bluuebunny/crossref_metadata_2025 huggingface dataset:
Evaluation of Wear in Compliant Tower Pile Systems
Authors: Johnson G.S. , Hasle A.E. , Figgers R.F.
Published: October 2008

One of the technical issues that must be addressed for compliant towers is the wear that may occur due to the relative motion between the piles and pile guides. This paper presents a method for adapting! accepted analytical procedures for wear of mechanical systems and applying them to compliant towers and other offshore structures. This method is used for the Compliant Concrete Tower in North Sea fatigue conditions to show that wear is expected to occur, but can be limited by proper material selection and design and construction detailing.

INTRODUCTION

Pile supported compliant towers have been proposed and are being considered for use in various deep water fields throughout the world (Ref. 1 to 4). These towers obtain optimum "de-amplified" dynamic response by maintaining a structural period on the order of 2 times the wave period. This high period is achieve through the axial flexibility of the foundation system, which consists of long pile/sleeve units. Each of these units is guided along its entire length between the mudline and the attachment point to the structure.

As the tower sways due to wave loads, axial couples are developed in the pile/sleeve system. Relative motion occurs between the pile/sleeves and guides, giving the potential for wear at those locations. For the Compliant Concrete Tower (Fig. 1), the critical location for wear is at the guide nearest the mudline, where the normal force on the Pile is a maximum (Fig. 2).

DEFINITION OF WEAR

Wear is defined as a change in the surface contour, such as a groove formed in a flat surface or a flat spot on a curved surface, that results when two bodies are pressed together and relative sliding occurs. Zero wear is not defined to be the complete lack of wear; rather, it is wear of such a magnitude that the surface finish in the wear track is not significantly different than the finish in the unworn portion. Roughly, this corresponds to a depth of the wear scar on the order of one-half the peak-to-peak value of the surface finish.

WEAR CALCULATIONS FOR OFFSHORE SYSTEMS

Once a detailed design of the pile guides at the base has been completed, the appropriate method for determining wear potential will be through testing. However, at earlier stages of the design, simpler methods are required to ensure that wear concerns will not damage overall feasibility of the compliant tower concept.

Although wear is not a typical problem with offshore designs, it commonly occurs with mechanical systems, and accepted methods of calculating expected wear have been in existence for many years. The preliminary analytical methods presented in this paper are adapted from procedures presented by Bayer, et.al., (Ref. 5) for wear caused by loading of constant magnitude and frequency. This approach has been modified for offshore load conditions by using a discretized wave environment over the life of the platform and determining accumulated wear, as is done in a standard fatigue analysis.
The Relevance of Interaction Rules in Evaluating the Growth of Multiple Fatigue Cracks
Authors: Morgan H.G.
Published: October 2008

Flaws introduced during fabrication or developing in service rarely occur singly. Contained in the popular codes used to assess the integrity of structures are rules defining how arrays of multiple flaws should be treated. The current US and UK codes adopt the same philosophy, - simplification of the flaw array into a single larger flaw of assessable shape. Recent experience in using these rules has suggested that in many cases, they may result in assessments that are excessively conservative, possibly resulting in premature rejection or repair of plant.

Although a number of solutions for Fracture Mechanics parameters relevant to interacting flaws have been derived, little has been done to investigate the mechanisms of crack interaction. This paper describes a series of fatigue tests performed to investigate by direct observation how crack growth rate and shape development are affected during interaction.

The results are encouraging in that only weak effects have been observed with no indication of grossly increased crack growth rates. A methodology for assessing simple flaw arrays is proposed which assumes individual growth until crack tip contact or overlap occurs, at which point crack shape is adjusted to a smooth semi ellipse and crack growth evaluation continued.

INTRODUCTION

Manufacturing or service-induced flaws rarely occur in isolation and are often clustered together in critical areas of a structure or plant. A common problem therefore in assessing plant integrity or remanent life is the determination of the behavior of groups of flaws and their influences on one another.

In structures subject to fatigue loading, or where failure by fracture is a possibility, Fracture Mechanics methods are increasingly used to calculate the growth rate, shape development and stability of crack-like flaws. It is the influence of the proximity of another flaw on the effective value of the stress intensity factor operating at a crack which will modify the way in which it grows.

The problem of flaw interaction is dealt with specifically by two of the currently used assessment codes which give guidance on the use of Fracture Mechanics, - ASME section XI Division 1:1986 (1) and BSI PD 6493:1980 (2). In these documents, multiple flaw arrays meeting specified criteria are "recharacterised" into single flaws of simple, easily handled shapes for which standard solutions exist. The criteria for interaction are nominally and loosely based on a particular increase in stress intensity factor (typically 20%) in the ligament between flaws, although the solutions used to calculate this are not given or referenced in the documents.

FLAW RECHARACTERISATION.

The rules governing the amalgamation of flaws are broadly similar in both documents, the rules in ASME XI generally being based on flaw depth or height and those in BSI PO 6493:1980'on flaw length. Recharacterisations are defined for coplanar and non co-planar surface breaking and embedded flaws. Each of the codes contains cases not addressed by the other.
Offshore Placer Drilling Technology: A Case Study From Nome, Alaska
Authors: Bronston M.A.
Published: October 2008

A drill system configuration capable of working in the arctic offshore environment was developed to evaluate the placer gold resources contained in the glacio-fluvial sediments offshore Nome, Alaska. This system is capable of working on the shore fast ice during the winter months and is deployed on a drill ship during the summer. Transfer of the drill between its summer and winter configurations is accomplished with a minimum of modification to the design.

The system is designed around the Becker drill, a percussive, reverse circulation drill capable of penetrating the indurated glacial sediments present in the offshore Nome environment. This drill has operated effectively to depths in excess of 50 meters below sea level.

Since early 1987, in excess of 2000 holes have been drilled using the system. The drill system, often operating in extreme weather conditions, has averaged approximately 10 holes per day of drill production. The ore reserves defined during these drilling programs have resulted in the recovery of greater than 70,000 fine ounces of gold to date, by the mining vessel Bima.

Introduction

In recognition of the rising interest in offshore exploration and mining technology, this paper describes a placer drilling system configuration which has operated successfully in the arctic offshore environment near Nome, Alaska (Figure 1). Since March, 1987, 2073 holes have been drilled during four programs, two summer and two winter. An average of 9.3 holes per day have been completed to an average depth of 10 meters per hole. The drill has also been used to conduct standard penetration tests for dredge design studies.

The system is modular, during the summer a ship is used as a platform, and during the winter under most conditions, annual or perennial sea ice may be used as a platform. The drill, a Becker AP-1000 diesel hammer drill, was originally mounted on a tracked carrier for work on tundra and other rough terrain. The drill was removed from the tracked carrier due to handling and weight considerations and mounted on a skid specifically designed to serve as a drill support structure in either summer or winter configuration.

Review of Previous Drilling Programs

Three drilling programs were completed in the offshore Nome area prior to 1987. Two programs were completed on the annual ice by mining .companies during the winters of 1964 and 1967 (Daily, 1964; Daily, 1969; Wojcik, 1974), and one was completed by the U.S. Bureau of Mines (U.S.B.M.) using a drill ship in 1967 (Nelson and Hopkins, 1972).

The Becker drill was used in both mining .industry drilling programs. These programs were conducted during the winter from the ice using a skid mounted drill, towed by a bulldozer.
A Subsea Wireline Service System
Authors: Cobb C.C. , Dines C.A. , Headworth C.S.
Published: October 2008

The costs, safety and complexity of performing downhole service operations on subsea completed wells are major concerns of offshore operators. Service costs in particular often become the deciding factor when determining the feasibility of developing small subsea fields. Of particular interest are slickline and conductor line operations. In the past, these operations were usually performed on subsea wells using a semi-submersible rig and a rigid riser.

This paper describes a subsea wireline service system in detail. The system includes a subsea lubricator stack that attaches directly to the subsea tree. The wireline unit, control systems and deployment equipment remain on the deck of the vessel. The system is designed to be deployed from a relatively small vessel such as a diver support vessel (DSV). However, operations can also be conducted from a semi-submersible rig, workboat or moored barge. A riser is not required. System components, deployment and retrieval procedures, scope of work and emergency disconnect system are described in this paper.

INTRODUCTION

In many areas of the world, especially in the North Sea, the trend in offshore field development is towards subsea completions. The number of subsea wells is expected to double by 1994, with as many as 1000 by the year 2000. Several factors have influenced this trend.Cost - subsea wells are generally more economical than platform wells.Marginal production from undeveloped fields - most of the remaining undeveloped fields are small with limited potential which cannot justify the expense of platforms.Harsh environmental conditions - high Tides, extreme currents and ice make platform construction impractical.Deep waters - some undeveloped fields are in areas where extreme water depths exceeding 1300' make platform construction cost prohibitive.

Subsea wells are completed as single well satellites or in multiwell templates. Production can be into a floating production system, platform mounted system or a shore-based facility.

Subsea well completion designs do not differ significantly from platform-based wells. The corollary to this is that well servicing requirements will, in general, be the same for subsea wells as for any other offshore well.

Conventional riser-based methods of reentering subsea wells are time-consuming and costly, and therefore not: compatible with the operating cost levels dictated by the majority of subsea developments.

The system discussed in this paper was developed over a number of years, and has been successfully used to carry out a comprehensive range of wireline operations from floating vessels in the North Sea. This system was originally designed to be deployed from a diving support vessel (DSV).
ROV Tooling and Interface Design for a Subsea Satellite Template
Authors: Jolly A.D. , Hughes E.W.
Published: October 2008

This paper describes the ROV intervention tooling and interface designs for the Troll Osberg Gas Injection (TOGI) template. The ROV tooling and interface hardware successfully completed shallow water tests in January 1989 and are presently undergoing final integration testing.

The paper is significant because the TOGI template is the deepest underwater production template ever installed in the North Sea. The template's dependence on cost effective and efficient ROV support has caused an increase in the level and sophistication of ROV tasks and hardware and these advances are applicable to many other deep water installations.

INTRODUCTION

Production systems are being installed in deeper waters than ever before and are requiring more unmanned intervention. In addition, production systems are becoming more sophisticated. These two factors are combining to expand the envelope of present remote subsea support, intervention tasks and hardware. The TOGI project represents an up-to-date example of present day ROV production intervention capabilities.

The TOGI ROV work packages are designed to be ROV independent so that they can be deployed and operated by work ROV's of opportunity. In order to accomplish this, the ROV tools are powered and controlled through a modular hydraulic power skid package, and the deployment of the tools is based on the fly-in method. The use of manipulators is minimized and all tool interfacing is done through the hydraulic power skid as shown in Figures 1 & 2.

TOOL DEPLOYMENT/DOCKING

Many previous remote work system tooling and work package designs relied on manipulator installation of the tools. Manipulators are necessary and useful tools for performing a wide variety of subsea work tasks. However, different manipulators have different operational envelopes and characteristics and it is extremely difficult to develop manipulator deployed tools that are manipulator/remote work system independent. At the same time, it is usually impossible to justify the complexity and expense of developing all remote work system interfaces and intervention hardware such that the need for manipulators is eliminated. For this reason, the TOGI remote work system tooling and production equipment interfaces were designed to provide both docking and actuation of various hydraulic and mechanical functions without the use of manipulators, while the operation of hydraulic isolation valves and the connection of surface hotlines were retained as manipulator tasks.

In order to ensure the manipulator based tasks could be accomplished by a variety of remote work systems, several full scale manipulator models were built to ensure that compatibility was maintained. The use of the models to determine final equipment location and physical interface design has been proven to be extremely beneficial.

As stated above, the best method of ROV tool deployment, in most cases, is by physically flying the tool into position with the ROV. This is commonly referred to as docking.
Accuracy of Offshore Subsidence Measurements With Seabed Pressure Gauges
Authors: Mes M.J.
Published: October 2008

Phillips Petroleum Company Norway successfully pioneered the development, installation and application of pressure gauges to measure offshore- platform subsidence as a function of time.

Four pressure gauge systems were installed in the Greater Ekofisk area. On each of the 2/4-E Tort 2/7-C Edda and 2/4-F Albuskjell platforms one system was installed, serving as platform bench mark references. One system was installed on the 2/4-H Hotel platform, for which the subsidence must be measured as a function of time. These systems are described.

The theoretical accuracy as well as the apparent accuracy from prototype data prove that with pressure gauges we can measure the progression of subsidence with a relative uncertainty of +/− 9mm

INTRODUCTION

Background

Phillips Petroleum Company Norway is the operator for the Ekofisk Field, located roughly in the center of the North Sea. Figure 1 shows a geographical location map. Several platforms, located near the center of this field, are connected by bridges. This group of platforms is referred to as the Complex. The field was discovered in 1969 and has produced oil and gas since 1971.

In November 1984 it was discovered that subsidence takes place in the central Ekofisk field. Several measurements systems were, used to obtain information on the total amount of subsidence and the subsidence rate. An experimental measuring system based on pressure gauges was installed.

This system consisted of pressure gauges suspended from the Hotel and Tor platforms. A sketch of this experimental system is shown in Figure 2. Over ten weeks of uninterrupted, usable data were obtained and interpreted. It was decided that the measurement system was attractive for further development. Permanent Pressure Gauge Subsidence Monitoring Systems were installed on the? 2/4-H Hotel, 2/4-E Tor, 2/4-F Albuskjell and 2/7-C Edda platforms during the summer of 1987.

Sea bed pressure gauges (SPG) serve the following requirements:they track the progression of subsidence accurately so that a reliable rate of subsidence is computed.Data collection and data interpretation is economic and not labor intensive.The SPG system provides continuous measurements so that discontinuous subsidence can be seen.
New Critical Service eRA Alloys
Authors: Rizzo F.J.
Published: October 2008

Over the past few years, there has been increasing activity in drilling oil and gas wells to depths greater than 20,000 feet. Because of this, extremely hostile environments such as those in Mobile Bay are encountered. Materials selection for these systems has become a problem. Standard alloys will not hold up under the expected operating conditions. High alloy CRA compositions are being evaluated for these applications. This work describes how the use of powder metal processing of 625 (UNS N06625), 625M (UNS N07626) and C-276 (UNS NI0276) and the unique properties of these materials can be helpful in solving many of these problems.

INTRODUCTION

Powder metallurgy (PM) technology has been in use for many years. It has only been in the last twelve to fifteen years that people have been using this technique to produce material for critical applications. This has occurred because of the advantages of PM processing, which include uniformity of microstructure and properties, the ability to produce near-net shapes, and the ability to make components from alloys which are difficult to fabricate.

The aerospace industry led the way to develop materials and processes for the manufacture of fully dense PM materials. There was a need for materials with very high strength-at-temperature .capabi1ities. Years earlier, the limit had been reached for materials made by the conventional cast and wrought or ingot metallurgy (IM) method. The approach with PM was to make high strength, homogeneous, near-net shape parts for turbine engine hardware. After ten solid years of successful flight experience and many thousands of components later, this technology is beginning to find its way into other industries.

In the oil industry, this technology is being used to make some of the new corrosion resistant alloys (CRA's). Many of these materials are highly alloyed which leads to segregation problems and fabrication problems with conventional manufacturing processes. With the PM approach, these problems are overcome. In critical service applications such as the sour gas wells in Mobile Bay, there are many areas where CRA's made via the PM route can fulfill the stringent requirements. This is an ideal situation to take full advantage of the flexibility of PM and a cost effective method to justify the use of the high alloy CRA's that will do the job.

PM HANUFACTURING PROCESS - POWDER

The PM process starts the same as many other material manufacturing operations do; with melting. When high alloy materials are being made, this usually means vacuum melting. Powder can be produced from any molten metal, but the type of CRA's being discussed herein require vacuum or inert gas handling to keep the residual elements as low as possible. At Crucible, this melting unit is a vacuum induction furnace with a nominal 5000 pound capacity. Attached to this equipment is an inert gas atomizer which is used to manufacture the powder metal from a molten stream. A schematic diagram of this system is shown in Figure 1.
Statistical Risk Methodology: Application for Pollution Risks for Canadian Georges Bank Drilling Program
Authors: Sharples B.P.M. , Stiff J.J. , Kalinowski D.W. , Tidrnarsh W.G.
Published: October 2008

Risk analysis assessment techniques have not been applied previously to determining environmental risks at specific locations for offshore exploratory drilling programs in Canada. Rational analysis of the potential causes of failure, combined with probabilistic methods, provides a quantitative measure of the risk of an oil spill and consequent pollution from exploratory drilling which has not been available previously. This approach has demonstrated that the risk of oil pollution is very small much smaller than is perceived by the general public. Methods are also suggested by which operators can further reduce this risk.

INTRODUCTION

In recent years, there has been increasing public concern about protecting the marine environment from the effects of accidents in offshore oil and gas exploration and development activity. This concern attained a very high profile over a proposal for exploratory drilling on Georges Bank, a seaward extension of the continental shelf that projects into the Atlantic Ocean between Cape Cod and Nova Scotia. Georges Bank supports a rich fishery which is important to fishing interest in both Nova Scotia and Massachusetts. Much of this concern stems from the public's perception about the risk of environmental damage. When Texaco Canada Resources Ltd. (Texaco Canada) announced plans to seek regulatory approval to drill two exploratory wells on the Canadian side of the bank, the issue of risk to the environment was paramount in the minds of the interest groups lobbying against such activity. In the public debate over the issue, the potential risks of drilling activity on the bank became badly distorted.

Without site specific information to answer the concerns of critics, Texaco Canada was unable to respond directly to the allegations that its proposal presented an unacceptable risk to the environments and fisheries of the bank.

To provide a factual basis for discussing these risks, Texaco Canada contracted Energy & Maritime: Claims and Appraisals, Inc. (EMCA) and Noble Denton & Associates, Inc. (NDA) to develop a methodology to quantify the risks of an oil spill resulting in environmental damage, and apply it to the Georges Bank area. The objective of this study was to put the potential pollution risk in proper perspective and then develop recommended strategies for managing this risk.

Data were collected on blowout casualties, oil spill events and combined with an existing database on nondrilling hazards for the years 1955-1987. The goal was to present the hazards in a simplified graphical form, together with quantification of the probabilities of occurrence of the initiating events that could lead to an oil spill. Probabilistic techniques were used to verify the roost likely size of such an oil spill, if it were to occur, from both historical data and rational analysis. Information on previous accidents was used to assist in formulating appropriate hazard mitigation techniques.
Borehole Mining: Improved Technology Expands Horizons
Authors: Dibble M.F.
Published: October 2008

Borehole mining as a viable concept for mining deep mineral resources has historically been long on promise but short on results. Application of this technology has been of particular interest in the east coast deep middle miocene phosphate deposits. These deposits are potentially enormous in scope and underlie sections of the coastal plain and the near offshore continental shelf.

The landslide portion of these deposits are generally too deep to mine using open pit methods, forcing development of alternative mining methods. A significant proportion of the offshore resources appear to occur at depths below the practical limits of cutter suction dredges and even exceed the limits of bucket ladder dredges. In most cases these deposits are covered by overburden that has to be removed and redeposited before the mineral section can be mined. The depth, thickness, composition of overburden, and environmental sensitivity of a particular location greatly impact mining system selection. It is probable that conditions will exist on portions of the continental shelf that will eliminate dredge mining as a choice for extracting a particular deposit. In these cases new processes such as borehole mining must be developed or the mineral resource will remain undeveloped.

A successful borehole mining test program for extraction of these deep phosphate resources was conducted in northeast Florida in 1984-1985. Based on the results of this test program, plus subsequent laboratory testing, and engineering studies a prototype borehole mining system was designed in 1986. The design contained several innovations that appeared to offer higher productivity and lower power consumption than the test unit actually used. Economic restraints, at that time, prevented the actual construction and testing of this unit.

INTRODUCTION

Extracting mineral values through a borehole is a very old process. Oil wells, brine solution mining, frasch sulphur recovery, etc. are only a few of the borehole recovery systems that have a major economic impact on our society. The extraction of mineral values in slurry form through a borehole has, as yet, not been as successful.

Some of the early work in borehole slurry mining started in the 1920's. Periodically new attempts have been made to develop this technology as specific requirements occured. Unfortunately, no major commercial projects have been developed as a result of this work.

The deep middle miocene phosphate resources generally occur from North Carolina to south Florida. Active test efforts were made to extract these phosphates on shore in North Carolina and Northeast Florida through a borehole during the later part of the 1960's. The cyclic nature of the phosphate industry, unfortunately, always seemed to start a down trend at times most inappropriate for ongoing research programs.

During the 1970's the United states Bureau of Mines (USBM) took an active role in the development of this technology. Test work was conducted on coal, oil sands, uranium bearing sandstone, etc, with varying degrees of success.
Operating Experience With Gas Compression Facilities on North Sea Platforms
Authors: Nilsen N.J.
Published: October 2008

This paper describes the gas compression facilities on Ekofisk Area platforms in the North Sea. It discusses the operator?s experience with several compressors, the problems encountered and the solutions to correct them.

It also describes the improvements that are being made to reduce unscheduled maintenance and improve the reliability of the equipment.

INTRODUCTION

Phillips Petroleum Company Norway (PPCON) has developed and operates seven oil and gas fields in the Norwegian Sector of the North Sea. This operation consists of 25 platforms, with export thru a gas pipeline to Germany and an oil pipeline to England. Oil and gas are partly processed on the outlying platforms, and then transported via subsea pipelines and/or bridges to the Ekofisk Center. This is shown in figure 1.

PPCON has an important role in the transportation of gas from Norwegian fields. The transportation system is used to deliver gas from Elcofisk Area fields and other fields in the Norwegian Sector.

Compressors have a critical role in this operation. They are used in the transportation of gas from the outlying platforms to the center and from the center to Germany. They compress gas and vapor, which has been collected from various parts of the process, before it enters the final stages of processing. Gas above sales quantities is compressed and reinjected into the reservoir.

Operation and maintenance problems developed on some of these compressors after they were installed. These problems had to be solved to operate satisfactory and to keep the maintenance to a reasonable level.

Standby compressor capacity was installed on all critical functions. Increased gas production and gas transport for other fields will require that all installed compressors operate simultaneously in certain functions. Increased reliability and reduced unscheduled maintenance are required to avoid costly installation of additional machines.

Some "key" compressors at Ekofisk Center are discussed below.

PROCESS

The process is the separation of oil and gas. The liquid goes to England and the gas is processed and piped to Germany.

At Ekofisk Center the oil and gas are routed to different separators depending on the pressure. The separators and the pressures at which they operate are:

(Available in full paper)

A simplified diagram of this process is shown in figure 2.

Machines recompressing the different gas streams are the Flash Gas Compressors, Booster Compressors, and the Pipeline Compressors.
Steam-Powered Offshore Oil Producing Platforms
Authors: Tronstad I.M.
Published: October 2008

1. ABSTRACT

This paper will briefly describe the parallel development of an oil production platform topsides with a conventional gas turbine driver system. This will include waste heat recovery and a topside with steam turbine drivers for large rotating equipment and steam heating system.

The base process configuration is outlined and key elements in the steam system design philosophy are described by diagrams, along with brief system explanations.

Principles regarding the layout of major steam system equipment are also described.

Comparative conclusions are drawn regarding weight and space requirements, as well as operation, maintenance and safety aspects.

Generally, the conclusions are promising and indicate potential for topside simplification by utilizing steam for heating and powering purposes.

2. INTRODUCTION

The conventional method of producing electric power on a large oil production platform is by gas turbine driven generators. Additionally diesels may be used for auxiliary purposes. Heating for processing or other purposes is accomplished by a heating medium or directly by fired heaters.

A heating medium heat source may be gas turbine exhaust.

Thermal efficiency has seldom been a major consideration since the local price of gaseous fuel is low.

Looking at the landbased pendant, you will often see a different picture, with back-pressure steam turbine driven power generators and the exhaust steam utilized directly as the heating medium.

There are many arguments for rechecking the validity of old conclusions, such as fluctuating oil prices, changing on-board value of associated gas, possibility for reduction of operation and maintenance cost, or better tools for production availability analyses, etc.

This paper describes the key elements in the development of a topside concept based on both a conventional system, as outlined above, and a topside based on steam turbine drivers for large rotating equipment and the use of steam as the heating medium. None of these concepts are optimized, and since the steam option is new, it will most likely have a greater potential for further savings than the conventional topside.

The development is done for the Heidrun field outside mid-Norway where Conoco Norway Inc. is operator.

3. SYSTEM DEFINITIONS

1. Heidrun field.

The Heidrun field is planned to be developed with a single concrete TLP platform with a concrete subsea crude storage. The platform will produce approximately 1450 m3/hr of stabilized crude, exported via offshore loading system. Gas will initially be reinjected. No decision has been made regarding later gas disposal. Gas-oil ratio will initially be approximately 100 Sm3/m3. Water depth is approximately 350 m.

2. Steam system basics.

The main elements in a steam system will be a high pressure steam boiler, steam turbines as drivers for rotating equipment, and steam operated heaters for various purposes. A simplified flow diagram is shown in fig. 1. In this case, the steam turbine is working against a low pressure (vacuum) condenser, cooled by seawater.
On-Bottom Stability of Jackups
Authors: Stewart W.P. , White R.M. , Rapoport V. , Devoy S.D.
Published: October 2008

A new approach to assessing the on-bottom stability of jack-up rigs during preloading is presented. A leaning Instability IS described, which may appear similar to a punch through, but is associated with soil stiffness and rig flexure. Suggestions for avoiding this instability are given.

INTRODUCTION

Jack-up foundation failures during preloading are generally associated with a so-called "punch through". When a strong soil layer overlies a weak layer punch through may occur. The typical example is a sand layer, or sand lens, above a weak clay. If spud can load exceeds the bearing capacity of the sand layer, the spud can will penetrate into the weaker layer. This may happen suddenly and penetration into the weaker layer may be deep. Buoyancy support on the hull may eventually prevent further spud can penetration.

In soft uniform soils, where bearing capacity increases continuously With depth, punch through failures are not expected. However, there have been foundation failures in such conditions, which appear to be punch through, but cannot be explained geotechnical by the normal punch through model. Ingram & Dutt (Reference 1) describe such an event in the Gulf of Mexico in 1982.

This paper describes a new method for calculating elevated stability of jack-ups during preload where spud can loads are steadily increased. It shows how unstable leaning, leading to rapid leg penetration, can occur in soft soils where punch through cannot be the failure mechanism. For the purposes of this paper, the term rapid leg penetration is used to describe leg penetration rates which are faster than the rig's jacking rate. This typically means 1.5 feet per minute. As will become evident in the paper, the speed at which events take place is a critical factor.

STABILITY REQUIREMENTS

Stability requirements (Operations Elevated) for Mobile Offshore. Drilling units are normally addressed by classification societies along with governmental regulatory bodies.

These requirements vary from being quite extensive as pertains to Environmental Forces, Soil Particulars, Overturning moments, Air Gaps and various Safety Factors, to being Just a general statement in regards to these variables.

The U.S. Coast Guard (USCG) and the American Bureau of Shipping (ABS) make a general statement in regards to operations. Elevated/Stability On Bottom. The ABS, in their publication Rules for Building and Classing Mobile Offshore Drilling Units, Section 3' Part 3.13 state :

Units Resting on the Sea Bed

"Units which are to rest on the sea bed are to have sufficient positive downward gravity loadings on the support footings or mat to withstand the overturning moment of the combined environmental forces from any direction. Realistic variable loads are to be considered."

Underwriter-approved Marine Surveyors can provide Location Approvals for Mobile Offshore Drilling Units Operations Elevated. '

It is standard in the industry that these location approvals be based on compliance with the approved Rig Operations Manual.
Advanced Composite Tubular Tendons for Deepwater TLP's
Authors: Odru P. , Sparks C.C. , Metivaud G. , Bono H.
Published: October 2008

This paper presents an original concept of composite tubular tether for TLPs, made from carbon fibers and resin. The technica1 advantages of the concept are compared with those of alternative tether solutions by considering their application to an example TLP in 1200 m of water. Economic aspects are presented.

INTRODUCTION

Tension leg platforms (TLPs) are promising concepts that are considered for future deepwater developments by many oil companies. The principal feature of such platforms is that they are anchored vertically by axially stiff tethers, which are maintained in tension by the excess buoyancy of the platform itse1f. Heave, pitch and roll movements are virtually suppressed and the platform behaves like an inverse pendulum. One of the major advantages of a TLP (over a straight semisubmersible) is that it allows the wells to be continued up to deck level, by means of individual production risers, with the wellheads insta11ed on the platform itself.

As many authors have pointed out, TLPs are weight sensitive structures (ref. 6). Successful applications in deep water will depend greatlyon the abi1ity to contro1 and limit weight. Savings can plainly be made in the structure, deck and production equipment, but it is particularly important for deepwater TLPs that they shou1d not be penalized by the weight of their tether (and riser) system.

TETHER DESIGN CONSIDERATIONS

It is normal practice to design a TLP so that, even in extreme conditions, the tethers remain subjected to positive (effective tension at all times. This implies that the minimum tether tension at keel level must always exceed the apparent weight of the tether itself.

Heavy TLP tethers would, of course, require increased buoyancy to provide increased pretension. This would imply a larger platform, which would tend to be subjected to increased environmental loads, 1eading to 1arger fluctuating forces in the tethers, requiring increased pretension. The prime motivation for reducing tether weight is to avoid this vicious circle.

Heavy tethers are subjected to sag, when the platform is offset under the influence of horizontal loads. As a result, platform offset and set down are increased. Vertical re1ative movements, between the platform and we11 heads, are also increased. Conversely, buoyant tethers (with positive upward sag) would give rise to reduced offset, set down and relative movement between platform and we 11 heads. There" is of course no difference between the pretension requirements of neutrally buoyant tethers and positively buoyant ones, since tethers cannot be allowed to go into compression at the upper end (anymore than at the lower end).

In the case of steel tethers, the apparent weight can be reduced by making the tethers from large diameter tubes, left air filled to give them buoyancy. This solution has been adopted for most TLP projects since HUTTON, which had virtually solid steel tethers (in 150 m of water, the tether weight prob1em does not arise).
The Behaviour of Flexible Risers in Waves
Authors: van Elsacker A.W.
Published: October 2008

In this paper results of flexible riser model tests will be discussed and compared with results of time domain simulations for regular and irregular wave conditions with current.

The model tests were performed at the Maritime Research Institute Netherlands (MARIN) with a large scale riser model instrumented to determine bending moments at a number of point along the riser. Motions of the riser were measured by means of underwater optical tracking systems. The riser top was attached to a mechanical oscillator programmed to carry out the motions of the attachment point of a tanker in irregular waves. Special attention was given to the correct phasing between riser motions and irregular waves. The calculations were performed with the MARIN riser analysis program DYNAFA.

Results of tests and calculations are compared for the riser top area. Correlation is obtained for both regular and irregular wave conditions. It is concluded that computations for irregular wave conditions provide valuable information in early design stage and can be used to extend the model test results.

INTRODUCTION

During the last years it has been recognized that permanent floating production units can be a viable solution for marginal field development in hostile environments like the North Sea. Due to the crucial role of flexible risers as link between oil well and floater, stringent demands are put on the safety and reliability of flexible riser schemes designed for these extreme conditions.

Current riser design procedures are mostly based on theoretical prediction methods for maximum internal load levels and bending radii, using numerical methods to simulate the riser behavior. Design procedures generally include static geometry analyses in near and far position to determine the required pipe length. In addition to this, deterministic regular design wave analyses are performed for several wave height/period combinations and corresponding harmonic top displacements [1].

In recent years a variety of numerical tools has been developed generally based on finite element or finite difference solution schemes offering different options for analysis, see refs. [2] to [6]. An indication of the computing efficiency of several of these methods is presented in [7]. Although most of the programs have been verified against simple analytical and/or model test cases, very little data is available on correlation between computational results and realistic test data for extreme design conditions.

To assess the overall behavior of specific flexible riser systems and to provide measured data to enhance theoretical prediction methods BP International, London, commissioned the Maritime Research Institute Netherlands (MARIN), Wageningen, to carry out representative scale model tests on several flexible riser configurations exposed to extreme environmental conditions and large top displacements.

MODEL TESTS

As part of a BP design study on permanently moored floating production system MARIN was requested to perform model tests for a tanker-based system, see Figure 1.
Foundation Problems in Coral Soils: A Case History-the Oil Terminal of Matanzas, Cuba
Authors: Puech A. , Bustamante M. , Auperin L.
Published: October 2008

This paper presents the case history of the MATANZAS oil terminal constructed on metallic piles driven into coral formations. Particular emphasis is laid on the difficulties encountered to characterize the coral formations, the results of the static and dynamic pile test program implemented and the installation procedures developed.

INTRODUCTION

An oil terminal for 250 000 Dwr supertankers has been built in the bay of MATANZAS (Cuba). The structures were founded on driven 32" and 48" D.D. steel pipe piles. The soil is composed of materials of coralline origin exhibiting a strong spatial heterogeneity and very low resistances to driving.

This paper is devoted to the foundation problems encountered during the construction works. Substantial design modifications were required. They were based on a comprehensive program of static and dynamic pile tests. The aim of the paper is to identify the difficulties associated with the specific soil conditions prevailing on the site, to present the results obtained from the pile tests and to describe the remedial procedures implemented.

The case of the MATANZAS terminal may be considered as a reference-project for the construction of structures founded on metallic piles driven into coral formations. Guidelines and recommendations are proposed; they may be useful for further similar applications.

THE MATANZAS OIL TERMINAL

The oil terminal is located in the bay of MATANZAS. The marine facilities. installed on a shallow submarine terrace, include (fig. 1):two unloading platforms PTl and PC2 installed near the -20m isobaths on top of a natural steep slope. These platforms are supported by twelve 48" O.D. steel pipe piles with 19mm-thick walls;breasting and mooring dolphins, respectively 70" and 86" in diameter:a 600m-long trestle for pipeline support and truck access connecting the platforms to the shore. The bridge is composed of sixteen 36m-long spans and supported by forty-seven 32" O.D. steel pipe piles with 16mm-thick walls. The average water-depth under the bridge structure is about 5m.

Typical sections Of the terminal facilities are shown on figures 2 and 3.

PRELIMINARY SOIL INVESTIGATIONS

At the terminal location, the first submarine terrace is sub-horizontal, 200m-wide with a depth increasing from 2 to 5m.
Monotonic Testing of a Model Pile Driven in a Calcareous Sandy Silt
Authors: Tan C.P. , Parkin A.K. , Yee Y.W.
Published: October 2008

A monotonic pull test was performed on a driven model pile in a large calibration chamber fried with incremented calcareous sandy silt, prepared as an analogue of material obtained in the vicinity of a new platform in Australia's Bass Strait. A cone electrometer test was performed prior to the installation of the pile. Strain gauges mounted along the pile enabled the evaluation of pile axial loads developed during the pile driving and pull test.

INTRODUCTION

Calcareous soils are found in many parts of the world where offshore platforms are built, for example Bass Strait and the North West Shelf of Australia, Arabian Gulf, Gulf of Mexico, Mediterranean Sea and the Brazilian Campos Basin. The carbonate formations occur in varying states of cementation, with classification ranging from well-graded sand to silty sand and sandy silt. There is, however, a lack of detailed information on pile behavior during driving and pull testing in the finer materials, and also on the geotechnical characteristics of such materials.

As part of an investigation of the behavior of axially loaded driven piles and grouted piles in calcareous soils, a monotonic pull test was performed on an instrumented pile driven into a large calibration chamber filled with calcareous sandy silt. ?Ile soil, designated Soil B, is an analogue of material found in the vicinity of a new platform in Australia's Bass Strait, and was produced by grinding a medium calcareous sand, designated Soil A, dredged from another site in Bass Strait [1].

Descriptions of the cone electrometer, pile driving equipment, model pile, chamber and other associated equipment are given in [1]. This paper describes the characteristics of the test soils, the procedures that were developed to prepare the test sample, and the results of the cone penetration and model pile test that were performed in this soil

PROTOTYPE AND ANALOGUE SOIL CHARACTERISTICS

The in situ soil to be modeled in this investigation has a particle size distribution of a sandy silt. It is cemented, with typical in situ void ratios of 0.95 ± 0.05.

Due to the unavailability of prototype soil to carry out model pile testing, an analogue material (Soil B) was produced by passing Soil A through a variable gap disc mill. Platey shells and elongated and coral fragments were broken down during the milling process. Grain size distribution curves for samples of the product were obtained during the process, and the gap between the discs was adjusted to compensate for disc wear when required.

The particle size distribution curve of Soil B is shown in Fig. 1 and the specific gravity was found to be 2.73. It should be noted that the material is extremely fine and to the authors' knowledge, no other model pile tests have been conducted in such fine cohesion less material.