bluuebunny/crossref_metadata_2025 huggingface dataset:
Prediction of Weather Downtime for Derrick Barges
Authors: Rawstron Philip J.M. , Blight Graham J.
Published: October 2008

As oil is found in more hostile environments, economic and rapid completion of offshore production facilities will depend more heavily on the sea keeping ability of construction vessels. However, the relationship between vessel motion characteristics and the percentage of time that the vessel will be able to work is not generally understood.

The net effect of the vessel motions on lifting operations can be reduced, in fact, to a single value represented by the vertical motion of the crane boom tip (or sheave centerline). The explanation for this is quite simply that the corresponding vertical motion of the hook controls the ability to safely pick-up or set-down a load. This single assumption presupposes that the lateral or swinging motions of the load are not a direct function, but rather an indeterminate consequence, of the lateral or horizontal motions at the boom end. Thus, vertical hook or boom end motion can be used as a parameter to qualify the operational tenacity of a derrick barge.

Barge operation reports and corresponding measured wave data were analysed to yield sea-states when work could be accomplished. Further analyses provided the maximum vertical boom end motions with which piling work or module lifts could be carried out. These maximum vertical motions can now be used as criteria todeterm1ne the expected number of working days for a particular vessel at a given location, according to the operation required.

A computer program has been developed to carry out the process, which is described in the paper. The method is based on the vessel motion characteristics, statistics of the expected weather and limiting criteria of vertical motion. The paper discusses the influence that equipment aids, such as heave compensators and tag-line .systems, might have on the predictions. A comparison is shown of the expected performances of typical heavy lift vessels in the North Sea.

INTRODUCTION

Model tests have been used for many years to determine the sea keeping characteristics of floating vessels. More recently computer programs have been developed which will perform the same task with equal reliability, but less expensively and much more rapidly. However, it is not immediately obvious from the fundamental motion responses derived by either method as to how efficiently a proposed vessel will operate in practice. All that can be done, provisionally, is to compare the motions of the proposed vessel with those of existing vessels. This procedure is clearly insufficient, even if motions in corresponding sea-states are compared, because it fails to recognize the effect of variations between the arrangement or geometry of different vessels. It also completely ignores the practical factors of the operation to be performed by the vessels.

For a derrick vessel the ability to work is primarily a function of the relative motions between the package being lifted and the foundation on which it is to be set down. The ability to work can also be controlled by relative motions at the time a package is to be picked up.
Self-Installed Deck On Offshore Platform
Authors: Orfali M. , Herve J.L. , Buzzichelli M.
Published: October 2008

This paper attempts to describe the development of "self-installed" decks on offshore platforms.

When North Sea operations first started platform installation was performed by means of conventional derrick barges i however, faced with harsh sea conditions, they did not prove very cost effective and installation was often delayed.

Thus, operators were driven to seek new concepts. Among them, TOTAL CFP, operator on the Frigg pipeline, chose to erect big cranes on bare platforms in order to complete installation. This concept soon proved to be a reliable one as it became far easier to take advantage of narrow windows of good weather than it would have been possible with derrick barges. Moreover, safety was increased.

After a few modifications, this installation scheme can be applied to a wide range of fixed platforms both in rough and calm seas.

In this paper, our intention is to explain why we chose this concept and to give the main results gained from our experience on the field.

INTRODUCTION

Within the scope of the Frigg operations, TOTAL, as operator on the pipeline developed several completely new techniques which involved the installation of lifting devices with a load capacity of 300 t to 800 t on platforms.

Once developed by TOTAL and BUZZICHELLI, these techniques enabled offshore lifting operations to be carried out rapidly in weather conditions which were unsuitable for conventional lifting devices.

APPROACH TO THE NEW CONCEPT

North Sea beginnings

When operations first started in the North Sea, production platforms were equipped by means of conventional derrick barges.

The latter can work in up to half a foot heave but roll is critical, the limitation being between 0.5 and 1° depending on the size of the load. In the North Sea, these thresholds are often over-reached resulting in long waiting for calmer weather and sometimes in postponements of operations for up to 8 months, should they not have been completed before the winter season.

Moreover, safety is sometimes affected. Indeed, once a package has been lifted a cargo barge becomes very lively, which lead to a very tricky situation, as it is impossible to drop the package down either on the platform or back on the cargo barge. Should by any chance, the load then start swinging laterally, it is almost impossible to control.

Big modules and huge barges : some improvement but many disadvantages.

In an attempt to improve North Sea operations huge derrick barges appeared. They could handle large and heavy packages (up to 2 000 tons) in slightly worse sea conditions.

It was agreed that the bigger the modules, the fewer lifts required to install them. This was in fact quite true, but several disadvantages should be pointed out.
Dynamic Response Of Flexible Offshore Structures To Regular Waves
Authors: Taudin p.
Published: October 2008

The highly nonlinear fluid solid interaction can be studied by an iterative calculation procedure described in the paper. It is a convenient method for the computation of the steady response of tubular structures.

An extension of the method is possible so as to obtain response curves through the wave periods. They describe the behavior of the structure in various sea conditions, from small and short waves to high and long ones.

The response curves can be used to calculate the spectral response to a sea spectrum if the structure is not highly nonlinear.

INTRODUCTION

An offshore structure is said to be flexible when the displacements, speed, and accelerations of the structures elements highly affect the wave and buoyancy forces. According to such a definition, a same structure can be flexible only for some sea states and rigid for others. A typical property of flexible structures is that the relation between the hydrodynamic forces on the motionless structure and the displacements is nonlinear.

Our purpose is to study flexible structures built with tubes, using the Morison's equation for the calculation of wave forces and to propose a low-cost iteration procedure for the calculation of the steady response to a large variety of wave periods and wave heights.

An objection to such a study may be that the theory of regular and cylindrical wave3 is not realistic. But we think that the large amount of information given by the iterative procedure about the behaviors will be helpful in the nonlinear spectral analysis (this is the theme of our present research).

On the other hand, we have noticed that the regular response is of a high practical interest. Thanks to the method presented hereafter, it has been possible to calculate what wave periods are the most dangerous for a structure, for a given wave height. It, has been possible to delimit in the H,T plane, the curves of equal fatigue duration, then to determine the towing feasibility of structures.

EQUATIONS OF MOTION

Basic Hypothesis and Simplifications

Structure Schematization

The structure is composed of tubular beams whose length and diameter are small, when composed to the wave length. Each beam can be cut artificially into small elements along which any function of space will be taken as constant. Then the structure will be represented only by a set of points corresponding to the center of gravity of the elements. Each of the following properties are defined for each point.

(Available in full paper)

Base of Displacement Fields

A set of n arbitrary vector functions of space are chosen:

(Mathematical equation available in full paper)

They represent a base of displacement for the structure. It is supposed that any displacement of a tubular element is represented by the formula.
Submarine Landslides In The Mississippi River Delta
Authors: Coleman James M. , Prior David B. , Garrison Louis E.
Published: October 2008

Systematic side-scan sonar and high-resolution seismic records from the shallow-water offshore areas of the Mississippi Delta have revealed widespread sub aqueous slope failures in bottom sediments. These failures have resulted in damage and loss to offshore structures and pipelines. The features occur on slopes with very low inclinations (ranging from 0.2° to 1.5°) and in water depths of 5-100 m. The types of features include collapse depressions, bottleneck slides, elongate slides and slumps, mudflow gullies, and overlapping mudflow lobes. Although movements include both vertical and rotational displacements, the bas1c mechanism can be approximated as down slope translation of shallow slabs of debris. Although movement rates of up to several hundred meters/year have been documented, it is postulated that large-magnitude surged may be inherent in these features. These submar1ne landslides result from complex temporal and spatial combinations of wave-induced stresses, sediment loading, and generation of high pore water and methane gas pressures.

INTRODUCTION

Numerous detailed marine surveys (high-resolution geophysical, side-scan sonar, and bathymetric data) have been conducted in the area offshore of the Mississippi River. These surveys have revealed the presence of a large number of distinct types of sub aqueous slope instabilities that can be generally classed as submarine landslides. Most of these bottom sediment mass movements are of sufficient magnitude to severely endanger bottom-emplaced petroleum facilities such as offshore drilling and production platforms, wellheads, and pipelines.

The modern bird-foot (Balize) delta of the Mississippi River displays three major distributaries (Fig. I) and has formed within the past 600-800 years.1 Seaward progradation rates of the distributaries vary from in excess of 100 m/yr to less than 50 m/yr, depending upon the specific distributary monitored. Sedimentation rates seaward of the delta shoreline vary spatially as well as temporally. Near the mouths of the distributaries, accumulation rates in excess of 1 m/yr have been documented, whereas in adjacent interdistributary bays accumulation rates rarely exceed a few centimeters per year. In offshore waters, in front of the delta, accumulation rates vary from a few tens of centimeters per year in 50-m-depth water to a few millimeters per year in water depths approaching 200 m.

Offshore slopes of the entire delta front are extremely low, rarely exceeding 1.5°. In the interdistributary bays, bottom slopes are generally less than 0.5° and are rarely greater than 0.2°. In water depths of approximately 5-80 m, bottom slopes range from 0.7° to 1.5°, and in depths of 80-150 m the slopes are less than 1°. In general, hydrographic maps indicate extremely irregular topography, the bottom displaying a large number of radial trending submarine gullies 2 in water 20-80 deep and broad, flat terraces seaward to water depths as great as 200 m (Fig. 1).
Wave Direction Measurement By a Single Wave Follower Buoy
Authors: Middleton F.H. , LeBlanc L.R. , Sternberger W.I.
Published: October 2008

This paper describes a system for measuring the direction of ocean wave propagation, as well as wave height and period. The system employs a conventional wave-follower buoy, modified so as to provide compass, pitch, and roll signals in addition to the vertical component of buoy acceleration. The modifications are indicated in the paper, and the buoy was used in rough water conditions in Narragansett Bay for a test. The pitch, roll, compass, and heave acceleration signals were all processed on the computer in the laboratory to provide a variety of results. This test condition was ideal in that the wind and wave conditions were changing steadily throughout the data recording process. The results presented in the paper include the wave height power spectral density, a polar tilt histogram for the buoy motion, and a family of polar plots of the average tilt angle in each 5° sector of azimuth of the buoy pitch/roll.

INTRODUCTION

This paper is the third in a series of OTC papers on the subject of the continuous development of wave-follower buoys, starting in 1975. The first paper was OTC 2424 in 1975 on the mathematical analysis of wave data making optimum use of fast computer techniques. The next paper was OTC 2597 in 1976 and it was devoted to ways to handle the problem of spectral compensation of acceleration data from a buoy to produce a true wave height power spectral density (PSD). This last paper also was condensed and published in the June 1977 issue of the Journal of Petroleum technology. It is the purpose of the present paper to extend this buoy development history to include not only the PSD of an open-sea wave field, but the direction of propagation of the dominant wave mode (or modes) that are present.

In many important ocean operations, either on the open sea or near shore, the direction of propagation is often of more importance than the significant wave height, or the power spectrum. This clearly is the case, for instance, for storm waves striking a beach, a harbor channel, a breakwater, or any sort of offshore structure. Up to the present time, anyone needing wave-incidence direction data has been forced to deploy complex, complicated, and expensive spatially distributed arrays of wave measurement devices. Using wave staffs on a structure requires spatial separation on the order of the wave length of interest in order to obtain reliable results.

The buoy used in this study was a Model WF-100 wave Follower Buoy manufactured by Coastal Data Service, Inc. (licensed to Hydro Products, San Diego) on loan to the U. of Rhode Island for purposes of the present development. It was necessary for Coastal Data Service to modify their standard buoy, by the addition of tilt and compass sensors, to make it possible to measure propagation directions. Some internal modifications were required to provide space for the necessary additional sensors.
Trigone Jack-Up Drilling Rig
Authors: Laplante Gilbert , Clauw Rene
Published: October 2008

The jack-up drilling rigs TRIGONE, built according to the North Sea regulations and the DNV classification requirements, for 350-ft water depth (i.e., for a wave height of 65 ft and a wind velocity of 100 mph), are the largest self-elevating units operating in the world. Their design has been developed bearing in mind their possible use as an early production platform. The rig is equipped with a skidding system allowing a drilling area of 21 × 21 in.

The legs have a length of 507 ft. They are built from cast joints, tubular bracings, chords, and racks. All steels are high tensile steel with impact notch tests at minus 40°C. Test programs for materials and welds have been developed especially by the Det Norske Veritas in collaboration with the builder. Welding procedures have been established by same under DNV survey.

Elevating units (gear and pinions) electrically driven, have a maximum holding power of two times the elevating power. The power of the jacking system permits elevation and holding in drilling conditions 2500 metric tons of variable loads and holding under storm condition more than 2000 metric tons.

INTRODUCTION

The two jack-up TRIGONE have been built by CYEM in its Dunkirk yard for the Norwegian ship owner DYVI OFFSHORE Co. These rigs are the first ones especially designed to work in 350-ft water depth in the North Sea. Design and particular studies of equipment installation have been approved by the Det Norske Veritas and Norwegian regulatories bodies. Construction in shop and erection at the yard also have been surveyed and approved by the same authorities.

These rigs have obtained DNV class notation I A I self-elevating unit, all certificates from the Norwegian authorities (Maritime Directorate), and also obtained the certificate of fitness for operations in U.K. waters.

GENERAL DESCRIPTION (Fig. 1)

The platform structure consists of a triangular hull supported by three legs made of triangulated steel bracings. In drilling position, the legs stand on spud tanks on the bottom of the sea.

The hull makes the buoyancy element during towing conditions.

Legs are located at the three angles of the hull and are used before drilling operation to elevate the whole hull.

The living quarters are in above decks in one structure, mounted on the main deck just aft of the forward leg well.

Hull (Fig. 2) - The hull is of triangular barge form. The primary structural system is of longitudinal and transverse bulkheads. The hull is surrounded with double bottoms (under and on the side of the rig). The machinery spaces are made of 13 technical rooms located just under the main deck.

Spud Tanks and Legs (Fig. 3)

Each leg consists of a hexagonal bearing tank (spud tank) with a conical point at the bottom, surmounted by a truss triangular in cross-section. The truss-type leg structure has three corner members consisting of two continuous racks joined by a plate forming the diameter of a circle.
Convergent Plate Margins for Field Geologists
Authors: Karig D.E.
Published: October 2008

The variability and complexity of active plate margins require that the identification of units of previously active margins in orogenic belts be done with care and sophistication. Tectonostratigraphic units from active margins vary not only as sedimentation and plate kinematic parameters change spatially, but also temporally, as these parameters evolve. Deposits from the trench, lower slope basins and upper slope or forearc basins may be distinguished using the distinctive structural styles, sedimentation pat terms, and lithologic content observed within each unit. The relative positions and characteristics among the units are equally important criteria.

INTRODUCTION

Recent geological and geophysical investigations, interpreted in the framework of plate tectonics, have produced a general geological model of convergent plate margins. Nevertheless, one of the most important objectives, that of identifying various elements of a previously active margin within an orogenic belt, remains elusive. The field geologist, if he is to interpret the evolution of the complexly deformed mountain belts formed by plate convergence, must be able to deduce the original tectonic settings of the various tectonostratigraphic units which are now deranged by younger overprinting and by large-displacement faulting.

The purpose of this paper is to review diagonistic characteristics of the various packages of rocks produced within a convergent plate margin with which these units might be identified after their morphology is destroyed. An admonition is immediately required. There is no typical arc system, but rather a broad spectrum of responses arising from a single basic process. As a result, in this setting, perhaps more than in any other, identification using simple criteria can lead to serious error. Only a comprehensive integration of data and an adequate knowledge of arc behavior is likely to produce the correct interpretation.

Convergent plate margins range from oceanic island arcs involving very little sediment to continental margin arcs which often have several kilometers of sediment on the descending plate. These differences are reflected in wide variations in size, shape, and lithologic content of units [Fig. 1]. Not only do convergent margins vary spatially, but also temporally as sedimentation and kinematic parameters change. To compound the difficulty, morpho tectonic units are the basis for subdivision in active arc systems whereas in orogenic belts, only lithologic structural entities (tectonostratigraphic units) remain. These may differ because a given tectonostratigraphic unit may be involved in several different morphotectonic units the arc evolves. For example, a block of trench turbidite rapidly becomes part of the lower trench slope. Later this same block of rock may become part of the basement beneath a forearc basin or even later may become part of the frontal arc, but when mapped in the field as a melange, it cannot be used to locate that frontal arc until temporally correlative information on other arc units is available.
In Situ Fatigue Analysis Of A Tubular Joint: Hydrodynamic And Mechanical Aspects
Authors: Barnoiun B. , Thebault J.
Published: October 2008

A very simple experimental structure has been designed to perform fatigue-corrosion analysis of tubular joints. The methodology is described, together with the results of the static tests needed to interpret the measured data. A complete set of S.C.F. values for every possible situation has been determined. Besides, the opportunity was seized to test the methods available for foreseeing vortex induced vibrations and to initiate a study of the behaviour of the pile submitted to hydrodynamic forces. The procedure and some results are indicated.

INTRODUCTION

Offshore oil development is built upon the capacity of present technology to erect production platforms in large water depths and to assess their ability to withstand long term action of the environment.

Calculation procedures of the jacket type structures are now well established. Nevertheless, some uncertainties remain about the input of these models, and particularly about the corrosion fatigue strength of the tubular joints that form the nodes of the lattice-work, and through which structural loads are conveyed. Moreover, there are still large gaps in the knowledge of the fluid action on cylindrical obstacles which constitute the basic element of this lattice, specially in the case where the dynamic properties of the structure allow for large relative displacements.

The experiment described hereafter is aimed at a better understanding of these problems, typical of the real environment complex action.

DESIGN OF THE STRUCTURE "S.E.D.A.T.I.S."

The system consists in a tubular joint clamped to the seabed through a heavy concrete base (12 metric Tons). The brace of this T. joint is extended by a 14 m. long vertical pipe. A detailed diagram of the experiment may be found on Fig. 1.

This very simple assembly, named SEDATIS (Systeme d'Essais Dynamiques d'Assemblages Tubulaires In-Situ), offers at least two valuable testing opportunities :from an hydrodynamic point of view:The pile is loaded as well by currents (as high as 2 knots at spring tides) as by waves, the height of which may reach 2 meters in Winter. Besides, the immersed length of the vertical part varies with the tidal elevation from 8 to 15 m. As a result, we have been faced with a great variety - and complexity - of loading cases. The Reynolds Number has a maximum value of about 3.105.from a mechanical point of view:The weld fillet is placed in a representative corrosive environment, and submitted to a true nonstationnary wide-band random loading.Besides, the overall loading spectrum has proved to have a mean frequency high enough to ensure a time compression of 10 to 1 (Le. 108 cycles in less than two years).

The design conditions were estimated as follows:

(Available in full paper)

and the pile has been designed using Morison's formula. The inertia terms are negligible in the design conditions for an outer diameter of: D = 18 cm.
A Proposal For The Permanent Disposal Of Nuclear And Other Toxic Wastes Utilizing Offshore Technology
Authors: Krutenat Robert A.
Published: October 2008

This paper is a proposal to apply the capabilities of offshore technology to a new area-nuclear waste disposal. Permanent disposal is accomplished by placing the wastes in boreholes drilled, for that purpose, in the basaltic basement rock of the oceanic plates. The boreholes are drilled at the very edge of the subduction zone allowing the descending tectonic plate to carry the wastes into the center of the earth. Sufficient distance of subducting oceanic plate exists west of Washington and Oregon and south of the Aleutian chain, to accommodate the waste output of the United States for several hundred years (assume boreholes 0.3m diameter, 1,000m deep on 50m centers). Waste disposal in the ocean is not new. The unique aspect of this proposal is the location of the boreholes in the subducting basement rock allowing the descending plate to carry the waste into the molten core of the earth. Several hardware configurations are suggested, with appropriate sketches, to implement the proposal. The annual cost for disposal using this method is estimated to be 0.072mils/kWh (nuclear) or $3.74 million per 1,000 MW "standard" reactor.

INTRODUCTION

The disposal of radioactive wastes from nuclear reactors and other activities of atomic energy research, is a problem facing world society. This problem arises from the long half-life of the radioactive waste products and the toxicity of the chemical compounds in which the radioactive material is found. The magnitude of the problem can be expressed as follows: About 2.23m3 of solid waste are produced by a "standard" 1,000 MW reactor each year.l Five hundred such reactors would produce approximately 1,115m3 of solid waste each year.

"Plutonium 239 has a half-life of nearly 25,000 years and 10 half-lives are required to cut the radioactivity by a factor of 1,000. Thus the buried wastes must be kept out of the biosphere for 250,000 years."2 The problem is: Where can the material be located so that it is impossible for it to find its way back into the environment?

Early in 1960, geophysicists realized that the crust of the earth was formed of large solid plates moving relative to each other.3 These large basaltic plates are formed at ocean ridges and return to the center of the earth at places known as "subduction" zones.4 The plates re-enter the earth at about 5 to 6 centimeters per year.5 Material contained in the basement rock, as well as most surface sediment, is being constantly returned to the center of the earth.6.

The solution to the problem is: Place the waste material in the basement rock of-an-oceanic plate at the edge of its subduction zone and allow the crustal movement to carry the waste to the of the earth. About 58 million years are required for a one-way trip.7.
Deepwater Pipelaying Method With A New Welding Technique And J-Curved Pipestring
Authors: Koenig D. , Muesch H.
Published: October 2008

Extensive studies have shown that the J-method of offshore pipelaying can substantially increase the water depth capability for 20 inch pipe to about 3, 000 m (9,850 ft) or for 40 inch pipe to 1,200 m (3,950 ft). This achievement is not presently possible with conventional S-method pipelaying. This method was rendered feasible by the development of a fast, new flash butt welding technique.

The entire J-pipe system is described and the technical practicability and potentialities for pipelaying are reported.

Extensive welding tests have been performed and more are in progress. The tests have shown that the improved flash butt welding technique can produce quality welds meeting the requirements of offshore laying. The prototype welding machine is almost complete and further test series are scheduled.

INTRODUCTION

In the first half of the seventies there has been a substantial increase in efforts toward finding technically and economically acceptable solutions for deep water pipelaying.

On one hand there are drilling activities in deeper and deeper waters. In 1974/75 drillers reached water depths of 600 to 700 m (2,000 to 2,300 ft). Pipelaying techniques are still relatively undeveloped for those water depths. In spite of decreased drilling activities in deep water, development is continuing. The 2,000 m (6,500ft) mark is forthcoming.

On the other hand the problem of transporting oil and gas across deepwater areas exists. Best examples are pipelines for the Norwegian Trench and the Mediterranian Sea. The utilization of that gas found offshore or onshore - without a pipeline is possible only by shipping the gas to consumers in a processed condition, as methanol, LNG, etc.

In 1975 a group of German companies commenced a deepwater pipelaying research and development programme. Many concepts for deepwater pipelining were assembled and evaluated. Special attention was paid to economic aspects, to the state of art, to experience with various pipelay techniques, and to the availability of essential technology in consideration of risks and time for research and development.

The most promising method J-pipelaying with flash butt welding - was singled out and further developed. This paper describes results from this development programme.

ARGUMENTS FOR J-CURVED PIPESTRING LAYING

Offshore pipelaying methods differ in the procedure for lowering the pipestring and in the method of connecting new sections. If the pipeline is lowered from the surface, the suspended pipe span forms a J-curve or an S-curve. If a pipestring is constructed by the bottom tow method, there is no such curve. The last method requires an on-bottom connection, the first and second mentioned methods can be performed by (1) an on-bottom, (2) a floating (i. e. between bottom and surface) or (3) a surface connection Many technical limits of the pipelay methods refer to these aspects. Other limits are economic ones.
Determination Of The Period Range Associated To The Design Wave
Authors: Arhan M.F. , Cavanie A.G. , Ezraty R.S.
Published: October 2008

Description of the paper

This paper presents two new methods of determination of the range of periods associated to the design wave. The first is based on the analysis of the extreme wave for each storm recorded, the second on an adaptation of our individual wave height-wave period joint distribution previously published. Data used in this work came from U.K.O.O.A. waverider recordings in the northern North Sea for values Of H1/3 greater than 6 m. A total of 340 hours of sea surface recordings collected from 21 major storms at point Stevenson over a three years period were analysed by the zero upcrossing method.

Applications

The methods developed are adapted to determine the range of periods to be associated to a given design wave height, once given sea state data over a reasonable period of time.

Results, observations, conclusions

In view of the nature of severe storms, a strong statistical relationship is observed between H1/3 and Tz.u.c at the storm peaks. For a given H 1/3, variations in Tz.u.c from one storm to another and variations in periods of individual waves for a given height in a given sea state both contribute significantly to the width of the period range for the design wave. Continuous zero upcrossing analysis over each storm gives the highest waves encountered from which can be determine a 14 to 17 second range for the period of a 30 m design wave. When an appropriate choice of the spectral width parameter, evaluated by comparison with data, is used in the theoretical individual wave height-wave period joint distribution a probability distribution for the period of the design wave is obtained. This second method, which depends only on mean sea state parameters (H 1/3, Tz.u.c) leads to results quite similar to those of the first method.

INTRODUCTION

The period to be associated to the design wave (fifty or one hundred year wave height) can be fixed only within a certain period band, because within a given sea state there is a continuous distribution of periods for a given height and because different meteorological conditions will lead to sea states having similar extreme wave heights but different associated periods. At the present time, in the North Sea, for a design wave close to 30 m in height, a range of periods between 12 and 18 seconds is advocated. Within the climatology working group of A.R.A.E. (Association de Recherche Action des Elements) we have sought for methods in order to determine this range of periods objectively and have applied them to data concerning the most severe storms at point Stevenson (61°20' N, 0°0' E), North East of the Shetlands over a three winter period (1973-1976). We are indebted to the U.K.O. O.A. (United Kingdom Offshore Operators Association) which kindly placed this data at our disposal for this study.
New Underwater Threshold Of The 1980S, Demands New Answers From Underwater Contractors
Authors: Galerne Andre
Published: October 2008

This discussion analyzes present and projected trends in underwater contracting, as related to exploratory drilling in constantly deeper waters in the quest for new fossile fuel sources. Its primary focus is on the problems facing diving contractors who must be careful in their efforts to survive and prosper in an industry that is expanding in all directions underwater, while new sub-sea technology has tended, as of late, to remain moot.

Three basic modes of underwater diving service - hyperbaric; monobaric; and remote control - are examined. Specifically, this analysis is aimed at the problems associated with supporting exploratory drilling in waters beyond 1500 feet deep, the depth at which anchors have to be abandoned in favor of dynamically positioned vessels.

A threshold has been reached comparable to the 200 foot watershed met in the early 1960s, which was overcome by the introduction of mixed gas bounce dives to 600 feet for short term projects, and saturation diving practices which are today used on long term projects to depths of over 1000 feet. Vital work beyond 1200 to 1500 feet, the author contends, will have to be carried out either by tethered unmanned vehicles compatibly designed to work on deep sea structures; by manned monobaric diving suits or bells; or more likely, by free swimming submersibles.

An analysis of present methods of under water intervention is given, followed by an explanation of the role free swimming submersibles will probably play in the future as an adjunct to deep sea drilling techniques. It concludes that manned submersibles will in effect be safer, with far greater operational capabilities, than either monobaric diving suits or tethered unmanned vehicles. It also notes that the problems of safety in deep water offshore operations may be the determining factor in resolving which options will be selected by oil companies, as governments, under whose jurisdiction they operate, move to ensure that diving accidents are kept to a minimum.

HISTORY

Two decades ago man's ingress into the underwater environment, although centuries old, was still in its infancy. Working dives to depths beyond 200 feet were rare and carried out commercially only on compressed air, mixed gas still being the domain of the united States Navy. Despite the fact the deepest known depths of the oceans had been plumbed by the bathyscaphe Trieste, mid and shallow water submersibles were just beginning to appear and none were being built for commercial application. When in 1961 a young Swiss mathematician, Hannes Keller, irrefutably demonstrated by going to a 1000 feet (in a hyperbaric chamber) that the depth barrier for man breathing an artificial atmosphere was far beyond what many experts deemed the absolute limit, there were still those who claimed manned intervention to such depths would never be practical, nor would there be justifiable application.
Tests On The Wave Power Generator "Kaimei"
Authors: Miyazaki T. , Masuda Y.
Published: October 2008

1. INTRODUCTION

Although the possibility of utilizing wave energy has long been considered, the only wave power generator put to practical use in the world today is a navigation lite buoy. Thus, the utilization of wave energy is still limited to a small scale. The Japan Marine Science and Technology Center (JAMSTEC) has been carrying out a five-year developmental research project on a large-output wave power generator since 1976. Since the objective of the research is the conversion of wave energy into electricity, a wave power generator, " Kaimei", was constructed in order to perform two stages open-sea tests during two years. From these tests, technical data on the wave power generation system was obtained and will contribute to the realization of actual harnessing of wave power. Below is a portion of the data from the open-sea tests.

2. PRINCIPLE OF CONVERTING WAVE POWER INTO ELECTRICAL POWER

Various applicable methods for the conversion of wave power into electrical power have been proposed by experts in the world. However, JAMSTEC has been concentrating its efforts on the research of a wave power generator employing air turbines. In this type of generator, a bottomless box is floated on the surface of the sea, and an entering or exiting flow of air is actuated by the upward and downward movement of the water in the box. An air turbine driving a generator are installed on the box and utilize these air currents, to generate electricity.

After various types of model tests, the steelmade 80-m long, l2-m wide Kaimei was constructed. The body of the Kaimei consists of four buoyancy rooms and a number of air pump rooms where wave energy is converted into air flow. The following are its characteristics.As a floating structure, the Kaimei is moored with chain and anchors.Since it is a ship-like buoy, always heading to the waves in order to reduce mooring force.The model tests have proven that wave energy can be absorbed sufficiently, even when the waves are received on the bow.The absorption of wave energy is related to the wave length, and maximum efficiency is achieved when wave length is 8O% of the buoy length.Calculation of the hull strength is facilitated by the design of the Kaimei as a ship-shape structure.There was no observable affect on output caused by differences in mooring systems.No mobile apparatus were installed at areas exposed in the water.The slow movements of the waves are transformed into high-speed movements by the use of air turbines.The energy converting apparatus does not directly receive the shocks from the waves.The Kaimei is not affected by the marine fouling to its surface.

3. OPEN-SEA TESTS

The open-sea tests were performed twice. The first tests, from August 1978 to May 1979, employed three generator units. The second, from August 1979, is a joint research project involving five countries as part of an IEA undertaking.
Fatigue Life Prediction Of Tubular Nodes
Authors: Lieurade Henri Paul , Gerald Jean P. , Putot Claude J.
Published: October 2008

Large scale specimens of X-joints were tested in fatigue, either in tension or in in-plane bending (R=O.1). Similar specimens (size ratio 1/2) were tested in order to estimate the scale effect. y was chosen here from 8 to 12, at the limit of the validity of any shell-theory. In each node, one brace joint was heat-treated and the other was left as welded. The specimens were instrumented by strain gauges.

During monotonic static loading, the measured distribution of stresses shows that the Effective stress concentration factor KE can be defined as a product of two factors KE =KG × KL, where KG reflects the Global geometry effect of the intersection of the two shells and KL the influence of the Local shape at the weld toe.

During the fatigue test, the crack growth and the evolution of strains along the weld-toe were monitored. On the basis of the obtained results an elementary theoretical model for life prediction is proposed which takes into account the stress redistribution due to the crack growth.

INTRODUCTION

The fatigue analysis of structures in general and offshore structures in particular is based on the comparison of a design stress with a specified S-N curve. This curve is usually obtained as the most conservative envelope of a constellation of dispersed points, corresponding to fatigue results obtained on different small-size and large-size models, put in a more or less clearly defined S-N coordinate system. The recent progress in fracture mechanics should allow a more rational approach to the definition both of the design stress (S) and of the fatigue life (N).

As a part of a research sponsored by the E CS C (European Coal and steel Community) the IRSID (French Steelmaker Research Institute) has tested ten X joints. In parallel, a French programme of theoretical and numerical studies was conducted at the Ecole Polytechnique in order to give a basis for the interpretation of test results, which would ensure more reliable design data for engineers.

The present paper gives an account of a part of this research. After a brief survey of the test programme, an approach to the definition of the design stress is presented in § 3 and an elementary model of the fatigue crack growth in tubular joints is proposed in § 5, which may help a more rational discussion of S-N curves.

TESTING DEVICES

Test specimens

The test specimens consisted of large size welded X-joints, represented in Fig. 1. The average values of actual dimensions of chords and braces, in particular the tube thicknesses, as well as the values of the usual non-dimensional ratios and the types of loading are given in Table 1.

The specimens were made from grade E 36 steel plate, according to AF NOR Standard. They were fabricated by a manufacturer familiar with offshore structures according to a specification which is typical of that currently used in North Sea practice. Weld size and configuration were chosen in accordance with API codes.
Wind And Wave Measurements From The First Year Of Drilling In The Baltimore Canyon
Authors: Forristall George Z.
Published: October 2008

From April 1978 through February 1979, wind and wave measurements were made from the Western Pacesetter II while it was drilling in the Baltimore Canyon area approximately 100 miles off the coast of New Jersey. An anemometer was mounted at the top of the drilling derrick and a Wave rider buoy was moored nearby. Approximately 20 minutes of data each three hours were recorded by a microprocessor based digital recording system onto cassette tapes. The data has been processed to yield average wind speed and direction, significant wave heights, and wave spectra. Seasonal and summary statistics have been developed which show, for instance, that a significant wave height of ten feet was exceeded 8.5 percent of the year. The highest measured significant wave height during the year was 19.5 feet. These statistics should be useful in planning future operations in the area. Study of the high frequency portions of the spectra revealed that over a substantial range, it was inversely proportional to the fourth power of the frequency and directly proportional to the wind friction velocity. The classical inverse fifth power equilibrium range appeared only for frequencies substantially above that of the spectral peak. Using these observations, it was possible to produce a consistent representation of the high frequency tail of the spectrum which should be useful for developing criteria for structural fatigue studies.

INTRODUCTION

During 1978 and the early part of 1979, Shell was one of a number of operators who tested leases which had been acquired in the Baltimore Canyon area off the coast of New Jersey (Figure 1). The drilling was done from the Western Pacesetter II. Since the entire east coast of the United States is essentially a frontier area for offshore drilling, additional environmental data on the area is very useful for planning future activities. Thus, in addition to participating in the joint industry instrumentation project described by Rose et al. (1979), we decided to make wind and wave measurements from the Pacesetter II. Since most of the instrumentation needed was already on hand, it was possible to start making measurements in April, shortly after the rig moved on site.

Because of the simplicity of the instrumentation, a reasonably complete data set was collected even though the system was operated without regular service checks. The measurements add significantly to the marine climatology data base for the area, as well as providing some interesting information on the characteristics of the high frequency tail of the wave spectrum.

We begin by discussing the instrumentation and its installation on the Pacesetter II. Our operating experience with this system may help others plan similar measurement programs. The data was first processed to yield histories of the significant wave height and wind speed and direction. From these histories, we calculated seasonal and summary exceedance statistics.
Kinematics Of Deep Water Breaking Waves
Authors: Kjeldsen Søren P. , Vinje Tor P. , Myrhaug Dag P. , Brdvig Per P.
Published: October 2008

Little is known about wave forces caused by extreme breaking waves in deep water. This study presents certain results from simulation of the kinematics of plunging breakers in deep water obtained both from carefully controlled laboratory experiments and from numerical simulation. In the experiments extreme waves are generated in three ways:The dispersion properties of gravity waves are used to generate a transient situation in which the energy of the individual waves in a wave train concentrates towards a specified point in time and space, resulting in one extreme wave;Very steep Stokes waves of finite amplitude are forced to break at a selected point in deep water due to superposition of a subharmonic disturbance;Very steep Stokes waves are forced to break at a selected point due to superposition of' a superharmonic disturbance.

In the numerical model, the development of the (breaking) wave profile, the particle velocities and the accelerations within the wave are simulated. We exact free-surface conditions. are satisfied, and good results are achieved up to the point where the overhanging crest hits the wave front.

Comparison of the plunging breakers generated in the wave flume and calculated numerically, shows good agreement with respect to the wave geometry. The measured and calculated velocities also agree well at depths larger than approximately one wave amplitude below the mean water level. In the wave crest near the plunging jet, the measured velocities are, however, 1.5 times as large as the calculated velocities. This may be due to the difference in the method of generating the plunging breakers in the two cases.

Experiments so far have now confirmed that transient, near breaking and breaking waves attain velocities up to 2.8 times the first order phase velocity.

The results may be applied to the operation and evaluation of stability of ships under severe sea conditions. Further, they provide the basis for the estimation of wave drag and inertia forces from extreme breaking waves in deep water with many applications for the design and safety of offshore structures.

INTRODUCTION

In order to evaluate safety at sea, a basic need is a prediction of the characteristics of extreme sea states. There is an extensive literature on the subject of the' mysterious disappearances of ships, both large and small. A high percentage of these mysteries can be solved by even a cursory study of so-called "freak waves" (a freak being "capricious change"), which are more correctly termed episodic waves. By definition an episode is a recurring event, and so it is with huge waves on the continental shelves. Episodic waves occur in virtually all such areas during certain predictable times of the year. The origin of episodic waves is not fully understood. It is believed that a shoaling mechanism, unique to a certain geographic location as well as a particular random phase relationship between waves, can account for the phenomenon.