bluuebunny/crossref_metadata_2025_split huggingface dataset:
Probabilistic Extreme Values And Their Implication For Offshore Structure Design
Authors: Och Michel K. 
Published: October 2008

ABSTRACT
The concept of the probabilistic extreme value and its application for offshore structure design is outlined. In particular, a method to evaluate the design extreme value based on the long-term prediction approach using a family of wave spectra in each sea severity is discussed. Some results of numerical calculations made on a semi-submersible-type ocean platform are presented.

INTRODUCTION
For the design of offshore structures floating in a seaway, it is possible to assess the expected structural safety by applying recent technology developed on the probabilistic approach. The assurance of structural safety against the wave-induced loads may be expressed in terms of probability of occurrence of possible structural failure.
One type of failure which the present study primarily concerns is that which takes place as soon as a single load exceeds the value critical for the structural strength. For the probabilistic evaluation of possible occurrence of such failures, the probability density functions for the loadings as well as structural resistance are required.
If the probability density function of the structural resistance is not known or cannot be established precisely (which may be the usual case at an early design stage), then one way to determine the design value is to estimate the extreme load which an offshore structure may experience in a lifetime, and then design the structure to withstand this extreme load. For this approach, it has been common practice to consider the probable extreme value for the assessment of the design value. Here, the probable extreme value is defined as the modal value of the probability density function which is applicable for the largest load in a specified number of observations.
The probable extreme value has a significant meaning in practice since the largest value which actually occurs in n-observations may be very close to this value. However, the probability that the largest value exceeds the probable extreme value is theoretically quite large, and hence it is not appropriate to consider the probable extreme value for design. For design consideration a certain margin above the probable extreme value is required as an assurance for safety, and this can be achieved by estimating the extreme value which is unlikely to be exceeded with a pre-assigned assurance.
This paper discusses the principle and underlying concept for evaluating extreme values of offshore structures floating in a seaway specifically for design consideration, and a method of estimating extreme values based on the long-term prediction approach is presented.

DESIGN EXTREME VALUE
The probabilistic extreme value of waves, and responses of offshore structures thereto, is defined as the largest value of the maxima (peaks or peak-to-trough excursions) which will occur in a specified number of observations or in a specified period of time. Let X be the response of an offshore structure in a seaway, and let Yn be the extreme value of the response in n-encounters with waves.
A Case History of 3-0 Seismic Application in the Gulf of Mexico
Authors: Bone M.R. 
Published: October 2008

ABSTRACT
Atypical faulted Tertiary structure in the Gulf of Mexico was surveyed to obtain dense 3-D coverage for the purpose of ultimately guiding field development efforts targeted on proven productive horizons.
Comparisons can be drawn between the interpretive results of this program and more conventional grid shooting.
The benefits gained from data density and 3-D migration are explored in this geologic setting. The interplay of the data density with the velocity evaluation and direct hydrocarbon indicators is reviewed along with various aspects of display versatility.

INTRODUCTION
The escalated use of oil and gas products by each individual in recent years has generated concern that the known oil and gas reserves are not sufficient to provide current and near future energy needs of this nation. Consequently, the oil and gas industry is expected to find more reserves and produce them at "tomorrow's cost" and receive "yesterday's prices" for its efforts.
With these facts in mind, the oil and gas industry is faced with three challenges. First, where to search; second, how to locate; and third, how to develop these new reserves efficiently.
Since new frontiers are not as plentiful as they have been in the past, it is necessary that a portion of these future reserves be located in problem areas evaluated by "yesterday's technology". These areas include oil and gas fields currently producing, fields considered depleted, areas with limited surface access, and areas with complicated subsurface geology which prevents the making of an accurate subsurface map.

CASE HISTORY DISCUSSION
The area selected for this discussion could be described by a number of the above problem areas. It is located Offshore Louisiana (figure 1) in about 15 feet of water and has wells producing from multiple zones within a series of faulted blocks. Subsurface maps made from the available 2-D seismic data and well information have defined the major fault blocks from which the present production exists. Amore detailed seismic evaluation of each block could possibly pinpoint smaller traps that were overlooked by previous efforts.
The remainder of this discussion will review the results from a three-dimensional (3-D) seismic survey which was used to improve the subsurface picture and in turn provide valuable information for optimum field development. This survey technique is now considered a production method in the industry and has proved to be a valuable asset to field development.
In areas where no access problems exist, the conventional "3-D swath" technique may be used; whereas, areas having surface restrictions may require an adaptation of the 3-D method, such as the Seisquare1 or Seisloopl methods (figures 2 and 3).
The initial step in any 3-D survey is to evaluate the range of dip, velocity, frequency and depth of the zone of interest expected in the area. These attributes control the spatial sampling interval required to obtain optimum results.
Horizontal Spar Wave Direction Monitoring Buoy
Authors: Brainard Edward C. 
Published: October 2008

ABSTRACT
The equilibrium position of a center tethered buoyant spar buoy on the surface of the ocean is parallel to the crests of the wave train.
This phenomenon has been used in the design of a wave direction monitoring buoy which permits reliable and inexpensive measurement of wave direction by an unattended buoy recording system when placed in a coastal or open ocean wave environment. The system design is discussed, with emphasis on the conceptual and operational aspect.
Field tests are presented and discussed which indicate close correlation between measured and observed data.

INTRODUCTION
The measurement of surface wave amplitude and period provides only limited understanding of the wave forces on an offshore structure, vessel, or shore-based construction project. With an ever increasing emphasis being directed towards the ocean in search of petroleum and minerals, and with the construction of billion dollar structures, such as nuclear power plants, the need has developed for an instrument to measure wave amplitude and period, and also wave direction.
A number of oceanographic instruments have been devised to measure wave direction, but the art has remained a relatively complex task. A multiple float surface buoy1, multiple bottom mounted pressure sensors, multiple wave staffs, the use of a 2 axis horizontally oriented solid state current meter2, or airborne radar techniques have been employed but have disadvantages in baseline environmental surveys due to the need for special platforms, lack of reliability, or the need for complex and costly computer processing.
The method described in this paper measures wave direction directly from a center tethered lightly ballasted horizontal spar. The design minimizes the effects of current and wind and can be used with moorings in any depth of water. The recorded directional data is the direction of the wave train in cases of waves originating from a common origin or a measurement of the direction of the maximum wave energy in sea conditions of multiple wave train origins.

BACKGROUND
The concept of the horizontal spar wave direction monitoring buoy evolved from work carried out by Braincon Corporation (now Environmental Devices Corporation) on the P1ank-On-Edge (P.O.E.) buoy. It was observed during model tank tests and at sea that the buoy, with the absence of current, would orient itself parallel to the wave crests. It appeared that the buoy would orient to experience minimum forces from the waves.
A number of requests were received from customers for a buoy which would provide wave direction data for such projects as beach erosion studies, pipeline surveys, docking facility investigations and O.C.S. environmental baseline studies. The requests also stressed the need for a simple and reliable device, which was easy to maintain, and cost effective. The general requirement was for a device which provided a good first order measurement, not one which detailed the fine structure of the waves. The basic question was "Would the proposed structure survive?".
Extreme Wave Heights In The Gulf Of Alaska
Authors: Augustine Frank E. , Maxwell F.D. , Lazanoff S.M. 
Published: October 2008

Abstract
Expansion of Outer Continental Shelf (OCS) leasing into frontier areas like the Gulf of Alaska has been accompanied by a need for more complete environmental design data such as extreme wave characteristics and seismic risk. The U.S. Geological Survey (USGS) recognizing this need, has instituted programs to gather such information. The results of one of these programs, extreme wave data, are presented herein.
Extreme wave heights due to severe extra tropical storms have been estimated for three deep water locations in the Gulf of Alaska. These estimates are based on hindcasts of the wave climatology for the entire North Pacific Ocean. Wave statistics were compiled for a thirteen year period from 1964 to 1977. An analysis of these statistics yielded extreme wave height as a function of return period. The wave characteristics were computed using the Spectral Ocean Wave Model (SOWM) at the U.S. Navy's Fleet Numerical Weather Central (FNWC). This model was found to be applicable for wave estimation in this area through a favorable comparison between computed and measured wave heights using data taken at several Pacific locations.
The results of this study confirm the severe winter wave climate of the Gulf of Alaska. Although predicted extremes for this area are somewhat greater than those predicted for the North Atlantic and the North Sea they are not as great as have been predicted recently by other investigators. Two of the three locations are sufficiently close to the present offshore lease areas in the northeastern Gulf that deepwater wave estimates for these locations would provide the initial conditions for design wave estimation for offshore structures in this region.

Introduction
Petroleum industry interest in the Gulf of Alaska, particularly in the northern continental shelf has been strong. The Department of Interior scheduled a sale for the summer of 1969; however, this was cancelled because of heightened environmental concerns following the January, 1969 Santa Barbara oil spill. The second call for nominations in November, 1974 resulted in even stronger interest than the first and a lease sale was conducted in April, 1976 (OCS Lease Sale 39). The approximate location of the lease area is shown in Figure 1. The expansion of OCS leasing into frontier areas like the Gulf of Alaska has been accompanied by the need for more complete environmental design data such as extreme wave characteristics and seismic risk data. Up to the present time practically no data have existed in the public domain with which judgments could be made relative to the adequacy of structures intended for service in the Gulf of Alaska.
Inherent to the basic responsibilities of the U.S. Geological Survey for OCS leasing is the specification of requirements which must be met by offshore operators in the design, construction or operation of fixed or mobile facilities in these regions. These requirements are currently promulgated through OCS orders developed by the USGS.
Instrumentation Requirements for Kick Detection in Deep Water
Authors: Maus L.D. , Tannich J.D. , Ilfrey W.T. 
Published: October 2008

ABSTRACT
Because of the reduced difference between fracture and pore pressure gradients inherent in deepwater drilling, gas influx during a kick must be limited to much lower volumes than on land or in shallow water. To do this, instruments capable of detecting a kick at a very early stage are needed.
Using a recently developed computer program capable of modeling a kicking well, the transient behavior of several well parameters was investigated to determine which parameters are most indicative of a kick and what sensitivity is desirable for instrumentation to measure these parameters. Results show that instrumentation to detect changes in return mud flow rate offers the greatest potential. Pit volume and standpipe pressure instruments can provide valuable backup detection capability, provided they can be made sufficiently sensitive. Standpipe pressure instruments have potential advantages for floating drilling operations, since they should be unaffected by vessel motion. Desired sensitivity levels, based on the computer analysis, are presented for each type of instrument.

INTRODUCTION
Increasing water depth reduces the difference between the mud weight required to balance formation pore pressures and that which will result in formation fracture.1,2 Figure 1 illustrates this point at the seat of a 3500-ft surface casing string. If a 0.5-lb/gal margin is provided relative to both the fracture gradient and the pore pressure gradient (seawater gradient assumed), the range of allowable mud weights is substantially reduced as water depth increases.
Another way of illustrating the effect of increasing water depth is to consider the "critical kick size". This is the maximum gas influx that can be contained within a shut-in well without fracturing the rock at the casing seat. Influxes greater than this size will cause an underground blowout when shut in. Figure 2 shows the critical kick size as a function of water depth for kicks taken while drilling a 12 ¼-in hole 5000 ft below a 3500-ft surface casing string. Mud weights of 9.5 and 10 lb/gal and under balance conditions of 0.25 and 0.5 lb/gal are considered. While kicks of 150 to 250 bbl are necessary to produce casing seat failure on land, in 5000 ft of water the critical kick size ranges from zero to 100 bbl for the conditions assumed in this example.
This analysis shows that there is a definite incentive in deepwater drilling operations to detect and control kicks at an early stage of their evolution. The accuracy, sensitivity, and reliability of the drilling instrumentation are key elements in determining at what stage a kick will be detected and controlled by the drilling personnel. We have investigated the time-dependent evolution of kicks under a variety of assumed conditions to better define what types of instrumentation are useful for early kick detection and what levels of accuracy and sensitivity are desirable. In this work we utilized a recently developed computer program that models the transient behavior of a kicking well.
Structural Loads On Twin-Hull Semisubmersible Platforms
Authors: Paulling J.R. , Hong Y.S. , Stiansen S.G. , Chen H.H. 
Published: October 2008

ABSTRACT
This paper describes a computational system for the determination of the wave induced structural loads on a twin-hulled semi submersible platform. The total loading consists of hydrodynamic components due to incident waves, diffracted waves, and platform motions. Additional motion-dependent forces result from the body mass.
The resulting forces are expressed in three alternative forms:Distributions of pressures or, equivalently, lumped nodal forces over the surface and interior of the platform.Shear and bending moment distributions along the platform length.Forces and moments in the cross structure between the two hulls.
The procedure is embedded in a comprehensive program package for the analysis of platform motions, dynamic loads and structural response.

INTRODUCTION
In an earlier paper, Paulling, et al (1977), we have described a procedure for computing the total wave-induced hydrodynamic loading and resulting oscillatory motion of a twin-hulled semi submersible platform. Some of the characteristics of the various force components were described and in particular, the frequency-dependence of both forces and rigid-body platform motions was discussed. The solution for the rigid-body motions requires knowledge of the total force and moment integrated over the entire body.
A knowledge of the distribution of these forces over the body is needed for purposes of structural analysis. The structural analysis procedure which is employed in any given situation may involve different levels of sophistication depending upon the nature of the structure itself and the end objective of the analysis. As one example, a ship or platform may be assumed to behave structurally like a simple beam. The longitudinal distribution of the shear force and bending moment in the vertical and horizontal planes are required in order to compute the stresses and deflections of the structure. If a more detailed determination of stress is required, or if the geometry of the structure is such as to preclude the use of beam theory, a distribution of loads over a nodal mesh throughout the structure may be required as input to a detailed finite element structural analysis. A description is given in a recent paper by Stiansen, Chen and Lee (1978) of a computational system in which the present hydrodynamic load computation is combined with the ABS DAISY finite element program to provide a complete analysis capability for twin hull semi submersible platforms. Intermediate degrees of structural analysis may be envisioned in which the resultant forces on relatively large segments of the structure, may be used as input to an analysis of interconnecting components. An example of this type of load is the system of forces and moments carried by the transverse structure which connects the two hulls of a catamaran.
In the present paper we present the development of a computational system for the determination of the load distribution on catamaran type semi submersibles. The output of this computation includes options for obtaining.
Analysis Of Fatigue Crack Growth Behavior In Welded Tubular Tjoints
Authors: Kawahara Masanori , Iwasaki Toshio 
Published: October 2008

ABSTRACT
This paper describes an experimental investigation on fatigue strength of welded tubular T joints. Fatigue tests were conducted under axial tensile loading in brace tube with various chord supporting spans, which enabled to vary the ratio of nominal bend1ng stress 1n chord to the nominal tensile stress in brace.
The results are analysed both by conventional S-N diagram approach and by a fracture mechanics approach with reference to fatigue crack growth behavior.
Fatigue cracks initiate, often as multiple surface cracks, at the toe of weld reinforcement of the chord-brace interface. Crack growth occurs 1n two d1rectlons; extending around circumference and increase in crack depth. A simple empirical formula enables to interprete general behavior of crack growth taking account of the role of local out-of-plane bending stresses.

INTRODUCTION
Fatigue strength analysis is one of the most important subjects in the safety estimation of offshore steel structures. Fatigue damages are apt to occur at the welded intersection of tubular joint, where stresses are much enhanced by the discontinuities or abrupt changes in geometrical shape of elements.
Fatigue life is, in convensional approach, estimated by a set of stress versus life curves based on accumulated results of large or small scale fat1gue tests. Recently, fracture mechanics approaches have been applied to fatigue analysis by using a relationship between the crack growth rate and the stress intensity factor range which may provide en effective means of more detailed analysis on the process of fatigue damage in real structures.
The basic formula in fatigue crack growth analysis is given as follows;
(Mathematical equation available in full paper)
where da/dN is the rate of crack growth and ?K is the stress intensity factor range.
When the crack propagates only in one direction, equation (1) may give a simple description of the whole itinerary of crack growth: Integration of (1) gives a life estimation formula for given load cycles and given initial and final crack sizes Analysis on fatigue crack growth in wall-thickness direction in tubular joints have been attempted by several authors such as, Pan and Plummer (ref 1) Hibberal and Dover (ref. 2), etc.
Real crack growth, however, occurs in much complex morphology. As discussed in ref. 2 cracks extend around circumgerence as well as grow in depth. Moreover, cracks initiate often at multiple sites. Somet1mes, they coalesce to form a long unified crack; sometimes, they interfere the growth each other.
Crack growth in such cases is to be described rather by simultaneous equations as,
(Mathematical equation available in full paper)
where suffix i denotes each crack growth direction to be respected. Equations (2) may be rearranged in the following two parts, choosing a specific direction i=l,
(Mathematical equation available in full paper)
Solution of equation (3) characterizes changes in crack shapes during growth which may be expressed in a form such as,
(Mathematical equation available in full paper)