bluuebunny/crossref_metadata_2025 huggingface dataset:
Factors Affecting the Measurement of Axial Stiffness of Polyester Deepwater Mooring Rope Under Sinusoidal Loading
Authors: Casey N. F. , Banfield S. J.
Published: October 2008

This paper presents laboratory testing results to explore the experimental and fundamental dynamic axial stiffness behaviour of polyester ropes for deepwater mooring systems. Effects of mean load, load range, load or stroke control, extensometer versus crosshead, peak trough versus linear regression, cyclic period and scale effect (60 to 600kN) have been studied.

Understanding the axial stiffness behaviour is essential for calculating peak line tension and platform offset for riser excursion and fatigue life. Also, the effect of bedding-in of the rope structure, which increases stiffness, is required to establish the test matrix for certification.

A thorough review of the methods of data collection, interpretation of data and calculation of stiffness has been conducted. For those methods reviewed, no significant differences in stiffness have been found except for crosshead, which tends to be higher than measured directly on the rope between splices.

The effect of cyclic period and scale effect upon dynamic axial stiffness has shown to be insignificant and for dynamic mooring system response can be ignored, which reinforces previous work. For drift and longer term effects, the effect is very small and needs to be considered whether any allowance is needed. The effect of bedding-in on axial stiffness has shown that to reach peak axial stiffness, very large numbers of cycles are required. Typically, however, around 80-90% of the peak axial stiffness can be reached within several hundred cycles. For full-scale certification, the costs and time of large numbers cycles maybe prohibitive, so a technique has been suggested for conducting these tests on scale samples since there is no scale effect of stiffness.

These results will add to the knowledge base for understanding axial stiffness behaviour from testing, data collection, stiffness calculation method and mooring/riser system design. It will also provide valuable data for the future rewrite of API 2SM, to ensure that certification test methods collect the appropriate axial stiffness data for polyester ropes. The information will also be of use to the regulatory bodies.

Introduction

The main purpose of the work presented in this paper was to investigate the experimental factors that can influence the measurement of dynamic axial stiffness of polyester rope when subjected to sinusoidal loading.

The factors addressed in this paper are:

- Testing medium (air or water).

- Constants and variables, in the calculation of dynamic axial stiffness.

- Linear regression stiffness versus peak trough stiffness.

- Extensometer derived stiffness v crosshead based stiffness.

- Machine load control versus stroke control.

- Effect of cyclic period upon axial stiffness.

- Load History.

- 60 kN sub-rope to 600 kN rope stiffness scaling.

A 60 kN sub-rope together with 600 and 2,500 kN ropes of parallel strand construction (PSC) were tested during this investigation.
Deep Water Navigation With an Inertial System
Authors: Mateker Emil J. , Ott Leo E. , Shave David G.
Published: October 2008

Integrated satellite-navigation systems have been employed extensively for positioning marine geophysical surveys in the past three years. Since most exploration has been in relatively shallow water, dead-reckoning with doppler sonar velocities and gyrocompass heading reference has proven adequate. For programs in deep water, we have mechanized an inertial system to provide velocity and heading data for real-time positioning and postprocess mapping with the precision required by the geophysical industry. This is accomplished through real time software control. Inertial-platform drift is determined initially by comparing satellite fix positions with inertial coordinates. Inertial drift curves are predicted, and ship position is controlled by compensating for this drift. As drift characteristics change, drift prediction is updated. Continuous experience at sea since early December, 1971, had demonstrated that system drift can be effectively controlled. On the basis of all satellite fixes regardless of elevation angle, for operations on or near the equator, drift rates have been less than 300 m/hr. about 70% of the time, and less than 450 m/hr. about 90% of the time. Research with more sophisticated prediction methods led to software changes, introduced in October 1972 which showed some performance improvement but also demonstrated a shortcoming in the error model. Software changes, to be introduced shortly, are expected to show a 20%-30% improvement in drift rates. At higher latitudes where more good angle passes are available, system performance will be even better!

INTRODUCTION

There are a variety of radio systems available for marine navigation. However, none are able to provide continuous, world-wide navigation with the accuracy required for marine geophysical surveys. Consequently, integrated satellite navigation systems were introduced in 1969 to satisfy this need and used extensively with varying degrees of success. These systems rely upon periodic position fixes from the Navy Navigation Satellite System. Orbital data and frequency measurements are used in the position calculation (Stansell (12)). On the average, an acceptable position fix is obtained about once everyone to two hours, depending upon latitude. Since most geophysical exploration has been in relatively Shallow water (less than 100 fathoms), dead-reckoning between fixes with doppler sonar velocities and gyrocompass heading has proven adequate.

In deep water, doppler sonar performance is degraded due to an inability to track the ocean floor. Under this condition the sonartracks a return from the water mass and hence the sonar velocities are in error by the amount of any sea current. Halamandaris and Ozdes (S) proposed a solution to the problem of deep water navigation with an integrated satellite doppler-sonar system by the use of a sea-current biasing technique.
Optimal utilization of Groundbased Navigation Systems In Worldwide Deep-Sea Navigation
Authors: Sender Friedhelm
Published: October 2008

The application of satellite-navigation is increasing rapidly. But for practical use the only presently available transit-system has still several limitations, so that conventional groundbased radionavigation systems have not lost their significance. The optimal utilisation of those systems especially in view of the cooperation with satellite-navigation will be discussed here.

Satellite-navigation is the modern technology successor of astronavigation -with increased accuracy at day and night as well as aII weather capability. The disadvantage is, that presently installed systems are using exclusively sateIIites orbiting at relatively low altitudes about the earth with an orbiting time around 90 minutes. So satellites are only sporadically "visible" to the user and since all actual orbits are about the poles satellite-fixes are available at a medium random rate of one hour with higher frequency at the polar and less at the equatorial regions. Because it takes up to 16 minutes to sample the data necessary for a fix the accuracy of the fix depends to high extend on the exact knowledge of the path of the ship during this time. So it proved obligatory to use satellite-navigation together with an additional all time navigation system to achieve high accuracy and always real time position-information. Still better results may be achieved by integrating more than two positioning systems and optimal correlation of respectively available data.

Sonar-Doppler-navigation is the most widely used system for integration with satellite-systems. It has nearly ideal characteristics in coastal shelf areas with a water depth lower than 600 - 1,000 feet. Disadvantages of Sonar-Doppler are:High installation cost for the transducers on ships' hull in drydock;

Low accuracy in deep-sea, where system switches to "water track" and sea current cannot be eliminated, apart from transients and track losses due to plankton movement and sound velocity dispersion.

In many shelf areas are fixed radio navigation aids installed, which makes Sonar-Doppler redundant. Radionavigation has at least theoretically and in deep-sea many advantages compared to-Sonar-Doppler. But its use in integration with satellite-navigation hitherto was limited. The reasons are probably:Very long range radionavigation is burdened with propagation irregularity problems;Most systems are only at daytime useable with full accuracy caused by ionospheric properties;Apparently there are not enough installations for full worldwide high accuracy coverage;For successful integration with satellite systems long range radionavigation date seems too noisy.

These problems inherent to radionavigation will be discussed in the following part.

Propagation problems of different systems

Radiowaves are normally propagating similar to the light in a straight direction. For long range radionavigation the receiver-transmitter line is vignetted by the horizon.
A Design Method For an Anchor Pile in a Mooring System
Authors: Reese Lymon C. , Beard Richard M.
Published: October 2008

A procedure is described for the design of a pile in soft clay to provide an anchor for a moored vessel. A method is presented for computing the angle of incidence of the anchor chain with the pile, and theories are presented for computing the behavior of the pile. The procedure can be employed to compute the dimensions of the pile and optimum positioning of the chain bracket along the pile. Example computations are presented.

Introduction

In the mooring of floating vessels at locations where the bottom consists of soft soil, ship anchors frequently do not give sufficient holding power. A possible solution to the problem is shown in Fig. 1. The scheme involves the welding of a bracket to a tubular anchor pile and the attaching of the end of the anchor chain to the bracket. A pile is then driven with a follower to the desired penetration below the ground surface. The follower is subsequently retrieved and, when tension is applied to the chain, the chain assumes a position such as that shown in the figure.

In the design of such a system the following information is normally at hand: water depth, shear strength and other properties of the soil, availability of construction equipment, tension on the anchor chain, and dimensions of the chain. The problems which are to be solved are to find the optimal length and diameter of the anchor pile, the position of the bracket along the pile, and the depth below the mudline to which the pile should be driven.

In order to obtain solutions to the above problems, one must employ methods for analyzing the behavior of a pile under lateral and axial loading and for predicting the position of the anchor chain in the soil. The following paragraphs give brief presentations of those methods and give example computations.

Behavior of a Pile Under Lateral Loading

A model of the problem to be solved for behavior under lateral loading is shown in Fig. 2. The soil in Fig. 2 is represented by a set of mechanisms. These mechanisms are used to depict a set of curves which are varying in character along the length of the pile, with respect to x, and are such that the resistance p is nonlinear with pile deflection y. The concept of p-y curves can be explained by Fig. 3.

In Fig. 3a a section is shown through a pile which has been driven into soil, with the soil stratum to be examined indicated at the depth below the ground surface x1. The earth pressure distribution around the pile after driving and prior to lateral loading is shown in Fig. 3b.
Testing of a High Seas Oil Recovery System
Authors: March Frank A. , Beach Robert L.
Published: October 2008

A system for recovery of petroleum products from accidental offshore spills has been developed and tested. The testing has included both model testing and full-scale testing of a 60-foot long system capable of recovering oil at a rate of 2000 gallons per minute.

The tests have shown the system capable of recovering spilled oil at high efficiencies (less than ten percent water in the recovered oil) at the 2000-gpm rate under environmental conditions ranging from calm to low state 5 Seas (eight-foot significant wave height) and in currents up to three knots.

The system efficiency (percent of oil in the recovery stream) is relatively independent of oil specific gravity, viscosity, oil film thickness, wind and wave conditions, oil-water interfacial tension, surface temperature and variation in significant oil type when towed at speeds of one to three knots. As indicated, efficiencies of 90-100 percent can be expected under these conditions.

The oil recovery rate is relatively independent of wind and wave conditions, surface temperature, oil specific gravity, oil-water interfacial tension and significant oil type variation. It is not affected by viscosity except for discharge hose pressure limitations above 10,000 SSU.

The oil recovery system is air and truck transportable for rapid deployment to a remote spill location. It is completely self-contained having its own power unit, control system, and pumps. In its preferred deployment configuration it is used with oil containment booms for funneling the oil to the recovery system.

A double weir system is employed to obtain the high performance of the system. The system is operated in such a way that oil thickens in front of the primary weir and then further thickens in front of the secondary weir, where it is withdrawn. Typically, the system concentrates free slicks from a thickness of 0.1 inches or less to a depth of approximately two feet front of the secondary weir.

The testing reported upon includes both model testing and full scale testing of the system. The work reported upon was partially supported by the U. S. Coast Guard.

INTRODUCTION

Accidental oil spills on coastal and ocean waters are a matter of increasing concern. It has been recently estimated(1) that over 200,000 metric tons of oil are annually discharged into the world's waters from accidental spills. This spillage not only represents a loss of product but also represents substantial cleanup costs and potential liability. When the spillage occurs offshore in a wave environment the oil spreads fairly rapidly and cleanup becomes a major problem. Only recently have methods been developed for cleanup of oil spills in an offshore environment. This paper deals with one of those methods.

There are two major elements to any successful cleanup operation which must be carefully coordinated: containment and recovery.
Mariculture In an Artificial Upwelling System
Authors: Roels O.A. , Babb J.S. , Hamm G.L. , Haines K.C.
Published: October 2008

We have established an experimental mariculture system on the north shore of St. Croix (U.S. Virgin Islands), using as raw material water pumped from a depth of 870m in the sea. This deep water is cold and rich in nitrate, phosphate, silicate and other dissolved nutrients necessary for plant life.

It is particularly free of human disease-producing organisms, shellfish parasites, predators, fouling organisms and man-made pollutants. Moreover, its chemical and thermal characteristics are constant. The St. Croix site was chosen because the ocean reaches a depth of 1000 meters approximately 1.6 km offshore.

The deep water is pumped into pools onshore in which blooms of diatoms are produced to feed shellfish.

To date, F1 hybrid (or racial cross) clams (Merc'en'a'ria mercenaria Linne X Mercena-r-iacarnp'echien'sis Say), the European oyster (Ostrea edulis Linne), and the bay scallop (Argopecten irradians Lamarck), have been grown to market size in the system in less than one year.

A taste panel judged the quality of the shellfish grown in the St. Croix system to be excellent.

Slow-growing shellfish are culled from the system and used to rear the spiny lobster (Panulirus argus), and the Maine lobster (Homarus americanus). Preliminary experiments with carrageenan and agar-producing seaweeds to purify the effluent from the shellfish tanks are underway.

Introduction

It is well known that deep ocean water is cold and rich in the dissolved nutrients necessary for plant life. In our experimental mariculture system on the north shore of st. Croix (U.S. Virgin Islands), water from 870 m depth in the sea is pumped into 4S,000-liter concrete pools onshore where algal blooms of 2 diatoms (Chaetoceros simplex and Bellerochea sp.) are produced as food for shellfish in a managed food chainTable 1 compares the nutrient content and other parameters of surface water in St. Croix and water from 870 m depth, pumped up through our deep-water pipeline. As is clear from Table 1, the deep water is cold and rich in nutrients: its cold temperature can be used for a wide variety of cooling applications and for sea-thermal power production by the Claude_ processin areas where the temperature differential between the surface and the deep water is large enough. Air conditioning, ice-making, cooling of electrical power generating plants and of desalination plants to avoid thermal and brine pollution, and fresh-water production by desalination are some of the cooling applications.The high nutrient content of the deep water compared to that of surface water makes it a valuable resource for mariculture. Moreover, the use of deep water in mariculture systems bas many other advantages over inshore or estuarine surface waters: it is relatively sterile i.e. it is free of human disease-producing organisms, of shellfish parasites, predators and fouling organisms.
A Geophysical Data Acquisition System for Marine Use
Authors: Ligon Jerry L. , Haill Harry K.
Published: October 2008

A minicomputer-oriented geophysical data acquisition system has been developed which offers simplified system operation, instantaneous floating point capability, and provisions for real-time quality control. The computer performs all routine tasks with the operator interacting only to begin or modify major functions. The system is designed to fully utilize the precision capability of the floating point format in both its acquisition and processing capacity. Special features allow quality control to be applied to the data during acquisition.

INTRODUCTION

A geophysical seismic data acquisition system has been developed which simplifies data collection while monitoring data integrity and which allows full precision on-site processing. This advanced system brings the digital recording format, including instantaneous floating point amplification, together with a minicomputer to provide improvement in data handling and flexibility over conventional systems. In brief, the most interesting features of this system which will be discussed aresimplified system operation and control,full precision instantaneous floating point capability, andquality control.

Having a minicomputer integrated into the acquisition system allows simplified set-up and operation of the entire system. Communication with the system by the operator is accomplished exclusively through the keyboard and display of a CRT terminal. The display terminal is used in either of two modes by the operator, monitor or interactive. As a monitor, the system can provide an immediate display of operational parameters or system conditions. As an interactive device, the system can guide the operator through desired changes in operating parameters within limits set by the system.

The full floating point amplification and the ability of the system to deal with this form of data with full precision is of course important. There are several significant advantages offered by instantaneous floating point data. A system using this data will not Boon become obsolete. The technique gives a wide dynamic range (174 db in this case). Large changes in amplitude can occur without amplifier distortion. Also, small signal resolution is improved. Measuring accuracy is constant due to the A/D converter being operated within a restricted range of voltage input. The floating point technique also requires no manual control or settings.

Quality control is easily applied with this type of system. There are several ways quality of data is insured. One way is by making available results of instrument tests quickly and in hard copy form. Also, continuous quality control of data as it is acquired is applied before it is recorded. Data quality is further improved due to the wide range of data processing that can be performed by the system which was previously done only at distant processing centers.
Sosie Uses a Pulse Coding System to Improve Marine Seismic Records
Authors: Barbier Maurice G. , Viallix Jean R.
Published: October 2008

An ever-increasing multiplication of transmission and reception points has been sought in reflection seismology, resulting in a better quality of the seismic sections. A limit has been reached, however, because no energy can-be transmitted during reception time. Sosie break this bottleneck in marine work by transmitting a continuous sequence of pulses at an average rate of about two pulses per second. If the pulse instants make up a successful designed code, a simple process can bring the returns back into their proper position.

The method can be implemented with any source, as long as its transmission rate is high enough, used as a single unit or alternately with other units of the same type. At the present time, sparkers and steam or air guns have been tested.

The implementation does not undergo drastic changes compared to the conventional method. The only additional piece of equipment is a coding-decoding device that, on the one hand, delivers the firing orders to the sources at prerecorded instants and, on the other hand, processes a trace to yield a single section for a check and preliminary interpretation on board. The best results are obtained when transmission and reception never stop.

Comparison sections demonstrate the advantages of the new process in shallow, normal and very deep water.

INTRODUCTION

Continuous improvements have been effected in reflection seismology at sea, resulting inmultiplication of transmission and reception points,increased order of coverage andintroduction of sources with controlled transmission.

The Sosie process takes maximum advantage of all these improvements. It is not a new source, but a better way of using existing sources provided they are capable of firing at least two to three times a second. The recorded data are a mixture of information coming from all depths, but it is easy at processing time to put everything back into position if the shooting instants are known accurately and if these instants make up a pseudorandom code.

The principle of Sasie was explained in some detail at the SEG meeting in New Orleans, and only a brief description of it will be made here. This paper will present the know-how that has been gained on implementation in the field, the processing and the presentation of recently shot comparison sections.

BRIEF DESCRIPTION OF THE PRINCIPLE

Sosie makes use of a coded sequence of discrete seismic pulses that have the same sign and essentially the same amplitude. The energy is released along the seismic profile according to the sequence, and seismic signal and transmission instants are recorded continuously.

Under these conditions, if we call y (t) the time function representing the code and S (t) each seismic pulse released, we transmit S (t) convolved with y (t) (Fig. 1).
Utillizing the Capabilities of Doppler Sonar to Solve the Navigating Problem
Authors: Jones Howard P.
Published: October 2008

Doppler Sonar, although a unique and self contained means of navigation, saw limited acceptance until Edo Western Corporation realized that a real need existed for such a system. Edo Western Corporation invested in an extensive research and development program to advance the system capabilities over those systems which were already being marketed. The results of this program can readily be observed in the Model 435 Series of Doppler Navigation Systems. This system possesses greater "over-the-bottom" depth capability than any competitive system, will operate in shallower depths than any competitive system, and has a greater low velocity sensing and updating capability than any competitive system, just to highlight a few features.

Now with these capabilities, the question still arises as to how this system might best be utilized to help solve the many navigation problems.

The modular building block concept of the Edo Model 435 Series of Doppler Sonars provides a selection of optional hardware which can be selected to provide inputs to a satellite navigation receiver computer system or total system navigation capability utilizing a gyro compass may be provided completely within the Doppler Sonar. In addition, the operating depth range over the bottom may be extended to ranges heretofore unattainable by selection of available modules. All of these features make the Model 435 Doppler Sonar series capable of providing solution to virtually any navigation requirement.

INTRODUCTION

Many new instruments and techniques have been developed in recent years for use in researching the vast world of the oceans. Increased interest in utilizing ocean resources has led to significant growth in ocean exploration; hence an increasing need for development of new and improved methods for ocean navigation. Increasing sophistication in the fields of geophysical exploration, commercial shipping, and research for marine fishery sources has now made the use of precise navigation systems a necessity.

This paper has been prepared to assist in acquainting those unfamiliar with doppler sonar as to the various types and the ways in which they may be used. Once an individual understands the basic operation and capabilities of Doppler Sonar, many other applications can readily be seen.

Doppler sonar systems provide a self-contained means of navigation which does not depend on shore based stations or any other contact. The doppler sonar is capable of operating at any time of the day or night, regardless of weather conditions; therefore, it represents an extremely useful means of navigation for most types of ships. One of the primary uses of a navigation sonar is in geophysical survey work where precise location is required at all times. The doppler sonar has been integrated into complete Satellite Navigation Systems to provide a continuous, real time input of ships velocity to the navigation computer.
State-Variable Feedback Compensation of Acoustic Transducers
Authors: Newman A.K.
Published: October 2008

Piezoelectric and electromagnetic acoustic transducers may be mathematically described by the state variable formalism well known in control theory, so that the powerful synthesis methods available in that discipline may be applied. Three transducer applications are considered. Each situation results in the standard ABC D matrix differential equation. The three equations are both completely controllable and completely observable. Therefore, general state-variable feedback is possible.

The well known Kalman observer1 is employed to obtain the internal state variables of the transducers. The Kalman observer is different for each transducer model, but it is an analog or digital computer that operates on the transducer input and an electrical transducer output that is part of the circuitry associated with the transducer for the production of an electrical output in addition to the acoustic output. The additional electrical output is critical to the compensation design. Once the internal transducer state variables are available, state-variable feedback may be used to arbitrarily place the closed-loop transducer poles. Reference is also made to a different approach using Luenberger observers,2 which allows a general compensation to be made by using only RC lag networks in a clever ladder array.

As long as the transducers are operated in the linear region, they may be compensated to have a transient response arbitrarily sharp to a pulse wave shape without ringing, as well as a very wide-band frequency response with very little phase shift. The Q and resonant frequency are, thus, also arbitrarily designable.

The compensation scheme may be compared to the well known negative damping factor or variable damping-factor idea used in some high fidelity power amplifiers that provide a variable adjustment on the effective internal impedance, even to the extent of obtaining a negative internal impedance. This new compensation scheme is able to provide an effective negative source resistance also, but the flexibility is extended to allow the designer to place negative mass, viscous damping, spring stiffness, resistance, capacitance, and negative inductance in desired places in the transducer equivalent circuit.

However, large input power is required for a design that places the closed-loop poles far in the left half plane; and such large power may drive the transducer into saturation, which would violate the design assumption of transducer linearity.

Further research is being conducted that will extend these results to nonlinear transducer models and allow the transducer to be driven nonlinearly so the net output is linear up to saturation and the distortion is drastically reduced.

INTRODUCTION

The history of acoustics is replete with methods for the improvement of acoustic response from transducers for microphones, loudspeakers and other transmitters.
Pore-Space Reduction in Sandstone - Controlling Factors and Some engineering Implication
Authors: Rittenhouse Gordon
Published: October 2008

The average porosity of Gulf Coast Tertiary sandstones commonly decreases more than 1 percent for each 1,000 f't of' burial. This reduction results from a combination ofgrain rearrangement,pore filling with crystalline material andphysical and/or chemical modification of the original sand grains.

Factors controlling grain modification aresand composition,fluid composition and movement,effective stress,temperature,time and the extent to which early pore filling has strengthened the rock.

Pore filling reduces only the pore space; grain rearrangement and grain modification, in contrast, reduce both porosity and thickness of the sandstone.

Of particular concern here is that reduction of reservoir pressure during production increases the effective stress. Reducing reservoir pressure from 4,000 to 1,500 psi, for example, would have the same effect as burying a sand and its associated shales an additional 5,375 ft or more. As a result, the sand thickness could be reduced by as much as 9.4 percent, unless pore filling or prior deeper burial enables it to withstand the increased effective stress. This would be in addition to the consolidation of the associated shales that may occur more slowly and be of greater or lesser extent.

An understanding of factors controlling pore-space reduction and determination of their probable relative importance in specific cases may be helpful in anticipating such problems as casing failure or subsidence of' platform relative to sea level.

INTRODUCTION

When deposited, well-sorted sands commonly have porosities that exceed 38 to 42 percent and may exceed 50 percent. After burial and post depositional modification, sandstones of comparable grain size and sorting usually have much less porosity - in some cases, 1 percent or less. It is the purpose of this paper to discuss the factors and processes involved in such pore-space reduction and to consider some engineering implications, particularly when pore space reduction is accompanied by reduction in rock volume. Such reduction in rock volume is commonly referred to as compaction by geologists and as consolidation by engineers.

Knowing what has happened to sands in the past, and Why, can be significant in predicting what will happen if stress or other conditions are modified by man's activities. Has the sand been more deeply buried than at present, or was pore filling early? If so, an increase in stress may have little or no effect; if not, volume reduction may result. How may the original composition of sands and the type and extent of their post depositional modification affect selection of samples for engineering tests? Before considering these and other problems an understanding of the factors controlling pore space reduction is necessary.
Fatigue Strength Of Welded Tubular Joints With Various Stiffeners In Offshore Structures
Authors: Teramoto Susumu , Matoba Masaaki , Kawasaki Tetsuro
Published: October 2008

This is a report of experiments and analyses made in a study on fatigue strength of tee and lambda types of pipe-pipe joints which are generally used for column stabilized drilling units or drilling platforms. The objective of this paper is to present a fundamental guidance and data to the designers engaged in this field by offering the fatigue strength of such joints under complicated stress and to find' the most effective type of joint.

There are many types of offshore structures to satisfy their various restricted service conditions. The fatigue strength is one of the important factors in strength of the structure under actual load. That is because safety and reliability during the useful service life of the structure are governed by its capability of registing against fractures (such as fatigue crack, brittle fracture, plastic collapse, buckling, etc.).

It may be assumed that the fatigue strength and its behavior of the pipe-pipe joints are particularly influenced by the factors of the structure design such as geometrical configuration and intersecting angle, reinforcement method and brace to chord radius ratio, as well as production process factors such as material, weldability, brace-chord fabrication, in addition to environmental factors (1-3).

In this study we have carried out the static and fatigue tests with regard to the above factors of structure design by using 22 typical scaled models (chord dia. 400 mmp) consisting of 8 different types of pipe-pipe joints in which the brace directly intersects the chord.

INTRODUCTION

Along with the recent tendency for more advanced ocean development, the ocean technology has been also developed to build off-shore structures capable of being operated under severer conditions. As is well known, various column stabilized drilling units and drilling platforms are now under operation, which have many problems of local structure strength resulting from the restrictions in their service conditions. These problems must be solved for further development of any new type of structure in the future.

Characteristics of these offshore structures are that large diameter pipes are used in full extent, forming a type of trussed Rahmen structure (Fig. 1). The joint structure of pipe members are decided taking various factors into consideration synthetically such as strength, member arrangement, hydro dynamical stability, weldability and fabrication, etc. For the purpose of applying the fatigue strength concept to the design of offshore structures, analysis of actual load (wave induced force involving dynamic pressure, tidal current) and the responses of the structure to these loads turn to be essential conditions (4).

It is an urgent problem to study the characteristics of fatigue strength of the joint of structural members, and rind its design procedure.
Automated Forecasting of Extratropical Storm Surges for the Northeast Coast of the United States
Authors: Pore N. Arthur
Published: October 2008

The National Weather Service has developed an automated technique for forecasting extratropical storm surges along the northeast coast of the U.S. The storm surge is the rise of water level above the normal tide and is caused mainly by the strong winds associated with extratropical storms over nearshore areas.

Empirical forecast equations have been derived for 10 locations from Portland, Me., to Norfolk, Va. The equations were derived using data from 68 storms that occurred from 1956 through 1969. Input data to these forecast equations are sea-level pressure values as forecast by the numerical weather model of the National Meteorological Center. A sample forecast equation is shown.

The forecast method was put into operation in Oct., 1971, on an experimental basis. Forecasts are transmitted via teletypewriter and consist of values projected out to 48 hours at 6-hour intervals. These projections are used by Weather Service forecasters as guidance in preparing actual storm surge forecasts. A sample teletype message is shown.

Forecasts of the devastating storm surge of Feb. 19, 1972, are discussed. Forecast charts of sea-level pressure are compared to sea-level pressure analyses, and storm surge forecasts are compared to observed storm surges. Experience so far has emphasized the extreme importance, of the input sea-level pressure forecasts. Indications are that the system is useful and will be expanded to include other locations.

INTRODUCTION

The coastal storm of March, 1962, affected the entire Atlantic Coast of the U.S. and produced record-breaking high tides at some locations. This storm was the most devastating storm on record, as it caused damage estimated at over $200 million. Figs. 1 and 2 show some of the damage at Virginia Beach, Va., and Rehoboth Beach, Del.

It is, of course, extremely important to forecast the flooding associated with such storms. The critical time to forecast such conditions is also the time when forecasters are burdened with the usual bad weather conditions associated with coastal storms, such as extensive areas of rain or snow. The main purpose of automating the method is to produce better storm surge forecasts by providing guidance forecasts to Weather Service forecast offices.

CHARA.CTERISTICS OF THE EXTRA.TROPICAL STORM SURGE

Storm surge is defined as the meteorological effect on sea level and is computed as the algebraic difference between the observed tide and the normal astronomical tide. Fig. 3 illustrates this definition with a 2 day length of tide record. Here the observed tide is showy by the upper solid curve, the normal (or predicted) astronomical tide by the dashed line, and the storm surge by the lower curve.
Forecasting and Hindcasting the Maximum Combined Sea In Storms
Authors: Golaman Joseph L. , Bujnoch John A.
Published: October 2008

1. Abstract

Forecasts of wind, waves and swells were made for a particular offshore operations area in the North Atlantic with accuracies of up to 97%.

The operational forecast system, using a dynamic model, is sensitive to accurate measurements of wind velocity and the combined sea at the verification site. Some of the waves forecast with the system were "twenty year waves", that occurred more than once in four years, so that a statistical forecast system could not possibly have been used to anticipate them. The system uses the moving storm as the center of coordinates with as many details of the wind field as can be applied directly through measurement or indirectly through physical principles of the atmosphere in mesoscale.

A simple circular storm model made from actual data for each case studied has resulted in hindcast accuracies never before achieved. This is shown to be a result of detailed wind information. The importance of detailed wind information for both forecasting and hindcasting cannot be overemphasized.

2. Introduction

The Institute for Storm Research is responsible for forecasting the state of the sea, on a daily basis for specific clients with specific needs. A working system is utilized which not only accounts for the known parameters but also satisfies "real time" prediction requirements without sacrificing accuracy. The resulting predicted values are refined by the use of diagrams in order to account for the local effects on fetch length, shoaling and refraction. On a long term basis (five years of use), the system which is independent of location, has proven itself to be the best for offshore operations in shallow water.

The Institute considers wave forecasting to be one of its primary products and is constantly analyzing, refining and updating its procedures to improve the product.

A most effective way of refining forecasting techniques is to perform a "post mortem" analysis of selected storms and compare the results to actual observations. This is done by wave hindcasting and forecast evaluation. It is not sufficient to merelyrecalculate our forecasts using observed data. A re-analysis of the basic system must be made to insure that, while results may be satisfied through statistical verification, physical laws governing the sea's response to particular atmospheric conditions are not violated.

Presented here, are the results of one such wave forecast evaluation study. The region of interest is the Grand Banks Area near Nova Scotia and Newfoundland particularly that area surrounding Sable Island.

Two Computing Methods

A method of computing significant wave heights has been developed to account for the proximity of the forecast point to a moving circular storm taking into account location, extent, intensity and movement.
New Offshore Recovery Advancements For Deep Water Completions
Authors: Adams John R. , Manuel John F. , Cook Roger W.
Published: October 2008

This paper presents a review of the development status of three subsea oil production systems under development by Subsea Equipment Associates Limited (SEAL). This international consortium owned by Bristish Petroleum, Compagnie Francaise Des Petroles, Groupe DEEP, Westinghouse and Mobil Oil Corporation, is presently testing two production systems designed for primary use in foreign fields. These systems should find application in the water depths greater th'an 150 feet with average well production ranging from 1, 000 to 10, 000 barrels per day. SEAL is also testing a subsea oil production in the Gulf of Mexico which is designed primarily for utilization with large numbers of domestic low-production wells which in turn generally require significant maintenance. Participating in these test projects from anon-owner standpoint are Continental Oil Company, Phillips Petroleum, Sun Oil Company and ELF/ERAP.

The SEAL Intermediate System (SIS) is described with particular emphasis on utilization of a Manned Working Enclosure (MWE) to permit men to perform maintenance operations in a dry environment at the sea floor. The MWE concept involves lowering the structure over the wellhead, expelling the water and transferring personnel by the use of a Personnel Transfer Bell (PTB). The status of this development project is reviewed.

The SEAL Shallow System is described including a review of present testing off the South Coast of France. This concept is similar to the MWE with the exception that the maintenance personnel are carried in the SEAL Bell. Once the bell is pulled to the subsea wellhead, water is expelled and the atmospheric pressure reduced to sea level to permit the men to enter the work chamber and perform the necessary operations.

The SEAL Atmospheric System is described in some detail in this paper. This system concept is based on the use of a large habitat-type structure permanently installed on the sea floor to house oil field equipment. Future applications of this concept would be as a manifold center, a test separator center or a complete oil production system. In a similar manner to the other systems, men are transported to and from the Subsea Work Enclosure (SWE) through the use of a Personnel Transfer Bell. In addition to the description of the production SEAL Atmospheric System this paper presents a review of the present test installed in the latter part of 1972 and details are presented on the installation of the base structure, the Subsea Work Enclosure and the Wellhead Connection Assembly. The present status of this test program is also presented together with plans for the future.

A common element in each of the three systems being developed by SEAL is the use of a Personnel Transfer Bell to transport maintenance workers to the subsea structure in a safe and efficient manner.
Development and Utilization of a Deepwater Pipeline Connector
Authors: DeJong J. , Brown R.J.
Published: October 2008

This paper describes how one-atmosphere chambers might be utilized for making deepwater pipeline connections and, as well, provide the means whereby valves, controls, etc., can be encapsulated and readily serviced subsea.

A new connector device is described. It makes use of the high seawater pressure available at depth for pulling one pipe end into close proximity with another and through a port located within a subsea one-atmosphere connector chamber Control of the entire operation and final joining of the two pipe ends by means of conventional welding is accomplished within the manned chamber.

Anticipated utilization of the connector includes deepwater pipeline construction and repair connections. Modified connector chambers can be used for line tie-ins and riser connections.

INTRODUCTION

Related to growing world wide energy requirements, the search for oil and gas has expanded to deepwater areas which only a few years ago were inaccessible. In 1968, after extensive evaluation of alternative approaches for producing oil from these greater depths, Lockheed adopted the philosophy that a one-atmosphere chamber system at the seafloor provided the maximum opportunity for man to do useful work and at the same time maximize utilization of proven oilfield hardware subsea.

This work capability was first demonstrated in 1970 in up to 900 foot water depths during prototype tests of a one-atmosphere diving capsule for servicing seafloor chambers. More recently in 1972, this capability was again demonstrated in the Gulf of Mexico with an encapsulated subsea wellhead completion in 375 feet of water. A crew of three worked on bottom 11 hours out of each 12 hour shift.

At the conclusion of the 1970 tests, one of the companies supporting the test program suggested that the technology of the subsea work chamber should be extended to include deepwater pipe to pipe connection capability.

The seed having been sown, development work was undertaken. Several types of pipeline chambers were identified and were examined as to functionality, overall technical feasibility, and economic advantage. At the same time, a concept was evolved for mobilizing the ambient seawater pressure available at depth to generate a high controllable force, parallel to the seafloor, for pulling two pipe ends together.

This paper briefly describes the pipeline chamber development work conducted to this date and outlines the pipe pull concept and the experimental program conducted to establish its feasibility. The paper concludes with a discussion of situations where this subsea pipe-pull work chamber capability might be utilized as an aid to offshore pipelaying.

SERVICE CAPSULE and SURFACE SUPPORT

Fig. 1 pictorially shows the surface support vessel, the onboard support vans, launch and recovery equipment, and the Lockheed service capsule necessary for transporting men and materials to any sea floor chamber.