bluuebunny/crossref_metadata_2025_split huggingface dataset:
The Behavior Of Steel Offshore Structures Under Accidental Collisions
Authors: De Oliveria Joao 
Published: October 2008

Abstract
In order to design steel offshore structures against impact loads due to accidental collisions caused by ships, it is necessary to evaluate the plastic energy absorption capability of its members. In this paper the local energy absorbed at the point of impact and in its immediate vicinity is first considered. Simple estimates based on upper bound solutions are discussed. Then the overall energy absorption capability of a tubular member is studied when it deforms in bending. The varying degree of axial and rotational restraint provided by the end supports is included in the analysis. A simple method for estimating the extent of damage resulting from a collision caused by a typical supply vessel is then suggested, and a specific example of application is discussed.

Introduction
The collision between ships and platforms or other installations in offshore oil and gas fields is a type of accident which can have catastrophic effects, and as such is a cause of growing concern. Several studies have dealt with the problem of collisions offshore from the point of view of probability of occurence and degree of severity. For example in [1], major failures recorded in the Gulf of Mexico between 19L,8 and 1977 were analysed, indicating that 797. were related to drilling and production, 7% to storms and 5% to collisions. On the basis of oil spilled, only 1% of volume spilled resulted from collisions. Thus it appears that collisions account for a relatively small percentage of all major failures offshore. However, other studies indicate that the probability of occurrence of such incidents is by no means irrelevant. In [2], for example, values for the probability of occurrence of a collision involving different types of vessels and an offshore platform are given. In the case of supply ships the probability is considerable (p &gt; 10−2), for oil tankers involved in offshore loading the probability is medium (P &gt; 10−3), while for passing ships not involved in offshore operations the probability is very low (P &gt; 10−7). Some studies sponsored by the UK Department of Energy [3] also address the question of the risk of shiRip1atformco11ision, and the general conclusion that can be taken from these various sources is that such risks cannot be ignored. In fact, not only surface ships should be considered as potential causes of accidental impacts, since the possibility of collisions involving submersibles is certainly also real, as discussed in [4].
Several studies have addressed the question of how to minimize the risks and extent of damage associated with collisions offshore, such as methods for improving navigation [5] and the development of special fendering systems [6]. However, practically no effort has been devoted to improving or adapting the structural design of platforms to this type of accidental load.
Observed Settlements Of Five North Sea Gravity Platforms
Authors: Lunne Tom , Myrvoll Frank , Kjekstad Oddvar 
Published: October 2008

Abstract
The observed settlements of five North Sea gravity platforms installed in the period 1975-1978 are presented. They are all founded on overconsolidated clays and dense sand. For each platform the systems for measuring settlements are described and some availablemethods for computing'immediate and long term settlements are reviewed. The foundation base witdth varies between 89 mand 140 m, and the net foundation pressures imposed on the sea floor varies between 200 and 300 kN/m2
The major part of the long term settlements occurs in the upper part of the foundation soils, within a depth of 30-40% of the foundation. width. One of the reasons for this being the presence of thick dense sand layers below 40-50 m depth.
Observed consolidation settlements range between 50 and 230 mm and are completed within 9 to 40 months. This is considerably faster than predictions based on soil properties determined from laboratory samples. Secondary settlements vary between 3 and 13 mm per year.
The observed data have been used to determine empirical relationships between strength and compressibility properties for the soil.

Introduction
The first gravity structure in the North Sea was installed in a water depth of 70 mat the Ekofisk field in July 1973 In the period 1975 to 1978 another 12 gravity structures were installed (Fig. 1) in water depth between 94 and 153 m (Eide et al2, Schjetne et al.3
Settlements have been measured for most of the 13 platforms with varying degree of success. Feedback from these observations helps the designer to modify the methods that have been used for predicting settlements. Thi s paper di scusses the settl ement records from 5 pl atforms on overconsolidated clays where the long term settlements have been measured relative to a deep fixed point. All the measuring systems have been designed and installed by the Norwegian Geotechnical Institute. The five platforms are (Fig. 1).
Beryl A - operated by Mobil North Sea Ltd., U.K. Brent B and Brent D - operated by Shell U.K. Statfjord A - operated by t10bil Exploration, Norway Ninian Central - operated by Chevron Petroleum, U.K.

Description Of Platforms
Four of the five platforms considered in this paper are of the Condeep type designed by H0yer Ellefsen and built by Norwegian Contractors. They have 19 interconnected cells of which 3 are extended upwards as towers that support the deck structure (Fig. 2). The first platforms installed, Beryl A, Brent B, and Brent Dhave very similar geometries. Statfjord A has a somewhat larger base area formed by series of smaller circular skirts underneath a cantilevered base slab.
Model Studies of Long Piles in Clay
Authors: Aurora Ravi P. , Peterson Edward H. , O&apos;Neill Michael W. 
Published: October 2008

Abstract
Model studies were conducted to study the fundamental behavior of long pipe piles driven into two stiff, canpacted clay soils. The purpose of this paper is to report on the phenanena observed and not to provide guidelines for design. The specific observations reported are:The effects of pile penetration and diameter on the magnitude of displacement required to mobilize maximum skin resistance:'Ihe time required for soil "setup";Skin friction development and proportioning at increasing penetrations:The influence of tension loading on the development of skin friction:Verification of a soil-to-soil failure; andEvaluation of driveability studies using model piles.

Model Data
This investigation was aimed at modeling the behavior of piles driven to varying penetrations in clay. A modeling factor, denoted as relative stiffness, was used to gauge the simulitude of the test pile to typical slender piles driven into clays. Relative stiffness simulitude was maintained to insure similar tendencies for deveopnent of load distribution and rapid progressive failure between model and prototype.
The relative stiffness is defined as the ratio of the pile flexibility (axial foreshortening under a generic load) to the vertical elastic displacement of the soil at the pile tip produced by skin friction. Assuming uniform skin friction and zero tip load, the pile flexibility, pp is given by;(Mathematical equation available in fullpaper)
Although it was possible to simulate relativestiffness, it was not possible to simulate the state of effective stress along the pile due to the restricted size of the model and lack of an apparatus to apply confining pressures.

Test Soils
Two test soils were used that represented reasonable values of liquid limit and plasticity index found in recent deposits in the Gulf of Mexico. Each soil was compacted in a test chamber at near standard Proctor effort at a water content several percent wet of optimum in order to effect as high a degree of saturation as possible. The soils were compacted in thin layers to achieve a uniform undrained shear strength. The first test was conducted in a kaolin soil, having a liquid limit of 53% and a plasticity index of 23%. The material classified as MH, according to the Unified Soil Classification System. The second test was conducted in a soil made of a mixture of 90% kaolin and 10% montrrorillonite, which had a liquid limit of 67% and a plasticity index of 37%. This soil is classified as CH in the Unified Soil Classification System. Approximately 90% saturation was achieved in both soils
Stepwise Cracking of Line Pipe Steels in Simulated Oilfield Environments
Authors: Steinberg Beverlee Gail , Kane Russell D. 
Published: October 2008

Abstract
Pipelines used in the gathering and transport of natural gas, natural gas 1iquids and other petroleum products may contain significant amounts of hydrogen suffice (H2S) and carbon dioxide (C02). When associated with brine these environments can cause internal corrosion and stepwise cracking of pipe line steels. Such phenomena are of increasing importance in the design and protection of offshore gathering lines as levels of H2S increase in these areas.
Stepwise cracking is observed in lower strength (less than or equal to 80 ksi yield strength) line pipe steels which are used in oilfield service. Cracking occurs internally through the accumulation of molecular hydrogen along internal surfaces such as nonmetallic inclusions or low temperature transformation structures. As hydrogen becomes trapped at internal surfaces, b1isters fond, usually along parallel planes, which begin to link up via small cracks. These cracks lower the effective wall thickness of the pipe and may eventually become through wall failures.
Research over the years has shown how various metallographic variables can be controlled to increase resistance to stepwise cracking. This study examines the effects of inhibitors on the stepwise cracking characteristics of API line pipe steel. The use of inhibitors was found to be available method of controlling the onset and propagation of stepwise cracking.

Introduction
Pipelines used in the gathering and transport of natural gas, natural gas 1iquids and other petroleum products may contain significant amounts of hydrogensulfide (H2S) and carbon dioxide (C02). In conjunction with aqueous water, these constituents cause internal corrosion of steel pipelines. Also, pipe line material s exposed to aqueous H2S containing environments may be susceptible to failure by either sulfide stress cracking (SSC) or stepwise (blister) cracking. A study was undertaken to provide an assessment of the state-of-the-art of preventing or minimizing the deleterious effects of these phenomena in the transmission of sour (H2S containing) gas. This investigation was undertaken to provide information regarding:the manufacturing of pipe line materials,operating procedures andfield experience.

Background

Mechanisms of Pipeline Corrosion Damage
When acid gases are present, three distinct forms of corrosion damage in pipeline may occur:Weight-loss CorrosionSulfide Stress Cracking (SSC)Stepwise (Blister) Cracking

Weight-Loss Corrosion.
Produced oil and gas frequently contain H2S and/or CO2 When dissolved in aqueous systems (water or brine), these gases fond acids of fairly low pH which attack and corrode materials commonly used for transmission lines. This attack may be localized, in the fond of pitting or crevice corrosion, or it may be general attack dependent upon the environmental conditions that exist.
The Ultrasonic Torch - A Novel Device for Monitoring Steel Structures Underwater
Authors: Dalgliesh E.H. , Casswell P.H. , Poole M.J. 
Published: October 2008

ABSTRACT
An Ultrasonic Non Destructive Testing device for the detection of cracks in the vicinity of node joint elements in offshore steel platforms is described.
Major design considerations were:The system should provide rapid screening of the welds for the presence of cracks.The work load on the diver should be as low as possible.A permanent record of the inspection should be available.
The resulting equipment known as the Ultrasonic Torch, fulfils these requirements and transfers the onus of data interpretation to a qualified inspector who is stationed at an above surface console.

INTRODUCTION
Offshore oil production is common place nowadays and in the U.K., of course, the main source of the North Sea means that structural safety of the platforms must be taken very seriously, and the integrity of the structure must be confirmed by regulars inspection. In the North Sea there are now well in excess of one hundred steel platforms, built, in general, from large steel tubes with the diameter matched to their function. The larger tubes are the supporting legs, with the size of cross bracing matched to the calculated stress. The various members are welded together at nodes, as typical structure having several hundred nodes, and at each node will be many meters of well-meant. The action of welding by melting the metal causes a physical change in the steel close to the weld in the region known as the 'Heat Affected Zone'. By definition the nodes are the joining points of the member, where the physical shape of the structure passes through discontinuities and where there will be stress concentrations. The combination of the affected metal and high stress means that the regions immediately adjacent to welds are the most susceptible to cracking. For this reason there is a requirement that at least an agreed percentage of the welds are inspected annually.
Gross visual inspection can be done with submersibles using TV camera, and general monitoring can be done with systems based on acoustic radiation from cracks, or vibration mode analysis. However the greater part of inspection work is carried out by divers, much of the task being visual inspection, with the diver reporting on the general state of the welds, the amount of undercutting, spatter and so on. Obviously the most important feature which could be revealed by the inspection is a crack, and should a crack be suspected, then other more specialized systems, such as MPI, will be used to provide information on the precise location, extent and type of crack.
Unfortunately a tightly closed crack may be invisible, and some method of initial detection is necessary. Whatever device is used it is going to be operated in what at best can be described as a hostile environment and the underwater tasks involved in the inspection work should be kept as simple as possible.
An Analytical Study of Stress Concentration Effects in Multibrace Joints Under Combined Loading
Authors: Gulati Kailash C. , Wang Wen J. , Kan David K.Y. 
Published: October 2008

ABSTRACT
This paper summarizes the results of a finite element study of stress concentration effects in multi-branch tubular joints subjected to combined in-plane loading. The joint configurations analyzed include T, K, non-overlapping and overlapping TK and overlapping TKT. Simple joints of the T and K type subjected to axial load and/or in-plane bending were studied first. The interaction of various branches of a multi branch joint were then investigated by introducing additional members in the neighborhood of simple joints. The study also compared the computed hot spot stresses for a T joint under combined loading with those estimated by the use of parametric equations and various superposition schemes.

INTRODUCTION
Fatigue of structural elements is a complex process involving a large number of influencing parameters. In addition to the magnitude and nature of cyclic loading, the parameters that influence fatigue life include the stress concentration effects, residual and mean stress levels, the metallurgical structure of the material and the corrosiveness of the environment. The common fatigue design practice utilizes a semi-empirical approach that expresses the fatigue life of structural elements as a function of the analytically or experimentally determined stress ranges and accounts for the effects of the other parameters through empirical relationship denoted by S-N curves.

Current Design Practice
The current offshore design practice concerning the fatigue life evaluation of tubular structures is based on the proven engineering approach depicted above. These applications are, however, particularly severe, both from the viewpoints of complexity and diversity of structural configurations and the complexity of loadings. A typical joint from the fatigue prone areas of a jacket structure is illustrated in Figure 1. The loads acting on the member ends of the multi planar joint were obtained from a global analysis of the jacket. A typical North Sea wave was used for this purpose. The nature of member end loads under changing magnitude and direction of the impinging wave is expected to be similar to that shown in Figure 1. Fatigue life evaluation of this joint requires an estimation of the hot spot stress history under combined loading and the provision of an applicable S-N curve.
The common tubular joint fatigue evaluation procedure utilizes parametric equations 1, 2, 3 to estimate the hot spot stresses at selected locations (crown and saddle points) of the tubular joint interface. These parametric equations are based either on a finite element analysis utilizing thin shell theory or on laboratory testing of simple joints subjected to simple loadings. The parametric formulation of stress concentration effects, to date, is limited to X, T, K and TK joints subjected to simple loadings of the axial, in-plane bending and out-of-plane bending types, applied separately to the joints.
A Powerful Marine Seismic Source Seismic Tuned Array of Steam Jets
Authors: Manin Michel 
Published: October 2008

ABSTRACT
The sudden ejection of steam into the water generates an acoustic wave which corresponds to the expansion peak of the signature. As the bubble expands its internal pressure decreases and the equilibrium position is overshot due to kinetic energy. Under the hydrostatic pressure, the bubble pressure falls and practically reaches the saturation pressure of the cold water. From this time, corresponding to the minimum pressure of the signature, the bubble starts its implosion according to Lord Raleigh's law. The implosion ends in a shock of the water against itself. The well-known qualities of a punctual steam seismic source are an exceptionally sharp impulse signature and no bubble effect. The basic system consists of 1 to 4 identical subarrays towed behind the seismic vessel. Each subarray comprises 4 guns emitting 4 different bubble periods and synchronized on the implosion peak (fig. I). Firing these 4 guns together results in a powerful and well shaped signal (fig. 2) with good spectral characteristics (fig. 3). Thus tuned, Starjet preserves the advantages of a punctual source and is characterized by an excellent repetitivity of the emitted signal, a clean pulse (no tail), good ratio between implosion and first peak, a 15 bar-meter peak-to-peak amplitude for each subarray.
This seismic source has to be used when penetration and definition are required in seismic surveys.
After 5 years of research and development Starjet's industrial use began in June 1982 with a series of tests conducted in the North-Sea and the Mediterranean. Seismic results are discussed in relation with the pulse characteristics and wavelet processing.

INTRODUCTION
Implosive sources are becoming ever more widely used because of their superiority in high frequencies in comparison with conventional airguns. Nevertheless there are three drawbacks with the general use of such sources.firstly, they generate a double impulse. The first when the bubble is formed under waterdischarge of the steam (explosion) in the case of steam sources; discharge of water in the case of water sources, with electric current used to cause the sudden generation of steam in the case of sparkers. The second impulse is of course the implosion itself. This double impulse in the time domain expresses itself by major oscillations of the frequency spectrum due to phase differences between the components of each of the two impulses. The resulting signal is generally difficult to deconvolve.secondly, the spectrum toward the low frequencies is limited by the maximum bubble period these sources can produce, since the minimum frequency emitted with a sufficient level is in fact the inverse of the maximum bubble period.thirdly, synchronizing a subarray of implosive guns is difficult in rough seas due to the fact that the time implosions occur depends on the local hydrostatic pressure at the instant of the shot according to the well-known Rayleigh's formula. The Starjet system presented here has been designed to overcome these three drawbacks.
High-Sensitivity Magnetic Profiling
Authors: Unterberger R.R. 
Published: October 2008

INTRODUCTION
A high sensitivity rubidium 87 magnetometer, designed and built by the author, is used at sea to make magnetic profiles over subsurface structures of interest. The Texas A&M University Research Vessel GYRE was used to launch, tow and recover a nonmagnetic fiberglass skiff that carried the magnetometer. To avoid magnetic field disturbances of the GYRE, the skiff with the magnetometer was towed 600 ft behind the ship. Loran C, and sometimes SATNAV, position data were used to determine the ship location. Two recording depth finders using 3.5 kHz and 12 kHz respectively were used to profile the bottom. Time marks were plotted on the magnetic and sonar data in accordance with WWV time signals received on 10 MHz. (15 MHz and 5 MHz were also available if there happened to be poor radio transmission at 10 MHz). Magnetic data were recorded in digital form on a strip chart recorder, using the last two digits of the six digit resonance frequency of the Rb 87 atoms.

THEORY OF MAGNETOMETER
The magnetometer is an optically-pumped rubidium 87 magnetometer. The system was patterne1 after a rubidium clock built by Dr. Peter Bender of the U.S. National of Standards. It uses rubidium 87 rather the rubidium 85 because the spin of Rb 87 is 3/2 rather than 5/2 for the more plentiful isotope Rb 85. Thus by obtaining Rb 87Cl from Oak Ridge National laboratory and reacting it with solid Ca in a vacuum, we were able to obtain 500 ml pyrex spheres with some solid Rb 87 in them.(Equation Available In Full Paper)
Approximately 1 mm (of mercury) pressure of neon gas is also introduced into the sphere to reduce collision broadening of the spectral line caused by rubidium atom spin-spin transfer.
The rubidium atoms have their absorption lines split in the presence of a magnetic field (such as the earth's, that we wish to measure). This natural phenomenon is known as the Zeeman effect. The splitting of the energy levels is given by the Breit-Rabi 2 equation.(Equation Available In Full Paper)
In this way we measure the absolute value of the earth is field H in terms of frequency. This is the optimum situation because frequency, and its inverse time, is the parameter we can measure best. By means of a servo oscillator that keeps the radio frequency oscillator locked onto resonance in the earth I s magnetic field of approximately 50,000 gamma, and a frequency multiplier of ten, we then can measure the earth I s magnetic field to 1 part in 3.5 million, because we measure the frequency to ± 1 count. Thus the sensitivity of the magnetometer is 50,000 gamma divided by 3,500,000 or a sensitivity of ±.01 gamma. This is 100 times more sensitive than the usual proton precession magnetometer which operates at a much lower frequency and is not a continuous instrument.