bluuebunny/crossref_metadata_2025 huggingface dataset:
Electrical Shock Hazard Protection System
Authors: Miller Harvey A. , Yemington C.R.
Published: October 2008

A new electrical shock hazard protection system actually predicts impending shock hazard and provides advance warning or shutdown of electrical equipment to protect deck handlers and divers working near electrical power cables. The increasing use of electrical diver support equipment, tethered vehicles, and high power remote subsea equipment has increased the need for a failsafe shock hazard protection system. At the same time, the higher voltages, higher power, and longer cable lengths now being used make it more difficult to get good protection without a high incidence of false alarms. With the new approach, it is no longer necessary to tolerate more false alarms to get better sensitivity. This new system is immune to false alarms from changes in submerged cable length, temperature, and electrical load Characteristics including balance and transients. It also gives an alarm for almost any type of malfunction within the protective system itself. On its first application in the ocean, this system operated hundreds of hours without false alarms. It then gave an alarm when it detected damage to the jacket of a cable between canisters on the remote vehicle. It subsequently shut off power when it detected further breakdown of the insulation around the conductors inside the jacket at the point of damage. Even though this ground fault occurred in an interconnect cable on a remote vehicle, the detection system at the 'surface sensed the condition immediately. Future applications for this device include use on any electrical equipment in the marine environment. Applications to high-voltage and highpower remote vehicles are emphasized in this paper.

INTRODUCTION

Picture yourself on a steel deck with seawater washing around your ankles while you try to manhandle an umbilical cable carrying 2300V electrical power. One of your foremost concerns is bound to be the integrity of the insulation in the cable. Designers of electrical equipment for use in this environment should be equally concerned, especially since techniques are now available to continuously test the insulation and predict shock hazards, while the equipment is running, without impacting normal operation.

The use of undersea electrical instrumentation and power equipment is rapidly growing in response to economic and safety demands of the expanding offshore industry. This is true for both manned and remotely controlled systems including underwater welding equipment, sophisticated manipulator bells, submersibles which are tethered to allow greatly increased mission time, and high-powered remotely-controlled vehicles such as trenchers. In the last few years, the power level has gone from 1kW to 2,000 kW; cable lengths from a few hundred feet to several thousand feet, and the transmission voltages are increasing to accommodate the higher power levels--from 24V to 120V to 480V to 2400V to 6600V. Each such extension increases the hazard to human life.

Undersea power cables, particularly working cables used in the equipments outlined above, are indeed vulnerable.
Pull-out Parameters for Buried Suction Anchors
Authors: Wilson Quentin , Sahota Baldev S.
Published: October 2008

Data obtained from a model of a self-burying suction anchor tested in a granular soil bed in the laboratory and from larger anchors tested at sea are presented. Scaling parameters for comparing the performance of buried suction anchors are proposed and discussed and a preliminary investigation made of an anchor group with a 1000 kN (225000 lbf) vertical pull-out capacity.

The procedures used and some of the problems encountered during sea trials with suction anchors are described.

INTRODUCTION

Suction anchors should be considered as a family of marine devices with capacity for resisting uplift force, each device having its own characteristics and field of application.

Anchors in the form of inverted suction cups have been investigated extensively and have been shown to be capable of application in sands, silts and clays 1,5,11. The use of large cup-type anchors as footings on a gravity structure has been proposed by Wang et al 10 with the intention that additional anchoring capacity should be developed by soil pore pressure reduction during extreme storm conditions. Large suction anchors have been tested by the Shell Rijswijk laboratory5,6 and the maximum allowable lateral force for a 3.8 metre (12.5 ft) diameter anchor reported to be 160 tonnes.

Cup-type anchors may be regarded as surface attaching devices, even though the anchor skirts penetrate the sea bed, since there is usually no soil above the anchor to contribute to pull-out resistance. A suction anchor with non-penetrating skirts of flexible material has been proposed by Schofield4 and investigated in the laboratory by Al-Awadi l Cup type anchors, which are of rigid construction, require some soil depth for penetration of the skirts but the non-penetrating anchors can be attached to slabs of rock or concrete.

The development of self-burying suction anchors of hemispherical shape has been described by Wilson and Sahota12 and it has been observed that, for a given anchor size and suction pressure, the buried anchors can develop greater pull-out resistance than the other types due to the effect of the soil above the anchor.

All suction anchors have certain features in common. A cavity within the anchor is connected to the water in the soil voids via a filter or porous part of the anchor body, so that a reduction in water pressure within the cavity, created by pumping, is transmitted to the soil pore water. The flow of water which develops between the surface of the submerged soil bed and the anchor filter creates a distribution of force in the soil, directed in the flow direction and at right angles to the equal-pressure or equipotential surfaces (Fig. 1). The flow lines and equipotential surfaces form a flow net which develops with time from initiation of pumping but which tends towards a steady state condition if no perturbations occur. The generally favourable increments in the soil effective stresses that are produced increase the pull-out resistance of the suction anchor.
Presentation of 3-D Seismic Results
Authors: Paturet Daniel , Chanliau Michel , Leflaive Remi
Published: October 2008

The processed seismic data of a 3-D grid can be presented using conventional seismic sections along the direction of acquisition and a perpendicular direction. They can also be presented using any chosen direction.

Like in 2-D seismic it is possible to use colors to enhance the variations of certain parameters such as amplitude or frequency.

Constant time horizontal sections are very helpful for the Interpreter Since they allow a direct isotime contouring.

Tomoseis sections, where color is used to represent the instantaneous phase, give the dip of the seismic events and thus help and improve the above contouring.

A selected seismic horizon can also be flattened for studying some of its characteristics such as amplitude and frequency. It is then easy to obtain sections similar to the constant time sections, but parallel to this reference level. This process allows direct isopach contouring.

Examples of these different displays obtained from actual 3-D offshore seismic surveys will be presented.

INTRODUCTION

Seismograph prospecting for oil is now a little more than 50 years old. During the first decades, the interpretator had to pick the reflected events on individual records (fig. 1), to check whether they correlate from each record to the next, then to draw a vertical time sections (fig. 2), then to read the times of selected horizons and transfer such readings on a location map, then to contour the isotime - lines, so as to eventually obtain an structural image of the subsurface (fig. 3).

In the mid-fifties, with the introduction of the magnetic recording, some of these tedious tasks were taken out from our poor interpretator, who directly disposed of a vertical time sections along the acquisition lines (fig. 4). The digital recording and processing, surprisingly enough, although it launched tremendous possibilities in the field of signal enhancement, dit not appreciably help in the data handling at the interpretation stage. It is only very recently that efforts were made to capitalize on the computers capabilities, in order to supply the interpretation with documents on which he can directly visualize the geological Implications of his seismic Interpretation.

Such objective assumes that, first, there is a spatial 'continuity of the seismic data collected; it is what has been designated under the general name of "3D seismic". For offshore surveys, it amounts to shooting a large number of parallel lines, and relying on the sea currents to spread the measurement points on a regular grid (fig. 5) : surface position - (fig. 6) : subsurface position.

PRESENTATION OF 3-D SEISMIC RESULTS

Another necessary condition is to have relocated the seismic energy where it has been reflected, and not where it has been recorded. This is migration stage of the data processing, the effect of which can be seen by comparison of fig. 7, 8, 9 (7 : grid stack - 8 : 3D migrated grid stack - 9 : 3D migrated grid stack 500 m apart, whereto the deep seismic event of fig. 7 has moved).
Analytical Approach To The Design Of Atension Leg Platform
Authors: Chou Frank S.F. , Ghosh Susobhan S.F. , Kypke Dean A.
Published: October 2008

INTRODUCTION

The economics of offshore petroleum production have changed radically over the past few years, and reservoirs thought too deep or small now contain the financial incentive to justify their development. The various techniques and concepts previously developed to exploit such conditions pave also become economically feasible, such as the Tension Leg Platform (TLP).

The costs of TLP systems can be justified for use in the deep water reservoirs which are currently attracting attention because, at the water depths being considered, TLP's are less expensive than alternative structures. The results of a comparative study done by Daily, Karsan and Kypkel, illustrated on Fig. I, compares the qualitative cost between conventional fixed structures, Guyed Towers and Tension Leg Platforms.

The cost comparison crossover points for the offshore structure alternatives shown on Fig. 1 depend on environmental factors as well as water depth. The severity of the sea environment, seabed conditions and topography possible seismic activity, projected life of the field and desireability of platform reuse are all cost factors. In general, the cost crossover points move to the left, i.e. to shallower water depths, with an increase in degree of any of the environmental variables other than water depth.

It is in the deeper water depths that the TLP cost curve increases at a lesser rate than conventional structures, because the platform structure does not change as you move into deeper and deeper water. In other words, fixed platforms and guyed tower structure size correlate with water depth; the deeper the water, the larger the structure size required. In fact the inability to attain the structure size and proportions required by conventional platform design probably set the upper limits on the water depths in which conventional systems can operate. Thus the cost curves for these two structures will increase more- rapidly than the TLP in deep water reservoirs, for once a TLP platform is paid for, nothing will change except the amount of the mooring system to be bought and the cost to install it.

The mooring system is the common concept linking all TLP design. TLP's are basically buoyant structures restrained at a draft greater than that required for hydrostatic equilibrium by use of a vertically oriented, taut mooring system. This mooring system is quite rigid axially while being relatively flexible transversely. TLP design studies by Peres and Kypke2 and Capanoglu3 show that the mooring system configuration effectively eliminates heave motion, and the horizontal offset can be limited to an acceptable percentage of water depth.

The TLP has been developed as variations to one or more of the basic configurations noted in Fig. 2, and the individual components of a TLP drilling and production system are noted schematically in Fig. 3. These components will be found in a variety of shapes, locations, and applications on anyone of the TLP variations developed to date.
Evaluation Of Internal Corrosion On Marine Risers By Divers Using Acoustical Holography Techniques
Authors: Stankoff Alain , Guenon Yves , Thomas Gerald
Published: October 2008

The proposed paper describes a method producing a three-dimensional acoustical image of the internal face of the riser. Inspection is carried out by a diver operating only at the moment from a lock-out submersible. An array of ultrasonic transducers is applied on the outside surface of the riser, and generates a sequence of focused Shear wave beams, which are reflected back to the transducer array by the defects on the other face. The point by point electronic scanning allows to construct a 3-D image of a 150 mm by 100 mm section of the riser. Each image includes approximately 5000 acoustical points, each one corresponding to a 20 mm by 5 mm by I mm riser element. Depth extension of defects in the metal is given to - 1 mm. All data are recorded on digital tape and processed by computer at the surface after each dive to produce detailed inspection documents. Approximate time for inspection of a I square meter riser surface (100 images), in operational conditions, is 3 hours.

Extensive laboratory tests are reported showing the capabilities of the method in evaluating the internal corrosion and especially the corrosion due to pitting. At the issue of these tests, the system is found to provide a detailed representation of the corrosion or erosion with satisfactory depth resolution. Several parameters are extracted from the data to characterize the corrosion status and to allow evaluation of the safety of the riser.

INTRODUCTION

Monitoring and maintenance of marine risers are essential for operators of offshore oil and gas production platforms, as any damage to the risers can result in a partial or total shut down of the production, with significant economical consequences. On the other hand, marine risers are specially exposed: highly corrosive fluids, thermal gradients, collisions with service ships, constraints due to connecting sealines instabilities, etc...

The significant damage can manifest itself either as:A geometrical deformation, easily identified by a visual surveySome external corrosion evaluated through visual surveys and potential measurementsInternal corrosion or erosion frequently localized (pitting).Fatigue cracks.

These two later events are generally evaluated through techniques derived from on land surveys, poorly adapted to the needs of saturation diving :Internal corrosion is evaluated either with instrumented pigs, requiring a shut down of the production and providing a very coarse information unable to identify pitting, or, with manual ultrasonic tests which are time consuming and inappropriate to provide a complete survey of the suspected areas.Fatigue cracks, are visualized through magnetic particles inspections which are costly, due to the high quality cleaning required, and provide only a description of the surface outlet of the cracks.
Drillability Profiles Derived From 3D Seismic Surveys
Authors: Owusu Kwame J. , Gardner Gerald H.F.
Published: October 2008

A computer algorithm is outlined by which velocities can be derived from three dimensional multi-fold data, based on three dimensional migration before stack. The operation of the algorithm is illustrated with synthetic data for a layered model. It is shown that the velocities derived are practically the same whether the input traces are digitized with 16 bits or 1 bit. The practical significance is that velocity analysis can be extended by this method to areas where the data quality is poor by using 1 bit data. Sign bit digitization is, in effect, an extreme form of trace equalization. The reduction in amplitude information by using sign bits is compensated for by the vastly increased number of measurements spread over an area around the well. A practical application for the method is the construction of drillability profiles based on velocity measurements, with emphasis on the detection of formations with abnormally high fluid pressures.

Introduction

Three dimensional, multi-fold, seismic surveys generate a notable amount of data for the analysis of subsurface problems. A typical marine survey, for example, may have source points 100 ft apart along parallel lines that are 200 ft apart. Thus, if the data are recorded with a 48 channel streamer, each square mile surveyed contains over 250,000 seismic traces. Three dimensional processing combines all these data to form an image of each point of the subsurface.

Similar quantities of data can be acquired on land with the use' of 1024 channel sign bit recording systems. The amplitude recovered on correlating a Vibroseis sign bit recording with a sign bit representation of the sweep signal is remarkably similar to that obtained using 16 bit digitization for both the recording and the sweep signal. This property suggests that in some data processing routines the input data can be made sign bit with little loss in accuracy but with a great gain in data compression and reduction in computation time. An example of this was given by Cochran for CDP stacking velocity analysis.

In this paper, we explore a velocity analysis based on three dimensional migration before stack. The input data are migrated for a wide range of velocities using the Kirchhoff method of migration. No approximations are made. The computation is simplified by transforming time to a logarithmic scale, as explained below. The display, instead of being the usual plot of semblance as a function of velocity and time, is a plot of power as a function of depth Z and time T. Maxima in this plot are picked and velocity obtained from the relation V = 2Z/T.

The method of computation is illustrated with 16-bit synthetic data to which noise has been added. These data are also reduced to sign bits and the analysis repeated. The velocities inferred in the two cases are shown to be virtually identical.
Probabilistic Fatigue Analysis Of Offshore Pipelines During The Laying Operation
Authors: Stephens Denny R. , Mandke Jayant
Published: October 2008

When a pipelaying operation is performed in deep water, a joint of pipe may take 'one or two days to travel from the lay barge to the sea bottom. During this time, it is subjected to dynamic loads due to barge motion, waves, and currents. The oscillatory stresses induced by the loads may result in significantfatigue life damage to the pipe before it reaches the sea bottom.

This paper describes the application of probabilistic techniques to determination of fatigue damage to offshore pipe during the normal layingoperation. Spectra1, ana1ysis techniques of offshore structures as described in the now classical papers of Vughts and Kinra and of Maddox and Wildenstein, cannot be applied directly in this situation.

Because of the movement of a joint of pipe with respect to the lay barge, the relationship between stress cycle amplitude and wave height (i.e., the stress transfer function) for a joint on the pipe will vary with time. This, in combination with the random nature of waves, will result in widely varying stress histories, and therefore fatigue damage, for the joints of pipe when they reach the sea bottom. It 'is necessary that the bounds on this damage be determined for design purposes in order to know the fatigue life remaining in the pipe.

Introduction

Although the deepest pipeline installation to date has been in 2000 feet of water along the transmediterranean line across the Sicily channel, future installations in water depths up to 9000 feet are being considered at present. The candidate pipelaying methods applicable to these depths are the so-called "J" curve method and perhaps to a limited extent an extension of the conventional "s" method. Both systems have yet to be fully developed for application in these depths. When laying pipelines in deep water, a joint of pipe may take two or three days to travel from the barge to the sea bottom. The laying method and the pipe-joining technique that is selected will influence this travel time. During this period, every pipe joint is subjected to dynamic loads caused by barge motion, waves and current. The oscillatory stresses induced by these loads can result in significant fatigue damage to the pipe by the time it reaches sea bottom. This paper discusses the method of somputing the fatigue damage imparted to the pipe during the laying operation from a surface vessel.

Offshore pipelaying can best be described as an incremental process in which joints of pipe are added to an offshore pipeline suspended off of a lay barge. After a joint of pipe is welded to a pipeline, the lay barge moves forward and permits that joint and the previous suspended joints to move toward the sea bed. A typica1 barge and pipe1ine configuration is shown in Figure 1.
Ice Force Prediction Using a Limited Driving Force Approach
Authors: Vivatrat Vitoon , Krelder John R.
Published: October 2008

This paper discusses the importance of environmental driving forces in determining ice loads on Arctic offshore structures. The concept of a limited driving force is discussed and methods arepresented to estimate the magnitude of the driving force.

Most techniques for calculating ice loads on structures assume that sufficient driving force is available to sustain failure of the design ice feature against a structure. This paper assesses limits to the environmental driving force in a first-year ice field by considering analytical models of ridge formation, wind and current driving forces and irregularities such as leads and tidal cracks. The predicted forces are compared to available in-situ measurements.

An example is presented to illustrate calculation of maximum rubble pile loads on a conicalstructure. Implications of the concept to selection of design ice loads for structures in exposed, dynamic ice areas are considered.

Average driving forces are estimated to be approximately 10-50 kips/ft for far-field, with some local concentration near fixed structures. These values for the average limited driving force should be considered only as order of magnitude estimates. However, they are sufficiently less than failure loads for many ice features to indicate that consideration of limited driving forces in an ice field may reduce estimated ice loads on Arctic structures.

Introduction

The design of safe, economical structures in the offshore Arctic requires accurate predictionof ice loads for a wide variety of ice conditions. Ice conditions offshore the northern coast of Alaska can generally be classified as a fast ice zone atransition zone, and pack ice. Near the shoreline the fast ice is relatively stable during the winter with limited movement and deformation. A transition zone, beginning at the outer edge of the fast ice, extends out into the Beaufort Sea where it merges with the arctic Pack. The transition zone, typically beyond 45-60 foot water depths, is characterized by more movement than the fast ice with intense ice deformation. Design ice loads increase significantly with water depth because larger ice features can occur without grounding, and the larger movements indicate a higher probability of encountering a large ice feature.

Gravel islands have been constructed and used for exploratory drilling in water depths of up to approximately 65 feet in the nearshore Beaufort Sea. As offshore development advances in the transition and pack ice zones, reducing the uncertainty in ice load prediction will provide more reliable and cost-effective designs.

The prediction of a design ice load involves several components. First, the design ice feature must be selected and the properties of the ice feature specified. The load required to fail the ice feature against the structure is calculated.
Analysis Of Physical And Mechanical Properties Of Deepsea Sediments From Potential Manganese Nodulemining Areas In The North Central Pacific
Authors: Tisot Jean-Paul , Gerard Bernard
Published: October 2008

The knowledge of geotechnical properties of potential manganese nodule mining areas sediments is necessary to study and determine in the best conditions the possibilities of collecting these nodules.

The sediments found in this area all along the geotechnical study fall into three categories. The most numerous are pelagic clay, but other materials from abiogenic origin can be found locally, that is to say: siliceous ooze or calcareous coze.

First, we are going to study. the sedimentologic description, then the index properties of the three soils. Furthermore, we shall describe the mechanical properties incluG Lng a synthesis of the laboratory test results.

A fact is obvious : the physical and mechanical properties of the three materials as well as their behaviour are very different.

:Introduction

Since 1970, AFERNOD has been studyinga2.25Million square kilometer area between the Clarion and Clipperton fractures (Fig. 1) in order to prospect the potential deposits of polymetallic nodules (LENOBLE, 1980). At the same time, theoretical and experimental works about harvesting these nodules have been conducted (GAUTHIER and CHARLES, 1979).

These works have shown that, whatever the systems used to harvest, an interaction between the nodule harvester device and the seafloor will appear. This interaction will mainly consist in a mechanical sollicitation of the sediment. HIRST and RICHARDS (1975), among others, have analysed this problem.

Therefore, since 1977 AFERNOD has decided to study the geotechnical properties of the existing sediments in the prospected area. When doing this researching work, we have tried to study the problem from different points of view.

The required samples have been obtained by of three different means :Box corer.Boomerang corer.. Piston corer.

A few soil mechanic tests have been done immediatly after the sampling on board; most of the samples have been stored and carried over to France to be studied in a laboratory.

The results shown here are a synthesis of the various experiments done on the whole of the samples picked up in a burial depth of one meter below thebottom.

Description Of The Sediments

Three types of sediments quite different one from the others have been encountered during the coring in the studied area.

The most frequently encountered sediment is pelagic clay very homogeneous and of a dark brown colour. Thissoil is mainly (80 %) composed of clay minerals (the fi'ne-st elements), the rest gel).erally have a biogenic origin (diatoms, radiolarias,.) and are more or less well preserved.
The Behavior Of Steel Offshore Structures Under Accidental Collisions
Authors: De Oliveria Joao
Published: October 2008

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

Introduction

The collision between ships and platforms or other installations in offshore oil and gas fields is a type of accident which can have catastrophic effects, and as such is a cause of growing concern. Several studies have dealt with the problem of collisions offshore from the point of view of probability of occurence and degree of severity. For example in [1], major failures recorded in the Gulf of Mexico between 19L,8 and 1977 were analysed, indicating that 797. were related to drilling and production, 7% to storms and 5% to collisions. On the basis of oil spilled, only 1% of volume spilled resulted from collisions. Thus it appears that collisions account for a relatively small percentage of all major failures offshore. However, other studies indicate that the probability of occurrence of such incidents is by no means irrelevant. In [2], for example, values for the probability of occurrence of a collision involving different types of vessels and an offshore platform are given. In the case of supply ships the probability is considerable (p > 10−2), for oil tankers involved in offshore loading the probability is medium (P > 10−3), while for passing ships not involved in offshore operations the probability is very low (P > 10−7). Some studies sponsored by the UK Department of Energy [3] also address the question of the risk of shiRip1atformco11ision, and the general conclusion that can be taken from these various sources is that such risks cannot be ignored. In fact, not only surface ships should be considered as potential causes of accidental impacts, since the possibility of collisions involving submersibles is certainly also real, as discussed in [4].

Several studies have addressed the question of how to minimize the risks and extent of damage associated with collisions offshore, such as methods for improving navigation [5] and the development of special fendering systems [6]. However, practically no effort has been devoted to improving or adapting the structural design of platforms to this type of accidental load.
Observed Settlements Of Five North Sea Gravity Platforms
Authors: Lunne Tom , Myrvoll Frank , Kjekstad Oddvar
Published: October 2008

The observed settlements of five North Sea gravity platforms installed in the period 1975-1978 are presented. They are all founded on overconsolidated clays and dense sand. For each platform the systems for measuring settlements are described and some availablemethods for computing'immediate and long term settlements are reviewed. The foundation base witdth varies between 89 mand 140 m, and the net foundation pressures imposed on the sea floor varies between 200 and 300 kN/m2

The major part of the long term settlements occurs in the upper part of the foundation soils, within a depth of 30-40% of the foundation. width. One of the reasons for this being the presence of thick dense sand layers below 40-50 m depth.

Observed consolidation settlements range between 50 and 230 mm and are completed within 9 to 40 months. This is considerably faster than predictions based on soil properties determined from laboratory samples. Secondary settlements vary between 3 and 13 mm per year.

The observed data have been used to determine empirical relationships between strength and compressibility properties for the soil.

Introduction

The first gravity structure in the North Sea was installed in a water depth of 70 mat the Ekofisk field in July 1973 In the period 1975 to 1978 another 12 gravity structures were installed (Fig. 1) in water depth between 94 and 153 m (Eide et al2, Schjetne et al.3

Settlements have been measured for most of the 13 platforms with varying degree of success. Feedback from these observations helps the designer to modify the methods that have been used for predicting settlements. Thi s paper di scusses the settl ement records from 5 pl atforms on overconsolidated clays where the long term settlements have been measured relative to a deep fixed point. All the measuring systems have been designed and installed by the Norwegian Geotechnical Institute. The five platforms are (Fig. 1).

Beryl A - operated by Mobil North Sea Ltd., U.K. Brent B and Brent D - operated by Shell U.K. Statfjord A - operated by t10bil Exploration, Norway Ninian Central - operated by Chevron Petroleum, U.K.

Description Of Platforms

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

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

Model Data

This investigation was aimed at modeling the behavior of piles driven to varying penetrations in clay. A modeling factor, denoted as relative stiffness, was used to gauge the simulitude of the test pile to typical slender piles driven into clays. Relative stiffness simulitude was maintained to insure similar tendencies for deveopnent of load distribution and rapid progressive failure between model and prototype.

The relative stiffness is defined as the ratio of the pile flexibility (axial foreshortening under a generic load) to the vertical elastic displacement of the soil at the pile tip produced by skin friction. Assuming uniform skin friction and zero tip load, the pile flexibility, pp is given by;(Mathematical equation available in fullpaper)

Although it was possible to simulate relativestiffness, it was not possible to simulate the state of effective stress along the pile due to the restricted size of the model and lack of an apparatus to apply confining pressures.

Test Soils

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

Pipelines used in the gathering and transport of natural gas, natural gas 1iquids and other petroleum products may contain significant amounts of hydrogen suffice (H2S) and carbon dioxide (C02). When associated with brine these environments can cause internal corrosion and stepwise cracking of pipe line steels. Such phenomena are of increasing importance in the design and protection of offshore gathering lines as levels of H2S increase in these areas.

Stepwise cracking is observed in lower strength (less than or equal to 80 ksi yield strength) line pipe steels which are used in oilfield service. Cracking occurs internally through the accumulation of molecular hydrogen along internal surfaces such as nonmetallic inclusions or low temperature transformation structures. As hydrogen becomes trapped at internal surfaces, b1isters fond, usually along parallel planes, which begin to link up via small cracks. These cracks lower the effective wall thickness of the pipe and may eventually become through wall failures.

Research over the years has shown how various metallographic variables can be controlled to increase resistance to stepwise cracking. This study examines the effects of inhibitors on the stepwise cracking characteristics of API line pipe steel. The use of inhibitors was found to be available method of controlling the onset and propagation of stepwise cracking.

Introduction

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

Background

Mechanisms of Pipeline Corrosion Damage

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

Weight-Loss Corrosion.

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

An Ultrasonic Non Destructive Testing device for the detection of cracks in the vicinity of node joint elements in offshore steel platforms is described.

Major design considerations were:The system should provide rapid screening of the welds for the presence of cracks.The work load on the diver should be as low as possible.A permanent record of the inspection should be available.

The resulting equipment known as the Ultrasonic Torch, fulfils these requirements and transfers the onus of data interpretation to a qualified inspector who is stationed at an above surface console.

INTRODUCTION

Offshore oil production is common place nowadays and in the U.K., of course, the main source of the North Sea means that structural safety of the platforms must be taken very seriously, and the integrity of the structure must be confirmed by regulars inspection. In the North Sea there are now well in excess of one hundred steel platforms, built, in general, from large steel tubes with the diameter matched to their function. The larger tubes are the supporting legs, with the size of cross bracing matched to the calculated stress. The various members are welded together at nodes, as typical structure having several hundred nodes, and at each node will be many meters of well-meant. The action of welding by melting the metal causes a physical change in the steel close to the weld in the region known as the 'Heat Affected Zone'. By definition the nodes are the joining points of the member, where the physical shape of the structure passes through discontinuities and where there will be stress concentrations. The combination of the affected metal and high stress means that the regions immediately adjacent to welds are the most susceptible to cracking. For this reason there is a requirement that at least an agreed percentage of the welds are inspected annually.

Gross visual inspection can be done with submersibles using TV camera, and general monitoring can be done with systems based on acoustic radiation from cracks, or vibration mode analysis. However the greater part of inspection work is carried out by divers, much of the task being visual inspection, with the diver reporting on the general state of the welds, the amount of undercutting, spatter and so on. Obviously the most important feature which could be revealed by the inspection is a crack, and should a crack be suspected, then other more specialized systems, such as MPI, will be used to provide information on the precise location, extent and type of crack.

Unfortunately a tightly closed crack may be invisible, and some method of initial detection is necessary. Whatever device is used it is going to be operated in what at best can be described as a hostile environment and the underwater tasks involved in the inspection work should be kept as simple as possible.
An Analytical Study of Stress Concentration Effects in Multibrace Joints Under Combined Loading
Authors: Gulati Kailash C. , Wang Wen J. , Kan David K.Y.
Published: October 2008

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

INTRODUCTION

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

Current Design Practice

The current offshore design practice concerning the fatigue life evaluation of tubular structures is based on the proven engineering approach depicted above. These applications are, however, particularly severe, both from the viewpoints of complexity and diversity of structural configurations and the complexity of loadings. A typical joint from the fatigue prone areas of a jacket structure is illustrated in Figure 1. The loads acting on the member ends of the multi planar joint were obtained from a global analysis of the jacket. A typical North Sea wave was used for this purpose. The nature of member end loads under changing magnitude and direction of the impinging wave is expected to be similar to that shown in Figure 1. Fatigue life evaluation of this joint requires an estimation of the hot spot stress history under combined loading and the provision of an applicable S-N curve.

The common tubular joint fatigue evaluation procedure utilizes parametric equations 1, 2, 3 to estimate the hot spot stresses at selected locations (crown and saddle points) of the tubular joint interface. These parametric equations are based either on a finite element analysis utilizing thin shell theory or on laboratory testing of simple joints subjected to simple loadings. The parametric formulation of stress concentration effects, to date, is limited to X, T, K and TK joints subjected to simple loadings of the axial, in-plane bending and out-of-plane bending types, applied separately to the joints.
A Powerful Marine Seismic Source Seismic Tuned Array of Steam Jets
Authors: Manin Michel
Published: October 2008

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

This seismic source has to be used when penetration and definition are required in seismic surveys.

After 5 years of research and development Starjet's industrial use began in June 1982 with a series of tests conducted in the North-Sea and the Mediterranean. Seismic results are discussed in relation with the pulse characteristics and wavelet processing.

INTRODUCTION

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