Soft Mooring System Research Articles

Rational Simplification of High Fidelity Wind Turbine Models Used for Mooring Analysis

This study investigates the use of simplified turbine modelling in analysis of a soft mooring system (horizontal plane periods > 250 s). Two simplified turbine models are developed, tested and compared to a high fidelity turbine model in a case study of the INO WINDMOOR 12 MW FOWT with a soft mooring system. The chosen simplified modelling strategies are a drag disc model, a de-coupled approach where the aerodynamic loads are pre-calculated in a fixed turbine simulation and a model with reduced degrees-of-freedom. Corrections to the pre-calculated force model is also developed and compared. The results show that the pre-calculated force approach gives better comparisons to the full model than the drag disc model, and that applying a controller correction to account for the wind turbine controller’s response to floater motions improves the predicted response.

A Numerical Study on the Hydrodynamic Performance of a Tanker in Bow Sea Conditions Depending on Restraint Conditions

The importance of accurate ship performance estimation is increasing for efficient ship operation. Ship performance has been evaluated through model tests in the past, but there are limitations in terms of facilities and costs. With the spread of high-performance computers, the method of evaluating the performance of a ship by numerical analysis, especially computational fluid dynamics (CFD), has become common. There have been many numerical studies on added resistance under various wave conditions for many years, showing a high reliability. Meanwhile, most of the studies were conducted under conditions where the degree of freedom (DOF) of the ship was limited due to computational complexity. In this study, we tried to compare the added resistance performance and fluid dynamics of S-VLCC with 6 DOFs in the regular wave conditions. One of the methods for utilizing the 6 DOFs is the soft-mooring system, which allows springs to be attached to the bow and stern to recover the non-restoring force of the hull. The second method considers the free-running condition. The virtual disk is used for the self-propulsion of the ship, and the rudder can be rotated to maintain its course. The propeller rotation speed and rudder angle are controlled through PID control. The bow wave (ψ = 180°) and oblique wave (ψ = 150°, 120°) conditions were considered, and various regular wave conditions from short to long wavelengths were regarded. The effects of restraint conditions on the added resistance and motion response amplitude operator (RAO), according to each wave condition, were compared. As a result, there was a difference in the roll motion for each restraint condition, and the y-direction force and yaw moment generated on the hull were compared to analyze the cause. In addition, we observed the change in flow characteristics by comparing the streamlines around the hull and the nominal wake on the propeller plane.

Hydrodynamic inertial forces on a single-point moored vessel due to time-varying current

Whilst the concept of the current drag and lift forces on ship-like vessels is widely used, the hydrodynamic inertial forces due to time-varying current are usually not considered and not captured in the calculation methods. The objective of this article is to examine such forces on a weathervaning vessel by applying classical formulations of fluid mechanics. Based on the general expressions for the inertial forces on a body moving in an accelerating flow of an ideal fluid, the formulation applicable for a single-point moored vessel is presented and illustrated by examples. The effect of an oscillatory current is considered further and two limiting scenarios are examined: a periodically oscillating current acting on a vessel with a fixed mooring point (turret) and oscillatory motion of the vessel in a stationary fluid; the two scenarios corresponding to a relatively stiff and soft mooring system, respectively. Both problems are solved by the direct separation of motions, a perturbation method known in vibrational mechanics, and expressions for the mean drift forces are obtained. In both cases, the mean surge force is found to be positive, acting towards the mooring point, and the mean sway force produces a restoring moment which tends to stabilize the vessel in the direction of the current flow. A calculation example is presented and results are discussed. This study concludes that, depending on the current regime, the inertial forces due to time-varying current may be of the same order of magnitude as the quasi-steady forces, their importance increasing for lower mean current speeds.

Experimental Data of a Hexagonal Floating Structure under Waves

Floating structures have a wide range of application and shapes. This experimental investigations observes a hexagonal floating structure under wave conditions for three different draft configurations. Regular waves as well as a range of white noise tests were conducted to quantify the response amplitude operator (RAO). Further irregular waves focused on the survivability of the floating structure. The presented dataset includes wave gauge data as well as a six degree of freedom motion measurement to quantify the response only restricted by a soft mooring system. Additional analysis include the measurement of the mass properties of the individual configuration, natural frequency of the mooring system as well as the comparison between requested and measured wave heights. This allows us to use the provided dataset as a validation experiment.

Experimental investigation of multiple oscillating water column wave energy converters integrated in a floating breakwater: Wave attenuation and motion characteristics

This article is Part B of a study into the proof-of-concept of a floating breakwater integrated with multiple Oscillating Water Column (OWC) Wave Energy Converters (WEC). The structure is designed for nearshore and offshore applications and the investigation was performed through model scale hydrodynamic experimentation. Part A introduced the concept and the experiments performed, followed by an investigation into the energy extraction performance of the installed OWC devices [1]. This paper covers the impact of the OWC WECs on the floating breakwater’s wave attenuation and motion characteristics. As this investigation took a proof-of-concept approach, a soft mooring system was configured to the device in order to establish the general behaviour of the structure. Results indicate that the pitch motion was the most detrimental motion in terms of OWC device performance, but this is expected to be mitigated with incorporation of realistic mooring arrangements. Additionally, findings infer that in majority of cases, OWC device integration has beneficial impacts on both the wave attenuation and motions of the breakwater structure, lending further credence to the economic and performance viability of the concept.

Motion and dynamic responses of a semisubmersible in freak waves

The present research aims at clarifying the effects of freak wave on the motion and dynamic responses of a semisubmersible. To reveal the effects of mooring stiffness, two mooring systems were employed in the model tests and time-domain simulations. The 6-DOF motion responses and mooring tensions have been measured and the 3-DOF motions of fairleads were calculated as well. From the time series, trajectories and statistics information, the interactions between the freak wave and the semisubmersible have been demonstrated and the effects of mooring stiffness have been identified. The shortage of numerical simulations based on 3D potential flow theory is presented. Results show that the freak wave is likely to cause large horizontal motions for soft mooring system and to result in extremely large mooring tensions for tight mooring system. Therefore, the freak wave is a real threat for the marine structure, which needs to be carefully considered at design stage.

Sensitivity analysis of air gap motion with respect to wind load and mooring system for semi-submersible platform design

A design of semi-submersible platform is mainly based on the extreme response analysis due to the forces experienced by the components during lifetime. The external loads can induce the extreme air gap response and potential deck impact to the semi-submersible platform. It is important to predict air gap response of platforms accurately in order to check the strength of local structures which withstand the wave slamming due to negative air gap. The wind load cannot be simulated easily by model test in towing tank whereas it can be simulated accurately in wind tunnel test. Furthermore, full scale simulation of the mooring system in model test is still a tuff work especially the stiffness of the mooring system. Owing to the above mentioned problem, the model test results are not accurate enough for air gap evaluation. The aim of this paper is to present sensitivity analysis results of air gap motion with respect to the mooring system and wind load for the design of semi-submersible platform. Though the model test results are not suitable for the direct evaluation of air gap, they can be used as a good basis for tuning the radiation damping and viscous drag in numerical simulation. In the presented design example, a numerical model is tuned and validated by ANSYS AQWA based on the model test results with a simple 4 line symmetrical horizontal soft mooring system. According to the tuned numerical model, sensitivity analysis studies of air gap motion with respect to the mooring system and wind load are performed in time domain. Three mooring systems and five simulation cases about the presented platform are simulated based on the results of wind tunnel tests and sea-keeping tests. The sensitivity analysis results are valuable for the floating platform design.

Analysis of the First Order and Slowly Varying Motions of an Axisymmetric Floating Body in Bichromatic Waves

The paper presents an experimental and numerical investigation on the motions of a floating body of simple geometry subjected to harmonic and biharmonic waves. The experiments were carried out in three different water depths representing shallow and deep water. The body is axisymmetric about the vertical axis, like a vertical cylinder with a rounded bottom, and it is kept in place with a soft mooring system. The experimental results include the first order motion responses, the steady drift motion offset in regular waves and the slowly varying motions due to second order interaction in biharmonic waves. The hydrodynamic problem is solved numerically with a second order boundary element method. The results show a good agreement of the numerical calculations with the experiments.