a mathematical model of multi-component structure under the combined loads (wind, wave and current), considering higher order motions and hydro dynamic loads with Langley's constraint matrix.This time, we have applied same model above to SALM buoy and have compared its nu merical results with the same from model test.Following conclusions are obtained through comprehensive discussion(1) Constraint matrix of multi-body system is effective(2) SALM buoy system in small waves can be analyzed in frequency domain.(3) SALM buoy system in large waves should be analyzed in time domain considering higher order motions and hydro-dynamic loads.