bluuebunny/crossref_metadata_2025 huggingface dataset:
Deployment and Operation of the 50KW Mini-OTEC Plant
Authors: McHale Frank A. , Jones W. Lloyd , Horn Henry M.
Published: October 2008

In 1979, the world's first successful at-sea closed cycle OI'EC plant was moored offshore of Kona, Hawaii and produced the first net useful electricity from the solar heat resource stored in ocean surface waters. The unique mooring combined the Ocean Thermal Energy Conversion (OI'EC) mId water pipe and mooring" tension leg in one polyethylene pipe. Operation of the electrical generating process was successful in waves up to 15-20 feet.

Introduction

The State of Hawaii, Lockheed Missiles and Space Company, and the Dillingham Corporation agreed in lq78 to design and build a demonstration Ocean Thermal Energy Conversion (OI'EC) plant to be installed on a floating platform. By August 1978 a Navy barge had been obtained. The actual instruction of the plant took place between December 1978 and June 1979, and the barge was placed on station in July 1979. On August 3, 1979, Hawaii's Governor George Ariyoshi was able to announce the world's first successful generation of net useful OTEC power from a closed cycle plant operating at sea. The plant continued operating successfully until November 1979, when available funds had been depleted, and at that time the Mini-OTEC returned triumphantly to Honolulu Harbor.

MOORING DESIGN REQUIREMENTS

The underlying requirements for Mini-OTEC were to supply 3200 GPM of 40°F. water from 2200 feet to a surface barge and to moor this surface barge, a 103 × 34 ft. rake-ended hull, in currents which were known to exceed two knots. Winds of 30 knots and waves of 25 foot significant height were assumed to be acting in the same direction with the ocean current. A system was required which mold be deployed within a weather window of only one day. The sea bed at the required mooring site (a State-sponsored, environmentally blessed research area) is a steep rocky volcanic slope with very rough topography. There are essentially no bottom sediments that would allow an anchor to "grab". The seabed slopes so -steeply that there was concern that an anchor might simply slide down the underwater hill, which is merely a continuation of the shoulder of the volcano above.

Research on candidate materials for the Mini-OTEC cold water pipe (CWP) had led to consideration of fiberglass, polyethylene, polyvinylchloride, rubber and polypropylene. Polyethylene was attractive as a CWP material due to its very smooth interior for flow characteristics and low susceptible to befouling. In addition, a fusion welding process was available that provided reliable joints. The polyethylene is very flexible and rugged and is not brittle even at low temperatures.

After selecting ultra high molecular weight polyethylene pipe as the CWP material, and in looking at the polyethylene properties, it was soon realized that the breaking strength of the pipe was quite high and a most attractive solution was to use the cold water pipe itself as the mooring line. This was considered to be a rather daring step at the time and much discussion ensued.
Use Of Fracture Mechanics In Tubular Connexions Fatigue Crack Growth
Authors: Putot Claude J. , Truchon Michel L. , Nguyen Quoc S.
Published: October 2008

The experimental studies carried out by IRSID within the framework of a larger european program, are backed up by a French program of theoretical and numerical studies. The purpose of this program is to relate the results obtained in the test specimen experiments to the behavior observed on large scale tubular joints tested in fatigue fracture mechanics. This paper gives some of the results and trends obtained, with the help of which it should be possible to make better life predictions than those in common use.

INTRODUCTION

Leaving aside the effects of corrosion which are by themselves the subject of numerous studies, the following basic information is required to enable the behavior of a joint working in fatigue to be interpreted:The character of fatigue crack propagation obtained by means of classic experiments performed in the parent material as well as in the heat affected zone (HAZ).Verification that the propagation laws, thus obtained, apply to cracks with curved fronts.The influence of biaxial and bimodal effects, always present during crack propagation in a tubular joint.

The experimental work was carried out on a E36-Z steel (AFNOR standard) which is generally used in the construction of offshore platforms. This material was hot-rolled into plates of 20, 40 and 75 mm thicknesses. Its chemical composition and mechanical properties are given in tables I and 2.

PROPAGATION IN A CLASSICAL TEST SPECIMEN

Figure 1 shows tests carried out on 20 mm and 40 mm thick C.T. specimens machined out in the longitudinal direction. The slope of the regression line is 4.3. Figure 1 also shows the-fatigue crack growth rate measured with CT specimens machined out in the short transverse direction. The slope is 3.1 - All these tests were performed with a stress ratio of 0.1.

In the heat affected zone (HAZ)

In a first set of tests C.T. test pieces were cut out of 40 mm thick welded joints so that the crack propagated in the H.A.Z. in a plane close to that describing the limit of the weld perpendicular to the plate: cut-out L-T (fig. 2a).

As shown in figure 3 the rates of propagation recorded, in as-welded joints, are for a given level of stress intensity factor, much slower than in the parent material. A second series of tests was worked out to approach the effective geometry of tubular joints. This is the L-S cut-out direction, as shown on figure 2b. The propagation characteristics observed on KF and KT test pieces corresponding to this series of tests, are recorded on figure 3. They show striking parallelism with short transverse results, in spite of a shift between the two regression lines.

Restrictions concerning the Paris line

Regression analysis carried out on 153 pairs of variables (C,m Paris law parameters) proved that Paris lines depend on only one parameter [I].
Modeling Of Piled Foundations
Authors: Kriger Glenn A.
Published: October 2008

A comprehensive set of guidelines for constructing linear models of single piles and pile- groups for foundations of offshore structures is presented. These models are used as boundary conditions at the base of the superstructure, thus permitting independent analysis of the superstructure from its supporting foundation.

The paper is, a "how-to-do-it" text for piled foundation modeling. It is also of value to those in related disciplines, such as geotechnical specialists who will gain insight into how their data is applied in analyzing structures supported by piled foundations. Discussions include the behavior and modeling of single piles and pile groups. Construction methods are presented for three types of pile models -- matrix, springs, and equivalent pile. The advantages and disadvantages of each model type are described. Linear and nonlinear foundation behavior characteristics are treated in depth. Factors which influence the approach to a modeling problem are outlined. Emphasis is placed on providing the reader with an understanding of the physical behavior of piled foundations and model construction. A step-by-step procedure for model synthesis is provided in the form of an example.

INTRODUCTION

In a fixed offshore platform, the steel jacket superstructure and its supporting piled foundation are more conveniently analyzed if treated separately. There are major structural and behavioral differences between the jacket and foundation, and the two do not lend themselves to similar analytical methods. This paper presents basic techniques for constructing linear models that simulate the foundation behavior at the super structure foundation boundary. Use of these models permits independent superstructure analyses. Selection of the model type and its degree of refinement are described from a global overview of the structure, available data and ramification of analytical results. Construction of the foundation simulation model follows routine procedures utilizing results of an independent foundation analysis.

SINGLE PILE BEHAVIOR

The load-deflection behavior of a single pile crown is of key importance in model construction (2), (4). Analysis of a pile embedded in soil is extremely difficult because of the infinite dimensions, nonhomogeneity and nonlinearity of the soil. As a practical necessity, the problem is usually simplified by treating the pile as a beam-column supported by nonlinear axial and lateral soil springs. Details of these analytical procedures are beyond the scope of this work, although application of the techniques presented here will require the availability of such an analytical tool. Although this material pertains to single piles, it forms the basis for understanding pile group behavior discussed later.

Figure 1 depicts a right hand orthogonal coordinate system which is used throughout this paper. Displacements "D" and forces "F" are shown in each of the six degrees of freedom (DOF). The pile behavior is studied by observing the force(s) required to produce displacement in each of the six DOF while holding all other displacements at zero.
Dynamic Behaviour Of Platform Cranes
Authors: Balfour James A.D. , Owen David G.
Published: October 2008

Mathematical models of varying complexity are developed to predict forces occurring in a typical offshore platform crane while lifting from a supply boat. Computer programs based upon these models are used to assess the usefulness of these simulations. It is shown that a simple two degrees of freedom model can be usefully employed at the feasibility design stage whereas a more sophisticated multi-degree of freedom model is required for detailed crane design. These programs are used to investigate the influence upon the induced loads of various operational parameters including the load lifted, the jib angle, the line speed and the instantaneous deck velocity. It is concluded that high dynamic loads will frequently arise in such cranes and that specifically written computer programs are necessary for effective derating. or design. A statistical approach to the assessment of supply boat motions is used to show that maximum deck velocities can greatly exceed those predicted by the simpler methods previously used.

INTRODUCTION

As oil and gas exploration moves into more exposed waters supply boats are required to operate in increasingly severe conditions. This means that the dynamic stresses induced in offshore platform cranes lifting from these boats are likely to increase for three different reasons. Firstly, there will be a tendency for supply boats to stand further off from the platform with the consequent increased lifting radii resulting in increased stresses. Secondly, higher stresses will be produced should higher line speeds be used to reduce danger to the supply boat, the supply boat personnel, and the load. Thirdly, larger supply boat motions mean that the effect of a misjudged lift will prove more severe. Since exploration has moved into the North Sea there have been numerous crane failures, and recognition of the dynamic aspect of offshore lifting is given in the derating rules used when lifting in conditions other than a flat calm. In spite of this, the accident statistics for offshore crane operations continue to make sad reading, indicating shortcomings in the derating criteria adopted.

It is self-evident that to design or derate an offshore platform crane effectively a study of the crane's behaviour over a wide range of conditions must be made. Unfortunately, due to the complexity of offshore cranes, an explicit solution is possible only if the crane structure is reduced to a very simple model. Such models have been studied previously (1,2,3" but they tend to give unrealistic results and they fail to give any information about the forces in many of the primary crane components.

For a more realistic prediction of response mathematical models of greater sophistication must be employed. Such models normally use step-by-step numerical techniques to integrate the equations of motion of the structure through the time domain starting from initial, known, conditions. Essentially the conditions at time "t" are used to predict the conditions a short interval of time "ilt" later.
Actual Measurement Of External Forces On Marine Hoses For SPM
Authors: Saito Hideaki , Mochizuki Takao , Fukai Toshio , Okui Kenzo
Published: October 2008

In single point mooring (SPM), the role played by the marine hose system for transporting oil is so essential that if the hose system develops any trouble, it will not only stop the functioning of the transporting of oil but also may cause oil pollution.

It is indispensable for engineers in developing and designing hose to know how much external loads act on the hoses and how the hoses break. In the Catenary-Anchor-Leg-Mooring hose system (CALM), much trouble occurred on the first off-the buoy hose, tail hose and under-buoy hose. Therefore field testing and/or simulated testing by model in a tank for measuring loads on these hoses, was conducted. From this testing, the relationship between waves and external loads could be generally known, and hoses could be designed more suitable for each location.

INTRODUCTION

The SPM comprises floating hose strings from a tanker to a buoy, the buoy, under-buoy hose strings from the buoy to the sea bottom, and a submarine pipe to land for conveying oil.

Recently, stocking crude oil has been increased and the mooring hours of a tanker shortened, so hoses transporting oil must have bores as large as 20" or 24". Thus, in any accident occuring on the hose system, losses might be larger and more serious than for ordinary hose. For this reason, the design must be greatly reliable. Furthermore, conventionary, values of external loads acting on hoses were estimated mainly through experience, and even if a hose developed trouble, the cause was not sufficiently examined.

Therefore, the authors measured the external loads of the first off-the-buoy hose in Tokyo Bay and the tail hose of a storage tanker in Java Sea (Indonesia) in field testing in order to find the external loads usually expected on marine hoses. On the other hand, in order to find how hoses break under such rough conditions at sea as typhoons, simulated testing with a model was conducted for the floating hose and the under-buoy hose in our wave tank. The enormous volume of data obtained by this testing was processed by a data recorder, A-D converter and computer, and many results were obtained.

This report describes test methods, maximum load, minimum bending radius acting on the hoses under various conditions of waves.

FIELD TEST METHODS AND ANALYTICAL METHOD

1. Field testing the first off-the-buoy hose in the sinking and floating hose system at the Keihin Kawasaki seaberth in Tokyo Bay. As shown in Fig. 1, a water-proof spool for measuring external loads was fitted between the buoy-inlet and a 20" first off-the-buoy hose. Strain gauges were attached to the spool in order to measure the bending moments, torsional torques and axial forces; the relation between the load and strain had been calibrated before hand.
Design And Installation Of The Main Pass 72 Mud Slide Resistant Platform
Authors: Irick Jack T. , Birdwell J. Rick , Henkhaus E.J. , Hayes Don A. , Mannina Glenn A.
Published: October 2008

Exxon, U.S.A., has installed a mud slide resistant drilling and production platform in 119 ft. of water in the Main Pass area, block 72. Mud slides as deep as 50 ft. are possible in this area where rapid deposition has produced a weak, unstable, under consolidated clay. The major configurationally difference between this platform and conventional Gulf of Mexico platforms is the four 154 in. diameter jacket legs required to resist the lateral forces caused by soil movement and to form a protective housing for the conductors. The foundation consists of four 144 in. diameter piles with 72 in. diameter insert piles driven in two legs and twelve 22 in. diameter conductor piles inside each of the other two legs. The design and installation of this structure are especially noteworthy due to loads imposed by the unstable bottom conditions of the site.

INTRODUCTION

Exxon's Main Pass 72 platform (Figure 1) is in the state-owned portion of the block and is approximately 6 statute miles from the mouth of Pass A Loutre in water depth of 119 ft. The regional seafloor slope is to the east-northeast and averages0.4°. Under consolidation and high pore pressure have decreased the sediment strength sufficiently to make the delta front prone to mass sediment movements, which are categorized as either creep, slide, or oscillation. Creep is a gradual soil deformation with time, typically associated with a slope. A slide is a sudden movement of a mass of soil down a slope. Oscillation is a cyclic movement of the soil, including both horizontal and vertical motion, driven by wave pressures on the soil surface. Consideration of soil motion in conjunction with the wave loading leads to the design concept of encasing the well conductors within the large diameter leg members. Most other platforms (e.g Reference 1) for mud slide areas have this common design characteristic. However, the platforms are quite different, primarily due to the large array of concepts for jacket and piling installation.

The structural design of Main Pass 72 platform was initiated in 1977. The design goal was to maximize the use of proven technology, particularly that required during the offshore installation.

PHYSICAL FEATURES

The Main Pass 72 platform (Figure 2) was designed to accommodate 24 wells and provide 16,900 sq. ft. of deck space on each of two levels for drilling and production. The platform was designed to be installed in 119 ft. of water and to withstand hurricane storm loads and those additional loads caused by moving soil.

The jacket has four plumb 154 in. diameter legs whose centers in plan form an 85 ft. square. The leg members penetrate 50 ft. into the mud to stabilize the jacket during the early stages of pile driving. Each leg is grouted to a 144 in. diameter pile driven to a penetration of 200 ft.
Hydrodynamic Coefficients For Macrd-Roughnesses
Authors: Nath John H.
Published: October 2008

Consideration is given to the drag of cylinders in fluid flow as influenced from the very large and lumpy roughnesses of marine growths. A means of characterizing the effective diameter is based on the Strouhal number of the roughened cylinder. This can be applied even to cylinders covered with thick accumulations of kelp, or heavy encrustations of mussels or barnacles, or any combinations. Thus, the effective diameter can be a function of Reynolds number. The effective roughness can be evaluated from force measurements. This paper presents results from measurements on laboratory models of marine growth roughened cylinders.

INTRODUCTION

A considerable fund of evidence has been developed on the force transfer coefficients for smooth and sand roughened circular cylinders in steady and unsteady ambient flow. In particular, the offshore industry and the U. S. government have sparked a considerable effort on the very complicated problem of determining wave and current forces on platforms at sea. It would be futile in the space alloted here to try to review the large number of reportings of others on this subject. Suffice it to say that the recent works of Borgman, Chakrabarti, Dalrymple, Dean, Garrison, Hudspeth, Keulegan and Carpenter, Morison et al, Sarpkaya and Wiegel (arranged in alphabetical order) and many others, have contributed greatly to the understanding of cylinders in the drag, drag/inertia and inertia regimes. Only the work of others that has direct bearing on a particular subject in question will be referenced in this paper. Even so, much work of others will be omitted and no doubt even overlooked. I apologize if anyone feels slighted and say at the outset that applicable work not mentioned herein and later brought to my attention will be much appreciated.

This paper presents the results of some studies on heavily roughened cylinders in steady flow. A relatively smooth cylinder was tested and a sand roughened cylinder in order to establish a "ground truth." This is part of a larger study presently in progress at Oregon State University of macro-roughened cylinders in waves and in oscillatory flow. However, testing is still in progress at this writing and results are not yet available for publication. (References, etc. at end of paper).

Heideman, et al (2) show a plot of the post-critical steady flow drag coefficient as a function of relative roughness, k/D, where k is the height of the roughness elements and D is the smooth cylinder diameter. It is reproduced here as Fig. 1 to illustrate that the drag coefficient still increases with increasing k/D at the limit of the plot, where k/D has a value of about 0.05 or so. However, it is known that marine growth thickness in the Santa Barbara channel (for at least one place) can be greater than one foot in the near surface levels on structural members greater than, say, two or more feet in diameter. One 3/8" steel wire rope, observed by the author and his diving partner, supported growth that had an approximate circumference of 2.2 feet.
The Variable Bore Ram - Present And Future
Authors: Williams Bolie C.
Published: October 2008

Introduction

Variable bore blowout preventer rams are a relatively new development. Since these rams increase the flexibility of the ram-type BOP substantially, they have become the subject of considerable current interest. This paper contains general background information on the variable bore ram (VBR), as well as discussions of the advantages and limitations which are to be considered when specifying BOP equipment and when designing and implementing well control procedures using VBRs.

DESCRIPTION

A variable bore ram (VBR) enables a ram-type BOP to close and seal on more than one size pipe. This is in contrast to the standard pipe ram which is designed to seal on only one size pipe. In the standard pipe ram, the elastomeric packing material is supported against extrusion by rigid steel plates which are shaped to fit closely around the pipe. In VBRs the elastomer is contained by a series of segmented inserts which cooperate to conform to any of several pipe sizes as well as tool joints, kellies and other parts of the drill string whose size lies within the range of the VBR.

VBR packers are actuated in the same manner as standard pipe ram packers. The packers extend forward of the ram block faces. When the rams are closed, the packers contact each other on the leading surfaces at the sides of the pipe bore. As the rams continue to close, the elastomer in the packers is displaced inward against the pipe. In standard pipe rams, the packers extend forward of the ram block faces only a fraction of an inch since the elastomer is only required to "feed" inward sufficiently to replace material lost through wear. In the case of the VBR however, much more radial displacement is required. There must be enough displacement to cover the pipe size range of the ram, plus enough to replace material lost through wear. In VBRs, the packers may extend forward of the ram faces an inch or more. There are practical limits to the amount that the packers may extend forward of the ram face. The most obvious limiting factor is the requirement that the ram packers may not extend into the wellbore when the rams are fully retracted. This limits the pipe size range of the VBR, especially in the case where they are designed to fit existing BOPs.

Another factor which limits the pipe size range of a VBR is the width of the ram block, or the width of the packer recess in the ram.

The annular area that can be closed off by the packer is proportional to both the packer width and the distance that the packer extends forward of the ram face. As a result, VBRs for large bore BOPs can have a greater pipe size range than VBRs for small bore BOPs. For example, the maximum pipe size range for the Cameron VBR the 11" bore BOP is 5" to 2-7/8", whereas in the 18-3/4" BOP, the VBR can cover 7-5/8" to 3-1/2" pipe.
Availability And Pricing Of Insurance For Offshore Operations
Authors: Petersen Warren T. , Sinnott John T. , Whalen Leo R. , Sammis Russel F.
Published: October 2008

Section I.

Fixed Structures And Pipelines - Installation Stage

Scope Of Available Insurance

With surprisingly few exceptions, the insurance industry has been able to meet the coverage requirements presented by the rapid change in offshore risks over the past dozen or so years. It has been necessary for the insurance industry to provide:Broad All Risk PerilsCoverage For Accidents Resulting From Error In DesignCoverage For Loss Of Income As A Result Of Delay In Start-upProtection For Loss Due To Certain Political Risks

Installation Risks

The installation of offshore fixed platforms confronted insurers with new technology. Hondo, Cognac and North Sea Condeep Platforms are examples. While the designers, builders and installers of these units demonstrated the soundness of their advanced technology, it must be remembered that the insurers did not have the ability to assess the design criteria and installation techniques. They did enlist the services of outside Marine surveyors, but, in agreeing to underwrite these new risks, had to accept as an article of faith that the "experts knew what they were doing". Having done so, they then had to weigh the risk of unexpected fortuitous incidents, acts of God and other happenings beyond the control of these "experts".

The need for underwl'iters willingness eo insure new technological risks will not diminish in the future. Tension Leg Guyed towers and Artie platforms will challenge insurers.

The insurance arrangements for large fixed platform installation risks often entail an owner controlled program. This would cover physical damage and third party liability risks. Protection is provided for the operator, non-operating partners and all contractors and subcontractors. The program would envisage coverage during all phases - fabrication, load-out, transit, installation and hook up.

Insurance on offshore pipeline installation risks involve broad All Ri~k perils coverage similar in scope to platform risks. However, insured amount requirements per risk are substantially less than with platforms. Furthermore, there is rarely a combined program for fabrication and installation. As such, our comments herein relate purely to the installation risk.

Deductible Patterns

The deductible approach on installation risks is fairly straightforward. While approaches can vary, minimum deductibles of $50,000 and $100,000 arerequired for platform and pipeline risks respectively. Larger deductibles can be negotiated but the scale of credits diminishes sharply. Very large deductibles rarely prove economic.

Pricing

The pricing of offshore installation risks is notas mysterious as is sometimes alleged. In the case of fixed platform rate workups, the value and period of each phase (fabrication, tow out, installation, hook up, etc.) is determined, and specific rates assigned to each phase. These rates can vary from say one-tenth of one percent of value for the fabrication stage to 3% for the tow out stage.
The Development of Corrosion-Resistant Tubing
Authors: Yoshida Toshio , Mann Tarlochan , Matsuda Shozo , Matsui Shigetomo , Atsuta Toshio , Toma Shouji , Itoga Koyu
Published: October 2008

Corrosion problems of deep well tubings and their control measures are discussed. As one of the economical solutions, metal-lined tubings are proposed. It is shown that the metal liner can be tightly fitted by the newly developed thermo-hydraulic fit method. The selection of corrosion-resistant material for the liner is discussed. Some designs of the tubing joint and their manufacturing methods pre proposed.

Introduction

With the seriousness of an energy shortage, the drillings of oil and gas wells have become more active. In the last few years, the number of drilled wells has increased rapidly as shown in Fig. 1. It should be noted that eighty percent of these wells were drilled in the United States. Fig. 2 shows the depth distribution of oil and gas wells drilled in the United States in 1977. It can be seen that the numbers of oil and gas wells are about the same, but, in general, there are more gas wells at deeper depths than oil wells. As shown in Fig. 3, well depth increases each year, and recently an exploratory well deeper than 10,000 m has been drilled.

As the well depth increases, the down hole pressure and temperature rise. For instance, in some severe wells at a depth of 6,000 m, the pressure and temperature are more than 1,000 bar and 200°C respec. tively. Therefore, the strength of the tubings used in such severe conditions is a very important factor for deep wells because they have to sustain their ownweight also. Figure 4 diagrammatically illustrates the transitions of design conditions with the increaseof well depth. Figure 5 shows the relationship between well depth and axial and hoop stresses of API standard tubings with various pipe 'dimensions. It isknown roughly that the strength of class P-I05 is required for the tubing of a 6,000 m well.

Another substantial problem in deep oil and gas wells is corrosion. This is mainly due to H2S and C02 which are contained in crude oil and natural gas The density of these contaminants is increased with well depth, and, together with high pressure and high temperature, the corrosion reaction is accelerated.

To solve all the above problems in deep wells, a new type of economical, high strength tUbing which has high corrosion resistance is expected. At present a super Ni alloy tubing, which is extremely expensive, tends to be used. As a solution, the authors propose the Tight Fit Tubing (hereinafter referred to as TFT) which is lined inside with corrosion-resistant alloy to meet the above three requirements for the tubing of deep wells. The thermo-hydraulic fit method used here for TFT has been developed to produce the corrosionresistant pipe, TFP, for corrosive flow lines as reported in Ref. 1.
Reliability Analysis of Offshore Structures in Towing Operation
Authors: Pajouhi K.
Published: October 2008

This paper presents a semi-probabilistic method for evaluating the reliability of tie-down braces for ocean going tow barges,-and a rational design method for tie-down braces compatible with effect of uncertainties in analysis and impact of result offailure.

In receht years, study of response of an offshore towing system to random sea became more important than before because of the increased"" water depths for which platforms are now being designed. Since uncertainties are unavoidable in determination of forcing functions, structural behavior and tolerances within fabrication and installation methods, therefore the tool of engineering analysis should include concepts and methods for evaluating the significane of uncertainties and its implication on the design and performance of the towing system.

Reliability analysis of the towing system is a multi-component analysis; tug, towlines, barge, tiedowns, and the structure (cargo) being in tow itself. The reliability, P, of a towing component can be expressed in termssof probability of failure, pf, defined as the probability of experiencing of this system component failure during the tow period (Pf = Probability [ Load> Resistance]).

The reliability of a tow system can not be based on the "weakest link of a chain approach", but it should reflect reliability of all the tow components in the system. This work is based on the following assumptions; i) a structural component can be in either safe or fail conditions, ii) tugboat propulsion and steering system can also be in either broken or unbroken states, and iii) occurrence of any storm along the tow route is independent of the others. The reliability model for tow system is described as a combined system, being composed of series and parallel system.

The evaluation of probability of occurrence of a design sea state is based on the MARKOV model. The basic assumption used is that given the present state of a seaway, the past states have no influence on the future. The theorem of total probabilities is used to evaluate the system reliability. As an alternative approach, the POISSON model is also used in computing the frequency of occurrence of the design sea state.

An optimum towing system design can be achieved using the reliability analysis in terms of initial cost, probability of failure, and cost of failure in relation with the time-lag in production, and impact on public confidence.

Introduction

The application of the structural reliability concept to offshore tow system requires descriptions of random variables; loads, and resistances for each individual component of the system. However, due to imperfect or non-existent data, the frequency distributions of loads and component resistances can not be predicted exactly Thus engineering judgement and subjective reasoning must be used in conjuction with available statistical data to modify the probabilities and produce an acceptable reliability assessment of the system.
Fatigue Considerations for Ocean Towage
Authors: Hambly Edmund C. , Edwards Antony J. , Kohli Colin C. , Miller Barry L.
Published: October 2008

Fatigue damage of structure, seafastenings, or barge during ocean towage can be a critical consideration in choice of barge and tugs, and in the selection of tow route and marine operations. This paper describes the procedures that were followed to assess the risk of fatigue damage, and the practical precaution aken to control its incidence, during the tow of the jacket for Amoco Norway's Valhall Production Platform from Morgan City, Louisiana, to Stord, Norway, during autumn 1980.

Damage criteria are identified and calculations demonstrated for:- survival of the unit in possible extreme conditions, exposure along the tow route, and derivation of Critical Motion Curve to limit rate of fatigue damage to an acceptable level. The analysis of motion induced stresses is explained, including the rapid methods of calculation adopted to derive hot spot stresses around a complex multi-chord overlapping joint, and to adapt standard S-N curves for direct use for Rayleigh distributed stresses. The theoretical survival life of the unit in maximum seas is shown to be very sensitive to the values determined for parameters such as SCF and wave period.

The paper reports the marine procedures adopted, and describes the fatigue audit that was maintained throughout the tow, which indicated that the theoretical fatigue damage was successfully controlled to a low level.

1.0 Introduction

The jacket for the Valhall Production Platform was completed for Amoco Norway by McDermott Inc. at their yard at Morgan City, and was loaded out onto the barge "Intermac 600" on 9th August 1980. Fig. 1 shows the 4737 L.T. jacket on the 500'x120'x33' barge. The tow commenced on 25th August when all we-ather forecasts were favourable, and the barge was towed by 4 river tugs to Bayou Boeuff. From there the 5000 HP "Dallas J- Adams" towed the unit 20 miles offshore to hand over to the deep draft, ocean going salvage tug "Atlantic", 140 ton bollaI'd pull, of Bugsier Reederei. (Throughout the paper the term "unit" is used to 1.0describe the total structure of the jacket, tie-downs and the barge.)

The tow route is shown in Fig. 2. The tow proceeded without serious barge motions until 8th September when Hurricane Earl passed nearby and the tug had to heave to for a period to limit barge motions to below the limits previously set for control of the rate of fatigue damage. Subsequently steady progress was maintained until a storm was encountered on 18th September near 44°N 21°Wwhen the tow line broke, for reasons not fully understood. This point was close to the predetermined point at which the "Atlantic" was to be joined by the Bugsier tug "Caribic", 84 ton bollard pull, for the remainder of the tow.
Observations on the Mode and Rate of Decay of an Artificial Ice Island in the Alaskan Beaufort Sea
Authors: Reimnitz Erik Reimnitz , Kempema Edward , Ross C. Robin
Published: October 2008

In the Arctic ice is abundant and cheap, and therefore has long been considered as structural material for offshore drilling platforms. In February 1979 Exxon constructed an ice island 400 m in diameter and 10 m thick, in water 3.5 m deep, about 6 km north o£ Prudhoe Bay, Alaska. The purpose of the experiment was to learn whether such an island could survive one summer and then serve as a platform for an extended drilling period the following winter. Our studies of the decay of this island are based on:surface observation and photography,diving observations and bottom photos,fathometer, sub bottom profiler and sides can sonar surveys, and spot measurements of water salinity, temperature, and turbidity around the island.

Thermal erosion, promoted by turbulent heat transfer from wave action and a relatively warm surface water layer surrounding the island, resulted in a 3- to S-m-deep notch at sea level, frequent calving, and a cantilevered, vertical scarp above sea level around the island. The winds, waves, and surface currents that transport warm river water past the island are predominantly from the east. Therefore, thermal erosion and calving were most rapid on the east side; they had truncated the island to the original center point by late August. By early September, the last remnants of the island floated away. The submerged part of the island, below the sea-level notch, consisted of an extensive pedestal reaching as much as 15 m beyond the island and feathering out to 10cm thick at the lip. The outer 5 m of this pedestal was elevated above the flat sea floor, forming a gap up to 25 cm wide that exposed the smooth base of the island to divers. Average island shore erosion, effected largely by calving, ranged from 2.5- to 5-m/day. The pedestal retreated across the sea floor at the same rapid rate, but melting, rather than calving, was probably the major cause of erosion. Hydraulic processes did not have time to develop cut-and-fill structures in the sea floor. In short, the test showed that unprotected ice islands in shallow, warm coastal waters have no chance of survival through the summer.

Introduction

Ice interacts with the marine environment in many different ways, and through these interactions becomes an important geologic agent. The U.S. Geological Survey has conducted research on ice related processes for many years. Recent petroleum developments in northern Canada and Alaska have focused attention on ice in the marine environment and on the effects of the ice on shores and the shelf surface. During the winter of 1978-1979, a circular ice island with a diameter of 400 m was built in the shallow waters of Stefansson Sound, about 6 km north of Prudhoe Bay, Alaska (Fig. 1). This large, firmly grounded mass of ice of known age, size, and shape provided a unique opportunity to study the effects of summer conditions until the island was destroyed by natural processes before winter.
Newest TFL Retrievable Safety Valves for Subsea Completions
Authors: Yonker J.H.
Published: October 2008

Two new novel retrievable ball type safety valves are now available in the TFL (pumpdown) configuration. These valves offer greater depths in a more simplified configuration to operators planning subsea completions. They were developed at the request of major producers for use in the North Sea and Southeast Asia.

Design of these new valves began in 1977 and early 1978 by engineers from Otis' Research and Development Department. One valve is a balance line valve with a novel fail-close leak protection feature. The other valve is a single line deep set valve which allows installation at greater depths than previously achieved. They have been successfully tested in test wells and test loops at the manufacturer and in two land test wells by the operators. The safety system packages for these valves are currently installed in two subsea wells - one in the North Sea and the other in the South China Sea.

The material presented in this paper shows how the capabilities of subsea production are expanded by the development of a truly fail safe balance line retrievable ball valve and a deep set single line ball valve which can be placed at or near the packer, below paraffin levels, or below the kickoff point.

IN1RODUCTION

As completion technology has kept pace with drilling technology in the push for oil into deeper waters and hostile environments, so too has pumpdown (TFL) completion and service technology kept pace with developments to give the industry viable alternatives in the completion of subsea installations. This holds particularly true for TFL retrievable ball safety valves; two new valves have been developed which help extend subsea completion technology.

In 1977 a design team consisting of R&D Engineers from Otis and engineers from a major North Sea producer met and established design criteria for a new subsea safety system. This system would consist of a tubing retrievable ball valve and a separate backup insert TFL ball valve both of which would be of the balanced type. The exact details of the design criteria for the tubing retrievable valve and the overall safety system are presented in detail in arc Paper No.4253 presented at the 1982 Offshore Technology Conference.

In early 1978 Otis was approached by a major producer from the Southeast Asia area regarding new TFL completion equipment. This equipment was to be for a new below-the-mudline subsea completion and was part of an ongoing development program in which five TFL subsea completions had been made with an above the mudline wellhead configuration.

Discussions centered around the updating and compatibility of the TFL completion and service equipment with the new system; but also in particular on the safety system. The safety system would, as in the past, be a single line system. However for this completion it would consist of both a tubing retrievable valve and an insert valve. Five previous completions had consisted of only a landing nipple and TFL insert valve which spoke well of the reliability of and confidence in the insert valve's operation and retrievability.
Model Anchor Tests in Cohesionless Soil
Authors: Walker G.R. , Taylor R.J.
Published: October 2008

Tandem (piggyback), parallel, and single model anchor tests have been performed in cohesionless soil. Tandem anchor arrangements have been found that result in a tandem system capacity that exceeds twice the capacity of a single anchor. These methods are operationally feasible and offer the potential for anchor weight savings.

INTRODUCTION

The Navy has been involved in an ocean test program of typical Navy and commercial drag embedment anchors. 1,2,3,4 An important aspect of this program has been the determination of effective methods of using the Navy's large inventory of STOCKLESS anchors. Tandem (piggyback) and parallel anchoring methods had been considered during this program and several methods have been found suitable with STOCKLESS anchors.

The Navy's expanding fleet mooring requirements and the capacity limitations of the STOCKLESS anchor has made it necessary to extend the test program to evaluating multiple high efficiency anchors. The overall effort consists of laboratory tests in cohesionless (sand) and cohesive (soft clay) soil, and then prototype field tests of the most promising anchor combinations.

This paper describes the laboratory test program in cohesionless soil and provides test results describing the effectiveness of the multiple anchor methods.

TEST APPARATUS AND PROCEDURES

Two model anchors were fabricated for this test program; both were identical 1/20 scale models of a 6,000 lb STATO anchor. A holed plate was attached to the anchor shank to enable a shank-shackle attachment which is subsequently described in the tandem anchor section. As with the STATO prototype the fluke angle could be adjusted, however, for the multiple anchor tests the fluke angle was set at 32°. The test chain between anchors models the effective diameter of 2-inch welded chain. The models can be seen in Figures 1 and 2.

The anchors were instrumented so that load and drag distance could be recorded continuously. The sand used was a dry poorly graded medium dense sand that was vibrated to a maximum relative density of65%. The soil was enclosed in a 9.5' × 2.5' × 1.3' deep box. The anchors Were dragged through the sand with a wire attached to a capstan driven by a variable speed motor.

SINGLE ANCHOR TESTS

Prior to multiple anchor tests the single anchor's (Figure 1) holding capacity was defined so that subsequent holding capacities of the tandem anchors could be compared to the standard holding capacity. The average amount of anchor roll was 4° and this amount of roll is considered minimal. All subsequent holding capacities are compared to the single anchor capacity using the parameter S, where S is the ratio of anchor capacity to the standard anchor capacity. The standard capacity for the single anchor with wire is 15.9 lbs.
Three-Year Experience in the Numerical Prediction of the Slow-Drift Motion of Moored Tankers
Authors: Molin Bernard
Published: October 2008

A numerical model is described which allows to simulate the slow-drift motion of moored tankers, in waves, wind, and current. Emphasis is put on the physical problems for which a better knowledge is required in order to improve the model. Some numerical applications are presented.

INTRODUCTION

For the past years IFP has been carrying out a thorough research program on wave-drift forces and slow-drift motion of moored tankers. In a previous OTC paper I it has been described how wave drift forces in regular waves could be obtained from potential theory, through diffraction-radiation codes. In irregular waves the drift force varies according to the wave envelope squared, and induces slow-drift motion of moored structures. Wind and current also play an appreciable role, and in some cases may cause dynamic instabilities of the system 1.

So far the only tools for designs of SPM systems have been model testing and rules of thumb based on previous experience and the calculation of mean wind and current loads 2. In most cases model testing remains the only alternative for the final design of a system however the need is felt for more handy and less expensive tools for preliminary designs or parametric studies.

In the recent past different attempts at solving this problem numerically have been proposed. Due to the many occurring non-linearities, simulation models are usually preferred ',4, eventhough they present some disadvantages regarding the statistical interpretation of the results. Models dealing directly with the statistics 5 or working in the frequency domain are so far restricted to academic cases.

The main difficulty in elaborating a simulation model for slow-drift motion consists in formulating properly the hydrodynamic exciting loads and damping terms. Since one is dealing with a resonant system the latter ones are as important as the former ones, even- though they have been kind of overlooked in many instances. The first paragraph consists in a short presentation of the simulation model. In the second one some considerations are given on the problem of properly formulating the hydrodynamic loads and damping. They aim more at pointing out where the problems are than at providing solutions. Last the third paragraph presents some results obtained with the model, and discusses their validity.

PRESENTATION OF THE MODEL

Figure 1 schematically illustrates the problem that is considered: a tanker moored by a hawser to a buoy anchored in 0, and exposed to waves, wind, and current coming from different directions in the horizontal plane. The wave-system, defined by its power spectrum S(w), is assumed to be mono directional, while the wind and current may be time varying.

The position of the tanker is referenced by the horizontal coordinates (xG,YG) of its center of gravity G, its heading, together with its vertical coordinate ZG.