bluuebunny/crossref_metadata_2025 huggingface dataset:
Comparison With Observation At Sea Of Period Or Height Dependent Sea State Parameters From A Theoretical Model
Authors: Ezraty Robert , Laurent Maryse , Arhan Michel
Published: October 2008

A theoretical joint probability density for individual wave heights and wave periods, originally developed to describe storm conditions, is here compared with some 2,000 routine wave recordings in the Bay of Biscay.

From the joint probability density, all other mean sea-state parameters (HT1/3, TH1/3, T1/3,) can be computed using H1/3, T and (Symbol available in full paper), the spectrum width parameter. The systematic discrepancy existing between theory and observation can, if necessary, be corrected empirically.

INTRODUCTION

The paper published by Cartwright and Longuet Higgins is widely used to predict the height of sea waves and to compute the significant wave height, h1/3, or similar variables h1/N, as well as the expectancy of the maximum, E(hmax), of a given number of waves, starting from mo respectively, the total energy and the width parameter of the spectrum.

To describe a sea state, it is necessary to have at least a characteristic period as well as a characteristic height. In the "zero-up-crossing" wave analysis, the following periods usually appear. T1/ 3 = mean of the highest third of the zero-upcrossing periods; TH1/3 = mean of the periods connected to the waves used to compute H1/ 3; T = mean of the zero-up-crossing periods.

In the same way, T1/N and TH1/N can be defined. Empirical relationships between these characteristic periods, based on observation at sea can be found in the literature.8

The theoretical model developed at C.N.E.X.O. is based on the theory of Gaussian noise as established by Rice7 and leads to an explicit formula for the joint probability density of wave heights and periods; this density is fixed, given three parameters a characteristic height, a characteristic period. Then, it is possible, by appropriate integrations, to relate the different average heights to the associated average periods that describe a given sea state. Although the narrow band spectrum hypothesis is not always satisfied in reality, computed values of mean quantities from observations at sea remain quite close to their theoretical equivalents; the discrepancy, being systematic, can be corrected, if necessary, for the theoretical model.

THE THEORETICAL MODEL

The model is developed, using as a starting point the joint probability density for a Gaussian noise signal to have a value?1, at a maximum and a second derivative, with respect to time ?3, in the notation of Cartwright and Longuet-Higgins.(Mathematical equation available in full paper)

To each positive maximum we have assigned a sinusoidal wave having for amplitude ?1 and period T given by(Mathematical equation available in full paper)
Oil in Marine Waters
Authors: Frey M. Gordon
Published: October 2008

Oil has been in marine waters for millions of years, entering from marine seeps as migrating oil reached the sedimentary surface. Most of this oil was biodegraded by marine bacteria. The lighter fractions evaporated and a portion resisted weathering, becoming tar balls or asphalt slabs. More recently, greater volumes of oil have reached the oceans, both as crude and refined products. Nature still cleans up most of what man spills, but special problems occur when large volumes of crude or ' refined products suddenly are dumped into a coastal environment. The industry has maintained a remarkable record in its offshore exploration and production program, contributing less than 1/½percent of the petroleum in marine waters.

MARINE SEEPS

From historical records, we have learned that oil has been found in marine waters for many centuries. Geologically, we recognize oil has found its way into sea waters for millions of years.

The early Greeks skimmed oil from the surface of the Aegean Sea and used it in their ancient lamps. Here in the United States, we learn from the journals of the early explorers, that the Indians on both the Gulf Coast and along the California shores used oil and tar for many purposes: affixing arrowheads to shafts, waterproofing baskets, and mending pottery.

Perhaps the earliest reference8 to oil and tar in the New World is from the records of the Hernando de Soto expedition2, which included an area of exploration from the Savannah River southward and westward to the Mississippi River and East Texas. Before his death in 1542, de Soto appointed Don Luis de Moscoso as his successor. After a discouraging expedition in East Texas, de Moscoso built seven rickety vessels and set sail down the Mississippi on July 2, 1543. After reaching the Gulf of Mexico, the Spaniards decided to sail west along the coast. Upon reaching the mouth of the Sabine, they found a scum called copee, which is like pitch. They beached their vessels and, using dry slabs - tar, caulked their boats. The pitch was formed from a large oil seep that was known to exist off the coast near the Sabine River as late as 1903.

About the same time, the journal11 of the Portugese explorer Juan Rodrigues Cabrillo in 1542 reveals that oil seeps were prevalent in the Santa Barbara area of Coastal California. Later references to land and/or marine petroleum seeps in the area include the records11 of Father Pedro Font, 1776; Captain Cook, 1792; and the records of Benjamin Silliman, 1864.4
Environmental Aspects Of Drilling Muds And Cuttings From Oil And Gas Extraction Operations In Offshore And Coastal Waters
Authors: Monaghan P.H. , McAuliffe C.D. , Weiss F.T.
Published: October 2008

Drilling mud and cuttings discharged from the more than 21,000 wells drilled offshore and in coastal waters of the United States have not caused detectable environmental damage either short-term or long-term. This is the conclusion from a study of reports of underwater observations during actual discharges of mud and cuttings, laboratory bioassay results for materials used in drilling muds, and actual well histories and laboratory results used in estimating the concentrations of mud components.

Discharged cuttings normally fall to the bottom and form a circular or elongate pile up to 150 feet in diameter and less than 3 feet high, thinning rapidly to the edges. Both measurements and calculations indicate that mud discharged during normal drilling quickly mixes with sea water and is diluted at least a thousand fold about 300 yards down-current. Divers have observed fish swimming back and forth through the mud and cuttings discharges, have observed organisms working in the cuttings piles, and have found barnacles growing on and in the exit of the discharge pipe.

The materials added in greatest amounts to drilling muds are barite (barium sulfate) and bentonite clays. Bioassay data indicate that these solid materials are nontoxic. Any acute toxicity for a typical mud should be due mainly to the dissolved lignosulfonate thinner. However, rapid dispersion and dilution quickly lower the concentrations (within a few minutes to an hour) below those found to be toxic in bioassay tests conducted for four days without provision for dispersion and dilution.

Mud and cuttings discharges apparently have not caused long-lasting effects, as a wide variety of mobile and fixed organisms are found on and under oil producing platforms. In the Gulf of Mexico, 10- to l5-yr old cuttings are no longer distinguishable from the normal sediments. Studies in Timbalier Bay (shallow water and a long history of petroleum operations) have documented no adverse effects on marine life or buildup of hydrocarbons metals.

INTRODUCTION

A well drilled for oil and gas is not simply a hole drilled from the surface to total depth. Rather, it is several concentric holes, the smallest being deepest, and each often lined with steel casing. This casing is held in place by placing cement in the annulus between it and the hole wall. The casings prevent collapse of the hole and/or uncontrollable flow of fluids into or out of the wellbore. Typical hole size and casing programs for 10,000- and 20,000-ft wells are shown in Table I. Also shown are the volumes of rock removed to drill each segment of the well. Note that doubling the depth triples the amount of rock removed and that this increase occurs in the shallow part of the hole.

In the rotary drilling process, a steel pipe rotates a bit against the rock face to break off chips of rock. As drilling proceeds, lengths of steel pipe are added to keep the bit in contact with the rock.
Newest Pumpdown (TFL) Completion Capabilities
Authors: Yonker John H. , Arendt Henry P. , Johnston Russell A.
Published: October 2008

The latest pumpdown (TFL) completion methods and capabilities are now being used with consistent reliability worldwide including the North Sea, Gulf of Mexico, Borneo, and on land United States.

These capabilities allow placing the H-member at well bottom, full alternate multiple zone dual completions, surface selective downhole shifter/locator system, and downhole gas lift and safety systems; all of which are adaptable to subsea manifolds and atmospheric chambers, riser systems, floating platforms and standard platform completions.

INTRODUCTION

The search for oil has pushed the oil industry into deeper and less accessible waters. The costs for production platforms and pipelines have increased accordingly. For a well to be profitable under these conditions it must be able to produce large amounts of oil and be easily maintained.

The result has been an increased demand for pumpdown (TFL) completions with large bores specially designed for deep water. These completions, in addition to the basic pumpdown (TFL) requirements, must also incorporate other certain features as discussed below.The capability to perform maintenance and service work on subsea wells with flow lines up to 20 miles long. This distance results from cluster and satellite wells being placed further from fixed or floating platforms.The capability to locate downhole equipment selectively. This is more a demand on the deep or long flow line wells where the pressure response is minimal.The capability to perform limited maintenance work on wells with BHT up to 450½F and BHP's over 20,000 PSI that are in excess of 20,000 feet deep.Maintaining pressure integrity and response and keeping fluid losses to minimum While performing service operations. This is very important in wells having low bottom hole pressures and subsea wells having extremely long flow lines.The compatibility of downhole equipment within the whole system.The incorporation of a circulation path (H-member) of some sort, whether downhole or in the wellhead.Surface controlled subsurface safety systems. This is a necessary safety precaution with the ever present threat of damage to wellheads and flowlines.

As will be discussed in this paper, recent innovations in pump down (TFL) equipment now provide these features and offer solutions to the problems of well maintenance.

DESCRIPTION OF EQUIPMENT

In the past three to four years much development work and testing has taken place to help answer the problems faced by deep water completions. The developments include wellhead plugs, pumpdown retrievable ball valves and tubing retrievable ball valves for pumpdown use, pumpdown kickover tools with side pocket mandrels and side-pocket H-members, 'Select-20' shifter and locator systems, upper H-members with emergency circulating shear discs, sliding sleeve H members for gas lift in tubing less completions, and equipment for servicing deep, high pressure land wells.
An Acoustic Navigation System For Site Survey And Geodetic Positioning
Authors: Sharpe Richard T. , Galyean Paul G.
Published: October 2008

An integrated navigation system which combines data from the Transit satellite system, a network of underwater acoustic transponders and an assortment of secondary sensors is described. Particular emphasis is placed on the techniques developed for relative and geodetic calibration of the acoustic grid. The system is scheduled for deployment in Spring 1977 for survey operations in the North Sea.

INTRODUCTION

Integrated navigation systems have evolved significantly during the recent past to keep pace with the needs of the offshore oil and exploration communities. As production and exploration activities move beyond the range of shore based navigation aids, there has been an increased incentive to produce accurate, portable navigation aids for use at sea. The use of underwater acoustic beacons has been one approach to this problem.

Acoustic navigation systems are typically comprised of the following components: a network of underwater transponders deployed on the ocean floor, a hull mounted or towed transducer connected to the vessel to be navigated and an interrogation or control unit. The transponders are designed to transmit a pulse of acoustic energy in response to an interrogate pulse issued by the interrogation unit. The time delay between transmission of the interrogate pulse and receipt of the reply pulse by the vessel's transducer provides a measure of the slant ranges to each of the underwater transponders. When properly calibrated, these systems can provide precise navigation over a limited area.

The system described in this paper is designed to use this type of acoustic structure in combination with the Transit Satellites and the various navigation aids discussed below.

SYSTEM CONFIGURATION

Fig. 1 is a block diagram of the system configuration. The navigation sensors include a gyrocompass, speed log or sonar, satellite receiver, Decca main-chain or Loran-C receiver, and the acoustic subsystem. The keyboard/printer is the primary operator interface device. The CRT is used for display of real-time navigation parameters. The track plotter is used to provide a graphic record of the vessel's path.

The data logging unit may be a magnetic cassette device or a reel to reel tape drive. In addition to recording survey data it is also used to log satellite data necessary for the geodetic calibration of the acoustic array. At the heart of the system is a Hewlett Packard minicomputer with a 24K word memory.

The acoustic subsystem is interfaced via an eight channel multiplexer to the CPU. Each channel in the multiplexer contains its own range counter unit, which uses a stable 100 KHz output from the satellite receiver as its frequency reference. This arrangement obviates the need for a full BCD interface with the acoustic interrogation unit, since only start and stop pulses are required to obtain range measurements.
An Investigation Into The Stress Distribution And Fatigue Strength Of Awelded Tubular T-Joint
Authors: Martin T. , McGregor J.
Published: October 2008

The prerequisites for fatigue analysis of tubular joints are knowledge of local stresses at critical areas and an appropriate design S/N curve. Cumulative fatigue damage can then be calculated for a given loading spectrum.

This paper describes an experimental investigation of an axially loaded small-scale T-joint to measure the maximum stress concentration and compare it with formulae from the literature, to obtain constant-amplitude fatigue data for comparison with proposed design curves and to investigate the suitability of Miner's rule from the results of variable-amplitude fatigue tests.

INTRODUCTION

With the expansion of offshore oil exploration to hostile waters such as the North Sea there is an urgent need for reliable fatigue analysis procedures to be included in design codes for offshore installations. The tubular welded joint is the most widely used constructional detail in steel jacket type structures, and is an economical method of febrication, which produces a joint with a remarkable reserve static strength1. It has been used extensively without unduly large failure rates in environments, such as the Gulf of Mexico, in which the high risk of damage tends to be confined to short periods of storm conditions when the static strength of the joints is critical. In other locations where the wave spectrum includes appreciable numbers of waves that are higher than approximately 20 per cent of the design wave height, there will be a high risk of fatigue cracking. The complex stress distribution in a tubular joint, which is caused by the interaction of the restraint of the welded joint and the deformation of the tubular members, gives rise to high stress concentrations, adjacent to the weld toe, at critical positions around the joint. These areas of local high stress are potential sites for the initiation of fatigue cracks and progressive failure.

PROPOSED ANALYSIS PROCEDURES

The fatigue analysis procedures proposed by the UK Department of Energy2 and the British Standards Institution have the same basic format as the AWS Fatigue Criterion which has been considered to be applicable to tubular joints1.

A designer using these procedures would have to carry out a suitable experimental or computational study, which takes account of geometrical parameters and modes of loading to establish stress concentration factors for each joint of the structure. He would then use the resultant local stress spectra at the hot spots in a cumulative damage calculation, based on an appropriate design SIN curve, to estimate the fatigue lives of the joints by Miner's rule.

An alternative technique of analysis, which can be used for chord members of some simple T- and K-joints, is based on the punching shear stress range. The nominal axial and bending stresses in the brace are used to calculate the spectrum of punching shear stress, Vp, using the formula 4

(Mathematical equation available in full paper)
A Probabilistic Approach to Structural Operational Safety
Authors: Planeix J.-M.
Published: October 2008

The aim of this paper is to introduce the notion of operational safety, i.e, the influence of the action of man on the safety of a system. Explicitly or not, this action must be taken into account when designing a structure. For a given operation, man has :to decide whether or not to attempt carrying out, as a result of his judgment of the environmental conditions, andif his decision is to go ahead, to perform it.

The decision and the eventual performance of the operation constitute the action of man, modifying the environmental loads. Both the decision and the performance of the operation involve errors, with respect to the conditions taken for design. These errors combine with the error inherent in any design analysis, due to calculation of environmental forces and modelization of the structure, to form the error in demand. Quality of materials, building, and maintenance also deviate from the-design standards, resulting in an error in capability. The design safety factor, giving an excess of design capability over design demand; must be such that safety is achieved, i.e. that, in operation, capability remains larger than demand, in spite of the influence of all the above mentioned errors, to an acceptably high probability. To carry out illustrative calculations, on a number of systems for marine work, Gaussian distributions are taken for the errors in the factors affecting capability and demand. These calculations show the influence of the various safety parameters on the number of safe operations allowed by the design.

INTRODUCTION

The problem of safety of units for work at sea, and more particularly of offshore units, is given more and more attention. Oil companies, operators, governments, and regulatory bodies strive to obtain a high degree of safety of these units, but, at the same time, owners (oil companies or operators) must make sure that this search for safety does not result in an uneconomical over-design of the units. To achieve both goals, an assessment of safety must be made, i.e the probabilistic distributions of capability and demand must be found, for each constitutive element of the structure, the failure of which is considered unacceptable.

At design, both demand and capability are evaluated and the design is based on a safety factor, or a set of safety factors, leaving some margin of design capability over design demand. In operation, demand and capability deviate from their design value. Safety in operations is achieved when the influence of these deviations does not absorb the margin of safety at design. The assessment of safety consists in finding out whether or not the design safety factor is insufficient or more than sufficient to ensure safety for an acceptably large number of operations.

Design demand results from an analysis of the motional and/or structural behaviour of the unit, in a given set of environmental conditions.
Experimental Study On The Behavior Of Concrete Structural Elements In Natural Sea Water
Authors: Peyronnet Jean Paul , Trinh Jacques Paul , Kavyrchine M. Paul , Seguin Michel Paul
Published: October 2008

This report describes an experimental study on the behaviour of concrete structures in marine environment conducted by C.E.B.T.P. and C.N.E.X.O. Two concrete structures, each composed of two U' form elements, one reinforced, the other prestressed, had been tested under dynamic loadings in two different environments : on land and in natural sea-water (splash zone).

The aim of the study is to investigate the behaviour of offshore structures in its service conditions, so the six months loading program the same in the two cases has been chosen for the simulation of wave induced forces in sea. The dimensions have been also adjusted for a good representation of the phenomena involved.

These tests give an opportunity to develop new means of control which could be applied in such constructions.

In the test conditions, natural sea-water did not induce much unexpected mechanical behaviour compared with that on land. However in sea water, cracks were the site of alterations (destruction and deposit) causing a higher level of residual deformation. Within the test duration, the steel of the reinforced concrete structure showed a beginning of corrosion, which did not reduce over all strength.

The state of corrosion of the reinforcements is evaluated by the electric potential method associated with measurements of the water content inside the concrete using capacity-resistance gauges. Strains in concrete are determined by vibrating wire gauges, and crack widths by an optical micrometer.

INTRODUCTION

For the extraction of marine resources, steel was at first mainly used, in relatively shallow waters. But for deeper sea-beds, concrete is now more employed and offers solutions which are attractive from the technical and economical point of view, especially for durability. Concrete protects embedded steel reinforcement from environmental attack. However this advantage is effective only if the concrete covering has a sufficient thickness and compacity and if cracking can be reasonably limited. But we need actually more tests in sea conditions to determine the limitations of these requirements.

Among the different problems concerning a concrete structure placed in the open sea (I), we, have chosen to study at first its behaviour in its service life, under dynamical loads and corrosive action at sea, and to compare it with its behaviour on land.

EXPERIMENTAL STUDY

This first stage of experimental research on concrete at sea, concerns essentially the comparison of the behaviour of bent elements placed either on land or at sea. These first tests also allow us to control that certain measuring devices will behave well in the marine environment.

TESTING PROCEDURE

To facilite the application of loads, U shaped structures have been adopted (fig. I). Four test elements of identical dimensions have been fabricated in the C.E.B.T.P. Laboratories at Saint-Remyles-Chevreuse near Paris.
Cathodic Protection Criteria for the North Sea
Authors: Eliassen Svein , Steensland Odd
Published: October 2008

Experiences from "the first generation" of offshore structures in the North Sea have shown that cathodic protection criteria that are sufficient in other offshore areas like the Gulf of Mexico cannot necessarily be applied to steel structures in the North Sea. This paper describes some of these experiences. It also gives some background information about the special environmental conditions in the North Sea and their influence on cathodic protection systems.

The main part of the paper is. devoted to the installation of a field testing equipment for measuring current output of anodes and the polarization level of a steel structure in service. Some preliminary results are given. The results show that the necessary practical current density for the North Sea when using sacrificial anodes may be in the order of 100 to 130 mA/m2. Special attention should be paid to calculation of the correct current output and distribute the anodes well to get an even current distribution.

At the end of the paper we have outlined some further developments that are necessary to establish the correct design of cathodic protection. A special item is the combination of a coating system and cathodic protection. Finally, a brief introduction to the DnV rules for cathodic protection systems is given.

INTRODUCTION

Corrosion and corrosion protection are the most important factors to be considered in the design, construction, and inspection of offshore structures and pipelines. If basic corrosion protection technology is neglected, this will result in expensive and serious mistakes, not only in terms of money, but in safety as well. Corrosion, in fact, has turned out to be a major problem for the operation of offshore platforms and production facilities.

For DnV, being a certifying authority in the North Sea area, special interest has been paid to pipelines, risers, and structural members, which are important to the over-all integrity of the structure. Of special concern are members that are difficult to inspect or repair and members that are exposed to very aggressive environments, for instance, the splash zone.

The submerged zone of the structures and pipelines is a part that may be defined as difficult to inspect or repair. This is especially true for the North Sea with its deep waters and rough sea conditions, which are making diving operations more complicated. The predominant way of protecting the submerged zone is by cathodic protection systems, which sometimes may be combined with a suitable coating. Thus, the performance of a cathodic protection system is a very important factor in obtaining reasonable life and safety of the structures.

Cathodic protection can be achieved either by sacrificial anodes, by impressed current systems, or by a combination of both methods. For North Sea applications, most of the systems so far have been sacrificial anode systems. This paper deals mainly with sacrificial anode systems.
First Reel Pipelay Ship Under Construction - Applications Up To 16 Inch Diameteh Pipe 3000 Feet Of Water
Authors: Friman Kenneth  R. , Uyeda S.T. , Bidstrup Herman  R.
Published: October 2008

There is an increasing need in the offshore petroleum industry to lay pipelines in deep and rough water, in multiple pipeline bundles, and in remote areas. The industry's first dynamically positioned Reel Pipelay Ship, the APACHE, now under construction, is one approach to these problems.

The paper presents a discussion of the design parameters, special features and details of the ship. Also presented are applications such as pipelines in very deep water where normal anchoring is not practical, pipelaying with limited offshore work time, pipelines in remote areas, subsea completions, bundled pipelines, and removal and reuse of pipelines. The paper includes a typical operating example.

The reel ship will have a capacity to carry up to 2,000 short tons of pipe on its permanent large reel and additional pipe on portable reels. This tonnage is equivalent, for example, to approximately six miles of 16-inch pipe, or 20 miles of a inch pipe. The ship will have the capability to lay pipe up to two miles per hour. The pipeline equipment is designed to allow the pipe to leave the ship at departure angles up to 60 degrees from the horizontal. This allows pipelaying in very deep water. The ship will have a 12.5 knot cruising speed and an operating range of 5,000 miles.

The reel ship, patterned after the reel barge presently operated by Santa Fe Engineering & Construction Co., significantly extends the range of the Reel Method of laying offshore pipelines.

INTRODUCTION

The first reel pipelay ship is presently under construction by Todd Shipyards Corp. in Galveston, Texas. The vessel will be owned and operated by subsidiaries of Santa Fe International Corp. It is scheduled to start work in January 1979.

The vessel, named Apache, (Figure 1) will be the first self-propelled vessel ever built for laying pipe from a reel. It will use a vertical reel, a concept first introduced by Santa Fe at the 1975 Offshore Technology Conference. The vertical reel, with the adjustable stern ramp, will allow the lay angle to be increased or decreased depending on water depth and pipe properties. The preliminary design basis is for the laying of 16-inch, 12-inch, and smaller pipes in approximately 2,000, 2,500, and 3,000 feet of water, respectively. This system constitutes one of the Apache's major advantages over existing equipment.

The concept of laying pipe with a reel is not unique, having been used successfully for many years in the Gulf of Mexico from both portable and barge mounted reels. The pipelay barge, Chickasaw, (Figure 2) equipped with a horizontally mounted reel has installed nearly five million feet of pipe up to 12-inches in diameter in water as deep as 350 feet. Using a special stinger attachment, the barge has demonstrated the ability to lay pipe to a depth of 1,000 feet in a test conducted in 1975.
Hawser System Design For Single Point Moorings
Authors: Woehleke Steven P. , Flory John F. , Sherrard James R.
Published: October 2008

This paper presents design information for mooring hawsers for single point moorings. It is based on a recent study, conducted by Exxon Research and Engineering for the U.S. Coast Guard, which will serve as a guide for the evaluation of SPMs used at U.S. deepwater ports. Information from many sources has been drawn together and analyzed to present a comprehensive study of synthetic rope properties of interest in SPM design. Much of the information has not been published before.

Properties of nylon, polypropylene, and polyester fibers, and the types of rope construction used at SPMs are discussed. The elasticities of ropes of various materials and constructions are compared. The methods which are used to rate large-rope breaking strengths are evaluated. Factors which affect new and used SPM hawser strength are discussed. Chafing chains, hawser floatation devices, and the methods by which hawsers are attached to the tanker are also discussed.

Recommendations are made for SPM hawser material and construction. Factors of safety for SPM hawser design for U.S. deepwater ports are also recommended. These factors of safety are higher than those previously adopted by the industry.

The data and findings presented in this paper will be of interest to SPM designers and operators and to other users of large-diameter synthetic rope.

Introduction

During 1977, Exxon Research and Engineering Company conducted a study for the U.S. Coast Guard to prepare guidelines for evaluating Single Point Moorings (SPMs) for U.S. Deepwater Ports. As part of the study, SPM hawser systems were examined, and this paper covers the major findings pertaining to hawsers.

The hawser system serves to moor tankers to the SPM. An SPM hawser assembly consists of a large synthetic rope spliced to form eyes at both ends or end-to-end spliced to form a loop, referred to as a strop. The eyes or ends of the strop are placed on thimbles, and chafing chains are attached to the thimbles. Floatation devices are provided to keep the chafing chains and hawsers afloat when a tanker is not moored. A typical SPM and hawser system with a tanker moored is shown in Figure 1.

The examination of SPM hawser systems focused mainly on their design. Information from many sources was drawn together and analyzed in a comprehensive study of synthetic rope properties of interest in SPM hawser design. Information was also gathered and analyzed on the associated hawser components, such as thimbles, floatation units, and chafing chains. Based on the study, guidelines were prepared for the design of SPM hawser systems. The complete study of SPM hawser systems, along with studies on other aspects of SPM design, is presented in "Guidelines For Deepwater Port Single Point Mooring Design" (l) available to the public through the National Technical Information Service, Springfield, Virginia 22151.
Dynamic Tow Motion Of a Platform In a Seaway
Authors: Chung Jin S.
Published: October 2008

An analytical method for the prediction and analysis of the motion and stability of a platform under tow in a seaway has been developed. Equations of motion and hydrodynamic coefficients are derived, which account for the effect, on plat form motion and stability, of tow speed, waves and heading relative to the prevailing wave direction. This method is an extension of a previous simple method of prediction for zero tow speed. Predicted motion is presented for a multihull platform subjected to the wave frequency of interest. The results show that heave is increasingly coupled with pitch as tow speed into a head sea increases, whereas there is little heave pitch coupling at zero tow speed. Surge, sway, roll and yaw show little coupling effect compared with heave-pitch coupling. As speed increases in a head sea, amplitude increases near the resonant heave period, possibly reducing longitudinal stability. Heave pitch coupling effects are strongly dependent upon tow speed or damping. At tow speeds above certain critical values, reduction in pitching moment or decrease of longitudinal stability may require damping devices such as fins.

There has been renewed interest or concern following the recent capsize of several jacket platforms when under tow in a seaway. Conventional analytical prediction or analysis of stability has not taken into account the effect of speed in the equation of motion. Present analytical prediction of motion is based on solution of coupled six degrees-of-freedom motion for a multihull platform at tow draft. The analytically predicted speed effects on the motion of the present semi submersible platform under tow in a seaway are qualitatively compared with model test results of a jacket platform under tow. The comparison shows good agreement in the trend of the speed effect.

This method can be used to aid in the analysis of dynamic stability or to establish requirements for towing a jacket plaltform in the actual ocean environment, given prevailing weather or wave conditions along the tow route. Also, it can be used to aid in deciding the tow-out date by analyzing weather and sea state forecasts, and in predicting platform motion and stability at various times of the month or year. Towing of semi submersible platforms, semi submersible-type offshore supply vessels or similar floating structures can be analyzed directly by the present method.

INTRODUCTION

Renewed interest or concern has been developed following several recent capsizing incidents of platforms or tall jacket platforms under tow at sea as a whole or in pieces. Floating structures such as semi submersible or jacket platforms designed for stationary operation in the ocean have usually been towed to operational or installation sites. Safe towing depends largely on local weather conditions prevailing along the tow route. That is as much a question of halting the tow during bad weather, as it is of getting to the site a few hours earlier.
Variations Of Waves And Currents In Storms In The North Sea
Authors: Freeman John C.
Published: October 2008

The North Sea has great variations in width and depth. These physical characteristics and the direction of the prevailing strong winds lead to a great geographical variation in the expected extreme values of waves and currents. Further variation is caused by the fact that an extreme storm in the middle latitudes has a typical small scale variation that is usually ignored. This feature has some of the characteristics of a hurricane in that it is an intense vortex caused by strong convergence and rising moist air. Since this vortex is embedded in an already well developed low cell in middle latitudes and is cold it is not isolated and easy to identify like a hurricane. The affect of this intense embedded vortex on maximum wind and maximum wave statistics for the North Sea (or any other middle latitude stormy area) is very important. In summary there is the same low probability at a middle latitude station of a hit by one of the intense vortices as there is at a semi-tropical station of a hit by a hurricane or typhoon.

This means that extreme wind and wave statistics for a middle latitude station must be based on studies of the eight or ten worst storms in a few decades rather than on the more readily obtainable history of thirty to forty storms in two or three years.

Examples of winds and waves in the North Sea in extreme storms are presented in this study.

INTRODUCTION

In temperate latitude locations such as the North Sea the usual practice of developing design conditions is to find a series of measurements for a (necessarily short) time and to project (by probability theory) the probability of extreme events by graphical or analytic means.

It is well known that this process will not work for an area such as the Gulf Coast of the United States where the extreme storm (the hurricane) is a rare event in itself.

The purpose of this paper is to show that the likely cause of extreme conditions in the North Sea is a rare event (once every 3 years as opposed to once every 10-15 years for a hurricane on the Gulf Coast) and that the same general type of analysis should be used.

Quite by coincidence the rare event in the North Sea has some of the same meteorological Characteristics as a hurricane. The appearance of the disturbance is modified by the fact that it is embedded in a large deep middle latitude low cell and because it is moving fast it is difficult to capture the extreme values by ordinary reporting systems.

THEORY OF COMPENSATION

We begin the discussion by showing the similarity with a hurricane and then investigate the effects of such a rare small disturbance on estimates of extreme conditions.
Critical State Soil Mechanics Model Of Soil Consolidation Stresses Around a Driven Pile
Authors: Miller Terrell W. , Murff James D. , Kraft Leland M.
Published: October 2008

This paper discusses results of a study investigating the radial, plane-strain consolidation of soil around a pile using a Critical State soi1 Mechanics [1, 2] model of the soil The study was conducted to assess the assumption that the stress path of the soil during radial consolidation would occur along the critical state line as a basis for developing an effective stress pile capacity formulation [3, 4].

The study showed that predicted stress paths deviated significantly from the critical state line. Furthermore, the results indicated that if initial pore pressures are high enough, consolidation of the soil at the pile wall occurs along a stress path characterized by a radial-Ko condition (Available in full paper). The implications of these results are that consolidalion pressures are model dependent and that excess pore-pressures are likely to be generated during pile loading. It therefore appears appropriate to modify effective stress pile capacity formulations [3] that are based on the soil being at the critical state at initial pile loading.

INTRODUCTION

Developing improved methods of predicting axial pile response has been an area of active interest by the offshore industry and the geotechnical community for some time. Until recently, the methods developed have been based on a total stress approach to soil behavior although it is widely appreciated that an effective stress approach is more fundamentally sound. The preference for total stress methods exists because effective stress analysis introduces major experimental and analytical difficulties. As a result, use of effective stress methods require many simplifying assumptions and it has not been clear they provide a real advantage. Furthermore, the total stress methods have proven to be sound, workable engineering tools, however many engineers feel that they have been extended to their limits and that further improvements can only be achieved through the more fundamental approach.

Recently several efforts have been made to develop axial pile capacity prediction techniques that are based on an effective stress representation of soil strength (e.g. Burland [5], Parry and Swain [6], Esrig, et al. [3]). Burland assumed that pile installation and loading did not change the soil's stress state. Parry attempted to remove an important assumption by considering changes in stress state due to pile loading. Although these methods do constitute important background work and have indicated some promise they do not demonstrate a clear advantage over total stress approaches when compared with available pile test data. In order to improve on these methods, the in-situ stress state and stress history of the soil around the in-place pile must be determined. This exercise requires several important tools; chief among these are accurate constitutive models of soil behavior, models of pile insertion and loading, and a means of solving difficult nonlinear equations.
Sedimentation In Coastal Embayments, Northeastern Gulf Of Alaska
Authors: Molnia B.F.
Published: October 2008

During the last two centuries, glacial-fluvial, glacial-marine, and littoral sedimentation have completely filled at least three major embayments in the coastline of the northern Gulf of Alaska. In addition, the same processes are presently responsible for significant shoaling in Controller Bay, Icy Bay, Yakutat Bay, Dry Bay, Lituya Bay and Taylor Bay. The three filled embayments are Tsivat Bay, Kaliakh Bay and Vancouver's Icy Bay. To document the depositional histories of these nine coastal embayments, comparisons of shoreline configuration and depth were made from eighteenth century maps of exploration, nineteenth and twentieth century nautical charts and maps, and numerous ground and aerial photographs. These comparisons often permit calculation of volumes of sediment accumulated and rates of sedimentation.

Sedimentation rates in the nine Gulf of Alaska coastal embayments vary, but may be as high as 2 - 3.75 m/yr. Icy Bay, Yakutat Bay, and Lituya Bay all have areas that accumulate more than one meter of sediment annually. Planning and development of harbor facilities in coastal embayments of the Gulf of Alaska must include consideration of long term deposition patterns in the selection of harbor sites. Without careful analysis of sedimentation patterns, harbors may be selected which could fill with sediment before the completion of development.

INTRODUCTION

The- Gulf of Alaska coastline has been mapped and explored since 1741. Numerous maps and charts, and more recently, ground and aerial photographs have been made which depict parts of the coastline and which serve as permanent records of the configuration of the coastline at specific points in time. Maps of exploration, early ground photographs, seismic surveys, recent nautical charts and maps, and 1926-1979 aerial photography document many significant changes in the configuration of the coastline of the Gulf of Alaska and in the depth of specific coastal embayments.

This paper will describe changes in nine Gulf of Alaska coastal embayments and discuss the implications of sedimentation on the onshore siting of resource development facilities. The nine enbayments, (Fig. 1) from west to east are:Controller BayTsivat BayKaliakh BayIcy BayVancouver's Icy BayYakutat BayDry BayLituya BayTaylor Bay

Many of the early maps of exploration are printed or drawn at scales that make it difficult to tell what is cartographic imagination and what is real. Others however, are accompanied by narratives of exploration complete with soundings and descriptions of positions. Detailed accounts of the voyages of Bering (Davidson, 1901), 0 Cook (Ellis, 1782), La Perouse (1798), and Vancouver (1798) form the basis for many of the earliest shoreline comparisons made in this paper. A detailed atlas by Tebenkov (1852), which incorporates many of the earlier maps of exploration, includes data compiled by the Russian American Company in the early nineteenth century and serves as a basis for comparison with the modern data.
Development of a New Welding Gun for Underwater Welding
Authors: Coriatt G. , Roggen R.
Published: October 2008

A new semi-automatic welding gun has been developed for underwater welding. The principle consists of producing a water-free zone within a rapidly rotating bell-shaped transparent cavity. A protective gas environment can thus be maintained within the vortex zone, providing suitable conditions for performing the welding operation in all positions. Butt and fillet welds on plates up to 1" thickness have been made. The encouraging results obtained indicate the possibility of a large field of application on offshore structures and in shipyards, where manual or automated welding of good quality is required.

The paper describes the conceptual design of the system, the test procedures which have been adopted and summarizes the most pertinent results obtained in the laboratory and under hyperbaric testing conditions.

INTRODUCTION

The technique of underwater wet welding has made some progress within the last few years, mainly by the development of new coated electrodes; suitable for operation in the presence of water. Nevertheless, the water in the welding zone limits the quality of welds for the following reasons:–the dissociation of the water in the arc zone yields atomic and molecular hydrogen, which increases the hydrogen level and hence the risk of hydrogen cracks;–the rapid cooling of the weld in the presence of water results in additional local quenching effects, which may increase the hardness, in the weld and the heat affected zone;–the presence and the rapid evaporation of water affects the stability of the arc which may cause the occurence of porosity.

For these reasons, large underwater dry welding operations, as for instance on pipelines in the offshore oil and gas industries, are generally carried out in dry habitat welding. The installation of such chambers around pipeline sections to be welded is a time-consuming and expensive operation.

In recent years, a number of devices have been developed and used with different degrees of success. These consist of carrying out the weld in a small welding chamber supplied with a protective gas environment and sealed either by means of flexible or spongy seals or else by discharging a water jet through an annular nozzle surrounding the gun. As will be explained below, none of these systems has proved to be truly successful in every day underwater welding applications, a fact which has led us to consider new approaches for underwater welding in suitably dry conditions, with a robust, reliable, light-weight hand-held tool.

The principle consists of producing a water-free zone within a rapidly rotating bell-shaped transparent cavity. A protective gas environment can thus be maintained within the vortex zone, providing suitable conditions for performing the welding operation in all positions (see Fig. 1).

Patents for this process have been applied for in several industrialized countries.