bluuebunny/crossref_metadata_2025 huggingface dataset:
Signature Measurements On The Water Gun Marine Seismig Source
Authors: French W.S. , Henson C.G.
Published: October 2008

Source signature data for the water-gun marine seismic source were collected 45 miles south of Grand Isle, La., in water depths of 1,800 ft. Signatures were digitally recorded using calibrated near-field and far-field hydrophones for both a single water gun and a pair of interacting water guns. Single-gun tests were recorded for all combinations of the following two parameters: firing pressures of 2,400, 2,000, 1,800, and 1,000 psi; firing depths of 10, 20, 27, 40, and 80 ft. Gun interaction tests were conducted at 10-ft and 20-ft depths with a firing pressure of 2, 000 psi and gun separation of 4, 8, 12, and 20 ft. In addition, repeatability tests were conducted. The experimental procedures and test results are presented.

INTRODUCTION

The water-gun marine seismic source generates an acoustic signal by means of an implosive mechanism rather than an explosive mechanism. The following characteristics are claimed by the inventor of the water gun (SODERA-Societe pour le development de la Recherche Applique, Toulon, France). (1) The source is a purely implosive device and the bubble pulse effect characteristic of explosive sources is eliminated; (2) the output spectrum is expanded toward higher frequency relative to an air gun while maintaining an output power comparable to an air gun; and (3) the water gun can be interfaced with the same onboard equipment as used for air guns. The above claims quite naturally catch the attention of those of us interested in petroleum exploration by the seismic reflection method. As a result, Amoco, Geophysical Service, Inc. (GSI), and SODERA conducted a joint test to measure the signature characteristics of both a single water gun and pair of interacting water guns. Mica T water guns with 80-cu in. air consumption were used. The tests were conducted in July 1976 aboard a conventionally rigged seismic reflection vessel in about 1,800 ft of water in the Gulf of Mexico.

Fig. 1 shows the operation of the water gun. The water gun is constructed much like an air gun except that one of the chambers is filled with water prior to firing. Upon firing, compressed air accelerates a shuttle through the water-filled chamber, ejecting the water at high velocity through exit ports. The shuttle stops instantly upon reaching the exit ports, while the ejected water continues to move away creating a cavity. The cavity increases in size until the outward velocity of the ejected water is reduced to zero by hydrostatic pressure. Sea water then collapses implosively into the cavity creating the acoustic signal. With the water gun, no air is released into the water to produce subsequent bubble oscillations.

Pressure signatures radiated by the water guns were measured for different firing pressures and firing depths. The single-gun tests were recorded for all combinations of the following two parameters: firing pressures of 2,400, 2,000, 1,800, and 1,000 psi; firing depths of 10, 20, 27, 40, and 80 ft. Signatures from two interacting water guns also were Measured.
Model Tests On Continuous Line Bucket Mining System
Authors: Yamakado Norio , Handa Keiji , Usami Takeshi
Published: October 2008

Authors have been studying the continuous line bucket (CLB) mining system, which is one of the mining methods to mine manganese nodules from deep sea floors, mainly through the model tests in an indoor test tank.

According to the results of the indoor tests, very small-scale field tests of the CLB mining system were carried out in depths of 50 to 8O m to pick up problems due to natural phenomena such as currents, winds, and waves and to understand an actual CLB mining system affected by these phenomena.

It is significant to circulate the loop line, which hangs many buckets, between the sea floor and the ship smoothly at the operation of the CLB mining system. The traction machine using umbrella-type wheels was adopted for these field tests.

As a result, satisfactorily smooth circulation of the loop line, which hung 75 buckets (about 1,500 cc in volume each), was obtained. It becomes clear that smooth circulation of loop line results in stable operation without the rope being tangled even when the ship alters her course. However, it may be entangled when the circulation is not smooth.

In order to avoid tangling the rope, some experiments on the method not to reduce the span between the descending and ascending lines were examined in these field tests. A continuous monitoring for behavioral patterns of buckets and rope line on the sea floor also was tried by using a pinger.

INTRODUCTION

Authors have been studying model tests of the continuous line bucket (CLB) mining system in order to have a deeper understanding of this system as one of the mining methods used for recovering manganese nodules found distributed over the deep sea floors. The model tests mostly were conducted in an indoor test tank. In such tests, however, there are limits to detection of the problem area resulting from alteration of natural phenomena such as currents, winds, and waves. For further development of the CLB mining system, it should be considered important that we find solutions for problems that surfaced on the basis of the experimental results obtained from field tests.

In this context, we conducted small-scale field tests on the applicability of the CLB mining system in Sagami Bay in water depths of 50 to 8O m.

In such field tests, the bucket behavior on the sea floor cannot be observed as in laboratory tests. It will have to be an indirect observation through the use of some measuring instruments. When a field test was conducted in 1975 at a site with 430 m of water depth, a Bucket Behavior Recorder, which was developed for this particular field test, was used to measure the dredging path of the bucket and the shape of the dredging line took on the sea floor. The data obtained by this Bucket Behavior Recorder were almost identical to those already obtained through laboratory tests.
Early Subsea Production Systems Advance The State-Of-The Art
Authors: Van Bilderbeek Ben , Reimert L.E.
Published: October 2008

Hydrocarbons have been found offshore in considerable quantity and quality. Whether this increasingly scarce commodity can be exploited to extend the age of fossil fuels depends on the development of offshore production systems.

During the summer of 1977, a modular template for a five-well early subsea production system was built in Aberdeen, Scotland. Shell Oil has purchased this system for installation in the North Sea. The system was designed eventually to be tied back and produced to a fixed platform.

A different type early subsea production (ESP) system, one using a unitized template, was employed by Petrobras as part of the Garoupa field development in the early part of 1978. Of particular interest is the unique 20-in. pile leveling system, which allows placement of the template onto the ocean floor in a horizontal position. As was the case for the Shell ESP system, these template wells also will be individually tied back to a fixed platform structure.

Various ESP systems have been employed and successfully operated in some of the most hostile offshore environments. One, therefore, can argue that the technological and economic viability of ESP systems has been proved, although general acceptance by a conventionally conservative industry cannot be claimed yet.

The ESP systems, subject of this paper, are specifically designed to render currently marginal fields economically viable and to extend hydrocarbon production into deeper waters. Another application for these systems is to provide cash flow during the early stages of a major field development when permanent production systems are being constructed.

INTRODUCTION

It will be known to most readers that the price per barrel of crude oil continues to rise to ever record levels. It may be less known that the cost of drilling and completion also is rising at astronomical rates.

Several reasons can be considered contributory to the rise in drilling and production costs such as inflation, politics, and, not least, field accessibility. The offshore industry is moving ever further offshore into deeper waters in their quest for finding economically viable hydrocarbon deposits.

The extreme high drilling and production costs for these deep-water offshore fields have prompted the oil industry to investigate alternative production systems that can provide substantial savings in over-all investment and development time required.

One such method has been called the early subsea production (ESP) system. Several variations of this approach are available, having in common the fact that the drilling stage of the field development can commence and continue while the production facility such as a bottom supported platform or a floating production vessel is under construction.

Two distinctly separate groups can be identified within these various systems:the ESP systems utilizing a floating production platform, andthe ESP systems tied back to a fixed or bottom supported production facility.
Test Of Various High Resolution Seismic Devices In Hard Bottom Areas
Authors: des Vallieres T. , des Kuhn H. , des LeMoal R.
Published: October 2008

Research of operational high resolution geophysical methods for offshore soils and sub-bottom survey in a "seismically hard environment" led to a comparison study of a wide variety of shallow seismic instruments available on the market in the summer 1976.

Different seismic source (pingers, air-gun, miniflexichoc, surface or deep towed boomers and sparkers), streamers (mono and multitrace) and recorders (analog and mono or multichannel digital) were tested in glacial deposits off the East Coast of Canada The combination of the selected devices (source, receiver and recorder), led to thirty different data acquisition possibilities.

The on site performances and capabilities of each tool and the subsequent data processing possibilities led to an objective assessment of the different pieces of equipment to be used according to the type of soil encountered and the depth of investigation desired. When necessary, improvements were recommended.

1. INTRODUCTION

Offshore high resolution shallow sub-bottom profiling has always been a problem in the so-called "hard bottom areas" (coral reefs, morainic drifts, cohesive formations).

In these environments, a large amount of the emitted energy is scattered resulting in poor-penetration with conventional high resolution seismic tools.

In 1976, a joint venture between CFP (Compagnie Française des Pétroles), IFP (Institut Français du Pétrole) and SNEA(P) (Societe Nationale Elf-Aquitaine Production) associated in the GERTH (Groupement Europeen de Recherches Technologiques sur les Hydrocarbures) with the EEC (European Economic Community) support, decided to run a survey in such a seismically "hard" environment in order to test and compare the largest possible variety of seismic data acquisition devices and methods. The purpose was not to develop any new system or concept but to operate the existing tools, including experimental devices, available on the market. The tested instruments were basically selected according to the desired investigation depths (from a few metres down to 200 m). Only reflection methods were considered.

The tests were conducted in a morainic environment off the East coast of Canada on an acreage held by the Labrador Group of which Eastcan is operator. The average water depth was between 150 and 200 m. Several zones presenting specific soil conditions were preselected. In each zone the tests were run on the same lines to facilitate the comparison.

2. TESTED DEVICES

The 1976 program allowed the testing of nine seismic sources, five types of streamers and seven different recorders (fig. 1 & 2).

2.1. The sources

The tested sources can be divided into five main types :

Four different sparkers were used :a high powered sparkarray : near surface towed, 3 to 9 electrodes, energy up to 21 kJa multielectrode sparker (MES)(l) : near surface towed "comb" like sparker using an array of 3 sets of 144 electrodes. Power from 1 to 3 kJ.
Shallow Water Drilling With Adeep Water Drillship
Authors: Magill James M. , Macan John J.
Published: October 2008

A working procedure has been developed and successfully used to enable a deepwater drillship to drill in shallow water. The paper describes how by a combination of recessing the wellhead and B.O.P. below the sea floor, in a sea-floor cellar, and by shortening the slip joint the deep water drillship CHANCELLORSVILLE was used to drill two wells in the Gulf Coast, in approx. 55 ft. of water. Experience gained during the first well was used to develop a more efficient procedure which was adopted for the second well.

INTRODUCTION

Until now, offshore drilling in shallow water depths has been greatly dependent upon availability of bottom supported drilling units, principally jack-ups or submersibles.

The concept that a drillship or drill barge employing innovative techniques, could be used economically in place of a jack-up in shallow water was not readily accepted until recently when Samedan Oil Corp. initiated its program to drill in shallow waters.

With the demand for bottom-supported rigs greater than the available supply, AOI proposed to use its drillship CHANCELLORSVILLE with a seafloor cellar as portrayed in Figure 1.

The typical minimum water depth for a drillship has been generally considered to be about 100 ft. This limitation is based upon factors requiring adequate spacing between drill floor and sea floor to physically accommodate the wellhead. BOP, ball joint, and slip joint.

Sufficient length of the riser system from ball joint to drill floor is necessary to allow movement (offset) of the vessel in the horizontal plane resulting from the combined actions of wind, waves, and current. Typical acceptable offset has been about 5% of water depth, or an offset which would limit the ball joint angle to about 5°.

Ball joints are capable of angles up to 10°. However, other considerations restrict allowable to a significantly lesser angle. Passage of drilling tools through the ball joint, and wear at the ball joint as drill pipe is rotated must be considered.

Sufficient vertical stroke of the slip joint should be present to enable the vessel to heave under the action of waves, and also for tidal changes.

Sufficient clearance between the vessel's hull and the BOP fixed to the sea floor should preclude contact when the vessel exhibits motions both vertically and horizontally from wave action.

As shown in Fig. 2, placement of the BOP and slip joint for CHANCELLORSVILLE in the normally designed manner would require a minimum water depth of about 99 ft.

Modification Needed

To work in water depths less than 88 ft. required certain modifications to the normal placement of equipment.

It would be possible to accept reduced water depth by merely setting the slip joint for reduced but acceptable downstroke. For example, 10 ft. downstroke would enable operation in water depths of about 76 ft.
Sea Bed Instability From Waves
Authors: Yamamoto Tokuo
Published: October 2008

The pressures and the effective stresses in seabeds induced by waves are treated analytically based on Biot's three-dimensional consolidation theory. The wave induced pressures and stresses in a bed are strongly influenced by the permeability and the shear modulus of the soil, the compressibility of pore water, and the thickness of the bed. As a design example, a stress analysis is made for the North Sea design wave and seabed conditions. The numerical results indicate that the North Sea design waves will liquefy the top up to 2 m of the sand beds and induce the sliding failures in the top up to 8.0 m of the sand beds. The theory has been verified by a laboratory experiment for the wave induced pressures.

INTRODUCTION

The subject of wave induced pressures and stresses in seabeds is important with regard to the design of foundations for various ocean and near shore structures, such as gravity type breakwaters, offshore oil storage tanks, and drilling rigs like EKOFISH at North Sea. The subject is also important when one considers the problems of the flotation of buried pipelines and the burial of rubble mounds, tetorapods, and other blocks by waves. However, the subject is not well understood because the dynamic behavior of soils is difficult to express mathematically.

When the water waves propagate over a porous bed such as a sand bed, the flow of fluid is induced in the porous medium and the porous medium itself is forced to deform. Thus, the bed response to water waves is actually a combination of fluid and solid mechanical effects.

There have been numerous investigations of the problem of the flow induced in a porous bed by water waves, including Liu (1973), Massel (1976), Moshagen and Tørum (1975), Nakamura, et al. (1973), Putnam (1949), Reid and Kajiura (l957) and Sleath (1970). However, they all assumed that the porous beds are rigid and non deformable. In addition, all except Moshagen and Tørum (1975) and Nakamura, et al. (1973) assumed that the pore fluid is, incompressib1e. The fluid motion in the porous bed is usually expressed by Darcy's law which, with the assumption of a rigid bed with isotropic permeability and incompessible water, leads to the Laplace equation for the pore water pressure. The consequence of this theory is that the pore-water pressure response is independent of the permeability of the bed material. The approach taken by Nakamura, et al. (1973) and Moshagen and Tørum (1975) is based on the assumption that the water is compressible while the porous bed is non deformable which leads to the heat conduction equation for the pore-water pressure. The conclusion from this assumption was that the pore-water pressure response is strongly dependent on the permeability of the bed material. This approach provides no information on the wave induced stresses in seabeds.
Explosive Welding - The Deepwater Pipeline Connection/Repair Solution
Authors: Redshaw P. , Stalker A.W. , Allen K.
Published: October 2008

After briefly describing the physical aspects of explosive welding in general terms, this paper outlines how the underlying principles are applied to the making of pipeline connections under water. Using the same basic weld geometry, two general connection types are available - a pipe flange and a sleeve weld. The associated systems of pipe preparation, flushing and drying, dewatering, welding, and subsequent inspection receive attention, and in all cases the minimal requirement for operator skill is emphasized. Results of mechanical property tests are presented for particular pipe sizes. Finally, practical applications are proposed with emphasis on the suitability of the technique for all water depths.

INTRODUCTION

During the early 1970's, the ability to make planned connections and undertake repairs on pipelines in deep water was seen as one potential problem area, and following a comprehensive examination of a number of alternative joining systems, it was decided that explosive welding offered several positive advantages. It is basically a very simple operation, requiring a minimum of equipment that, once the parameters have been developed for a particular set of circumstances, gives very reproducible results. There is no demand on the manual skills of divers operating in saturation diving conditions and preliminary indications are that the process is relatively insensitive to water depth. All these factors would suggest that explosive welding offers great potential as a pipeline-joining technique that could eventually be a remote operation controlled from a submersible. Thus, in 1973 a work program was initiated to develop an explosive welding technique for pipelines that would offer to the oil industry both a technically acceptable solution and one that would afford a considerable economic advantage over other currently available systems.

This paper describes the joining process that has culminated from this program and that has achieved full process approval from the major classification societies operating in European waters. In addition to the actual welding process itself, there is a range of ancillary engineering operations equally relevant to the success of the over-all operation and these also receive attention.

EXPLOSIVE WELDING TEDHNOLOGY

The process depends on the use of explosives to accelerate metal components into a high-velocity oblique collision. The physics of the process have been amply described elsewhere (Conclusion 4) and will only receive brief attention in this discussion.

In order to achieve a weld, two conditions must be satisfied:the impact pressure must exceed a certain value (at least 10 times the yield stress); andthe collision-point velocity must be subsonic.

The impact pressure depends on the velocity of the flyer pipe, and since the acceleration of the flyer takes a finite time, it is usual to arrange for a gap between the components in order to satisfy the first criterion.
Development Of An Alignment System For The Three-Part COGNAC Platform Jacket
Authors: Piter E.S. , Sterling G.H. , Cox B.E.
Published: October 2008

This paper discusses the system incorporated into the three-part Cognac "A" Platform jacket to guide the sections into lateral alignment as they were positioned offshore. A brief description of the alignment appurtenances and data on the in-place misalignment are given.

INTRODUCTION

During the early phases of the Cognac design, the decision was made to fabricate the Cognac Platform in three separate sections (Reference 1 and 2), (Figures 1, 2, and 3=Available in full paper). The Base Section was launched from a barge in an upright position. It was then lowered to the seafloor and anchored by 24 large diameter skirt piling (Reference 3). This section was 178½ feet in height measured from the centerline of the bottom horizontal to the top of the upward looking mating flanges.

The Mid Section was fabricated and launched in a horizontal position, ballasted to vertical, lowered and set on top of the Base Section. This brought the total height of the jacket to 498½ feet. The Top Section, which was 527½ feet in height, was fabricated and installed similar to the midsection and extended the platform so that the jacket :walkway was 14 feet above the waterline.

The primary mating components were the ten jacket legs through which large diameter tubular connection pins were installed and grouted in place (Figure 4). These pins extended through all three jacket sections and terminate just above the mudline at the bottom of the Base Section.

Two of the jacket legs (Rows XL and X4) terminate at approximately 400 feet below the waterline. The other eight legs extend above the waterline.

At each mating interface, the jacket legs terminate with large mating flanges that contain a rubber seal for containment of the grout as it was pumped into the jacket leg/pin annulus.

Other items which were fabricated as an integral part of each jacket section, and had to line-up after installation, included eleven pipeline pulltube risers, sixty straight conductor pipe guides and two curved conductor pipe sleeve guides. The proper alignment of these items was not necessary to insure the structural integrity of the platform; however, each of them could be utilized only if properly aligned.

Precepts to be followed during the design and selection of the jacket alignment system were developed and alternate alignment systems were studied. Relative cost comparisons were made, taking into account the impact on the construction schedule that each system would have.

The two most important functions of the alignment system were:To provide a guiding system that would allow the upper jacket section to be set down upon the landing supports of the lower section and released from the lowering lines without structural damage resulting from an underwater collision between the mating jacket sections.To provide a method of aligning the corresponding jacket legs of the mating sections to an accuracy that would allow installation of the leg connection pins.
Permafrost Beneath The Beaufort Sea: Near Prudhoe Bay, Alaska
Authors: Sellmann P.V. , Chamberlain E.J.
Published: October 2008

The occurrence and properties of subsea permafrost near Prudhoe Bay, Alaska, were investigated by drilling and probing. Nine holes were drilled and 27 sites were probed with a cone penetrometer. The deepest drill hole was 65.1 m below the seabed, while a depth of 14.1 m was reached with the cone penetrometer. Engineering and chemical properties were determined from core samples and point penetration resistance data were obtained with the penetrometer. Thermal profiles were acquired at both the drill and probe sites.

Temperatures below 0°C were observed in all the drill and penetrometer holes logged, although frozen sediments were encountered only occasionally. Seasonally frozen sediments were observed near the seabed at each site. The degree of ice bonding, or strength, could be related to seabed temperature and was greatest in shallow water (<2 m). The penetrometer resistance and thermal data indicated that deeper ice-bonded sediments occur, for example approximately 12.7 m below the seabed in 2 m of water off the Sagavanirktok delta.

Of eight holes drilled offshore, it appeared that four encountered bonded permafrost. In general, the position of the ice-bonded permafrost interface was extremely irregular. The depth below the seabed to this interface at various distances from shore along the line studied was 28.8 m at 1 km, 65.1 m at 3.5 km. 44.1 m at 6.8 km, and 29.5 m at 17.2 km.

Shallow, overconsolidated marine sediments were found in the upper fine-grained section at all of the drill sites investigated; the degree of overconsolidation varied considerably among the sites. This fine-grained section was up to 10 m thick and covered sands and coarse gravels.

INTRODUCTION

Scientific and engineering investigations conducted by several nations bordering the Arctic Basin have documented the existence of subsea permafrost. Many questions concerning distribution, and particularly the properties, of these perennially frozen sediments still remain.

The earliest quantitative studies commenced around 1953 in the USSR. The Soviet investigations have continued, with emphasis on field observation and theoretical studies.1

Few studies were conducted in North America prior to the extensive activity stimulated by petroleum exploration in the arctic regions. Early direct observations and theoretical models of subsea permafrost by Lachenbruch and others2,3 were based on studies made near Barrow, Alaska. Lewellen4,5 followed these investigations with drilling and probe studies in the same region.

In the Canadian Beaufort Sea, probably the first evidence of permafrost and ice-bonded sediment was obtained during an Arctic Petroleum Operators Association drilling program.6 Shearer et al.7 observed the occurrence of bingo-like features on the outer shelf. Hunter et al. followed these studies with an extensive investigation of the distribution of ice-bonded permafrost. These studies, which were largely based on examination of seismic records, showed extensive subsea permafrost.
An Assessment Of Grouting Materials, Placement Methods, And Monitoring Equipment For Offshore Structures
Authors: Callis C. , Knox C. , Sutton D. , Wiley S.
Published: October 2008

Grout placement techniques, technology of grouting materials, and methods for monitoring grout quality and grout placement are presented.

Placement methods using new generation high pressure packers and inflation-grout manifolds are discussed. High pressure packers have made single stage grouting jobs a practical reality. Reliable inflation-grout manifolds have eliminated many lines previously needed for packer inflation and placement of grouting materials.

In addition to traditional grouting materials, new materials to promote high bond strength, control self-desiccation, and minimize shrinkage are discussed. Grouts are classified according to use and typical examples are given. Methods and equipment for controlling grout density and grout placement are included.

INTRODUCTION

Platform grouting is the process of placing a cementitious material into the annulus between a jacket leg and pile. This operation has advanced to the point now that an operator has a variety of methods, materials, and tools from which to choose. For example, at least five different methods for grouting platform legs are commonly used with variations existing within each method. Terms for each of these methods, although not standard, might be (1) conventional two stage, (2) packer, (3) balanced pressure, (4) two stage delayed set for insert piles, and (5) two stage inner string for insert piles.

Recent advances in methods, materials, and tools have helped make platform grouting more trouble-free and dependable. This paper will review these advances and provide an overview of current platform grouting in general.

GROUTING METHODS

Conventional Two Stage

As shown in Fig. 1, this method usually involves a mud wiper seal at the bottom of the annulus. This serves to help reduce mud contamination of the annulus as the pile is driven through the jacket leg and it also helps support the first stage of the grout. This technique normally involves running two separate grout lines from the surface. The first stage line enters the annulus just above the mud wiper seal. Second stage line enters the annulus 5 to 30 ft above the first stage grout line depending on the length of plug required. First stage grout is normally a quickset type and fills up to the second stage grout line entry level. While the first stage is setting, water should be circulated through the second stage grout line to displace any grout that may have covered the second stage port. Once the first stage grout is set, the second stage is injected to the desired height in the annulus.

An improvement to this procedure, shown in Fig. 2, makes it possible to accomplish the grouting through only one grout line from the surface. One line is run from the surface to the lower grout inlet. A pipe is run from the upper grout inlet to a sleeve-type flow control valve installed in the grout line at the same elevation as the upper grout port.
OTEC Cold Water Pipe Program Status
Authors: McGuinness Terence , Scotti Richard S.
Published: October 2008

In this paper, the results and current work efforts of the Ocean Thermal Energy Conversion (OTEC) Cold Water Pipe (CNP) program will be presented. The program, sponsored by the Department of Energy and developed and technically managed by the National Oceanic and Atmospheric Administration, is one of the largest civil ocean engineering activities underway at the present time. The uniqueness of the OTEC GNP is derived from its physical magnitude, namely, pipes that are being designed with a diameter of 9 meters" and a length of 1000 meters. Details of the three phases of the program: the analytical procedures used to predict pipe responses, the model test projects and the large scale at-sea tests, will be discussed. The procedures used in planning the program will be highlighted. Problems and their solutions will be specified. Projects currently underway or just completed will be synopsized.

INTRODUCTION

The cold water pipes (GNP) required in Ocean Thermal Energy Conversion (OTEC) Plants present a formidable challenge to ocean engineers and marine constructors. The GNP size alone is impressive: 1000 meters in length, 9 meters to 30 meters in diameter depending on the OTEC plant size. The magnitude of the pipe forces and the pipe structural responses, plus pipe development and deployment procedures, pose additional problems for the designer. The uniqueness of the GNP as a marine structure, however, lies not so much in its unprecedented mass as in its need to be a flexible structure which interacts dynamically with the hostile ocean environment. This factor amplifies the concern for structural fatigue to the point where only abnormally low stress levels can be tolerated and factors of safety based on stresses alone become meaningless. There is only one GNP in an OTEC plant, which must be designed to survive all environmental conditions for the duration of the plant, namely, 30 years, and not be damaged nor cause damage to the other parts of the OTEC plant.

A federal program has been formulated over the past four years, directed toward the solution of the major GNP problems. The end products of the program will be designs, data and procedures that can be utilized in the final design and development of several viable alternative GNP concepts.

In this paper the current status of the CWP program will be presented. The program formulation will be discussed and the major elements of the program will be explained.

PROGRAM OUTLINE

The original concept of an OTEC GNP was that it was a simple vertical tube appendage attached rigidly to an OTEC platform. By making the pipe large in diameter, the cold water flow could be limited to 2 meters/sec to reduce pipe head losses to a minimum. To ensure pipe survivability, pipe concepts were made rigid from such materials as concrete and steel.
On The Accuracy Of Different Finite Element Types For The Analysis Of Complex, Welded Tubular Joints
Authors: Hoffman Robert E. , Sharifi Parviz E.
Published: October 2008

There has recently been great interest in the use of three-dimensional finite elements to study local stresses in the weld areas of complex (reinforced, overlapped) tubular joints. This paper examines displacement and stress solutions obtained for selected tubular joint configurations modeled with two such thick shell and brick elements. These solutions are compared, with the aid of an a-posteriori convergence (error) criteria, to those from a well known arid widely used thin shell element. It is concluded that the three-dimensional elements examined can behave very well in typical tubular joint problems. They, however, appear to suffer more instability in irregular mesh situations than do thin shell. The proposed error criteria seems to indicate well those areas where the thin shell approximation is most suspect.

INTRODUCTION

For many years thin shell theory has served as a basis for studying stress concentration factors of tubular joints. With various thin shell finite element types, detailed parametric studies have been performed in the hope of obtaining information consistent enough to establish design curves. It is not the purpose here to review this great volume of past work, but instead to compare in the tubular joint context the behavior of one well established thin shell element with two versions of publicly available three-dimensional elements.

The approach selected is to first discuss the three-dimensional element formulations outlining their salient features. The results of a small test problem are then examined and observations made. An overview of an element residua}C basis for comparing solutions of predominately thin shell problems is presented in the section ERROR ANALYSIS. A short discussion on its potential for application to of three problems solved with both thick (compatible and incompatible) and thin shell elements are compared from the displacement, stress and residual error standpoints.

ELEMENT DESCRIPTIONS

In this study, two types of three-dimensional isoparametric solid elements are used. One is an eight noded "brick" element. The other is a sixteen noded "thick-shell" element. As described in the following section, various "incompatible" modes are introduced into the stiffness formulations of these elements to improve their flexural behavior. Both the basic (compatible) and the incompatible versons of these elements are utilized in this study.

Element Formulations

The 8 and 16 noded isoparametric elements under consideration are shown in Figure 1. The global Cartesian coordinates are denoted as (X1, X2, X3) and the element local coordinates as (available in full paper), both right handed systems. The local coordinates range from -1 to +1 with (0,0,0) located at the centroid of the element. In the isoparametric family of elements, the local and global coordinates are related as follows (see for example Reference [1]):

(Available in full paper)
Fatigue Strength Of Tubular T- And X-Joints
Authors: Dijkstra O.D. , de Back J.
Published: October 2008

The results of fatigue tests on 40 tubular T and X-joints are presented. The chord diameters of the joints are 168, 457 and 914 mm. The most important conclusions are that the lifetime decreases with increasing joint size and that in the high cycle rance (N ? 107) some test results of the large joints fall below the AWS-X curve.

INTRODUCTION

The weather conditions which induce a fatigue crack are more severe in the North Sea than in the Gu1f of Mexico, Because of these severe conditions, a research project, sponsored by the European Coal and Steel Community (ECSC) on the fatigue behaviour of steel in offshore structures, entitled "Fatigue and corrosion fatigue behaviour of offshore steel structures", has been carried out. The research has been carried out in the U.K., France, Germany, Italy and the Netherlands. There is a special cooperation between Norway and the participating ECSC countries. In the programme, flat plate specimens and tubular joints have been tested. In addition, theoretical work (e.g. finite element analysis on tubular joints) has also been carried out. An interim report of the program was presented at a siminar in November 1978 in Cambridge.

In this paper, the results of tests on tubular joints, carried out in the Netherlands under the sponsorship of the ECSC, are discussed. Also, the results of additional work (strain measurements, crack measurements and calculations) sponsored by MaTS (Netherlands Marine Technological Research) are discussed.

TEST PROGRAMME

Forty tests on tubular joints were carried out by the Stevin Laboratory of Delft University of Technology and the Institute TNO* for Building Materials and Building Structures (IBBC-TNO). T- and X-joints were tested with chord diameters of 168, 457 and 914 mm. Table I gives a survey of the test programme.

Three small T-joints were loaded with in plan8 bending. All other joints were axially loaded. One X- joint was tested with both chord and brace axially loaded. Some joints have one (Tl and Xl) or two (T2) additional unloaded braces. Two joints are tested with a random load and all others with a constant amplitude load (R = Fmin/Fmax = 0 or -1). Four joints have been tested in artificial seawater, one of them with cathodic protection.

TEST SPECIMENS AND EQUIPMENT

The test specimens were made by a manufacturer with experience in offshore structures. The specimens were welded in a fixed position with both chord and brace horizontal. Fig. 1 shows the weld shape of a large joint. The steel quality of the joints is in accordance with the standards mentioned in table II. Fig. 2 shows the test rig for the large joints. To avoid secondary effects, all supports in the test rigs are hinges.

STRAIN MEASUREMENTS

General

Before starting the fatigue tests, the strain distribution and the maximum (hot spot) strains in the joint were measured under a static load smaller than the fatigue load. The results are related to the nominal strain in the loaded brace (SNCF = Elocal/Enominal).
Fatigue Tests On Tubular Kand Kt Joints Under Axial Brace Loading
Authors: Wylde John G.
Published: October 2008

The results of fatigue tests on 40 tubular T and X-joints are presented. The chord diameters of the joints are 168, 457 and 914 mm. The most important conclusions are that the lifetime decreases with increasing joint size and that in the high cycle rance (N ? 107) some test results of the large joints fall below the AWS-X curve.

INTRODUCTION

The weather conditions which induce a fatigue crack are more severe in the North Sea than in the Gu1f of Mexico, Because of these severe conditions, a research project, sponsored by the European Coal and Steel Community (ECSC) on the fatigue behaviour of steel in offshore structures, entitled "Fatigue and corrosion fatigue behaviour of offshore steel structures",has been carried out. The research has been carried out in the U.K., France, Germany, Italy and the Netherlands. There is a special cooperation between Norway and the participating ECSC countries. In the programme, flat plate specimens and tubular joints have been tested. In addition, theoretical work (e.g. finite element analysis on tubular joints) has also been carried out. An interim report of the program was presented at a siminar in November 1978 in Cambridge.

In this paper, the results of tests on tubular joints, carried out in the Netherlands under the sponsorship of the ECSC, are discussed. Also, the results of additional work (strain measurements, crack measurements and calculations) sponsored by MaTS (Netherlands Marine Technological Research) are discussed.

TEST PROGRAMME

Forty tests on tubular joints were carried out by the Stevin Laboratory of Delft University of Technology and the Institute TNO* for Building Materials and Building Structures (IBBC-TNO). T- and X-joints were tested with chord diameters of 168, 457 and 914 mm. Table I gives a survey of the test programme.

Three small T-joints were loaded with in plan8 bending. All other joints were axially loaded. One X- joint was tested with both chord and brace axially loaded. Some joints have one (Tl and Xl) or two (T2) additional unloaded braces. Two joints are tested with a random load and all others with a constant amplitude load (R = Fmin/Fmax = 0 or -1). Four joints have been tested in artificial seawater, one of them with cathodic protection.

TEST SPECIMENS AND EQUIPMENT

The test specimens were made by a manufacturer with experience in offshore structures. The specimens were welded in a fixed position with both chord and brace horizontal. Fig. 1 shows the weld shape of a large joint. The steel quality of the joints is in accordance with the standards mentioned in table II. Fig. 2 shows the test rig for the large joints. To avoid secondary effects, all supports in the test rigs are hinges.

STRAIN MEASUREMENTS

General

Before starting the fatigue tests, the strain distribution and the maximum (hot spot) strains in the joint were measured under a static load smaller than the fatigue load. The results are related to the nominal strain in the loaded brace (SNCF = Elocal/Enominal).
Experimental And Analytical Studies Of The Elastoplastic Behavior Of Offshore Pipelines During Laying
Authors: Kimura Tetsuo , Idogaki Satoshi , Takada Katsumi , Fujita Yasushi
Published: October 2008

With more and more offshore pipelines being laid in deeper water, the need for larger laying equipment has increased larger barges, stronger tensioners and longer stingers. The primary reason is that, in deeper water, the pipeline is suspended at a steeper liftoff angle, and consequently a longer over bend portion is required between the liftoff point and the welding stations In order to help relieve stress in the pipeline.

If the over bend portion can be curved until the plastic region of the pipe is reached, it will be possible to use shorter stingers and lay pipe in deeper water without any special equipment investment.

We took note of these possibilities and conducted integrated experiments consisting of material tests, pipe bending tests and pipe well denting tests. We also developed a continuous beam elastoplastic analysis program, a pipe section analysis program and a cylindrical shell analysis program. Using both test data and programs, we established a precise elastoplastic design method for laying offshore pipelines.

Measured results show good agreement with calculated results.

This elastoplastic design method accurately predicts the capacity of pipe to bear environmental loads, thereby making it possible to set optimum limitations for laying operations.

However, it should be noted that, just above the yield point of the pipe, slight variation environmental loads cause significant changes in the strain and deflection. It would therefore be desirable in actual laying operations to use a stinger that permits control and restraint of pipe curvature and deflection.

INTRODUCTION

The elastoplastic design method for laying offshore pipelines makes it possible to use shorter stingers, which results in reduced liquid forces acting on the stinger pipeline system. This in turn results in improved operating rates and increased water depths in which pipe can be laid, as well as reductions in cost. Moreover, optimum limits set on laying operations by this design method ensure operational safety.

Three problems arise when large strains are induced in the pipe during laying. It is essential for the elastoplastic design method to solve these problems. The three problems are:NonlinearityThe bending stiffness of pipe bent beyond Its proportional limit becomes nonlinear due to the nonlinearity of the steel's stress-strain characteristics and the ovaling of the pipe section. Therefore, a nonlinear analysis must be made of the portion of the pipeline between the lay barge and the ocean floor.Local bucklingIn our experiments, all pipe specimens with a D/t ratio of 56 or over failed due to local buckling. In the case of specimens with a D/t of 56, local buckling occurred after the yield point was reached, whereas specimens with D/t ratios of 78 and 97 buckled locally before their yield points were reached. Thus, the elastoplastic design must give full consideration to local buckling.