S. Nallayarasu

Estimation of incident and reflected waves in regular wave experiments

Abstract This paper presents a laboratory technique for measurement of reflected waves by a submerged horizontal plate. A simple method is proposed to decompose the composite wave record obtained from the wave reflection experiments in the wave flume. The data collected were used to compute the reflected wave height by means of two and three probe methods proposed by other investigators and compared with the directly computed reflected wave using the measured incident wave. The experiment was carried out for a horizontal plate of 1 m length spanning the full width of the flume with wave periods ranging from 0.8 to 1.8 sec with 0.1 sec increments and wave heights of 5, 10 and 15 cm. The methods using two and three probes with phase measurement yield better results than the three probe method without phase measurement, and in general they have a wider range of application. The method using three probes without the phase measurement generally fails due to numerical instability of the scheme. The results obtained by the proposed method are compared with the two and three probe methods and a comparative analysis of the four methods is presented and discussed.

Influence of Heave Plate on Hydrodynamic Response of Spar

Spar technology has been used in offshore oil and gas exploration successfully for number of years for drilling, production and storage in deepwater. The motion response of floating structures especially the heave response in particular, is very important to the selection of suitable drilling and production equipments. Reduction of heave response can be achieved by attaching a heave damping plate to the keel of a Spar. This has been used in the past. Experimental and numerical studies on such devices of various diameters under regular waves has been carried out and presented. The experiments were conducted on a 1: 100 scale model of Spar designed for a water depth of 245m with a payload of 10000 tonnes and the numerical analysis was carried out using ANSYS AQWA software. Numerical and experimental values of RAOs for surge, pitch and heave compares reasonably well. Measured and numerical results of RAOs for surge, pitch and heave and the effect of diameter ratio, wave steepness, and mooring line pretension were presented and discussed. An optimum heave response is achieved when the heave plate diameter is 20% to 30% larger than the diameter of the spar.Copyright

Experimental investigation and CFD simulation of heave damping effects due to circular plates attached to spar hull

ABSTRACT Damping elements are used in ships and offshore structures as response reduction devices for maintaining the motion response within acceptable limits. The design of these elements relies on a combination of empirical approach and past experience. In this paper, an investigation of the wave excitation and hydrodynamic reaction forces acting on floating structures with and without damping elements is presented taking spar platform as the candidate structure. Understanding the flow pattern and vortex structure around these circular heave damping plates helps correlate the energy dissipation and damping. In the present study, measurement of hydrodynamic damping and inertia forces and response were carried out to establish the relationship between wave frequency and damping ratio. Flow pattern around the damping elements has been simulated using computational fluid dynamics (CFD) approach. Furthermore, flow visualisation and velocity measurement using particle image velocimetry (PIV) technique has been carried out in a wave flume for comparison and evaluation of the CFD approach. It was observed that the damping varies with wave period in a nonlinear fashion and it varies linearly with wave height. It was also observed that increase in added mass combined with viscous damping is a key factor for the effectiveness of heave plates in reducing motion response of spar platforms. Of the various configurations tested, the one with spar with double heave plate at the keel has higher damping and superior motion response compared to other configurations. The velocity pattern and vortex formation around the circular plates obtained through PIV study and CFD simulation showed good agreement.

Hydrodynamic response of spar and semi-submersible interlinked by a rigid yoke – Part I: regular waves

Experimental and numerical investigations on the hydrodynamic response of a spar and semi-submersible linked system under regular waves (Part I) and random waves (Part II) are presented. These studies were performed for two cases: the first case with the spar alone and the second case with a semi-submersible linked to the spar. The influence of the semi-submersible on the motion response of the spar and its mooring system are evaluated and presented together with acceptance criteria used in the industry. Experimental results are compared with numerical results and found to be acceptable.

Wave induced pressures and forces on a fixed submerged inclined plate

Abstract The diffraction of water waves by a submerged fixed inclined plate is modelled using the finite element method (FEM). Wave induced pressures and forces on the submerged fixed inclined plate due to incident monochromatic waves are analysed using the finite element model. The fluid domain is discretised using 8 node isoparametric elements with 3 node quadratic line elements on the free surface, bed and radiation boundaries. The accuracy of the numerical technique is demonstrated by comparing the FEM results with the analytical solution obtained by long wave solution for a thin submerged fixed horizontal plate in shallow water. The horizontal and vertical force coefficients are computed for various wave periods and inclinations of the plate. The reflection and transmission coefficients are also computed and presented for various submergence depths. The possible use of the submerged inclined plate as a breakwater and its advantages over the horizontal plate are discussed.

Effect of hull geometry on the hydrodynamic response of spar in regular waves

The hydrodynamic response of the floating production systems, especially deep draft spars, is of major concern in the offshore industry for its influence on the selection of suitable drilling and production equipment. A number of response reduction methods are being investigated in view of better motion response and improved design methods. In the present study, experimental and numerical investigations were carried out on spar hulls with different shapes to obtain the motion response characteristics and to compare them with that of the classic spar. The spar hulls were designed for a common topside weight of 10,000 tonnes with a displacement of 65,000 tonnes. Experiments were carried out on 1:100 scaled models of spar with circular disk, enlarged base, eight-cell spars and six-cell spars for a water depth of 300 m and numerically validated using ANSYS AQWA software package. Experimental and numerical values showed good agreement especially at lower wave periods. Among the cell spars, the six-cell spar showed better motion response characteristics than the eight-cell spar, both in heave and surge, with limited difference in pitch. The spar with enlarged base was found to be more stable in pitch and less stable in heave than the spar with circular disc at keel.

Experimental and Numerical Investigation on Hydrodynamic Response of Spar with Wind Turbine under Regular Waves

The necessity of offshore wind turbines to generate wind power is multifold and well known. The focus of offshore based wind farms are mainly due to abundant availability of unobstructed wind power and land use restriction. Therefore, the development of floating type supports for offshore wind turbines is getting more attention. In this paper the experimental and numerical investigation on the motion response of Spar supporting the wind turbine under regular waves is presented. The experiments were conducted in three stages viz. (a) Spar with stationary wind turbine under regular waves. b) Spar with rotating wind turbine under calm sea conditions. c) Spar with rotating turbine under regular waves. Numerical simulation of the Spar response due to incident waves and the wind turbine torque is carried out using AQWA software. The results obtained from experiments are then compared with that obtained from numerical analysis. The measured Response Amplitude Operator (RAO) compares well with that obtained from numerical analysis. The comparison of measured response and simulated response for wind turbine rotation case shows reasonable match. The combined effect of wind turbine rotation and incident wave on the hull response is 10%, 16% and 32% higher than that of the response due to incident wave alone for heave, surge and pitch responses respectively.

Damping Characteristics of Heave Plates Attached to Spar Hull

Damping elements are used in ships and offshore structures as response reduction devices for maintaining the hydrodynamic response within acceptable limits. The use of such elements so far has been based on past experience or using empirical based design approach. An investigation of the wave excitation forces acting on these structures with and without damping elements may give an insight into the damping effects of such elements. Also understanding the flow pattern and the magnitude of flow variation around these elements may be useful to improve the design methods. In the present study, measurement of damping forces and response were carried out to establish the relationship between wave frequency amplitude and damping ratio. It was observed that the damping varies with wave period non-linearly and linearly with wave height. Also it was observed that increase in added mass combined with viscous damping is a key factor for the effectiveness of heave plates in reducing motion response of spar platforms. Of the various configurations tested, the spar with double heave plate at the bottom has higher damping compared to other configurations.Copyright

Wave Slam/Slap Loads on Structural Members in the Air-Gap

The offshore platforms shall be designed with deck elevation that maintains sufficient air-gap with maximum predicted wave crest elevations. However, due to compelling reasons of hydrocarbon process and changes to water levels/seabed subsidence may have structures in the close vicinity of the sea level or submerged. Hence the assessment of wave induced forces on the deck structures especially the old platforms become very essential. Review of literature indicates that the previous studies focused mainly on the circular sections and not much information is available on the non-circular sections. The current study focused on measurement of wave induced slam/slap loads in a laboratory wave flume and estimation of force coefficients based on drag based empirical equations. Further, it is of interest to focus on the flow around the open sections (non-circular) using a flow visualization technique (Particle Image Velocimetry) to understand the flow pattern around the elements. The results indicate that the slam and slap coefficients for I sections are smaller than the channel sections and the values of coefficients are few times higher than the circular sections.Copyright

Hydrodynamic response of spar hulls with heave damping plate using simplified approach

The evaluation of hydrodynamic response for variety of hull configurations has to be obtained during initial design for concept screening studies. A number of response evaluation techniques including finite element, boundary element and panel methods have been in use for some time. These techniques may employ first- or second-order solutions but may take considerable effort and time. The geometric parameters used in the industry for spar hulls indicate that the structures fall within the Morison regime (D/L < 0.2, where D is the diameter of the spar and L is the wave length). A simplified solution technique, which may offer quicker solution with certain approximations, may be acceptable during initial design process. Such an approximate solution has been applied to other floating body problems in the past. Application of semi-empirical methods such as Morison equation for slender structures has been very successful due to its simplicity and ease with which the wave-induced loads are computed. A simplified solution process together with heave damping plate in the form circular disc attached to the keel of the spar has been proposed in this study. The computation of added mass due to the presence of damping plate has been carried out using simple geometric proposition and is verified by experimental studies. The computation of response of rigid floating body can be obtained by the solution to the equation of motion involving six degrees of freedom. The simplified solution uses the force calculated using Morison equation and damping, added mass coefficients obtained from the experiments to calculate the linear responses. Experimental studies were conducted in a laboratory wave flume on spar models to obtain the hydrodynamic damping, added mass and motion response characteristics. The computed responses in terms of response amplitude operators have been compared with that obtained from WAMIT and experimental results.