bluuebunny/crossref_metadata_2025 huggingface dataset:
Concrete TLP's in Water Depths Beyond 3,000 ft
Authors: Hannus H.G. , Engebretsen K.B. , Børresen R.H.
Published: October 2008

Concrete hull TLP's are established as a viable concept for field development in water depths up to 3,000 ft. The first platform is now under construction for the norwegian Heidrun field. However, as the water depth increases, the platform natural periods tend to increase, which also will increase the tether fatigue. Is there a limit for the concrete hull?

This paper describes a large size platform for Gulf of Mexico conditions at different water depths. A comparison is made with a steel hull alternative, under identical functional requirements. In particular, the tether extreme and fatigue responses are analyzed.

The results demonstrate that concrete hull platforms are competitive also beyond 3,000 ft. High natural periods can be acceptable for the tether system provided that the platform geometry gives low excitation at roll/pitch resonance as well as low responses at higher periods.

Introduction

In the last few years, concrete has been introduced as a construction material also for floating production platforms. The concrete hull TLP has been found feasible and economically attractive in water depths up to 3,000 ft, as demonstrated by Wilson /1/ and Lokken /2/. The tether system, that moors the platform, is typically designed for extreme storm condition, which also provides adequate fatigue life (assuming appropriate component design).

As the water depth increases, the fatigue life becomes more critical, as the tether stiffness decreases and the natural periods increase. In order to reduce the response at resonance, the natural periods can be reduced by increasing the tether stiffness. This means that the tether system is no longer designed for stress, but for stiffness (steel cross sectional area) " For Gulf of Mexico conditions, a practice is about to emerge, which claims that the natural periods in heave, roll and pitch should not exceed four (4) seconds.

This paper sets about to investigate the relevance of such a limit, which may be questioned on the basis that the tether responses are a function of the platform geometry. For comparison, a concrete and a steel hull are analyzed with identical tether system.

Design Basis

The environmental conditions for this study are based on published information, the extreme conditions presented in Table 1 are taken from Brewer /3/, while the fatigue conditions, the wave scatter diagram, is taken from Kinra and Marshall /4/. Three waterdepths are considered; 3,000 ft, 4,500 ft and 6,000 ft.

The platform requirements represent a large size Gulf of Mexico field. The topside mass, including payload and deck structure, is 30,000 tons (27,216 metric tonnes). This carrying capacity is defined for 3,000 ft (914 m), as the hull geometry is kept identical for larger water depths, the deck weight has to be reduced to allow for the increased tether weight. The total riser tension is 14,000 kips (62.3 MN). The riser tension is taken as independent of water depth (assumes that buoyancy is attached in very deep waters).
Reliability Analysis of the Behavior of Dropped Objects
Authors: Colwill A.D. , Ahilan A.V.
Published: October 2008

Dropped objects, such as drill pipe, pose a potential hazard to sub-sea structures. A program has been developed to simulate the submerged trajectory of such objects, and verified against experimental data. A study was conducted to determine the design impact velocity of dropped tubulars, to aid in the column design for a floating structure. Initially the maximum attainable velocity was determined. Subsequent multiple drop simulations produced a quadratic surface of maximum impact velocity with respect to the initial drop parameters. The lack of a consistent definition for the 'design' velocity prompted the application of reliability techniques which established a relationship between impact velocity and probability of occurence.

INTRODUCTION

Small objects such as scaffolding bars may be dropped frequently with little consequence to sub-sea structures and equipment. However, occasional drops of larger objects such as drill pipe, containers and B.O.P. stacks pose potential greater risks, due to the likelihood of significant damage resulting from their impact.

Dropped tubulars, such as drill pipe/casing are particularly prone to rotational effects of hydrodynamic loading, and can turn horizontally upon entry into the water column. This can pose a significant threat to the columns and pontoons of floating offshore units where wall thickness may be reduced, as far as is feasible, to maximise topside capacity.

A study of the trajectory and maximum velocity of dropped objects aids in the design of column and pontoon configurations for floating structures. However. designing to the maximum impact velocityderived from a combination of conservative assumptions (due to a lack of better guidance) may lead to uneconomic solutions. Analyses employing multiple simulations and reliability techniques allow a more rational approach to be adopted, producing design velocities which can be associated with an acceptable probacility of occurence.

DROPPED ELEMENT TRAJECTORY ANALYSIS

Any object dropped off a platform, from the deck or during lifting operations, will undergo three distinct phases during its passage to the seabed.Falling in air.Entry into water, body partly submerged.travel through water co1umn.

The aerodynamic effects are generally small in comparison with the gravitational force on the body, therefore attention is focused on the motions of thedropped object during b) and c). A computer program has been developed for Dropped ELement Trajectory Analysis "DELTA", and utilised for the assessment of the likely maximum horizontal velocities from dropped drill pipe/casing.

The 2-D motion of the dropped object in the global system is modelled by differential equations relating lateral, vertical and translational forces. The dropped object is discretized into 20 segments, and integrations are conducted numerically. The forces and moments, computed in a local system, are updated every time step before transformation into a global system. A full description of the theory utilised in the model is presented by Luo & Davis (1992).
Testing and Certification in Europe
Authors: Gundlach H.C.W.
Published: October 2008

In 1.985 a "White Paper" was published by the European Commission concerning the creation of a single market, with free movement of goods, services, people and capital within the European Community. It was clear that differences in standards, tests and certification created barriers to trade. To 1.evel down these barriers to trade a "New Approach" was started and a CE-mark was introduced. This mark for products is a tool to cross the borders but not a sign of quality.

INTRODUCTION

In Europe technical barriers to trade have arisen for a variety of reasons, one of them being the legislative requirementsof different Member states protecting the health and safety of their people. Also a wide variety of other non-legislative reasons ranging from simple purchaser preference, through the use of local standards, to insistence on local testing and certification build up serious barriers. (Fig. 1)

Elimination of these barriers to trade is dictated by their nature, whether they are in the regulated/ mandatorysector, that is to say due to legislation, or are due to some other reason. To gain a clear insight into the industry'sview on different types of barriers to trade, a questionnaire was sent out to over 20.000 companies in all 1.2 Member states of the European Community. The questionnaire was inquiring how the companies evaluated the barriers to trade as a whole. The different barriers to trade were rankedas follows:Technical standards and regulationsAdministrative barriersFrontier formalitiesFreight transport regulationsValue-added tax referencescapital market controlGovernment procurement restrictionsImplementation of Community law

Technical regulations were thus considered by the industrialists as the most important single category of barriers to trade. Also repeated testing and certification of the same product in different Member states constitute technical barriers in the common market. In order to level down the legal barriers to trade, in 1.985 the Council of Ministers agreed a resolution on a"New Approach to Technical Harmonisation and Standards". The New Approach Directives set out "Essential Requirements" which stipulate in broad terms must be achieved in order to protect health, safety and environment, rather than how it should be achieved.A product must satisfy the Requirements of all relevant Directives before it can be placed on the market.

A Directive is legally obliging Member states to implement the regulations in national laws. The broad nature of the "Essential Requirements", together with majority voting introduced under the New Approach, makes agreement on the continent of Directives more straightforward and allows greater flexibility in product development.

In recognising the diversity of testing, inspection and certification activities, the EC has drawn up a series of standard conformity assessment techniques, known as "Modules", for use in the New Approach Directives. Theserange from manufacturer declaration, through a variety of routes involving design and type approval, to full third party certification..
Low-Frequency Motions of Tankers on Tandem Offloading Condition
Authors: Feikema G.J. , Huijsmans R.H.M. , Aalbers A.B.
Published: October 2008

In this paper a number of hydrodynamic aspects of tandem mooring operations of floating VLCCS will be discussed. The effect of shielding of the wind and current on the export tanker are established by means of model test experiments. The interaction of two floating vessels on the wave drift forces exerted on each other is treated by means of potential theory calculation methods. One of the findings of this study is that the current loads and wave drift forces on the export tanker are reduced with respect to the undisturbed case, provided that the F(P)SO heading angle is close to 180 deg. However, for larger heading angles even an increase of the longitudinal forces on the export tanker can be observed.

INTRODUCTION

Tandem Mooring

Nowadays, oil off loading from storage tankers is common practice due to the presence of several dozens of tanker-based FpSOS and FSOS moored at sea. With the in Creasing number of marginal fields in development it may be expected that more (easily transferable)tanker-based F(P)SOS will follow.

For the economy of the oil production system it is beneficial if the 'weather window? for oil offloading is large: an export or off take (OT) tanker should be able to stay connected and keep loading in relatively high sea states.

Model tests and experience have learned that a tandem mooring arrangement can be maintained in higher sea states than a side-by-side mooring, so that all present F(P)SO designs assume that offloading takes place by an export tanker which is hawser moored to the F(P)SO stern.

Operability

The limitations for the operability of tandem off loading are determined by two aspects:The low frequency drift motions of the export tanker with respect to the F(P)SO, which may be due to wave drift forces as well as galloping (mooring instability).The wave induced (high frequency)relative motion between the two tankers in way of the bow hawser, at the time that the low frequency excursion is at a maximum.

Improvement of the operational limits of tandem offloading has been focused on optimization of bow hawser characteristics and bow hawser length. Properly chosen elasticity will help reduce the peak loads due to the second aspect mentioned above, while a properly chosen bow hawser length will avoid galloping, ref. [1] and [2].

Still, low frequency 'fishtailing? motions of the export tanker form the basis for the limitations in the operability of tandem moored offloading. Modern methods to improve on this are in development: thruster assisted bow hawser mooring and dynamically positioned(DP) offloading. In thruster assisted mooring the bow (tunnel) thrusters of the export tanker are used to maintain a good alignment of the bow hawser, whilst the main propeller provides some astern thrust to avoid slack. In DP offloading the bow hawser is not necessary: an automatic thruster control system takes care that the tanker keeps position behind the F(P)SO, [3].
Operation of a Dynamic Positioning Offshore Vessel
Authors: Reinertsen V. , Halvorsen C.P. , Alvheim N.
Published: October 2008

This paper describes the criteria used in the design of a dynamically positioned vessel. There are two main criteriaClass RegulationsGuidelines including the user's particular requirements

This paper is presented in four parts:Experience in Operating DP Controlled Vessels,Comparison between DP and Anchor Mooring Controlled Operation both using a Monohull Vessel,The Basic Design Philosophy for all DP Vessels with the Class Notation "DYNPOS".Description of a Latest Generation DP Vessel with the DnV Class Notation "AUTR" and Corresponding ERN 99.99S9.

EXPERIENCE IN OPERATING DP CONTROLLED VESSELS

Ugland Companies have been involved in the design and operation of DP vessels since 1976, including specialised offehore installation vessels and shuffle tankers. Considerable experience has bean gained in the intervening years, all of which haa been put to good effect in the specification for the Ugland Maxita described in this paper.

However, the technological developments have been further advanced in the design of a duel purpose vessel currently under construction. She will incorporate the greatest DP control capability of any installation vessel and hence will offer the maximum operational flexibility in the offshore industry during the Summer months,

Advanced features will include:Open System ArchitectureMan/Machines InterfaceDecision Support

The design of the vessel has been accomplished as a result of cooperation between Ugland and the Finnish National 6oard of Navigation who are the Owners,

During the Winter months, she will be employed as an Icebreaker in Finnish waters; In this capacity she will also provide the most efficient service available,

The use of dynamically positioned vessels, and their application, has increased tremendously during the last 10 years, The flexibility of this type of vessel is substantial, as is the benefit to the user. The standard and complexity of the systems available has improved, and redundancy has become a key issue. Systems today have a high level of safety, and from a technical point of view, reliable operation are to be expected.

In the past, more than 50% of all DP "accident" have been due to human error on the part of the operator of the DP system. However, under current Rules In the North Sea, certification of the DP operator is a mandatory requirement, and this is a step in the right direction. On board the vessel, the system require highly skilled operators and service personnel,

Consequential analysis performed on-line by the computer, and based upon worst case failure, Is now a standard requirement during operation, This provides a good operational guideline for the operator

COMPARISON BETWEEN DP AND ANCHOR MOORING CONTROLLED OPERATION BOTH USING A MONOHULL VESSEL

The main objective for all parties involved in offshore work is to make the operations as safe as possible. The safe working limits for each activity are based upon this fundamental principle.
Drag Embedment Anchor Performance Prediction in Soft Soils
Authors: Stewart W.P.
Published: October 2008

A rational approach to calculating the drag embedment performance of marine anchors in cohesive soils is presented. The method considers the geotechnical and gravity/buoyancy forces acting on all components of the anchor system including various areas on the anchor and the attached mooring line which may be either chain or wire. The solution proceeds in a piece-wise linear manner until an equilibrium position is found for the anchor. This equilibrium position is described in terms of the anchor burial depth, the anchor drag distance, and an anchor angle of rotation in the soil. The equilibrium position may be such that the anchor holds the applied line load and no further movement occurs, or it may be such that the anchor can dig no deeper and horizontal movement is predicted with a given maximum resistance.

The solution technique requires a computer program. Such program has been developed and the results from the program have been compared with large scale tests performed in the Gulf of Mexico in 1990.

HISTORICAL ANCHOR DESIGN

This paper is concerned primarily with relatively large marine drag embedment type anchors which areused in the offshore industry. However, it is worth considering the evolution of anchors in general. The first patent that was filed for describing an anchor was that by Hawkins in 1821. This patent described an anchor consisting of two flukes which were articulated about a pin perpendicular to the shank of the anchor. This concept has remained in use today and falls now into two main categories of stockless anchors and stock anchors.

Stockless anchors are widely used today on warships and on merchant ships. They are not typically used inthe offshore industry.

Articulated stock anchors are more widely used incommercial applications today than stockless anchors. The Danforth anchor is a typical example developed in the late 1930s. These anchors are used on barges and semi-submersible drilling rigs. Names in common use include Danforth, LWT, and Offdrill.

The requirements of the offshore industry to provide ever larger capacities of anchors led to a scaling up ofexisting models. Stockless and stock anchors weighing up to 30 tons in air have been built. Special anchor handling vessels have been developed specifically for the placement and retrieval of these anchors. In the last decade, the increasing use ofpermanent mooring systems for floating production has resulted in the design of anchors specialized to certain types of soil conditions. In the soft cohesive soils typical of many deep water areas in the world, increasing use of anchors fabricated from steel plates has occurred. These fabricated anchors replace cast steel anchors. They are more efficient in terms of their holding capacity compared to their weight. However, they are very specific to individual applications and cannot be regarded as general purpose anchors.
The Disconnectable Mooring System Of The FPSO Vessel At Huizhou, South China Sea, Lives Up To Its Promises
Authors: Dodelin Daniel Forest
Published: October 2008

The system chosen to develop the HZ-21 and HZ26 oilfield offshore the People's Republic of China was a tanker based FPSO incorporating an innovative disconnectable mooring design to allow the tanker to leave the area in the event of an approaching typhoon. This paper gives an outline of the design and construction, and describes the installation and first 18 months operational experience.

INTRODUCTION

The HZ-21 and HZ-26 fields, jointly developed by the ACT OPERATORS GROUP (ACT-OG) comprising Agip, Chevron, Texaco and the Chinese NanhaiEast Oil Corporation, lies in the South China Sea approximately 100 miles S.E. of Hong Kong in 116 meters of water.

The area is notorious for typhoon conditions where a combination of wind, wave and storm driven currents result in far more severe weather conditions than encountered, for instance, in the North Sea.

However, the seasonality, low frequency andpredictability of typhoons made the choice of a disconnectable rather than permanent mooring system more attractive. In addition to the relative costs, down time due to disconnection would be minimal since the production facility in the field required evacuation and hence production would be halted in any case.

The system involved locating an internal turret inthe forepeak structure of the FPSO supported by a three race roller bearing allowing the tanker to weathervane around the mooring (refer to Figure 1). Connection of the turret to the submerged mooring buoy carrying the eight catenary anchor legs is effected by means of a mechanical connector once the mooring buoy has beenwinched up to the tanker from its neutral buoyancy position 35 m below mean sea level (refer to Figure 2).

The design concept is described in greater detailin OTC 6251, Innovative Disconnectable Mooring System for Floating Production System of HZ-21-1 Oil Field at Huizhou, South China Sea, while SPE 22377, First 12 Months' Experience of Disconnectable Mooring System for HZ-21/HZ-26 FPSO, gives a review of the first year of operation of the system.

MOORING SYSTEM

The Buoyant Turret Mooring (BTM) was designed to allow reconnection in 3.5 m significant sea states. As with any disconnectable mooring system there are three major elements:the mooring buoy which carries the risers and the anchor legs,the turret around which the tanker weathervanes,the swivel which provides the connectionbetween the earthbound risers and the piping on the rotating tanker.

Mooring Buoy

The mooring buoy carries the eight catenary anchor legs, comprising of wire rope and chainterminated with a high holding power anchor as well as the risers from the production platform. The buoy is of novel design since the buoyancy is controlled to allow the buoy to sink, when disconnected, to a depth of 35 m where the net buoyancy is equal to the suspended weight of the chains and risers.
Sliding Mode Acoustic Servoing for an Autonomous Underwater Vehicle
Authors: Marco D.B. , Healey A.J.
Published: October 2008

Currently in use at the Naval Postgraduate School is a fully functional experimental autonomous underwater vehicle (AUV). The vehicle serves as a test bed for research in autonomous control, autonomous obstacle avoidance, automatic fault detection, guidance and control at slow speed. It measures over seven feet (two meters) in length and weighs about 430 pounds (215 kg). Maneuvering control is provided by four rudders and dive planes, has twin propulsion motors, and onboard sensors for speed, depth, angular rates and positions and four ultrasonic sensors for range information. All shipboard action is controlled by a 68030 microprocessor running under an OS-9 operating system and a G-96 bus with code written in 'C' language. All systems are powered by lead acid gell batteries for a test mission duration of about two hours.

This paper presents some experimental modeling results upon which computer simulations of the dynamic positioning performance are based. Details are given of the vehicle modeling, the influence of thruster dynamiclags, Sliding Mode control design including integral control for the compensation of ocean current effects, and a Kalman filter design for the estimation of target range and velocity from noisy sonar data. The filter can remove transient fault anomalies which are common insonar data.

INTRODUCTION

This paper provides an analysis of the effects of ocean current, thruster dynamic lags and acoustic sensor noise on the motion control of an underwater vehicle in the longitudinal direction. The context and importance of this work lies in the increased use of ROV's for deep ocean intervention and inspection tasks. With the ever increasing emphasis on reduction of costs, it is the contention of the authors that the use of fully autonomous vehicles will be needed for routine underwater jobs because they can be deployed from platforms and will not need the support of expensive surface assets. One of the missions foreseen for vehicles of this type is to provide inspection of underwater facilities. Motion control using onboard sonar to 'servo'to a target will be a typical operation.

Current research at NPS and elsewhere is aimed at designing robust controllers for underwater vehicles, the slow speed behavior of which are highly uncertain. The robust control of ROV's has been addressed by Yoerger and Slotine (1985), and the design and operation of the Jason supervisory control system hasbeen described by Yoerger et. al. (1986), in which a surface operator drives the vehicle using a manual teleoperated link from a surface ship. More recently, Fossen (1991) has studied the same subject. At NPS we are exploring the behavior of fully autonomous underwater vehicles which are programmed prior to launch and for which there is no tether. After recovery, the data stored on board is uploaded for data visualization and postprocessing. All mission functions are controlled by the onboard autonomous controller..
Testing Program Probes Capabilities and Potentialities of TFL Systems
Authors: Chioetto Stefano , De Ghetto Giambattista
Published: October 2008

In autumn 1991 AGIP concluded a test campaign on 3 in. Through Flow-Line (TFL) equipment, at a purpose-built test-site at Cortemaggiore, Italy. Testing included over 220 runs in a live onshore well, using a large number of different tools and functions.

TFL Side-Pocket Gas Lift and Jet Pump installation and operation techniques were also tested.

This article is a concise summary of the results and conclusions of the project.

Potential prospects for the future of TFL systems are evaluated in the light of present trends in offshore productiondevelopments and existing alternatives to TFL.

INTRODUCTION

Long-term .trials on 3 in. TFL equipment and procedures were run by AGIP for 3 years, at a test facility built for the purpose in Cortemaggiore, Italy (Figs. 1, 2). The site was set up next to an old, depleted oil well that had watered out and was recompleted to accommodate TFL for testing purposes (Fig. 3). Test operations were organized in two phases:the preliminary phase (March 1988, April 1990) of tests on the surface, where the various tools and operations were tested in a short flow-line loop (280 m.) and in a horizontal mock-up TFL well completion;the main phase (July 1990, June 1991), where the tools and operations weretested in the well.

OTC paper 6433 offers a thorough description of the test facilities, andsynthesizes the results from the preliminary phase of surface tests 1. The present article is the sequel to that first paper, and describes the second phase of the testing, performed in Well Corte 57.

Well and reservoir characteristics

A Residual Oil Saturation test performedin the Spring of 1990, just before the work-over to re-complete Well Corte 57 for TFL testing, yielded the following data:Water Cut - 100 % approx.;Salt concentration of formationwater - 210 gr/l;Static BHP - 2767 psi;Static THP - 41 psi;Static BHT - 52°C;P.I. - 1.4 (m3 /d)/(kg/cm2);Sand volume fraction - 1% approx.

The well completion specifications are as follows (Fig. 4):vertical well;Perforated interval = 1740 - 1745 m.;Inner casing profile = 9 5/8 in. 1.0. to 1448 m.; 6 5/8 in. 1.0. liner from1047 m. to bottom.

The well completion is composed of a double TFL string, 3 1/2 in. * 3 1/2in., down to the depth of 1009 m., where an H-Member Cross-Over Head provides the lowest circulation point for TFL operations. Below this:the "primary" string follows as a 3 1/2 in. TFL string down to the Foot-Valve (1029 m. depth), and, from there on, as a non-TFL single string to the perforations; a prepack screen is installed at the bottom of the string to protect TFL tools from sand;the "secondary" string follows to the depth of 1017 m, where it ends with a No-Go type KN Landing Nipple.
An Economic Early Production and Testing System for the Deepwater Gulf of Mexico
Authors: Grecco M.G. , Tein David
Published: October 2008

This paper presents the results of a study which was conducted to show that an Early Production and Testing System is technically feasible and can be economic under certain conditions, providing the oil price, well rate, and duration of production are adequate.

An Early Production and Testing System (EPTS) is a collection of conventional offshore production equipment which is used to producecrude oil from wells which are drilled for the purpose of exploration. The system consists of: a subsea completion tree, a freestanding riser, a buoyant flowline and a moored vessel. The moored vessel provides support for the oil processing equipment and processed crude oil storage.

Produced crude is offloaded into shuttle tankers for sale and thus the generation of, early cash flow.

Besides the early production andcash flow, vital well productivity and crude characteristics are obtained. With the extended well testing, more data can be supplied for modeling the reservoir anddetermining the economics of potential field development. This information will lead to a more accurate characterization of the reservoir, and a better design of the permanent production facilities.

EPTS for various water depths, well production rates, production durations and oil prices were evaluated for the Gulf of Mexico environment. A sensitivity study on these variables was conducted and the results were analyzed to give guidance on the economic use of an EPTS.

The results of the study show that the EPTS can be designed using conventional offshore components andconstructed and installed at a reasonable cost. The concept appears to be technically feasible, even to 8,000 ft water depths.

The EPTS can be economic and provide a positive cash flow, provided the well rate and production duration are adequate for a given oil price. Profits of tens of millions of dollars are possible.

INTRODUCTION

The successful development of an offshore oil field depends on having adequate information about the reservoir production characteristics and an accurate estimate of the field development costs. From thisinformation a decision can be made on whether or not to develop the field and the type of production concept which should be used, to be the most cost effective.

An Early Production and TestingSystem (EPTS) is one method which can be used to supply information to make decisions on the reservoir'sproduction characteristics and what type of field development concept would be most cost effective. These systems are designed to remain on location during the exploration and delineation phase of a field development. The initial exploration well and subsequent delineationwells are produced for an extended period of time so that the wellproduction characteristics can be defined and the generation of an early cash flow can be established.

The purpose of this study is to identify under what conditions it would be economically beneficial toconduct an extended test or establish early production utilizing an EPTS.
Dewatering and Hydrocarbon Commissioning of 36-in. Forties Charlie to Cruden-Bay Pipeline
Authors: Tanner R.N. , Putnam J.S.
Published: October 2008

This paper summarises the activities involved in the dewatering, oil fill, and hydrocarbon commissioning the 172km (107 mile) 36" Subsea Forties Export Pipeline, (36" sealine), from Forties Charlie, (FC), to Cruden Bay, (CB). The starting point for this paper is a discussion of the factors affecting the selection of the dewatering/oil fill strategy adopted. The consequent hardware requirements, and equipment selection, are then described, followed by a brief description of the status of the pipeline prior to the commencement of it's dewatering and oil fill. Finally a summary of the actual dewatering/oil fill operation, and the subsequent transfer of FC export flow from an existing 32" sealine to the 36" sealine is given.

INTRODUCTION

The original 32 inch Forties Crude CM Export Pipeline was constructed between 1973 and 1975 to transport crude oil from the Forties Field in the United Kingdom (UK) Sector of the North Sea to the shore at Cruden Bay north of Aberdeen on the Scottish coast.

Once ashore the crude oil is transported by a 36 inch Iandline for stabilisation at Kinneil, Near Edinburgh, followed by storage and export via the Hound Point terminal, or further processing at the BP Grangemouth refinery. Fig. 1 shows the layout of the Forties Pipeline System (FPS), of which the 32 Forties Crude Oil Export pipeline (32" sealine), formed part.

Originally the FPS transported crude oil from the Forties Field only. Since 1983 however, when crude from the South rae Field entered the FPS, Crudes from a number of North Sea fields have been transported to utilise the ullage left by the decline in production from the Forties Field. The number of crudes transported by the system grew steadily after 1983 and by 1988 had reached nine.

Internal inspection of the 32 inch pipeline during 1988 revealed that significant corrosion of the pipe had taken place and while the pipeline remained fit for its original purpose, it was predicted that progressive reductions in operating pressure would be required as corrosion continued, such that the pipeline could reach a capacity limitation for existing entrants during 1991.

Various solutions were put forward, including the use of flow improvement chemicals, partial or total replacement of the 32" pipeline or replacement with a 36 inch pipeline. The chosen solution which minimised production downtime, provided for longevity of operation, and provided a business opportunity for the transportation of further crudes from other fields was to replace the 32 inch pipeline with a new 36 inch pipeline.

The Project to replace the pipeline was sanctioned in June, 1989 with pipelay commencing in April 199o, the timing being governed by the predicted down rating program me of the existing pipeline, together with the anticipated high level of North Sea construction activities during the 1990's.
Use of Sized Salt Mud in Horizontal Completions
Authors: Sargeant J.P. , Siamos Anastasios , Eriksen N.H.
Published: October 2008

Norsk Hydro has used a sized salt drilling mud in the completion of three horizontal wells and five open hole gravel packs. This paper explains why this system was chosen, describes the mud properties and reviews the different completion operations used.

INTRODUCTION

Three horizontal wells were drilled into thin oil layers between aquifers and gas caps and completed with prepacked screens for sand control. Two of the wells are on the Troll field, while the third is on, the oseberg Gamma North structure. The objective in each case was to produce at high rates with a m1nimum drawdown in order to reduce gas coning. It was critical for efficient reservoir drainage that, the entire section produced. This implied that all mud cake should be removed prior to production to prevent impairment of the screen and reservoir.

The five vertical open hole gravel packs are installed in the Troll subsea wells that supply the Oseberg field with gas for pressure maintenance of the main 011 produc1ng reservoir (TOGI) successful completion of these wells was critical for pressure maintenance and production on Oseberg.

In order to achieve the high completion efficiency required, the sized salt mud was chosen because apart from its good properties as a drilling fluid, the mud cake was easily dissolved in undersaturated brine. Hence this avoided the use of acid with its detrimental corrosive effect on the chromium steels used in the completions.

HISTORY

Prior to the horizontal well drilling program, a calcium chloride brine system with calcium carbonate for weight and fluid loss control had been used on Troll gravel packed appraisal wells. The size distribution of the carbonate particles was specified in order to minimize fluid loss in this reservoir which has several darcies permeability. The filter cake was removed using hydrochloric acid before the gravel was placed. Injection into the reservoir was obtained very quickly, and it was normal to pump the gravel with complete losses. This system was considered to be a success despite the need for post pack acid jobs.

Saturated salt systems had been used for many years for various applications including drilling horizontal wells (ref.l&2). We had used a saturated salt system on the Oseberg field as a kill pill. The pill consisted of saturated NaCl brine, Xanthan polymer, salt, lignosulphonate and magnesium oxide for fluid loss control. Laboratory testing on cores indicated, however, that this system caused formation damage. The return permeability was reduced to 87% and 54% of original in short and long term tests receptively. The lignosulphonate and magnesium oxide were replaced by a modified corn starch recently made available, which in addition to being non-damaging also enhanced the fluid theology.

To gain experience with this system, it was used to core the reservoir section on an exploration well. We discovered that instead of the expected thin, easily dissolved filter cake, we obtained a thick rubbery filter cake that we could not dissolve.
A New, Small Semi Concept for Marginal Fields
Authors: Molin Bernard
Published: October 2008

A new type of semi-submersible platform is proposed, which is claimed to be well suited for the development of offshore marginal fields, where light and economic supports are required. It consists in shrinking and perforating the pontoons which become flooded and act as passive dampers, Duoyancy is then insured by the columns only.

Introduction

The performances of such floaters are demonstrated by theoretical and numerical analyses, and comparisons with model test results. Semi-submersible platforms are floating structures characterized by relatively deeply submerged elements - pontoons - and vertical columns piercing the free surface and supporting the deck. Most often they are being used as drilling units - in which case minimization of their vertical motion is a major design objective. Here we are concerned with their utilization as production supports - in which case heave minimization becomes a less stringent requirement if flexible risers are in use.

The design of semi-submersible platforms obeys a number of rules or constraints, the most obvious ones being of hydrostatic nature: that the displaced volume should balance the weight in all circumstances, and the metacentric heights be larger than some minimum values. For voluminous and deeply submerged pontoons this means that the columns should be sufficiently far apart. The other design criteria are more subtle, and of a dynamic nature: they result from the existence of a so-called cancellation frequency, at which the vertical dynamic loads on the columns and l)on toons cancel out (notwithstanding phase cancellation effects which playa role at somewhat larger wave frequencies).

For usual designs the heave natural frequency is just slightly below the cancellation frequency, As the heave motion is 180 degrees out of phase with the free surface elevation in between these two frequencies, semis are designed in such a way that their cancellation frequency be smaller than the wave frequencies they are likely to encounter in their lifetime. For North Sea conditions this means cancellation periods around 20 seconds. The result is that these semis have voluminous pontoons, that typically account for 70 to 80 %of their total displacements (the higher the cancellation period, the larger the pontoons' displacement, as compared to the columns').

For drilling units this large available volume permits deballasting and sailing off location quickly to another drilling site. For permanently moored structures used as production units, it may be wondered whether the cost of such voluminous pontoons is not prohibitive when the topsides weights are limited. It may also be wondered whether their motion response is not too good, since reduced vertical motion means higher deck elevation and increased structural cost. Cutting off the pontoons would significantly decrease the structural cost (with the columns alone still providing enough buoyancy to support the deck and topsides!), but then the motion behavior would be frankly too bad, with the heave, roll and pitch natural frequencies in the wave frequency range.
Automatic Control of Bulk Cement Tank Levels
Authors: Wienck D.A. , Pitts A.J.
Published: October 2008

The delivery of dry bulk cement to the cement mixer can be a difficult task. The level of cement in the surge tank must be controlled as the flow into and out of the surge tank changes. An automated system has been designed that is capable of maintaining a proper level in the surge tank. The system is primarily designed for use on offshore rigs and reduces the need for constant attention by an operator.

This paper will cover:the bulk level control system requirements,the selection of components and,the operation of the system.

INTRODUCTION

In the past, steady flow separators on offshore rigs were operated by fairy available person on the rig. Typically, this was any rig crew member who was not busy during the cementing job. This meant that often the operator was untrained and, due to the turnover of personnel, rarely the same person twice. To determine the level of cement in the tank, the operator used the sight glasses which were often dust covered or tapped on the tank with a hammer to determine the level by a difference in sound.

To overcome these operating problems, an automated bulk level control system was designed to control cement flow into the surge tank. The system monitors the level in the surge tank and adjusts the inlet flow as needed.

SYSTEM REQUIREMENTS

The normal practice of handling bulk cement is for the operator to watch a dust covered sight glass and attempt to keep a uniform Level of material in the steady flow separator based on this sight reading. Some rigs are equipped with a scale to weigh the steady flow separator (surge tank). This tells the operator the weight but not the level in the tank. Possibly 10%of rigs have scales. Using the scale, a lightweight cement blend can overfill while the same weight of heavier blend may result in the correct material level.

Proper control of the material is not an easy job. Dust collects on the inside of the sight glass and makes the surge tank look full when it is empty. The light installed on the inside of the tank helps, but frequently burns out. Typically, the only reliable indication of the level in the tank is found by tapping the outside of the tank with a hammerand trying to listen to the difference in tone between full and empty at that location.

Several undesirable events can happen with inexperienced operators. The surge tank could unintentionally empty. When dry cement stops flowing to the cement mixer, this causes a drop in the density of the cement slurry.

Alternately, the surge tank could overfill. This could plug the vent piping with dry cement, making it difficult to remove the dry cement or the excess conveying air from the tank. The result is change in the density of the cement slurry.
Ocean Bottom Receiver Location: A Model-Based Solution To Locating Receivers On The Ocean Bottom By Using Geophysical Data
Authors: Edington B.L. , Starr J.G. , Cafarelli W.J.
Published: October 2008

The ocean bottom cable method (OBC) of seismic data acquisition involves placing stationary receivers on the ocean floor and using an air gun array towed by a separate boat as an energy source. The location of each receiver group after deployment is a function of water depth, prevailing water currents and the dynamics of the cable falling through the water. Ocean Bottom Receiver Location (OBRL) positions the receivers accurately by utilizing first break information, estimated water and water bottom velocities, shot locations and water depths.

The major advantages of the OBRL method are speed and accuracy of the solutions and that no additional equipment is required for locating the receivers. In addition, the near real time processing of the data allows for adjustments in the shooting and deployment patterns to assure proper subsurface coverage.

INTRODUCTION

The ocean bottom cable (OBC) method and its use for 2-D and 3-D surveys has been discussed previously (Rigs by et al., 1987). This method is currently used in water depths ranging from very shallow to 100 meters and is not impeded by obstructions such as drilling rigs, production platforms, or boat traffic. Figure 1 depicts a recording boat anchored on prospect and attached to the receivers on the water bottom via a telemetry cable. The gun boat towing a short air gun array is separate. This configuration allows for work in obstructed areas and versatility with the acquisition geometry. The minimum configuration of an OBC crew is:The source boat towing only an air gun array and thus Highly maneuverablethe cable boat, responsible for cable deployment and retrieval,the recording boat carries the dog house and all recording Instrumentationa chase boat, used as a water taxi and picket duty along The deployed cable.

Stationary receivers on the water bottom allow for versatility with the acquisition geometry. Patch, swath or radial designs are commonly used in 3-D surveys. The swath technique involves laying out approximately 24 km of bottom cable in one or two lines and recording multiple shot lines parallel to the eceiver cable as shown in figure 2. Figure 3 depicts a patch geometry in which several receiver cables (in this case 4) are laid out with multiple shot lines orthogonal to the receiver lines. To make full use of the acquisition geometry, receiver and shot positions are predetermined on a map before the survey commences. The cable boat deploys the receivers at these locations, however, currents and other factors cause the receiver positions on the water bottom to be different than that of the surface drop position. It is necessary in an OBC survey to locate the receivers on the water bottom. OBRL locates the receivers accurately by using first break analysis recorded by the receiver groups and a single air gun or air gun array as a source