Showing papers in "International Journal of Offshore and Polar Engineering in 1991"

Time Scale For Wave/Current Scour Below Pipelines

Abstract: A non-dimensional formula has been developed for the time scale of the scour process below a marine pipeline, based on the presently available data. The main part of the data originates from ISVA. Both the current case and the pure-wave case are considered. The results indicate that the non-dimensional time scale is proportional to the -5/3 power of the Shields parameter. In the study, the time scale of scour, involving a change in the wave climate has also been investigated. The results indicate that the time scale is governed by the Shields parameter plus the two Keulegan-Carpenter numbers corresponding to the waves before and after the change takes place.

Onset Of Scour Below A Pipeline Exposed To Waves

Abstract: Experiments are carried out in a wave flume to investigate whether tunnel erosion will take place or otherwise. The main parameters studied are the initial burial depth and the Keulegan-Carpenter number, KC. The results indicate that the critical burial depth beyond which no scour occurs is a function of KC number. The larger the KC number, the larger the critical burial depth. Also presented in the paper are the results of bed shear-stress measurements in the vicinity of a pipeline exposed to an oscillatory flow as well as the pressure distributions around a pipeline mounted on the bed. The latter information shed light onto the mechanism responsible for the onset of scour in waves.

Three-dimensional hydroelastic response of a very large floating structure

Abstract: A simplified analysis procedure based on 3-D hydroelasticity, which can be used to determine the motions and intermodule forces of a multi-module, very large floating structure (VLFS), is described. While the procedure is applicable to an arbitrary geometric layout of the modules, the modules are considered rigid, and hence all deformations occur in the module connectors. The procedure is used to analyse the response of a 5-module VLFS in both regular and irregular seas. The effect of fluid and structural coupling of the modules on the response is evaluated.

On The Breaking Of Energetic Waves

Abstract: The historical development of attempts to understand and quantify the breaking of energetic gravity waves is reviewed, and some new theoretical results are presented. The importance of at-sea observations, and of the appearance of wave breaking near the center of wave groups is emphasized. Discrepancies between laboratory measurements of wave steepness at breaking, (H/gr)* = 0.021, and recent measurements at sea, (H/gT2)*avg = 0.0067 are explained in terms of the fetch dependence of the breaking of modulated waves, as shown experimentally by Su and Green (1984). With regard to mechanism, the role of non-linear resonant wave interactions leading to modulations, and of wave deformation is stressed. Finally we outline a new theoretical analysis of the breaking mechanism, central to which is the role of resonant (sideband) instability as an intermediary mechanism for rapid wave deformation leading to breaking within wave groups. Results are shown to be in excellent agreement with observational laboratory experiments of Bonmarin (1989) and observations of breaking in wave groups at sea. surements at sea. Much of the research is exploratory (i.e., fact finding), and the various efforts have not been well-coordinated. A special difficulty is the ambiguous relation between controlled experimental studies, and measurements at sea. For example, there has been no systematic study of the effect of scale on breaking, nor is it yet entirely understood which aspects of real sea waves are crucial to an understanding of breaking (i.e., wind, currents, spectral shape, directionality, etc.). Despite these difficulties, as we show, comprehension of at least one important breaking mechanism for energetic waves may be within our grasp. For that we have to understand wave instability and non-linear wave interactions. We begin with a brief and selective review of the search for a breaking criterion in terms of wave steepness, or other simple local conditions. Here we show that neither the idea of a limiting wave steepness nor widely accepted laboratory values of limiting steepness, (H/gT2)* = 0.021, are in accord with measurements at sea. We explain this discrepancy in terms of the fetch dependence of breaking inception for waves of varying steepness, as shown by Su and Green (1984). Finally a picture begins to emerge that is consistent with available laboratory and sea observations, not the least of which is the tendency of waves to break at sea within wave groups. Central to this picture is the role of resonant (sideband) instability as an intermediary mechanism for wave deformation leading to breaking within wave groups. Finally we outline a new theoretical analysis of this intermediary mechanism; this allows calculation of the deformation and the time rates involved, which are shown to be in excellent agreement with observational laboratory experiments of Bonmarin (1989). Other conclusions are drawn that are in accord with observations. INTRODUCTION Those long waves spread under the peak in the energy spectrum are of greatest concern for the safety of ships and marine structures, and coming inshore for the integrity of the shore environment. Furthermore, at-sea measurements of breaking waves show that the breaking waves are about twice as energetic as the wave of mean height (Holthuijsen and Herbers, 1986). Here we review our understanding of the breaking of these energetic waves at sea in deep water. We have the advantage that they are predominantly long-crested, so that we can approximate them, at least prior to breaking as planar waves. Breaking is itself characterized by a forward motion of the crest, a steepening of the forward face of the wave, and an accompanying increase in the water velocity at the crest, q*. From everything we know, when q* becomes as large as the wave celerity, c, then the irreversible overturning process becomes inevitable (Fig. 1). Therefore, this condition, q* ~ c, is a good criterion to separate breaking and nonbreaking waves in practice; Melville and Rapp (1988), in a tank experiment using a laser velocimeter, have proven this criterion and have shown that wave height itself, or even rapid rises in wave height, cannot distinguish breaking waves without error. Upon observing breaking waves in deep water, it is natural to ask: Why do sea waves break? What is the hydro-mechanics? Are there appropriate criteria for the inception of breaking? How can we model the breaker as it evolves? Although the subject is 140 years old and great progress has been made in recent decades, these large basic questions still remain largely unanswered. That is why the fundamental study of breaking waves is so challenging. As we shall see, our current knowledge of breaking is derived from a combination of: theoretical analysis and computation, controlled experimental studies, and actual observations and meaReceived June 24, 1991: revised manuscript received by the editors February 5, 1992. The original version (prior to the final revised manuscript) was presented at The First International Offshore and Polar Engineering Conference (lSOPE-91), Edinburgh, United Kingdom, August 11-16, 1991.

Numerical Analysis Of 2-D Nonlinear Cable Equations With Applications To Low-Tension Problems

Abstract: Two efficient techniques are presented for analyzing cables under zero or low tension. First, an explicit finite-difference algorithm that is stable for zero-tension problems is presented. Secondly, an implicit finite-difference algorithm, which incorporates the effects of bending stiffness, is discussed. The importance of bending stiffness is shown for low-tension problems and the method proves stable, regardless of the cable tension magnitude. Therefore, the implicit method provides an efficient means with which to study low-tension cable problems in a more physically accurate way.

Comparison Of Higher-Order Boundary Element And Constant Panel Methods For Hydrodynamic Loadings

Abstract: This paper presents the description of a higher-order boundary element method (HOBEM) for calculating linear hydrodynamic loadings on large floating bodies and comparison with constant panel methods and HOBEMs that are employed in conjunction with the hybrid boundary integral equation procedure, for a variety of structural configurations. It was concluded from the study that HOBEM has several important features: it uses many fewer boundary elements and much less computer time, with higher accuracy than conventional methods. In addition to these, the computer code for hydrodynamic loadings can easily be used for finite element structural analysis.

Numerical Simulation Of Experiments On Multiplanar Tubular Steel X-Joints

Abstract: Current design codes, used to predict the ultimate static load of uniplanar and multiplanar X-joints in circular hollow sections, are mainly based on extensive tests on simple uniplanar joints. Very few test results on multi planar joints are available for verification. Therefore, an experimental research programme, consisting of 3 uniplanar and 9 multiplanar Xjoints in circular hollow sections, has been carried out for the determination of the stiffness, the ultimate static load and the deformation capacity. The results of these experiments are presented in IJOPE, March 1991 (van der Vegte, 1991b). In addition, numerical simulations of these experiments have been carried out. The results of these numerical analyses are presented in the present study. Good agreement exists between the numerically and experimentally determined load-deformation curves. Finally, the numerically determined ultimate loads are compared with the values of the ultimate loads obtained from several design codes and recommendations.

Identification of low-order dynamic models for deeply-towed underwater vehicle systems

Abstract: The horizontal dynamics of deeply-towed underwater vehicle systems can be effectively modelled by nonlinear partial differential equations. However, high resolution numerical solutions are of limited use in controller design, where methods for systems of very high or infinite order are not well developed. This paper examines the possibility of finding low-order dynamic models in differential equation form, in order to present a more tractable control problem. A learning model method is used, and the identification process is novel in that a verified high-order model provides the primary data set. This approach allows a priori characterisation of system responses in regimes or scenarios for which no experimental data exist. The performances of the reduced-order forms are verified through comparison of model output with actual sea data obtained during recent deepwater tests.

Localization For Automated Inspection Of Curved Surfaces

Abstract: This paper deals with localisation of curved surfaces, meaning the optimum positioning of a target surface obtained by some manufacturing process with respect to an ideal design surface. The localisation problem is formulated as an optimum parameter estimation problem involving rigid body translations and rotations obtained through unconstrained minimisation of a distance norm. An iterative localisation algorithm has been developed which terminates when the magnitudes of rigid body translations and rotations become smaller than preset threshold values. To reduce the cost of localisation for large amounts of data arising in high precision applications involving free-form surfaces, methods to improve the efficiency of the process based on coherence considerations of input data are investigated. An example based on actual measured data from a manufactured sculptured surface illustrates our technique.

Radiation And Diffraction Forces Acting On An Offshore-Structure Model In A Towing Tank

Abstract: The integral-equation method is applied to calculate the effects of tank-wall reflections upon the hydrodynamic forces acting on a model of offshore structure. The Green function satisfying the tank-wall boundary condition is provided by first considering an infinite number of mirror images and then seeking a closed-form analytical expression for the resultant infinite series. By the analysis of energy and momentum conservation, the formulas are derived, giving damping coefficient, wave-exciting force, and drift force in terms of only the Kochin function. Numerical computations are performed for a structure, composed of four vertical circular cylinders with horizontal base, both in the open sea and in a towing tank. It is shown that the tank-wall effects on the second-order drift force are greater than those on the linear forces and resultant motions.

Analysis Of Nonlinear Response Of An Articulated Tower

Abstract: A semi-analytical method is employed to investigate stability of the nonlinear response of an articulated tower. Local and global bifurcations determine the possible existence of complex nonlinear and chaotic motions which cannot be obtained through evaluation of an equivalent linearised system.

Biplane Axial Turbine For Wave Power Generator

Abstract: Experimental investigations directed towards improving the overall characteristics of the self-rectifying turbine for a wave power generator are reported. First, biplane axial turbines with various setting angles have been manufactured and tested in a computer-controlled wind tunnel, which can simulate arbitrary oscillating flows. The effects on the turbine performance of the blade setting angle, gap-to-chord ratio, solidity and arrangement of the biplane rotor have been examined. The results have been compared with those of the air turbine with self-pitch-controlled blades and the biplane Wells turbine from the viewpoints of the running characteristics and starting characteristics. As a result, a choice of design factors has been suggested. Next, the hysteretic characteristics of the turbine have been examined in order to establish a design method. The hysteresis is more sensitive to the solidity and arrangement of the biplane rotor and less sensitive to the setting angle and gap-to-chord ratio.

Three Dimensional Analysis Of A Marine Riser With Large Displacements

Abstract: A formulation to analyse the large displacement problem of a marine riser with two unequal principal moments of inertia of cross sections in three dimensional space is presented. The method involves the utilisation of the equilibrium equations in the axial direction and the stationary condition of an energy functional. The stationary condition of an energy functional. The co-ordinates X and Y, twisting angle F, and tension T are the four dependent variables, while the arc length s in the equilibrium state is the independent variable which is changed to depth Z in the numerical implementation. A finite element method was developed to solve the problem of a riser with equal principal cross sectional moments of inertia. An example is given.

Three-dimensional effects on hydrodynamic forces acting on an oscillating finite-length circular cylinder

Abstract: The hydrodynamic forces acting on finite-length circular cylinders in an oscillating flow was investigated experimentally Forced surging tests were carried out on finite length circular cylinders whose ratio of length to diameter varied from 1 to 10 The drag coefficients, the added mass coefficients and the lift coefficients were obtained, and were compared with those of 2-D circular cylinder and a finite-length circular cylinder with end plates We also studied the flow fields around the circular cylinders using the hydrogen bubble technique

Time variant wave drift damping and its effect on the response statistics of moored offshore structures

Abstract: A new method for investigating the wave drift response statistics of nonlinear compliant offshore structures is described. Compared to the typical spectral bandwidth of the slow-drift response of moored structures, the wave drift excitation forces may be considered as broad band. It is well known that the response of dynamic systems to broad band stochastic excitation in many cases is well approximated by modelling it in terms of a multi-dimensional Markov process. By exploiting this observation, it is shown how the theory of Markov diffusion processes can be applied to the case of wave drift response. The power of the method developed in this paper lies in its ability to provide accurate response estimates for almost any nonlinearity. At present, the most serious drawback is the (practical) limitation to single-degree-of-freedom (SDOF) system.

Force Coefficient Estimation From Random Wave Data

Abstract: A summary of alternative methods of estimating drag and inertia coefficients from random wave and wave force data is given Eight such methods are summarised, with six of these providing a single pair of force coefficients for each force record, and the remaining two providing variable coefficients The methods are compared by using them to provide force coefficients from numerically simulated wave and force records which derive from linear wave theory and the Morison equation with specified force coefficients These force coefficients are initially taken as constants, but in order to provide a more realistic representation of measured forces, simulations have also been obtained with variable force coefficients On the basis of the comparisons made, the method of least squares applied to the force time series is found to be the simplest and most accurate method by which constant force coefficients may be estimated

The Computation Of Mean Drift Forces And Wave Run-Up By Higher-Order Boundary Element Method

Abstract: A higher-order boundary element method (HOBEM) is used for the reliable computation of the firstand second-order wave loads on arbitrary three-dimensional bodies. The accuracy and efficiency of the present method are demonstrated through the convergence tests as well as comparison with analytic solutions and axisymmetric results. HOB EM is shown to be efficacious especially for the computation of body-surface velocities and velocity potential and its derivatives on the waterline and related integrals. For illustration second-order mean forces and wave run-up on vertical circular cylinders, hemispheres and the stationary ISSC tension leg platform (TLP) are presented.

Impact Damage Of Long Plastic Cylinders

Abstract: The objective of the paper is to assess the local damage of long tubular members and pipes caused by the impact of a rigid mass. The formulation is general and covers a wide range of events: low velocity-large mass impacts, as encountered in collisions; medium velocity impacts caused by dropped objects; and projectile and missile impacts. By making assumptions on the cross-sectional deformed shape of the cylinder, the two-dimensional shell problem was reduced to a one-dimensional problem of a plastic string resting on a rigid-plastic foundation. It was shown that the deformation propagates away from the point of disturbance with a constant plastic wave speed and diminishing amplitude. The instantaneous velocity and deflection profiles, the final deformed shape of the shell, and the maximum deflection attainable under impact were calculated. A parametric study was performed by changing the mass and velocity of the impacting object over several order of magnitude. An approximation to the dynamic solution was also obtained by using the static solution of the shell under "knife" loading and comparing the plastic work of the deformation process to the kinetic energy of the impacting mass. The approximation was compared to the dynamic solution and good agreement was shown for a range of masses and impact velocities encountered in offshore applications. Finally, use of the proposed methodology was illustrated by predicting the local damage caused by a frill-collar accidentally falling on one of the brace tubular members of an offshore platform.

Bottom Breakout Of Objects Resting On Soft Clay Sediments

Abstract: Bottom breakout force is the force that is required to free an object resting on a soft saturated clay sediment. This paper summarizes the results of a laboratory study relating to the bottom breakout force of a cylindrical object on three different soft clay sediments. Two important parameters for the estimation of the bottom breakout force are the plasticity of the soil and the in situ rest time of the object on the sediment.

The Shape-Effect Of Buffer On The Longitudinal Vibration Of A Pipe-String In The Deep Sea

Abstract: In order to analyze the longitudinal vibration of the pipe-string in the deep sea, first, the drag and added mass coefficients of various buffer-models vibrating axially in water were evaluated by the method developed by the authors. Then, the forced longitudinal vibration of the pipe-string equipped with a pump-module and a buffer was analyzed theoretically by introducing the fluid forces evaluated with the above-obtained coefficients. Furthermore, the axial stress induced in the pipe-string was calculated. The results indicate that the buffer whose shape causes a higher drag force is more useful for reducing the amplitude of the vibration and the axial stress in the pipe-string, and that the highest-drag buffer used in this study causes a half amplitude of the vibration and about 63% axial stress at the first resonance as compared with those produced by the lowest-drag model.

The influence of prebuckling deformations and stresses on the buckling of spherical shell

Abstract: From the Von Karman large deflection equation of the plate and by assuming that a plate has an initial deflection in form of a spherical cap, the equilibrium equation, of a spherical cap subjected to hydrostatic pressure is obtained, simplified and solved in the same way as an equilibrium equation of a beam on elastic foundation subjected to the axial and lateral loads. The influence of prebuckling deformations and stresses on the buckling of the spherical shell may be evaluated and the relation between buckling strength of the spherical shell and the column is obtained by analysing the buckling problem of the beam on elastic foundation. The presented formula for the calculation of the stability of the spherical shell gives a lower limit of buckling pressure and is in good agreement with test data recorded in literatures.

Dynamic elastic-plastic analysis of offshore framed structures by plastic node method considering strain-hardening effects

Abstract: The Plastic Node Method (PNM) considering combined strain-hardening effects is applied to the dynamic elastic-plastic analysis of offshore framed structures under earthquake loads. In the PNM, a strain-hardening rate for plastic hinges under general loadings. A tangential stiffness matrix of beam-column members can be obtained simply by matrix calculation. Several examples including the earthquake response of a three-dimensional jacket platform are presented, and the validity and effectiveness of the proposed method are demonstrated.

Elastic Static Analysis Of Chain Links In Tension And Bending

Abstract: The significant loading conditions applied to each link of a mooring chain during its operation are first identified. Then the resulting stresses and displacements within the elastic limit are calculated using both analytical and finite element methodologies. The calculations are performed for both studded and studless links. A comparison of the obtained analytical and numerical results shows good agreement. Finally, recommended values for the effective modulus of elasticity of a mooring chain are derived.

Nonlinear Behaviour Of Free Spanning Pipelines Exposed To Steady Currents:Model Tests And Numerical Simulations

Abstract: Flexible pipe model tests in steady current, carried out as part of submarine vortex shedding (SVS) project are briefly outlined as far as the effects on hydroelastic synchronisation induced by the pretensioning level in the models, the static sagged shape and the gap ratio are concerned. The results are presented and extensively discussed. The mathematical model set up to interpret the above results is described and the theoretical findings are used to explain possible mechanisms of hydroelastic synchronisation, coupled with the non-linear dynamic behaviour of the model. Applications on real free spanning pipelines are evidenced with examples.

Stochastic Linearization Method For Prediction of Extreme Response of Offshore Structures

Abstract: The standard stochastic linearization method applied to a nonlinear dynamic system is based on a mean square deviation measure to derive the equivalent linear parameters. Experience indicates that the resulting equivalent linear system may have approximately the same mean square response as the original nonlinear system. For predicting extreme response, however, this procedure is not equally suitable. In the case of strong nonlinearities in the dynamic model, application of the standard method of stochastic linearization may lead to significant overestimation of extreme responses. In the present paper are described some initial efforts to develop stochastic linearization methods applicable for prediction of extreme response levels. INTRODUCTION cantieri, Registro Italiano Navale (RINA) and Tecnomare. NONLINEAR EQUATION OF MOTION In this paper the following (normalized) equation of motion describing the response of a compliant offshore structure will be assumed: so that the damping and restoring force terms can be separated. For the sake of illustration and explicit derivations, it is assumed in this paper that the functions g(X) and heX) are given by the relations:

The Effect Of Unsteady Motion On The Drag Forces And Flow-Induced Vibrations Of A Long Vertical Tow Cable

Abstract: Drag coefficients and flow-induced vibrations of a long vertical tow cable are measured under steady and unsteady towing conditions. The steady-state drag coefficients range from 2.2 to 2.5. For unsteady towing conditions, the drag coefficient was lower by as much as 40%, depending on the frequency content ofthe planar ship motion. For purely oscillatory motion, the drag coefficient decreased as the frequency of motion increased. The reduction in the drag coefficient may be related to the amplitude modulation of the flow-induced vibrations of the cable, which are magnified during unsteady operations. When the surface ship changes speed, differences in the normal component of the velocity along the cable are present because of time delays in the response of the bottom of the cable to inputs at the top. The longer the cable, the greater are the delays. This creates large velocity gradients in the oncoming flow which are responsible for the intensification of the amplitude-modulation above the level that is observed during steady-state towing conditions. The overall effect of the amplitude modulation is a reduction in the hydrodynamic drag forces.

Surge drift motion of a moored vessel in random waves

Abstract: The results of experimental measurements of the surge drift motion of a soft-moored barge in random waves are compared to a numerical simulation employing a complete quadratic frequency response function for surge motion. Simulations are based on a Volterra theory and experimentally determined bi-frequency wave drift damping coefficients. Estimates of typical statistical parameters derived from a large ensemble of model tests and time domain simulations are found to be in good agreement, and illustrate the weakly non-Gaussian nature of the surge response to Gaussian wave excitation.

A strategy for assessing weld metal pop-in

Abstract: Interpretation of CTOD weld data is often complicated by the occurrence of pop-in. This paper argues that existing procedures for classifying the severity of pop-in are inadequate. An alternative strategy based on crack arrest toughness is suggested and illustrated by practical example.

Reliability Analysis Of Tension Leg Platforms By Domain Crossing Approach

Abstract: This paper presents a reliability analysis model for a tension leg platform (TLP) against severe storm events by the domain crossing approach of the random tensile stress in the tendons Two limit conditions are considered; ie, the exceedance of the ultimate tensile capacity and the occurrence of the negative tension In order to consider the correlation effects among the failure events of the tendons at four corners, the system limit state is defined in terms of the TLP motions in the vertical plate; ie, heave, roll, and pitch To investigate the validity of this methods, numerical analysis is carried out for two TLP's with different structural dimensions Numerical experiments were also performed to investigate the sensitivity of the reliability to several parameters, such as the cross-sectional area of the tendons, the mean tension in the tendons, and the uncertainties of the severe storm data