Experimental Study of Wave Force Distribution on a Monopile Structure

As the demand for offshore wind energy continues to grow, the strive to understand the wave forces acting on the substructure of the wind turbines continues. In regard to wind turbine design, it is vital to consider not only the total wave force, but also the local wave forces. Local forces are particularly important for the design of secondary structures as e.g. mooring platforms. Typically, however, experimental studies mainly concern total forces or idealized local forces. We present here a rather simple way to measure local forces along a model monopile. The study is conducted in a wave flume of 28 m in length, in which waves are generated by a piston-type wave maker at a water depth of 0.515 m and shoal onto a bed of slope 1:25. A model monopile is installed and subjected to forcing from a series of both regular and irregular waves. In the experimental set-up, the model monopile is fixed at the bottom and the top and consists of seven independent cylindrical sections. The cylindrical sections are connected by force transducers which measure local shear, and so the associated local forces may be determined. The measured local forces are compared to the force distribution given by Morisons equation combined with linear theory and Wheeler stretching, which is a force estimate commonly used in the industry. This study shows that the total force is rather well captured by Morison’s equation. The force distribution estimated from Morison’s equation, however, shows larger discrepancies from the measured forces. This encourages for further measurements. In this study, we show that it is possible to measure force distribution on a model monopile in a simple and cost-effective manner. The aim is here to demonstrate the method and we will later present a larger body of work associated with the outcome of the measurements.

Relocatable Tide Prediction and Storm Surge Forecasting

The marine environment represents a large and important resource for communities around the world. However, the marine environment increasingly presents hazards that can have a large negative impact. One important marine hazard results from storms and their accompanying surges. This can lead to coastal flooding, particularly when surge and astronomical high tides align, with resultant impacts such as destruction of property, saline degradation of agricultural land and coastal erosion. Where tide and storm surge information are provided and accessed in a timely, accurate and understandable way, the data can provide: 1. Evidence for planning: Statistics of past conditions such as the probability of extreme event occurrence can be used to help plan improvements to coastal infrastructure that are able to withstand and mitigate the hazard from a given extreme event. 2. Early warning systems: Short term forecasts of storm surge allow provide early warnings to coastal communities enabling them to take actions to allow them to withstand extreme events, e.g. deploy flood prevention measures or mobilise emergency response measures. Data regarding sea level height can be provided from various in-situ observations such as tide gauges and remote observations such as satellite altimetry. However, to provide a forecast at high spatial and temporal resolution a dynamic ocean model is used. Over recent decades the National Oceanography Centre has been a world leading in developing coastal ocean models. This paper will present our progress on a current project to develop an information system for the Madagascan Met Office. The project, C-RISC, being executed in partnership with Sea Level Research Ltd, is translating the current modelling capability of NOC in storm surge forecasting and tidal prediction into a system that will provide information that can be easily transferred to other regions and is scalable to include other hazard types The outcome, an operational high-resolution storm surge warning system that is easy to relocate, will directly benefit coastal communities, giving them information they need to make effective decisions before and during extreme storm surge events.

A Methodology to Characterize Internal Solitons in the Ocean

Internal waves in the ocean occur in stably stratified fluids when a water parcel is vertically displaced by some external forcing and is restored by buoyancy forces. A specific case of such internal waves is internal tides and their associated currents. These currents can be significant in areas where internal waves degenerate into nonlinear solitary waves, known as solitons. Solitons are potentially hazardous for offshore engineering constructions, such as oil/gas pipelines and floating platforms. The most efficient mechanism of soliton generation is the tidal energy conversion from barotropic to baroclinic component over large-scale oceanic bottom obstructions (shelf breaks, seamounts, canyons and ridges). In this paper, a methodology is provided to compute diagnostics and prognostics for soliton generation and propagation, including the associated currents. The methodology comprises a diagnostic tool which, through the use of a set of theoretical and empirical formulations, selects areas where solitons are likely to occur. These theoretical and empirical formulations include the computation of the integral body force (1), the linear wave theory to compute the phase speed and the empirical model proposed by (2). After the selection procedure, the tool provides initial and boundary conditions for non-hydrostatic numerical models. The numerical models run in 2D-V configuration (vertical slices) with horizontal and vertical resolutions ranging from 50 to 200 m and 5 to 10 m, respectively. Examples are provided for an open ocean location over the Mascarene Plateau in the Indian Ocean. Validation of diagnostics and prognostics are provided against ADCP and satellite data.

Investigation of Higher-Harmonic Wave Forces and Ringing Using CFD Simulations

Typical offshore structures are designed as tension-leg platforms or gravity based structures with cylindrical substructures. The interaction of waves with the vertical cylinders in high sea states can result in a resonant response called ringing. Here, the frequency of the structural response is close to the natural frequency of the structure itself and leads to large amplitude motions. This is a case of extreme wave loading in high sea states. This understanding of higher-order wave forces in extreme sea states is an essential parameter for obtaining a safe, reliable and economical design of an offshore structure. The study of such higher-order effects needs detailed near-field modelling of the wave-structure interaction and the associated flow phenomena. In such cases, a Computational Fluid Dynamics (CFD) model that can accurately represent the free surface and further the wave-structure interaction problem can provide important insights into the wave hydrodynamics and the structural response. In this paper, the open source CFD model REEF3D is used to simulate wave interaction with a vertical cylinder and the wave forces on the cylinder are calculated. The harmonic components of the wave force are analysed. The model employs higher-order discretisation schemes such as a fifth-order WENO scheme for convection discretisation and a third-order Runge-Kutta scheme for time advancement on a staggered Cartesian grid. The level set method is used to obtain the free surface, providing a sharp interface between air and water. The relaxation method is used to generate and absorb the waves at the two ends of the numerical wave tank. This method provides good quality wave generation and also the wave reflected from the cylinder are absorbed at the wave generation zone. In this way, the generated waves are not affected by the wave interaction process in the numerical wave tank. This is very essential in the studies of higher-order wave interaction problems which are very sensitive to the incident wave characteristics. The numerical results are compared to experimental results for higher-order forces on a vertical cylinder to validate the numerical model.

Wave Buoy Performance in Short and Long Waves, Evaluated Using Tests on a Hexapod

Offshore wave conditions can be measured using wave buoys, which are generally designed for wind waves. Longer waves (swell or bound second-order waves) are very relevant for certain maritime structures. The accuracy of the instrumentation in a typical wave buoy in long and short waves was therefore studied, and it was investigated if the buoy can be applied in longer waves. A Waverider buoy was placed on a hexapod, which applied regular and irregular prescribed motions in multiple degrees of freedom. The hydrodynamic response of the buoy in waves and the effect of its mooring system were not evaluated; the buoy was assumed to follow the orbital motions of a wave and to rotate with its slope. The tests showed that the buoy sensors measure accelerations and rotations with periods between 1.5 and 35 s very well. Vertical displacements derived from the accelerations by the buoy are accurate for the period range of 2 to 20 s. In longer waves, the motions are significantly underestimated, even though the accelerations are accurately measured. This will not lead to large errors in normal operation, as the energy of such long waves is generally low. This explains why the buoy also performs well when it is subjected to irregular motions (less than 2% error in the significant wave height of a half-hour measurement in realistic irregular sea states with peak periods between 5 and 20 s can be expected). It can be concluded that the buoy accurately measures accelerations. The accuracy of the derived displacements decreases when very long swell wave energy (> 20 s) is present. Review of the internal integration procedure may be considered when there is a specific interest in measuring longer waves.

Investigation on Mechanical Properties of Fibreglass Reinforced Flexible Pipes Under Torsion

Fibreglass reinforced flexible pipe (FRFP) is regarded as a great alternative to many bonded flexible pipes in the field of oil or gas transportation in shallow water. This paper describes an analysis of the mechanical behavior of FRFP under torsion. The mechanical behavior of FRFP subjected to pure torsion was investigated by experimental, analytical and numerical methods. Firstly, this paper presents experimental studies of three 10-layer FRFP subjected to torsional load. Torque-torsion angle relations were recorded during this test. Then, a theoretical model based on three-dimensional (3D) anisotropic elasticity theory was proposed to study the mechanical behavior of FRFP. In addition, a finite element model (FEM) including reinforced layers and PE layers was used to simulate the torsional load condition in ABAQUS. Torque-torsion angle relations obtained from these three methods agree well with each other, which illustrates the accuracy and reliability of the analytical model and FEM. The impact of fibreglass winding angle, thickness of reinforced layers and radius-thickness ratio were also studied. Conclusions obtained from this research may be of great practicality to manufacturing engineers.

Numerical Investigation of Energy Saving Device Effects on Ships With Propeller-Hull Interactions

Energy saving devices (ESD) such as propeller ducts, pre-swirl stators, pre-nozzles, etc have been explored as a more economic and reliable approach to reduce energy consumption for both in-operation and newly design ships over the past decades. Those energy saving devices work in the principle of reducing ship resistance and improving propulsion efficiency as well as hull-propeller interactions. Potential saving from various types of ESD have been reported in literature from the range of 3–9% [1] for propulsion efficiency dependent on different measures. Deployment of those devices on actual full-scale ships has been limited over the past years. One of the key obstacles in application of ESD is the lack of confidence in measuring its efficiency on full-scale ships in actual operational conditions. Advances in computational fluid dynamics (CFD) has provided an alternative approach from model scale test to better understand uncertainties in prediction of ESD efficiency in full-scale ship operations [Shin et al, 2013]. In this work a high fidelity CFD model is presented for investigation effects of pre-nozzles on propulsion efficiency and ship resistance. The model is based on the Reynolds Average Navier-Stokes (RANS) solver with different turbulent models including a hybrid detached eddy simulation (DES) approach for predictions of complex near body flow features as well as in the wake regions from hull and propeller. The model is validated with model test for both towing and self-propulsion conditions. Finally a study of pre-nozzle effects on propeller efficiency as well as hull-propeller interaction is presented and compared with available experimental data (Tokyo 2015 Workshop). The current work constitutes a fundamental approach towards designing more efficient ESD for a specific hull form and propeller.

Ocean Wave Non-Linearity and Wind Input in Directional Seas: Energy Input During Wave-Group Focussing

There has been speculation that energy input (wind) can play an important role in the formation of rogue waves in the open ocean. Here we examine the role energy input can play by adding energy to the modified non-linear Schrödinger equation. We consider NewWave type wave-groups with spectra which are realistic for wind waves. We examine the case where energy input is added to the group as the wave-group focuses. We consider whether this energy input can cause significant non-linear effects to the subsequent spatial and spectral evolution. For the parameters considered here we find this to have only a small influence.

Experimental Investigation on Resistance Reduction by Means of Air-Bubbling Technique

The paper refers on a research activity, focused at DREAMS Lab of the University of Genoa (Italy) and still under development, oriented to experimental application of air-bubbling techniques on flat plates and hull models. In this study the reduction in the frictional resistance by air bubbling generated by customized pneumatic circuits is tested, both on the lower surfaces of flat plates characterized of different geometries of holes and on a hull model tested in towing tank. The effective shape of air bubbles is observed, and changes in the local frictional drag are measured, using flexible and low cost thin sensors at different levels of flow rate and pressure of injected air. In towing tank tests the experiments compare hull without and with holes on the bottom, modifying the characteristics of speed, pressure, flow rate and areas interested to the air injection. Systematic tests campaign has been developed, using also actuation pneumatic workbenches expressly designed for the experiments.

Experimental Study of Ship Response due to Internal Viscous Cargo Motions

Potential liquefaction of some cargoes (Nickel ore, iron ore, ...) is a major risk for the maritime industry. The difficulties to simulate accurately the behaviour of these materials as well as their interaction with a bulk carrier model leaded us to use a non-Newtonian highly viscous fluid to model a liquefied ore. An experimental approach is presented in this paper. Roll responses of a ship model as well as details on the internal free surface behaviours are investigated for different loading conditions: solid cargo, fresh water and viscous fluids.