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Proceedings Papers

Publisher: Offshore Technology Conference

Paper presented at the Offshore Technology Conference, May 6–9, 2019

Paper Number: OTC-29289-MS

... the (1988) SNAME T&R Bulletin 5-4. 4 OTC-29289-MS The presented measures of variability in reported wind loads, or data scatter, is standard deviation and relative standard deviation ( r) the standard deviation divided by the

**mean****value**, expressed as a percentage. Coordinate System. Two reference...
Abstract

This paper documents the results from the 2017 Society of Naval Architects and Marine Engineers (SNAME) OC-8 Panel Comparative Wind Load Study. Initial unpublished results were presented at a one-day panel at the 2017 SNAME Maritime Convention; however, the final results are brought together for the first time in this paper. A blind, comparative study was organized through the SNAME OC-8 Panel in 2017 to assess the relative accuracy and repeatability of existing wind load estimation methods. Twenty-five companies and organizations throughout the world participated in this study, which encompassed three available wind load estimation methods: empirical building block procedures, wind tunnel testing, and Computational Fluid Dynamics (CFD). To permit an ‘apples-to-apples’ comparison, the same representative semisubmersible design was used by all participants, including a single physical model shipped consecutively to each of the five wind tunnel facilities participating in the study. The most significant finding from the study is the remarkably low variability in wind tunnel and CFD results relative to the empirical building block method incorporated in the U.S. Code of Federal Regulations (CFR), classification rules, and industry codes for stability calculations. Moreover, only wind tunnel and CFD results were able to accurately quantify the contribution of a lifting force and its effect on the overturning moment. The lessons learned from the comparative study will be incorporated into a long-awaited revision to SNAME's wind tunnel testing guideline, and has inspired the development of a new industry guideline which will broadly address wind load estimation methods in design, including the use of CFD throughout the design spiral.

Proceedings Papers

Diego Felipe Barbosa de Oliveira, Diogo Ferreira Alves Pereira, Gustavo Echenique Silveira, Pedro Andrade Lima Sá de Melo

Publisher: Offshore Technology Conference

Paper presented at the Offshore Technology Conference, May 6–9, 2019

Paper Number: OTC-29564-MS

... constraint

**mean****value**reservoir management injection rate reservoir model waterflooding application field application optimal reservoir management black line upstream oil & gas enhanced recovery water injection rate simulation run oil production implementation artificial intelligence...
Abstract

Reservoir management in offshore fields is a challenging task, particularly for mature fields because of a typical excessive production of water and/or gas. Due to several constraints on facilities capacity, assisted reservoir management process can deliver solutions to operate properly offshore fields, seeking for increasing oil production with better assessment of water and gas production and injection. Optimal Reservoir Management (ORM) can be applied aiming to maximize reservoir performance and to deliver well controls applicable to field operations. In this work, we implemented an assisted optimization procedure to maximize overall oil production for a field offshore Brazil in Campos Basin. We applied our ORM technique in an important field offshore Brazil, where cumulative oil production is maximized by optimally controlling water rates through injecting wells. Injection rates can vary in time, honoring operational requirements of smoothness. Geomechanical limits on injection pressures are considered to avoid loss of rock integrity and platform constraints on overall production and injection are imposed all times. Our approach deals with reservoir uncertainties described within a large set of calibrated simulation models to decide on optimal injection rates, taking into account possible risks. The model-based ORM under uncertainty that we developed has shown gains in total oil production over 20 years of operation up to 7.2% with respect to the base strategy currently applied. On average, results show an increase around 4% in oil production, with concomitant reduction in total water production and in the overall water injection. A pilot test in the actual field has been implemented to verify the consistency between modeling and reality (data observation), in order to guarantee that the gains forecasted by our study are feasible. We have chosen an area in the field to proceed an optimal injection control pilot, aiming to check the quality of the uncertain models in comparison with the observed data in practice. The pilot area has been selected based on aspects related with geological description, connectivity expected in the reservoir and operational constraints. The results of 8 months of pilot show clear coherence between models and reality within the uncertainty range expected to the reservoir of interest. To the best of our knowledge, it is the first time that an offshore field is actually operated based on a set of controls obtained by an assisted optimal reservoir management procedure, although it was performed still in a pilot scale. Results suggest robust benefits under reservoir uncertainties consideration and large-scale application shall take place soon, but it is out of the scope of this work. The pilot provided more confidence on field applications, leading to broader perspective for full-field implementations.

Proceedings Papers

Publisher: Offshore Technology Conference

Paper presented at the Offshore Technology Conference, April 30–May 3, 2007

Paper Number: OTC-18947-MS

... otc 18947

**mean****value**diameter maximum value offshore technology conference minimum value standard deviation operation strength pipe size wt material deviation requirement pipe spec transverse value tensile strength OTC 18947 Deepwater SCR Manufacturing Criteria R.T. Hill, EWI...
Abstract

Abstract The successful design and installation of large diameter SCR relies on close control of the mechanical properties and the dimensional tolerances used in the manufacture of the line pipe product. Statistical assessment of the resultant properties and process control data has shown in the ability to provide pipe dimensions of up to 20 inch OD and 1.22-inch wall for an SCR application in over 7900 FSW. This paper describes the manufacturing process used to produce a large diameter deepwater riser with closely controlled mechanical and dimensional properties. A statistical presentation of the resultant line pipe properties is included to demonstrate the effectiveness of the production process control parameters. Introduction The use of steel catenary riser (SCR) systems has gained prominence within the past few years. Predominately small diameter SCRs in the range of 6-inch to 12-inch have been used for infield flow line tie-backs; with diameters of up to 18-inch diameter being installed for export service. With the introduction of deepwater semi-submersible production platforms the installation and operation of large diameter SCRs has bought about increased requirements in the mechanical and dimensional characteristics of the pipes. Meeting the vessel response associated with a semi-submersible hull design, regardless of the draught and mooring system, can be difficult. Supplemental specifications have been and will continue to focus on more stringent criteria, such as controlled strength levels, enhanced stain capacity and improved fit-up and fabrication performance. To withstand the demands of high installation loads, and extreme operating conditions, such as loop currents, and hurricane force winds and waves, seamless and longitudinal submerged-arc welded line pipe are being manufactured using the latest in steel making, rolling and line pipe production technology. Unique to the use of longitudinal submerged-arc welded line pipe is the ability to meet pipe end dimensional requirements to minimize offset conditions in the girth welds (hi-lo conditions), thereby reducing stress concentration factors that are used in determining the life expectancy of the riser system. With seamless pipe the ends can be machined (boring operation), whereas longitudinal double submerged-arc welded pipe must obtain these tolerances as manufactured. Deepwater SCR Requirements There are a number of material and dimensional requirements deemed necessary to insure the safe installation and continued operation of a deepwater riser system. The ones chosen for presentation include: Chemical Composition Mechanical & Toughness Properties Dimensional Characteristics Manufacturing Controls Specifics of each will depend on the installation technique and service conditions. Chemical Composition Control of the chemical composition predominantly focuses on weldability, resistance to environmental stress checking and the ability to secure high strength, ductility and toughness. To assist field and shop fabrication welding, the chemical composition is restricted in the form of a carbon equivalent (CE) or cold cracking parameter (Pcm). Usually values of 0.38% and 0.20% are specified as maximum values. A sulfur content of 40 ppm is also required to insure toughness and resistance to environmental degradation. For weld metals, the hydrogen and oxygen contents are verified to insure the integrity of the as deposited properties. Also limits will be specified for chemical elements used in sulfur reduction and de-oxidation of the steel so as not to impair the welding operation.

Proceedings Papers

Publisher: Offshore Technology Conference

Paper presented at the Offshore Technology Conference, May 1–4, 1995

Paper Number: OTC-7757-MS

... failure rate decision support system frequency standard deviation

**mean****value**reliability analysis strength random variable upstream oil & gas OTC 007757 Model Uncertainty in Offshore Geotechnical Reliability R. B. Gilbert, The Univ. of Texas at Austin and W. H. Tang, Univ. of Illinois at...
Abstract

ABSTRACT Model uncertainty is a dominant source of uncertainty in offshore geotechnical reliability. Caused by limitationsin data quantity and quality, model uncertainty leads to uncertainty in the failure rate for a design. [n this paper, an approach is presented for evaluating model uncertainty and incorporating it into reliability analyses. This approach provides the ability to estimate statistics of the failure rate. The failure rate is of interest because (1) it represents an objective measure of performance, (2)it should enter into design decisions since it reflects quality, and (3) it can be observed, providing the ability to reduce model uncertainty by analyzing observed successes and failures. The value of reliability analyses will be enhanced greatly if a physical meaning (i.e., the rate of failure) can be assigned to the results. INTRODUCTION Foundation design for offshore structures represents one of the earliest applications of reliability methods in geotechnical engineering. There are substantialuncertainties associated with the offshore environment concerning storm loads, site characterization and foundation behavior. In addition, costs for offshore construction are large and the consequences of failure are significant. Because of these large uncertainties and costs, considerable attention has been devoted during the past 20 years to develop reliability-based design pproaches for offshore foundations (e.g., Bea Wu et al 1 ). The primary product of a reliability analysis is the probability of failure for a component or system. For example, Tang and Gilbert2 evaluated the reliability three pile foundation systems constructed during the early 1980's. They obtained probabilities of collapse over a 20-year design life ranging from 0.005 to 0.01. Lacasse and Nadim 3 presented failure probabilities ranging from 0.00007 for bearing capacity failure of a for gravity structure to 0.02 for failure of a platform system under seismic loading. In both papers, the authors contended that the calculated probabilities were not 'absolutd' quantities but expected probabilities, at best, hich could be used for comparing different designs. 11-wsecalculated probabilities were not based purely on actuarial data, and therefore contained a large measure f subjective judgment and interpretation. In fact, eotechnical reliability analyses are somewhat unique inhat this component of judgment and interpretation due tolimited data, which we will refer to as model ncertainty, is dominant. In recent years, several efforts have been directed at calibrating estimated failure probabilities for offshore structures 4,5 . Which the value of a reliability analysis is greatly enhanced if a physical meaning can be assigned to the failure probabilities, a clear understanding of model uncertainty is required to achieve this object paper, we provide a definition of model uncertainty anddistinguish it from random uncertainty, identify sources of model uncertainty, and discuss how to evaluate and represent model uncertainty in reliability analyses. DEFINITION OF MODEL UNCERTAINTY Random Events Uncertainties abound in offshore foundation design; soilproperties, loading conditions and foundation behavior are not known with certainty at different points in time and space. There is a risk that a foundation will not perform as designed because of these uncertainties.

Proceedings Papers

Publisher: Offshore Technology Conference

Paper presented at the Offshore Technology Conference, May 6–9, 1991

Paper Number: OTC-6783-MS

... sea state reliability probability mooring system safety assessment line dynamic tension reliability analysis coefficient resistance assessment line strength reliability technique variation subsea system link strength strength variability safety factor water depth

**mean****value**...
Abstract

ABSTRACT The paper deals with the reliability of offshore mooring systems. Reliability techniques have been applied to assess mooring systems designed by the quasi-static method. The study identifies that the variability of mooring line strength due to uncertainties in link strength and dynamic tension response are the two crucial parameters which affect the system safety, and further study is carried out to investigate their effects. It has been concluded that due to the simplistic and deterministic approach adopted, the quasi-static method is unable to address the variability of design parameters, and therefore mooring systems so designed often have inconsistent safety levels. 1. INTRODUCTION Mooring system failure frequently occurred in the past (about average once every 2 or 3 years per rig) and questions have been raised as to whether the presently used quasi-static method is adequate to cope with mooring system designs in extreme conditions. In this paper, mooring systems designed using this method have been assessed using reliability techniques. The main objectives of the study are: To perform a reliability analysis of a typical semisubmersible mooring system in order to identify the general acceptance level of mooring system safety To assess the reliability of mooring systems designed according to the present quasi-static method To identify the critical design parameters and assess their effects on mooring system safety. To perform the reliability analysis, typical North Sea environmental conditions are modelled and the mooring system static and dynamic responses are predicted. Probabilistic descriptions of tensions and line strength are then defined to compute the probability of failure. The technique is first applied to the mooring system of a semisubmersible unit, Sedco 700 series in 150m water depth designed according to the quasi-static method. The reliability analysis has shown that the variability of chain link strength and the line dynamic response are the two most important parameters which affect the probability of failure. The study has concluded that mooring systems considered to be at the limit using the quasi-static method have inconsistent safety levels. The study has also identified a number of areas which warrant future studies. 2. PROBLEM DESCRIPTION As most engineering design problems, the design of offshore mooring systems can be broadly categorised into two main approaches: Allowable working stress approach Probabilistic or reliability approach In design approach (a), the system resistance capacity required can be determined by: (mathematical equation) (available in full paper) where Q is the resistance, L is the load and SF is the safety factor. This approach implies that both the load and resistance can be perfectly predicted as shown in Fig.1, and when successfully designed, the probability of system failure is zero. However, in reality, due to the randomness of environment and our ability to predict the system response precisely, uncertainties are inevitable. Therefore every system has a finite probability of failure. This is why the design method (b), reliability based approach can be utilised for a better description of system safety.

Proceedings Papers

Publisher: Offshore Technology Conference

Paper presented at the Offshore Technology Conference, May 7–10, 1990

Paper Number: OTC-6455-MS

... fabricators. 2 Not surprisingly, however, very significant Defect aspect ratio differences between the values of W for different types of defect were detected. The defect aspect ratio was defined as the ratio of the length L to the width W. The

**mean****values**of this ratio For the purposes of distribution...
Abstract

ABSTRACT This is one of a series of papers describing an analysis of the occurrence, size and type of defects in the welds of the lower hull of the Hutton Tension Leg platform found during a detailed survey of the structure in the late stages of fabrication. This paper gives the results of an analysis of the dimensions of embedded defects detected and sized by ultrasonic testing, and involving the examination of approximately 27 km of weld by the method. Analysis has been undertaken of characteristics such as defect width, defect length and aspect ratio. Sufficient data were available to investigate the influence of parameters such as the type of welded joint, welding process, welding procedure and defect type on the defect dimensions. Results are given in terms of the statistical distributions of the sizes of embedded defects for different types of welded joint and types of defect, and are based on a sample of approximately 8300 defects. INTRODUCTION This paper describes the conclusions reached during a detailed analysis of the dimensions of embedded defects in the welds of the lower hull of the Conoco Hutton Tension Leg Platform (TLP). The data that have been analysed are part of a large data bank compiled from the results of the non-destructive examination (NDE) of about 27 km of weld by ultrasonic testing (UT) and about 76 km of weld by magnetic particleinspection (MPI) - a total of 121,270 UT and MPI examinations. These data have provided considerable scope for analysis and a general overview of some of the results has already been published by Baker et al. 1 A subsequent more detailed analysis of the frequency of occurrence of defects and the occurrence of multiple defects has been given by Kountouris and Baker. 2 This paper is concerned with the distribution of the size of embedded defects and with the relative frequenty of occurrence of defects of different types. Information on the sizes of surface-breaking defects has been reported elsewhere. 3 In addition, a number of detailed report a have been prepared which discuss various aspects of the analyses undertaken. 4–6 Knowledge of the occurrence rate and size distributions of defects in completed structures and knowledge of the factors which influence them are useful in the planning of NDE procedures and also in the safety assessment of structures which could fail by fatigue and fracture, see e.g. Ref. 7. It must be stressed, however, that because the structural form of the Hutton TLP is radically different from that of typical jacket structures, having details more akin to those found in ships' hulls, care should be taken in extrapolating the conclusions from the TLP to other forms of offshore construction. BRIEF DESCRIPTION OF THE STRUCTURE AND NDE HISTORY The Hutton TLP is well known as the first tension leg structure and was installed in the North Sea in 1984, with oil production starting later than year.

Proceedings Papers

Publisher: Offshore Technology Conference

Paper presented at the Offshore Technology Conference, April 27–30, 1987

Paper Number: OTC-5543-MS

.... This data base was completed with the environmental data for the same period of time: Significant Wave Height (Hs), Significant Wave Period (Ts),

**Mean****value**of Wind speed for a 10 minute reading (W),**Mean****value**of Current speed for a 20 minute reading (C), and their geographical directions...
Abstract

ABSTRACT Measurements of movements were carried out on board a compliant platform in the North Sea during one year. Data processing established relationships between platform response parameters and environmental parameters. Comparisons between the measured results and the predicted platform design behaviour show that observed movements are similar to calculation predictions. INTRODUCTION In order to acquire better understanding of the dynamic behaviour of a compliant structure, and to confirm the validity of calculation tools used for this behaviour prediction during design, a mean is to analyse measurements of movements carried out on board an existing compliant platform. This paper is related to the description of movements recorded from March 1984 to March 1985 on board an articulated column, installed in a -depth of 102 m of water in the North Sea, statistical treatments carried out on movements and environmental conditions. The concept of articulated column which is being developed by ELF AQUITAINE since 1964 has been described several times (Ref. 1). DESCRIPTION OF THE PLATFORM, DATA ACQUISITION AND DESIGN BEHAVIOUR CALCULATIONS NORTH EAST FRIGG Field Control Station is an articulated column operating since 1983 in a depth of 102 m of water, 16 km from the FRIGGFIELD in the North Sea. It supports the production control system of a subsea well cluster (see figure 1). A concrete gravity base is connected to the column by a universal joint. The column is a steel cylinder (Ø 8 m) with a float (Ø 14.6 m) at the upper part, and supports an unmanned deck (650 t) at + 131 m above mud-line. A solid ballast in the lower part of the column body counteracts the uoyancy tank (see figure 2) (Ref. 2–3). Two linear accelerometers on the methanol deck and one roll and pitch sensor in the living quarters measure inclination and acceleration along North-South and East-West directions with a 4 hz frequency, for 17 minutes, every 20 minutes. Some of these "raw data" sequences were recorded. For each sequences of 17 minutes, the following reduced data are computed and recorded on board: Mean Inclination of the column (I mean), Root Mean Square Inclination (IRMS), Maximum Inclination (Imax), Root Mean Square Acceleration (ARMS), Maximum Acceleration (Amax), and their respective geographical directions. This gives us a large data base of around 19000 reduced data for each parameter over one year. This data base was completed with the environmental data for the same period of time: Significant Wave Height (Hs), Significant Wave Period (Ts), Mean value of Wind speed for a 10 minute reading (W), Mean value of Current speed for a 20 minute reading (C), and their geographical directions.

Proceedings Papers

Publisher: Offshore Technology Conference

Paper presented at the Offshore Technology Conference, May 5–8, 1986

Paper Number: OTC-5206-MS

... out on the effect of test length on the results. ote 5206 wave response extreme event frequency component upstream oil & gas rayleigh distribution statistics fpso mooring system stiffness assumption nonlinear dynami cs oscillation

**mean****value**surge response subsea system...
Abstract

1.0 INTRODUCTION The designer of Floating Production Systems need to determine the safe working limits for a floating platform over a period of many years. One of the most important parameters is the loads experienced by the mooring system over the life of the system or between inspections. This paper studies one such system and probabilistic description of the mooring loads. The technique depends on separating the slowly varying component from the wave frequency motions and making several assumptions concerning the statistical independence and the nature of the resulting distributions. The paper differentiates between line dynamics and mooring dynamics. Line dynamics are considered as a separate phenomena that is due to the dynamic oscillations within the mooring line while mooring dynamics are considered as the global motions of the system. Here we will not consider line dynamics, this does not imply that the line dynamics are unimportant. On the contrary, we believe that under certain circumstances they are very important and should be included in any final design evaluation, but rather that they are beyond the scope of the present investigation. DESCRIPTION OF THE SYSTEM The system modeled is a turret moored tanker with an eight point chain mooring pattern. Modeled at a scale of 1 : 53 using Froude scaling. The prototype system consisted of a 200,000 DWT tanker moored with 5000 ft of 5 1/4" chain in 1476 ft of water. The system is described in Table 1. Figure 1 gives a detailed layout of the system. A stock tanker model was outfitted as an FPSO with facsimiles of the production equipment decks in addition to the deckhouses to stimulate the wind loads. A Turret was modeled and located just aft of the watertight bulkhead between tanks 1 and 2 for a typical tanker. The breakout torque of the turret was modeled to permit weathervaning. Because of the impracticability of modeling the actual mooring system at a reasonable scale a horizontal mooring configuration was modeled (see figure 2) and the data extrapolated using STAMOOR (ref 1) to determine the actual mooring line tension. This procedure was shown to be accurate as shown in Figure 3. This is a plot of mooring line tension against surge displacement with the spring stiffness plotted through the data. It can be related directly to the displacement through the non-linear spring stiffness. At the lower turret exit a simulated flexible riser system was attached. Modeled for visual inspection only the risers were connected in a J configuration to a submerged buoy platform. This platform supported the risers connected to the bottom of the test tank. Table 2 gives the complete environmental parameters for the tests performed. TEST DESCRIPTION The tests analyzed in this paper consisted of collinear wind, wave and current parameters were set independently and superimposed upon one another to arrive at the complete environment. This is perhaps not the most realistic but it does have the advantage of simplicity and allows each individual component of the environment to be measured accurately. The principal effect of the wind and current was to add a large mean offset to the system on the order of 30 feet. Due to the predominance of the slowly varying response of the system an investigation was carried out on the effect of test length on the results.

Proceedings Papers

Publisher: Offshore Technology Conference

Paper presented at the Offshore Technology Conference, May 7–10, 1978

Paper Number: OTC-3066-MS

... coefficients has received little attention. Currently there is no rational procedure to generally account for coefficient uncertainty except to use arbitrary, and potentially unrealistic, guidelines such as the

**mean****value**plus a multiple of the standard deviation. The purpose of this paper is to provide a...
Abstract

INTRODUCTION The wave forces on an offshore structure are determined by a wave theory (e.g., Stokes or Stream Function) which relates the water kinematics (velocity and acceleration) to the wave parameters (height and period), and a theory which relates the resulting pressures on the structure to the predicted water kinematics (e.g., Morison Equation or Refraction Theory). Generally, the Morison Equation is used which incorporates two force coefficients-the drag and inertia coefficients. The wave parameters experienced by a structure during a storm are random. Also, inferred values of the force coefficients from field measurements indicate a random scatter from wave to wave due to the random nature of the processes involved and imperfect wave and hydrodynamic theories. Therefore, for offshore structures the prediction of wave forces and ultimately the selection of design criteria involve both the random nature of the wave parameters (e.g., height) and the uncertainty in the force coefficients. Procedures for selecting wave heights for design criteria have received considerable attention and are well established; however, the problem of considering the uncertainty in the force coefficients has received little attention. Currently there is no rational procedure to generally account for coefficient uncertainty except to use arbitrary, and potentially unrealistic, guidelines such as the mean value plus a multiple of the standard deviation. The purpose of this paper is to provide a rational framework for dealing with the uncertainty in force coefficients. This framework is statistical and incorporates into the force statistics the uncertainty of the force coefficients and the random occurrence of the wave parameters. BACKGROUND The wave forces on an offshore structure are generally determined by the Morison equation (Mathematical Equation Available in Full Paper) and Q D and Q r are the drag and inertia forces (respectively) per unit length acting normal to a structural element; CD and Cr are the drag and inertia coefficients (i.e., the force coefficients), v and v are the water velocity and acceleration normal to the element, D is the element diameter and P is the mass density of water. As shown in Appendix I, the maximum total horizontal forces on a vertical cylinder during a wave cycle are (Mathematical Equation Available in Full Paper) which are obtained by integrating (1) over the water depth. F D is the total drag force; F r is the total inertia force; H is the wave height and s is the wave steepness (height/wave length). The assumptions used to derive (2) are deepwater waves and linear wave theory. Similar expressions have been presented in Ref. 1. Except for the properties of the water and cylinder (p,D), (2) depends only on the force coefficients, C D and C r , and wave height, H, if the steepness is small (exp(2ns)=1) or constant. It is noted that for these conditions the total forces are independent of wave period.

Proceedings Papers

Publisher: Offshore Technology Conference

Paper presented at the Offshore Technology Conference, May 2–5, 1976

Paper Number: OTC-2467-MS

...° 54 N. ,long. 75° 43 W. I3.0 BUOY A I v :

**MEAN****VALUE**OF 11 SAMPLES WITH 1.04< u (ft/secz) C 2.38 2.5 l :**MEAN****VALUE**OF 11 SAMPLES wITH ~54 < u (ftlsd <3.74 0 : STANOARO DEVIATION OF HEAVE K 2.0 0 2 E : 0 g ~ 1.5 $ z a H z 0 ~ 1.0 $ I v < l : .5 v l v 9 l I I I I I I I I I I I I I 1,2 -1.4 1.6 1.8...
Abstract

ABSTRACT Response Amplitude Operators (RAO's) are obtained by the application of existing theory to measurements of the transient response of a buoy to steps of heave and angular displacement. These results are compared with RAO's computed from sea and motion spectra of two buoys of the same type deployed in an exposed sea 10cat1on. Excellent correlation has been obtained between the two methods. With this validation, the expeditious step response technique can be applied with confidence to the comparative evaluation of buoy designs and other applications that involve the prediction of buoy motion. INTRODUCTION The light source of an aids to navigation buoy signal light is placed at the focal point of a 360 degree Fresnel lens. This provides a peak light intensity in the horizontal plane when the buoy is upright. Because of the finite size of the light source filament, the light beam is the horizontal plane is character1zed by a vert1cal divergence angle. This vertical divergence angle, defined by half the peak intensity, is typically 2.5 degrees above and below the nominal horizontal plane of the light. Angular motion of the buoy can move the signal beam out of the line of sight of a potential user. The motion of the buoy depends upon both the physical design parameters of the buoy, and the seaway characteristics. Thus, for the same optical characteristics, the delectability of the light signal could vary in the same seaway with different buoy designs and with the same buoy under different sea conditions. Present day buoys have evolved from recognizably similar designs of more than one hundred years ago. No one set out to design these buoys with a specified and demonstrable set of dynamic characteristics and, until recently (ref. 1) there was no particular interest in characterizing existing designs. Efforts of the U. S. Coast Guard to do so, stem from two recognized needs. The first, is to provide a necessary input into an assessment of the probability of The opinions or assertions contained herein are the private ones of the author and are not to be construed as official or reflecting the views of the Commandant or the Coast Guard at large. The simple situation of a buoy's motion in a seaway, consists of three elements; the input (waves), the transfer function (buoy dynamics), and the output (motion). In principle, if we have two of these elements we can obtain the third. (It is recognized that wind, tide, current, passing water craft, etc. may also be present). This paper presents and compares the result of two techniques for determining from full-scale tests, the transfer function of buoy response to waves. One technique uses the analysis of the statistical wave and motion data to calculate the transfer function. The transfer function is expressed in the form of the Response Amplitude Operator, (RAO).

Proceedings Papers

Publisher: Offshore Technology Conference

Paper presented at the Offshore Technology Conference, April 28–May 1, 1973

Paper Number: OTC-1902-MS

... The liquid and plastic limits of 519 Recent general trend establishedin a particular clay samples were included in this analysis. correlation. The liquid limit ranged from about 30 to 130 with a

**mean****value**of about 74. The Evaluationof Correlations plasticityindex varied between about 10 to 90 with a...
Abstract

ABSTRACT A thorough statistical study of engineering properties of the Gulf of Mexico clays was conducted. An extensive amount of data covering a wide range of offshore clay properties was assembled and analyzed. The data represent the results of strength, consolidation and classification tests on clay samples from the upper coast of the Gulf of Mexico. The analysis included a total of 759 samples from depths ranging from about 2 ft. to 446 ft. The samples were obtained by 65 undisturbed-sample borings drilled in water depths varying from 3 ft. to 382 ft. Several forms of single and multiple correlations were developed among the engineering properties of Recent and Pleistocene offshore clay deposits. The established relationships were evaluated statistically. The significant correlations were compared with similar equations developed earlier by other investigators for offshore and on land clay formations. It is concluded that several valuable relationships exist between certain properties of Gulf of Mexico clays. Examples are correlations between the plasticity index and liquid limit, between the undisturbed cohesion and preconsolidation pressure, and between the compression index and liquid limit. These correlations agree favorably with results of previous studies. The consolidation and strength characteristics of the clays can be practically predicted from the index properties of the soil. Equations relating the compression index, preconsolidation pressure, remolded and undisturbed cohesion of the Recent and Pleistocene clays, with such soil properties as the liquid limit, moisture content and unit dry weight, are reported. INTRODUCTION Significance Correlations which establish reliable relationships between soil properties can be quite useful in soil engineering practice. The value of such correlations is even greater in connection with offshore studies due to the adverse conditions faced in collection of soil data. Securing sufficient information in marine soil investigations involves considerable cost and difficulty. The prediction of consolidation, strength and other characteristics of undisturbed offshore deposits from the index properties of the soil is particularly helpful. Previous Studies Earlier investigations have shown that there are reliable correlations between soil properties of the Mississppi River deposits (15). McClelland (11, 10) studied the engineering characteristics of the Gulf of Mexico soils and established valuable relationships between the properties of offshore clays. Casagrande (3) related the plasticity index and the liquid limit of fine-grained soils for classification purposes. Skempton (16) correlated the compression index of remolded clays with their liquid limits; Terzaghi and Peck (17) developed a similar relationship for undisturbed, normally consolidated clays of moderate-to-low sensitivity. Objectives and Scope This study is a continuation of earlier work on correlation of engineering properties of the Gulf of Mexico clays to confirm and to extend these relationships. An extensive volume of data was analyzed statistically to determine interrelationships among the offshore clay properties.

Proceedings Papers

Publisher: Offshore Technology Conference

Paper presented at the Offshore Technology Conference, April 18–20, 1971

Paper Number: OTC-1508-MS

... reverses. 2 Wiegel et ale show the co- efficient of mass, CM, to be a normally distributed random variable. The

**mean****value**is 2.5, the standard deviation is 1.2, and the coefficient of skewness is approximately 0.0. The**mean****value**is somewhat greater than ,he -cheoretical value of. 2. 0, Lamb. A...
Abstract

1. Introduction Wave forces exerted on structures are of vital interest to offshore drilling stations and various other marine structures. Because the piles constitute the major or entire support for the whole structure the engineer must ensure that those piles successfully withstand the forces to which they are subjected. In this regard, the most important piece of information is the occurrence of wave forces at the site of the proposed structures. The design problem is complicated by a number of factors. First, wave characteristics change as a wave approaches the shore. Secondly, local wind-generated waves become superimposed on large ocean waves, creating a complicated surface. Thirdly, the forces exerted by waves of similar properties vary greatly. This variation is caused by fluctuations in fluid particle velocities as well as eddies and turbulance around piles caused by the rapidly reversing flow. Consequently, a deterministic approach to wave force prediction appears to be impossible. As an alternative method in design, an engineer might determine his design by using a cumulative distribution function (CDF) of wave forces created from field data. If the distribution of wave forces is known, the engineer can then make reliability statements about his design. The method used here utilizes a multiple linear regression analysis to develop a relationship between wave properties and the parameters 1 used in the Morison force equation. These parameters include velocity and acceleration of water particles and the drag and mass coefficients. Using Monte Carlo simulation these regression relationships are then utilized to develop the probability density formation of wave forces. 2. Description of Force Equation The forces exerted by ocean waves on a pile is one of the most complicated fluid flow problems because of the presence of both unsteady and oscillary motion. The generally accepted formula for a wave force on a pile is the so-called Morison formula, (Mathematical equation available in fullpaper) The first part of this equation represents the for drag caused by surface shear. The second part is the acceleration force on the displaced volume of fluid including the virtual mass effect. The relative importance of inertia force increases with the ratio of the pile diameter to wave height. Although the particle motion in waves is orbital, the method is based on flow. The fluid density and the pile diameter D are assumed to be constant. It is also assumed that the drag force and the inertia force can be treated separately, and the total force can be obtained by adding the solutions linearly. In previous investigations 1 , 2 , 3 , 4 , 5 , 6 water particle velocity and acceleration were calculated by stokes 7 irrotational theory. The theory considers only waves of small but finite wave steepness.

Proceedings Papers

Publisher: Offshore Technology Conference

Paper presented at the Offshore Technology Conference, April 18–20, 1971

Paper Number: OTC-1503-MS

...

**values**are indicated by big circles. The samples were pulled to destruction or at least 2 million times. DISCUSSION OF RESULTS The ruptures occurred in the same place in the bent part of the link with location according to Fig. 4. In Fig. 5, the**mean****values**of Fig. 3 have 11-856 GILBERT de LA VAL OTe...
Abstract

Fatigue tests have been carried out on anchor chain-cable. Fatigue tests on steel normally are performed on specially machined specimens. Such a specimen is given an exceptionally smooth surface in order to eliminate as much as possible the probable influence of surface defects on the fatigue strength of the steel. In order to obtain, in this case, a more realistic picture of the fatigue strength of anchor chain-cable than is possible with the ordinary fatigue test, these tests have been carried out on the chain links proper. Three-link samples of 2-in. oilrig chain (Fig. 1) were tested at six different maximum loads. Chemical composition: C = 0.27 percent Si = 0.27 percent Mn = 1.60 percent P = 0.024 percent S = 0.012 percent N = 0.008 percent Mechanical properties: yield strength 47 kp/sq mm 67,000 psi tensile strength 70 kp/sq mm 100,000 psi elongation = 24 percent impact resistance = 11 kpm at O°C, Charpy V-notch Testing requirements: proof load = 147 mp 322,000 lb breaking load = 222 mp '" 488,000 lb Before fatigue tests all samples were subject to proof load. At each maximum load five samples were tested. The tests were made in an AMSLER 100 mp tensile testing machine with pulsator at the Government Testing Institute, Stockholm, Sweden The frequency used was 285 load changes/min and the amplitude varied between the maximum load and a minimum load of 5 mp. The tests were carried out at room temperature in dry conditions. Records of test results can be studied in the table (Fig. 2) and diagram (Fig. 3) which show the number of load changes at rupture (small circles) at different loads. Mean values are indicated by big circles. The samples were pulled to destruction or at least 2 million times. Discussion Of Results The ruptures occurred in the same place in the bent part of the link with location according to Fig. 4. In Fig. 5, the mean values of Fig. 3 have been transformed to stress at maximum load. This stress value is simply the mean stress on the chain section obtained by dividing themaximum with the cross-section of the link [2 (?D 2 )/4] It is !mown that stresses in some parts of the link under load are considerably higher than this mean value. It is perhaps allowable to define a fatigue limit for chain-cable. This limit could be defined as the maximum load at which the chain sample withstands at least 2 million load reversals without rupture.