bluuebunny/crossref_metadata_2025 huggingface dataset:
Cathodic Protection of Offshore Support and Security Craft
Authors: Morgan John
Published: October 2008

INTRODUCTION

A hundred years ago Thomas Edison investigated in his laboratory the corrosion of iron ships' hulls in seawater. He carried out these experiments in model form and predicted a catastrophic corrosion problem. He was not content to leave it there and with his usual genius designed and indeed operated an impressed current cathodic protection system on board a United States ship.

It was not a great success both because of the materials he had available but also because in scaling down the experiment he had in reality investigated the turbulent conditions that are found at the stern of a vessel associated with a steel propeller and with steel brackets and nozzles.

Perhaps this paper repeats much of Edison's work, it certainly proposes the same solution and application to cure the same conditions that were reproduced a hundred years ago in that famous laboratory.

PROPELLER AND TURBULENT WATER CATHODIC PROTECTION

The current that is required to achieve cathodic protection depends on the rate of arrival of oxygen at the cathode surface and an empirical relationship exists between speed and the cathodic protection current density. At a highly turbulent surface however the corrosion is enhanced by differential oxygen cells caused by variations in the partial pressure of the oxygen in the water. This type of attack, often called impingement corrosion, is evident on many vessels, principally in the stern area, on the propeller blades, on the leading edge of the rudder and on the A and P brackets.

A propeller blade will suffer a more extreme form of attack which is usually called cavitation damage and this is the corrosion of the metal at the point of cavity collapse. Cavitation is usually considered to be a mechanical attack and it has been claimed that it can be reproduced with ultrasonics. However if we consider the collapse of one of these cavities we can formulate an electrochemical mechanism for the corrosion.

The cavity that will be formed in the liquid will be at extremely low pressure and because of this will be completely ionised. The liquid surface of the cavity as it collapses will travel towards the metal of the propeller blade and will pick up an electric charge from the ionised gas. It will probably do this unequally and there will be a resultant charge left on the surface of the metal. This charge will disperse into the liquid and metal and in doing so will cause corrosion.

If we-consider a cavity of 1 cc capacity at 100th of an atmosphere pressure and forming two singly charged ions in the space then we can estimate a current that might flow if the whole of the charge were completely separated at each collapse.
Applications for Olver Lockout Submersibles
Authors: Holland Robert H.
Published: October 2008

This paper will describe recent industrial applications of diver lockout submersibles operating in the North Sea. Types of tasks which can be accomplished more efficiently and economically with diver lockout submersibles will be identified. Innovative solutions to problems presented by lockout submersibles will be discussed, as well as specific oil field applications of diver lockout capabilities and comparisons of diver lock out capabilities with other types of underwater work.

INTRODUCTION

The technique of divers exiting and reentering the confines of submarines while submerged is not new. In fact, it has been used in varying forms by the armed forces since World War II. From these generally clandestine type operations has evolved the diver lockout from submersibles as exists within the offshore oil industry.

HISTORY

This process of evolution has not been one of joint development, but more one of converging the paths of two individual services set up to supply the ever increasing demands of an industry under pressure to satisfy energy needs of modern civilization. These demands have caused the search for oil and gas to expand from land to the sea bed of the continental shelf, which in turn produced the requirement for the underwater worker. Initially, while in shallow water, the scope of operation of these underwater workers/divers was satisfactory, but as exploration, construction, and the supporting maintenance moved further a field and into deeper water, so the diving technique had to change. Now the diver needed a bell or chamber from which to dive and larger support systems, such as deck decompression chambers, on the surface to maintain him. The handling of the bell from the surface and the ability to position that bell in the close vicinity of the job required a platform of considerable size that could be positioned accurately and held in that position irrespective of the natural elements.

This was achieved by either heavy mooring or dynamic mooring systems, which on the one hand is a time consuming process with ever present danger of anchors dragging and causing damage to subsea equipment, or on the other hand a very sophisticated and expensive system which is generally installed in the larger type of vessel.

While the demands on the diver were increasing depth-wise and his range of operation from the surface base reduced, so the submersible was developing to cope with the demands for survey and inspection work over on an every increasing area. The "shirt sleeve" environment of the submersible provided the ability for survey and inspection of as much as 8 or 9 km of pipeline per day, where in many cases, a diver would be hard pressed to cover as much as 1.5 km per day. Also with the use of manipulators certain simple engineering tasks could be performed by the occupants of those submersibles.
LNG: From Ship to Shore
Authors: Peraino J. , Chase B.L. , Plodowski T. , Pupko H.
Published: October 2008

An LNG (liquefied, natural gas) unloading terminal - the largest in the U.E and second only to one in Japan - has been designed by Raymond Technical Facilities Inc. (RayTech), for Columbia LNG Corp. and is nearing completion at Cove Point, Md., on the Chesapeake Bay. Tankers will be unloading Algerian ING at offshore berths from where it will be piped to shore, still as a liquid at -260 °F through a mile-long sunken-tube tunnel. This is the first such application of tunnel construction for carrying LNG. The original terminal design had provided for a conventional pile-supported trestle with pipeway and roadway from the offshore berths to the onshore storage and regasification plant. However, in view of the situation of the terminal in a relatively unspoiled, recreational area, and, particularly of the wetlands area with its value to the over-all local ecology, objections were raised by environmentalists resulting in the substitution of the tunnel for the more visible trestle.

This concept entailed major changes throughout the project. However, once the change was judged worthwhile and economically tolerable by the owners, the work has proceeded as expected with the added attraction of the preservation of many acres of land as a wildlife refuge and recreational area.

INTRODUCTION

With depletion of oil and gas fields within the U.S. coming simultaneously with vastly increased energy demand, the importation of natural gas from foreign countries has become increasingly necessary. By liquefying the gas (by reducing its temperature to -260 °F and, thus, its volume to 1/600 of the original) and by the development of special ships for its importation into the U.S. from the oil and gas fields or Africa, the Middle East, Indonesia, Alaska, etc., a much needed new source of energy has been made available.

Columbia LNG Corp., a subsidiary of the Columbia Gas System Inc., in 1970 started planning for LNG importation through a terminal second in size only to one already operating in Japan. The marine terminal and pipeline for the $30$ million complex at Cove Point, Md., will permit the unloading of the equivalent of a billion cu ft/D (28 million cu m) of natural gas imported from Arzew, Algeria.

Raymond Technical Facilities Inc. (RayTech) a wholly owned subsidiary of Raymond International Inc., was first authorized by the owners in July 1970 to perform site investigations and to prepare preliminary designs and budget estimates of construction cost for the marine terminal to berth, unload, and service the new generation of 125,000 cu m ING tankers coming into use. RayTech was later committed to perform the final design including the preparation of working drawings, specifications, and bidding documents for all structures from the berth line to the contract limit interface, at the onshore tunnel shaft.
Implications Of Plate Tectonics For Offshore Gulf Of Alaska Petroleum Exploration
Authors: Rogers John F.
Published: October 2008

The Gulf of Alaska is coincident with a converging plate boundary, along which the Pacific plate under thrusts the North American plate. Evidence of this interaction shows up in the deformational pattern of offshoreTertiary strata, as well as the sedimentary record.

A broad belt of compressional structures attributable to plate movement can be traced along the continental shelf and slope of the western Gulf of Alaska, parallel to the Aleutian trench. This field cuts diagonally across the continental shelf of the central Gulf of Alaska, splaying into the Fair weather fault system in an area north and west of Yakutat. Structures within this compressional field show evidence of sequential development - from northwest to southeast.

Tertiary sediments record a history of southeastern migration of the "Aleutian" trench, and a tectonic incorporation of early Tertiary oceanic", trench and slope sediments in a late forming continental shelf, all in response to episodic, but persistent plate convergence.

The implications of plate tectonics for exploration is significant. Knowledge about timing of structural growth, and distribution of reservoir and source rocks is vital to an exploration program. Plate tectonics is not an oil finding tool but a unifying concept to which one may relate the required interpretations about structure and stratigraphy.

INTRODUCTION

The concept of global plate tectonics is becoming more and more important to petroleum geologists exploring the Gulf of Alaska. There, two segments of the earth's crust, the Pacific plate and the North American plate, interact in an area where one major O.C.S. lease sale has recently been held, and others are scheduled for the future.

The most promising areas for petroleum potential in the Gulf of Alaska are in the northern Gulf where O.C.S. Sale 39 was held in April 1976, and in the western Gulf where O.C.S. Sale 46 is tentatively scheduled for November 1977 (Fig. 1). In both areas, petroleum potential lies in Tertiary sediments. Folding and faulting took place in Tertiary time. Both stratigraphy and structure have been affected by the plate-tectonics environment of the region and the time.

The regional geology of this area has recently been described by various U.S.G.S. workers.1, 2 Fig. 2 is a vastly simplified geologic map. Mesozoic rocks border the Gulf of Alaska onshore. They are highly deformed and metamorphosed to some extent, and probably have no petroleum potential in the region. Early Tertiary sandstones, shales and volcanics of deep marine (in the west) and shallow marine and non-marine (in the east) environments form the next outcropping belt seaward. They too, are highly deformed. Petroleum potential in these rocks depends on their being found in a location where the degree of deformation is less, and where there is reasonable expectation of reservoir sand development. Late Tertiary rocks outcrop in the next, more seaward out Crop belt.
The Effects Of Cyclic Loading On The Behavior Of Anisotropically Loaded Soft Clay
Authors: Arango Ignacio
Published: October 2008

While clayey soils subjected to cyclic loading may show a decrease or an increase in undrained shear strength during and immediately after the stress applications, their moduli have been observed to decrease considerably. Cyclic loads produced by earthquake or storm wave action may, even at relatively low strain level cause considerable shear deformations in soft clays which are under initial shear stresses, as in sloping delta and bay deposits.

Laboratory test data are presented which show the effects of cyclic strain amplitude, rate and number of applications on the strength and modulus of clays. The use of the laboratory data to the solution of typical field problems is illustrated with an example.

Laboratory shake table tests on soft clay banks appear to validate the analysis procedure. Moreover, the results of the application of the method to the field case appear to be reasonable. It is concluded that this simplified method is a practical and reasonable way to obtain estimates of the dynamic deformation of sloping soft clay deposits.

INTRODUCTION

The expansion of industrial and commercial installations in highly populated near-water areas requires, in many cases, the use of land with poor subsurface conditions. This situation is illustrated in Figure 1, where fill is shown overlying a soft San Francisco Bay Mud deposit for subsequent use in a highway extension in the area.

Cyclic loading of the clay produced by water wave activity, or ground shaking induced by earthquakes in the region requires that the engineer search for appropriate answers to questions, such as:Is the site stable during and immediately after the cyclic loading?; or,What is the order of magnitude of the permanent deformations that the fill may sustain? Answers to both of these questions can be given at this time with a reasonable degree of confidence.

It is the purpose of this paper to review the experimental evidence that may be utilized to answer the first question, and to present a simplified method which provides an answer to the latter. The experimental validation of the method and its application to a practical case are also presented in the paper.

CLAY STRENGTH UNDER CYCLIC LOADING

Castro and Christian (1) have recently reviewed the findings of various investigators on this subject. The authors concluded that when a saturated undrained soil is subjected to cyclic loading followed by undrained compressive loading to failure, the static shear strength is equal to or very close to the static undrained shear strength for samples that failed without initial cyclic loading. For example, tests on San Francisco Bay Mud (2) indicated that for cyclic strains less than one half of the strain to failure in static tests, the loss of strength after cyclic loading was less than 10 per cent of the original peak strength.
Underwater Survey Using An Inertial Navigation System
Authors: Stankoff Alain , Tait R. A.R.
Published: October 2008

Recent trials of an Inertial Navigation System onboard a manned submersible have proved that underwater distances over several hundred metres can be measured with an accuracy of about ±15 cm.

The system is completely self-contained, independent of transponders or beacons and free of underwater environment disturbances. It permits, in addition, to navigate over very long distances with accuracies comparable to those obtained with long or short base acoustic systems.

This means that a new tool is now available to the offshore industry to simplify and/or improve the efficiency and the accuracy of underwater jobs such as tie-ins, measurement of flow-line position and length, pipe-line surveys, installation of platforms, sea bottom profiling, Navigation in acoustically disturbed environment, etc?

INTRODUCTION

Inertial navigation systems have, in the past, found application in, particularly, fighter aircrafts and space vehicles. Such aircrafts required the system to be light enough, self contained and largely immune from external influences. Navigational data is presented on a plane perpendicularly to the local vertical and oriented with respect to the True North. The system basically measures the components of the acceleration along the reference axes and compute the position by integrating them twice. Due to various imperfections, the resulting integrals are drifting as a function of time, and, for aircraft systems, the order of magnitude of the drift is I Nm/hr which is unacceptable for underwater usage. To offer convenient navigation performances required the system to be completed by an additional information relative to the INS velocity drift. Unlike aircraft, submarines can easily stop during their survey, and stay motionless on the seabed. Then, during such a station, the inertial system is informed that the speed is null, can measure its own drift and takes it into account to correct the navigational data until another stop occurs, where the drift data will be updated. The accuracy of the correction depends on the residual motion of the system during the drift check and of the time interval between two stations. Preliminary laboratory results had shown that such an inertial system, corrected every minute, could give the position within a few centimeters, which was at least one order of magnitude better than conventional underwater positioning systems. Furthermore, in the underwater environment, being self contained and immune from external influences offered immediate advantages over transponder systems. As it was necessary to demonstrate the performances of such an accurate system over significant distances, but impossible to verify the absolute accuracy over pre-determined distances underwater, the present evaluation was performed in two steps :First, the absolute performances were accurately checked on a well-known circuit onshore, with the system mounted on a vehicle and driven at speeds simulating the submarine motion.
The Installation Of A Deep Water Dry Subsea Completion System
Authors: Stone William H.
Published: October 2008

INTRODUCTION

The dry subsea completion described in this paper is not the first, but is the initial non-experimental "off the shelf" installation of a deep water one atmosphere air, shirt sleeve environment, dry habitat subsea completion.1–4 The following is a chronological account of this installation from the selection of the dry system to the commencement of actual oil production from the well. The selection of the total production system, the testing at a dry land test site, simulating underwater conditions of the complete system including the platform facilities and the TFL pump down system, the drilling and completion of the actual well, the installation of the wellhead cellar, the flowline connection, the underwater assembly and the start up of the actual production are included.

It is the author's intent that this paper will convey to operational engineering personnel a fundamental understanding of how this project was successfully accomplished. This paper is not a detailed technical treatise of the dry system for the scientific reader as what information is available elsewhere; However, if the reader wishes to delve into details of specific aspects of the system's various components, the many, vendors who contributed to this project are named to provide a means for the reader to obtain this information.

OBJECTIVES OF THE PROJECT

As exploratory drilling advanced into ever deeper waters, the need for sophisticated production systems to produce reserves in deep water became essential. However, the industry has very little experience in deep water production. Deep water is defined as below any depth divers can function effectively. Therefore, in 1974, Union Oil Company of California's management created the "Deep Water Subsea Completion Project" with the following goals:To complete and produce a well on the sea floor that world enable the company to gain experience and expertise in deep water production.To simulate, in an existing shallow water field, conditions below depths that divers can function effectively.To utilize functional hardware and methods whenever possible using "off the shelf", rather than experimental hardware or methods. Functional is defined as having been successfully used in the field.

The project's primary purpose was to gain deep water subsea production experience, not to begin a research and development program. The entire project was planned and implemented by the company's Gulf Coast Regional Operations personnel, not by a research or staff study group. The project was divided into the following phases:A study of- the "state of the art" of available systems.Selection and procurement of equipment.Dry land testing.Offshore installation.Well production.
A New Concept For Single Point Mooring Offshore Terminals
Authors: Di Tella V. , Di Campanile A. , Di Paruzzolo A.
Published: October 2008

The paper describes a new type of offshore tanker terminal and the related specially developed method of analysis of its dynamic behaviour.

The system, called articulated leg mooring system "ARLEMS" is an improved single point moo r in g buoy capable of mooring large tankers and of with standing severe meteorological-oceanographic conditions.

It is essentially formed of an upper structure supporting the rotating table, connected to a basement standing on the sea bottom, by three tubular legs. Two cardan joints at the ends of each leg allow the upper structure to move horizontally.

The advantages of such a system, as well as those of the particular kinematics, are examined in detail. The important role played by cardan joints is pointed out.

Two basic configurations of the system, the "buoy ant body buoy" and the "buoyant legs buoy" and their basic applications are outlined.

The method of analysis of the dynamic behaviour of the buoy is based on the non-linear motion equations of the legs, taking into account the influence of the different forces and the particular kinematics of the system.

A computer program solving non-linear equations by a step-by-step integration technique has been developed; movements of the buoy, as well a s loads on the bow hawser, on the legs and on the joints, are evaluated for two typical water depths: 27,5 meters for the "buoyant body buoy" and 105 meters for the "buoyant legs buoy".

INTRODUCTION

The large amount of oil exchange all over the world has caused a great demand for oil terminals. Because of frequent lack of suitable natural harbours near oil fields, it has been necessary to provide off shore oil terminals; besides, these installations can be located far from port traffic, thus reducing pollution risks.

Various types of offshore oil terminals have been studied, experimented, realized; among the latter:fixed berth;tower mooring system;multiple buoy mooring (M. B. M.);single point mooring (S. P. M.). With the rapid increase of tanker size, the S. P. M. system has prevailed mainly because:fixed berths are very expensive;tower mooring system is expensive and rather exposed to collision hazards;M. B. M. system is not capable of mooring large tankers and requires much time for mooring operations. The catenary S. P. M. system is widely used today; its basic elements are:a cylindrical hull buoy anchored to the seabed by means of four, six or eight chain legs;a turntable, mounted on the buoy body, equipped with lights, valves, piping, floating hoses and bow hawser;a central swivel, mounted on the inner part of the buoy, to transfer the oil from fixed submarine hoses to rotating flow-lines. The growth, in recent years, of offshore oil production has called for oil terminals capable both of operating in great water depths and of withstanding severe meteorological-oceanographic conditions.
Marine Seismic Surveys
Authors: Espey H.R.
Published: October 2008

The frontiers of petroleum exploration are principally in offshore areas where seismic surveys provide the only technical data for prospect evaluation. The marine seismic industry has grown from 15 crews at work in 1960 to 75 crews today. Worldwide expenditures on marine seismic surveys last year totaled $450 million. The average cost per mile of marine seismic data today is the same as it was 9 years ago. New technology has reduced operating costs counteracting inflation. Significant improvements have been made also in data quality and resolution.

MARINE SEISMIC SURVEYS

In offshore areas such as the Atlantic OCS and the Bering Sea, seismic surveys commonly provide the only source of technical data on which decisions can be made concerning lease acquisition. In such frontier areas few wells are available for use in conventional exploration techniques. Other methods used on land, such as surface geology, photo mapping, and satellite imagery, are not applicable in marine areas. Last year, the petroleum industry spent more than $450 million on marine seismic surveying, including the associated data processing and interpretation. This may seem like a large amount of money, but it is almost negligible compared with the additional outlay that is made based on the geophysical data. For example, the Atlantic offshore sale last year was preceded by 68 crew months of seismic work at a cost of $35 million. In that sale the petroleum industry bid $3.5 billion based principally on the seismic data.

Geophysics for many years has played a key role in petroleum exploration, but it is only in the past 15 years that its use in offshore areas has become significant. In terms of the number of seismic crews in the field, most of the activity is onshore (Fig. 1). Only 15 marine crews were at work in 1960, but this has increased to an average level of 75 in 1976. These figures are for the free world only. Essentially all of the work in 1960 was carried out in the Gulf of Mexico, while today marine crews are surveying the shelf areas off of every continent. Due to higher mobility, a marine crew typically records 10 times as much data as a land crew. For this reason, it is more useful to look at statistics in terms of line-miles of data recorded (Fig. 2).

In 1960, only 21 percent of the seismic data was recorded by marine crews. Last year, more than 800,000 line-miles of data were recorded offshore and this represented 67 percent of the total seismic activity. This reflects the increased dependence on offshore areas for new major discoveries. The increases in activity in 1973 and 1974 reflect the response to the energy crisis. The drop in activity in 1975 and 1976 is principally due to adverse government regulations and tax policies in the United States, Canada, and the Far East.

In 1971, the petroleum industry began placing an increased emphasis on finding new reserves in the United States.
A New Offshore Pile Aligning System
Authors: Boyadjieff George , House Horace , Roussel Herbert
Published: October 2008

In offshore construction work related, to platforms, one of the difficult operations has been the connection of add-on sections to pin piles as they are driven into the sea floor. The reason for this difficulty has been the relative motion of the derrick barge and the fixed platform. This paper describes the design of and the application of a pile aligning system that makes this operation much easier to accomplish. It emphasizes the design of the system and its field operation conducted in the Gulf of Mexico. It describes the design concepts, the actual hardware used to implement these concepts, and the successful operation of this hardware on the actual job site.

INTRODUCTION

Steel platforms are conventionally pinned to the ocean floor with large diameter piling, either driven through the legs of the platform and/or through guides around the base of the platform. When platforms are set in appreciable water depth it is not feasible to handle the pipe in one section and it becomes necessary to add-on additional sections as the pile is advanced to the ocean floor and then driven into the ocean floor. The platform itself is grounded to the ocean floor, whereas the derrick associated with a derrick barge for handling the pile add-on sections is floating on the ocean surface. Consequently, there is relative motion between the section of piling to be added-on and the pile that has already been installed in the leg or guides around the platform itself. The amount of motion is, of course, a function of the wave action or sea state on the derrick barge. When the weather becomes a factor, it becomes a common practice to wait for calm seas even though the operation may be costing upwards of $50,000 per day to stand by.

Piling commonly used today is either 48" diameter or 54" diameter, with some piling over 90" in diameter. The accepted method of joining the add-on sections to the pile is by welding. Because this welding must be performed in the field, the connection time can approach three to twelve hours per connection, or longer depending on wall thickness. A time saving solution has been to use mechanical type connectors which theoretically eliminate the welding time and pay for themselves even though they may never be recovered. Experience to date, on the other hand, has proven that the motion described above makes it virtually impossible to align the connectors and successfully connect them. Sometimes the operation can take longer than welding would have in the first place; consequently, there has been a reluctance to use these connectors.

Finally, in the case of pile that is driven' through guides around the base of the platform (skirt piles), it is sometimes desirable to cut off the piles at the base of the platform and to remove the upper section of the pile since it is not needed to improve the stiffness of the jacket and thus considerable savings can result.
An Approach to the Maximum Storm
Authors: Goldman Joseph L.
Published: October 2008

Four models of extreme storms were constructed that could retain their dynamic stability long enough to generate a fully developed sea. Two time-dependent distributions of wind, wave and swell are computed for each model for use in a realistic computation of directional wave spectra. The G-4 storm model in its stationary mode contains a maximum deep-water combined sea of over 200 ft. height with over 30 second-period.

While the 4 models, G-I ' through G-4', may not represent the ultimate conditions that can be realistically contemplated, they have been prepared to represent a step in the direction of determining the ultimate extreme conditions for a given location. These models represent storms that can occur within our present major climatic cycle based on global climatological and weather information of periods less than 200 years and on near continuous records of daily Northern Hemisphere pressure distributions of about 75 years.

Soaring costs due to limited resources, dictate greater care in attaining the proper design of offshore structures. When applied covertly these models can provide the environmental design criteria to help decrease the range between over-design and under-design.

INTRODUCTION

Prior to the advent of semi-permanent offshore operations that were fixed to a particular site, the hostility of the offshore environment was confined to accounts in ship's logs. A summary of these accounts by Bigelow and Edmondson (1947) indicated that extreme conditions in the Pacific were about twice as severe as in the Atlantic. The Pacific maxi= wave height was 112 ft., and the Atlantic maximum wave height was 60 ft. The famous Arakawa and Suda report (1953) of the 1935 typhoon that affected the 28 ships of the Japanese Fleet indicated 81 ft. significant waves. At the time of their report, all wave observations over 65 ft. were discounted. A later comparison of the 1935 typhoon with Hurricane Carla by Freeman and Davis (1972) indicates that the high waves may indeed have been accurate observations.

Historical accounts of high waves encountered by ships in the Atlantic indicate an increase with time. In January, 1894, a ship off the west coast of Scotland reported many 30 ft. (significant) waves during a storm with some reaching 43 ft. (maximum). In February, 1900, in a storm off the northwest coast of Scotland, waves of 35 ft. were reported offshore from Peterhead. During February, 1907, an ocean liner 400 miles southwest of Iceland encountered a 'huge swell with waves up to 41 ft. (maximum). During December, 1911, a ship encountered up to 40 ft. waves (maximum) indicating a storm in the Bay of Biscay. Calculations of wave heights, from data supplied by ships officers, indicated a wave of 60 ft. (maximum) encountered southwest of Iceland during February, 1923. Photographic evidence of multiple waves over 36 ft. (significant) were obtained from a ship south of Newfoundland during March, 1929. These storm events have been confirmed by Freeman and Davis (1972) with weather maps.
Bottom Stability as a Factor in Platform Siting and Design
Authors: Garrison L.E. , Bea R.G.
Published: October 2008

The stability of bottom sediments is a critical factor in the emplacement of offshore platforms. It should first be investigated early in the pre-sitting stage when reconnaissance surveys might reveal areas to be avoided or conditions that must be accommodated. More detailed studies of specific sites should follow to characterize local soil properties. In the design process, sea floor soils and the foundation elements of the platform should be considered as two components of a single system that together comprise the platform substructure. The strength and load-deformation characteristics of the soils, just as any other element of the platform system, must be determined for the projected lifetime of the facility.

Principal sea floor instabilities are downslope mass movements of sediments, sediment moved about by various transporting agents, and faulting. These can be recognized and characterized by a five-part program of interdisciplinary study consisting of geology, geophysics, environmental mechanics, soil reconnaissance, and the development of analytical models.

To provide the basic research for understanding these problems, acquire adequate information on bottom stability in frontier areas, and build and maintain a data base for input to analytical models, a cooperative effort between industry and government will be necessary.

INTRODUCTION

Experience in Outer Continental Shelf (OCS) areas has shown that in many cases a primary factor in platform sitting and design is the potential instability or the bottom sediments. Moreover, available information on sea floor conditions in frontier OCS areas indicates that similar concerns exist there. These problems are, for the most part, being successfully resolved with available technology, but at a considerable cost in overdesign and redundant measures. Although rapid progress is being made toward understanding the nature of bottom instability and improving the means of quantifying it, much remains to be learned. Meanwhile, the economy and safety of platform design can be improved simply by the early identification of sea floor stability problems, and by the careful assessment of expected soil performance.

The following discussion will briefly review the problems of bottom instability as they affect platform sitting and design, the techniques for recognizing and analyzing soil performance, and a possible approach by government and industry toward gathering the information necessary to resolve these problems.

THE PLATFORM SYSTEM

An offshore platform consists of two major components. They are:The substructure, which consists of the soils in the vicinity of the platform and the foundation elements and well-conductors that receive their support from these soils, andthe superstructure, which consists of the braces, legs, and other steel and concrete elements that bridge the distance from sea floor to deck level, as well as the deck girders and beams that support the drilling and production equipment.
In-Line And Transverse Forces On Cylinders Near A Wall In Oscillatory Flow At High Reynolds Numbers
Authors: Sarpkaya Turgut
Published: October 2008

The in-line and transverse forces acting on circular cylinders placed near a plane boundary in a sinusoidally oscillating fluid in a U-shaped vertical water tunnel have been measured. The drag and inertia coefficients for the in-line force have been determined through the use of the Morison's equation. The transverse force has been expressed in terms of a single lift coefficient. The results have shown that the effect of wall-proximity is to increase the force coefficients for-relative gaps of e/D less than about 0.5. In this range of e/D values (0<e/D?0.5) the non-linear interaction between the shear layers emanating from the top and bottom of the cylinder is reduced and the frequency of oscillations in the shear layers is a synchronised. For e/D larger than about 0.5, the regular vortex shedding resumes, more or less unimpeded by the presence of the wall, and the lift, drag and inertia coefficients nearly assume their free-cylinder values.

INTRODUCTION

There are numerous problems associated with the design and installation of pipelines in deepwater offshore. The results such as the ones presented herein touch upon only one facet of the submerged pipeline design and do not deal with the consequences of pipeline forces due to temperature, pressure, seabed friction, buckling, scour, hydroelastic oscillations, etc. Suffice it to say that the reliable design information is most likely to come from a combination of laboratory studies, field experience, and careful documentation of the causes of failures.

The present investigation is fn extension of that previously reported by this writer1 and was undertaken for the purpose of determining the in-line and transverse forces un cylinders near a wall at relatively higher Reynolds numbers.

It is a well-known fact that for very small values of the period parameter UmT/D, the flow about the cylinder does not separate, the wake-dependent drag force is very small or negligible and the in-line as well as the transverse forces are essentially from the wave temporal acceleration. Under such circumstances, the appropriate coefficients may be easily evaluated from the potential theory. When the cylinder touches the boundary, a net force exists away from the wall. However, if even a very small gap exists between the cylinder and the wall, then a large net force exists toward the wall. The inertial forces alone do not usually give rise to the maximum load situation for small structures, such as pipelines located on or near ocean bottom, and the separation effects become extremely important not only in the determination of the magnitude but also the direction of the forces.

It is important to recognize that the separation effects are not free from the effects of the boundary layer which forms on the bottom plane.
Dynamic Response Of Agravity Platform Under Random Wave Forces
Authors: Nataraja R. , Kirk C.L.
Published: October 2008

A spectral analysis of a typical multi-tower gravity platform is carried out considering soil-structure interaction. The modal frequency response function and the linear hydrodynamic load transfer functions are determined taking into account diffraction effects. The displacement and wave loading spectra are evaluated for sea states described by the JONSWAP wave height spectrum relevant to the North Sea environment. Considerable simplification is achieved by a reduced interaction model together with complex notation for wave forces. A detailed parametric study using various values of soil shear modulus yields r.m.s. values of deck displacement and tower elastic deformation.

1. INTRODUCTION

With the increase in the rate of world oil consumption, exploration and production has moved into deeper and environmentally more hostile waters. The first phase of this development favoured fixed platform with concrete gravity structures being designed for water depths up to 150m and the steel jacket for water depths up to 300m. The first concrete gravity platform, Ekofisk I, was installed in the North Sea in June 1973 and by the end of 1977 there will be more than 12 concrete platforms in operation. These are generally of a multi-tower (2, 3 or 4 towers) configuration with a large base caisson of cellular construction extending up to 1/3 of the height of the platform, or a manifold type of construction with a large diameter base raft up to 150m in diameter.

In designing a gravity platform it is necessary to consider the effect of wave forces on both the towers and the caisson in order to calculate the structural and foundation stresses. A detailed analysis of a rigid four tower gravity platform supported on a rigid foundation under deterministic and random wave forces is given by the authors in ref l.

The present paper gives a random dynamic analysis of a typical multi-tower gravity platform (Condeep Brent 'B') taking into account flexibility and damping of the towers and of the foundation The random wave forces are calculated for a fully developed sea state represented by the J.O.N.S.W.A.P. wave height spectrum11, which is typical of the North Sea. Due to the relatively large diameter of the towers hydrodynamic drag forces are small compared with the inertial forces on the towers and the diffraction forces on the caisson and can be neglected, enabling a linear spectral analysis to be used. The fluid inertia loading on the towers is evaluated using Morison's equation and diffraction theory where relevant. The wave forces and moments on the caisson are computed using the results of wave diffraction analysis2,10

The spectral analysis is carried out by formulating the wave force transfer function for unit wave height using complex notation, taking account of the phase differences of the waves due to spatial separation of the towers. The frequency response function is based on a real eigenvalue analysis through a modal formulation of the soil-structure dynamic model.
Deep Water Drilling From Amoored Semisubmersible
Authors: Kobus L.C.S. , Meyers H.M. , Bounds V.G.
Published: October 2008

A new series of floating-drilling semi submersibles, the Zapata 55-2000 class, have been in operation fore total of 24 months. During that time, the first two rigs of the four rig series have drilled at twelve locations in the 900-l400 feet water depth range in the Gulf of Mexico. This paper describes the design features of these innovative semi submersibles and their unique deep water mooring systems. Operational techniques and problems peculiar to drilling in 1000 feet of water are discussed. This paper describes actual operating experience that can be of use in advancing normal floating drilling operational capabilities into deep water.

INTRODUCTION

When the BLUE WATER I was being redesigned and modified to serve as the pioneer semisubmersib1e drilling unit by a team of major oil company technical experts in 1959-1961, the 200-300 ft. water depth range was considered to be "deep water" for floating drilling operations. (Figure 1)

That historic "sit-on-bottom" submersible, which was redesigned and modified to operate in a"semi"-submersib1e or floating mode, was the forerunner of the semisubmersib1e offshore drilling rig. After the concept of drilling while floating on a Semi submersible "Island of Steel" was proven by the BLUE WATER I project to he technically, operationally and economically feasible, a whole new class of offshore drilling rigs came into existence. The first new semisubmersib1e to be designed and built was the innovative eve-formed once OCEAN DRILLER in 1963. Since that time, 123 more semi submersibles have been constructed and put into operation.

Zapata's design efforts in the late 1960's and early 1970's were directed towards the development of a super class pf semi submersibles, with capabilities to operate safely and effectively in the harsh environments of the North Sea, the Gulf of Alaska and the stormy regions of the extreme southern latitudes of the Atlantic, Pacific and Indian Oceans off the Antarctic continent. The Zapata 55-3000 design class and the ZAPATA UGLAND were the resultant products of that design effort.

However, in 1972, Zapata set its sights on the development of a lesser capability class of semi submersibles that would be more suited and could safely, effectively and economically operate in deep water (1000-2000 ft.) on the continental shelf and slope areas off the Eastern, Southern and Western coasts of the United States and as well as the Eastern and Western coasts of Canada.

Evaluative study efforts resulted in Zapata teaming with the noted design consultants, Frieda and Goldman, in 1973, to modifying one of their standard designs to suit Zapata's specific design requirements. The product of that joint-design effort between Zapata and Frieda and Goldman is the Zapata SS-2000 class - the ZAPATA CONCORD (1975), ZAPATA LEXINGTON (1976), the ZAPATA SAPATOGK (1976) and the ZAPATA YORKTOWN (1976). (See Figures 2,3,4 and 5).
Challenges Associated With The Design Of Oil-Gas Separation Systems For North Sea Platforms
Authors: Penick Dudley P. , Thrasher William B.
Published: October 2008

The conditions encountered in North Sea oil-gas separation facilities design and operations require that the system be both highly selective and extremely flexible within the space and weight limitations of a production platform. To better understand the approaches used by process engineers in attempting to solve these difficult and sometimes conflicting challenges, this paper discusses the objectives, some of the more significant factors which influence the potential solutions to these objectives, and some of the approaches utilized to meet these challenges.

OBJECTIVES

1. Produce Specification Crude Oil

Crude oil specifications may be either for offshore tanker loading or for delivery to an oil pipeline at the platform. Crude oil specifications are usually defined relatively simply, through limitations on vapor pressure and on BS&W (basic sediment and water) content. BS&W is normally limited to a nominal percentage, such as 0.5%, and meeting this specification is outside the scope of this paper. The oil-gas separation system in most cases? does not significantly affect whether or not the oil will meet the BS&W specification, since for those oils where this is a problem special emulsion treating is required independent of the oil-gas separation system.

Crude oil vapor pressure is the specification which most influences the design of oil-gas separation systems. For offshore tanker loading, the oil may be limited to a vapor pressure in the range of 8 to 14 pounds RVP (Reid Vapor Pressure). RVP refers to a standard method of vapor pressure testing utilizing a specific test cylinder assembly and determined at a temperature of 100°F. RVP is not identical with TVP (true vapor pressure), which is the actual vapor pressure exerted by a liquid in equilibrium with a vapor at any given temperature. Oil leaving an oil-gas separator operating at 15 psia and 1000F will have a TVP of 15 psi at 100°F. Because of the nature of the RVP test cylinder, TVP of a liquid at 100°F will always be slightly higher than RVP. The amount of this difference is a function of the liquid composition and can be estimated.

Crude oil vapor pressure limitations for pipeline delivery may be comparable to those for tanker loading. However, a pipeline can be specifically designed as a high vapor pressure system to handle gas liquids components mixed with the crude oil, and it is quite possible that North Sea oil pipeline systems will be developed in this manner .It is not expected to be feasible to recover NGL components separate from oil and gas on the offshore platforms and construct separate NGL pipeline systems. It also may not be possible in every field to design a separation system which meets both gas specifications and low crude oil vapor pressure specifications unless an intermediate product is also made. This will be more fully discussed later in this paper.