Generation of Wave-Field Time Histories by the Modulated Discrete Fourier Transformation

In the time domain simulation of the response of an offshore structure under random waves, the time histories of the wave field should be generated as the input to the dynamic equations. Herein the wave field is the wave surface elevation, the water particle velocities and accelerations at structural members. The generated time histories should be able to match the given wave-field spectral descriptions, to trace the structural member motions if it is a compliant offshore structure, and be numerically efficient. Most frequently used generation methods are the direct summation of a limited number of cosine functions, the Fast Fourier Transformation, and the digital filtering model. However, none of them can really satisfy all the above requirements. A novel technique, called the Modulated Discrete Fourier Transformation, has been developed. Under this method, the wave time histories at each time instant is a summation of a few time-varying complex functions. The simulated time histories have continuous spectral density functions, and the motions of the structural members are well included. This method seems to be superior to all the conventional methods in terms of the above mentioned three requirements.

Drag Force Reduction and VIV Suppression of Circular Cylinders Using Radial Water Jets

Deep-water oilfield developments demand accurate predictions of vortex induced vibrations (VIV) of risers and free span pipelines subjected to ocean current. In order to prolong operational life of such structures, VIV suppression devices such as helical strakes or shrouds are often employed. Such devices will, however, imply certain disadvantages such as drag amplification and increased operational costs. Therefore the quest for effective suppression devices with a minimum of such drawbacks is still ongoing. The present paper presents a novel approach for VIV suppression based on radial water jets from prescribed patterns of circular openings in the cylinder wall. Jet flow will introduce a disturbance that will change VIV amplitudes. The alternation of the flow pattern must be understood to have both 2-dimensional (2-D) and 3-D effects. 2-D effects will influence the local pressure on the cylinder surface by altering the separation point as well as creating a general disturbance to the flow, while the 3-D effects involve changes in correlation of the vortex shedding process along the span of the cylinder. Results will be presented from experiments in a towing tank testing four 2m long spring supported cylinders with diameter 0.1m and three different patterns of radial water jets. One cylinder has three straight rows of holes at angular positions 0° and ±120° with respect to the ambient flow. The second has two straight rows of holes at positions ±120° with respect to the ambient flow, while the last has one straight line of holes blowing directly upstream. Since the first can be said to consist of the two latter, comparing them to each other gives valuable information in order to understand the physics of the first. The volume flow rate and reduced velocity have been varied in the tests. Oscillation amplitudes, frequencies, added mass and drag force coefficients are presented and compared to a smooth cylinder.

Wave Action on Double-Cylinder Structure With Perforated Outer Wall

Based on an eigenfunction expansion of velocity potential and a linear model between the pressure difference between two sides of a perforated wall and the fluid velocity inside it, a semi-analytic linear solution has been acquired for wave interaction with a combined cylinder with an solid interior column surrounded by a coaxial exterior column with perforated wall at a section in azimuthal direction. Numerical experiments have been carried out to examine the influences on the wave force and wave run-up on the combined cylinders with perforated wall by the porous coefficient, the size of the perforated section, and the ratio between the radii of the interior and the exterior columns. This paper also presents the comparison between the numerical experiments results and the physical experiments results. It is acceptable of the comparison of these two results. The combined cylinder may reduce both the wave run-up and the wave loads on it through combination of certain parameters.

Modelling of Seabed Interaction in Frequency Domain Analysis of Mooring Cables

Mooring cables under wave loading interact dynamically with the seabed; this interaction is nonlinear and can be modelled in full only by performing lengthy time integration of the equations of motion. However, time domain integration is far too computationally expensive to be carried out for all load cases. A new method of modelling the interaction between a cable and the seabed in the frequency domain, but without considering frictional effects and impact, is therefore proposed. The section of cable interacting with the seabed is truncated and replaced with a system of coupled linear springs, with stiffnesses linearised from static catenary equations. These springs would model the behaviour of the truncated cable and hence the time-varying boundary condition at the touchdown. The entire cable-spring system is then analysed in the frequency domain with a centred finite difference scheme. The proposed method has shown to increase the accuracy of frequency domain analysis in certain cases with affordable computational overhead.

The Ocean Thermal Gradient Hydraulic Power Plant and Its Scope

Heretofore, the concept of developing power from the tropical oceans, (Ocean Thermal Energy Conversion, or OTEC) has assumed the mooring of large platforms holding the plants in deep water to secure the coldest possible condensing water. As the Ocean Thermal Gradient Hydraulic Power Plant (OTGHPP) does not depend, on the expansion of a working fluid, other than forming a foam of steam bubbles. It does not need extremely cold water as would be dictated by Carnot’s concept of efficiency and the 2nd Law of Thermodynamics. Plants may be based on or near-shore on selected tropical islands, where cool but not extremely cold water may be available at moderate depths. This paper discusses the above possibilities and two possible plant locations, as well as projected power outputs. The location and utilization of large of amounts of power on isolated islands, where cabling of power to major population centers would not be feasible are discussed. Two that come to mind are the reduction of bauxite to produce aluminum and the of current interest is the electrolyzing of water to produce gaseous hydrogen fuel to be used in fuel cells, with oxygen as a by-product.

Hydroelastic Modeling of a Compliant Offshore Tower: Formulation

Offshore oil and gas can be produced using a variety of platform types. One option, the compliant offshore tower, has proven to be an economic solution in moderately deep water (300–600m). In this paper, the wave-induced global dynamic responses of a compliant tower in wind, current and waves are studied in the context of fluid-structure interaction. A beam undergoing transverse and axial motion models the vertical member of the tower. The beam is supported by a linear-elastic torsional spring at the bottom end and a point mass and a buoyant chamber is located at the top free end. The fluid forces on the beam are modeled using the Morison equation while the hydrodynamic forces on the chamber are obtained based on the three-dimensional diffraction-radiation theory. By applying Hamilton’s variation principle, the equations of motion are derived for the coupled fluid-structure interaction system. The non-linear coupled system equations that emanate from this new approach can then be solved numerically in the time domain.

Numerical Simulation of Vortex Shedding From a Circular Cylinder in the Presence of a Free Surface

CFD simulations of crossflows around a 2-D circular cylinder and the resulting vortex shedding from the cylinder are conducted in the present study. The capability of the CFD solver for vortex shedding simulation from a circular cylinder is validated in terms of the induced drag and lifting forces and associated Strouhal numbers computations. The validations are done for uniform horizontal fluid flows at various Reynolds numbers in the range 103 to 5×105. Crossflows around the circular cylinder beneath a free surface are also simulated in order to investigate the characteristics of the interaction between vortex shedding and a free surface at Reynolds number 5×105. The influence of the presence of the free surface on the vortex shedding due to the cylinder is discussed.

The Diffraction of Multidirectional Random Waves by Trapezoidal Submarine Pits

A numerical model is presented to predict the interaction of multidirectional random surface waves with one or more trapezoidal submarine pits. In the present formulation, each pit may have a different side slope, while the four side slopes at the interior edge of any given pit are assumed equal. The water depth in the fluid region exterior to the pits is taken to be uniform, and the solution method for a random wave system involves the superposition of linear-wave diffraction solutions based on a two-dimensional boundary integral equation approach. The incident wave conditions are specified using a discrete form of the Mitsuyasu directional spectrum. The results of the present numerical model have been compared with those of previous theoretical studies for regular and random wave diffraction by single or multiple rectangular pits. Reasonable agreement was obtained in all cases. Based on these comparisons it is concluded that the present numerical model is an accurate and efficient tool to predict the wave field around multiple submarine pits of trapezoidal section in many practical situations.

Progress of COSMOS (CO2 Sending Method for Ocean Storage) and OACE (Ocean Abyssal Carbon Experiment)

The storage of liquid CO2 at an ocean floor, one of promising measures to mitigate the global warming, requires 3500 m depth for the gravitationally stable storage, a breakthrough technology and a reasonable cost to realize, although it has large advantages such as the sequestration term longer than 2000 years. However CO2 can be sent to the ocean floor by shallow release, if we can use the nature that the cold CO2 to be shipped by a CO2 carrier is much denser than the ambient seawater even at shallow depths. The National Maritime Research Institute (NMRI) conducted several joint field CO2 release experiments with the Monterey Bay Aquarium Research Institute (MBARI, USA) since 1999 under the auspices of the NEDO, and proposed the improved COSMOS, CO2 Sending Method for Ocean Storage, in which CO2 is released into 200 m depth as slurry masses (mixture of dry ice and cold liquid CO2). Since 2002, under the NEDO Grant, the NMRI started a new international joint research, OACE, Ocean Abyssal Carbon Experiment with the MBARI and the University of Bergen (UoB, Norway), in order to accumulate the basic data on the long-term stability of stored CO2 and its environmental effects around storage site.

Dynamic Compression of Rigid and Flexible Risers: Part I — Experimental Results

Dynamic compression is a critical issue for the viability of submerged lines used in offshore applications, especially for deepwater operations. The subject has been addressed both numerically and analytically. However, few experimental data exist in literature for validation purposes. The aim of this first paper is to present experimental results on the dynamic compression of rigid and flexible risers, obtained in towing-tank tests. Two small-scale models have been built, one emulating the dynamic behavior of a steel catenary riser (SCR) and the other corresponding to a much more flexible case. Uniform circular motion has been applied to the top of the line, representing the floating system oscillation. Four different amplitudes have been considered, each one of them with five different frequencies. The influence of current velocity has also been evaluated. Tension has been measured at the top. In this work the small-scale models and experimental setup are described and some comprehensive results are presented and discussed. In a companion paper, comparisons between theoretical (numerical and analytical) and experimental results are presented.