A wavelet for predicting the time-domain response of vertically tethered systems

The response wavelets for tether tension and the displacement of a package on the end of a tether are derived from a linear, frequency-domain model of vertically tethered systems using inverse fourier transform techniques. These wavelets are convolved with a record of ship displacement to predict the instantaneous tension and displacement to an accuracy of better than 20% for a typical system. Although snap loads are non-linear and the response cannot be calculated while the tether is slack, the onset of zero tension — a precursor to snap loads — is predicted by the wavelet. Thus, extreme and potentially hazardous conditions are foreseeable with the wavelet and this information can be used in real-time to guide operations at sea.     

深海垂直吊放缆索系统固有频率和母船临界运动幅值快速计算方法

探索一种可描述缆索系统垂向吊放动力学行为的解析模型.给出它的自封闭方程、固有频率和简谐运动表达式,确定了母船和中际站之间的运动和张力传递函数,模拟结果能很好地符合实验估计值.导出沿着缆索的张力表达式,可用来预测缆索松驰和突变载荷的发生.     

深海遥控潜水器多体系统非线性耦合动力特性模拟

建立带缆遥控潜水器(TROV)系统空间运动模型,探讨支持船-吊缆-中际站-脐带缆-潜水器多体之间的强非线性耦合运动机理。潜器的运动考虑为六自由度,缆索分段的三维动态方程中采用了"凝集参数"模型与平均切向量非线性流体动力载荷处理技术,通过计算非均匀缆索的动张力和瞬态构型,预报导致脐带缆保护层及其内部光电传输芯线结构破坏的巨大瞬间突变载荷,对避免谐振,延长缆索寿命和最大限度地扩大ROV系统安全操作的范围,确保潜水器安全入坞和回收,节约试验费,避免作业事故都具有重要意义。     

A submarine-launched wave measuring buoy

A wave buoy, the Submarine Deployed Sea State Sensor (SUDSS), was developed to directly measure surface waves from a submerged moving submarine. The SUDSS is adapted from the Submarine Launched Expendable Bathythermograph (SSXBT) by replacing its temperature probe with a vertical sensing accelerometer. The SUDSS, launched from the aft signal ejector, utilizes the SSXBT cylinder, its lifting body, and filament wire spool-out mechanisms. Upon surfacing it oscillates vertically in phase with waves of frequencies below 0.5 Hz producing a voltage signal that is conducted via the filament wire back to the submarine and processed to produce a 10–12 minute wave record. On-board data analysis determines spectra, sea state, and the partial variance of spectral energy which between 0.2–0.5 Hz estimates the local wind speed.An accelerometer calibrator for wave frequencies was assembled consisting of a computer-controlled motor-driven swing arm which rotates an accelerometer as a simple harmonic oscillator; different rotation rates providing a variety of frequencies. Absolute accelerations are determined from the arm radius and its angular velocity. Wilcoxen accelerometers were chosen, providing a linear output of 1 v/g₀ (sensitivity) for simulated waves from 0.09–0.25 Hz (4–11 s periods).The SUDSS fitted with a buoyant tether signal cable, can be deployed from a surface ship for rapid wave/sea state measurements.     

“Batfish” a depth controllable towed body for collecting oceanographic data

“Batfish” is a streamlined vehicle developed to house fast-responding oceanographic sensors. It is towed behind a ship or small vessel and its depth is controlled from the vessel by a manually or automatically produced command signal. Variable-angle wings permit the vehicle to be lowered and a novel control surface, which eliminates the need for heavy ballast, assures lateral stability. There are two models: the standard and the wide-wing Batfish. The standard Batfish has collected temperature and conductivity data at depths of up to 200 m when towed at 10–25 km/hr, and the wide-wing Batfish at depths to 400 m when towed at 10–16km/hr.     

Analysis of hydrodynamic behaviors of gravity net cage in irregular waves

Based on the lumped-mass method and rigid-body kinematics theory, a mathematical model of a gravity cage system attacked by irregular waves is developed to simulate the hydrodynamic response of cage system, including the maximum tension of mooring lines and the motion of float collar. The normalized response amplitudes (response amplitude operators) are calculated for the cage motion response in heave and surge, and the mooring line tension response, in regular waves. In addition, a statistical approach is taken to determine the motion and tension transfer functions in irregular waves. In order to validate the numerical model of a gravity cage attacked by irregular waves, numerical predictions have been compared with the experimental observations in the time and frequency domain. The effect of wave incident angle on the float collar motion, mooring line tension and net volume reduction of the gravity cage system in irregular waves is also investigated. The results show that at high frequencies, the cage system has no significant heave motion. It tends to contour itself to longer waves. The variation amplitude of mooring line forces decreases as the wave frequency increases. With the increasing of wave incident angle, the horizontal displacement of the float collar increases.     

Tension during parametric excitation in submerged vertical taut tethers

The construction of a suspension bridge with floating pylons or a submerged floating tunnel requires the installation of a mooring system. The option of taut vertical tethers, similar to those used in tension-leg platforms, has been suggested in preliminary designs. The environmental loading on the tether, mainly due to wind waves and swell, results in a parametrically excited system. Certain loading conditions develop instabilities that translate into large horizontal motion. However, the effects of parametric resonance on the tension values have rarely been investigated. This paper aims to clarify the relation between lateral displacement and tether tension and to quantify the extreme tension values in the event of parametric resonance. The presented analysis is based on a full numerical model of the tether that includes geometric and hydrodynamic nonlinear effects. This model is used to investigate a representative example that illustrates parametric resonance and multiple parametric studies to assess the effects of the excitation frequency, amplitude, initial pretension, tether length and inclination angle on the tether’s response. The results reported here provide the basis for a recommendation on designing a tether under parametric resonance regarding the ultimate extreme values and fatigue life.     

Biology ofGonostoma gracile Günther (gonostomatidae)

Basing upon the total of 1,081 samples collected by a large plankton net (160 cm in mouth diameter) in the Pacific Ocean, the geographical and vertical distribution ofGonostoma gracile were studied. The species is distributed in the water masses of the Kuroshio, the Kuroshio Extension, the North Pacific Current, the Oyashio, the North Pacific Subarctic Water and the western North Pacific Central Water. The center of distribution lies in the Kuroshio area off Japan. Vertically, the species occurs between the depths of 200 and 1,000 m, mainly 300–700 m, both during daytime and at night. A part of population might come up to 0–200 m at night, although its biomass is negligibly small in comparison to that remaining in 300–700 m layer. Postlarvae are found in 300–700 m layer, mainly at 300–500 m, and apparently do not undertake diurnal vertical migration.     

New pop-up type Ocean Bottom Seismometer

We designed a new pop-up type Ocean Bottom Seismometer (OBS) in order to study micro-earthquakes in off-shore areas. With a 57 cm O.D. sphere of high tension aluminium alloy, the OBS system, including one vertical and one horizontal geophone, can safely operate on ocean floors of up to 6000 m depth for seismic observations. The amplified seismic data and the time code are directly recorded on the four-channel cassette deck for periods of up to one month. The frequency response curve throughout the recording and play-back system is flat for the range, 1–15 Hz (–3 dB). The anchor release and the geophone clamp are operated by an acoustic command signal.So far, we have deployed our OBS's 42 times in the ocean. All of the OBS's deployed have been recovered safely. Seismic data has provided seismological evidence for a number of processes associated with tectonism along subduction zones and spreading ridges (e.g., Eguchi et al., 1986).     

Dragged surge motion of tension leg platforms and strained elastic tethers

This paper presents an analytical solution for the dynamic behavior of both the platform and tethers in the tension leg platform system when the platform system is subjected to the wave-induced surge motion and the flow-induced drag motion. Along with the analysis the coupling problem of a two dimensional tension leg platform interacting with a monochromatic linear wave train in an inviscid and incompressible fluid is being considered. The scattering problem and radiation problem are first solved independently and then combined together to resolve for all unknowns. The dynamic behavior of the platform and tethers was further solved based on these solutions. The material property and the dimensional effect for the tether incorporated in the tension leg platform system are both taken into account in the analytical analysis. Corresponding to the variation of material properties and tether dimensions, it was found that the dynamic behavior of both the tether of tension leg platform and the platform itself is closely related to the material property and the dimension of the tether.     

Evidence that biological activity affects Ocean Bottom Seismograph recordings

Brief and impulsive signals of uncertain origin appear regularly on records from Ocean Bottom Seismographs (OBS) of several institutions. These signals have been recorded on nearly all deployments of the Texas OBS, including sites at depths greater than 7000 m. At some sites, they account for over 90% of the events recorded. They are of short duration (usually 0.5–4.0 s) and have a characteristic frequency (usually in the range of 4–18 Hz) that differs from site to site. When networks of OBS instruments are deployed, the signals are not recorded simultaneously by different instruments. Neither the frequency content nor the distribution of durations of these signals is similar to what is observed for known earthquake events.We present evidence suggesting that the signals are of biological origin, perhaps caused by animals touching the OBS units. (1) The distribution of these signals on instruments deployed at depths shallower than 1000 m shows a 24 h periodicity, while there is a 24 h periodic pattern on instruments deployed at sites deeper than 1000 m (where there is no visible light). (2) The frequency of occurrence of signals is similar to the vertical distribution of biomass in the oceans, i.e., they appear most frequently on OBS instruments deployed at very shallow depths. (3) Biological material has been found attached to several OBS units upon recovery.University of Texas Institute for Geophysics contribution number 468.     

Mediterranean deep-sea amphipods: composition, structure and affinities of the fauna

The recent revision of the Mediterranean amphipod fauna includes 415 species (i.e. 8% of the worldwide fauna). The number of endemic species is estimated to be about 190 (i.e. 46% of the total fauna).The deep-sea fauna, which includes all those species inhabiting depths >150m, comprises 154 species (37.1% of the total fauna), belonging to 89 genera and 25 families. 84 of these species inhabited depths down to 400m, 47 down to 1000m, and 31 to depths >2000m. Considering the upper limits to their bathymetric ranges, 49 species are restricted to depths >150m, 15 to >400m, 9 to >1000m and just 5 to depths >2000m. 62.9% of the gammaridean fauna is restricted to the continental shelf at depths shallower than 150m. Only 11.3% of this fauna inhabits depths >1000m. and only 2.2% is restricted to these deeper depths.The composition of this deep-living fauna has complex zoogeographical relationships. There are four main categories of species: eurybathic, Atlantico-Mediterranean bathyal, endemic bathyal and endemic abyssal species.The deep-living species include 71 (46%) which are endemic. There are also 71 species which zoogeographically are Atlantico-Mediterranean, 4 which are cosmopolitan, 5 Atlantico-Pacifico-Mediterranean, one Indo-Atlantico-Mediterranean and one Pacifico-Mediterranean. The 71 endemic species belonging to 52 genera. Three of these genera are endemic and five others were previously known only as Indo-Pacific.The presence of bathyal endemic species belonging to bathyal genera, and the affinity of some other species otherwise occurring only in the Indo-Pacific, confirms that a Tethyan component persists in the amphipod fauna, and indicates that during the Messinian Crisis the Mediterranean did not completely dry up. Thus a relict amphipod fauna was able to survive in some of the residual basins.     

Random dynamic response of a tethered buoyant platform production riser

A new normal mode spectral analysis method is presented for calculating r.m.s. riser deflections, bending stresses and lower ball joint angles. Forces on the riser consist of: (a) non-linear fluid drag taking account of the relative velocity due to tethered buoyant platform (TBP) motion, riser elastic deflection and wave induced fluid velocity, (b) wave induced fluid acceleration, (c) inertia forces due to TBP acceleration, and (d) buoyancy. The non-linear fluid drag forces are linearized using Tung and Wu's approximation based on the r.m.s. relative fluid velocity and current. A wide range of results is presented for risers in water depths up to 1000 m and it is observed that 6 normal modes are sufficient for calculating bending stresses. A static analysis is also presented for bending stresses due to wave and current induced drag forces and riser offset.     

A case study of a modified gravity type cage and mooring system using numerical and physical models

A modified gravity-type cage, developed by SADCO Shelf Ltd., was examined using numerical and physical models to determine if the cage and mooring system is suitable for an exposed site south of the Isles of Shoals, NH. The 3000-m/sup 3/ SADCO Shelf Submersible Fish Cage has angled stays between the upper framework and the ballasted bottom rim (in addition to net) to resist the horizontal shear deformation. The mooring system consists of three legs-each made up of a taut vertical chain and an angled rope, both leading to deadweight anchors. Normalized response amplitudes (response amplitude operators) were found for motion response in heave, surge and pitch, and load response in the anchor and bridle lines, in regular (single frequency) waves. In addition, a stochastic approach was taken to determine the motion and load transfer functions in random waves using a spectrum representative of seas at the selected site. In general, the system motion had a highly damped response, with no resonant peaks within the wave excitation range of 0.05 to 0.45 Hz. The anchor line force response was at all frequencies below 5 kN per meter of wave amplitude. The physical model tests showed consistently more conservative (larger) results compared to those for the numerical model.     

Seakeeping assessment of fishing vessels in conceptual design stage

The main idea of this paper is to identify functional relations between seakeeping characteristics and hull form parameters of Mediterranean fishing vessels. Multiple regression analysis is used for quantitative assessment through a computer software that is based on the SQL Server Database. The seakeeping attributes under investigation are the transfer functions of heave and pitch motions and of absolute vertical acceleration at stern, while the ship parameters influencing motion dynamics have been classified into two groups: displacement (Δ) and main dimensions (L, B, T), coefficients that define the details of the hull form (C_WP, C_VP, LCB, LCF, etc.).Four multiple regression models having different parameter combinations are here investigated and discussed, giving way to the so-called ‘Simple Model’, ‘Intermediate Model’, ‘Enhanced 1 Model’ and ‘Enhanced 2 Model’. The obtained results are more than satisfactory for seakeeping predictions during the conceptual design stage.     

深海采矿车布放回收中脐带缆轴向张力响应特性研究

在深海采矿车布放回收中,准确预报脐带缆在底部自由悬垂边界条件和顶部波浪载荷随机激励等共同影响下的动力响应是关键点之一。基于三维势流理论与集中质量法,结合水面支持船运动和海流联合激励,对深海布放回收中脐带缆的顶端张力等力学特性进行研究。结果表明：在布放回收过程中,轴向张力在结构载荷中占主要成分;由波浪引起的轴向张力极值与均值差距最大可达到54%;轴向张力的频谱存在双谱峰,分别为 0.14 Hz的主峰和0.10 Hz 的次峰;当浪向角为180°时,轴向张力处于峰谷。     

A shipborne wave-recording system with digital data processing

A shipborne wave-recording system which consists of a sonic wave gauge, accelerometers, gyroscopes and a computer system is described. Signals from the measuring apparatus are fed directly into a shipborne digital computer system at a prescribed sampling rate. The time series of wave heights and the acceleration are transformed into Fourier series using an algorithm of Fast Fourier Transform. Errors contained in the observed wave heights due to ship motion are corrected in the Fourier series by using the Fourier coefficients for the vertical acceleration. Power spectra and waveforms can also be calculated in a short time with this system from Fourier coefficients. Examples of the observational results obtained in the central part of the East China Sea in 1969 are presented.     

An investigation on the vertical motion sickness characteristics of a high-speed catamaran ferry

Low frequent motions of vessel may cause motion sickness in rough seas. These undesirable effects induce fatigue of crews during the navigation. The motion sickness is always an important criterion for the high-speed craft design. Modern ferry designs have been marketed with a great emphasis on the seakeeping performance. This research has been carried out by investigating the results on the vertical motion sickness incidence (MSI) study for a 40 m wave-piecing catamaran at seas. The primary purpose of this research is to investigate the vertical motion sickness characteristics of a high-speed catamaran ferry. Two mathematical models, three-dimensional translating–pulsating source distribution technique and three-dimensional pulsating source distribution technique, are used for predicting the vertical acceleration responses of the wave-piecing catamaran in oblique waves. The comparison between numerical predictions and experimental data shows a good agreement except that around the pitch resonance region in FP vertical acceleration motions. Based on the experimental observation, the discrepancies may be caused by the nonlinear effects of centre bow during large pitch motions in waves. The comfort assessments are based on the ISO-2631/1997 standard with the hydrodynamic analysis for determining the acceleration levels in different locations on the vessel. The effects of seating location, wave heading and duration of motion exposure on seasickness are discussed.     

豪华邮轮耐波性及救生艇砰击载荷特性研究

利用基于三维势流理论的Wasim软件,系统研究了在不同海况下大型豪华邮轮的耐波性能及作用在救生艇上的砰击载荷。首先计算豪华邮轮在规则波和不规则波中的运动响应,分析航速、浪向和海况对豪华邮轮运动响应的影响规律,然后计算救生艇在不同海况下砰击载荷的变化规律,根据变化规律评估救生艇在实际航行中的安全性。结果表明：豪华邮轮运动响应幅值随着航速和海况的增大整体呈增大趋势,规则波中横摇运动响应幅值在浪向90°时最大;当豪华邮轮处于4级和6级海况时救生艇不发生砰击;当豪华邮轮处于8级海况且航速大于10.29 m/s时救生艇发生砰击,为保证救生艇的安全,邮轮应避免在浪向120°和浪向150°下航行,此时建议邮轮以低于12.35 m/s的航速迎浪180°航行。     

A rational approach to intact ship stability assessment

In this study, asymptotic and total stability of the non-linear free and forced pure rolling motions of a ship are investigated. A ship performing a rolling motion is taken as a dynamical system. Lyapunov's direct method is employed in the analysis. By generating a time-invariant Lyapunov function, conditions and the domain of asymptotic stability are obtained for free rolling motion. Results of the work on “boundedness” and “uniform boundedness” of the solutions of the equation of forced rolling motion, done by Özkan (1977), that is, conditions of total (practical) stability and its domain in the phase-plane are given and illustrated.