bluuebunny/crossref_metadata_2025 huggingface dataset:
Development and Testing of an Active Motion Compensation System for Production System Support
Authors: Maxwell J. Patrick , Hughes E.W.
Published: October 2008

An active motion compensation system has been successfully tested which will allow hardware such as control pods and choke modules to be lowered and mounted to seabed installed hardware from a DP monohull multi-service vessel. The compensation system attenuates vessel vertical heave effects on the liftline and sub sea equipment in up to sea state five. This system enables an MSV to provide cost effective support to deepwater production facilities by allowing shock and vibration sensitive sub sea equipment to be run, set and recovered from the MSV. Testing instruments monitored the vessel, liftline, a subsea test package and the compensator control. Offshore testing is essential to determine the system's capabilities and suitability for intended purpose.

INTRODUCTION

Production systems are being installed sub sea at greater depths and in more severe climates. At depths greater then 1,000 FSW, dynamically positioned (DP) monohull multipurpose service vessels (MSV) can provide cost-effective support, work overs and maintenance to control and maintain sub sea systems. A monohull MSV equipped with a motion compensation system can provide a cost-effective alternative to drill ships and semi submersibles.

The motion compensator pre-engineeringspecified the operating parameters, including a testing program to coincide and guide system development, and confirm that operating parameters were met. The design and operating parameters are as follows:Working depth of 4,000 FSWHandle loads from 2,000 to 40,000 poundsNot sensitive to load variationsVessel heave periods of 6 to 13 secondsDouble heave amplitude of 15 feetMaintain sub sea package +/- six inches from a fixed referenceMaximum velocity of sub sea package is 1 foot/second (ft/sec)Interface with existing winch systemFit in deck space allotted

Due to the wide load range specified, the requirement to set down and recover equipment, and the level of compensation required, an active motion compensation system was selected rather than a passive unit. The handling system layout, including the winch, sheaves, and motion compensator is shown on Figure 1. Figure 2 shows the details of the motion compensator hydraulic skid.

The active motion compensator's hydrauliccylinder drives a traveling sheave assembly that interfaces with the liftline. In the compensating mode as the vessel heaves down the cylinder rod retracts, shortening the liftline. As the vessel heaves up the cylinder rod extends paying out cable. An accelerometer package mounted by the over boarding sheave measures vessel vertical motion. The compensator control for processing accelerometer outputs 'is located on the bridge. outputs are double integrated to determine vessel heave displacement. This signal regulates cylinder hydraulics so the traveling sheave mirrors vessel heave. The motion compensator control process is illustrated in Figure 3.
Integrity Testing of Three Drilled and Grouted Piles
Authors: Briaud J-L. , Dupin A.M.
Published: October 2008

THE PROJECT

The work described in this article was done to evaluate the ability of oil field grout logging technology to determ1ne the integrity of drilled and grouted piles.

Three drilled and grouted piles were constructed at one of the two Texas A&M University sites for geotechnical experimentation. At the site the soil is a stiff to very stiff clay. One boring with Shelby tube sampling and pressure meter testing was drilled. Three cone electrometer soundings were also performed. Unconsolidated Untrained Tests and some Drained Direct Shear Tests were performed. Figure 1 gives a summary of the soil data.

The drilled and grouted pile designed for logging purposes was 110 ft (33.5 m) long, and had an average diameter equal to 30 in. (0.76 m). The steel casing was 20 in. O.D. (0.51 m) and had a wall thickness equal to 0.75 in. (19 mm). The hole was drilled dry, the grout was placed at the bottom of the hole and the closed end steel casing was lowered to the bottom squeezing the grout up in the annulus. From 0 to 20 ft (0 to 6.1 m) the annulus was made of loose cuttings of clay, from 20 to 40 ft (6.1 to 12.2 m) the annulus was left unfilled by creating a 360° chamber void, from 40 to 60 ft (12.2 to 183 m) a 45° vertical channel void was created in the grout curtain, from 60 ft to 85 ft (18.3 to 26 m) the pile was fully grouted with a 5 in. (127 mm) thick grout curtain and from 85 to 110 ft (26 to 33.5 m) the pile was fully grouted with a 2 in. (5 mm) thick grout curtain. Figure 2 shows the pile.

The two drilled and grouted piles designed for capacity determination were 40 ft (12.2 m) long and 8 in. (203 mm) in diameter. The steel casings were 5 in. (127 mm) in diameter. The holes were drilled with betonies drilling mud, the grout was tremied to the bottom of the hole and the closed end casings were slowly lowered in place. The grout filled the annulus from 40 ft (12.2 m) depth to 5 ft (1.52 m) depth. The no defect pile had no purposely designed defects, while the defective pile had a 45° defect from 5 ft (1.52 m) to 25 ft (7.6 m) depth and a 45° defect from 15 ft (4.57 m) to 35 ft (10.7 m) depth at 180° from the other defect. Figure 3 shows the defective pile.

The logging pile and to a lesser extent the two load test piles were logged by Schlumberger with a series of sonic, ultrasonic, gamma-gamma, and neutron logging tools. A well head was placed on the three piles so that the logging could be done at various pressure levels.

The two load test piles were subjected to various tension load tests after the logging. This article gives the details of the logging part of the work.
Dual Sensors for Improved Resolution in Marine Seismic Data
Authors: Barr F.J. , Sanders J.I.
Published: October 2008

The use of both pressure and velocity detectors at each station in a water-bottom cable has been theoretically investigated and field tested. As a result of this study, we present a method for combining the two signals recorded at each station in such a manner that receiver-station, water column reverberations associated with each reflection wavelet are essentially eliminated from the data.

In addition to being less operationally complicated than using pressure detectors at two depths, we've found the dual-sensor method to be less sensitive to noise sources in the water column.

INTRODUCTION

The performance of 3D surveys using pressure detectors in water-bottom cables is well established (Rigsby et al., 1987). The method has proven itself effective for obtaining full 3D coverage in areas too congested with obstacles for normal towed streamer operations.

But the ghost reflection from the air-water interface which accompanies each reflection wavelet is a problem inherent in the method. Of course it shares this problem with the towed-streamer method, but the time delay between reflection wavelet and ghost is greater because the detectors are farther removed from the air-water interface. And as water depths increase to hundreds of feet, concern grows regarding the effectiveness of disconsolation techniques in removing the ghost reflection and other water-column reverberations.

Two basic approaches have been proposed in the literature for eliminating the ghost reflection. The first involves recording signals from detectors at different depths and performing a wave field separation (Bell and Cox, 1987; Brink and Swenson, 1987).

The second, and operationally more straightforward method utilizes paired pressure and velocity detectors at identical locations (Haggerty, 1956; Gal' Erin, 1974; Loewenthal et al., 1985). This method takes advantage of the fact that the two types of detectors generate signals of the same polarity for upward traveling waves, but of opposite polarity for downward- traveling waves (the ghost reflection).

ACOUSTICS AT THE WATER-BOTTOM

To determine how to most effectively combine the signals from the pressure and velocity detectors, it is necessary to solve the elastic wave equation subject to appropriate boundary conditions for the model shown in Figure 1 (White, 1965). In this figure, A's represent compression waves and B's represent shear waves.

Converting the solutions for pressure and vertical particle velocity at the water-bottom from the spatial and temporal frequency-domain to the time-domain, one obtains the pressure and velocity detector signals shown in Figure 2 (assuming detector transduction constants of unity and infinite bandwidth).
Fatigue Design and Overall Safety of Grouted Pile Sleeve Connections
Authors: Ingebrigtsen T. , Loset Ø. , Nielsen S.G.
Published: October 2008

This paper covers two main subjects. Firstly the main results obtained in a joint industry research project on the strength of grouted pile sleeve connections in fixed offshore steel structures are presented. The testing and the specimens are described and guidance is given for both static and fatigue design. Secondly the main elements and results of a safety calibration study on the ultimate strength of such connections are presented.

INTRODUCTION

The purpose of the research project was to provide new and improved design formula for the capacity calculation of axially loaded, grouted pile sleeve connections. This was obtained through new and already established test results on reduced scale specimens. Both plain pipe and shear key connections were tested. The project 1 produced61 test results on the static strength of grouted pile sleeve connections. Together with other available data these have formed the basis of the development work. The total data base comprises202 test results.

In the first half of the 1980's Department ofEnergy (DEn), UK', and American Petroleum Institute(API)6 issued revised design regulations for grouted pile sleeve connections. DEn used an overall safety factor of 6.0 (4.5) for the grouted pile to sleeve connection. API, which uses the allowable stress method, does not express the overall safety factor explicitly. It is implicit in the allowable stresses specified. An overall safety level of about 4.5 is aimed at7. This is significantly greater than that used for other types of structural members and connections. A level of 1.5-2.0 on characteristic values is generally used in the offshore design codes.

In the studies the level of the material factor was calibrated within in safety format of the Norwegian Petroleum Directorate(NPD)8. The DEn and API design equations were used. The 202 test results were valuable support for the study together with records from offshore installation work concerning obtained grout strength and frequency of packer failure.

The NPD-regulations differentiate between uncertainties connected to the loads and those to the material. This is appropriate for a safety calibration study

STRHNGTHINVESTIGATION

GEOMETRY OF SPECIMENS

The cross sectionalize of piles and sleeves used are mainly 1/5 to 1/3 compared to offshore size. A few specimens of full scale diameter and thickness have been tested. The principal geometry of the grouted pile-sleeve connections tested is shown in Figure 1.

All specimens were fabricated from standard size seamless pipes satisfying the requirements of ASTM - A106 grade B.

To eliminate the effects of variations in surface condition the outer surface of inner pipe (i.e. that in contact with grout) was sandblasted, with a preparation grade BSa 2% in accordance with Swedish StandardS1S 05-5900 - 1967. Then the surface was allowed to corrode slightly in humid air, but not directly subjected to water.
Systems for ROV Actuation and Repair of Pipeline Ball Valves
Authors: Baugh B.F. , Tillinghast W.S.
Published: October 2008

ABTRACT

The selection, design, and testing of 12 remote ROV to ball'alve interfaces on the Conoco Jolliet sub sea pipeline system in 1080 feet of water in the Gulf of Mexico are discussed. Test results and graphics information developed are presented.

The selection of the public domain RU-2 profiles and this development program provided 5000 ft.-Ibs. torque capability, remote repair sealant injection capability, position indication, and same profile size application on all'alve sizes considered. The resulting installed profiles will allow competitive bidding of service operations during the lifetime of this equipment.

INTRODUCTION

This paper describes the decisions leading to the installation of 12 remote ROV interfaces on ball'alves in a sub sea pipeline system in the Gulf of Mexico. The pipelines connected the Jolliet TLWP (Tension Leg Wellhead Platform) in 1760 feet of water in the Gulf of Mexico back to more conventional facilities in 616 feet of water. The Unified Connection Skid housing most of the'alves and attached profiles discussed was in 1080 feet of water. It served as a connection point in the flexible pipeline system where further production can be conveniently connected.

Additional information on the overall project can be found in the 1990 OTC paper no. 6359 titled "Jolliet - The Project". More specific information about the pipeline project which these ball'alves are a part of can be found in 1990 OTC paper no. 6403 titled "The Deepwater Pipeline System on Conoco's Jolliet Project"

This subject is of particular interest because it reflects a change in the nature of the ROV related business from one of limited capability manipulators and/or proprietary profiles to one of multipurpose public domain profiles. The initiation of public domain profiles is considered to be key in the development of a'olume of economic sub sea applications for ROV type service systems.

ROV?s operations have characteristically been limited as:Being a flying eyeball (TV camera),Doing low capacity manipulator tasks (mostly unplanned tasks),Using non-repetitive prototype tooling with a high cost to benefit ratio, andProviding service operations which are locked into a specific service contractor. This project takes a substantial step toward:Providing multifunction tooling to fit a maximum number of applications,Providing repetitive construction,Providing standardized tooling within the public domain, andProviding a'iable basis for economic competitively bid service operations. Almost certainly this project will be recognized as a significant step in the development process of sub sea service systems. The developed technology will find numerous applications on sub sea completions, templates, and BOP stacks. It is even being reviewed for a Navy project.
At Sea Test: Hawaii Deepwater Cable Program
Authors: McHale F. , Van Ryzin J.
Published: October 2008

In order to investigate and demonstrate the technical feasibility of laying a Submarine power cable in the open ocean, in depths exceeding 6000 feet, over difficult volcanic island seabed conditions, a computer assisted control system for cable vessels was developed. The control system was successfully tested at sea in November 1989, achieving three-meter placement accuracy while laying cable along a pre-selected curved route on the seabed.

INTRODUCTION

A major challenge to laying a proposed power cable between the islands of Maui and Hawaii in the state of Hawaii is the 1900-meter deep Alenuihaha Channel, known for its difficult currents and waves, with an especially difficult rough bottom terrain, necessitating a very narrow and winding cable path. The United states Department of Energy sponsored the Hawaii Deep water Cable Program (HDWC) with the objective of determining the feasibility of laying and operating a Submarine power cable in these waters. After the channel bottom was surveyed by the HDWC program, the best cable route identified during the surveys was found to narrow in one area to a width of 80 m. Since three cables are required for the final power system, it was decided that an at-sea test would be necessary to determine the feasibility of laying a cable to an accuracy of ±11 m on the seabed.

In addition, the rough and steeply sloping bottom terrain, leading to the possibility of unacceptable cable spans on the bottom, required the cable be laid with controlled tension in the various portions of the route.

Consideration of the high tensions to be encountered during laying, with 1900 m of cable suspended from the cable ship, led to a cable design with contra helically wound armor layers. This results in a cable which cannot be coiled in a tank, as in the typical case of submarine communications cables. The power cable being considered here requires storage on a revolving reel or turntable on the laying vessel, and must be kept under some tension at all times. The HDWC program developed an integrated control system to control the laying of this cable in these special conditions, and carried out an at-sea test program, laying the cable in the actual ocean positions, and recording data for use by the designers of the future commercial cable.

CABLE ROUTE SURVEY

The future permanent power cable will have a wet weight of about 27 kg/m and a diameter of about 12 cm. When laid over the sea bottom in the Hawaiian Islands, the cable will encounter bottom roughness due to old lava flows, submerged shorelines and reefs, escarpments and boulder fields. The cable may be laid over obstacles or suspended between obstacles in brief spans; the possibility of damage due to bending over obstacles, or fatigue caused over time by vortex-induced oscillations induced by tidal currents, must be avoided
Wave-Impact Loads on Large-Diameter Horizontal Cylinder Placed Above Still Water Level
Authors: Davies R.L. , Martin J.G.
Published: October 2008

This paper gives information on results of studies performed in relation to the installation of the Protective Barrier around the Ekofisk Tank, in particular the wave loads and wave impact forces on large diameter cylinders. The studies have been conducted on 10.15m in diameter, 40m long cylinders (prototype scale), placed above still water level. in such a way that they could be partially submerged by storm waves. Both vertical and horizontal impact forces have beenanalysed. The paper presents the model tests which have been undertaken as well as typical results. The paper also presents the theoretical model which has been set up and results from the corresponding mathematical simulation.

INTRODUCTION

The Ekofisk 2/4 Tank is 1inked to adjacent platforms by four bridges (See Fig.l). The combined effect of the subsidence at theEkofisk field and of the wave enhancement created by the Protective Barrier results in the bridges being partially submerged by the storm waves (See Fig.2). Studies were undertaken in order to evaluate the consequences of partial Submergence of the bridges; two concepts were envisaged : the shroud concept where the bridges would be enclosed in a large protective cylinder. or the bare bridge concept where the bridges will be made as transparent as possible to the waves by stripping away all cladd1ng and other unnecessary obstructions. The bare bridgesolution is the one which has actually been implemented at Ekofisk; however, studies concerning the shroud concept were carried up to a basic design stage.

In the shroud concept. the bridges were totally enclosed in a protective cylinder (See Fig.3). resting on the Protective Barrier at one end and at the adjacent jacket at the other end. Depending on the size of the bridge to be protected, the necessary shroud diameters were ranging from 8 to 12 meters. the maximum Submergence (referring to the bottom chord of the shroud) being in the order of 14 meters. The designer was then faced with the problem of the determination of the wave forces on large diameter cylinder submitted to impact loads. A bibliographic review showed that there had been many publications dealing with the problem of wave impact loads. that the theory was really well established, and that model tests had been conducted. However, the studies and model tests mainly dealt with the problem of vertical slamming forces on horizontal jacket members situated in the splash zone, the aim being to determine slamming coefficient for the design of those members. In our case we had to dea1 not only with the impact force, but with thewhole time history of the wave force over awave cycle. with the reaction at both ends of the shroud, with oblique impacts and also with the horizontal component of the force. Based on existing publications a theoretical model was set up. At the same time model tests were undertaken. as they were deemed necessary to calibrate any the oretica1 mode1. The mode1 tests. the theoretical model and thecorresponding results are presented hereafter.
Propagation of Solid Particles in Single and Multiphase Flow Pipes and Pipelines
Authors: Lingelem M. , Nuland S.
Published: October 2008

1.ABSTRACT

The transport of glass particles in a 32 mm ID plastic pipe has been studied using water, viscous water-glycol solutions and water with 500 ppm of corrosion inhibitor added. Particle sizes ranging from 30µm to 200 µm in batches from 0.1 g to 109 has been used. The particles have been activated in a nuclear reactor so that the particle velocities in very dilute suspensions can be measured using radiation detectors.

A critical velocity is determined, above which the particles are suspended in the flowing fluid. This critical velocity increases with viscosity. Above the critical velocity, the particle velocity gradually approaches the average fluid velocity, and for the most viscous fluid may even exceed it. Turbulence is necessary but not sufficient to suspend the particles in the flow.

To remove most of the particles a velocity three times the critical one is needed, the presence of film forming, amine type corrosion inhibitor makes removal of 30µm particles very difficult.

2.INTRODUCTION

Certain amounts of sand may be produced from sand reservoirs, even when completion techniques have been employed to prevent sand production. In long transport lines for untreated hydrocarbons, the accumulation of even small amounts of particles may cause problems when the line is cleaned by pigging. For slurry transport it is well known that a critical velocity exists, below which the particles movein the form of dunes, above that velocity they are suspended in the fluid. Turian et al. has published a survey of correlations for this critical flow, but it is difficult to determine if these correlations apply at the low particle concentrations of concern in this application. Most correlation indicate that the critical velocity approaches zero as concentration approaches zero, which is definitely not the case.

When long hydraulic lines are used to control wellhead equipment, extreme cleanliness is required to ensure reliable operation of the equipment. The line is typically welded together of short pipes, and prior to the installation all particles thathave settled into the pipe has to be flushed out. The particles may be a product of the welding process, or they may be characterized as normal industrial dust as will be present in most industrial environments where such jobs are performed. The appropriate standards require that particles down to 10µm are removed. When such particles have settled, they will be buried deep into the laminar sublayer of flows of attainable velocities. The problem is most acute for long length of pipes (pipes in excess of 50 km has been experienced) that need to be cleaned. The burst strength of thepipe limits the pressure drop that can be applied, resulting in a limited flushing velocity.

For such long pipes it is also important to know the velocity of the particles in order to determine the time needed to clean the pipe. Due to slip between liquid and particles, the particles may not travel at the bulk velocity of the liquid.
Improved Mechanical Connectors for Subsea Tree Applications
Authors: Singeetham S.P.
Published: October 2008

This paper describes the design, analysis and testing of "The Split Clamp", a device which can be used for strengthening any flanged connection. For a given space envelope, the split clamp connection is significantly stronger than any conventionally used flange or clamp connection.

A split clamp device was designed and built for a modified 11"-5,000 psi API flange. This prototype assembly was tested to 20 ksi internal pressure and to1.1 million ft-lbs bending moment with various torque values on the flange bolts. Finite Element Analysis(FEA) was also conducted and the results were compared.

INTRODUCTION

API flange bolted connections have traditionally been used to fasten several forged body components together for many subsea wellhead and completion systems. As equipment is i nsta11 ed in ever deeper waters, the bending moment associated with drilling or completion risers grows. Recently, many subsea equipment assemblies have used large diameter high pressure flanges (e.g., a 13-5/8-IOK flange on a 4" × 2"-5K working pressure tree) to withstand high bending moments on tree end connections. These larger flanges have diameters much larger than what is usually needed for the rest of the forged body. This results inspecially shaped forgings or larger and heavier forgings which are expensive.

The split clamp devices were originally designed for use on deep water (3,200 - 5,800 ft) guideline less subsea completion trees being installed offshoreBrazil. Riser Analysis [1] was conducted to determine the bending moment on the guide1i ne1ess tree end flanges. It was determined that when tie 9-5/8" completion riser is subjected to the environmental conditions shown in Table 1, the maX1mum bend1ngmoment on the connection at the base of tree canbe as high as 1.1 million ft-)bs. The bending moment capacity of an API 68 1l"-5K flange at 5,000 PSl internal pressure is 350,000 lbs [2] and that of a 6BX I3-5/8"-10K flange is 800,000 ft-lbs. This necessitated significantly stronger end connect10ns, on the trees, than the above specified API flanges which are typically used on subsea trees.

The split clamp device increased the strength of the connection without increasing the overall size of the subsea tree composite block valve.

DESIGN

The split clamp device (see Figure 1) is designed for insta11 action on any flanged connection. It consists of a split ring (see Figure 2) and annular outer ring. The device is assembled by firstthreading the outer ring over the lower half (threaded) of the flange connection. Next, flangehalves are made up with the AX type gasket 1ns1de. The flanges are made up and preloaded with the flange bolts in the same manner as conventiona1 flanges, except the preload value is higher than recommended for a conventional flange. The two halves of. the split ring are then mounted and bolted together in agroove around the upper half of the flange connection.
Environmental Considerations of Nodule Mining in Central Indian Basin
Authors: Sharma R. , Rao A.
Published: October 2008

The status of benthic population and their physical and chemical environment in the CIB have been evaluated. The information on nodule populations and their association with benthic activity leads to prediction of potential mortality of benthic fauna as well as the area of seafloor and the volume of sediment expected to be disturbed due to nodule mining. The resulting environmental implications due to large scale mining are expected to be in the form of 'plume' at the seafloor, turbidity in the water column and addition of bottom waters to the surface resulting in change in productivity patterns. Hence, the design of mining system should ensure that the sediments are seived off at the seafloor, strip mining be carried out to allow repopulation by nearby organisms and that there is no effluent discharge in the water column or at the surface to avoid change in ecology due to transportation by currents. However, the location of the CIB is such that the impact of mining on environment is likely to be minimal.

INTRODUCTION

India has embarked on the second stage of its endeavour towards exploitation of nodule deposits in the CIB. After having evaluated the potential resource available and having acquired exclusive rights under the UN law of the sea, it is conducting seafloor surveys for identification of mine-site and areas for relinquishing. At this stage, a review of various environmental characteristics of this area is essential, so as to use this information in the design of the proposed mining system as well as to predict the potential impact.

The mining activity is expected to cause large scale disturbance not only at the seabed from where nodules will be lifted, but also in the water column as well as at sea surface resulting in widespread changes in the marine ecosystem. The potential impact of mining, based on model studies and pilot mining tests, is known to be varied and to some extent unpredictable1 - 11 and needs a cautious approach towards the problem of environmental conservation.

This paper evaluates the existing environmental status of the CIB as well as the association of nodules with seafloor features. Based on this information, an attempt is made to predict potential disturbance in terms of seafloor area as well as volume of sediment and mortality of benthic organisms due to large scale mining activity in the area. Certain design parameters to minimise the deletrious effects of mining have also been dicussed.

DISTRIBUTION OF NODULES AND ASSOCIATED FEATURES

The seabed comprises of bottom sediments, diverse sedimentary structures mainly of biogenic origin, polymetallic nodules, massive rocky exposures and mineral encrustations, as well as the abyssal benthos. Of the three main varieties of sediment found in the Central Indian Basin, siliceous ooze predominates the area. There is a thin strip of calcareous foraminiferal and coccolithous ooze in the west, while in the south there are red clays12.
Design Improvements in Box Dredges To Enhance the Collection of Manganese Nodules
Authors: Fernando V. , Sonawane A.V.
Published: October 2008

Design of three box dredges along with the technique and modifications effected to enable bulk sampling of manganese nodules is presented. Incorporation of a sled, angled cutting edges, retention plates and increased size of the dredge resulted in significant increase of the dredge haul. Consequent field data is included. Design of a simple feeding concept to enhance the concentration of nodules at the dredge mouth is briefly dealt with for future application.

INTRODUCTION

It was from a routine dredge haul in the Atlantic Ocean that manganese nodules were first discovered in 18731. This century old technique of drag-line dredging continues to be universally used within the Oceanographic community for collecting manganese nodules and consolidated substrate materials. The analysis from the samples collected from dredge hauls have been the subjects of numerous publications.

The universal use of the bucket or box dredge can be attributed to its dual ability to function both as a reconnaissance survey tool and as a bulk sampler. Other favorable characteristics are its simple design, low cost, the flexibility it allows to operate in almost any water depth and adaptability to almost any research vessel having the necessary deck gear like a winch, wire rope and an 'A' frame. Generally considered to be an archaic and blind sampling method, often they are the only tools available for bulk sampling of seafloor substrate2. Compared to its wide spread use, sparse literature relating to improvement in dredge designs for collecting manganese nodules and crust seem to exist3–8.

The purpose of this paper is to present the evolution of designs and refinements carried out in the dredges to enable bulk sampling of manganese nodules and encrustations during surveys in the Central Indian Basin (CIB).

HISTORY AND BACKGROUND

For India, the National Institute of Oceanography's Research Vessel R.V. Gaveshani collected the first nodules on 26th January 1981 from the Indian Ocean. Extensive exploration in the CIB lead to the recognition of India as a "Pioneer Investor" in sea bed mineral exploration in April 1982. The first dredge was designed during this period for collecting manganese nodules. The 10mm diameter cable on board R.V. Gaveshani having a breaking load of 5.4 tons restricted the development of a larger dredge. The period August '82 to March '88 saw accelerated activity in the Central Indian Basin. Three chartered vessels M.V. Skandi Surveyor, M.V. Farnella, M.V. G.A Reay in addition to the Indian research vessel ORV Sagar Kanya were deployed during different periods for the project 'Surveys for Polymetallic Nodules'. The requirement of tons of manganese nodules for development of metallurgical process resulted in the design and operation of various types of dredges.

DESIGN CONSIDERATIONS

Ship size and deck machinery apart, the diameter and strength of the wire rope chosen for dredging directly influences the design of a dredge.
Concrete TLP for Sub-Arctic Conditions
Authors: Karsan D.I. , Borreson R.
Published: October 2008

Conoco And Norwegian Contractors have recently completed a study on the feasibility of using floating structures for production of hydrocarbons in the Norwegian Barents Sea. This location is in relatively deeper water depths of about 500M, and occasional Icebergs and other ice features may be encountered. A survey of feasible floating system configurations resulted in the selection of a deep draft concrete TLP with four corner columns and one central riser protection column. Comer columns are protected by cable type fendering units. Other fender types were also studied. This study demonstrated the feasibility of using TLP's in deep Sub-Arctic waters for oil and gas production. The paper presents the studied hull and fender configurations and associated geometric, cost, and schedule data.

INTRODUCTION

The area of interest lies in the Northern Barents Sea in the vicinity of 74 degrees North Latitude and 16 Degrees East Longitude. Sea ice can extend into this region as often as three years in every ten year period, and as much as 40% of the sea can be covered with ice. Additionally, icebergs may drift into the area. The structures used for production must therefore be able to withstand the ice floes and pressure ridges that may occur.

Water depths in parts of the area can be as deep as 500M (16500). For these water depths, and expected magnitudes of iceberg impact loadings, fixed platforms, such as a concrete Gravity Based Structure (GBS), will be costly. Therefore, only compliant floating structures have been considered.

DESIGN CONDITIONS

Since the objective of the study was to develop a platform concept, only the hull, mooring, and riser systems were evaluated. Deck was simulated as a box with 100M square plan area and 45M height. Payload, including the deck structure was 35,000 metric tons (tonne) dry and 52,000 tonne wet weight (including riser pretension, snow, and ice on deck), with 200, OOOBOPO plus associated gas production capacity, 46 each 9-5(8in 00 production and two 18in. OD pipeline risers. Design life was 50 years.

100 Year ULS wave, wind and current conditions were represented by a maximum wave Height of 24.3M with 14.5 S period, an associated 1-minute sustained wind speed of 40M/S, and a surface current of 0.85M/S.

Maximum 100 year ULS Iceberg size was represented by a 500,000 tonne ice mass with 20M sail height, 60M keel draft drifting at a velocity of 0.6M/S. Iceberg crushing strength was taken as 2.0MPa for global force calculations and 4.0MPa for local design. No other environmental effects were combined with loads from the 500,000 tonne iceberg.

Maximum 100 year ULS iceberg plus wave loading combination was represented by a 50,000 tonne iceberg (also defined as the 10 Year Iceberg), set to motion by a 7.7M significant height and 13.0S spectral peak period wave (also defined as the 10 year wave). A wave/iceberg Interaction study indicated that the significant iceberg impact velocities generated by this combination will be in the order of 2M/S.
New Developments in Prediction of Mooring System Damping
Authors: Huse E.
Published: October 2008

Previous OTC papers (Refs. 1, 2, 3) have shown that drag forces on the mooring lines can represent the dominating contribution to the surge damping of moored vessels. They have also shown that in order to calculate this damping effect correctly one normally has to take into account the effect of the wave-frequency motions of the vessel, superposed on the low-frequency, resonant motions, whose damping we wish to predict. The present paper suggests a simple and practical procedure for this purpose. It applies the concept of "apparent drag coefficient" in combination with an iterative use of the catenary equations to obtain the low-frequency damping. The range of applicability of this procedure covers most practical mooring configurations. Only in cases of very deep water, heavy lines, and suspended clump weights it may be necessary to use a complete FEM procedure to get the effect of wave-frequency motions and line dynamics accurately described.

The present paper also publishes for the first time results from investigations of the possible heave damping effect of the mooring system. It turns out to be normally negligible for moored ships, moderate but significant for semisubmersibles, and can be dominating the heave damping of spar buoys.

1. INTRODUCTION

An important feature of most moored offshore structures is the pronounced oscillatory motions at their low, resonant frequencies. Together with the motions at wave frequencies they determine peak offsets, peak mooring line loads, riser performance, and other important engineering aspects of the structure. These low frequency oscillations take place outside the frequency range of the waves. They are excited by second order wave forces, by non-linear viscous reaction forces to wave frequency responses, and to some extent by wind gusting.

Since we are dealing with low-damped systems at resonance, the LF (= low frequency) motion amplitudes are very dependent on system damping. Main contributions to for instance the surge damping of a moored vessel are due to wave drift, drag and skin friction on the main structure, and drag forces on the mooring lines.

Until just a few years ago it was customary to assume that the mooring system influenced the motions of a moored structure only by its horizontal stiffness, determining the surge and sway resonant periods of the structure. Any effect of the mooring system upon the damping of resonant motions was neglected, assuming that the drag area of the mooring lines was too small to be significant, compared to the drag damping of the structure itself.

Then in 1986 a first publication was issued, showing that the mooring system can be the main source of surge damping of a moored structure (Ref. 1). Further OTC publications in 1988 and 1989 (Refs.2 and 3) presented different attempts at calculating the surge damping effect of the mooring system, in addition to presenting more experimental information on the subject.

The objective of the present paper is to give a survey of some recent developments of numerical methods to predict mooring line damping.
Loss Control by Subsea Barrier Valve for Offshore Gas Pipeline
Authors: Kunsch J-P. , Sjøen K. , Fanneløp T.K.
Published: October 2008

Platform-mounted shut-down valves for offshore pipeline systems, are vulnerable to fire and explosion hazards. Should this primary barrier fail, the full inventory of the pipeline could be consumed in the fire. A secondary subsea barrier would reduce substantially the loss of pipeline inventory, but also expose the platform to a new hazard. Valve failure could lead to the formation of gas or oil plumes which could be carried towards the platform. The choice of subsea barrier location, requires a trade-off between these safety concerns. The paper presents one prerequisite for such a trade-off; an analysis of the potential gas loss for different secondary-barrier positions and valve-closing rates when a guillotine break occurs at platform deck level.

The paper gives a detailed study of the flow characteristics close to the valve and over the length of the segment between the barrier valve and the rupture. Particular attention is paid to the boundary conditions at the valve. These have been formulated based on the method of characteristics. Computer simulations using the simplifying assumption of isothermal flow have been compared to the adiabatic case. The differences between these cases have been found to be small.

Problem Discussion.

We consider the situation depicted in Fig. 1. The subsea pipeline is connected to the process equipment on the platform through a vertical segment. In the case of a fire or explosion on the platform, a possible pipeline failure would be at deck level, as indicated by the symbol R. At the broken end a counterpressure of about one bar exists. Two subsea barrier valves, one upstream at location A, the other downstream at location B, are installed to prevent a major loss of gas in case of flowline rupture. It is assumed that the barrier valves start to close at the same time as the guillotine rupture occurs. The aim of the present investigations is to determine the mass loss and flow variables in the pipeline. A good physical discussion of the pipeline break problem without valve is given by [1],[2] and a more detailed mathematical treatment is developed in [3],[4].

The length L of the pipeline segment between the valve and the broken end as well as the valve closing time TClose are considered to be two dominant parameters influencing the rate of outflow. The range of lengths L considered varies from an upper limit of 2000m to a lower limit imposed by the riser height (or ocean depth). The closing time is chosen in the technically realizable range of 40 s to 100 s. Two cases are of interest, one representing flow towards the platform (A) and one away from it (B). The pressure level will in general be higher with flow away from the platform. More important is the pressure gradient in the steady state. For Case B, the break leads to a flow reversal from the steady state direction.
Diverless Maintenance of a Subsea Barrier Valve
Authors: Jefferies A. , Partridge C.C.
Published: October 2008

Subsea barrier valves are used to prevent backflow of an offshore pipeline in the event of an upstream line rupture. These valves are either a ball valve or check valve configuration. As offshore production systems and their associated pipelines move into deeper waters, the maintenance of these valves becomes .more difficult and expensive due to the cost of diving operations. Eventually these valves are placed in locations where the water depth precludes the use of divers for maintenance tasks. It is then necessary to develop alternate maintenance methods for these valves. This paper outlines the development, testing, and deployment of a check type barrier valve for use in water depths beyond diver access. The system follows a maintenance through retrieval approach, using an intervention tool run from the surface on guidelines to retrieve the primary active components to the surface for replacement or repair.

INTRODUCTION

A 10" ANSI 900# (+) check type barrier valve system was developed for installation on a TLP drilling template in the Norwegion sector of the North sea. This template is installed in a water depth of 1010 ft, placing it beyond the depth normally associated with saturation diving operations. With the practical elimination of diver intervention to the barrier valve, and with the increased emphasis on subsea hardware maintenance and repair, the barrier valve system was configured for diverless operation and maintenance.

The barrier valve system includes a retrievable actuator which is used to place the valve in a locked open position to allow the passage of an intelligent inspection pig. This actuator is primarily controlled hydraulically via an acoustic/hydraulic power pack (provided by others). In the event of a control system failure, the valve may be opened via an ROV override feature built into the actuator.

The unique feature of this barrier valve is the ability to retrieve the internal working components as a single cartridge without diver intervention. This is achieved by using a collet type connector to attach the valve cover to the valve body and by using a hydraulically controlled valve intervention tool (VIT) to release and set that connector. The complete system is depicted in Figure 1.

SYSTEM DESIGN AND FUNCITONAL REQUIREMENTS

The barrier valve system was designed to interface with the other systems in the TLP production system, consequently these interface points were documented and set as limiting design criteria. Also, while staying within these criteria, the system was to be able to meet specific functional requirements. The limiting criteria and functional requirements for the system are documented below.

Design Criteria

The barrier valve system was designed to operate in a 10" gas pipeline at the base of the TLP riser, 1010 ft from the surface. The valve was designed for a maximum working pressure of 2285 psi, but unlike ANSI valves which are tested to 1.5 times working pressure, the barrier valve test pressure was 4570 psi (2 × working).
TOGI: Multiphase Flow From Troll to Oseberg
Authors: Lingelem M.N.
Published: October 2008

The TOGI 20" pipeline has been designed to transport unprocessed well fluid 48.5 Km from a subsea installation to an offshore production platform.

The paper describes how the pipeline has been designed to handle potential problems related to multiphase flow transport and gives data from measurements carried out during operation.

INTRODUCTION

Transport of unprocessed well streams have been with the oil and gas industry since it started, The use of the technology however, was limited to short gathering systems and it is fair to state that the underlaying technology was poorly understood,

In recent years the potential of exploring the use of multiphase transport and in particular the transport of unprocessed well streams has been clearly seen. In connection with wellhead platforms and sub sea developments especially the potential savings are most significant.

For multiphase transport lines having a starting point on shore or at a manned offshore platform it is possible to control the liquid accumulation through pigging. In this way production at low flowrates where liquids tend to accumulate can be tolerated as regular pigging (e.g daily) can be carried out to remove excessive liquid.

Multiphase transport lines originating subsea can normally not be pigged on a regular basis as the pigging operation requires the use of a drill rig, For such lines the importance of detail understanding of the multiphase flow is paramount, and the determination of a proper operational envelope is one of the most important tasks of the early design.

The TOGI development.

The TOGI production system will transport gas from the Troll field through a 20" pipeline 48 Km to the Oseberg field where the gas is processed, compressed and injected into the gas cap of the Oseberg reservoir (Fig l). The gas injection improves the total oil recovery by in excess of 7% over the originally planned water injection system.

The TOGI subsea station is located at 303 m of water depth at the south western part of the Troll East field. There are five wells remotely controlled from Oseberg by an electro-hydraulic control system. Methanol is pressurized at Oseberg and injected through a service line into the wells at Troll.

The pipeline has a 0.47 m inner diameter. The riser has an inner diameter of 0.46 m. From the Oseberg B-platform where there are facilities for receiving pigs, there is a 18" 0.41 m inner diameter line approximately 225 m long connecting the pipeline to the slug catcher located in the process facilities on the A-platform (Fig. 2).

Peak daily production is between 10.5 and 12 106Sm3/d. Minimum production rate is 5 106Sm3/d.

The TOGI fluid is at its hydrocarbon and water dew points at reservoir conditions. Maximum liquid drop out is around 3 % (mass) or 0.4 % (vol). No liquids are produced from the reservoir.

The reservoir pressure will fall around15 bar over the 11 year of planned production.

There are several potential problem areas associated with transport of unprocessed well fluid.