bluuebunny/crossref_metadata_2025 huggingface dataset:
The Distribution of Ice Pressure on Offshore Pile Structure and the Failure Mechanics of Ice Sheet
Authors: Saeki H. , Tanaka S. , Ono T.
Published: October 2008

The total ice force acting on offshore pile structures, located in cold regions, has already been investigated by many researchers. However, few papers have described the distribution of ice pressure on the structures and the failure mechanics of ice sheet. It is necessary to study them in order to design the pile structures, keeping in mind safety and economic factors. The results of our experiments on failure mechanics of an ice sheet are useful for dynamic analysis. For analysis of stress and, especially, local buckling of structures, it is essential to examine the distribution of ice pressure acting on the structures.

This paper describes a systematic study of these aspects through field tests with three rectangular piles (20, 40, 60 cm in width) in Saroma Lagoon to clarify the effect of aspect ratio. It is clear from our experiments on ice pressure that the distribution of ice pressure can be classified into two types according to the strain rate ë (= v/4B, v: penetration velocity of piles B: pile width) defined by Michel and Toussaint in each aspect ratio, B/h (h: ice thickness). It is our hypothesis that the failure periods of ice sheet are determined by the strain rate and the aspect ratio mentioned above.

INTRODUCTION

In order for exploration and production in the oil begins of the offshore Arctic to be possible, it will be necessary to build structures capable of withstanding forces caused by the movement of the surrounding ice sheet. It is important to explain the interaction between the structures and the ice sheet so that they may be designed more rationally.

Total ice force acting on the structures has previously been discussed, mainly in regard to the interaction. However, they can't be properly designed only on the basis of the total ice force. Stress analysis of local buckling on structures, especially those having a large diameter and thin wall, should be carried out. The equation for the total ice force acting on the structures, arrived at by the authors, can be expressed as follows:(Mathematical equation available in full paper)

If local buckling occurs at the area of contact with the ice sheet, the value of the above shape factor, namely C, will increase and the total ice force is considered to become larger. The distribution of ice pressure must be obtained in order to examine whether or not local buckling occurs on the wall of the structures.

Moreover, the structures are severely damaged by the interactive interference between them and ice sheet. In fact, the marine lighthouse in the Bosnia Sea has received such damage. Dynamic analysis through numerical simulation should be carried out in order to prevent structures from receiving this damage. Maattanen, and Matlock et al. have studied the above problem, however, few papers have been issued, which describe the dynamic analysis from the perspective of failure periods and failure modes of ice sheet.
Shear Strength of Adfreeze Bond and Its Effect on Global Ice Load Applied to Mobile Offshore Drilling Units Under Arctic Conditions
Authors: Gershunov E.M.
Published: October 2008

Theoretical results of calculating the global ice load which can be imposed on a conical and/or cylindrical type offshore structure as a result of the shear failure of the ad freeze bond are discussed. It is shown that the ice load required to fail the adfreeze bond essentially depends upon the shear strength of the adfreeze bond and the cone angle of the structure. If the adfreeze bond is strong enough, the ice load required to fail it in shear can be greater than the load imposed on the structure due to ice which fails in bending. In other words, under certain circumstances, the global ice load caused by the shear failure of the adfreeze bond, rather than the load due to the bending failure of ice, can govern the design. Experimental results confirm that conclusion.

INTRODUCTION

A major feature of a sloping sided structure is that it provides the possibility for the ice to ride up on the structure and fail in bending rather than in compressive crushing. Due to the fact that the ice fails in bending, the global ice load imposed on the structure is substantially less than the load which would be imposed on the same structure if it did not have a sloping side. Generally, this is true if one makes the simple assumption that the adfreeze bond between the structure and ambient ice field does not exist or is very weak. However, if the ice forms a strong adfreeze bond around the structure, the bending failure of the ice can occur only after this adfreeze bond fails. The primary objective of this paper is to show that under certain circumstances the structure can be exposed to a substantially larger global ice load due to adfreeze than it would be simply due to the load associated with a bending failure of the ice.

This phenomenon is the direct consequence of a strong adfreeze bond which can .be formed at the structure-ice interface.

STRENGTH OF ADFREEZE BOND

Reference 1 is an extensive literature search in which over 20 sources of experimental information were reviewed. The results of that study (Reference 1) show that the strength of the adfreeze bond depends upon numerous conditions including such things as salt concentration in water, temperature differential between air, water, ice and structural wall which is subjected to the ice, surface characteristics, freezing rate and crystal formation in ice, method used to apply protective coatings and the types 'of coating used. Since there is no reliable means of calculating the strength of the adfreeze bond, the designer must obtain data from model or actual field tests. However, at the present time, there is a lack of uniformity in the test conditions and procedures which have been used.
Analysis and Design of an Ice Wall Framing System for an Arctic Drilling Structure
Authors: Schlechten J.R. , Fernandes R.L. , Dolan D.K. , Bivens H.R.
Published: October 2008

The exterior shell of a concrete base structure for an arctic oil drilling platform must be designed to resist extremely high local ice pressures. Stringent draft criteria for deployment of these structures in shallow waters require that the exterior shells, commonly called ice walls, have minimal weight in conjunction with maximum strength.

These conflicting requirements are satisfactorily balanced by the selection of an arch shape on the interior face of the wall. This geometry induces arching action within the ice wall plate to resist ice loads in compression, thereby minimizing principal tension within the concrete. The development of principal tension in a concrete structure limits its ability to resist external forces. This paper describes the design of a pre-stressed concrete ice wall which satisfies these load, weight, and material constraints.

INTRODUCTION

An exploratory oil drilling rig for use in Harrison Bay of the Beaufort Sea of Alaska is being designed for SOHIO Construction Company. This facility is intended for use in the ice-infested waters of Harrison Bay in water depths ranging from 40 to 60 ft.

The SOHIO Arctic Mobile Structure (SAMS) consists of a 345-ft octagonally shaped concrete base structure. (See Figure 1.) The height of the base structure varies from 50 ft in the central bays (under the drilling facility) to 70 ft in the exterior bay. A steel perimeter superstructure extends the exterior bay of the structure to a total height of 112 ft above the bottom of the concrete base. The drilling facility for SAMS is located in the 257-ft octagonally shaped area enclosed within the perimeter superstructure and above the concrete base structure. Spuds (shear piles) 7 ft in diameter are to be driven into the underlying soils to prevent SAMS from sliding due to a combination of high ice forces and weak soils.

The global ice forces and local ice pressures were the primary loads that controlled the design of the concrete base structure. Design of the structure's exterior shell (or ice wall) is controlled by the extremely high local ice pressures that develop as an ice feature is crushed against it.

The exterior shell of the base structure is the primary ice-resisting element and constitutes approximately 20% of the base structure weight. Thus, considerable effort was expended to minimize its weight to meet stringent draft criteria, including the use of light weight concrete. The fully saturated, reinforced concrete is expected to have an average in-place unit weight of 133 pcf. The design strength of the lightweight concrete is for 7000 psi (measured at 90 days).

An arch shape on the interior face of the ice wall was selected to induce an arching action within the ice wall. The arching action resists ice loads by compression, thereby minimizing principal tension.
Predicting Offshore Soil Conditions
Authors: Campbell K.J.
Published: October 2008

This paper describes a practical method for developing predictive soil models for large offshore areas. The models are developed by synthesizing geologic background information, high-resolution seismic reflection data, and soil boring results. The geologic information provides reasonable limits for the predicted range in soil conditions. Seismic data define the three-dimensional relationships among soil units and provide a framework for relating soil borings to one another. The soil borings 'calibrate' the model by defining soil stratigraphy and geotechnical properties at key locations. Components of a soil model are: a soil province map showing areas where soil conditions are similar; soil cross sections to show the vertical relationships among soil units; and a representative soil profile for each soil province. Soil models can be used to predict general soil conditions throughout a study area, but not exact soil conditions at specific sites. Reliability of predictive models varies according to geologic complexity; the amount, type, and quality of data used; and the experience of those who develop the model. The method described has been used successfully to develop models for several Alaskan OCS areas and other frontier areas.

INTRODUCTION

The purpose of this paper is to describe a practical method for predicting general soil conditions offshore, and to discuss the limitations and factors affecting the reliability of this method. The method is most applicable for predicting soil conditions of large frontier areas tens of miles on a side, but ranging downwards to areas about the size of an individual lease block. Knowledge of general soil conditions is needed for conceptual and preliminary design of foundations, for various pre-lease economic analyses, for feasibility studies, for preliminary drilling and construction planning, and for planning geotechnical site investigations. Predicting soils at individual sites is not specifically addressed here, but aspects of the general method and its limitations as described here would also apply to site investigations.

The method consists of first developing a geologic model and then transforming the geologic model into a predictive soil model. The models are developed by synthesizing geologic background information, high-resolution seismic reflection data, and soil boring data. The method involves considerable interpretation and judgment, but is the most rational method readily available today for predicting general soil conditions for large areas.

Geophysical techniques are being developed that will complement the seismic data now used. These techniques are anticipated to provide direct identification of materials and to significantly increase the reliability of regional soil models. However, none of these techniques are expected to be perfected for geotechnical applications and be routinely available on an operational basis for at least several years and probably much longer.

GENERAL APPROACH

The method described here involves three basic steps;
Rate of Loading Parameters for Vertically Loaded Piles in Clay
Authors: Briaud J.L. , Garland E. , Felio G.Y.
Published: October 2008

The analysis of 152 laboratory tests and 32 pile load tests confirms that for clays, the faster the rate of loading, the higher the undrained shear strength and the higher the pile capacity. The data shows that the gain in undrained shear strength due to increasing rate of loading increases with increasing water content, plasticity index, liquidity index, over consolidation ratio but with decreasing undrained shear strength

A simple model is proposed to quantify the rate of loading effects on undrained shear strength. The viscous exponent n which is the main parameter of the model can be measured by conventional laboratory tests or possibly by cone penetrometer testing, or as a last resort by the proposed empirical correlations to index properties.

This model is used to develop rate dependent t-z curves and a computer program to predict the response of a pile subjected to a certain rate of vertical loading. The model and the program are checked by comparing the predicted and measured behavior of two piles

INTRODUCTION

The present practice for the design of vertically loaded offshore piles is to calculate the pile length which will allow to resist two times the dead load and one and a half times the sum of the dead load and the extreme storm load. The design is based on static ultimate pile capacity calculations and no explicit consideration of dynamic loads is made.

The data presented in this article is the partial result of an ongoing research project at Texas A&M University sponsored by the American Petroleum Institute. The purpose of the project is to study the explicit influence of rate of loading and cyclic loading effects due to waves on the capacity of long offshore piles in clays. This article is dealing with the rate of loading effect only.

RATE OF LOADING MODEL

An undrained shear strength Su is measured at an average rate of shearing Su/t where t is the time to failure. It has been shown as early as 1951 (Casagrande and Wilson) that the value of Su is dependent upon the rate at which the soil is sheared. Others including Whitman (1970), Bea-Audibert (1979) and Lacasse (1979) addressed this problem. A value SUI is obtained for a time to failure t1 while a value SU2 is obtained for a time to failure to Two models can be used to describe the dependency of upon. The first model is:(Mathematical equation available in full paper)

This model leads to a straight line on a log-log plot and is the one proposed by Riggins (1981). Laboratory data collected from six different studies published in the literature show that Su versus t increases faster than a straight line on a semi log plot and is closer to being a straight line on a log-log plot (Briaud and Garland, 1984). As a result, the second model (Equation 2) was selected.
Erosion, Cavitation, and Abrasion Resistance of Choke Trim Materials
Authors: Seger F.G. , Maroofian I.
Published: October 2008

An experimental investigation was performed to determine the relative erosion, abrasion and cavitation resistance characteristics of selected materials. Testing was conducted under controlled laboratory conditions to simulate service conditions encountered in production and injection chokes. The testing effort is ongoing.

The data accumulated allow informed material selection of conventional and novel trim for all chokes, valves, flow metering orifices, fixed beans and other devices used during drilling, completion and production of offshore and onshore oil and gas wells.

Sintered silicon carbide and tungsten carbide with minimum binder content were the most erosion and abrasion resistant of the materials tested. Cobalt base alloys bar and nickel chrome alloy 625 bar proved to be most cavitation resistant.

INTRODUCTION

Wear of wellhead chokes and other pressure reducing devices is a serious and common problem. Erosion, abrasion, and' cavitation are three types of wear, which is defined 1 as the volume of material removed. As the choke is adjusted to throttle fluid flow, a series of localized, time dependent and very complex flow fields are established in the region of the throttling point, such as the plug/seat gap in a globe valve.

Each flow field is a function of the valve position, fluid phase and properties, pressure drop across the choke and ratio of pressure drop to upstream pressures. Choke trim design parameters, such as trim contours and clearances also greatly affect the flow field.

The nature of the flow field in the gas choke is of particular concern when erosion of the trim material is considered. When the ratio of the pressure drop to the upstream pressure becomes critical, the velocity at the choke gap is sonic.

Small fluid borne particles move with a velocity approaching the mean velocity of the stream, and are influenced, depending on size, shape and density, by the large scale turbulence of the stream. The high velocity conditions at the choke throat and' along the downstream trim are therefore particularly conducive to high rates of erosion. Tests were performed on several materials to determine the wear (volume loss) rates in accordance with special erosion procedure.

Abrasion wear is associated with surfaces sliding along one another, and is greatly accelerated by fluid borne particulates. Those particulates with sharp corners, such as sand particles so often found in natural gas and crude oil are especially effective in promoting material removal when the choke is being adjusted frequently.

Other factors influencing abrasion wear include fluid lubricity, clearances between sliding parts, particle hardness, forces on component parts, and hardness and surface finish of the component parts. Low viscosity fluids, such as gas, provide poor lubricating qualities and negligible protection against abrasive wear.
New API Equation for Grouted Pile-to-Structure Connections
Authors: Karsan D.I. , Krahl N.W.
Published: October 2008

A new equation for the design of axial load transfer in grouted pile-to-structure connections, with or without shear keys, has been written into the latest edition of API RP 2A. The general form of the equation was obtained from an ultimate strength formulation for a predicted failure mode, then safety factors were applied. The reliability of the equation was studied by comparison with the results of 147 tests, and results are presented. The paper al sodiscusses 1 imitations on the equation, compares it to the equation in the U. K. Department ofEnergy Code and presents some numerical examples.

ORIGIN OF NEW EQUATION

In 1982 the American Petroleum Institute (API) Task Group on Fixed Platform Criteria asked member Demir 1. Karsan to form a mini committee on grouted pile-to-structure connections in order to review the current state-of-the-art and identify possible changes in recommended design practice. The first meetings of this new committee generated considerable discussion of grouted pile-to-structure connections, both with and without shear keys, and of tests which had been performed on this type of connection. Committee Chairman Karsan and member Nat W. Krahl reviewed the available test data and, in late 1982, presented to the committee two equations, one for estimating the ultimate strength of such a connection, and the second a modified form which could be used for design.

The ultimate strength equation was based upon test observations that two separate mechanisms occur during the failure of a grouted tubular connection containing shear keys: first, a slippage between the steel and grout, and second, a crushing of the grout against the shearkeys (see Fig. 1). Such a connection normally fail s in a ductile manner, so that the ultimate strength of the connection is the sum of the two separate sources of strength. Before final failure occurs, diagonal cracks tend to open across the grout, frequently between diagonally opposite shear keys, or from one shear key to the opposite pipe. The ultimate strength equation presented to the committee stated that;(Mathematical equation available in fullpaper)

EVALUATION OF NEW EQUATION BY API COMMITTEE

Eqs. 1 and 2 were of sufficient interest that committee members issued a call for all available test data of grouted pile-to-structure connections so that a comprehensive reliability analysis could be performed on the equations. Additional data were indeed made available to the committee, so that data from a total of 201 separate tests were accumulated. The committee was of the opinion that a minimum unconfined grout strength should be specified for actual field connections, and a minimum unconfined strength of 2,500 psi (17.25 MPa) on 28-day old specimens was chosen. Hence, in comparing Eq. 2 to the test data, all tests were excluded in which the measured grout strength was less than 2,500 psi (17.25 MPa). 147 tests remained, including 62 tests of plain pipe (without shear keys), and 85 tests of pipe with shear keys.
Field Results of Curved Conductors in the Main Pass Area, Offshore Louisiana
Authors: Broussard G.C.
Published: October 2008

Over the past fifteen years, the oil industry has developed the use of curved conductors in the Gulf of Mexico as an effective means of increasing horizontal displacement when drilling from offshore platforms. This paper reviews the results of utilizing curved conductors on several platforms. A discussion of the concept is presented as a preview, followed by drift angle performance of actual field results compared to design expectations. Case histories from four Main Pass area platforms are described where curved conductors were successfully used. Bearing performance is illustrated on a per platform basis before and after driving by rig operations. Conclusions are based on a comparison of directional programs for both curved and vertical conductors to the same target.

INTRODUCTION

As oil companies have expanded their search for oil and gas farther offshore in the Gulf of Mexico, they have encountered increasing difficulty in economically removing hydrocarbons for use. One common problem which first surfaced nearly two decades ago was the drilling for shallow sands in the range of 3000 ft. to 6000 ft. from a platform in water depths to 450 ft. With conventional methods, the horizontal displacement of drilling operations imposed certain limits on the area covered by a single platform, prompting the creation of curved conductors in order to increase horizontal displacement of wells and reduce the number of platforms required to develop these shallow prospects.

A curved conductor initiates the well angle deviation in the jacket typically above sea level instead of 200 ft. below the end of a driven conductor as is normally encountered with vertical conductors. A curved conductor allows a directional well to achieve a greater horizontal displacement when the well angle of deviation is built to its maximum. Angles greater than 55° have been found to cause mechanical problems for completion and workover operations because friction resistance exceeds the weight factor of tools, thereby making well work difficult. A curved conductor also allows a directional well to reach a target at a smaller average angle than would be encountered by a vertical conductor. Figure 1 illustrates a comparison of wells drilled through both type conductors and built to 55° average angle. As shown by Figure 1, horizontal displacement is significantly greater with a curved conductor. Also illustrated is a comparison of a straight versus curved conductor to the same target requiring a smaller average angle by using the curved conductor.

DESIGN EXPECTATIONS

Two variables are important to the successful use of curved conductors in a directional program. They are the bearing and rate of build angle. The ideal design expectations assume that the conductor's bearing and rate of build angle remain the same before and after driving. However, there is limited control over the drift angle and bearing during driving.
Seawater Corrosion Fatigue of 2¼ Cr-1 Mo and 4130 Steels for Marine Riser Application
Authors: Cozens K.O. , Burnside P.J.
Published: October 2008

This paper reports on a corrosion fatigue study of 4130 and 2 1/4 CR-1 Mo steels immersed in seawater which addressed the reduction of fatigue life for these steels in a seawater atmosphere, and developed fatigue life design curves. The study has evaluated the effect of shot peened and nitrided surface treatments on sea water corrosion fatigue, and demonstrated that a significant loss of fatigue life is caused when unprotected 4130 and 2 1/4 Cr-1 Mo steels are fatigued in seawater. Shot peening of the material surface improves the seawater fatigue life for these steels, but does not restore it to the full atmospheric fatigue life of these steels in air. The test results for the effect of nitriding material surfaces prior to seawater fatiguing were not conclusive, but nitriding may be beneficial. Manufactures of subsea hardware need to take appropriate precautions to minimize or negate the life shortening effect of seawater corrosion fatigue to 4130 and 2 1/4 Cr-1 Mo steels.

INTRODUCTION

The study of the effect of seawater Immersion on 4130 and 2 1/4 Cr-1 Mo fatigue life was divided into two parts, a literature view, and speech tests which were used to develop specific design limits. These two steels were selected for evaluation because of their frequent use in the manufacture of oil tool, and because their use has ken considered for marine drilling risers. The study was expanded to include an examination of the effect of shot peen and nitride surface treatments on the seawater fatigue life of the materials. These surface treatments are recognized as proving fatigue properties of steels in air atmosphere, but their effectiveness in proving fatigue life of steel immersed in seawater is unknown and hence, was the reason for inclusion in this study.

In reviewing the literature on the effect of corrosion on steel fatigue life, the authors found no literature pertaining directly to corrosion fatigue of 2 1/4 Cr-1 Mo steel, but did find literature pertaining to corrosion fatigue on other low alloy steels. Because 2 1/4 Cr-1 Mo steel is a number of the low alloy steel grouping, it is expected to follow the same general set of rules by which other family numbers abide. Hence, the following discussion of steel corrosion fatigue is considered applicable to both steel seven though the literature survey possessed no information specifically on 2 1/4 Cr-1 Mo steel.

Fatigue of unprotected steel immerse in Seawater will always possess a shorter fatigue life than fatigued steel in an air atmosphere. Hence, all air fatigue data should be down graded to account for the loss of properties when subjected to seawater fatigue. However, it is important to note that there are certain variables, when appropriately altered, may maintain fatigue properties of steel immersion in seawater equivalent to those exhibited within air. Table 1 (1) summarizes the basic variables of fatigue in seawater and how they effect fatigue life of steel.
The Arctic Cone Exploration Structure: A Mobile Offshore Drilling Unit for Heavy Ice Cover
Authors: Byrd R.C. , Coleman R.K. , Weiss R.T. , Boaz L.B. , Sauve E.R. , White R.M.
Published: October 2008

INTRODUCTION

The offshore oil and gas industry has been required by circumstances in recent years to place increasing emphasis on the very hostile, relatively remote, oil provinces. The Alaskan Beaufort Sea, ranks near the top of the list of new offshore development areas in terms of both remoteness and the harshness of its environment. Nevertheless, this area and the nearby Chukchi Sea offer among the best prospects for major oil finds of the U.S. Outer Continental Shelf waters remaining to be explored.

Two lease sales have been held to date in the Beaufort Sea offshore Alaska's North Slope, the most recent being OCS Sale No. 71, which was held in October 1982. The first major offshore sale in this area took place in December 1979. The majority of tracts leased in the 1979 sale are in water depths of less than 50 ft and can be explored using gravel islands. However, over 70% of the tracts made available for Lease Sale 71 are in water depths beyond 50 ft. A similarly high percentage of tracts scheduled for future lease sales in the Beaufort and Chukchi Seas are in depths beyond the range of economical development from gravel islands. Even within the 50 ft depth contour, multi well drilling programs do not look attractive from gravel islands. Industry has responded to this situation in the last year and a half by offering designs for a number of different caisson based mobile drilling units. These have generally been designed for water depths between the range of 25 and 70 ft and for operations in relatively sheltered waters where they will not be exposed to the most severe ice conditions.

This paper reports on the results of a rig development program which began in 1981 to design a mobile drilling unit which could operate beyond the 50 ft depth contour in the most exposed ice conditions. The Arctic Cone Exploration Structure (ACES) project has produced the design of what is likely to be the prototype for heavy-duty, bottom-founded mobile rigs for arctic offshore drilling. The current ACES rig design has focused on developing the drilling capability for the water depth range of 50 to 110 ft. Approximately 68% of tracts made available during the recent OCS Sale 71 fell within this depth range.

The latest phase of the ACES rig design was sponsored by Exxon Company, U.S.A.; Shell Oil Company; and Standard oil Company of California/Chevron. An earlier conceptual design phase of the program included three additional U. S. oil and gas companies. Brian Watt Associates, Inc. (BWA) of Houston, Texas, was the prime contractor for the design. Zapata Off-Shore Company of Houston was responsible for the drilling related systems.

The objective of this paper is to show the level of effort which has gone into the development of the ACES mobile rig design. The results of that effort are presented, including the design criteria and performance objectives used.
A Prestressed, High-Strength Grouted Connection for Offshore Construction
Authors: Elnashai A.S. , Carroll B.C. , Dowling P.J.
Published: October 2008

This paper concerns newly developed prestressed grounted connections for steel tubu1ars in offshore applications. A mechanical prestressing method has been devised and shown to give an increase in ultimate bond strength of 6 or more times that of a plain bonded connection.

The method was developed by small scale testing using models with varying degrees of prestress. The test information was appreciably augmented by the results of a parametric study carried out using a new non-linear finite element program.

The information obtained from the small scale tests and numerical study was used to design a series of large scale models. During the testing of these large models an alternative method of prestressing using expansive grout was also shown to produce satisfactory results. This test series confirmed that the results of the small scale tests and computer studies could be safely extrapolated to full scale connections.

Based on these studies a new comprehensive equation is proposed for designers which accounts more accurately for pile length/diameter ratio than existing design rules and which for the first time predicts the increase in bond strength in prestressed connections.

INTRODUCTION

Grouted connections are used in forming the main structural connections between steel jacket structures and their foundation piles, and in repairs to damaged tubu1ars. Both plain and shear keyed connections have been adopted in practice. Plain connections are known to have very unreliable and low bond strengths, since they are very sensitive to shrinkage and imperfections in the tubu1ars. To overcome these problems shear keyed connections, using weld beads, have in recent years become the more common solution.

Billington and his co-workers (1,2) have carried out extensive testing of both types of connection and have presented an empirical design formula.

An alternative to the use of shear keys for strength enhancement, is the use of a radial prestress in the grout annulus, which will increase the frictional bond strength. The radial prestress may be generated either by employing a mechanical system of active stressing, or a passive chemical system, using expansive grout. In both cases, the tubu1ars are being used in the most efficient manner, since both hoop and axial strength are utilised, and the annulus provides an ideal environment for the triaxial confinement of the grout. A new mechanical system of stressing, based on the high pressure grouting, has been developed at Imperial College, the general details of which are shown in Figure 1.

The validity of the prestressing concept was the first established by small scale model testing, the details of which are discussed below. In parallel with the test program, a nonlinear numerical model was developed and calibrated to the available experimental data. It was then used to perform a large number of analyses to quantify the effect of geometric configuration and applied prestress on the ultimate bond strength of grouted connections.
Geology and Hydrocarbon Potential of Bombay Offshore Basin
Authors: Sahay B.
Published: October 2008

DESCRIPTION :

The Western Continental Margin is a passive margin having six sedimentary basins namely Kutch, Saurashtra, Bombay Offshore, Konkan, Lakshadweep and Kerala. Out of the six basins Bombay Offshore is the biggest basin having an aerial extent of 1,20,000 Sq. Km. Bombay Offshore basin which is an intracratonic basin, is the southern extension of Cambay basin and encompasses the Gulf of Cambay and adjacent shelf part of the Arabian sea. It came into existence during Upper Cretaceous time as a result of faulting in the Deccan t['ap basement. This basin is divided into several tectonic units such as Cambay Gulf Shelf to the north followed to the south by Dahanu depression, Bombay Ratnagiri shelf occuyping central and southern part of the basin and Western clastic basin fringing Bombay Ratnagiri shelf.

RESULTS:

As a result of seismic surveys carried out five different reflectors have been met. They are H-1, H2, H-3, H-4 & H-5. Of these reflectors horizon-5 is confined only to the north and north Western region and probably marks the top of the basement. Geophysical surveys both reconnaissance, semi-detail or detailed carried out have helped in the delineation of big and small structures in the Bombay Offshore basin. The important structures delineated are Bombay High, Panna, Bassein, Dahanu, South Tapti, Heera, Ratna and other small structures indeed, area.

A full sedimentary section from Palaeocene to Recent of over 4 Km. resting on trappean basement has been met. In few places Pre-Cambrian basement instead of trappean basement also occurs. The oil and gas bearing horizons of Bombay Offshore basin, range in age from Middle Miocene to Early Eocene and are mostly wackstone/ mudstones. and chalky limestones.

Evaluation by drilling of important structures in Bombay Offshore Basin like Bombay High, Panna, Bassein, Heera, Ratna, South Tapti, etc., have resulted in the discovery of big and small oil and gas fields. Besides this oil and gas indications have also been met in several other structures, too.

The Bombay Offshore Basin has only Tertiary sediments with Eocene being absent in Bombay High. However, west of Bombay High the Palaeogene and Miocene are represented by a thick sequence of limestones and thin shales having poor source rock characteristics. In the south east, along the Panna-Bassein structural trend, Palaeogene consists of limestone and shale. In the north east, Palaeogene is represented by thick shales, which are considered to be source rock for Bombay High (Miocene oil accumulation), Panna, Heera and Ratna (Palaeogene oil accumulation).

SIGNIFICANCE OF SUBJECT MATTER :

This paper briefly gives the review of exploration activity in the Bombay Offshore basin, which has been a prolific hydrocarbon bearing province in the Offshore area of India and where big oil and gas fields have been already found.
Rov Involvement During Installation, Inspection, Maintenance,And Repair Of Offshore Structures
Authors: Watt A. , Smith G. , Goldberg L.
Published: October 2008

This review paper addresses the role of the Remotely Operated Vehicle (ROV) during Installation, Maintenance and Repair of Offshore Structures. Tasks encountered by the ROV are discussed along with problem areas where some research and development is still required.- Two areas specifically discussed are the removal of marine growth for weld inspection and the bulk removal of marine growth in order to reduce structural weight and drag loadings.

INTRODUCTION:

The ROV is now well established as a viable tool for supporting drilling operation and conducting route and pipeline surveys. Personnel associated with offshore structure installation, repair and maintenance have been slower to accept the ROV. In many cases this is simply due to the unawareness of an individual as to the capability. It can also rightly be argued that the ROV is still very limited in some roles. It is the intention of this paper to highlight the tasks which an ROV can complete successfully and illustrate areas where the ROV is limited and the reasons why. Further research and development must be encouraged by the oil companies and research groups to overcome these shortcomings if the ROV is to be utilized to cut costs associated with installation, repair and maintenance of offshore structures. With successful research in the areas discussed another hurdle will be overcome in the history of the ROV resulting in significant cost savings to the end users ie the offshore structure owners and operators.

PLANNING INSTALLATION OF OFFSHORE STRUCTURES

When designing any offshore structure, the designer takes into consideration many factors. These include: method of installation, water depth (beyond diving range?) maintenance routines, repair techniques, expected structural life, etc. Likewise the ROV operator must consider which tasks have to be completed and choose the ROV best suited to the work. He can choose from his armory of:–Eyeball Class ROV's–Inspection Class ROV's–Workhorse Class ROV's

Eyeball Class: A small ROV which has limited capability but can adequately take still and video pictures. Some eyeballs can carry miniature manipulators and tools

Inspection Class: A slightly larger ROV with improved but limited manipulator capability. It also as a limited payload of 50 to 100 lbs.

Workhorse Class: A high powered ROV which can carry a payload in excess of 200 1bs and usually carries two full sized manipulators. It has the capability of interfacing many items of equipment, some of which can be custom designed for the application. This class is able to carry out the majority of the work tasks encountered offshore.

OFFSHORE STRUCTURE INSTALLATION PHASE

The offshore installation phase starts with the tow-out of the structure to the chosen site. Normal practice is to pre-lay a transponder network to allow accurate positioning of the structure relative to the seabed target area. The barge moves in and at this point the ROV can begin working.
Progress In Computer Simulations Of SPM Moored Vessels
Authors: Wichers J.E.W.
Published: October 2008

In the last ten years computer simulation programs for SPM moored vessels have been developed by several authors. At present the application of such programs, if at all, is limited to some preliminary calculations in the very early design stage. The reasons for the reluctance to apply such computation methods in the more final stage of the design is clearly due to the failure of reliable input data and uncertainties concerning the procedures to describe the governing physical phenomena more accurately.

In this paper experimental and theoretical results for the input and the methodologies involved in simulations of the behaviour of and the mooring forces induced by a large tanker moored in wind, irregular waves and current will be presented.

INTRODUCTION

A vessel moored in the open sea can perform high and low frequency motions. The low frequency oscillations can be an important problem. The large horizontal excursions that occur can cause large forces in the mooring system.

As is schematically indicated in Fig. 1, in the computation procedure distinction can be made in a low frequency and a high frequency part of the equations of motion of a moored vessel. Nowadays reasonably reliable computations can be carried out concerning the high frequency input for the equations of motion. For the low frequency part, however, uncertainties exist concerning the input of the hydrodynamic reaction forces and the wave drift force excitation in current. Since, in general, an SPM system is a weakly damped system, reasonance peaks in the modes of motions in the horizontal plane can often be clearly distinguished. Therefore in order to predict the motion responses correctly, attention must be paid not only to the excitation but also to the damping.

In this paper results will be presented on input data and computation procedures in order to clarify the mentioned uncertain terms in the equations of motion of the low frequency part.

In order to obtain the low frequency hydrodynamic damping an extensive model test program has been carried out. .A review of the experiments is shown in Fig. 2. Some results will be given in this paper. To show the computation procedures examples for 1-DOF and 3-DOF will be highlighted.

For the 1-DOF of a linearly moored 200 kDWT tanker the following computation procedures will be dealt with:–wave drift force and wave drift damping in still water;–wave drift force and wave drift damping in current;–viscous surge damping in still water and in current;–solution of the equations of motion in the frequency domain.

For the 3-DOF case a formulation of the coupled equations of motion for tanker motions in the horizontal plane (in still water and in a current field) has been experimentally determined. In the past descriptions of the equations of motion for the low frequency tanker motions in the horizontal plane have been given by many authors, ref. [1] through [11].
Deepwater Mooring Operations In The Gulf Of Mexico
Authors: Zumwalt C.E.
Published: October 2008

In the twelve years since exploratory drilling has begun on the Gulf of Mexico's continental slope, mobile offshore drilling units' mooring systems have become increasingly sophisticated and the technology for their design and use has become well established.

With the advent of sophisticated computer analysis software, improved weather forecasting, seafloor surveys, high holding capacity anchors, permanent chain chaser systems, upgraded anchor boats, Loran-C based rig positioning systems and effective planning and coordination, it is now possible to routinely moor a rig quickly and safely in from 600 to 2000 feet of water in the Gulf of Mexico.

There is increasing interest and competition in drilling on the Gulf of Mexico's continental slope. This paper reviews the new equipment and technology being used in the Gulf to safely and efficiently moor mobile offshore drilling units on the continental slope.

INTRODUCTION

The Gulf of Mexico's continental slope begins at the 600 feet of water depth line where the continental shelf ends and extends out to over 10,000 feet of water. Only depths to just under 3,000 feet have been drilled from moored, mobile offshore drilling units (rigs).

The continental slope from Texas' southern border to southern Florida is under the dominion of the united States government which has divided it into 12 lease areas each of which is subdivided into many approximately 3 mile square lease "blocks"1. These lease blocks are shown in Figure 1. With the exception of the Destin Dane area, Florida's continental shelf area is not a promising area for future drilling and, for this reason, these lease areas are not designated in the figure.

Of the over 280 exploratory wells drilled on the continental slope from moored rigs, the majority have been drilled in the Mississippi and Green Canyon areas as tabulated below 2

Particularly since the 1983 federal lease sale in these areas, exploratory wells are being drilled in increasingly deeper water as shown in Figure 2. Note that most of the wells were drilled from chain moored rigs even in the deeper water areas. This is due to several factors.

One factor is that the majority (65%) of rigs are chained moored vs a combination of wire rope and chain and that most of the rigs available in the Gulf of Mexico are also chain moored3 .Another factor is the introduction in recent years of higher strength chain commonly designated Super Oil Rig Quality or K4 chain which allows chain moored rigs to safely drill in over 2000 feet of water under most environmental conditions.

With the increasing rig competition the emphasis on efficient and safe mooring operations has intensified. In these deeper waters large amounts of mooring equipment must be recovered, stored and redeployed each time the rig is moved. To improve mooring efficiency and safety several trends have developed in the industry.
Wave Dynamics Of Jackup Rigs
Authors: Grenda K.G.
Published: October 2008

A study was conducted to investigate the dynamic response to wave loading of a typical mid-sized jack-up drilling rig in the elevated position. Results of the study revealed the potential for significant dynamic amplification of wave loads in water depths exceeding about the 150 ft range. Major parameters studied were water depth, damping ratio, and current velocity. The increase in wave-induced leg stresses due to the inclusion of dynamic effects was calculated to be on the order of 20% and 150% for the subject rig in 150 and 300 ft of water, respectively. Sensitivity to damping ratio was found to be quite high, while the effect of currents was minimal.

The dynamic analysis procedure used in the study incorporated a time domain simulation method with a simplified wave-load model. This procedure allowed for explicit treatment of wave-load nonlinearities while maintaining reasonable computer costs. The simulation results were cast in the form of probability distributions from which extreme dynamic response parameter were evaluated. These extreme response parameters were then used to define dynamic amplification factors which were easily incorporated into structural evaluations of the jack-up rig.

INTRODUCTION

Recent structural assessments of several jack-up rigs demonstrated the potential for significant dynamic amplification of wave loads in water depths exceeding about the 150 ft range. Incorporation of the wave dynamic effects into structural analyses resulted in substantial reductions in calculated limiting environmental conditions for the rigs in the 150 to 200 ft water depth range. Had the effects of dynamics been ignored in the structural analyses of these rigs, different conclusions would have been reached regarding the structural suitability of several of the rigs at the proposed drilling sites.

Because of these findings, a study was initiated to investigate the potential impact of dynamic effects in even deeper water. A single jack-up rig, selected as typical of the mid-sized jack-up rig fleet, was chosen for analysis in this study. The major parameters studied were water depth, current velocity, damping ratio and degree of spud can fixity. These latter two parameters were included since there exists considerable discrepancy in the offshore industry as to what levels of damping and what degree of spud can fixity are appropriate for dynamic analyses of jack-up rigs.

The purpose of this paper is to present the findings of this parameter study by, first, presenting results in terms of base shear and overturning moment dynamic amplification factors (DAF?s) and, secondly, by presenting the effects of a given DAF on the limiting environmental conditions calculated for the rig.

DESCRIPTION OF ANALYSIS RIG

The rig analyzed in this study is a typical mid-sized, three-legged, independent leg jack-up rig capable of operating in water depths of 300+ ft. The specific rig details are not of consequence to dynamic amplification except as they affect the fundamental natural period of the rig.