bluuebunny/crossref_metadata_2025_split huggingface dataset:
The Realities Of Radio Positioning
Authors: Moyer James P. 
Published: October 2008

ABSTRACT
The field of radio positioning has grown from the simple rotatable antenna and AM receiver of the direction finder to a wide assortment of complex electronic systems with a correspondingly broad variety of performance and operational characteristics. The intelligent use of any specific radio positioning system must, therefore, 'require a reasonable understanding not only of its performance capabilities under jdea1 conditions, but also the constraints that determine its true effectiveness in the environment of actual operation. The cost and quality (cost effectiveness) of the positioning end product in the geophysical industry is a direct function of the quality of the numerous interfaces which should exist between the ultimate user and the service organizations providing the geophysical and positioning data. A review of available generic radio positioning systems serves to indicate the need for intelligent system selection and operation. A first order analysis of the interfaces which can exist throughout a routine geophysical mission points out numerous opportunities to improve the end product cost effectiveness by adequate mission definitions, thoughtful mission planning, and operational communications and control. Based upon a general appreciation for the operational mission problems, some specific areas are identified where timely attention and communications are most likely to reap positive benefits for all those associated with the survey.
Finally, current trends indicate that radio positioning systems will provide increasingly satisfactory survey data over at least the near future.

1.0 INTRODUCTION
Survey control for the vast majority of marine geophysical data collection missions is currently provided by a class of navigation systems which can be generically defined as radio positioning. While the low frequency end of these systems (10.2 KHZ) overlaps into the acoustical range and the high end (laser) reaches the visual, all share the common characteristic of producing positioning information based on measurement of electromagnetic signal propagation above the earth's surface, and wholly within the earth's atmospheric envelope. With radio positioning systems operating across such a wide frequency spectrum (distributed into discrete bands by international and national priorities) It Is obvious that even ideal performance characteristics will be quite diverse. In fact, the only generalization of ideal performance ‘the higher the frequency, the greater the accuracy’ can be a tenuous notion in the presence of the operating environment.
Since radio positioning systems play such an important role in marine geophysical activities, a reasonable appreciation of their performance under operational conditions can lead to the production of an improved quality survey end product at reduced cost. Thus, the cost effectiveness of the overall geophysical mission can be significantly affected by the levels of understanding and attention afforded the positioning subsystem in the total mission system.
On the Mechanics of Pressure Ridge Formation in Sea Ice
Authors: Parmerter R.R. , Coon M.D. 
Published: October 2008

ABSTRACT
One of the most strild.ng features of the Arctic Ocean is the pressure ridges, which are linear accumulations of ice rising to several meters above the ocean surface and tens of meters below. These ridges present a formidable obstacle to both surface and submarine shipping and to transportation on the ice surface. In order to better understand these structures, a computer model of the ridge formation process has been developed that generate ridges whose are in good agreement with observed data. This model is described in a paper published in the Journal of Geophysical Research. Experience with the computer model has suggested additional simplifications that yield analytical models of certain features, such as force displacement relations, maximum height of ridges, and location of cracks in the ice sheet. These analytic models are the subject of the present paper. Significant dimensionless parameters are identified, and parametric studies are presented of the functional relationships between these parameters. The maximum height, crack location and required force are found as friction of the mechanical and geometrical properties of the ice sheet. The problem of forming a ridge from a refrozen lead is considered in detail. Initially, the ridge forms from the thin ice of the lead. Once the lead has closed, the ridge building process must either stop or continue by breading ice from the thick parent sheet. Typically, a force increase of two orders of magnitude is required to make the transition from building a ridge from refrozen lead ice to building it from the parent ice sheet. The large forces are toward the upper end of the range of forces available in the ice sheet, in agreement with the observation that most ridges are built from thin ice blocks. When forces are available and the parent ice is fractured, it is shown that ridges of limited height but unlimited lateral extent can occur. These correspond to the hummock fields that are occasionally observed. The proposed models agree with present observational knowledge and provide predictions that can be tested by future field experiment.

INTRODUCTION
From the time of man's earliest travels into the arctic, he has known of the existence of ice ridges. These ridges rise several meters above the ocean surface and may extend tens of meters below. Until recently, however, little has been known of the cross-sectional profile of ridges or of the mechanisms by which ridges are formed.
Knowledge of ridge size and distribution is of interest because ridges present formidable obstacles to both surface and submarine transportation systems. The forces required to form ridges are an important part of the rheological behavior of pack ice since the compressibility of the pack is determined, in part, by the formation of ridge structures.
Ice Model Testing
Authors: Biewer F.N. , Grinius V.G. , Hammett D.S. 
Published: October 2008

ABSTRACT
The characteristics required of an ice model in order for model testing to yield meaningful results are discussed. The various materials that have been used for ice simulation are examined for their applicability in the offshore areas of interest. The problems associated with obtaining scaled values of the mechanical and elastic properties are cited. The author list the for the synthetic ice model that was developed and a number of photographs of the ice testing are included.

INTRODUCTION
The opening of the ice regions of the globe to oil exploration and production has placed a requirement on the testing laboratories and facilities for model testing of equipment in an ice environment. In the past the interest in ice in the commercial field was restricted largely to icebreakers in opening routes through the ice fields for the passage of ships engaged in commerce. The epic journey of the Manhattan in 1969 seeking routes and methods for the economical, safe, and efficient transport of Arctic oil to the commercial markets represented full scale experimentation by the petroleum and shipping industries. Model testing was utilized extensively in the design of the Manhattan and several of the current ice model testing techniques were developed as a direct result of the requirement to test models of the prototype ship. Ice model testing, however, remains as a new and challenging task for the model test basins.

REQUIREMENTS FOR THE ICE MODEL
Rather than discussing the highly technical aspects of ice modeling and repeating the excellent work of others in describing ice, it is appropriate to concentrate rather on the problems associated with creating a proper ice model. For those desiring more detailed information on ice itself, a brief bibliography has been appended. Model work has as its objective the re-creation or simulation, at a ratio less than unity, those features, properties, or characteristics which are important in the investigation at hand. Certain scaling laws, when properly applied, will enable the testing laboratory to predict prototype performance with an accuracy which is satisfactory for prototype design.
The model ice which is used may be either real ice or it may be a synthetic material. The nature of the material itself is not a fundamental requirement but the properties of the materials are basic to the study. Using accepted testing techniques we are required to scale properties such as flexural strength, compressive strength, shear strength, and modulus of elasticity of real ice in order to arrive at a-model ice. If a model scale of 1:50 is used with Froude scaling, the real ice properties listed above must be multiplied by the model scale, 1. e., 1/50, to give the properties of the model ice.
Wave Induced Pressures on a Submerged Horizontal Plate
Authors: Durgin William W. , Shiau Jong C. 
Published: October 2008

ABSTRACT
An analytic method is presented whereby the differential pressure distribution on a submerged horizontal flat plate, 'induced by the passage of gravity waves, can be calculated. Plates having cord lengths of the same order of magnitude as the wave length are considered, making the analysis applicable to large offshore structures. The results of an experimental program, designed to test the validity of the theory, are presented and show good agreement.
In the analysis, the plate is imagined to be rigidly suspended at an arbitrary fixed depth and then replaced by a vortex sheet whose strength varies both temporarily and specially. The velocity potential of this sheet is adjusted such that, when combined with the potential of the oncoming waves, boundary conditions on the plate, the free surface, and the bottom are approximately satisfied. The final vortex strength distribution can be determined allowing the velocity field and hence the pressure distribution on each side of the plate to be calculated.
The experimental program was conducted using 4 × 4 × 40 ft. flume with a generator capable of producing waves having a broad frequency and amplitude spectrum in varying water depths. A flat plate of sandwich type construction was instrumented with small pressure transducers whose outputs were processed and recorded on an oscillographic recorder. Results are presented as instantaneous differential pressure distributions for associated wave positions, sequentially as the wave progresses. Analytic and experimental results are compared and show good agreement.
An advantage of the analytic approach is that pressure distributions as well as total forces may be readily obtained. This is important in designing a structure against maximum local differential pressure. The method can be extended to more complex shapes and finite length plates.

INTRODUCTION
The prediction of forces, or more precisely, the pressure distributions, experienced by submerged structures due to the passage of gravity waves has become important with the advent of large offshore structures. If the size of a structure is small compared to the wave length then the forces can be evaluated by the assignment of suitable drag and inertia coefficients using an approach similar to that of Morison (5) in which case it is necessary to determine the coefficients experimentally for any given geometry (2). If the size of the structure is a significant fraction of the wave length the above method is invalid and a more complicated analysis such as the diffraction theory of Garrison and Rao (1) is needed. Unfortunately, the application of diffraction theory for a given geometry is exceedingly difficult unless one assumes the structure size is small compared to the wave length, which may not be realistic.
Computerized stress Analysis of All - Metal Marine Loading Arms
Authors: Gallo D.J. , Forbes D.J. , Huebsch P.J. 
Published: October 2008

ABSTRACT
Over the years the means used to transfer petroleum cargo between ship and shore have evolved from all-hose assemblies to the present all-metal, double counterweighted marine arm.
With recent trends toward larger tankers, higher cargo transfer rates, and more demanding operating specifications, arms have become bigger (10-inch to 24-inch diameters and 100 feet long) and far more structurally complex. Stress analysis of these arms is more demanding because of the spatial and frequently indeterminate nature of the arm structure. Except for the simpler, early 1960 vintage arms, stress calculations are tedious, time consuming, and often, of necessity, imprecise.
Consequently, Esso Research has developed a computer program capable of rapid and exact arm analysis. In a single run, the program automatically applies a range of loading combinations and utilizes a stiffness matrix technique to calculate stresses in all members of the arm. The analysis can be made with the arm in a variety of attitudes including: stored maneuvering, operating, maintenance, washing, and hydrostatic testing.
The following areas are covered: the criticality and complexity of loading arm design, the three-dimensional stress analysis required, and the computer program developed to examine arm stresses.

INTRODUCTION
One of the most common means for transfer of petroleum cargoes at conventional tanker berths is the double-counterweighted, all-metal marine loading arm. It is a three-dimensional articulated structure consisting of a fluid carrying piping system stiffened with pinframed or rigidly connected structural members on one or more of its basic components. At least six swivel joints in the fluid-carrying piping allow the arm to be maneuvered from a stored position to and within an operating envelope.
The all-metal loading arm has evolved over the last 25 years from the simplistic manual and semi-automatic hose systems shown in Illustration 1. Some of the shortcomings of these early systems, which spurred development of the double-counterweighted arm, are:All-hose structureslimited loading ratedifficult handlingcontinual adjustment to maintain proper hose bend radiusHalf-hose/half-pipe armscontinual adjustment to maintain proper hose bend radiusAll-metal, freestanding armslarge reaction imposed on tanker's Manifold
For the most part, however, these systems presented few difficulties for the design engineer. While stress analysis of the prototype arm shown is complicated by its three-dimensional nature, the loads can be hand calculated throughout the arm by using the equations of statics.
Over the past seven years loading arms have steadily increased in size. Arm reach (total length of inboard and outboard tubes) has extended from 46 feet to 100 feet to keep pace with the growth of tankers from 50M dwt to 300M dwt. Another factor contributing to longer arm reach is the wider range of vessels loading arms are required to service.
A Mediterranean Sea Wave Spectral Model
Authors: Pore N. Arthur , Stevenson Norman M. , Cardone Vincent J. 
Published: October 2008

ABSTRACT
An operational Mediterranean wave spectral model has been developed through the cooperative efforts of the U. S. Naval Oceanographic Office (NAVOCEANO), Fleet Numerical Weather Central (FNWC) and New York University (NYU). Since April, 1972, the model has been operating in a real-time environment (analyses/prognoses to 48 hours). The model is a modified version of the original NYU North Atlantic wave model and has two main parts:wave energy growth based on a modified version of the Miles Phillips growth mechanism and dissipation at individual grid points andwave energy propagation from grid point to grid point. 'The Mediterranean wave model uses a conic conformal grid, permitting the assumption of equal spacing between grid points. There are 455 sea points with a mesh length of 67 km.
The wave model driving force is a modified version of the FNWC marine wind model that has a mesh length of approximately 370 km. For use in the Mediterranean, the winds are interpolatec between marine wind field grid points. At analysis/preanalysis times, wind reports from synoptic ship files are reanalyzed in the Mediterranean wind program so that local wind phenomena, such as mistrals, are included in the wind field.
The Mediterranean wind and wave model was evaluated with ship reports, one wave buoy and an airborne wave laser. The wave computations appear to be as good as the wind input allows.
The on-time analyses and 24-hour forecasts of wave heights are transmitted to Fleet Weather Central, Rota, Spain.

INTRODUCTION
A Mediterranean wave spectral model has been developed through the cooperative efforts of the U. S. Naval Oceanographic Office (NAVOCEANO), Fleet Numerical Weather Central (FNWC) and New York University (NYU). Since April, 1972, the model has been operating in a real-time environment (analyses/prognoses to 48 hours). The Mediterranean wave spectral model is a modified version of the original NYU North Atlantic wave model and has two main parts:wave energy growth and dissipation at individual grid points andwave energy propagation from grid point to grid point.

WAVE SPECTRAL MODEL
The growth of wave energy in this model is accomplished with an equation that combines modified versions of the Miles instability theory and. Phillips resonance mechanism. 2 The model evaluates all computed spectra to ensure that the calculations never exceed the limits of the Pierson-Moskowitz4 fully developed spectrum for the given wind speed. For computational purposes, the energy spectra are broken into matrices of 15 frequency bands by 12 directional bands as shown in Table 1. The energy in all the frequency bands except the highest frequency band are propagated through the grid system by a "jump" technique.
Consideration of Freak Waves for Design of a Jack-Up Structure
Authors: Schellen Thomas E. , Jahnke Thomas , Künzel Jochen 
Published: October 2008

Abstract
To ensure adequate safety of marine structures in extreme weather, it is conceivable that standards are needed that account for the characteristics of ultimate limit seastates based on wave conditions with a return period higher than 100 years. Such standards must include an analysis of the reserve strength available when a marine structure is subject to an extreme event of a freak wave. For the generation of freak waves traditional potential flow methods are not well suited to accurately predict wave loads, because phenomena such as wave run-up on the structure’s legs and impact-related loads on the hull are not accounted for. Therefore, wave effects were predicted with advanced computational fluid dynamics techniques. The purpose here was to determine the safety level under freak wave conditions. We selected a typical mobile self-elevating drilling unit stationed the North Sea and investigated its structural response under survival conditions and, in addition, under two extreme wave conditions representing freak waves. Based on a comparison of the resulting stresses with the structure’s rule based design capability, we assessed the reserve strength capacity still available under freak wave conditions.

Introduction
Marine structures, such as mobile offshore drilling units, are designed to operate at different locations throughout the world under varying environmental conditions. To assess their structural strength and design safety, two loading conditions are generally investigated, namely, the operating condition and the survival condition. For both loading conditions, the unit’s operating manual generally specifies limiting environmental parameters, such as maximum wave height, current velocity, and wind speed for the given water depth. It is assumed that these environmental parameters are known for the operating location of the rig and that weather forecasts are reliable enough to allow sufficient reaction time to prepare for worsening weather conditions.
Weather forecasts do not give notice of freak waves arising under extreme weather conditions; that is, such waves occur unexpectedly. The occurrence of a freak wave could be treated as an accident, similar to an earthquake. From this point of view, it would be rational to require the structure to remain standing and that critical structural components remain intact to prevent global failure. Under these circumstances, with all safety factors equal to 1.0, plastic deformation may be acceptable to avoid failure (ultimate strength).
Extreme Wave Effects on Deepwater Floating Structures
Authors: Buchner Bas , Bunnik Tim 
Published: October 2008

Abstract
Extreme waves can lead to damage to floating offshore structures as a result of airgap problems, greenwater on the deck or slamming to the hull. As the physics of these problems are different, there is no single way of identifying and characterizing extreme waves.
As part of the investigations into the effect of extreme waves on deepwater floating structures, this paper focuses on the challenges of the numerical prediction of platform response due to extreme waves. This will be done by using an improved Volume Of Fluid (iVOF) method. Two case studies are presented, which both required specific extensions of the methodology. First green water simulations on a FPSO are discussed, requiring the coupling of a linear diffraction code to the iVOF method as part of a domain decomposition. Second the dynamic response of a TLP to an extreme wave is studied, requiring the integrated analysis of the wave loading and platform response.

Introduction
Hurricanes Ivan, Katrina and Rita in the Gulf of Mexico showed the importance of extreme waves for all types of offshore structures [1-5]. Extreme waves can lead to damage to floating offshore structures as a result of airgap problems, greenwater on the deck or slamming to the hull. As the physics of these problems are different, there is no single way of identifying and characterizing extreme waves:
- For airgap problems, the crest ampltidue of the waves is the most important aspect. Therefore, it is important to know the ratio Ac/Hs between Crest Amplitude (Ac) and Significant wave height (Hs). In [6] the detailed analysis of an observed extreme wave in a model basin is described. It was shown that even in a wave with moderate steepness (Hs=11.9 m, Tp=15.3 s), generated with a random phase model, extreme wave crests can be observed. The measured wave had a Ac/Hs ratio of 1.59. As a pilot study into the understanding of the occurrence of this type of extreme waves, [6] describes the spatial development of this wave in the basin, see Figure 1.
Figure 1: Spatial development of a wave in a model basin (from [6]) (Available in full paper)
It should be noted ofcourse that even for fixed platforms the crest amplitude is not the only factor, the structure itself can clearly enhance the local wave elevation [7].
- For floating structures such as TLPs, Semis and Spars the wave loading and response is even more complex, as was shown in recent tests on the Snorre TLP [8]. The dynamic response of the platform to the wave impact can be the main factor in the survivability of the platform.
Figure 2: Airgap tests on the Snorre TLP (from [8]) (available in full paper)
In [8] the important observation was made that extreme wave and airgap problems belong to the group of (what was called) ‘badly behaved’ problems. A ‘badly behaved’ problem is a problem with a step or discontinuity in the response. For a fixed platform or TLP it can for instance occur that the deck is not hit in the 100 year wave as the maximum wave crest just passes below the deck.