bluuebunny/crossref_metadata_2025 huggingface dataset:
Probabilistic Extreme Values And Their Implication For Offshore Structure Design
Authors: Och Michel K.
Published: October 2008

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

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

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

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

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

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

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)
Rescue of Diver Under Pressure
Authors: Galerne Andre
Published: October 2008

As divers plunge deeper under saturation conditions, they face increasing dangers as any illness or injury becomes a tremendous threat to their lives because of the limited medical treatment that can be provided to the diver during decompression at a remote site or on offshore rigs. In addition, in the case of an emergency rig evacuation, divers under pressure are helpless.

The above problems have been solved by a helicopter diver-under-pressure-transfer system, developed by IUC and recently demonstrated under simulated emergency conditions. The successful demonstration has shown for the first time that a diver or divers could be safely and rapidly transported while under pressure from a remote site (in this case ''Venture Two", a rig in the North Sea) to IUC?s North Sea Hyperbaric Center at Aberdeen, Scotland.

The major design criteria that the complete system weigh less than 2,600 lb and withstand an internal pressure equal to 750 ft was achieved by the engineers at Southwest Research Institute by sing a titanium alloy combined with sophisticated titanium welding techniques perfected by the manufacturer, TIFAB.

INTRODUCTION

In the North Sea oil fields, among the most dangerous diving conditions in the world, divers are called on to perform heavy construction tasks at deeper depths each year. As the working depths of the divers increase along with increasing number of divers employed in deep work, the risk of serious injury to a diver increases. With men living at pressure for weeks at a time, the increasing use of saturation diving makes the possibility of a medical emergency, such as appendectomy, heart attack, head injury, perforated ulcer, burns, infections, bodily injury, or accidental amputation, a major concern.

An injured or sick diver who had been working at 200 m would face at least a week's decompression eliminating the possibility of an immediate transfer to a shore-based hospital. Any effort to decrease the decompression time would jeopardize the diver's condition seriously; in fact, sound medical practice would be to prolong decompression due to the unknown effects of decompression on damaged tissues or organs. In cases where the victim's survival depends on immediate surgery or intensive care the only possibility that had existed was to location a qualified doctor and fly him and the necessary medical equipment to the rig. While some people feel that surgery or intensive medical care can be performed in the rig's deck decompression chamber the majority of people are of the opinion that offshore decompression chambers are not ideal locations for providing medical attention. Excessive noise vibration, possibly poor communications and lighting plus the limited equipment, space, and the poor hygienic conditions would result in a very high probability of failure and result in death or permanent injury to the diver.
High Accuracy Directional Surveying Of Wells Employing Inertial Techniques
Authors: Morgan D.G. , Scott A.G.
Published: October 2008

Drilling of multiple wells from offshore platforms has greatly increased the accuracy requirements for directional surveying. To meet this need a 3-dimensional method of surveying deviated wells to within one foot to the 13 3/8" Casing Shoe has been developed. The basis of the equipment is an inertial system which consists of a gyro stabilised instrument cluster with three accelerometers measuring the displacements. The advent of Inertial techniques offers the opportunity to complement, monitor and improve conventional survey techniques.

INTRODUCTION

The drilling of multiple wells from offshore platforms has greatly increased the accuracy requirements for directional surveying. Conductors are commonly drilled with seven or eight foot spacing and the accuracy required in this situation is essentially beyond the capability of conventional directional surveying equipment. With such spacings between wells the possibility of well collision cannot be excluded particularly since the density of wells may increase in future.

One may anticipate an increasing loss of production during concurrent drilling and production operations due to the uncertainty of hole position.

The need to define the bottom hole position in cases of loss of well control has increased. The source of concern arises from the doubts about the accuracy of conventional surveys and the doubt that the position of a deep well could be known within the detection range of homing devices.

TECHNOLOGY

Taking the original premise - that there is concern and doubt about the accuracy of conventional surveys - a premise which will not be rigorously evaluated in this paper but which surveys will endeavour to illustrate - the first question which arises is to define alternate systems.

Reliability and accuracy of gyro systems has existed for aircraft navigation and a natural progression arose to examine the more sophisticated navigation systems of military and commercial aircraft.

Sophisticated inertial navigation systems were developed in the 1960's for missiles and all weather combat aircraft. More recently they were applied as survey equipment in the artillery and topographical surveys. The navigation performance for aircraft is typically 1 nautical mile per hour - accuracy sufficient to assist transatlantic crossing the system is, however, capable of increased accuracy under different operating conditions.

THE SYSTEM

An Inertial Navigating system can be defined as one which is independent of information from outside the system except for gravitational effects.

The System consists of five main parts:Inertial platform containing accelerometers and gyroscopes mounted on three gimbals.Electronics control and computing system which processes the output signals from the inertial platform and calculates displacements and monitors the servos which stabilise the inertial platform.CDU (Control and Display Unit) which performs two-way communication between the system and the operator when initiating the system.Random Access Memory store for data storage.
Re-Examination Of P-Y Curve Formulations
Authors: Stevens J.B. , Audibert J.M.E.
Published: October 2008

In the analysis of piles supporting offshore structures, the key element in predicting the response to lateral loads is the determination of the appropriate lateral load - deformation relationships (p-y curves) for the soil. The present practice of constructing p-y curves is based on the results of lateral load tests on instumented piles and strength-deformation characteristics of the soil. (Matlock,1970; Reese et al, 1975; and API RP 2A, 1978)

A study was undertaken to compare the results from lateral load tests on piles up to 59 inches in diameter driven in soft to medium clays to that predicted using the above ways of constructing p-y curves.

In general, it was found that the predicted pile deflections were significantly greater than the observed deflections. Also the maximum bending moments were underestimated.

Two factors were identified as sources of the differences; The first was the assumed linear dependence of the soil-deformation characteristics on pile diameter. The second was the lateral soil resistance at shallow depths was greater than those given by the empirical methods.

Based on the study results, modifications are suggested to the present methods of constructing p-y curves.

INTRODUCTION

P-y curves are generally used to characterize the lateral soil resistance-deformation relationship for piles supporting offshore structures. These curves are related to the strength-deformation characteristics of the soil as obtained from conventional soil tests. The methods of constructing p-y curves for clays are empirically derived from the results of lateral load tests on instrumented piles.

The use of empirical methods for design purposes is justifiable when the design conditions approximate those upon which the method is based. One of the important considerations in using empirical methods in geotechnical engineering is the scale effect. Ideally the size of the test element should be the same as the actual element. However, in most cases, it is impractical to perform a full scale test.

The methods presently used for constructing p-y curves for clays were based on piles having diameters ranging from 6 to 24 inches. With the trend of installing platforms in deeper waters and more hostile enviroments, piles with diameters ranging from 5 to 8 feet are common. An extreme example is a number of 15-ft diameter piles driven in the Scheldt River delta to support a cable way. (Lubking, 1977) Piles of these diameters are much larger than those upon which the empirical methods are based.

Lateral load tests on instrumented piles comparable to those presently used offshore have been published. In addition, other researchers have presented results which suggest ways to account for scale effects. This paper compares the test results to those predicted using the present methods of constructing p-y curves and suggests some modifications.
Piezometer Measurement For Deep Penetration Marine Applications
Authors: Preslan W.L. , Babb L.L.
Published: October 2008

Design analysis and planning for installation of offshore structures requires information about the pore pressures in the foundation soils. Pore pressures may be used to calculate effective stresses for use in the study of slope stability, for bearing capacity studies of gravity structures, and for the evaluation of axial capacity of piles. Also, pore pressure information is required for hydrogeologic studies.

A special piezometer has been manufactured for measurement of pore pressures at deep penetrations in sand or clay soils arid has been used at offshore sites in the Gulf of Mexico. The device will operate to pressures amounting to over 2000 ft of seawater and has been used successfully at a penetration of 200 ft in a water depth of over 650 ft. The accuracy is generally within two feet of head of seawater. The piezometer and installation system are believed to be unique and the measurements are believed to be the first of their kind in deep-water applications.

The piezometer and its operational characteristics are described and practical applications and field data are presented. The influence of the borehole and installation method on the measured pore pressure is analyzed, and techniques for estimating in situ values from short term tests in clays are presented. The influence of in situ gas on the-measured pressure is examined along with geochemical methods of evaluating the concentration, physical state and probable effect of that gas.

INTRODUCTION

The application of effective stress techniques in marine geotechnical investigations is currently hindered by the difficulties inherent in measurement of the pore water pressures that may exist within in situ marine sediments. To date, measurements in the marine environment have generally been in shallow water and the piezometer tips have been installed to relatively short penetrations below the seafloor. When effective stress analyses nave been attempted for projects involving large thicknesses of sediment, the in situ pore water pressure has been estimated using correlations with other more easily measured parameters.

This paper describes a wire-line piezometer that has been used to measure in situ pore water pressures in the course of routine offshore geotechnical investigations. Such investigations are typically carried out from an anchored vessel using a rotary drilling rig to advance a borehole below the seafloor. At specified penetrations below the seafloor, a wire-line sampling tool is lowered down the inside of the drill pipe and out the open-center drill bit to sample the relatively undisturbed sediment below the bit. At selected locations between sample intervals or at the conclusion of sampling operations the wire-line piezometer described here is lowered through the drill pipe and inserted into the sediment below the bit. The pore water pressures sensed by the piezometer are transmitted to recording devices on the vessel via the wireline.
Small Scale Tests For The Determination Of P-Y Curves In Layered Soils
Authors: Allen Junius D. , Reese Lymon C.
Published: October 2008

Preliminary results of a series of lateral-load tests on a small-scale pile in layered soils are presented. The major objective of the tests was to measure soil resistance p as a function of deflection y along the length of the pile in a stiff upper layer. The soil profiles all consisted of a stiff, cemented clay overlying a soft clay with the thickness of the stiff layer being different for different tests. The pile was loaded transversely a short distance above the soil surface and measurements of moment along its length were made for each load increment. Deflection and slope at the top of the pile were also measured.

Measured load-deflection curves are compared to load-deflection curves computed by using p-y criteria for homogeneous soils. The results indicate that the p-y criteria used for the analyses give consistently reasonable or conservative results. Further analyses of the test results are expected to show the effects of layer thickness on the resistance offered by a stiff surface layer to lateral pile movement and assist in the development of guidelines for predicting pile behavior in layered soils.

INTRODUCTION

Pile foundations in the marine environment are frequently subjected to lateral loading due to wind, wave, tidal current, ship impacts, and submarine slope movements. Accurate estimates of foundation response to such loads allow more economic design of foundations and superstructures. A method of analysis which is now in widespread use for predicting the behavior of laterally loaded piles involves the solution of the fundamental equation of beam bending,

(Available in full paper)

The equation can be expressed' in finite difference form and a digital computer is essential for solution of any realistic problem. The soil resistance as a function of deflection is expressed in the form of p-y curves. Criteria for constructing p-y curves have been presented by Matlock (1970) for soft clay, Reese et a1. (1975) for stiff clay below the water table, Welch and Reese (1975) for stiff clay above the water table, and Reese et al. (1974) for sand. These authors have presented recommendations for both static and cyclic loading. Gill and Demars (1970) have presented a set of p-y criteria for static loading.

Most of the p-y criteria were based on the results of one or several lateral load tests in fairly homogeneous soils, coupled with earth pressure theory. Results of analyses based on the criteria for homogeneous soils have been compared to additional field load test results by Stevens and Audibert (1979), Bushan et al. (1979), Ismael and Klym (1978), Lee and G1lbert (1979), and Meyer (1980). Suggestions for modifications have been made by Stevens and Audibert (1979), Lee and Gilbert (1979), Meyer (1980), and Sullivan (1977). The evaluations which the p-y criteria in homogeneous soils have undergone have brought some degree of confidence to the use of these methods.
A Riser Angle Positioning System (Raps)
Authors: Dean Quenton W.
Published: October 2008

A system has been developed to provide adequate position reference backup for a dynamically positioned vessel's primary position reference. This system employs two or more riser angle indicators, one located at the top and one at the bottom of the riser with any additional sensors located at intermediate points along the riser. A mini-computer, either a DEC PDP-8E or a PDP-ll-03L, is required to compute the vessel offset from the target using the riser angle data. The system was tested aboard the Discoverer Seven Seas drillship in 4,876 feet of water off of St. Johns, Newfoundland on August 15, 1979, and has been used aboard the ship since. This ship is positioned by a Honeywe11 Automatic Station Keeping (ASK) system uti1izing the Honeywell RS-sB acoustic position reference system as the primary position reference.

With the aid of the test results and the operation al data, the performance of the Riser Angle Positioning System (RAPS) and the theory of operation will be presented in this paper.

INTRODUCTION

RAPS is an integrated reference system employing the hard wired angle sensors and the primary acoustic reference system. Response from the lower riser angle indicator system originally supplied by Honeywell as a backup system to the acoustics has become increasingly suspect while drilling in water depths greater than 2000 feet. The shallow water acceptance of the taut wire system employed on other Dynamic Positioning System (DPS) vessels led to the development of RAPS using both a top and bottom riser angle indicator to provide reliable position references in up to 6000 feet water depth. The inertial reference systems being offered by certain manufacturers also led us to consider using the motion within the riser as an aid in determining the position of the vessel. The excellent performance of the RS-SB system encouraged-an adaptive system that would provide bumpless transfer between RAPS and the RS-5B system.

The, basic design objectives of the integrated reference system, RAPS, were:To develop an algorithm capable of determining the location of the end of the riser with respect to the top of the riser knowing at least the angle and bearing at the top and bottom of the riser.To develop algorithm constants by comparing the calculated position to a known position i.e. the RS-sB data.To adhere to a "no one sing1e fau1t wi11 cause a drive off" design objective. This would require extensive filtering and logic to determine the RS-sB data valid before altering the algorithm constants. It would also require logic to validate the riser angle raw data and the calculated RAPS data.

COMPONENT SET-UP (ASK-RAPS)

The importance of having accurate, reliable signals can not be over emphasized. The ASK has held position excellently on the position reference it has received. However, it has been the stability of the position references which have led to the development of RAPS.
Predicting The Drivability Of Large Diameter Offshore Piling
Authors: Lang George R.
Published: October 2008

Large diameter piling are increasingly being used in offshore structures to house the well conductors. In order to predict the drivability of these large diameter members, it is desirable to utilize the wave equation theory developed by Smith and others, which has been used for a number of years to examine the drivability of conventional piling. This paper examines the questions:"Does the wave equation theory accurately predict the drivability of large diameter piling?" and"If so, what variations from the standard input data are required?"

Three recently installed structures were analyzed and the predicted blow counts and driving stresses were compared to those measured in the field. The first of the structures was a 96- to l68-inch diameter, tapered, self-supporting caisson, and the other two were mudslide platforms with l25-inch diameter piling. Two variations from the standard Smith input data were required to predict the drivability of these piles. The first variation was a result of the fact that large diameter piling will usually require a tapered drive head because of the smaller diameter of the standard pile cap. In addition, large diameter caisson well protectors are usually tapered above the mudline to make more efficient use of the steel. Formulas were derived to simplify calculation of the stiffness and weight of a tapered, non-prismatic pipe segment. Then a study was made comparing the use of tapered drive heads with standard pile caps to the use of a special large diameter pile cap. The second variation from the standard input data is related to the problem of soil skin frictional "set-up" and end bearing resistance. The wave equation theory was found to accurately predict the drivability of large diameter piling with only a slight variation from the standard Smith method of analysis.

INTRODUCTION

Special precautions are sometimes taken to protect the well conductors in an offshore drilling platform. One way of protecting the wells is by housing them inside a large diameter member. In a mudslide structure, the piling may serve the dual purpose of supporting the platform and housing the wells to protect them from unstable soil movements. In some situations a self supporting caisson well protector can be used in lieu of a platform to support the well cluster when dynamic or fatigue problems exclude the use of self-supporting well conductors. The caisson may be a constant diameter pipe or the more efficient tapered member.

The design engineer must determine the answer to two questions. He must decide whether or not the pile can be driven to the design penetration or if drilling or jetting will be required. Secondly, he must be able to predict the driving stresses induced in the pile. New massive hammers are available and the engineer needs to know whether a large hammer is required or if a smaller and cheaper conventional hammer will suffice. Naturally, the engineer would like to utilize the one dimensional wave equation for the evaluation of the drivability of large diameter piling.
Special Offshore Piledriving Test
Authors: de Sitter Wolter R.
Published: October 2008

Predictions of the bearing capacity of foundation piles generally are based upon soils investigations on site and in the laboratory. In addition, load tests on completed piles have been performed. These provide the checks on prediction methods and experimental validation of established design methods. Actual load tests on large offshore piles however, are very few and tests on piles with bearing capacities of 20 to 40 MN (4.500 to 9.000 kips) would lead to prohibitive costs. Hence an increasing importance is attached to the significance of the Soil Resistance during Driving (SRD) as a yardstick for - or at least a check on - the ultimate static bearing capacity. The SRD may be determined by dynamic measurement of strains and accelerations in the pile and also by post-analysis of blow-counts. On a test site in the Netherlands a 84" O.D. batter pile (6:1) was driven by a Hydroblok type HBM 4000 hammer to 70 m penetration. The pile was instrumented to determine SRD. Accelerations were determined by a new strain difference technique as an alternative to conventional accelerometers. An extensive soils investigation on site and in the laboratory was included in the test program.

Post-analysis of blow-count lead to values close to the static bearing capacity at successive levels predicted according to the Cone Penetration Test (CPT)-method. The SRD obtained by measurements proved to be a lower bound for the predicted static bearing capacity (80%).

Offshore field evidence from the driving of anchor piles in the Eastern Scheldt show the same correlation between SRD Obtained by post-analysis of blow counts and predicted static bearing capacity on the basis of Cone Penetration Tests, except in cases where the CPT's indicated extremely high values for cone resistance and local skin friction.

INTRODUCTION

The delivery procedure for two Hydroblok HEM 4000 hammers included an endurance driving test. The opportunity to gather information of a more general nature on the behaviour of the interaction between soil, pile and hammer generated sufficient international support of the industry, certifying authorities and scientific organisations to include soils investigations and instrumentation of the pile in the test program. In this paper some of the results and additional field evidence are presented. The methods used to obtain SRD values and static bearing capacity are summarized and discussed. Data on the test programme and additional field evidence are introduced, followed by a summary of the results and conclusions from comparisons of SRD values and predicted bearing capacities. The objective of the paper is to contribute to the development of methods to determine axial bearing capacity of piles during the actual driving process. Controlled pile-driving presents the opportunity to collect fairly accurate information on the soils during driving. The problem needing the most attention in the future is the interpretation of this information with respect to the in service behaviour of the piles during the lifetime of the construction.