bluuebunny/crossref_metadata_2025 huggingface dataset:
A Preliminary Case for the Existence of Hurricane Alleys in the Gulf of Mexico
Authors: Cooper C.K.
Published: October 2008

A review of hurricane intensity in the Gulf of Mexico suggests there are two alleys where hurricanes are roughly 20% more intense than elsewhere. This paper develops a physical basis for the alleys using previous modeling and observations, and a statistical correlation of historical hurricane intensity and ocean temperatures. While the evidence for alleys is persuasive, final acceptance will depend on results of further research.

INTRODUCTION

Historically, designers and engineers have generally assumed that hurricane paths are random. This is based largely on the work of Haring and Heidemanl and Ward et al.2 who analyzed the ODGP wind and wave hindcast of 26 historical hurricanes in the Gulf of Mexico. While they found signMcant variations between sites, analysis showed that the differences were not statistically significant. In other words, the hindcast data suggested one region of the Gulf was as likely to experience a severe hurricane as another.

The recently completed GUMSHOE (GUlf of Mexico Storm Hindcast of Oceanographic Extremes) study enhanced ODGP by including more storms and shallow-water wave physics. As in ODGP, substantial spatial differences appear in GUMSHOE with three regions standing out a mild region off central and eastern Texas, and two severe regions off Mississippi and western Texas (denoted by L for low, He for eastern high, and Hw for western high, respectively). The regions are clearly evident in Figure 1 which shows contours of the central pressure deficit (peripheral minus central pressures), a good measure of hurricane intensity. Two dark arrows superimposed cmthe map denote "alleys".

77hepressure differences between the severe and the mild regions correspond to a 3-4 m difference in maximum wave height using the parametric model of Coopcr3. Existing RP2A criteria suggest a spatially uniform wave height. If designers had confidence that the differences in regional severity were due to a real physical phenomena and were not statistical aberrations, they could improve the reliability of their designs. Haring and Heidemanl and Ward et al.2 do not provide the necessary confidence - they suggest the regional differences are statistically insignificant. However a recent analysis of the GUMSHOE statistics by Chouina.rd4 suggests there is a statistically signitlcant variation over length scales of 100 km.

A close look at the physics of hurricanes and the climatology of the Gulf waters seems to provide a physical explanation for the alleys in Figure 1. The explanation: the two alleys reflect the influence of subsurface warm water on hurricane intensity. Figure 2 gives some support for this hypothesis. It shows the mean water temperature at 60 m depth in the Gulf of Mexico during hurricane season. A comparison of Figures 1 and 2 shows the two alleys correspond closely to the two warm water regions. The two regions in Figure 2 originate from the Loop Current and its eddies which bring warm Caribbean water into the Gulf as summarized by Elliot5.
Spatially Weighted Trim Stacking: A Technique for Prestack Noise Suppression
Authors: Holcombe H.T. , Wojslaw R.S.
Published: October 2008

Spatially Weighted Trim Stacking (SWTS) is a nonlinear seismic noise suppression process that works well on raw pre-stack data. It is a robust, efficient, and algorithmically simple operation that is best performed on CMP gathers prior to any other pre-stack data processing. Generally it improves signal-to-noise and increases trace-to-trace coherence, thereby enhancing the effectiveness of downstream pre-stack processes, such as DMO and multiple suppression. Also, it virtually eliminates transient noise and interfering crew noise in marine data.

INTRODUCTION

The fundamental premise of the Spatially Weighted Trim Stacking (SWTS) process is to make use of prior knowledge of some inherent characteristics of seismic data to identify and reject noise. The key assumptions are (1) that seismic data are coherent along some predominant local dip, and (2) that the amplitude varies slowly along that dip.

The process is a simple, sample-by-sample operation performed in the x-t domain as a roll-along process. First the predominant local dip is determined, over a specified number of traces, immediately adjacent to the trace containing the sample in question. To remove outliers, an alpha-trim is then performed on the samples along the predominant dip. The data sample is then replaced with a normalized, spatially weighted stack of the remaining sample values.

The "alpha-trimmed mean" or "trimmed mean stack", originally proposed by Watt and Bednar, 1983, has been shown to be effective in suppressing some types of transient and coherent noise (see Haldorsen and Farmer, 1989). Further, automatic dip determination and weighted stacking are common in many seismic data processing algorithms. However, to our knowledge, the simultaneous combination of dip determination, alphatrim, and weighted stacking (as implemented in the SWTS process) has not previously been tried. In our testing efforts SWTS has usually been more effective, in dealing with a broad range of noise problems, than the various spectral techniques in common use (see Luh, 1991, and Galbraith, 1991). Computationally, it is far more cost efficient than the F-K, F-X, and tau-P techniques when operating on pre-stack data.

P DETERMINATION

The automatic dip determination implemented in SWTS is somewhat unique and operates in the following way. A roll-along buffer is set up that contains an odd number of traces (typically 9 or 11) symmetric about the trace containing the data sample in question. A dip search is conducted over a specified dip range, in both lateral directions independently, from the sample in question to the edge of the roll-along buffer. A maximum semblance criterion is used for selecting each dip. The two maximum semblance values are compared and if they are close, all the sample amplitudes along dip are saved for the subsequent trim and weighted stack. If they differ by a significant amount, only samples for the dip of higher semblance are saved. This facilitates proper handling of edges and event terminations.
Recent Model Tests With Suction Piles in Clay and Sand
Authors: Steensen-Bach J.D.
Published: October 2008

The beneficial effects of suction on the break-out resistance of short, hollow piles (I.e. suction piles) in clays as well as sands have been verified by model tests. For sands increased break-out resistance is proportional to the developing suction inside the pile. For clays the increase in break-out resistance is due to a suction induced transition from local failure along the shaft to a reversed bearing capacity failure.

INTRODUCTION

As a part of a research programme on the utilization of suction piles as foundation for small light-weight platforms of mono tower type model tests were carried out in connection with the development of a design procedure. This paper presents model test results and their preliminary interpretation.

Steel tubes closed at the top and installed in the seafloor by the differential water pressure obtained when pumping from the pile interior is termed suction piles.

The literature on suction piles deals primarily with the conditions for installing and anchoring by the differential water pressure created from continued pumping. The possibility of utilizing the differential pressure created by the piles themselves, when subjected to uplift is investigated in this paper.

TEST PROGRAMME

The total test programme analyzed includes tests on Kaolin l, Nivaa Clay and G12 sand2 . Variable test parameters comprise:Pile diameterEmbedment/diameter ratioSoil typeUndrained shear strength of claysLoad type (monotonic, sustained)Regeneration time prior to loadingRate of monotonic breakoutMagnitude of sustained breakout load

THEORETICAL CONSIDERATIONS

Breakout from clays

The short term breakout capacity of embedded objects has been suggested determined by the use of plasticity theory on general shear failure mechanisms. Model tests at embedment ratios less than 1 support this3, 4. At larger embedment ratios 3 different failure mechanisms, (i)-(iii), have been recognized 1,2 . as illustrated in Fig. 1:General shear failure in terms of a reversed bearing capacity failure is formulated [1]: (available in full paper)

The value of N which is determined at the time of maximum breakout force F, and inserting a as the ultimate value in [II, is termed N1 The value of N that fulfils the vertical equilibrium equation [2] for the clay plug at the time of maximum breakout force, (inserting a as the ultimate value), is termed N2 (available in full paper)Local tension failure occurs when the inside soil-wall friction and suction exceed the tensile strength of the soil [3]. A clay plug inside the pile is then torn apart from the seafloor leaving a hole: (available in full paper)

(iii) If no suction is allowed to develop inside the pile, the soil-wall friction cannot exceed the tensile strength of the soil and failure develops as a local shear failure along the pile shaft [4]:(available in full paper)

Notations used in [1],[2],(3) and (4) are outlined in Fig.2.
Deltas as Petroleum Provinces: Past, Present, and Future
Authors: Wescott W.A.
Published: October 2008

Deltas have proven to be important worldwide petroleum provinces for two main reasons. Firstly, they are point sources from which sediment is introduced onto continental shelves and into marginal basins, where the interaction of fluvial and basinal processes frequently result in favorable associations of reservoir, source, and seal facies. Secondly, the progradation of deltas forms a geomorphic bulge which projects seaward and allows land and shallow offshore drilling rigs to reach deep targets which could only be reached by more expensive rigs in deeper water areas adjacent to the delta. Significant hydrocarbons have been discovered at least eighteen deltaic provinces worldwide. Almost 30 billion BO and over 100 TCF of gas have been produced from these deltaic provinces, which contain estimated ultimate reserves of 50+ billion BO and 350 TCF of gas. Fields with reserves greater than 500 MMBO occur in five deltaic basins and fields with more than 100 MMBO have been found in thirteen deltas. Future exploration efforts will focus upon deepwater delta-front environments and smaller deltas in more remote areas with hostile drilling environments, such as those in the circum arctic region.

INTRODUCTION

Ever since Herodotus described the land at the mouth of the Nile River as similar in shape to the Greek letter delta around 400 B.C. deltas have become one of the most studied of all depositional environments.1'2 Deltas form where fluvial systems enter a body of water and have been geologically defined as: "The low, nearly flat, alluvial tract of land at or near the mouth of a river, commonly forming a triangular or fan-shaped plain of considerable area, crossed by many distributaries of the main river, perhaps extending beyond the general trend of the coast, and resulting from the accumulation of sediment supplied by the river in such quantities that is not removed by tide, waves, and currents. Most deltas are partly subaerial and partly below water."z Deltas are significant components of many coastlines and their deposits make up a substantial volume of the stratigraphic record.1,3,4,5,6,7 Mfosdte7rn deltas are diverse in form and vary greatly in size and geomorphic setting. Deltaic morphology and three dimensional distribution of sediments is the result of a complex interplay between fluvial discharge, sediment supp" and the wave and tidal energy of the receiving basin. They occur at almost a" latitudes and climates, from the arctic the tropics.3
Impact of Offshore Drilling in the Eastern Gulf of Mexico
Authors: Shinn E.A. , Lidz B.H.
Published: October 2008

Six exploratory oil well drill sites were examined with a two-person submersible in water depths ranging from 21 to 149 m (70 to 490 ft). Time since wells were drilled ranged from 15 months to 17 years. The area of visually detectable cuttings around recently drilled wells in depths greater than 45 m (148 ft) ranged from -2,539 m2 (27,320 ft2) to -13,352 m 2 (143,668 ft2). At the shallowest site, the area with cuttings was only -20 m2 (215 ft2).. All visually observed impacts were mainly mechanical, such as debris, and anchor or jackup-leg scars. No lasting impact was observed at the 17-year-old site.

INTRODUCTION

Public concern over offshore drilling, especially off California and south Florida, has risen steadily since the early 1970s. Although two National Academy of Sciences reports 1,2showed offshore drilling to be safer than tankerage, events such as the IXTOC- 1 blowout, the Exxon Valdez tanker spill, and more recently, the Persian Gulf wars have served to punctuate public concern. Political pressure, and public concern, exacerbated by a lack of credible environmental data and highlighted by a recent National Academy of Sciences panel review Report3 and a State of Florida Governor's Report,4 led to a drilling moratorium on the southwest Florida shelf.

The purpose of this paper is to present observations, made on the basis of direct underwater examination, of six exploratory well sites in the eastern Gulf of Mexico. The wells were drilled between 1972 and 1990. This study builds on an earlier study of five well sites off the Florida Keys, where wells were drilled between 1959 and 1961. That study also included two deep-water sites drilled in 1981 nd 1986. Both studies were funded by the Minerals Management Service (MMS) EnvironmentalStudies Program. The first study, conducted in 1988, has been published in the biological literature,5 as an MMS technical report,6 and was visually presented in a 21- minute video.7 Field work for the second study was conducted in November 1991, and a thorough analysis of the data and bottom samples that were collected is in progress.

Well sites in the 1991 study ranged in depth from 21 m (70 ft) to 149 m (489 ft), and the youngest site had been spudded only 15 months before our examination (Fig. 1, Table 1). The oldest site was drilled 17 years before our examination. Another site, Gainesville Block 707, had been the subject of an extensive pre-drilling assessment and monitoring study during drilling. 8 The results of those studies were rendered questionable when Hurricanes Danny (Aug. 16-16, 1985) and Elena (Aug. 31-Sept. 3, 1985) swept the bottom while the study was underway. Evidence of drilling impacts were for the most part destroyed, and the study was terminated. 9,10
Analysis and Design of Anti-Recoil System for Emergency Disconnect of a Deepwater Riser: Case Study
Authors: Young R.D. , Hock C.J. , Karlsen Geir , Miller J.E.
Published: October 2008

Emergency disconnect of the drilling riser is required when a dynamically positioned deepwater floater has either a driftoff or a driveoff. This cannot be anticipated; therefore, the riser tension setting cannot be adjusted to a preferred value. Hence, following emergency disconnect, highly tensioned risers have been known to accelerate into the diverter housing and lift the rotary table several feet. This paper describes an analysis method for quantifying this problem and how the method was applied to design an anti-recoil system for Sonat's Discoverer 534 Drillship. The ship was being upgraded to operate in 6000 feet of water.

The analysis method described herein was compared to results of a full scale disconnect test in 3700 ft of water. As described in a companion paper (1), the comparison of analysis to test results was very good.

INTRODUCTION

In late 1989, BP Exploration initiated plans for upgrading Sonat's Discoverer 534 (0534) Drillship to a 6000 ft operating water depth (1). Several primary systems were installed or modified. One of these was the riser tensioner system to which a riser anti-recoil system was added. The purpose of this system is to avoid accelerating the riser into the diverter and substructure following an emergency disconnect.

Fig. 1 shows a sketch of upper riser arrangement for the 0534. Except for the latch mechanism, this configuration is typical of most floating drilling rigs. Also typical is the pneumatic/hydraulic system (as shown in Fig. 2) for providing tension to the drilling riser. These tensioner systems are designed to provide near constant tension whether the riser is connected or not. Therefore, if the tension was high and the situation called for prompt disconnect, then the riser could obviously be accelerated into the diverter/substructure. This accelerated riser motion is termed "riser recoil". The release of strain energy stored in a long riser also contributes to the recoil action.

Designing an "anti-recoil system" requires some way of accurately predicting how the riser/tensioner system will respond to proposed system changes.

The riser/tensioner system is too complex for hand calculation and too large for practical experimental study. Therefore, computer simulation is the most efficient tool for designing an anti-recoil system. This was done for the Discoverer 534.

THE EMERGENCY DISCONNECT PROBLEM

Drilling in water depths exceeding 3500 ft is generally performed from a DP (dynamically positioned) floater. Sometimes, these vessels experience driftoffs or driveoffs. When such happens there is no time for reducing top tension prior to disconnecting the riser. The top tension would be high if heavy mud was in use. Furthermore, it was determined that keeping the mud in the riser is not desirable for emergency disconnect. Thisreduces the riser's hanging weight and mass, thus increasing the riser's acceleration,
Methodology for Comparison for Alternative Production Systems (MCAPS)
Authors: Stahl B. , Shoup G.J. , Geyer J.F. , Vinnem J.E. , Cornell C.A. , Bea R.G.
Published: October 2008

The MCAPS project was aimed at providing engineers and managers with a tool for comparison of alternative production concepts based on a risk assessment approach. The approach extends the usual economic comparison to include random accidental and failure costs, in addition to personnel and spill risk consequences.

The MCAPS methodology is a full scope, life cycle risk assessment procedure targeted at offshore production systems. The benefits include an improved understanding of risk mechanisms, identification and mitigation of hazards, improved cost-effectiveness of design, and improved safety.

Observations are made concerning quantitative risk analysis, present worth values, risk costs, and notional veraus actuarial risks.

Introduction

Background

Considerable skepticism concerning the usefulness of probabilistic risk assessment was evident at an internationalworkshop on application of risk analysis to offshore operations held at the National Institute of Standards and Technology (NIST) in 1984 (Ref. 1). A different experience was provided at a similar workshop held at NIST in 1991, where the main issue was not whether to use these stu. dies, but how they should be utilized most effectively. This is an important shit% in emphasis. However, conditions in 1991 were not the same as they were in 1984Several fires and explosions have occurred on platforms, Piper Alpha being the best known.Recommendations made by Lord Cullen in the Piper Alpha inquiry report (Ref. 2) caused a change in the UK regulatory regime to a risk management and riskassessment basis, largely in parallel with the Norwegian regulatory regime (Ref. 3 and Ref. 4).

The application of risk assessment in a regulatory context is extensive in the United Kingdom and Norway. Such application is discussed in Ref. 5.

The MCAPS project has to be evaluated in the context of these important differences. The project was initiatedin 1987 and carried out in 1988 and 1989, with participation by 18 organizations including oil companies, regulatory agencies and engineering contractors. Thefinal project report was presented in 1990 (Ref. 6).

Objectives

The MCAPS methodology is an engineering procedure that can assist the process of making rational comparisons among design alternatives for offshore production systems. Currently, such comparisons are often made primarily on the basis of initial cost estimates without explicit consideration of risks and related life cycle costs, which can be several times the initial costs. Use is made of recent developments in structural system reliability and full scope risk assessment. The latter includes consideration of identifiable hazards to the operation, including those involving the riser and well systems, production and utility equipment, and the environment.
Effect of Current Fluctuations on the Motion Response of Offshore Structures
Authors: Huse Erling
Published: October 2008

The present paper develops the necessary mathematical formulae for calculating the motion response of moored offshore platforms due to current fluctuations. Numerical calculations are done for a single vertical cylinder, like a spar buoy, and an 8 column semisubmersible. The low coherence in the current field gives rise to very significant reduction of the motions. Model test results generally confirm the theory.

Recently published data show considerable current fluctuations in the sea. Nevertheless, a general conclusion from this study is that the corresponding motions are in most cases negligible compared to wave-induced motions.

NTRODUCTION

Present-day design codes, rules and regulations for determination of design loads and offsets of moored offshore structures are based on the assumption of a constant current. Nevertheless, in the sea the current is not constant but fluctuating in time and space. Recently some interesting information on current spectra measured in the sea have been published, see Refs. 1 and 2.

The scope of the present paper isto develop a simple procedure for calculating the motion response of moored platforms due to the current fluctuations,to suggest necessary current spectra and coherence functions to be used in practical applications of such procedures,to verify the procedures by model tests, andto provide general information on the magnitude of motions induced by current fluctuations.

GENERAL OUTLINE OF THEORETICAL METHODS

Let us first consider platform structures that consist of one or more elements where a strip-wise application of the Morison formulation seems logical. Typical examples are slender towers, spar buoys, and ordinary multi-column semisubmersibles. There are two ways of evaluating the effect of current fluctuations on the motion response of such structures:by straight-forward simulation in the time domain, orby frequency domain analysis.

Using the first method one would generate a realistic time series for the current velocity at the various parts of the structure, taking into account the correlation versus frequency and distance in the turbulent current velocity field. Calculation of the instantaneous current force at each time step can be done in a strip-wise manner, applying the Morison formulation with the true relative velocity as input, Knowing the mooring system restoring force characteristic, the damping, and the total mass, the motion response is obtained by a straightforward integration of the equation of motion by for instance a Runge-Kutta procedure.

In order to do a frequency domain analysis one has to make certain simplifying assumptions, which may not always be valid. However, the frequency domain analysis has the advantage of presenting the results in the form of analytical expressions, which provides more direct insight into the physics of the process. In the next 0 sections we shall develop the frequency domain formulation for a spar buoy and a multi-column semisub.
Floating Production and Offshore Loading Using Dynamic Positioning
Authors: Christensson C.G. , Omberg D.O.
Published: October 2008

This paper will deal with positioning strategies for a floating production unit and an offloading shuttle tanker. The positioning of the two vessels in a tandem configuration while weather vaning around theterminal point is a complicated task. Experience gained from ten years of operation, 12shuttle tankers and 19anchored units, will shed light on some of the problems encountered along the way and their solutions.

Introduction

SIMRADALBATROSSA/S has long experience with positioning and station keeping of vessels with large ?traditional" hull shapes. Already in 1978, a DP system was installed on the "Seaway Sandpiper", a 20,000 dwt. bulkship converted to a dynamically positioned stonedumper.

In 1981, M/T Wilnora was the first tanker to be converted for dynamically positioned loading at the Articulated Loading Platform (ALP) on the Statfjord field. The many advantages of this concept soon became apparent to the operator of the shuttle tankers at Statfjord, K/S Statfjord Transport A/S, and soon all the shuttle tankers in operation on the Statfjord and Gullfaks fields were converted to DP operation. The concept has since been developed further, both with respect to positioning strategies and number of reference systems. In the British sector of the North Sea, both ESSO and Shell are using dynamically positioned tankers.

The floating production ship may be held in position usingjust a mooring spread or its position maybe maintained by a wholly redundant Dynamic Positioning system. However, the most common method is to combine the use of anchors with some form of thruster control. The Petrojarl I, a turret moored production ship owned and operated by GolarNor Offshore A/S, is already in operation in the North Sea. She is currently producing at a rate of 25,000 b/d from two subsea wells via two flexible risers, on the Amerada Hess operated Angus field in the British sector of the North Sea. A new floating production ship (a Tentech 850 design) is currently under construction for Ocean Production A/S and Smedvig A/S at the Astano shipyard in Spain. This vessel will be equipped with both a wholly redundant Dynamic Positioning systemand a Position Mooring system, for operation in a turret moored configuration.

The Position Mooring System

This section will give a brief introduction to the most essential functions of the Position Mooring system (PM) for anchored vessels. PM systems have been in use for 10 years on 18 moored units, used for oil/gas drilling and production in rough North Sea conditions. Accommodation rigs moored very close to freed installations and connected with a gangway also benefit from the use of these systems. The main objective of the system is to automatically monitor and control of the vessel?sposition and stability within the actual anchor pattern,
Spread Moored FPSO for Very Shallow Water Depth
Authors: Liles E.G.
Published: October 2008

The paper describes the design process for an asymmetric, disconnectable spread mooring system for a tanker-based FPSO offshore West Africa in 50 feet of water. The mooring system design process was driven by cost effective solutions to the challenges of this particular oil reserve. The final mooring design is installed and operating successfully today. This mooring system design was a unique solution using engineering innovations to solve the challenges of ultra shallow water.

INTRODUCTION

This mooring system was designed for a tanker-based FPSO owned and operated by our company and leased to a major oil company in West Africa. The tanker was purchased, converted, and installed in 11 months after the contract let date. The entire FPSO was designed with a critical look at the cost effectiveness of the systems since every dollar spent had to be balanced against the lease rate paid.

The oil field is located 70 miles south of Port Gentil, Gabon 3 miles offshore in fifty feet of water. The environmental conditions are shown in Table 1. There are 3 major storm conditions: thunderstorm, South Atlantic swell, and southeasterly storms. There is one separate operating storm condition and one offloading storm condition.

There are two oil wells drilled at this location that defined the reservoir. These are supported by a small 4 leg jacket lattice structure. The reservoir showed a possible 15,000 bopd production. There was no pumping support on the jacket, and the wells would flow via their own pressure until reservoir depletion or until the reservoir was considered to be economically unfavorable. The FPSO vessel had to have a minimum of 350,000 bbl total storage capacity for saleable crude oil. The lessor could not let the contract for the FPSO vessel until the mooring system proposed was deemed acceptable by their research and development group.

CHALLENGES

The design of this mooring system at this location was problematic due to the complexities shown below. These challenges were met through the use of new ideas and state of the art technologies combined with upper level management that was willing to take the risk of utilizing new technologies.

Challenge 1

The extreme shallow water depth was the major challenge and is the reason for many of the other challenges. Water depth and the clearance of the tanker hull to the seafloor, both static and dynamic, drove the size (storage capacity) of the tanker chosen. The depth of water was also the cause of much theory modification on the wave and current interaction forces and excitations. The water depth created diffraction and reflection considerations of the wave train. Model basins had difficulty in generating the proper wave train characteristics in water depths this shallow. The expected excursion envelope in this shallow water depth also created problems for the flexible flowlines.
OCTOS1000 Template Manifold Transport and Installation Using Anchored Rigs
Authors: Awad S.P. , Moretti Milton , Santagueda Francisco
Published: October 2008

This paper describes the OCTOS 1000 template manifold transport and installation method for its first location in Alhacora field, offshore Brazil. The original idea consisted in using a barge and a dynamically positioned rig But as it has imposed some operational restrictions, another method had to be developed. The selected one foresees the use of a PETROBRAS owned semi-submersible to transport the template sea fastened to the rig lower deck, allowing a dry tow of the structure towards its final location.

The method described may be used to transport and install other kinds of templates and manifolds, using a rig'of opportunity as it requires minimum modifications on the rig. Its selection mainly depends on the foundation project and on the levelling devices of the subsea equipment, OCTOS 1000 concept features that simplify the installation task, and also on the relation between the overall dimensions of the equipment to be installed and the clearance area below the rig lower deck.

INTRODUTION

The exploitation of deep water oil fields is the principal goal of several studies and projects carried on by PETROBRAS. Considering the existing alternatives for Campos Basin Albacora and Marlim fields development, subsea completion and floating production system was the chosen one, basically due to the evaluated costs and to our know how on those systems for shallow water fields. However, some components of this scenario had to he upgraded in order to overcome the challenge of going deeper.

The OCTOS 1000 template manifold project represents an evolution on the basic concepts for adeepwater template design, based on simplicity, reliability, safety and field experience. Orthogonally shaped with a central manifold and seven wells spaced in a radial way, it is linked to the production platform by the 8th arm's sales lines base. The foundations design consists on a central foundation and eight radial ones located at its eight arms extremities.

Installation planning and designing followed thesame principles of the main project but aiming to maximize the use of our own human and material resources, in order to do it "in house".

FORMER STUDIES

The OCTOS 1000 transport and installation couldbe easily performed by a heavy lift vessel with dynamic positioning, for example, being this idea putaside at first to avoid high daily rates and mobilization fees. As one of the premises of the whole project was 1,000 m (3,281 ft) water depth rating, contracting an anchored lift barge or semi-submersible to perform the joh would result in a difficult anchoring and positioning over the location and in high costs also, due to the small availability of those units. The research was directed to dynamic positioned (OP) rigs, preferably with a long term contract under way.

The structure design and installation initial studies considered the drillship Pacnorse II or the semi-submersible Sedco 709 as the rigs which could launch the OCTOS 1000 into the water.
Submudline Silo System: A Cost-Effective Alternative to Conventional Subsea Protection Structures
Authors: Figenschou Arild , Wikdal J.A.
Published: October 2008

A submudline protection silo system has been developed. The silo is a cylindrical shaped steel structure having a diameter of 9 metres and a total weight of 125 tonnes. The installation method described in the paper is presented in a step by step manner. The structure is transported from shore to the offshore site by a supply vessel. The silo structure is transferred from the supply vessel to a drilling rig. When the silo is placed on the seabed, an installation tool is run by the drilling riser and docked into the silo. The penetration of the silo into the seabed is achieved by using standard equipment onboard a drilling rig only. The excavation process is fulfilled by water jetting and the soil is removed by electrically driven mass pumps. The penetration force is achieved by creating a suction underneath the silo floor. The silo is equipped with systems to remove drill cuttings and excessive cement during drilling operation. The top of the silo has a roof system to protect the X-mas tree from falling objects. Maintenance and inspection of the silo x-mas tree is done by a guided ROV through a hatch system in silo roof.

INTRODUCTION

This paper presents a development work on submudline silo made by Kvrerner Eureka a.s on behalf of Saga Petroleum a.s. The main purpose of the work was to develop a simple and cost efficient method to install a silo system on one of Saga Petroleum's future wells in the North Sea.

DESIGN DATA AND PHILOSOPHY

Functional requirements The silo concept shall meet the following functional requirements: The silo concept shall be equipped with a standard drilling template. Drilling of a well and well completion shall be possible using standard drilling procedures and drilling equipment. The silo structure shall have sufficient internal space for a standard blow-out preventer. The silo concept must have a system to discharge the spoils during the 36 inch hole drilling operation. The silo shall have a protection roof. A discharge system to remove sediments or spoils during operation. The X-mas tree shall be retrievable without disconnecting the production pipeline.

Design loads

The silo structure, including the roof system, shall be designed for dropped objects, wire pulling (fishing gear) and impact from fishing gear. The following loads are used in this study:(available in full paper)

Design philosophy

The following presents the main philosophy for thisproject: To the maximum extent, make use of standard and well proven drilling related technology. Give priority to simplicity and existing technology. Try to avoid "point of no return" during installation. It should preferably be possible to carry out the silo installation and well completion as a continuous operation All methods, solutions and design details chosen shall be carefully weighed against the impact on the concept total cost.
Burst Strength of Corroded Pipe: "Flow Stress" Revisited
Authors: Klever F.J.
Published: October 2008

If corrosion damage occurs in a pipe, assessing the remaining burst strength is essential in operational safety management. In this paper a model is presented to predict the burst strength for defects that take the form of a large area of metal loss, where a continuum mechanics approach is more appropriate than a fracture mechanics approach.

The model results in an explicit criterion for failure which can easily be applied in an operational setting, and provides fundamental insight in the effect of the depth and the circumferential length of the defect, the material grade and the axial stress. A recipe is available which shows how a burst strength analysis can be done with a pocket calculator. Apart from data on the depth and circumferential length of the defect, it assumes that both the yield strength and the ultimate tensile strength of the pipe material are known (or estimated).

The first comparisons between the model predictions and test results are very encouraging. It thus could form the basis for the development of a reliable defect assessment procedure for pipes with patches of metal loss. There is remaining work that needs to be undertaken. It would be useful to analyse the effect of different stress" strain curves, and to incorporate additional curve fitting formulas in the model. The next step is to further verify this burst prediction tool by means of an extensive test programme, preferably in a joint industry project.

INTRODUCTION

Corrosion damage in the form of larger areas of metal loss is not uncommon in pipes, transporting oil and gas. With pipelines and risers approaching their design life in increasing numbers, adequate fitness-for-purpose assessment procedures for various circumstances should be available. Even for new pipes these procedures should be in place, in view of the trend to transport untreated hydrocarbons where corrosion monitoring is performed during operation [1]. The evaluation of the remaining strength of these pipes, with respect to both collapse and burst, is essential in operational safety management.

Using the ANSI/ASME B31G guideline [2] to assess the burst strength for this type of corrosion is questionable, since it would then be used beyond the test data on which it was based. It may lead to substantial underestimation of the strength [3], but may be unreliable for high grades of steels or under unfavourable levels of axial stress.

In this paper a model is presented to predict the burst strength of a pipe with an idealised corrosion patch that has an infinite axial length. This continuum mechanics model incorporates the necessary effects of finite strains and displacements. The analysis results in an explicit formula for the burst pressure that provides fundamental insight in the interrelationship between depth and circumferential length of the defect, the material grade, and the axial stress.
Alternate Methods for Installing ESP's
Authors: Robison C.E. , Cox D.C.
Published: October 2008

In place of using a workover rig, this paper introduces: two different ways of running and pulling electric submersible pumps (ESP's), the conditions that make the methods attractive, the completion options for such installations, and computer generated graphs to show the expected production capacities for several combinations of pumps and completion schemes.

INTRODUCTION

ESP's are typically installed on threaded pipe joints with a workover rig; and while this is the industry standard, a number of alternatives are now available. Two cable deployed methods have been demonstrated in live wells and the results are a matter of record. This paper will describe two additional deployment options; using coiled tubing and using a hydraulic workover un it, both of which use proven techniques and equipment.

BACKGROUND

Conventional workover rigs have been used to install and retrieve ESP's due to availability, cost and familiarity of the method. However, in remote or offshore areas there may be a significant length of time an ESP is shut down awaiting service due to logistical problems in mobilizing the ri g or in the cost associated with a workover. It is in these areas that an alternative means of servicing ESP wells is desirable.

Deploying ESP's on cable has been presented previously. 1,2,3,4,5 In one configuration, the strength cable is integrated with the electrical power cable so there is not a requirement to insta11 supportive devices between the power cable and strength cable. A modified coiled tubing injector is then used to run the pump and cable into the well. This results in the most straightforward installation and retrieval method of any option available.

Perhaps overlooked, coiled tubing offers a viable means of deploying ESP's. The modular design of a coiled tubing unit makes it easier to mobilize and more flexible as to location than a workover rig. Coiled tubing runs smoothly and continuously without the sudden jerks and starts of rig installations. This is easier on the ESP. In a 6000 foot installation, coiled tubing eliminates 200 threaded joints; each a possible leak point. A standard ESP wellhead can also be used.

ESP's have been installed with a hydraulic workover unit (sometimes referred to as snubbing units). Like workover rigs, threaded pipe joints are used. Although the process is a bit slow, in those cases where offshore deck space is at a premium it may offer the only means of servicing the pump short of a cantilever jack-up rig. The length of time needed to schedule and deploy such rigs can be considerable; and while waiting, the well produces no revenue. The time to mobilize a hydraulic workover unit will usually be much shorter so that the additional service time to deploy or retrieve the ESP is not significant.

DESCRIPTION OF SYSTEM

The components of the coiled tubing system are shown in Fig. 1. The system is comprised of a tubing injector, a work window, a spool of coiled tubing, a spool of electrical power cable, and a hydraulic power pack and operator's console (not shown).
Laboratory Testing of Reflected Pressure Pulses in Small-Diameter Tubing
Authors: McStravick D.M. , Rothers David , Blum Gerhard
Published: October 2008

This paper reports on analysis and laboratory testing of induced hydraulic transients in 1/4 inch tubing. The form of the reflected pulses gives information as to the end condition of the far end of the tubing. This paper gives results of testing with up to 1500 ft. of 1/4 inch tubing. Classic open and closed end conditions show the expected opposite signs in the reflected pulse. The intermediate condition of a chamber at the end of the line shows a combination open end response followed by a closed end response.

Introduction

1/4" Tubing is commonly used to actuate hydraulic safety valves and for chemical injection lines. The safety valve closure mechanism is controlled by an internal hydraulic actuator which shifts from a small volume position (closed) to a larger volume (open) condition. The valves are remote from any personnel and so there is no local confirmation of valve actuator status. Hydraulic transients which are normally a nuisance in hydraulic systems and can show up as "water hammer" in hydraulic systems and even household plumbing, can be used to supply some information about the far end of a hydraulic line. Potentially these pressure pulses can give the actuator status (open or closed).

This paper reviews the laboratory testing to investigate reflected hydraulic transients both in regard to arrival time and the pressure pulse shape.

The classic analysis of water hammer shows that if flow is stopped suddenly (a valve is closed) a positive pulse is generated. This positive pulse will travel to the end of the conduit and reflect as a positive pulse if the end of the conduit is closed (ref.l). If the end empties into a large reservoir (an open end condition) the positive pulse will reflect as a negative pulse (ref. 2 and 3). Water hammer pulses result from changes in flow velocity; pulses an be generated in a closed system (that has no flow) using a plunger type device. This method results in pulses that will travel down the tubing and undergo a reflection as in the water hammer analysis. In a liquid filled tube the reflected pulse can carry information as to the far end condition ofthe tubing. This is shown schematically in figure 1. A positive pulse reflects at a closed end as apositive pulse; a positive pulse reflects at an open end as a negative pulse (ref. 2). As will be seenlater in the paper for the intermediate case of a chamber (a small volume reservoir) at the end of the line, a positive pulse will reflect as an initial negative pulse followed by a positive pulse.

Wave Velocity

Although liquids are relatively incompressible for many analyses, the elastic nature of liquid for compression waves is a key aspect to phenomena of sound transmission, water hammer, sonar etc.
A Simulation Of The Formation Of The Mississippi Fan
Authors: Williams G.N. , Warren J.D. , Parmley K.L.
Published: October 2008

A methodology has been developed which allows the structural history of a multi-layered marine sediment deposit such as a sub-marine fan to be recreated by simulating the fan's growth, layer layer, from the beginning of its existence until the present time. The technique is based on the concept of geotechnical stratigraphy and involves the simulation of two processes: the deposition of new sediment layers, and the consolidation of existing layers, This research describes the development of a numerical modeling and simulation technique for application in the field of geotechnical e engineering

INTRODUCTION

The advent of the electronic computer during the last half of this Century has produced spectacular advances in the applied sciences. In the process, a completely new mode of research, the computational approach, has come into existence. One of the primary tools of this new approach is the technique of computer modeling and simulation which allows a physical system to be studied by computer manipulation of its mathematical model

The primary objective of the research described hem was to develop new modeling and simulation methods for the study of multi-layered marine sediment deposits such as submarine fans In particular, the methods were applied to the large Mississippi Fan located in the Gulf of Mexico. Since consolidation is generally the most important process affecting fan geometry, the modeling and simulation techniques were designed around this concept. The goal was to develop techniques which could provide insight into how the formation and evolutionary processes active within the Mississippi Fan have changed the fan's geometry during the roughly two million years of its history. The basic approach was to recreate the fan's structural history through the use of a numerical simulation of the consolidation process.

The research can be partitioned into the following basic task:Develop an appropriate simulation model which quantifies the consolidation processDevelop a simulation database containing a discretized representation of fan structure.Collect and process sufficient data for defining current fan geometry.Perform a digital simulation to generate the fan's structural historyDevelop a visualization to show how the fan structure has changed during its past history

THEORY AND GOVERNING EQUATIONS

SUBMARINE FAN FORMATION

The Mississippi Fan is a large deep-sea turbidite deposit located in the northeastern Gulf of Mexico. It is generally. fan-shapedand composed of predominantly terrigenous sediments which have accumulated at the lower end of the Mississippi submarinecanyon. The fan is marked by a seaward bowing of the bathymetric Contours1 and extends seaward from the continental shelf of Louisiana to abyssal depths (Figure 1). The west Florida carbonate platform forma the fan's eastern flank, while the Texas-Louisiana Florida continental slope borders on the north and west. The fan's deeper portions merge with the Florida plain to the southeast and the Sigsbee plain on the southwest2.