bluuebunny/crossref_metadata_2025 huggingface dataset:
Parametrization And Preprocessing Of Ocean Color Image Data For Studies Of Coastal Oceanography
Authors: Mueller James L. , Ball David L. , Middleton Elizabeth M.
Published: October 2008

Methods are presented for processing ocean color image data into forms suitable for quantitative oceanographic studies. A radiative transfer technique is outlined for determining approximate corrections for atmospheric transmission. atmospheric path radiance. sun glitter. and skylight reflection from the sea surface as functions of scan angle. The radiometric accuracy of these corrections is perhaps no better than ± 20%. but they nevertheless serve to make an overall relative correction for the systematic scan-angle dependent radiance variations with which these effects badly mask the ocean color pattern in images. Further processing methods include removal of geometric scan distortions from each image. and geographic registration (accurate to 0.5 km. or a single pixel) using 'visible landmarks. Work in progress to account for effects of bottom reflectance in ocean color spectrum analyses is reviewed. Experimental results with ocean color spectra from different world locations suggest that it may be possible to standardize the parametrization of ocean color spectra with characteristic vectors.

INTRODUCTION

Ocean color images obtained from space with scanning radiometers have several potential uses for studying and monitoring oceanic phenomena. Some oft-cited examples include making maps of sediment transport in estuaries and river plumes. monitoring the dispersal of water pollutants. making maps of phytoplankton (chlorophyll) distributions in coastal waters. watching for "red tide" outbreaks. and tracking the drift of dredge-spoil from offshore mining sites.

The utility of ocean color imagery has been partially realized with LANDSAT Multi-Spectral Scanner (MSS) data. even though the MSS was not designed for water color measurements (its radiometric gains are too low and its spectral bandwidths are too wide). Better ocean color image data will become available in 1978. when the Coastal Zone Color Scanner (CZCS) flies on the Nimbus-G satellite. The CZCS will have four narrow-band channels between 400 and 700 nm, a broad-band channel in the 7001100 nm region, and a thermal infrared channel in the 10–12 µm atmospheric window. Radiometric gains on CZCS will resolve the low radiance levels emanating from the sea and will saturate over most land scenes. Finally, Nimbus/CZCS coverage of a given scene will be repeated approximately every 36 hours. compared to 18 days for LANDSAT/MSS. In order to simulate CZCS data for algorithm and software development and for pre-launch experiments. a 10channel ocean color scanner (OCS) is currently operated aboard a NASA U-2 aircraft.

Multispectral radiometric ocean color measurements with. e.g. the OCS. are multidimensional data. and therefore. somewhat awkward to handle, display and interpret.
Design Of Floating Concrete Platform For Deep Water Oil Production
Authors: Kure Gregers , Hjertas Bjørn
Published: October 2008

The paper describes a concrete platform design, Conprod, intended for use as floating production platform in hostile areas such as the North Sea, the North Atlantic and the Gulf of Alaska. The Conprod platform is composed of the same structural elements as the Condeep gravity platform.

Theoretical calculations of platform response in irregular waves are described. The system of structure, mooring lines, added mass and hydrodynamic damping is represented by a finite element model, suitable for solution in the frequency domaine. The springs and forces are assumed to have linear characteristics.

Model tests are performed at a scale of 1:60. Some preliminary results are presented and compared with the theoretical calculations.

A twelve leg spread mooring system is proposed. The mooring lines are composed of chain, clump weights and wire rope. Restoring force and peak tension for the proposed system are compared with a conventional chain system.

For the floating production facility are briefly discussed. A total construction period of 2 years is feasible.

INTRODUCTION

Rapidly increasing development costs, particularly in the North Sea, have initiated an increasing interest in alternative techniques for offshore oil production. In many respects the concept of using a floating platform seems to be highly promising. Some of the features related to this type of field development are:reduced time lag from discovery to first productionreduced early capital investmentscapability to operate in very deep watermobility, the floating facility can be moved from one location and installed at anotherflexibility, the production facility can be utilized for extensive reservoir testingfeasible for oil production in earthquake areas. In June, 1975, the first floating production facility was brought on stream on the Argyll Field in the North Sea by Hamilton Brothers Oil Co. The operational experience during the first year seems to prove the viability of this semi-permanent production system.

A complete production system utilizing a floating platform comprises:a number of subsea completed wells with flowlines to a riser basea production riser through which oil flows from the riser base to the floating production platforma floating production platform with production equipmenta system for transfering the produced oil to shore, normally an offshore loading system. A number of different concepts for a floating production platform have been proposed during the last years, one of which is the Conprod described in this paper. The Conprod platform is structurally based on the same ideas as the Condeep gravity platform.

DESIGN PHILOSOPHY

The base for the Conprod design has been to utilize high quality concrete to create a floating platform capable of operating in water depths from 120 m to 500 m. The Conprod concept is feasible for world wide operation and can be constructed and equipped in any deep water harbour.
Analysis Of Driving Of Foundation Piles
Authors: Faa Stephen H.C. , Matlock Hudson H.C. , Meyer Patrick L.
Published: October 2008

A computer method has been developed to analyze the driving of foundation piles by impact and vibration, with the hammer acting at any point along the pile. A mandrel or follower may be included in the analysis.

While retaining the discrete-element concept proposed by E. A. L. Smith, the method employs an implicit (Crank-Nicolson) type of numerical solution. Soil supports may be any desired nonlinear functions of pile displacement history. Degradation in ultimate soil resistance under the action of inelastic rebound and reload of the pile may also be introduced.

Introduction

The analysis of pile driving requires some method of modelling, or simulation, of the hammer, the pile, and the supporting soil. The dynamic interaction in the system includes the elements of hammer impact, wave propagation along the length of the pile, and soil resistance associated with movement of the pile.

Methods of dynamic pile driving analysis most commonly used in practice are based on work by E. A. L. Smith (Ref 1). Smith utilized the advent of the digital computer to perform a one-dimensional computer simulation of the behavior of the system consisting of the hammer, the pile, and the soil. This method has become widely known as the "wave equation" analysis.

The name is somewhat misleading because this analysis is a discrete-element mechanical analogy of pile driving which does not directly employ the conventional mathematical equation for one-dimensional wave propagation within the solution process.

The pile driving problem requires numerical flexibility to handle varied boundary conditions, soil restraint forces, and non-uniform cross-sections which cannot be accommodated with the use of closed form mathematics. A more appropriate method of analysis is a numerical discrete-element solution such as Smith proposed. However, there has been some concern expressed recently about the accuracy of driveability analysis (Ref 2).

Discrete-element concept, the present method employs an implicit (Crank-Nicolson) type of numerical solution to yield the desired computational accuracy, stability and reliability.

There has been an increasing demand in the industry recently for a rational pile driving analysis to include a more flexible driving system. In the present method of analysis, the hammer can act anywhere along the pile, and a mandrel or follower may be included in the analysis. In order to account for forces other than those from the direct impact of the hammer system, a force-time relationship can be prescribed by the user at any desired point along the pile.

The soil supports are not limited to an approximate, simple elasto-plastic shape. They can be any nonlinear function of the pile displacement history, to reflect the actual soil response along the pile. The system is allowed to rebound and reload inelastically. A degradation in the ultimate soil resistance under the action of this rebound and reloading may also be included.
Dynamic Ice-Water-Soil-Structure Interaction Of Offshore Timers Including Nonlinear Soil Behavior
Authors: Haldar A.K. , Swamidas A.S.J. , Reddy D.V. , Arockiasamy M.K.
Published: October 2008

This paper describes the dynamic analysis procedures, both deterministic and stochastic, for predicting he dynamic response of offshore tower - foundation systems to ice-forces. Representing the hydrodynamic interaction by the added water mass concept, the foundation - structure interaction is investigated using i) a lumped parameter (also called half space continuum and soil springs) model, and ii) a plane strain finite element model. The nonlinear soil characteristics are treated in terms of displacement and strain dependency along the depth of the foundation. The ice force records are generated artifically from sample records. The force input is specified in three different ways:Time history records,Response spectra, andPower spectral density functions.

Rationalisation of the existing design requirements is based on the numerical results obtained.

1. INTRODUCTION

The most serious problem which occurs due to the ice floe movements in an offshore site is the oscillatory or ratcheting type of ice failure which imposes Forced vibrations on the structure. Matlock et al (1) First analysed a cantilever pier, idealised as a damped, single-degree-of-freedom system subjected to a postulated saw-tooth type of deterministic loading.

Reddy and others [2, 3, 4, 5, 6, 7] carried out extensive studies on the response of framed and monopod offshore towers to ice forces. The responses were determined using response spectrum and power-spectral density methods. Procedures for developing artificial ice force records, based on the measured values of Blenkarn [9] at Cook Inlet, were formulated. The influence of the nonlinearity on the soil foundation of the tower responses was examined by Haldar [8].

The purpose of this paper is to describe the different types of modeling criteria for soil-structural interaction behavior of offshore towers. Two types of towers are investigated:A pile-supported three-dimensional tubular space frame with a platform - Fig. l(a).Model of a circular cylindrical steel tower supported by a large circular base or hull treated as a gravity type monopod platform.- Fig. 2.

2. MODELING OF THE STRUCTURAL-FOUNDATION SYSTEM

2.1 Structural Idealization

2.1.1 Framed Tower:

The tower-pile-soil-water system is roughly similar to that analysed statically by Feibush and Keith (10). The members are assumed to be rigidly connected and the added water mass b assumed equal to the mass of the water displaced. The structural modeling is based on a two-dimensional representation of the tower assuming a constant dimension equal to the base length perpendicular to the plane. The masses per unit length of the members in the plane of the frame are computed by summing up the structural mass, the mass of the water contained in the tube, and the displaced water mass. The masses of the members perpendicular to the plane are assumed to be lumped at the horizontal cross brace level joints.
Super-Sized Semisubmersible Performance In The North Sea
Authors: Kobus L.C.S. , Meyers H.M. , Haines S.W.
Published: October 2008

The super-sized 30,000 ton displacement ZAPATA UGLAND has been drilling in the North Sea since. August 1974. This paper summarizes the experienced weather, the associated rig motions and the drilling performance in its operations in the North Sea. This paper highlights the design, equipment and operational aspects of the ZAPATA UGLAND that have been key factors in its splendid performance in the North Sea. A discussion is presented of model test data and the experienced motions and performance. This repot concludes that (1) super-sized semi submersibles, such as the ZAPATA UGLAND, are the optimum type of unit to use in year around operations in deeper waters of the North Sea and (2) self propulsion for a semi submersible is operationally effective and reliable.

INTRODUCTION

As the offshore industry extended into deeper waters of the North Sea and other severe environmental offshore regions, a new class of super-sized semi submersibles were designed, developed, constructed and put into operations. The criteria for classing a semi submersible as "super-sized" is based upon its operating displacement exceeding the working displacement of the pioneering Mohole platform the prototype and forerunner design for all twin-hull semi submersibles.

In 1969, Zapata initiated design studies to determine the optimum characteristics and carrying capacities for a super class design of semi-submersible drilling units. After initial studies of existing and proposed designs were compared with the trend of industry requirements, Zapata developed design parameters for a new unit.

These included the following key features:Ability to continue operations in seas of up to 40 to 50 feetAbility to remain moored in seas of up to 100 feetVariable deck load capacity of 3,000 tonsLoad capacity enabling all movements to be made with anchors and anchor chains stowed aboard the vesselSelf-propelled, with a target speed of 9 knots in calm waterMooring system for 1000 to 1500 ft. maximum range of water depths.

A detailed study of designs tested by Bethlehem Steel at the Netherlands Ship Model Basin indicated one of its designs was highly suitable for further development according to Zapata's target objectives. In 1971, Zapata initiated a joint program with Bethlehem to modify the design for the parameters Zapata had selected. This program included designing the structure, selection of the lower hull form, model tank tests of the new design and strength analyses concurrently with selection of the optimum drilling and auxiliary equipment.

DEVELOPMENT PROGRAM

The resulting design consisted of catamaran lower hulls with six stability columns supporting a water-tight upper hull. The lower hulls were given a sea-barge configuration with faired bows to provide optimum heave dampening action and propulsion characteristics. The trim upper hull was designed with clear lines, minimizing the amount of machinery and storage located on the upper deck exposed to the elements.
A Single Anchor Leg Mooring System For The North Sea
Authors: Foolen J. , Pincemin M. , Smulders L.
Published: October 2008

I. INTRODUCTION

In 1974 SBM, Inc. designed and built a SALM for the Pulai Field in the South China Sea, where the water depth is 245 feet. The terminal was designed to serve a permanent storage tanker of 190,000 DWT, which will remain moored in the 100 year storm with a significant wave height of 23.5 feet. At that time only two SALMs were installed (Okinawa and Brega). The third one was to be installed at Tembungo, also in the South China Sea. All three were of the base-(riser)-chain-buoy type. (Fig. 1) Extensive model tests were conducted for Pulai at the Netherlands Ship Model Basin in Holland. The test showed that a riser-sparbuoy SALM, in which the chain between riser and buoy is eliminated, has more desireable motion characteristics. In addition lower mooring forces were measured. (Fig. 2).

The knowledge obtained from designing and building the Pulai BALM was used to start the study, model tests and design of a riser-sparbuoy SALM for 533 feet water depth, to be installed at the Thistle Field in the North Sea. The terminal will serve 80,000 DWT tankers in wave heights of 15' significant. Survival conditions are 100 foot waves and 100 knot winds. In this paper the theoretical background, as well as construction of this SALM will be discussed. (Fig. 3).

II. THEORETICAL BACKGROUND

For the design of an offshore terminal the mooring force is one of the main design criteria. However, in the severe conditions of the North Sea, the forces acting on the structure during survival are as important. The Thistle study and model tests showed that motions of the SALM, induced by the wave action are of extreme importance. It was shown by computer simulations and confirmed by model tests that correctly chosen dimensions and weight distributions on riser and buoy reduce the bending moments in the riser. Slightly different buoy dimensions and weight distributions will increase the bending moments occuring in an otherwise identical riser. Moreover, the mooring force level can be reduced significantly by applying the following theory.

The basic reason for the motion and force phenomena are the wave period and the energy level of the waves. The wave period can vary between 5 and 20 seconds. Energy levels of waves below 5 and above 16 seconds are so low that they may be overlooked. In the wave spectrum for which a SALM is designed, two frequency levels are of importance. From vibration theories it is known that a single column like the riser can vibrate in its own natural frequency, known as the beam frequency. On the other hand, the combination riser-buoy has its pendulum frequencies. Any of the frequencies can be of the same order of magnitude of the frequency of the high energy level waves, if the dimensions and weight distributions are not correctly chosen. Resulting resonance would put severe strains on the system as a whole.
Natural Backfilling Of Pipeline Trenches
Authors: Mortensen Preben , Fredsoe Jorgen
Published: October 2008

Sea pipelines are for various reasons frequently laid in trenches in the seabed rather than on the seabed itself. In certain areas it is requested, for security reasons, that the pipeline should be covered to a certain extent, either through natural backfilling or through artificial backfilling. For the planning of the pipeline protection, it is therefore important to be able to calculate the rate of natural backfilling. The present paper describes how the natural backfilling may be calculated on the basis of the following parameters: currents, waves, bed material, shape of trench and water depth. The backfilling (sedimentation) in the trench is partly caused by a gravity effect on the bed load transport and partly by settling of the suspended load due to decreasing transport capacity over the trench. The physics behind these two sedimentation mechanisms are described. A theoretical calculation of the sedimentation, due to gravity effects on the bed load transport is developed in two steps. The first step contains only current, crossing the trench at an arbitrary angle. In the second step, it is shown how the same solution may be applied to bed load transport caused by wave action. The sedimentation due to settling of suspended load is also calculated for currents alone and for combined currents and waves. From the developed mathematical model, it is possible to calculate which portion of the suspended load will be able to settle as the current passes the trench. Four examples of how to calculate natural backfilling are presented. Two examples in current only and two examples in combined currents and waves.

INTRODUCTION

The aim of the studies presented in the present paper has been to provide mathematical models for calculation of natural backfilling in pipeline trenches under the assumption that the sea bed consists of non-cohesive sediments and that the depth of the trench is small compared to the water depth, see Fig. 1. The local scour around the pipeline is not considered in the present paper. The transportation of non-cohesive sediment is normally separated into the transportation of bed load and suspended load. In many contexts, such a distinction is of minor importance, but for the present purpose it will be demonstrated that such a distinction is necessary. As an illustrative example, the especially simple case where the alignment of the dredged trench is parallel to the direction of the current is considered. In this case, no suspended sediment is carried from the undredged regions to the trench by the current, if secondary currents are neglected. As long as the variations in water depth are small, the magnitude of secondary currents is negligible. However, while the suspended load is carried in the current direction, the direction of the transport of bed load deviates from the current direction due to gravity effect on the bed load moving on the slopes of the dredged trench.
Case Study: Oil Recovery Beneath Ice
Authors: Allen Alan A.
Published: October 2008

Approximately 6,000 gala of No. 2 diesel oil were spilled during the winter of 1977 from an onshore storage tank in Nome, Alaska. The oil saturated the snow and soil adjacent to the storage tank and penetrated the ice cover of Snake River. Oil containment and recovery operations involved the removal of contaminated snow, soil, and ice; sorption of oil with 3M sorbent materials and with available snow at the site; and the collection of oil from beneath an ice cover 3 to 4 ft thick using an oil mop. The oleophilic, closed-loop rope mop was fed beneath the ice between two holes, one for the oil mop wringer assembly and collection reservoir and the other for the system's tail pulley. Recovery totaled approximately 2,000 gal over a period of 1 week with air temperatures approaching -30 °F (-35 °C). Recovery rates of 2 gal/min were achieved with the rope mop beneath the ice, while oil-water ratios were in excess of 95 percent.

INTRODUCTION

Setting

On March 22, 1977, 6,000 gal of No.2 diesel oil were spilled in Nome, Alaska, when a 25,000-gal storage tank was filled beyond its capacity. The storage tank, one of two used for fuel storage at the City of Nome Power Plant, is located adjacent to the Snake River just about a mile and a half from its mouth on the northern coast of Norton Sound (Fig. 1).

Since the spill was not detected until the following day, the nearly 6,000 gal of diesel oil were allowed to move freely out over the snow-covered terrain, up and over a modest bank along the river, and down into an open melt zone created by a hot water discharge from the nearby power plant (Fig. 2). The oil had sufficient time to saturate the snow and soil between the storage tank and the river, and to move into and beneath the ice layers surrounding the exposed melt zone.

Environmental Conditions

The air temperatures before and during the spill and the subsequent cleanup activities were consistently at or below 0 °F (-18 °C). Temperatures frequently dropped to -20 to -30 OF (-29 to -35 °C) and remained there for days at a time. Cloud cover, reduced visibility, and precipitation were minimal throughout the operations except for a few hours of light snowfall. Winds normally were light during the cleanup efforts, except for a few brief periods during which winds approached 15 to 20 knots.

One to 2ft of snow covered most of the ground near the storage tanks, while lesser amounts were typical over open areas where drifting was less pronounced. Snow at the spill site and over the ice-covered river was loose and dry in consistency, granular in structure, and yet sufficiently compactible to form effective barriers (or berms) for the temporary containment of oil.
Tests On Two High-Strength Shori Tubular Columns
Authors: Marzullo Marc A. , Ostapenko Alexis A.
Published: October 2008

As part of an investigation of the local buckling of high-strength steel tubular columns as used in offshore construction, two 1.52-m (60-in.) diameter, 2.43-m (8-ft) long columns were tested.

One column was made from nominal 345-MPa (50ksi) yield stress steel and had a transverse weld located at midheight. The buckling stress of this column when plotted against ? = (Et)/(FyD) fell on a smooth curve formed by the test results of other columns fabricated from-345-MPa (50-ksi) steel that had no transverse welds and were tested previously at Lehigh U. This indicated that the presence of a transverse weld had no apparent effect on the buckling stress.

The second column had only a longitudinal weld and was made from a higher-yield stress, 690-MPa (l00-ksi), steel. The buckling-stress value relative to the yield stress of this column when plotted against a fell, above the curve, formed by the 345-MPa (50-ksi) columns. This indicated that the effect of the yield stress may not be non dimensionalized by the popular parameter a and that local buckling stress of 690-MPa (100-ksi) tubular columns may be higher than the stress that would be predicted on the basis of the 345-MPa (50-ksi) tests.

Both tests as well as other Lehigh tests demonstrate that the present API and AIST rules for local buckling of tubular columns are quite conservative.

The paper gives a description of the specimens, the test setup, the test procedure, and the general specimen behavior. In addition, the initial imperfections of one column and the effect of prebuckling lateral deformations on the location and pattern of initial local buckling are described.

INTRODUCTION

Tubular columns fabricated from high-strength steels by cold-rolling and welding are used in many engineering structures. Of particular interest are the columns having thicknesses greater than 3 mrn (1/8 in.) in which local buckling is expected at stresses between the proportional limit and the yield stress. In this stress range, the design of fabricated members with respect to local buckling is very uncertain because of considerable discrepancies between the current design recommendations as indicated in Fig. 1.

The principal design curves shown are: AISI (American Iron and Steel Institute,1 API (American Petroleum Institute),2 DNV (net Norske Veritas),8 Donnell and Wan,4 and Plantema.7 The ordinate is the ratio of the buckling stress to the yield stress, Fcr/Fy, and the abscissa is the nondimensional parameter.

(Mathematical equation available in full paper)

The small open circles in the figure represent the pertinent test results obtained by Wilson and Newmark9,10 on mild steel specimens, Fy = 214 to 280 MPa. In spite of the wide scatter, these results together with those on specimens manufactured by other means (not shown) served as a basis for the design curves.

In order to investigate how well these test results and design curves apply to tubular columns made of high-strength steel, eight tests were conducted at Lehigh U.
Behavior Of Anchors In Different Soil Conditions
Authors: Puech Alain , Meunier Jacques , Pallard Michel
Published: October 2008

The application of soil mechanics concepts to the problem of anchor holding power clearly illustrates the insufficiency of the concept of efficiency by itself, in that it is merely an empirical relationship between the weight of the anchor and its holding power. Laboratory and in-situ tests have been performed to analyze the behavior of special adjustable-configuration anchors as a function of the nature of the soil and to determine the influence of leading geometric and mechanical parameters on the kinematics. Holding power appears mainly linked to fluke surface area, depth of anchor burial and the mechanical properties of the soil. In-situ tests performed on small-sized anchors confirm the general analysis and provide bases for comparing the leading anchors used in offshore operations.

INTRODUCTION

The anchoring of floating supports (barges or semi submersible platforms) used during operations associated with the exploration for or production from offshore hydrocarbon fields raises specific problems that are essentially different from the anchoring problems encountered in traditional offshore operations:tensions on anchoring lines are especially great in excess of 100 tonnes for drilling platforms;the movements tolerated for the support are extremely limited so that anchoring points must be absolutely fixed (no dragging);anchoring time, which may vary with the type of operation, may be relatively long (several months in the case of drilling supports or several years for production supports).

On the other hand, the mechanical properties of sea floor soil are generally known as the result of geophysical and geotechnical surveys.

The rational calculation of an anchoring system involves:choosing a type of anchor especially suited to the problem at hand, i.e. taking the above-designed criteria into consideration;making an accurate estimate of the holding power of the type selected as calculated from soil data.

Lack of information concerning anchor behavior and. the insufficiency of the concept of efficiency, as is lamented by most users, do not enable a satisfactory answer to be given to these two questions of prime importance.

The joint research project undertaken in 1976 by the Institut Français du Pétrole (IFF) and the Centre National pour l'Exploitation des Oceans (CNEXO) had the following two main objectives :gaining better understanding of the phenomena characterizing anchor behavior and kinematics;improving ways of predicting anchor holding power as a function of the soil encountered.

The complexity of the problem required the is plementation of highly diversified experimental and theorical methods :laboratory tests on scale models,in-situ tests of medium sized anchors,development of a computerized program for calculating holding power by finite elements,two dimensional analogical model testing to calibrate the numerical model.

This paper reviews the leading experimental findings made during this project. The numerical findings will be published later on.
Experimental Study On The Liquefaction Of Sandy Seabed Under Offshore Oil Storage Tanks
Authors: Kobayashi Hiroshi , Nagatsuka Makoto
Published: October 2008

As one of activities of the 1977-fiscal year for the R&D project of offshore oil storage system being conducted under the supervision of Japan Ocean Industries Association and in the budget of Ministry of International Trade and Industry, a series of experimental study on the liquefaction of sand foundation have been performed to investigate the aseismatic stability of offshore oil storage tank of gravity type. Acceleration of shaking table, tank model, sand bed, pore-water pressure of sand bed and settlement of tank model etc. are measured to describe experimentally the phenomenon of liquefaction and study the liquefaction potential in relation to the stability of structure. Excess pore-water pressure can be expressed with equations to a good degree of accuracy and liquefaction potential of ground is assessed on the basis of soil test in laboratory. In addition to the improvement of physical information of ground for better assessment of liquefaction potential, prevention of liquefaction and anti-liquefaction design of structure are the problems for further discussion to be solved according to situations. The results will be reflected upon the recommendations for the aseismatic design of offshore oil storage system prepared by Japan Ocean Industries Association (JOIA).

INTRODUCTION

Quantitative observation of liquefaction of sandy seabed is performed by means of experimental study of offshore oil storage tank model of gravity type in order to study liquefaction potential of seabed and stability of overlying structure. The influence of existence of structure and sea water around structure upon liquefaction potential of seabed are also discussed.

There are such problems of the ground underlying structure as shear failure, settlement due to consolidation, sliding of structure and liquefaction. The studies on these for conventional structure have been applied to the design and construction of marine structure and contributed to the improvement of safety.

Liquefaction of ground has been well known in earthquake region as such phenomena as blowup of sand, unusual settlement of structure, etc. The study on liquefaction has been gradually promoted up to date. However, it is scarcely performed on the liquefaction of seabed for offshore structure of gravity type. The structure of this type are going to be constructed in earthquake region and the liquefaction of seabed will be one of the important items for discussion.

Rise of pore-water pressure due to seismic shake causes reduction of vertical effective stress, leading up to liquefaction of ground when such damages as settlement of overlying structure and surfacing of embedded structure will occur. Liquefaction of ground was already taken up for study by Taylor(l) and Mogami et al(2) in the 40's and 50's. Sunami(3) made a detailed observation of liquefaction and settlement of sand bed in a shaking box. Seed(4) tried to predict liquefaction of ground with applying the result of cyclic triaxial compression test.
Nitrogen-Oxygen Saturation-Excursion Diving
Authors: Lambertsen Christian J.
Published: October 2008

Isobaric inert gas counter diffusion is an unusual, hazardous and newly discovered circumstance of modern diving in which differential rates of inert gas movement in opposite directions in a tissue can lead to gas super saturation and gas bubble disease without change in pressure (isobaric state) (1)(2). The expressions of the phenomenon include severe cutaneous itching, development of gas lesions in the skin, incapacitating vertigo from vestibular derangement, and lethal gas bubble embolism. Since the problem has been recognized for only a few years, it is not yet commonly understood. Because it has implications to safety in diving operations, in decompression and in therapy of decompression sickness, this description will define conditions in which potential hazard due to isobaric counter diffusion exists and conditions in which it does not.

BACKGROUND

Over the more than 100 years that decompression sickness has been known, the gas used in most diving has been air. When helium diving was introduced in about 1935, it was used, with necessary oxygen, from start to finish in a dive, without gas switching during decompression. Such use of helium alone continued in naval diving, and then expanded with increasing interest in deep diving for commercial purposes. Its use in conjunction with other inert gases was suggested to facilitate deep diving or decompression from it.

GAS SWITCHING AND "DEEP TISSUE COUNTERDIFFUSION"

Among the methods proposed to complement use of helium-oxygen alone were those providing for speeding the elimination of helium by breathing another inert gas-oxygen mixture during decompression (3)(4). In this manner, switching to air breathing during decompression after a deep helium dive enables the more rapidly exchanged helium to be eliminated faster than the more slowly exchangingli1trogen is taken up by the same tissue compartments. The advantage - a more rapid lowering of tissue total inert gas pressure is aimed at providing more rapid decompression or safer decompression. While more rapid elimination of helium must result, it has been noted that the switch to nitrogen may be associated with development of vertigo, indicating vestibular dysfunction (5). This occurrence does not yet have a clear or demonstrated explanation. The method should be a rational and effective procedure for reducing tissue total gas pressure in all tissue compartments as long as abrupt switch of airs is avoided, precautions are taken against isobaric counter diffusion, and the pressure of the switch to air is low enough to avoid prominent nitrogen narcosis or oxygen toxicity. It differs entirely from the use of nitrogen in a mixture with helium for voice modification or other purpose.

The second form of gas switching involves changing to helium breathing after prolonged periods of work while breathing air or nitrogen oxygen mixtures at increased pressure (Fig. 1). This procedure has been employed experimentally (6)(7)(8)(9), in excursion from nitrogen saturation states and in special situations in the therapy of decompression sickness.
Remote Acoustical Mud-Gas Ratio And Density Sensor
Authors: Roberts L.R.
Published: October 2008

INTRODUCTION AND ABSTRACT

The prompt and accurate measurements of mud properties are well known drilling-response indicators.1,2 An automatic, remote sensing instrument is under development which will measure the following parameters when installed in riser joints down to 3,000 feet depth: mud/gas mass ratio, mud flow rate, and mud density. Measurements will be multiplexed along a single cable and interfaced to the computer-display system of the data recording system.

The purpose of this paper is to discuss the theory of operation, the engineering factors to be considered, and the experimental development fixture which is nearing completion. The test results will be a subject of a subsequent paper when available.

The evolving concept is directed to finding the engineering solutions to the many anticipated environmental and physical problems. These include high pressures, cuttings, and abrasives in the riser, interference from the drill stem, ambient noise conditions, gas bubble size distribution, gas solubility and water density cutting.

THEORY

The measurement of the three properties will be discussed separately, beginning with the mud/gas mass ratio (?). We define ? as the ratio of the mass of mud to the mass of gas in a unit volume. Since mud is inherently much more dense than gas, even when gas is compressed at 3,000 feet depth, the normal operating range for ? is about 106 - 107. Values of 105 or lower would represent significant gas-cutting of mud.

The theory of operation is based upon the behavior of the speed of sound (c) in a mixture of fluids. For these purposes both mud and gas may be properly considered fluids since neither will support a shear stress. The speed of sound in mud itself as a fluid is determined largely by its porosity, the volume proportion of interstitial liquid to solid matter3. For porosity greater than about 0.5 the speed of sound is almost exactly that of the surrounding liquid. Normal drilling muds with density up to 20 lb/gal will thus behave as fluids acoustically.

Conceptually, as gas is added to the mud the speed of sound in the mixture will decrease from its nominal value of about 1,600 m/sec, since the speed of sound in any gas is much lower than this. When present, the solids in drill cuttings will have a slight opposite influence. The precise value of the speed of sound depends upon the type of gas, temperature and pressure in a known way; the relationship is4.

(Mathematical equation available in full paper)

A curve of c as a function of log10? for typical conditions is included as Figure 1.

The quantities ?1, ?2, c1, and c2 can either be directly measured or deduced from auxiliary measurements. The theory of operation is, therefore, to measure c and then to "compute" ?. Actually, rather than "compute," it is intended to use a statistical estimator for ? which not only handles the various uncertainties in the problem but also provides a predictor for future values.
Ultra Large Fire Fighting Installations In North Sea Emergency Support
Authors: Flyen T.
Published: October 2008

As a result of the oil exploration activities in the North Sea, a number of new oil fields have gone into production. To protect these installations, new, heavier and more complicated fire fighting equipment and systems have been developed. This paper describes briefly various design criterias for Fire Fighting Vessels seth forth by Det norske Veritas and other public agencies and the solutions developed to comply with these requirements. One section is devoted to a theoretical study of a supply vessel hull movements in various sea states to establish a set of monitor control design parameters. Practical experience and recommendations are listed and a summery of present and future development is given.

INTRODUCTION

Recent years have shown a spectacular increase in fire fighting equipment for the protection of North Sea Oil field installations. Not only have the size of the equipment grown but the fire fighting systems have also become far more complex and requires more advanced engineering to give optimum performance.

The fire protection systems may be divided in two major groups:–Platform self protection fire fighting systems–Emergency Support Vessel based Rlatform Fire Protection Systems

This last group is the subject of this paper. We would like to emphasize however at this early stage that the Ultra Large Fire Fighting Systems are not designed for extinguish hydrocarbon fires, but to cool platform structures and prevent them from collapsing.

We have for some time felt the need for a classification of system requirements and engineering specifications in this field. The following is a summery of the experience we have gained from developing various equipment and systems for North Sea Fire Fighting Vessels.

It is not our intention to answer all questions. There is undoubtly room for improvement in existing equipment, especially concerning monitor hydrodynamics, jet performance and system controls. But we would like to establish a basis for further development and to point OUX some of the most common problems encountered in designing fire fighting systems for todays North Sea ESVs.

THEORY & DEFINITIONS

FIRE FIGHTING PROBLEMS

Offshore fire fighting problems may be divided into 5 major categories.Platform Fire ExtinguishingPlatform Structure Self ProtectionPlatform Crew Rescue OperationsPlatform Cooling by Emergency Support VesselsEmergency Support Vessel's Self Protection

The subject of this paper is category D with some references to category E. It should be remembered however that this is only one part of the fire fighting problems and that the intentions with the equipment and systems described herein are not to extinguish hydrocarbon fire, but to cool the platform structures to keep them from collapsing in a fire.
Model Testing of OTEC Plant Platform and Cold Water
Authors: Barr Roderick A. , Sheldon Lewis R.
Published: October 2008

INTRODUCTION

In 1975 the Department of Energy (then ERDA) initiated a significant effort to develop ocean thermal energy conversion (OTEC) as a commercial energy source. Significant technical effort has since been devoted to investigation and solution of technical problems associated with OTEC. Certain areas, such as heat transfer, biofouling of heat exchangers and structural loading of the cold water pipe (CWP), have emerged as key OTEC technical problem areas.

The design of OTEC plant CWPs which can operate without failure for 30 to 40 years is a difficult problem, particularly when the severe environmental conditions which can occur at most proposed OTEC sites (Puerto Rico, the U.S. Gulf of Mexico, Hawaii, etc.) are considered. Over the past four years a number of analytical methods have been developed for analyzing CWP dynamic and static loads. These methods have been used as a primary basis for most of the CWP designs and studies carried out to date. The more widely used methods are based on different methods of approach and assumptions, and hence are of different potential accuracy for different CWP designs.

It is essential that analytical methods be carefully validated if they are to be used with confidence to design CWP's. Scale model test data provide the best basis for validation, and validation efforts to date have relied primarily on data from laboratory and at-sea model tests. This paper discusses model testing of OTEC plant platform/CWP configurations in waves, with emphasis on simularity or scaling requirements and two sets of laboratory tests carried out at HYDRONAUTICS Incorporated. The use of the test data for validation is also considered.

SCALING REQUIREMENTS FOR OTEC MODELS

Three types of scaling are applicable to seakeeping or other model tests of OTEC plant platforms and CWP's. These are: dynamic scaling; hydrodynamic scaling, particularly of the CWP; and structural or elastic scaling of the CWP. The first two types are considered in most ocean engineering testing, while the latter type is typically not considered. This section considers scaling or similarity cosiderations for OTEC models with emphasis on CWP elastic or hydroe1astic scaling. A more detailed discussion of scaling is given by Barr and Johnsonl

DYNAMIC SCALING

For any test in ocean waves, which is the environment of greatest interest for OTEC plants, it is essential to observe Froude scaling, in which the Froude number, F, is identical in model and prototype:

(available in full paper)

where V is a characteristics velocity, l is a character istic length and g is gravitation acceleration. This scaling makes acceleration invariant between model and prototype and requires that

(Available in full paper)

where the subscripts m and p denote model and proto type, respectively, and ? is the linear scale ratio of prototype to model size. An important implication of Equation [l] is that:

(Available in full paper)
Advanced Use Of The Radiopositioning System Pulse/8 In The North Sea
Authors: Fritzner Hans E.
Published: October 2008

One of the most commonly used radiopositioning systems in the North Sea, Pulse/B is discussed. The background is the recording and processing of a large volume of data from both survey vessels and anchored, semisubmersible drilling rigs in the Norwegian Sector 58°-62°N. Methods in order to improve the accuracy are presented. This include the Pulse/8 system both in hyperbolic and circular mode as well as utilization of the differential technique. Processing of data such as filtering and computations of three-way fixes and fixed errors is also discussed. For the hyperbolic mode, the results show that filtering and the use of three-way fixes will improve the repeatability well below the claimed 50 meters within most of the actual area, 25-30 meters can normally be found. When thorough establishment of the fixed errors is carried out for a location, the absolute accuracy will be of nearly the same order as the repeatability. More advanced use of the system wil improve the accuracy further, in certain areas possibly even below 20-25 meters. Finally, the circular mode of the Pulse/B system has been found to be, an attractive system in new areas with limited activity.

INTRODUCTION

During the last years, one of the most commonly used radio positioning systems in the North Sea offshore activities has been Pulse/B. This 100 kHz, pulsed and Loran C-like radio positioning system has six transmitting stations located as shown on Fig. 1, and operated by a joint venture between Decca Survey Ltd. and A/S Kongsberg Vapenfabrikk (from now on to be called the "Contractor".) As shown, a repeatability of less than 50 meters is claimed within most of the North Sea.

In this Paper, some methods with the intention of improving the accuracy are discussed. The background for this discussion is the recording and processing of a large volume of Pulse/8 data from both survey vessels and anchored, semisubmersible drilling rigs in the Norwegian Sector 58°-62°N.

The methods include the system applied both in hyperbolic and circular mode as well as utilization of the differential technique. Various methods of data processing are also discussed such as filtering (smoothing) and computations of three-way fixes and fixed errors.

Most of the data presented have been recorded on board anchored rigs. Under normal weather conditions, the movements of the rigs can be regarded as negligible. Therefore, the readings can be treated as recorded under stationary conditions.

In this Paper, the emphasis is laid upon the experiences from Pulse/8 data projects. Therefore, descriptions of the system and navigational and statistical theory are limited to a minimum.

THE OPERATION OF THE SYSTEM

On each of the six transmission stations, free-running, high stability cesium oscillators are giving the time reference for the pulses.