bluuebunny/crossref_metadata_2025 huggingface dataset:
High-Sensitivity Magnetic Profiling
Authors: Unterberger R.R.
Published: October 2008

INTRODUCTION

A high sensitivity rubidium 87 magnetometer, designed and built by the author, is used at sea to make magnetic profiles over subsurface structures of interest. The Texas A&M University Research Vessel GYRE was used to launch, tow and recover a nonmagnetic fiberglass skiff that carried the magnetometer. To avoid magnetic field disturbances of the GYRE, the skiff with the magnetometer was towed 600 ft behind the ship. Loran C, and sometimes SATNAV, position data were used to determine the ship location. Two recording depth finders using 3.5 kHz and 12 kHz respectively were used to profile the bottom. Time marks were plotted on the magnetic and sonar data in accordance with WWV time signals received on 10 MHz. (15 MHz and 5 MHz were also available if there happened to be poor radio transmission at 10 MHz). Magnetic data were recorded in digital form on a strip chart recorder, using the last two digits of the six digit resonance frequency of the Rb 87 atoms.

THEORY OF MAGNETOMETER

The magnetometer is an optically-pumped rubidium 87 magnetometer. The system was patterne1 after a rubidium clock built by Dr. Peter Bender of the U.S. National of Standards. It uses rubidium 87 rather the rubidium 85 because the spin of Rb 87 is 3/2 rather than 5/2 for the more plentiful isotope Rb 85. Thus by obtaining Rb 87Cl from Oak Ridge National laboratory and reacting it with solid Ca in a vacuum, we were able to obtain 500 ml pyrex spheres with some solid Rb 87 in them.(Equation Available In Full Paper)

Approximately 1 mm (of mercury) pressure of neon gas is also introduced into the sphere to reduce collision broadening of the spectral line caused by rubidium atom spin-spin transfer.

The rubidium atoms have their absorption lines split in the presence of a magnetic field (such as the earth's, that we wish to measure). This natural phenomenon is known as the Zeeman effect. The splitting of the energy levels is given by the Breit-Rabi 2 equation.(Equation Available In Full Paper)

In this way we measure the absolute value of the earth is field H in terms of frequency. This is the optimum situation because frequency, and its inverse time, is the parameter we can measure best. By means of a servo oscillator that keeps the radio frequency oscillator locked onto resonance in the earth I s magnetic field of approximately 50,000 gamma, and a frequency multiplier of ten, we then can measure the earth I s magnetic field to 1 part in 3.5 million, because we measure the frequency to ± 1 count. Thus the sensitivity of the magnetometer is 50,000 gamma divided by 3,500,000 or a sensitivity of ±.01 gamma. This is 100 times more sensitive than the usual proton precession magnetometer which operates at a much lower frequency and is not a continuous instrument.
Seal Integrity and Connector Clamping Force at the Subsea Wellhead
Authors: Owens J.H.
Published: October 2008

The importance of the relative stiffness of the hydraulic connector and the wellhead hub for subsea wellhead connections is demonstrated using existing design principles for bolted flanges and clamps. Connector stiffness is determined experimentally for one design, and a method utilizing finite element analysis is demonstrated for another. A 3-D finite element model is used to determine wellhead seal radial loading after hub separation has occurred on one side of the hub interface, as it might under a severe moment. The results suggest that the normal load between hub seal pocket and seal OD reduces markedly as the gap increases, and more of the internal pressure loading on the seal is resisted by hoop stresses in the seal itself. However, even at a gap of 0.090 inches, the loading on the seal appears to be uniform around the seal pocket. At a hub separation 0.090 to 0.120 inches, all OD normal load is lost on a portion of the seal near the separation.

INTRODUCTION

In a drilling operation, the connection between the blowout preventer stack, with the lower marine riser package and riser above it, and the wellhead near the sea floor must be made remotely, often at great sea depths. It must be well pressure tight under relatively severe bending, shear and tensile loads. A heavy, steel, pressure energized seal is used, and the joint is made up using a hydraulic clamping device, a marine hydraulic connector, which preloads the seal and generates a heavy, axial compression between the hub faces of the joint. The load, referred to as the connector clamping force, is generated primarily to ensure that the hubs do not separate and move relative to each other under external loads, thus chafing the seal, wearing it out, and eventually causing a leak.

That connector clamping force is important for maintaining joint and seal integrity is well established by field experience. Connector clamping force and its high sensitivity to sliding friction on working surfaces are discussed in a previous work by the author [1]. Substantial theoretical and experimental work regarding hub separation and stresses for API clamp connections has been done by [2].

In what follows, the interaction of the three principal parts which make up the subsea wellhead connection, seal, hub, and connector is considered. First, the importance of the relative stiffness of the connector and hub is explained using basic design principles usually applied to flange connections. Second, the stiffness of an existing hydraulic connector design is measured experimentally, and then finite element analysis is used to determine how the connector-hub stiffness ratio affects the magnitude of the external tensile force required to cause hub separation.

Lastly, the interaction between the steel wellhead seal and the hub seal pocket is investigated when an external moment separates the hub faces on one side.
Offshore Sand and Gravel Mining
Authors: Padan J.W.
Published: October 2008

This paper reviews the status of mining offshore for sand and gravel on a world-wide basis. It discusses the technology for exploration and evaluation of sea floor mineral targets, as well as mining, transportation, and processing. Large operations in Japan and Europe are described, based upon personal observations of the author. The U.S. situation is outlined and opinions offered as to the outlook for the future.

INTRODUCTION

Sand and gravel are broad terms used to cover almost any ground rock or mineral. Size distribution and performance specifications are more important that mineralogy per se but quartz is the most common constituent. Sand, of different size bands, ranges from 0.06 to 2 mm diameter: gravel from 2 to 8 mm.

Sand and gravel together comprise a low unit value mineral commodity (--$2.55 per metric ton in the U.S. in 1982) that is used in nearly every country in the world for construction aggregate and fill. This commodity is mined from the seafloor in several areas of the world for those same purposes and, in addition, for the restoration of stormeroded beaches.

Commercially important marine sand and gravel occurs as beds and lenses of unconsolidated aggregate, generally Pleistocene or younger, lying at or near the surface of the continental shelf under 20 to 30 meters of water. Glacial deposits are common in Northern latitudes; submerged beach and river deposits are mined elsewhere. Aragonite "sand", precipated from seawater, is mined in The Bahamas for construction aggregate as well as for use in the glass industry.

INDUSTRIAL OPERATIONS

Deposits currently mined offshore occur where favorable marine geology, metropolitan (not necessarily coastal) markets, navigable water, and a favorable legal regime occur in juxtaposition. The two largest industries have developed in Japan and Northern Europe. Each is discussed blow.

Japan

Rapid depletion of onshore deposits of construction aggregate has caused Japan to tap sand deposits in coastal waters. In recent years a fleet of about 900 small vessels has recovered 15 to 20% of all aggregate mined in Japan. Most of the mining is for sand for use as fine aggregate, which is costly to grind from crushed rock. Coarse aggregate is supplied increasingly by crushing rock. Annual production is 60 to 70 million tons and is expected to increase. By way of comparison, this rate of mining is double the rate of mining off Northern Europe although the latter operations have received more attention in the Western literature.

Most of the mining occurs off Western Japan, as shown in Figure 1. About two-thirds of the production comes from the Seto Inland Sea; most of the remainder comes from the north coast of Kyushu, off Fukuoka. In general, the seafloor upplies western Japan while eastern Japan is relatively uncrowded and so is able to utilize on land deposits of sand and gravel. Tokyo's huge demand is satisfied from onshore because of its great distance from marine deposits.
Comprehensive Dynamic Analysis of Offshore Loading Concepts
Authors: Natvig B.J. , Berta Marco
Published: October 2008

A computer based metod for comprehensive analysis of Offshore Loading concepts is discussed. The method differs from much of the previous published work in that all of the following effects are accounted for:Non-linear time domain analysis in irregular seas.The frequency dependent added mass and potential damping are tackled by convolution integrals.Still water damping, wave damping and current damping have been included in accordance with advice given from NSMB, WageningenTwo tankers, connected by a non-linear hawser, may be moored in tandem.The moored tanker is modelled using all 6 degrees of freedom.Slowly varying wave drift and non-constant wind forces wrt. the instantaneous ship headings are included.Non-linear inertia forces, as they appear in the ship manoeuvering equations, are included.Weather vaning stiffness due to wind and current are included. The way the computer program is organized to account for any types of mooring is a special feature of this program. Examples of different mooring applications are given in the paper.

INTRODUCTION

Exploration for hydrocarbons in the waters outside Northern Norway was started 3 years ago and the prospects now are promising. A major research programme to prepare for the many complicated problems yet to be tackled in this environmentally difficult area was started in 1980 by Statoil and several other oil companies. Amongst the larger of the over 20 research projects thus started are two projects for. Offshore Storage and Loading in 350 meters of water. These two projects are sponsored by Statoil and Agip and the project execution is shared between Aker Engineering - Oslo and Tecnomare - Venezia. In the first project several existing and proposed Offshore Storage and Loading concepts are evaluated for merits and capabilities. The second project concerns the further development of the most promising concepts.

The need to evaluate several different storage and loading concepts in an unified manner necessitated a more general analysis tool than was readily available. The decision was therefore made that such a computer based system were to be developed as part of these projects. An early version of this program, which is called Sea Off Load, is now running. A feature of the program is that it is structured in such a way that it will allow for the insertion of any mooring system that may be required. This will result in a family of programs which will have the description of the environment, the basic ship formulation, the assembly and solution of the equations of motion all in common. A block diagram of the structure of Sea OffLoad is shown in figure 1. At the present time, the modelling of a catenary moored large volume storage buoy together with an off take tanker is being formulated. For this configuration, which is only mentioned in passing, the storage buoy is substituted for the moored tanker.
Diffractions Reveal Drilling Hazards
Authors: Fulton T.K. , Hsiao R.T.
Published: October 2008

1983 OTC PAPER 4507

DIFFRACTIONS REVEAL DRILLING HAZARDS

Seismic waves are diffracted where there is a sudden change of curvature or acoustic impedance along a surface such as boulders embedded in clay, across faults, the eroded edges of channels, and at the edges of shallow gas drilling hazards. Diffractions reveal these situations, especially if they are enhanced at the expense of reflections and refractions and organized as in an Offset Panel. The Offset Panel was introduced by FuIton and Darr (1981) as a powerful too I for the analysis of seismic data. Their paper described a method by which conventional exploration seismic data are used to detect shallow gas drilling hazards.

The work of Berryhill indicates that boulders, which are one-half the dominant wavelength in size, can be expected to generate diffractions. Hazard seismic data from the North Sea presented here show energy in the 500-700 Hertz band corresponding to wavelengths of II to 8 feet. Unfortunately, the spacial sampling of the hazard data is four wavelengths, much too great to resolve boulders. A spacial sampling of 2 to 3 feet would be more appropriate for these data anday allow for the detection of individual boulders or zones containing fewer boulders.

Experience with the Offset Panel to detect shallow gas drilling hazards along the line of the profile suggested the Diffraction Panel to detect gas hazards nearby. In this report we demonstrate the use of the Diffraction Panel to detect buried channels and shallow gas drilling hazards.

INTRODUCTION

This paper deals with engineering and drilling hazards, such as:"Boulder beds" that might alter the design of both offshore structures and their piles.Near-seafloor gassy mud whose shear strength and velocity decrease as the amount of free gas in the mud is increased.Channels and/or near-sea floor faults that may alter the relative strength of the ocean floor.Boulder beds that may be costly in rig time required to penetrate them." Over pressured " shallow gas or shallow faulting which may vent deeper gas. All of the above hazards show a velocity contrast with respect to adjacent strata. Diffracted seismic waves occur at these velocity contrasts and are commonly called diffractions. The scattering of seismic energy may also occur at these contrasts. The subject of diffracted and scattered waves is complex. The events themselves have a dip different from that of reflections. A primary objective of this paper is to demonstrate that a simple display can aid their analysis.

Diffractions can best be observed in common offset profiles. They may be enhanced on a number of common offset profiles arranged in an "Offset Panel" format forease of interpretation.

The Offset Panel display was described as a powerful analysis tool by Fulton and Darr (1981). The display simply arranges common offset profiles vertically by offset and horizontally by Common Depth Point. This arrangement forces reciprocal source and receiver effects to form predictable patterns.
North Sea Physical Model Construction, Data Acquisition, and Interpretation
Authors: Nelson H.R.
Published: October 2008

This paper describes the building, data acquisition and interpretation of a complex physical model, SALNOR, built to illustrate a "typical" complicated N6rth Sea geological structural sequence. The model was built as a proprietary project for statoil who then released it to the Allied Geophysical Laboratories (AGL). The model consists of seven layers that represent the Top Palcocene, Top Cretaceous, J-Uniformity, Top Brent, Base Brent, Top Stratfjord, and Base Statfjord. The respective model material used were 3120 (red), 184 (clear), 3110 (white), 3120, 3110, 3120; and 3110 RTV silicone rubber to represent pre-cretaceous rocks.

The steps required to build the model are detailed, along with photographs of the model as construction progressed. The 3D data set across the model consisted of 240 traces on 240 lines. Several other data sets across this model have since been collected. In order to graphically illustrate the complex structure, the model was cut into 16 square blocks. "Wells" were also core-d in the center of each of these 16 blocks. This allows the seismic data to be directly tied to the reflection sources. It is interesting that when data was collected across the reassembled blocks, there was not excessive diffraction noise created by the vertical cuts and holes.

The initial interpretation was done on paper sections. Later the raw data and 3D migrated data were evaluated using the interactive interpretation capabilities of the Adage vector refresh graphics system. The interactive computer graphics interpretation is illustrated with a 16 mm movie. The movie specifically illustrates the value of rapidly animation through a sequence of vertical, horizontal or mixed vertical and horizontal seismic sections. The movement brings "alive" the geologic structures and stratigraphic sequences and allows the interpreter to make a rapid evaluation of the best prospects.

The model data can be used to evaluate and to teach different interpretation techniques. However the real value is the demonstrated ability of physical model data to accurately represent a complex offshore geological problem, and in the confidence this brings to marine seismic data acquired, processed, and interpreted in a parallel fashion.

INTRODUCTION

In late 1981 Ingebret Gausland from Statoil requested SAL to devise a workshop on 3D interpretation methods that interpreters working in the North Sea would find applicable to their work. It was decided that the best procedure would be to build a model that represented typical North Sea geology and structure and to base the workshop on it. 1 From this basic concept, the SALNOR model was designed. Figure 1 is a contour map of the only horizon that was contoured, the J-Unconformity. The two layers above this were to be flat, and the layers underneath of constant thickness dipping and parallel to each other within each separate fault block. Slight deviations from the planned dimensions occurred during construction and introduced an added degree of realism.
Tieback Requirements for Early Production Systems
Authors: DaMota A.C.
Published: October 2008

The idea of drilling subsea wells, installing a wellhead jacket and tying back to surface is very attractive since it can lead to early production capability.

However, conventional tieback systems are not suitable, when a production platform and/or a second intervention of a drilling rig is required to carry out the tieback.

INTRODUCTION

Early production systems are quite new in the North Sea. Shell Expro considered it for the first time in early '77 for the Fulmar Field where, before installing the main production platform in Summer, '80, a wellhead jacket was launched during the Summer of '79 over a modular template. This template was designed and manufactured in 3 months (March/June '77) although it was not landed until '78 with the help of a Semi Submersible. The same rig ("ocean voyager") drilled the first three wells before being released to allow jacket installation.

A jack up rig (Dyvi Beta) drilled a 4th well shortly after and tied back and completed the remaining wells in the Summer of 1980. The design and execution of these tie backs to subsea wellheads served to highlight not only the drawbacks of the dual BOP stacks, i.e. 20 3/4" X 13 5/8" subsea systems, but also the problem of misalignment.

Similarly the experience gained during the design of the S.E. Auk template for tie back or subsea completion of a single stack subsea system was more enlightening as a wider range of suppliers became involved.

Unfortunately the well was dry ... In this case, again tie back to subsea wellheads was considered and the main lesson learnt was the need for multipurpose subsea wellheads.

Early '81 a similar project was considered, this time for Leman E mudline suspension system tie back. Once more suppliers gave of their best.

This paper reflects the Subjects discussed, i.e. conventional systems (mud line suspension and platform tiebacks) the comments made and suggests different approaches to solve the problem of tying back predrilled wells.

Aware of potential problems posed by tiebacks, several operators (Agip, Amoco,B.P. Esso Norway, Marathon, Montedison, Norsk Hydro, Petrobras Philips and Statoil) met with Shell to discuss lessons learnt from past operations.

CONVENTIONAL MUD LINE SUSPENSION SYSTEMS

Application

Primarily they were designed to reduce weights at deck level on free standing connectors and to speed up the abandonment. Secondarily the possibility of suspension and re-use was envisaged. Both subsea or platform completions were considered. For the latter tieback to surface of subsea strings was required.
Tarsiut Winter Alert Monitoring
Authors: Wright B.D. , Weaver J.S.
Published: October 2008

Tarsiut Island is the first caisson retained island that has been built for drilling operations in the Arctic Offshore. Because of its novelty, a real time monitoring program involving the predicted and observed performance of the island under ice loading was established to ensure safe and efficientdri11ing operations and to satisfy government requirements.

To accomplish this program, an extensive state of the art instrumentation, data acquisition and monitoring system was developed and utilized to collect ice action and island response information. The data was used in real time to assess island stability and provide an alert warning to drilling personnel. The alert levels were related to drilling activities to ensure the integrity of the operation.

This paper describes the methodology, procedures and experiences associated with the Tarsiut winter monitoring program.

INTRODUCTION

Although many artificial islands have been built in the Canadian Beaufort Sea (De Jong and Bruce, 1978), the Tarsiut structure represents a significant new extension to Arctic drilling technology. This island, constructed by Canadian Marine Drilling under contract to Gulf Canada Resources, was the first caisson retained island used for exploratory drilling in ice infested waters (Stenning and Fitzpatrick, 1983). During the winter, spring and summer of 1982, Gulf Canada as operator for the Tarsiut group*, dri11ed the Tarsiut N-44 and N-44A wells from the island as a part of its Beaufort Sea exploration program. The N-44 well, drilled on the same geological structure as the Tarsiut A-25 discovery of 1980, yielded 556 m / day (3500 bbl /day) while the subsequent N-44A delineation well, side tracked from the same well bore, was less prolific yielding 346 m3/day (2175 bbls/day).

In addition to the novel use of caisson technology to reduce the fill requirements for island construction, the Tarsiut structure was located in 21 m of water making it the deepest artificial island ever built in the Beaufort Sea. The island was al so located in an area where dynamic ice motion was expected for at least part of the winter season.

Tarsiut was designed on the basis of theoretical analyses, model tests and full scale experience at other islands (Croasdale and Marcellus, 1978). The design work indicated that under expected ice conditions and loadings, there should be no hazard to the island from the stability or operational viewpoint. However, because Tarsiut was novel, there was a finite probability that the ice conditions might be different than expected or that some interactions not predicted by experience, theory or model tests might occur. This was recognized by Gulf Canada, its partners and the Canadian regulatory agencies and procedures were implemented to ensure that the effects of the ice environment on the stability and operations from Tarsiut Island were known or could be anticipated at all times.
Acquisition and Processing of Gulf Coast VSP Data
Authors: Landgren K.M. , Grubb Karel
Published: October 2008

This paper describes the acquisition and processing of vertical seismic profile (VSP) data from the Gulf of Mexico, including details of an integrated logging/seismic acquisition system. Standard field techniques and typical processing parameters are discussed, and a real data set is used to review the steps in VSP processing. Wellsite processing of field data is also presented.

INTRODUCTION

Vertical seismic profiles are an outgrowth of seismic velocity surveys. Originated in the 1950's, they were developed in the Soviet Union during the two following decades. I In recent years, the awareness and popularity of VSP techniques have spread to the West. Today, developmental activity is underway in all phases of VSP technology: acquisition, processing, and interpretation. As a high-resolution seismic method, vertical seismic profiling is assured of a prominent role in hydrocarbon exploration.

Acquisition of VSP data in the offshore environment requires a marine seismic source, a downhole sensor, surface instrumentation, and a wireline for sensor positioning and signal transmission. Historically, borehole seismic surveys have been performed in conjunction with open hole logging surveys, using the logging wireline interfaced with stand-alone seismic acquisition equipment. The introduction of a computerized logging system, designed for the digitization and taping of borehole logging data, opened the way for the development of an integrated surface system for the acquisition of both logging and seismic data. Such a system was developed, and it was first introduced in the Gulf of Mexico in 1981.

ACQUISITION

A typical setup for well seismic data acquisition is shown in figure 1. The well logging unit is usually mounted on the drilling rig. A downhole sensor and a marine seismic source and sensor complete the necessary acquisition equipment.

The standard seismic source in the Gulf of Mexico is the air gun (figure 2). As an impulse source, the air gun provides adequate energy and the bandwidth necessary for data acquisition. Its signature is complicated by the bubble effect, which results in oscillations with a period of about 100 ms. Frequently, a kit is used which limits the bubble effect, at the expense of reducing the effective chamber size and thus the radiated energy. Another approach to signature improvement is the use of arrays of guns with different volumes. Gun arrays display both reduced bubble effect and increased radiated energy. A hydrophone is used to monitor source signature.

Having produced a pulse of seismic energy, it is necessary to detect the pulse after a series of transmissions and reflections through the earth. The sensor for the work described in this paper consists of a uniaxial stack of four Geospace HS-1 geophones and associated electronics mounted in a downhole tool. During acquisition, the tool is coupled to the formation by an arm under springtension. The tool is designed to operate in well environments normally encountered in the Gulf of Mexico (figure 3).
MIG-Welding of Offshore Steels at Pressures From 1 to 30 Bar
Authors: Terlinde G.T. , Mueller L.G. , Schwalbe K.H. , Beaven P.A. , Szelagowski Peter
Published: October 2008

Hyperbaric welding was performed in an unmanned chamber at pressures up to 30 bar (300 m depth). A flux-cored and a solid wire were used as consumables; two typical offshore steels served as base materials. The study, which mainly focussed on the weld metal properties, included chemical analysis, detailed microstructural characterization, tensile and charpy impact testing, and for selected conditions a fracture mechanics analysis. When using argon as pressurizing gas, the impact toughness decreases only slightly at higher pressures «15%); the room temperature tensile properties and COD values for crack growth initiation show no pressure dependence. The solid wire has slightly lower impact toughness values than the flux-cored wire. An attempt is made to explain the mechanical properties on the basis of the observed microstructures.

INTRODUCTION

For more than a decade extensive efforts have been made in the development and application of underwater welding /1–11/. This is mainly due to the repair and construction work which has to be carried out on underwater parts of pipelines or platforms installed by the offshore industry /12–14/. Various methods are used depending on the water depth and the type of work; recent results have been summarized in numerous reports eg /2,3,5/. Underwater wet welding, for example, is characterized by the welder being in the water and the weld region being either wet or covered by a small gas filled chamber. Dry, or hyperbaric, welding is performed with the welder and the weld area in a gas filled chamber, which is held at the pressure of the surrounding water. In addition, 1 bar.

Welding is possible by keeping the pressure in such a chamber at 1 bar. There are two important factors which complicate underwater welding, namely the pressure and - especially for wet welding the water. These can, for example, cause changes in the arc geometry, temperature and stability /2,3,15/, variations in the metallurgical reactions /1–6,12,16,17/, strong effects on the cooling times /4-7,9–11/, and increased hydrogen concentrations in the weld /13/. Although code quality underwater welding is possible today to a limited extent, in most of the studies a loss of toughness has been observed with increasing water depth /1,4,5/; a complete understanding of this embitterment has not yet been achieved.

The GKSS research center has started an extensive marine technology program, which includes underwater welding and which is focussed on both technological development and more fundamental research. As part of this underwater program, a series of tests with dry hyperbaric MIG welding will be described in this paper. The tests were carried out at pressures from 1 bar to 30 bar. The materials selected were a pipeline steel StE 445.7TM (=X65TM), and a structural steel StE36 (500) which is used for platform components. Based on preliminary tests two commercial C-Mn-Si wires, one a flux-cored wire with basic flux, and, the other a solid wire, were chosen.
Dynamic Positioning in Single-Point Moorings
Authors: Sørheim Hans Roar , Gregersen Steinar Roar , Jenssen N.A.
Published: October 2008

This paper deals with the concept of and a system for complete control of the slowly varying bow motion of a bow-moored ship using dynamic positioning.

System performance is shown by simulated case studies with an offshore 1oading tanker. Significant improvements of motion behaviour and mooring force performance are demonstrated. A complete positioning system is presently runn1ng successfully onboard a 125000 dwt tanker in the North Sea. Some preliminary results are shown.

INTRODUCTION

Mooring of large ships at offshore terminals are done by single-point mooring. The ship is connected to the terminal through a single mooring line, or hawser, which allows H to position itself according to the preventing weather and sea conditions, like a weather-vane, and thereby to remain moored with a minimum mooring force when the direction of forces changes.

A major problem faced by single-p01nt mooring In hostel areas is due to the motion of the ship and the terminal. The ship tends to oscillate slowly relative to its mooring point with a following oscillatory behaviour of the mooring stresses, and with the effects of wave following motions superimposed. When the mooring force exceeds a prescribed value, loading must be stopped and mooring abandoned. The low frequency ship motionstems from unstable ship mooring configuration and/or dynamic force excitations from wind and wavesl.

From practical experience a certain amount of astern thrust is found to have a favorable effect on the slow motion, but it is questionable as a remedy for reducing peak hawser tension.

Dynamic positioning (dp) has been applied with great success to freely moving ships and moored vessels for more than ten years. While keeping the ship In a fixed position is typical to most dp system applications, the desirable objective in single-point mooring is to keep the distance between the mooring terminal and the ship's bow as nearly constant as possible, while maintaining the overall weather-vane capability.

In this paper a review of the essential slow motion behaviour of a bow moored tanker 1s presented with emphasis on stability and mode dynam1cs. The des1gn and the theoretical performance of a dp-system to meet the single-point mooring positioning objective Is presented and, finally, results from full scale tests with a 125000 dwt tanker during offshore loading are shown.

ANALYSIS OF SLOW MOTIONS IN SINGLE-POINT MOORING

Only static (elastic) properties of the mooring terminal affects the slow motion behaviour, while the high frequency (wave following) propert1es are reflected in the mooring force. Hence, the terminal is left out in the following analysis.

Governing equations

The governing equations describing the slow motion of a bow moored ship In wind, waves and current are reasonably well establ1shed. A detailed description is presented In1.

Applying some symmetry assumptions, the equations of motion In moored ship coordinates.
Model Tests With a Single-Point Mooring System In ShortCrested Waves
Authors: Høulie Morten , Stansberg C.T. , Werenskiold Per
Published: October 2008

Laboratory experiments with a deep-water singlepoint moored tanker model in irregular waves are reported. A rigid mooring arm is used. Wave induced forces and motions in both long-crested and short-crested sea are investigated. Short-crested waves are usually believed to decrease the wave loadings. In many cases this is not necessarily the case. The aim of the present paper is to point out possible significant differences between wave loadings of short-crested and long crested waves.

INTRODUCTION

The oil industry is applying floating production system in increasing numbers for the development of off shore fields. There are a number of floating production schemes currently in use or under design, which use a floating surface support vessel. The most important factor in successful design of any production riser concepts is thorough knowledge of the environmental conditions and the loads they apply to the structure. Both for determining design and operational ability, it is important to know whether or not short-crested sea should be taken into account. The results and conclusions from this paper are primarily useful when working with single point mooring problems. The magnitude of differences in responses in short-crested and long-crested waves indicate that the results will be of interest in analysis of non-rigid systems in general.

In this paper we are going to describe tests which were conducted in both long-crested seas, with and with out wind at various heading angles, and in short-crested seas. In this paper, special emphasis is given to compare results from unidirectional head seas and multidirectional seas. These tests were a part of an extensive model test program carried out in order to investigate a deep water SALMRA (single anchor leg mooring rigid arm) moored storage tanker system. The tested SALMRA system is a double articulated system design for 375 m water depth and for North Sea conditions. Main features of the system are a gravity base, a structural riser, a large top buoy to provide the restoring capacity and a rigid arm of the truss type for permanent tanker mooring. Fig. 1 shows the system in general.

EXPERIMENTAL SET-UP

Ocean basin

The model experiments were carried out at the Norwegian Hydrodynamic Laboratories (NHL) ocean basin. Fig. 2 shows a plan view of the Ocean Basin and the towing tank. Further, a cross-section of the Ocean Basin and some characteristic data are shown.

In the basin, two different wave generators are installed. In one end a hydraulically driven double flap generator is located, which is able to generate unidirectional waves with a maximum wave height of 90 cm referring to regular waves. A beach is located in the other end of the basin.

At one of the sides of the basin, a 62.5 m long wave generator is located. It consists of 144 individually controlled flaps which are able to generate unidirectional or multidirectional waves.
Shallow Seismic-Derived Acoustic Core Logs
Authors: Caulfield D.D.
Published: October 2008

A small micro-computer system with analog-to-digital sampling intervals to 30 microseconds has been developed for field processing of shallow seismic data. This digitizing capability plus an absorption model for marine sediments has allowed the development of a system for predicting acoustic impedance versus travel time.

Utilizing regional data tables for soil type versus acoustic impedance and absorption such as Hamilton and Bachman has allowed for the actual prediction of soil type, density, and grain size as a function of depth. The prediction accuracy is closely related to the signal-to-noise (S/N) of the received data, while the depth of penetration and resolution is related to the seismic frequencies employed.

The model is discussed in detail with two years of data examples versus actual core data from the Beaufort Sea and Vancouver offshore areas given for review. Application areas such as dredging, pipeline surveys, and material location for offshore construction are discussed. In addition, limitations and further signal processing requirements are reviewed.

INTRODUCTION

Caulfield, et. al.1,2 introduced concepts for the extension of Hamilton's reflection modeling into the prediction of sub-bottom material types through correction of signal levels by simplified absorption and other correction models. This paper discusses a number of system improvements and summarizes practical applications to data acquisition in the Beaufort Sea and the Vancouver Harbour regions.

The major improvements have been achieved by the application of standard deep seismic techniques to high frequency digital analysis problems. These improvements are summarized as follows:Signal-to-noise (S/N): The S/N is calculated before any corrections are made to the data. Only reflections that have a S/N greater than 5 db are considered as valid data points. This has improved prediction accuracy by at least 5 percent.Travel Time Layer Identification: A simple algorithm is used to calculate the exact travel time. The first reflection is taken as an image of the transmitted signal, and subsequent traces are cross-correlated to this to determine the exact position of the layer. Obviously, accurate travel times can be determined for layers of good S/N.Iterative Solutions: Beginning with approximate densities for the region from calibration cores combined with accurate travel time information a first solution is calculated. New density values and velocity adjusted travel times can then be applied to successive iterations. Iteration ceases when the error difference between the output of the derived synthetic model and the observed input signal amplitudes has been minimized.

The entire program has been aimed to develop systems that are practical to aid in field interpretation of data and or laboratory processing at minimum cost. Some of the above algorithms require extensive processing times which could be improved with parallel processors, etc. As is evident from the identity(Mathematical equation available in full paper)
Ice/Berm Interaction Study Using Rotary Sidescan Sonar and Acoustic Profiling Systems
Authors: Good R.R. , Anderson K.G. , Lanziner H.H.
Published: October 2008

Tarsiut Island, in the Canadian Beaufort Sea, was the first dredged caisson retained island built for exploration drilling operations in the Arctic offshore. Due to the island is configuration location, a large first-year ice rubble pile would result from the ice/structure interaction. This paper outlines how a rotary side-scan sonar and a mechanically scanning, narrow-beam acoustic profiling system were used to determine the geometry and the contact area of the underside of heavily rubbled first-year ice. The results of this study are to be used to further the understanding of the nature and mechanism of the ice/structure interaction in Arctic offshore structures.

The sonar transducers were deployed through augered holes in the ice thus permitting horizontal rotation. The systems were also configurated to allow an indirect remote means of data acquisition of the underside of the ice as well as the beam/seabed surrounding the caisson retained island.

Of main interest, was the geometry of the deep draft grounded ice at the edge of the ice rubble annulus, knowing the geometry and the contact area of the grounded rubble was required for use in understanding the ice load transfer mechanism between the rubble field and the island beam. Thirty-six under-ice profiles were taken from each of 14 locations in conjunction with a 360 degree side-scan sonograph acquired from each hole so as to encompass the circumference of the island. Aerial photographs were used to provide the above water geometry of the rubble field.

Sonar profiles were ground truthed utilizing manually drilled holes to establish the level of accuracy, and to allow necessary calibration corrections. The combination of these two systems utilizing remote indirect methods in this unique application provided Gulf Canada Resources Inc. and the participants in APOA Project No. 198 with both cost effective, quantative and qualitive engineering data.

The definitive resolution and detail of the data over such a large extent had not been previously attainable in so little time.

INTRODUCTION

To ensure safe stable platforms for offshore exploration, artificial islands are commonly being wilt in the Beaufort Sea. Two main types of artificial islands have beenused to date; sand with long-sloped beaches, in up to 20 m of water, and sand-supported steel or concrete caissons.

Tarsiut is a caisson retained island on a sand beam designed to reduce the quantities of fill required and the time needed for construction.

Although many artificial islands have been wilt in the Canadian Beaufort Sea (Figure 1), the Tarsiut structure represented a significant new extension to the Arctic drilling technology. The island (Figure 2) was designed and constructed under contract to Gulf Canada Resources Inc. by Canadian Marine Drilling Ltd., and was the first caisson retained island used for exploratory drilling in ice-infested waters (Fitzpatrick and Stenning, 1983)3.
Full-Scale Measurement Tests on the New Semisubmersible Polycastle
Authors: Tamaki I. , Sødal F. , Takanashi S. , Nakayama K. , Inui T. , Kobayashi M. , Nakamura K.
Published: October 2008

Full scale measurements onboard the newly built semi-submersible unit POLYCASTLE were carried out in the North Sea in February and March 1983.

The objective is to discuss the measured results of motion and stress by comparison with the design estimates through computer calculations and model experiments.

It is said in conclusion that the design efforts of the POLYCASTLE have been well appraised both in operation and safety points of view.

INTRODUCTION

A great number of semi-submersible units has been recently put in operation by offshore industries. But recent incidents in the past years have given rise to concern about the safety of semi-submersible units.

After the disaster of the A. L. Kielland, March 1980, the Norwegian authorities and others have renewed their rules and requirements introducing the thought of safety by redunduncy.

The initial design of the POLYCASTLE commenced enthusiastically in May 1980 to apply the new rules and requirements, and she was delivered to the Rasmussen Offshore A/S early in 1982. Her maiden operation in the North Sea began in July 1982.

In the circumstances described above, full scale measurements onboard had been advocated to estimate the design efforts.

The measurements were carried out in February and March 1983 during the stormy season at the Magnus Field of the North Sea, and were concentrated on the incident wave to the unit, motions of six degrees of freedom and stresses acting on the structural members of the horizontal and transverse diagonal bracings. Each measurement took 30 minutes four times a day. The measurements were carried out 87 times (22 days)

(PRINCIPAL PARTICULARS) (Available in full paper)

MEASUREMENT INSTRUMENTATION

Incident waves were measured by a non-contact radio wave-type wave height meter, which was installed on the upper deck facing to the sea. (Fig. l) The meter indicates directly the relative oscillation of the water surface; the vertical displacement of the unit was eliminated by doubly integrating the vertical acceleration simultaneously measured by the accelerometer.

Pitching, rolling, and yawing motions were measured by the gyroscopes, and swaying, surging and heaving by the accelerometers.

As shown in Fig. 1, stresses at the horizontal and transverse-diagonal bracings were measured by the strain gauges which were sealed hermetically with a rubber compound. The sealed wire codes were used to reduce the noise level.

Other environmental factors were also measured: wind by the anemometer (measured at 42 meters above sea level), current by the propeller type current meter, and wave direction by observation, etc.
The Use of Vertical Seismic Profiling in Geotechnical Site Investigation
Authors: Justice J.H. , Hinds R. , Stirbys A.F.
Published: October 2008

Field experiments carried out in the Beaufort Sea in 1983 indicate that the vertical seismic profile (VSP) has potential as a valuable tool for determining geotechnical material properties required in site surveys preparatory to construction or drilling. If both shear and compressional waves as well as density can be obtained from borehole surveys, then elastic moduli of interest can be inferred. Experience shows that the potential and economics of this type of survey are good. However, more work should be done to identify reliable sources of shear waves in order to obtain a full interpretation of the elastic moduli of interest in near-surface material.

INTRODUCTION

Several of Canada's major oil companies are actively exp1oring the oil resource potentia1 of the continental shelf of the Canadian Beaufort Sea. Gulf Canada Resources Inc. (GCRI) is an exploration operator in this region (Figure 1).

At potential exploration drilling sites, complete and accurate geophysical and geotechnical evaluations of the upper 100 m of bottom sediments are required to:assess the stability of gravity base structures and artificial islands under large horizontal ice loads during year-round operations.estimate the available holding power of the mooring system of floating drilling units.identify any shallow drilling hazards such as shallow gas, permafrost and gas hydrates.identify potential borrow material. Present site testing techniques include:high resolution shallow seismic mapping over a 5 sq. km area on an approximate 250 m to 500 m grid pattern to give a three dimensional image of site stratigraphy complemented bygeotechnical sampled boreholes and in-situ tests to obtain detailed engineering parameters and stratigraphy of a specific foundation site. Vertical seismic profiles (VSP) were introduced into the geotechnical program in 1983 on a trial basis. Currently it is unusual to use this method in the geotechnical context offshore. The purpose of the VSP was:to provide a link between seismic and geotechnical data,to provide some conventional geotechnical parameters,to provide a continuous seismic log of an intermittently sampled borehole. This paper describes the VSP operations, presents some preliminary results and outlines some improvements that should be made in operational technique. It appears that the VSP has application as a routine procedure in any offshore geotechnical investigation.

DESCRIPTION AND THEORY OF VSP

The VSP is a method of measuring seismic wave velocity by timing a wave front between two points a known distance apart. The measurement is made between a point on or close to the seabed and a point in a borehole a known distance below the seabed. Measurement is made, therefore, on an approximately vertical ray path. Multiples, tube waves, etc., are allowed for in the interpretation.