Heave Plates Research Articles

An investigation of the effects of wind-induced inclination on floating wind turbine dynamics: heave plate excursion

A current trend in offshore wind is the quest for exploitation of ever deeper water sites. At depths between 50m and 100m a promising substructure is the column-stabilised semi-submersible floating type. This solution is currently being tested at full scale at the WindFloat and Fukushima Forward demonstrator sites in Portugal and Japan respectively. The semi-sub design class frequently adopts passive motion control devices based on the water entrapment principle, such as heave plates, tanks, and skirts. Whilst effective for small inclinations, these can underperform when the structure is inclined under wind loading. This study examines the alteration of potential hydrodynamics due to wind-induced trim (geometric non-linearity) and its impact on the wind turbine׳s wave response with focus on heave plate performance. Firstly it is shown by using the boundary element approach that wind trim affects wave loading in the ocean wave band between 5s and 15s, and introduces hydrodynamic coupling typical of non-symmetric hulls. These features are incorporated in frequency-domain dynamic response analysis to demonstrate that said effects bear a significant impact on the turbine׳s motion in waves. Accounting of heave plate excursion improves the assessment of the seaworthiness of floating wind turbine concepts, potentially leading to new design constraints.

Hydrodynamic Responses of Spar Hull with Single and Double Heave Plates in Random Waves

Heave plates have been widely used to enhance viscous damping and thus reduces the heave response of Spar platforms. Single heave plate attached to the keel of the Spar has been reported in literature (Tao and Cai 2004). The effect of double heave plates on hydrodynamic response in random waves has been investigated in this study. The influence of relative spacing <TEX>$L_d/D_d$</TEX> (<TEX>$D_d$</TEX>-the diameter of the heave plate) on the hydrodynamic response in random waves has been simulated in wave basin experiments and numerical model. The experimental investigation has been carried out using 1:100 scale model of Spar with double heave plates in random waves for different relative spacing and varying wave period. The influence of relative spacing between the heave plates on the motion responses of Spar are evaluated and presented. Numerical investigation has been carried out to investigate effect of relative spacing on hydrodynamic characteristics such as heave added mass and hydrodynamic responses. The measured results were compared with those obtained from numerical simulation and found to be in good agreement. Experimental and numerical simulation shows that the damping coefficient and added mass does not increase for relative spacing of 0.4 and the effect greater than relative spacing on significant heave response is insignificant.

Smart' Buoy Could Turn Waves into Platform and Subsea Power

Technology Update The use of moored, oceangoing “smart” buoys that can harvest energy from waves could be an efficient and economic means of supplying electric power for various offshore oil and gas operations, including well trees, monitoring systems, and autonomous underwater vehicles (AUVs). Potentially, such technology could reduce, or in some cases eliminate, the use of diesel-powered generators on offshore facilities. The PowerBuoy (Fig. 1), developed by Ocean Power Technologies (OPT), is new to the oil industry but has been used for a number of years in the defense and utility sectors. The autonomous buoy system consists of a surface float, a spar containing a power takeoff (PTO), a battery system, and a heave plate that constrains the spar’s motions. The system is capable of delivering energy from a few kilowatts to several hundred kilowatts, with future evolution planned to deliver even more power. The process begins with the rising and falling of waves, and the resultant mechanical stroking is converted by a specially designed PTO to drive an electric generator. This power is transmitted to external equipment by means of an underwater power cable or directly to payloads integrated into the structure itself. Continuous power is then available with the option of larger, timed power bursts. Advanced internal control systems continuously monitor the various subsystems and the surrounding environment and optimize performance of those systems with data transmitted to shore in real time, providing health and status updates on itself and its attached payloads. With this information, the operator gains a high level of control. Depending on the model of the buoy, the system is designed to need no maintenance for up to 3 years, providing potential savings in operating and life cycle expenses compared with existing power generation alternatives. Advanced control algorithms have been developed that actively assess oncoming waves to tune the internal PTO dynamically to ensure that it extracts maximum power. In the event of especially large oncoming waves, the buoy will automatically protect itself by locking the float and PTO subsystems and continuing to supply electric power to its payloads by means of the embedded energy storage system (i.e., batteries). The power management system of the buoy manages the state of charge of the battery to ensure efficient overall system operation and to preserve battery life.

Hydrodynamic Response of Spar with Single and Double Heave Plates in Regular Waves

Hydrodynamic Response of Spar with Single and Double Heave Plates in Regular Waves

Hydrodynamic characteristics of an oscillating circular disk under steady in-plane current conditions

The heave motion performance of a Spar platform could be improved significantly by installing heave plates. The heave plates not only increase the vertical added mass of the platform but also provide extra damping in heave. This paper presents an investigation of the hydrodynamic characteristics of a heave plate under steady current conditions using three-dimensional direct numerical simulations. A circular disk of diameter D is forced to oscillate along its axis sinusoidally. Three aspect ratios of the disk, i.e., td/D=0.1,0.05 and 0.02, are considered, respectively. The current is introduced in the plane of the disk. The influences of three parameters, the Keulegan–Carpenter number (KC), the reduced current velocity (Vr) and the aspect ratio of the disk, on the flow structures, the current drag force, the added mass and the damping of the heave plate are examined both qualitatively and quantitatively. The ranges of the KC and Vr numbers considered in this study are 0≤KC≤1 and 0≤Vr≤0.25, respectively. The frequency number β for the oscillating disk is fixed at 200.

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

ABSTRACTDamping 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.

A Review of the Hydrodynamic Characteristics of Heave Plates

Heave plates can increase the added mass and viscous damping of marine structures. It's a more economical way to improve the stability of the marine structures' heave movement by adding heave plates compared to other methods. Many scholars have studied on the influencing factors of theirs properties. This paper describes the findings of each factor, and the results are compared. Some experimental results have still not been explained, so it is needed to continue to study heave plates.

Experimental investigation of the hydrodynamic characteristics of heave plates using forced oscillation

The heave plate is the key component of a Spar platform as it can effectively improve the heave response of the platform system by providing additional damping and added mass. This paper investigates the hydrodynamic coefficients of heave plates by using forced oscillation model tests. The effects of variables such as the Keulegan–Carpenter (KC) number, frequency of oscillation, plate depth, thickness-to-width ratio, shape of the edge, perforation ratio and hole size on the hydrodynamic coefficients were analyzed. Experiments using a group of three solid or perforated heave plates were also carried out and the experimental results were compared with those for a single plate. The relationship between the spacing of the heave plates and hydrodynamic coefficients was studied.

Experimental and numerical study of the effects of heave plate on the motion of a new deep draft multi-spar platform

The heave motion of a floating structure is critical, as a favorable heave characteristic permits dry tree systems, amongst other benefits. The heave response can be suppressed by installing heave plates. However, the associated hydrodynamic effects, which include viscous damping and added mass, are very complicated. Moreover, there are limited experimental investigations to understand the heave plate effects on actual platform designs. This paper aims to study the abovementioned issue based on a novel deep draft multi-spar (DDMS) platform, for four different configurations (with/without upper and lower heave plates). A set of experiments (free decay, regular and irregular wave tests) are conducted, and compared with time domain and linearized frequency domain analyses. Amongst other things, the investigations and discussions include the added mass and damping coefficients for the platform and heave plates, comparison of experimental and numerical results, and the influence of the heave plates on the surge, heave and pitch motions.

Dynamic response analysis of floating offshore wind turbine with different types of heave plates and mooring systems by using a fully nonlinear model

A finite element model is developed for dynamic response prediction of floating offshore wind turbine systems considering coupling of wind turbine, floater and mooring system. The model employs Morison's equation with Srinivasan's model for hydrodynamic force and a non-hydrostatic model for restoring force. It is observed that for estimation of restoring force of a small floater, simple hydrostatic model underestimates the heave response after the resonance peak, while non-hydrostatic model shows good agreement with experiment. The developed model is used to discuss influence of heave plates and modeling of mooring system on floater response. Heave plates are found to influence heave response by shifting the resonance peak to longer period, while response after resonance is unaffected. The applicability of simplified linear modeling of mooring system is investigated using nonlinear model for Catenary and Tension Legged mooring. The linear model is found to provide good agreement with nonlinear model for Tension Leg mooring while it overestimates the surge response for Catenary mooring system. Floater response characteristics under different wave directions for the two types of mooring system are similar in all six modes but heave, pitch and roll amplitudes is negligible in tension leg due to high restraint. The reduced amplitude shall lead to reduction in wind turbine loads.

Research on the hydrodynamic characteristics of heave plate structure with different form edges of a spar platform

The hydrodynamic characteristics of heave plates with different form edges of Truss Spar Platform are studied in this paper. Numerical simulations are carried out for the plate forced oscillation by the dynamic mesh method and user defined functions of FLUENT. The added mass coefficient C m and the damping coefficient C d of heave plate with tapering condition and the chamfer condition are calculated. The results show that, in a certain range, the hydrodynamic performance of heave plate after being tapered is better.

Frequency Response Analysis of Truss Spar by Considering the Viscous Damping

A calculation model for Truss Spars motion response is established under the wave action by combing the potential flow theory and the modified Morison equation. The high order boundary element method is used to acquire the wave exciting force and the relevant hydrodynamic information. By numerical simulation, the surge, heave and pitch s are obtained, and the parameter analysis for influence of Morison drag coefficient on responses are conducted. The calculated results indicate that the surge and pitch motions are only sensitivity to the drag coefficient in the low frequency range, and consequently the viscous damping would play an important role when the platform undergoes 2nd order wave slow drift force. For the heave response, the heave plates installed on the Truss Spar provide large vertical viscous damping which distinctly affects the heave .

Perdido Development Project - Spar and Moorings

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper OTC 20885, ’Perdido Development Project - Spar and Moorings,’ by Curtis Lohr and Keith Smith, Shell International Exploration and Production, prepared for the 2010 Offshore Technology Conference, Houston, 3-6 May. The paper has not been peer reviewed. A spar platform was selected for the Perdido development during system selection in 2005 to provide a stable platform for direct vertical-access drilling and producing operations in nearly 8,000 ft of water. A combination polyester/chain mooring system was selected to place less vertical load on the floating structure (compared to an all-steel mooring system). Introduction The Perdido spar was Shell’s first spar-host production platform, but had the benefits of being developed as a joint venture with Chevron and BP. Technip was selected as the spar and mooring contractor. The spar comprises a cylindrical upper section (hard tank); a trussed midsection (truss) formed from tubular legs and braces with horizontal stiffened-plate flats, or heave plates; and a bottom section (soft tank) used to carry solid ballast and provide flotation during installation. Some key characteristics of the Perdido spar are listed below: Overall length—555 ft, draft 505 ft Hard tank—118-ft outside diameter (OD)×264-ft length, with 214-ft hard-tank draft Hull weight—approximately 20,000 tons including structural steel, appurtenances, and hull systems Nine mooring lines Two heave plates Three strakes A 46×46-ft moonpool Six riser slots Four-leg topside and truss Octagonal soft tank Six hydraulic tensioners supporting five production risers and one drilling/completion riser The spar was designed to accept a single-lift topside by limiting the spar’s OD to 118 ft and by placing storage tanks for diesel fuel, methanol, dead oil, low-dosage hydrate inhibitor, and drill water inside the upper compartment of the spar rather than on the topside. The spar was fabricated in Finland. Fabrication began in November 2006, dry transport to the Gulf of Mexico began in May 2008, and the wet tow offshore occurred in August 2008. One challenge that was unique to the spar fabrication was installation of components for the hydraulic riser-tensioning system in the main hull-fabrication yard. Tolerance for this system was extremely tight, and installation requirements were discussed well in advance to plan the work. Jigs to ensure that the installation of components was within tolerance were designed, fabricated, and used. Mooring System The Perdido mooring system uses nine polyester/chain lines averaging more than 2 miles in length, with the vast majority of each line’s length being polyester. The nine taut lines are oriented in a 3×3 pattern and consist of 9.68-in. polyester-rope main sections and short lengths of 5.28-in. chain at each end. The spar has an “active” mooring system to reposition it over drilling locations within an approximately 350-ft-diameter area. Because polyester is more difficult to handle in a winch, the top of each mooring line consists of a short length of chain, called the platform chain, which is connected to the spar. The platform chain runs through a below-water fairlead mounted on the outside shell of the spar. A dedicated chain jack controls each platform chain.

Conceptual Design of Monocolumn Production and Storage With Dry Tree Capability

A new concept and a preliminary study for a monocolumn floating unit are introduced, aimed at exploring and producing oil in ultradeep waters. This platform, which combines two relevant features—great oil storage capacity and dry tree production capability—comprises two bodies with relatively independent heave motions between them. A parametric model is used to define the main design characteristics of the floating units. A set of design alternatives is generated using this procedure. These solutions are evaluated in terms of stability requirements and dynamic response. A mathematical model is developed to estimate the first order heave and pitch motions of the platform. Experimental tests are carried out in order to calibrate this model. The response of each body alone is estimated numerically using the WAMIT® code. This paper also includes a preliminary study on the platform mooring system and appendages. The study of the heave plates presents the gain, in terms of decreasing the motions, achieved by the introduction of the appropriate appendages to the platform.

Hydrodynamic performance of solid and porous heave plates

Heave plates have been widely utilized in floating offshore structures as they can provide additional damping and added mass to improve the hydrodynamic response of the system. This study investigates the hydrodynamic characteristics (added mass and damping) of oscillatory solid or porous disks using model scale experiments. All experiments were conducted via forced oscillation model tests using a planar motion mechanism (PMM). The hydrodynamic coefficients of the solid or porous disk obtained from the force measurements are analysed and presented. The sensitivities of the damping and added mass coefficients to both motion amplitude and the disk porosity are examined.

Spacing effects on hydrodynamics of heave plates on offshore structures

The nonlinear viscous flow problem associated with a heaving vertical cylinder with two heave plates in the form of two circular disks attached is solved using a finite difference method. Numerical experiments are carried out to investigate the spanwise length effects on the hydrodynamic properties, such as added mass and damping coefficients. Over a Keulegan–Carpenter (KC) number range from 0.1 to 0.6 at frequency parameter (β=7.869×107), calculations indicate that a KC-dependent critical value of spanwise length L/Dd (L—the distance between the disks, and Dd—the diameter of the disks) exists. A significant influence of L/Dd on the vortex shedding patterns around the disks and the hydrodynamic coefficients is revealed when L/Dd is smaller than the critical value due to strong interaction between vortices of different disks. Beyond that limit, however, both added mass and damping coefficients are shown to be rather stable, indicating the independence of the vortex shedding processes of different disks.

Design analysis of a truss pontoon semi-submersible concept in deep water

Truss pontoon semi-submersible (TPS) is a new offshore structure concept in industry, where a truss spar is used to create the added mass by the heave plates. In the present paper, the effect of the heave plates on the vertical motion of the floating structure is demonstrated. A TPS is analyzed by utilizing the linear diffraction theory as well as the linear part of the Morison equation. The close agreement of the analysis results with the experimental results suggests that the simplified Morison equation can be used for the present analysis without sacrificing the quality of the results. However, good engineering judgment is required for estimating the values of the hydrodynamic coefficients as well as the amount of damping introduced in the structure. It is also found that the heave plates indeed introduce large added mass and considerable damping in the system motion in the vertical direction such that the resonant oscillation becomes less of a problem. This suggests that the TSP concept may have merits as a heave-controlled floating production structure in the deepwater development.

Heave motion suppression of a Spar with a heave plate

Vortex shedding flow of an oscillating vertical cylinder with a disk attached at its keel is considered. This configuration is of interest for the offshore oil and gas industry. A finite difference method is employed to solve the incompressible Navier–Stokes equations in the primitive-variables formulation. Test cases were used to guide selection of the size of flow domain, numerical parameters, and to verify that the resultant method was both convergent and accurate. Numerical simulations have shown that the geometry configurations of the cylinder and disk, such as aspect ratio of the disk t d/ D d and diameter ratio, D d/ D c have significant influence on the vortex shedding modes and associated hydrodynamic properties, e.g. hydrodynamic damping and added mass coefficients. These in turn affect the performance in heave motion control of the structures.

Low KC flow regimes of oscillating sharp edges. II. Hydrodynamic forces

Three vortex shedding modes, i.e. independent vortex shedding, interactive vortex shedding and uni-directional vortex shedding, for flow induced by oscillatory vertical cylinder witha disk attached at its keel were presented in Part I. The occurrence of these modes were shown to depend on the Keulegan–Carpenter ( KC) number and disk thickness–diameter ratio. Associated with these distinct changes of vortex shedding patterns are distinct increases in the hydrodynamic damping. A quantitative method of identifying the vortex shedding flow regimes is proposed, and the scaling laws for heave damping estimation of cylinder+disk configuration are presented. A sample calculation on a classic Spar platform show that addition of a heave plate increased the heave damping fourfold.

Heave Plate Design With Computational Fluid Dynamics

Computational fluid dynamics (CFD) methods are used to predict the hydrodynamic loads on heave plates. A series of numerical simulations for plates undergoing simple harmonic motion is used to determine the force and moment resultant histories. Then a least mean squares method is used to obtain the appropriate Morison coefficients for the plates under a variety of sea conditions. Finally, force and moment histories are predicted for a plate on a floating platform excited by a random sea motion.