Tensegrity Structures in the Design of Flexible Structures for Offshore Aquaculture

Aquaculture is the fastest growing food producing sector in the world. Considerable interest exists in developing open ocean aquaculture in response to a shortage of suitable, sheltered inshore locations. The harsh weather conditions experienced offshore lead to a focus on new structure concepts, remote monitoring and a higher degree of automation in order to keep the cost of structures and operations within an economically viable range. This paper proposes tensegrity structures in the design of flexible structures for offshore aquaculture. The finite element analysis program ABAQUS™ has been used to investigate stiffness properties and performance of tensegrity structures when subjected to various forced deformations and hydrodynamic load conditions. The suggested concept, the tensegrity beam, shows promising stiffness properties in tension, compression and bending, which are relevant for development of open ocean aquaculture construction for high energy environments. When designing a tensegrity beam, both pre-stress and spring stiffness should be considered to ensure the desired structural properties. A large strength to mass ratio and promising properties with respect to control of geometry, stiffness and vibration could make tensegrity an enabling technology for future developments.

The Effect of Bow Shape on the Springing/Whipping Response of a Large Ocean-Going Vessel: Investigated by an Experimental Method

Wave induced vibrations often referred to as springing and/or whipping increase the fatigue and extreme loading in ship hull girders. Both effects are disregarded in current ship rules. Various numerical codes exist for predicting the wave induced vibrations, but so far they are not considered reliable. Another means to investigate the importance of the high frequency response, although more resource demanding, is to carry out full scale measurements and/or model tests. Recently, full scale measurements of blunt ships have been carried out by DNV, and in this paper one of these ships was considered and tested in a towing tank to evaluate the additional fatigue damage due to the wave induced vibrations. Different excitation sources may excite the 2-node vertical vibration mode depending on ship design, and it is not straight forward to determine which is more important. The relative importance of the excitation mechanisms are investigated by two approaches in this paper. The first approach separates the whipping from springing to illustrate their relative importance based on basic theory in combination with model test results. The linear and second order transfer functions are utilized in this procedure. The second approach deals with the effect of the bow design on the additional fatigue damage. Three different bows were tested. The first bow design is identical to the real ship. The second bow design is a simplified version of the first one, by removing the bulb and flare. The third bow is fundamentally different from the two former blunt bows. Bow three is sharp pointed with a vertical sharp stem and vertical ship sides. The results indicate that the importance of whipping depends on the sea state, but that it is of similar importance as springing for the sea states that contributes most to the fatigue damage. Moreover, the difference in the additional fatigue damage due to wave induced vibrations for different bow shapes is moderate. This indicates that vessels with pointed bows and without pronounced bow flare, such as LNG vessels, may have a similar contribution from wave induced vibrations. Modern container vessels, which are more slender, but with pronounced bow flares should be further investigated.

Time Domain Hydroelastic Analysis of a Flexible Marine Structure Using State-Space Models

This article deals with time-domain hydroelastic analysis of a marine structure. The convolution terms in the mathematical model are replaced by their alternative state-space representations whose parameters are obtained by using the realization theory. The mathematical model is validated by comparison to experimental results of a very flexible barge. Two types of time-domain simulations are performed: dynamic response of the initially inert structure to incident regular waves and transient response of the structure after it is released from a displaced condition in still water. The accuracy and the efficiency of the simulations based on the state-space model representations are compared to those that integrate the convolutions.

Investigation of a Cup Anemometer in Oblique Flow for Measurement of Wind Velocity in Offshore Installations in the Field of Aquaculture

In Germany the combination of environmentally friendly changed wind energy with aquaculture is seen as an opportunity for multiple use of natural marine resources. Loads and stresses of “open ocean” offshore installations depend on wind velocity. For measurements of wind velocity within the field of wind energy conversion the only cost effective device is the cup anemometer. It has a linear relation between number of revolutions and velocity. Questionable is the measuring result if the wind vector is not within the ‘measuring plane’. Results of CFD calculations of the flow around an anemometer cup are given. With the force coefficients (lift and drag coefficients) an anemometer in inclined flow is calculated. It will be shown that the measured velocity within the measuring plane is higher than the cosine component of velocity.

Experimental Investigation of Hydrodynamic Coefficients of a Wave-Piercing Tumblehome Hull Form

A systematic series of calm-water forced roll model tests were performed over a range of forward speeds using an advanced tumblehome hull form (DTMB model #5613-1) to examine the mechanisms of roll damping. This experimental investigation is part of an ongoing effort to advance the capability to assess seakeeping, maneuvering, and dynamic stability characteristics of an advanced surface combatant. The experiment was performed to provide data for development and validation of a semi-empirical roll damping model for use in validation of ship motion and viscous flow simulation codes, as well as to provide a basis for future work with additional experiments, contributing to the development of an improved analytical roll damping model. Two hull configurations were tested: barehull with skeg, and bare hull with skeg and bilge keels. Measurements of forces and moments were obtained over a range of forward speeds, roll frequencies, and roll amplitudes. Stereo particle-image velocimetry (SPIV) measurments were also taken for both zero and forward speeds. Test data was used to calculate added mass/inertia and damping coefficients. Two different system modeling techniques were used. The first method modeled the system as an equivalent linearly-damped second-order harmonic oscillator with the time-varying total stiffness coefficient considered linear. The second technique used equivalent linear damping, including higher-order Fourier components, and a non-linear stiffness formulation. Results are shown, including plots of added inertia and damping coefficients as functions of roll frequency, roll amplitude, and forward speed and SPIV measurements. Trends from the experimental data are compared to results from traditional component roll damping formulations for conventional hull from geometries and differences are discussed.

Numerical Simulation of Engineering Wave for Zhongzui Bay

For providing a better shelter condition, it is necessary to build a breakwater in Zhongzui Bay. In order to know whether mooring area meets the requirement after engineering construction and compare the mooring area between solid breakwater and permeable breakwater, a numerical simulation method is used in the sheltering harbor of Zhongzui Bay. The used Mild-slope equation which describes wave refraction, diffraction and reflection, considers the steep slope bottom and effect of energy dissipation. It has been validated to fit for simulating wave transformation in the coastal zone. Under extreme high water level and design high water level, wave fields in the calculation area of three wave types in three different return periods are simulated by using this method respectively. In addition, wave height in front of breakwater can be provided. Then the wave parameters and the mooring area of two occasions, with and without breakwater, are gained in calculation area. Based on these results, some conclusions are presented in the end.

Case Studies on ECA-Based Flaw Acceptance Criteria for Pipe Girth Welds Using BS 7910:2005

The use of an engineering critical assessment (ECA) approach to derive flaw acceptance criteria for pipe girth welds has become common practice. It allows the maximum tolerable size of weld flaws to be determined on a fitness-for-purpose basis, offering substantial advantages over the conventional workmanship approach. BS 7910:2005 is widely used to derive ECA-based flaw acceptance criteria for pipe girth welds. It offers a flexible assessment framework within the context of the well-established failure assessment diagram (FAD) approach. However, it can be relatively complex to apply and it may lead to assessments that are more conservative than codified pipeline-specific procedures. This paper illustrates, through practical case studies on assessing the significance of circumferential girth weld flaws, some of the options available to the user of BS 7910. The case studies cover the selection of the FAD (generalised or material-specific, with and without yield discontinuity), tensile properties (specified minimum or actual values); fracture toughness properties (single point CTOD values including δ0.2BL and δm, or full CTOD resistance R-curve), and welding residual stress (assumed to be uniform through the pipe wall with a yield strength magnitude, or considered to have a through-wall distribution associated with a specific level of welding heat input).

Study of the Wave Drift Damping for a Very Large Crude Carrier

Pull-out test and decay tests in still water and in waves for the surge motion of a VLCC in ballast condition are carried out at LabOceano. The pull-out test associated with the mean drift displacement in regular waves is used to determine mean drift force. From the decay tests the damping coefficients are adjusted using the Froude energy method and the procedure based on the logarithm decrement. For the decay test in waves, the response is subdivided in the mean drift contribution, the regular wave response and the transient response. The wave drift damping is considered as an increase on the linear damping in still water. So, we introduce an additional damping to the linear part of the damping coefficient in still water and simulate the decay test in waves. Comparing the results from the simulation with the experiments the wave drift damping contribution is adjusted. Finally, the mean drift results are compared with the results obtained with the potential theory. The wave drift damping coefficients obtained from the experiments are compared with coefficients obtained with a formulation proposed in the literature.

How to Increase the Capacity of an Existing Sea Water Intake

Sea Water Intakes supplying water for desalination, cooling systems, or other parts of many important industrial components, such as refineries, and power plants are usually built in the shore lines and close to the target systems. In some conditions, such as expansion of the refinery, or other plants and in order to avoid building a new sea water intake because of lack of the suitable land or expending a lot of money, it is required to enhance the amount of withdrawing water and increase the capacity of the existing sea water intake. In such conditions, several important factors such as, minimum required submergence depth for the pumps, maximum allowed current velocity at the entry of the suction chambers, maximum allowed current velocity inside the intake and near the filters, and the flow pattern should be checked. In this paper, it is tried to describe these factors and restrictions. In addition, a case study sea water intake located in South Pars Gas Field at the northern shore line of the Persian Gulf in the province of Boushehr, Iran, is considered and the mentioned factors and restrictions for increasing the capacity of sea water intake from 25,000 to 35,000 is discussed. Besides, a hydraulic mathematical model has been used to check the flow line of the developed system. At the end the results are discussed.

Three-Dimensional Large Amplitude Body Motions in Waves

Three-dimensional, time-domain, wave-body interactions are studied in this paper for cases with and without forward speed. In the present approach, an exact body boundary condition and linearized free surface boundary conditions are used. By distributing desingularized sources above the calm water surface and using constant-strength panels on the exact submerged body surface, the boundary integral equations are solved numerically at each time step. Once the fluid velocities on the free surface are computed, the free surface elevation and potential are updated by integrating the free surface boundary conditions. After each time step, the body surface and free surface are regrided due to the instantaneous changing submerged body geometry. The desingularized method applied on the free surface produces non-singular kernels in the integral equations by moving the fundamental singularities a small distance outside of the fluid domain. Constant strength panels are used for bodies with any arbitrary shape. Extensive results are presented to validate the efficiency of the present method. These results include the added mass and damping computations for a hemisphere. The calm water wave resistance for a submerged spheroid and a Wigley hull are also presented. All the computations with forward speed are started from rest and proceed until a steady state is reached. Finally, the time-domain forced motion results for a modified Wigley hull with forward speed are shown and compared with the experiments for both linear computations and body-exact computations.