bluuebunny/crossref_metadata_2025 huggingface dataset:
Case History: Decommissioning, Reefing, and Reuse of Gulf of Mexico Platform Complex
Authors: Hakam Aly , Thornton Win
Published: October 2008

During the creation of the Galveston 189 platform complex artificial reef several firsts were achieved: a permanent shallow water, low profile reef; a permanent reef site with easy access for the general public; and the largest diamond wire cut made in the Gulf of Mexico. In addition to these firsts, the procedures required during the decommissioning of an offshore platform, the severing techniques used and the variety of disposal options available for obsolete offshore oil and gas structures are reviewed.

Introduction

One requirement in every offshore oil and gas lease, in State or Federal waters, is the removal of the structure at the end of its useful life. In the best case, the end of life for a platform is defined as 15 months after production activities on that lease cease. The removal of an offshore facility is a multi-phase process. The first activities are the planning and permitting. Generally the planning and permitting phases will take a couple of weeks; in special cases, such as offshore California, they will require years of regulatory activities. Once the permits are received, the bid packages evaluated, and contracts in place the work onsite starts. The wells are permanently plugged. The pipelines are then cleaned and capped, and sometimes even removed. The production equipment is flushed and made gas free. The structure is removed. And finally the site is trawled with a shrimp net to prove the site is clear of objects on the seafloor.

The reefing of oil and gas platforms in the Gulf of Mexico is considered a win-win solution for the operator, regulators and the environment. When a platform is installed offshore an artificial reef is automatically created. By providing a method to maintain the hard bottom strata created by the platform the existing marine life around the platform stays virtually intact. The regulatory agencies involved are the Minerals Management Service (MMS), Army Corps of Engineers (COE), United States Coast Guard (USCG) and State Wildlife Departments. These regulators have control over the location, placement and materials that are reefed. The operators have the opportunity to reduce removal costs and liability. Upon acceptance of the created reef the ownership of the reefed materials transfers to the local State agency. To assist in the maintenance of the reef sites and outreach programs the operators typically donate one-half of the cost savings to the Artificial Reef Program in the State that they are working with. For the GA 189 platform complex the reef donation amount was $300,000.

Background

The Galveston 189 platform complex standing in 60' of water approximately 12 miles off Galveston, Texas was installed in 1981. The complex consisted of a four-pile main production platform, GA 189 B, set up to produce 10 mmcf/d and 2,000 bo/d. The production platform was bridge connected to two satellite structures, GA 189 C and GA 189 D. The GA 189 C platform was a four pile well protector with a minimal deck.
Design of a 25.5-in Titanium Catenary Riser for the Åsgard B Platform
Authors: Nygård M.K. , Sele A. , Lund K.M.
Published: October 2008

The Åsgard B semi-submersible production platform is located on the Haltenbanken in the North Sea in 305 m water depth. A free hanging 25.5" (ID) titanium riser has been demonstrated to be a robust engineering solution for a large diameter riser from a semisubmersible platform in this water depth. Pre-engineering covering all main aspects was carried out during the spring of 1999. The purpose of the riser is to accommodate the launching of a multi-diameter pig from Åsgard B and to serve as a gas export riser. The riser is presently a backup solution to the base case of subsea pig launching.

Presently, there is no other application of a catenary riser in titanium. So far titanium has only been applied to one top tensioned riser, and components, such as stress joints in a limited number of risers. A limited number of free hanging steel risers have been installed in deep water. Isolation joints have been provided at both ends of the titanium riser. In addition to the riser pipe itself the use of titanium is therefore limited to the taper joint in the pontoon support and parts of the isolation joints. Taper joints in titanium as well as the components in steel are considered proven technology. This accommodates the use of proven designs for all components.

In spite of not being critical to performance some technical issues are recognised as areas where further technical development is desirable. The most important aspect is considered to be the qualification of welding and NDT procedures. The most highly stressed welds need to meet strict requirements for initial defects. The requirements are determined through fracture mechanics analysis. It is believed that there is a potential for relaxing these criteria by further screening and grouping of available fatigue data in order to reduce uncertainties inherent in the current data.

A base case with a uniform 28 mm titanium wall thickness has been developed, with alternatives allowing for a nonuniform wall thickness. The base case gives simpler purchasing logistics, while the advantages of the alternatives are to reduce the required amount of titanium, to simplify production and to reduce the interface forces applied to the platform.

A well suited fabrication site which accommodates a straight riser of approximately 700m is located at Aker Verdal in Mid-Norway. Procedures for assembly of the riser, utilizing two weld stations, have been developed.

Load out, tow out and installation methods which are insensitive to varying weather conditions have been developed. Surface towing may be applied for the tow out. To limit weather window restrictions provision has also been made for mid water tow out. The installation procedure has initially the riser suspended between two tugs. One end is transferred to the platform whereafter the rear tug moves over onto a different heading to position the riser in the anchor in a cranked configuration. Contingencies for bad weather have been developed for all phases.
Correlation for Design of Laterally Loaded Piles in Soft Clay
Authors: Matlock Hudson
Published: October 2008

A program of research on laterally loaded piles for offshore structure has included field test with an instrument pile, laboratory model testing, and development of correlation for design. The work has been sponsored by a group of five oil companies.

Three loading condition are considered to be particularly pertinent to the design of laterally loaded pile in soft normally consolidated marine clay. These are (1) short time static loading, (2) cyclic loading such as would occur during the progressive build-up of the storm, and (3) subsequent reloading with forces less than previously maximums.

Good general agreement exist between conventional static-loading ultimate-resistance concept and experimental results, provide due allowances are made for the reduced vertical restraint at shallow depth, where it is insufficient to cofine plastic flow to horizontal planes. Force-deformation characteristic based on approximate theory produce satisfactory agreement between computed and experimental behavior of the pile-soil system.

The mechanism of cyclic loading characteristic are qualitatively illustrated by typical results from laboratory model studies. Deterioration in resistance because of cyclic loading is most severe at shallow depth and with large lateral deflection of the pile. A correlation based primarily on result with the instrumented pile tested at Sabine, Texas, gives satisfactory prediction of pile deflection and moment over a wide range of loading condition.

Estimates of response for reloading after cycling at a higher load made by considering that most of the lateral soil resistance is estimated for deflection smaller than those previous attained.

The correlation are summarized and recommendation given for their use in design.

Introduction

The ability to make reasonable estimates of the behavior of laterally loaded piles is an important consideration in the design and construction of many offshore installations. This is particularly true in the Gulf of Mexico where large lateral forces are produced by winds and waves associated with hurricane and where the foundation materials in the critical zone near the mudline are often found to be very weak clays.

A program of research on laterally loaded piles sponsored by five oil companies is the primary basis for the correlation in this paper. At this time it is intend only that the results be summarized in a form directly in design, but publication of the background research is planned for the near future.

Requirement for Analysis and Design

There are many different ways in which piles or caissons may be subjected to effect of lateral forces. One such case is show in Fig 1a represent a pile and a leg of a jacket-type structure.

The structural analysis problem amounts to that of a complex beam-column on an inelastic foundation. For pile separated by spacing of several diameter or more, the Winkler assumption is useful to facilitate the analysis. This means that the soil is considered as a series of independent layers in providing resistance p to pile deflection y (Fig 1b)
The Continuous Sweep VIBROSEIS®Method
Authors: Broding R.A.
Published: October 2008

A procedure has been developed for the VIBROSEIS® method of seismic exploration wherein the sweep or pilot signal is applied continuously to the vibrator during the acquisition period.. This method allows for increasing the duty cycle of the vibrators to as high as 1000/0. An increase in vibrator utilization is equivalent to increasing the vibrator power. A description of the method is given and examples of sections obtained with the continuous sweep method are compared to sections obtained with the normal sweep/listen method.

INTRODUCTION

In the VIBROSEIS method, the power output or resulting signal-to-noise ratio is directly proportional to the vibrator "on" time. This results from considering that the peak power in the pulse compression process is the pulse compression ratio.

(available in full paper)

or, the signal-to-noise ratio is proportional to the vibrator 1\ on" time.

In considering data processing costs, the time required for correlation is important.

Thus, in considering a maximum "on" time for the vibrators, we must also consider a minimum correlation time. A first consideration for maximizing the lion" time is to lengthen the sweep time. However, this also lengthens the correlation time.

The sweep repetition rate cannot be faster than the section time of interest. This is necessary to meet the requirement for having a unique pilot signal, i. e., for never repeating over the section time or depth of interest. Thus, the maximum sweep length time for a minimum correlation time is the desired section time.

In marine VIBROSEIS operations, the time for data acquisition is the time required to traverse one station interval. With a ship speed of 6 knots or 10 feet per second, the time for data acquisition using a 228-foot interval is then 22.8 seconds. Normal procedures of equal sweep and listening periods, do not permit two acquisitions per interval when the section time is 6 seconds or more. Therefore, it is common to shorten the listening period to reduce the cycle time and so permit two acquisitions to be obtained. For example, using a sweep length of 6.8 seconds and a 4. 5-second listening time, two acquisitions require 22.6 seconds or essentially the maximum time allowable at 6 knots for a 228-foot interval. The effect of shortening the listening period is to restrict the usefulness of the last part of the sweep on deep events. In effect, this produces a time variant filter with an increasing frequency or upsweep such that the high frequency end of the sweep is not available for that portion of the section that is greater in time than the listening period. This procedure is very useful for increasing the vibrator duty cycle without deteriorating the data quality. However, the method is restricted depending on the sweep used and the section time desired. Therefore, the method of continuous sweeping has been developed to permit a major increase in vibrator duty cycle or "on" time.
A Jackup-Installed Platform for Marginal Fields
Authors: Bunce J.W. , George P.J.
Published: October 2008

An innovative platform for marginal oil and gas field developments is described. Its principal features are that it can be installed by a jack-up drilling rig as part of the drilling program, it does not require foundation piles and has superior boat impact resistance when compared with other designs. This paper describes the design of the structure, with particular emphasis on jack-up installation aspects, boat impact resistance and foundation design. A number of practical applications are described.

Introduction

A large number of innovative platform structure designs have been developed in recent years that are cheaper to construct and install than conventional three or four legged steel jackets.

Ref. 1 gives a good summary of the range of platform designs currently available.

The platform designs usually comprise a superstructure supported on a single vertical caisson (monotower) with bracing near its base that attaches to piles driven into the sea floor. Such structures are simpler than conventional jackets, making them cheap to construct. They are also lighter, so they can sometimes be installed by a jack-up drilling rig rather than by a specially chartered crane vessel, making them cheap to install. The guyed caisson platform design introduced here is believed to have advantages over these established designs in that it is still cheaper and easier to construct and install.

In addition, it has superior resistance to boat impact forces, which will usually govern the design of small platforms in moderate water depths. Further, some of the conductors and risers can be on the outside of the central caisson, without the requirement for a boat impact frame. This allows the conductors to be spaced well apart, which leads to a good layout of the wellheads and production equipment in the superstructure and aids drilling operations. For multiple conductors it also has the effect of reducing the wave loads and the self-weight of the structure.

Platform General Description

Figure 1 shows an example of the structure design for a central North Sea site in 60m water depth with a 5,000kN topsides operational weight, 9 well slots and an export riser. The platform is unmanned and requires only limited access during normal operation.

The platform comprises a superstructure supported above the wave crest level on a guyed caisson founded on a shallow spud-can on the sea floor. Six of the conductors are supported on the outside surface of the caisson through guides; the export riser and other conductors are inside the caisson. The caisson is 2500mm-diameter fabricated from high strength steel plate with maximum thickness 50mm.

Three pairs of wire rope guy lines are attached to the caisson. One end of each line is attached to the caisson near the top, above the water surface with the other end attached to an anchorage at the sea floor. Anchorages may be vertical load plate anchors, suction anchors, piles or anchor blocks, depending on the soil conditions and the loads to be resisted.
A New Seabed Penetrometer
Authors: Meunier J. , Sangouard D. , Lhuillier B.
Published: October 2008

There is today no operational system able to perform geotechnical measurements with standard 36 mm CPT cones in deep sea with a depth of investigation of 20 to 30 metres below the sea-bed. The advantage of the new system is that it will allow surveys in deep sea in a more cost-effective manner.

GEOCEAN-SOLMARINE, a French Marine Contractor with deep water geotechnical tools capabilities and IFREMER, a French Research Institute for exploitation of the sea, joined to design and build a new seabed penetrometer which will be operated by GEOCEAN-SOLMARINE for industrial operations and by IFREMER for scientific applications. The IFREMER's version is now under assembly. The first tests at sea of the penetrometer are expected for mid-2000.

Introduction

The current trend for offshore oil developments is to move into deeper water. There will therefore be a growing need for reliable soil design parameters in deep water, for the design of structure foundations, for the anchoring of floating production and storage systems, for pipeline laying and for slope stability problems.

Scientists are also interested in in-situ measurements for different topics: sediment characterisation, slope stability, environmental studies, hazard mapping, stratification logging, indication of stress history and lithology deposits,...

The tool designed by GEOCEAN-SOLMARINE and IFREMER is a frame which is lifted over the seabed and which remains during the duration of the measurement. The penetrating system is based on a flexible rod which is pushed by friction into the soil. For scientific applications, the tool is called "Penfeld", the penetration force is limited to 40 kN which allows a penetration of 20 metres into the soft soils encountered in deep seas. Several locations can be investigated during the same deployment of the system. The industrial version called "DS7000" will be built with higher capacities.

In a first step, the penetrometer will be equipped with a conventional cone including the measurements of point resistance, shaft resistance and pore pressure. Further a new cone will be developed specially devoted to high pressure environment, including differential pore pressure and complementary sensors such as a gamma-densimeter and a thermometer.

Need of sea bed measurements in deep sea

The offshore activities are now moving from the continental shelf to deep seas. This change modifies the needs in soil surveys.

On the continental shelf, the main object of the surveys for foundation design of platforms was to obtain information on the soil for the design of the piles for the anchoring of platforms. The piles were 50 metres, 100 metres long, even more. A typical survey consisted generally in a cored borehole over all the length of the pile, and an instrumented borehole, using a wire line Cone Penetrometer Test performed by length of 3 metres.

In order to perform such a survey, a geotechnical vessel with facilities in drilling was needed.

These methods are operational only in water depths less than 200 metres.
Subsea Intervention From a NonDrilling-Rig-Type Vessel
Authors: Franklin Robert , Collie Ian , Kochenower Chris
Published: October 2008

Deepwater technology has been creeping up the learning curve as the different oil and gas operators move out into deeper and deeper water depth. With the new technology comes new ways of performing older more routine jobs. EEX became the first company to decommission and abandon a deepwater development project. Being first, they had to find new and better ways to do the work. The use of a non-drilling intervention vessel that was self-powered and dynamic positioning simplified the project and allowed EEX to save money and time over the estimated time and money required for a conventional drilling rig and derrick barge approach to the problem.

Discussion

As the Gulf of Mexico development activity moves out into the deeper waters, operators must find more cost-effective ways to exploit the oil and gas resources. Service companies must also change to meet the challenges that are found in the deepwater Gulf of Mexico. In 1998 EEX Corporation (EEX) recognized that the Garden Banks 388 Field, in 2,100 feet of water, was maturing and would be reaching its economic limit by mid-1999. Management at EEX began reviewing the techniques needed to properly decommission and abandon the field.

In shallower waters, the decommissioning and abandonment of a producing field has evolved from the use of large rigs and extensive management / logistics procedures to a more stream lined procedure with a turnkey or specialist in the decommissioning and abandonment process. The end result is the same, sometimes better, but the time required to complete the task and the cost of the operation is cut dramatically. In many of the cases the wells can be plugged and abandoned with a coil tubing unit and a wireline / logging unit, rather than using a large drilling or workover rig. The larger drilling / workover units could take one to two weeks to rig up and place in operation and many found that the equipment was overkill for the nature of the operation. Coil tubing and wireline / logging units often can be rigged up in a matter of hours and the plugging and abandonment operations could be completed in less time than it took to rig up the larger units. To say the least the time and cost savings were significant. EEX felt this same evolution was possible in the deepwater environment.

Decommissioning

EEX wanted to be able to have the FPF and the FSR decommissioned and brought back to the dock, by late August or early September, to avoid the peak of the hurricane season. By doing so, it would reduce the risk of damage to equipment or the loss of valuable time due to possible bad weather.

The Floating Production Facility (FPF) was used to decommission the production equipment, riser, manifold, jumper lines, flowlines and sales lines running to Eugene Island 315. Seawater was used to flush the system and it was pumped into the wells to push the wellbore fluids into the formation and establish the injection rates for each of the wells.
Fatigue Curves for Polyester Moorings - A State-of-the-Art Review
Authors: Banfield S. , Versavel T. , Snell R.O. , Ahilan R.V.
Published: October 2008

The gathering momentum for the use of polyester ropes for long term production system moorings in deep water has instigated a thorough review of their fatigue performance. The traditional view of designers has been to use spiral strand steel wire rope T-N curves to assess the fatigue performance of polyester ropes on the basis that available evidence indicates that the latter's fatigue performance is superior to spiral strand wire rope. The authors have examined all the available fatigue testing data and the proposals for polyester specific T-N curves and have identified a number of issues pertinent to the derivation of fatigue design data for fibre ropes and propose an improved design curve.

This paper systematically identifies the various issues and proposes a rational way of including the available data (including runouts) for the development of a T-N curve for polyester ropes. The new data was used to re-visit the fatigue life and reliability for an example West of Shetland FPSO moored using polyester ropes (Ref.1).

Introduction

Synthetic fibre ropes have undergone a rapid testing and development phase over the last five years through a number of Joint Industry Projects (Ref. 2,3,4 and 5) and other studies. Ref. 2 is a recently launched JIP which utilises the latest materials with marine finish, terminations and rope designs.

Due to their good behaviour in fatigue, testing fibre ropes to quantify their fatigue life can be an expensive process and individual tests at loads representative of FPS conditions are predicted to run for man-years even when accelerated several fold. Thus, most fatigue testing has been conducted at high load levels and accelerated rates to produce data within reasonable time and budget.

This paper will review the data to show the true potential of polyester fatigue life and to provide a realistic design T-N curve.

Terminated Rope Breaking Strength

Rope strength is defined as Minimum Breaking Strength (MBS) with terminations and is guaranteed by the manufacturer. The average measured breaking strength will typically be 10% higher based on the most recent technology.

As shown in Table 1, a material, rope and termination review of the public domain data shows considerable scatter in the margin of ABS (Actual Breaking Strength) over MBS between -7% to +36%. This illustrates that care needs to be taken in analysis of data when selecting the basis of rope strength and that a common basis for strength can reveal new information. The column titled theoretical ABS is calculated from the yarn content to give the theoretical strength of the rope to allow for terminations, variability (manufacture of rope, not terminations) and obliquity (effect of the lay angle). This gives a strength expected using a well made, designed and terminated rope with a high quality fibre. This now gives a margin well within 10% for the spliced ropes, with the exception of number 5 for which the terminations may not be reflecting the true potential of the rope.
Offshore Mineral Terminals -Artificial Islands And Open-Sea Shiploading Of Dry Bulk Materials
Authors: Soros Paul
Published: October 2008

Undersea m1n1ng is expected to enter an era of economical production of high volumes of low cost minerals from increasing water depths to be competitive in world markets, transport costs between mine and ultimate consumer become critical.

Typical transport systems between mineral source and ocean carriers are considered, with storage and shiploading facilities being usually the major problem.

Open-sea mineral loading is illustrated with Port Latta, in successful operation since 1968. The possibilities in this field are demonstrated by a review of all loading facilities now planned in the world for 250,000 DWT and larger vessels, engineered by Soros Associates.

The logical basis for incorporating artificial islands in transport systems between ocean mining and ocean carrier is explained.

The actual design problems and project economics are illustrated with two cases of widely different scale. One is for a 1.5 million ton per year movement, the other is designed for a throughput of 20 million tons per year.

Introduction

Undersea mining at shallow water depths, say up to 90 feet, or dredging, to use a less glamorous word, is one of the lowest cost methods to produce raw materials in large quantities. At larger water depths, production costs are still high, restricting commercial development to high value, low volume minerals. It is a fair assumption that with improved technology these production costs will decline, and undersea mining will become a source of low cost, high volume minerals also from increased water depths.

Our low cost, high Volume mineral, to be competitive with land based sources will be in the range of 25¢ - $5.00 per ton to mine. After being mined from the ocean at a low cost, it will share a fate with its land-based competition - the final selling price and therefore profitability, will greatly depend on the cost of transportation between mine and ultimate consumer.

Transport System from Undersea Mine to Ocean Carrier

Disregarding the special case of a single consumer close to the source of the mineral, the transportation costs for ocean mined minerals will be made up of ocean freight and the cost of the system to process and move the mined material into the ocean carrier.

The elements of such a system would be shiploading, storage, transportation between mining and storage, and processing, as required. Ideally, the mining operation would be at a short distance from firm, flat land suitable for storage, inside a protected harbor that can accommodate large bulk carriers. Unfortunately, reality may be quite different.

The most critical item is likely to be the port for shiploading. An existing port or natural harbor may be too far from the mining operation, may not have room for mineral loading or will-be able to handle only small or medium size bulk carriers.
A Description of Experiments and Results Obtained in the Correlation of Geophone and Hydrophone Response
Authors: Lechenger Louis
Published: October 2008

Experiments were conducted in enclosed pressure tanks using sources capable of exciting the tank and its contents with a sine wave motion, and measuring the-output of calibrated hydrophones within the tank at a precisely known depth.. Other hydrophonegeophone tests were made in and around a large open water tank sunk in the ground using the Dinoseis* as an energy source. Further testing was done involving larger ground area and a large open body of water.

The object of these tests is to correlate geophone response with hydrophone response and to correlate laboratory test conditions with the environment found in the field.

INTRODUCTION

There are many occasions in seismic prospecting when information correlating the response of an array of geophones on land and a group of hydrophones in shallow water could be more than just informative.

OBJECTIVES OF TESTS

It is the object of this report to reveal and analyze the results of some of our experiments in the correlation of geophone and hydrophone response. Experiments were conducted both in the laboratory and at an outside test environment. The main factors to be checked experimentally were correlation of electrical signal amplitude between geophones and hydrophones and the phase relationship between these output signals.

TEST ON SHAKER TABLE

In the beginning, an approach to the problem was made by mounting a container of fluid on our calibrated shaking table. A known velocity was imparted to the fluid and the elctrica1 output of a calibrated hydrophone was recorded with the hydrophone suspended at various depths in the fluid. Figure 1 shows the physical set up for this test. The voltage output of the shaking table calibration coil is directly proportional to the velocity of the table and is related by the transduction constant, 2.55 inches per second per volt. This transduction constant was obtained by measuring the peak-to-peak table displacement optically and the calibration coil voltage while accurately counting the frequency at which the shaking table is driven.

Tests were run and data was calculated based on the assumption that the excess pressure sensed by the hydrophone is related to the acceleration of the fluid by the relationship P =? h a/g where ? is the density of the fluid, his the depth of the acoustic center of the hydrophone, and a/g is the acceleration of the mass of fluid in g's due to the velocity of the shaker at a given frequency. A simultaneous value of the pressure wave was obtained by measuring the electrical output of the hydrophone and calculating the amplitude of this pressure wave based on the transduction constant of the calibrated hydrophone. The hydrophone used in this experiment was the Geo Space MP-15 streamer phone which has a capacity of 8,000 pfd and a transduction constant of 1.0 volt per psi. The MP-15 is shown in Figure 2. The main characteristic which influenced the choice of this hydrophone was its very flat sensitivity vs. depth response characteristic.
Deep Water Hydropnone
Authors: Heaps Stanley
Published: October 2008

A general purpose hydrophone has been developed which £features high sensitivity, wide band operation, and high pressure capability. An internal preamp overcomes the usual problems related to cable noise and cable capacitance. The internal battery is charged through the amplifier output leads. This hydrophone was originally designed for detecting low frequencies at oil well-depths in geophysical applications. These density and high frequency response have been improved to adapt the unit to a number of oceanographic problems.

The pressure sensitive element of the hydrophone is a hollow cylinder of lead zirconate. Its mechanical resonance above 40 KHz determines the maximum usable frequency. Other resonant modes of vibration of the metal housing land amplifier assembly restrict the 3 db band width to 8 KHz.

Figures I thru VII, at end of paper. The transistorized preamplifier has three stages with direct coupling. The feed-back 's factory adjusted to provide a choice of hydrophone sensitivities. Its high input impedance in conjunction with the zirconate cylinder establishes the low frequency cut off below 1/2 Hz. The output impedance is determined by the output coupling capacitor of 500 microfarads. This is near 5 ohms at 60 Hz. The minimum load resistance of 300 ohms permits coupling of this hydrophone to the input of most seismic amp audio amplifiers.

A diode switch permits the preamp battery to be charged through the amplifier output leads. The preamp, having no switches or adjustments, runs continuously.

Rugged construction permits the hydrophone to withstand the high pressure at deep depths and the usual rough handling of the outdoor environment. A plastic cone serves to protect the sensitive element from impact. The two conductor electrical cable serving as mechanical support enters the stainless steel housing through molded fittings with an o-ring seal.

INTRODUCTION

To meet industry's requirements, a hydrophone was developed for use in deep oil well bore holes for acoustic velocity measurements of the geological formations. In this application there was no need for high frequency response above 500 Hz, and the design was tailored to permit rapid assembly by nontechnical people.

The hydrophone is novel because of its internal preamplifier. This serves to isolate the pressure sensitive crystal from the cable capacitance and the recording amplifier input impedance. The self-contained battery is charged occasionally through the amplifier output leads hence only two-conductor cable is needed.

The hydrophone has been pressure tested for 10,000 ft. depths. On the barrel it is identified as "Geo Space Corporation Hydrophone Model MP-I0". (Fig. 1 is a photograph of-the hydrophone.)

Fig. II shows a response graph of the hydrophone. Here we see that it has sensitivity at 40 KHz, but a resonant condition near 2 KHz exceeds the 3 db tolerance for good linearity. This marks the upper end of the flat response band of the hydrophone.
A Probabilistic Approach to the Wave-Structure Interaction Problem
Authors: Gomez Alberto Gomez
Published: October 2008

The analysis of the interaction of wave forces and structural response is a typical case of a random process. In the dynamic analysis of the platform only random excitations are of major concern, while the other parameters of the system can be reasonably considered to be deterministic. In this paper, a mathematical model for the deterministic analysis of wave-structure interaction developed by the author in a previous investigation 1, is modified and presented as a stochastic model. Maximum response of a platform using a random excitation is compared with the results obtained from the deterministic model. The nature of the wave-structure interaction is, such that the maximum response can be obtained with a few passes of the most critical wave.

INTRODUCTION

The reliable prediction of the response of offshore platform to random wave action requires knowledge about the interaction of the wave and the structure during motion. With the structural system defined in a deterministic way for its topological and mechanical properties, and with the wave known or estimated as a random excitation, random response follows from application of the proper theory.

The forces exerted by waves on circular piles, plates, ships, and walls have been predicted by analytical methods and measured in the laboratory. Investigators agree in considering that water particle velocity and acceleration are the governing parameters of the hydrodynamic forces exerted by waves on structures. The velocities and accelerations should be relative values with respect to the body in motion and not the values with respect to ground. This fact is often overlooked because most structures designed in the past were fairly rigid; but for some flexible offshore platforms having velocities and accelerations in the same order of magnitude as the water velocities and accelerations computation of forces using velocities and accelerations, with respect to ground, yields completely unrealistic results. 1

DETERMINISTIC MODEL

The author developed a mathematical model for the deterministic study of wave-structure interaction with basis on the following principles1:The force exerted by the wave on a body can be computed by the formula proposed by Morison, O?Brien, Johnson, and Schaaf2(available in full paper)where V and A are the ater particle velocity and acceleration, respectively; Cl and C2 are constants defined by the geometry of the body.For a structural system composed of lumped masses, the equation of motion for each one of the masses can be written as:(available in full paper)where YI (tn) is the acceleration of Mass I and time tn; FI (tn) is the hydrodynamic force of Mass I and is a function of time; M I is the lumped mass at point I; and R I (Yn) is a function of the configuration of the structure at that instant.

The value of RI (Yn) can be computed from the expression available in full paper)
Marine Navigation System
Authors: Mitchell Donald K.
Published: October 2008

In certain types of offshore operations, particularly minerals exploration, there is a need for a small fully automatic navigation system which presents position data in such a form that it can be used to steer the vessel without need for further computation or reference to specially prepared charts. Such a system with an effective range in excess of 20 miles and an accuracy of 50 feet was built and tested. It gathered position data by digitally counting the travel time of a microwave pulse from the vessel to two beacons at known locations. This time information is transferred to a small computer which uses this time information to compute the position of the vessel with respect to a predetermined rectangular coordinate system. By appropriate choice of the coordinate system, the coordinates can be used directly to provide steering information necessary to keep the vessel on the desired course.

INTRODUCTION

When extending subsurface survey techniques from land to water, the greatest changes in techniques and equipment occur not in the actual gathering of information but in the methods of determining where that information was actually gathered. The transits and chains of land surveys can be used under some conditions, but an electronic navigation system is generally required. These systems have ranged from the complex through the extremely complex and sometimes require more equipment and technical manpower than the rest of the instruments needed for the survey. I have seen a gravity survey where one man with a gravity meter conducted the survey and three men with the navigation system told him where he was.

The navigation system had more than adequate accuracy under good conditions, but it had a number of serious drawbacks in operation. The boat had to be in a known position to start up the system and had to return to a known position to recalibrate if the signal was ever lost for any reason. Atmospheric disturbances from thunderstorms to skywave propagation around sunrise and sunset upset the system and occasionally rendered it totally useless. Data came out in the form of distances to two remote stations at fixed points and the rough position could be determined by looking at specially prepared charts, but the accurate position was computed at a remote location and had a turnaround time as long as a month.

In addition to the technical problems, there were the human factor s in trying to find competent men willing to sit in a trailer house at each of the remote locations day after day so that they could perform those few tasks which were necessary to the operation of the navigation system.

REQUIREMENTS ON THE NAVIGATION SYSTEM

When Geophysical Research Corporation became involved in offshore exploration for minerals, it became apparent that there was a need for a navigation system designed for that type of work. A range of 20 miles and an accuracy of 50 feet would be adequate for most mineral surveys, but the system must not slow down the surveyor complicate it any more than absolutely necessary.
Vertical current structures in the Deep Gulf using EOF analysis
Authors: DiMarco S.F. , Reid R.O. , Nowlin W.D.
Published: October 2008

Empirical orthogonal function (EOF) analysis was performed on current meter records and model outputs in the deep regions of the Gulf of Mexico. The results of the EOF analysis were similar for model and observation stations located on the continental slope and rise. The vertical structure of modes, generally, shows a surface-intensified mode containing a high percentage of total variance and a bottom-intensified nearbarotropic mode beginning at roughly 800 m and extending to the bottom.

Introduction

We examined current meter records and model outputs of the University of Colorado Princeton Ocean Model (CUPOM) to construct empirical orthogonal functions (EOFs) which describe the vertical current structure in the deep Gulf of Mexico. EOF analysis, also known as principal component analysis, has become an accepted tool to quantify patterns of variability present in large sets of time-series data1. To facilitate analysis of the EOF modal time series, all EOFs done in this paper used singular value decomposition (SVD) to provide both the modal amplitudes and principal component time series. Further, both east-west (U) and north-south (V) current velocity EOFs were done simultaneously. Therefore, percentages of variance reported are based on the variance contained in both the U and V time series.

Data

Current meter observations. Analyzed were datasets that consisted of simultaneous point measurements of current velocity of 5 or more distinct depths and which lasted at least 4 months2. Initial EOF analysis concentrated on observations in water depths greater than 700 m during the period 1980-1988. The analysis was done on raw unfiltered data from the eastern, central, and western Gulf of Mexico. The analysis was performed using those data that were available at all depth levels, i.e., irrespective of any temporal gaps.

CUPOM data. Model data were generated from a version of the Princeton Ocean Model adapted for the Gulf of Mexico. The model has 1/12° horizontal resolution and 24 vertical sigma levels. The model hindcast covered the years 1993-1998. EOF analysis of model output was performed using data from cross-and along-isobaths as well as from single locations. The across-isobath lines went from 200-3000 m total depth across the continental slope at longitudes 88°, 90°, 92°, 94°, and 96°W and across the slope meridionally in the eastern and western Gulf at 26°N. Along-isobath lines were in the central Gulf along the 500, 1000, and 2000-m isobaths between 89°W and 94°W. In this paper we concentrate on the along-isobath line at 2000 m and the cross-slope line at 90°W (Fig. 1).

Results

Fig. 2 illustrates the typical modal structure seen from slope locations around the northern Gulf of Mexico. The data used to construct the modes of Fig. 2 were from CUPOM model year 1993 and taken near 27.2°N and 89.9°W and at the edge of the Sigsbee Escarpment. Unfiltered hourly current velocity time series at all 24 sigma levels were used to construct the EOF modes.
Reliability Through Data Reconciliation
Authors: van der Geest Robert , Broman William H. , Johnson Tina L. , Fleming Ray  H. , Allen John O.
Published: October 2008

We propose a flexible simulator of an oil well that may be used for several forms of online, real-time reconciliation of data from existing measurement equipment. It turns out that data reconciliation used as part of the instrumentation philosophy on an oilfield constitutes a dissimilar, redundant level of instrumentation. This additional level of instrumentation may be used to make the existing measurement hardware more reliable by verifying the validity of measurement data. In addition, the technology may be used to replace permanently installed hardware in times of failure. Greater reliability is expected to contribute to greater system uptime by reducing the need for workovers.

Introduction

Data reconciliation is a technique that has traditionally been used in process control to verify measured data by reference to a process model. In this paper we consider a more active form of data reconciliation in which we use a model of the process to estimate a number of unknown variables on the basis of other, known variables in the process. Such estimated variables may be seen as virtual measurements of the process; therefore estimators based on data reconciliation are sometimes referred to as virtual sensors. They are also known as software sensors or soft sensors, which distinguishes them from their hardware counterparts. It turns out that soft sensors may be used with great success in the operation of chemical processes for supplementing, and in many cases even replacing, permanently installed hardware.

There is a big potential for soft sensors in offshore oil production. First of all, even though the added value of permanently installed sensors is undisputed, downhole equipment is particularly expensive to install because of the nature of offshore operations. Therefore it is appealing to limit the number of downhole sensors to what is minimally required in order to get sufficient insight into the production process, and to use inferential techniques to make the picture complete. Secondly, it is widely recognized that downhole measurement equipment and the corresponding communication systems are prone to failure during installation in the well, during special events in the life of the well, or after a few years in operation due to harsh living conditions. Furthermore, it is prohibitively costly to repair or replace downhole equipment in offshore wells. Software sensors, as opposed to hardware sensors, typically do not break down. In fact, even though a soft sensor depends on the availability of hardware measurements that may be subject to errors, it does not necessarily depend on the availibility of aparticular piece of hardware. A soft sensor will work as long as the total set of available data contains enough information for the system to make up its mind.

In this paper we propose a flexible simulator of an oil well that may be used for several forms of online, real-time data reconciliation..