bluuebunny/crossref_metadata_2025_split huggingface dataset:
Reliability Analysis of Offshore Structures in Towing Operation
Authors: Pajouhi K. 
Published: October 2008

Abstract
This paper presents a semi-probabilistic method for evaluating the reliability of tie-down braces for ocean going tow barges,-and a rational design method for tie-down braces compatible with effect of uncertainties in analysis and impact of result offailure.
In receht years, study of response of an offshore towing system to random sea became more important than before because of the increased"" water depths for which platforms are now being designed. Since uncertainties are unavoidable in determination of forcing functions, structural behavior and tolerances within fabrication and installation methods, therefore the tool of engineering analysis should include concepts and methods for evaluating the significane of uncertainties and its implication on the design and performance of the towing system.
Reliability analysis of the towing system is a multi-component analysis; tug, towlines, barge, tiedowns, and the structure (cargo) being in tow itself. The reliability, P, of a towing component can be expressed in termssof probability of failure, pf, defined as the probability of experiencing of this system component failure during the tow period (Pf = Probability [ Load&gt; Resistance]).
The reliability of a tow system can not be based on the "weakest link of a chain approach", but it should reflect reliability of all the tow components in the system. This work is based on the following assumptions; i) a structural component can be in either safe or fail conditions, ii) tugboat propulsion and steering system can also be in either broken or unbroken states, and iii) occurrence of any storm along the tow route is independent of the others. The reliability model for tow system is described as a combined system, being composed of series and parallel system.
The evaluation of probability of occurrence of a design sea state is based on the MARKOV model. The basic assumption used is that given the present state of a seaway, the past states have no influence on the future. The theorem of total probabilities is used to evaluate the system reliability. As an alternative approach, the POISSON model is also used in computing the frequency of occurrence of the design sea state.
An optimum towing system design can be achieved using the reliability analysis in terms of initial cost, probability of failure, and cost of failure in relation with the time-lag in production, and impact on public confidence.

Introduction
The application of the structural reliability concept to offshore tow system requires descriptions of random variables; loads, and resistances for each individual component of the system. However, due to imperfect or non-existent data, the frequency distributions of loads and component resistances can not be predicted exactly Thus engineering judgement and subjective reasoning must be used in conjuction with available statistical data to modify the probabilities and produce an acceptable reliability assessment of the system.
Fatigue Considerations for Ocean Towage
Authors: Hambly Edmund C. , Edwards Antony J. , Kohli Colin C. , Miller Barry L. 
Published: October 2008

Abstract
Fatigue damage of structure, seafastenings, or barge during ocean towage can be a critical consideration in choice of barge and tugs, and in the selection of tow route and marine operations. This paper describes the procedures that were followed to assess the risk of fatigue damage, and the practical precaution aken to control its incidence, during the tow of the jacket for Amoco Norway's Valhall Production Platform from Morgan City, Louisiana, to Stord, Norway, during autumn 1980.
Damage criteria are identified and calculations demonstrated for:- survival of the unit in possible extreme conditions, exposure along the tow route, and derivation of Critical Motion Curve to limit rate of fatigue damage to an acceptable level. The analysis of motion induced stresses is explained, including the rapid methods of calculation adopted to derive hot spot stresses around a complex multi-chord overlapping joint, and to adapt standard S-N curves for direct use for Rayleigh distributed stresses. The theoretical survival life of the unit in maximum seas is shown to be very sensitive to the values determined for parameters such as SCF and wave period.
The paper reports the marine procedures adopted, and describes the fatigue audit that was maintained throughout the tow, which indicated that the theoretical fatigue damage was successfully controlled to a low level.

1.0 Introduction
The jacket for the Valhall Production Platform was completed for Amoco Norway by McDermott Inc. at their yard at Morgan City, and was loaded out onto the barge "Intermac 600" on 9th August 1980. Fig. 1 shows the 4737 L.T. jacket on the 500'x120'x33' barge. The tow commenced on 25th August when all we-ather forecasts were favourable, and the barge was towed by 4 river tugs to Bayou Boeuff. From there the 5000 HP "Dallas J- Adams" towed the unit 20 miles offshore to hand over to the deep draft, ocean going salvage tug "Atlantic", 140 ton bollaI'd pull, of Bugsier Reederei. (Throughout the paper the term "unit" is used to 1.0describe the total structure of the jacket, tie-downs and the barge.)
The tow route is shown in Fig. 2. The tow proceeded without serious barge motions until 8th September when Hurricane Earl passed nearby and the tug had to heave to for a period to limit barge motions to below the limits previously set for control of the rate of fatigue damage. Subsequently steady progress was maintained until a storm was encountered on 18th September near 44°N 21°Wwhen the tow line broke, for reasons not fully understood. This point was close to the predetermined point at which the "Atlantic" was to be joined by the Bugsier tug "Caribic", 84 ton bollard pull, for the remainder of the tow.
Observations on the Mode and Rate of Decay of an Artificial Ice Island in the Alaskan Beaufort Sea
Authors: Reimnitz Erik Reimnitz , Kempema Edward , Ross C. Robin 
Published: October 2008

Abstract
In the Arctic ice is abundant and cheap, and therefore has long been considered as structural material for offshore drilling platforms. In February 1979 Exxon constructed an ice island 400 m in diameter and 10 m thick, in water 3.5 m deep, about 6 km north o£ Prudhoe Bay, Alaska. The purpose of the experiment was to learn whether such an island could survive one summer and then serve as a platform for an extended drilling period the following winter. Our studies of the decay of this island are based on:surface observation and photography,diving observations and bottom photos,fathometer, sub bottom profiler and sides can sonar surveys, and spot measurements of water salinity, temperature, and turbidity around the island.
Thermal erosion, promoted by turbulent heat transfer from wave action and a relatively warm surface water layer surrounding the island, resulted in a 3- to S-m-deep notch at sea level, frequent calving, and a cantilevered, vertical scarp above sea level around the island. The winds, waves, and surface currents that transport warm river water past the island are predominantly from the east. Therefore, thermal erosion and calving were most rapid on the east side; they had truncated the island to the original center point by late August. By early September, the last remnants of the island floated away. The submerged part of the island, below the sea-level notch, consisted of an extensive pedestal reaching as much as 15 m beyond the island and feathering out to 10cm thick at the lip. The outer 5 m of this pedestal was elevated above the flat sea floor, forming a gap up to 25 cm wide that exposed the smooth base of the island to divers. Average island shore erosion, effected largely by calving, ranged from 2.5- to 5-m/day. The pedestal retreated across the sea floor at the same rapid rate, but melting, rather than calving, was probably the major cause of erosion. Hydraulic processes did not have time to develop cut-and-fill structures in the sea floor. In short, the test showed that unprotected ice islands in shallow, warm coastal waters have no chance of survival through the summer.

Introduction
Ice interacts with the marine environment in many different ways, and through these interactions becomes an important geologic agent. The U.S. Geological Survey has conducted research on ice related processes for many years. Recent petroleum developments in northern Canada and Alaska have focused attention on ice in the marine environment and on the effects of the ice on shores and the shelf surface. During the winter of 1978-1979, a circular ice island with a diameter of 400 m was built in the shallow waters of Stefansson Sound, about 6 km north of Prudhoe Bay, Alaska (Fig. 1). This large, firmly grounded mass of ice of known age, size, and shape provided a unique opportunity to study the effects of summer conditions until the island was destroyed by natural processes before winter.
Newest TFL Retrievable Safety Valves for Subsea Completions
Authors: Yonker J.H. 
Published: October 2008

ABSTRACT
Two new novel retrievable ball type safety valves are now available in the TFL (pumpdown) configuration. These valves offer greater depths in a more simplified configuration to operators planning subsea completions. They were developed at the request of major producers for use in the North Sea and Southeast Asia.
Design of these new valves began in 1977 and early 1978 by engineers from Otis' Research and Development Department. One valve is a balance line valve with a novel fail-close leak protection feature. The other valve is a single line deep set valve which allows installation at greater depths than previously achieved. They have been successfully tested in test wells and test loops at the manufacturer and in two land test wells by the operators. The safety system packages for these valves are currently installed in two subsea wells - one in the North Sea and the other in the South China Sea.
The material presented in this paper shows how the capabilities of subsea production are expanded by the development of a truly fail safe balance line retrievable ball valve and a deep set single line ball valve which can be placed at or near the packer, below paraffin levels, or below the kickoff point.

IN1RODUCTION
As completion technology has kept pace with drilling technology in the push for oil into deeper waters and hostile environments, so too has pumpdown (TFL) completion and service technology kept pace with developments to give the industry viable alternatives in the completion of subsea installations. This holds particularly true for TFL retrievable ball safety valves; two new valves have been developed which help extend subsea completion technology.
In 1977 a design team consisting of R&D Engineers from Otis and engineers from a major North Sea producer met and established design criteria for a new subsea safety system. This system would consist of a tubing retrievable ball valve and a separate backup insert TFL ball valve both of which would be of the balanced type. The exact details of the design criteria for the tubing retrievable valve and the overall safety system are presented in detail in arc Paper No.4253 presented at the 1982 Offshore Technology Conference.
In early 1978 Otis was approached by a major producer from the Southeast Asia area regarding new TFL completion equipment. This equipment was to be for a new below-the-mudline subsea completion and was part of an ongoing development program in which five TFL subsea completions had been made with an above the mudline wellhead configuration.
Discussions centered around the updating and compatibility of the TFL completion and service equipment with the new system; but also in particular on the safety system. The safety system would, as in the past, be a single line system. However for this completion it would consist of both a tubing retrievable valve and an insert valve. Five previous completions had consisted of only a landing nipple and TFL insert valve which spoke well of the reliability of and confidence in the insert valve's operation and retrievability.
Model Anchor Tests in Cohesionless Soil
Authors: Walker G.R. , Taylor R.J. 
Published: October 2008

ABSTRACT
Tandem (piggyback), parallel, and single model anchor tests have been performed in cohesionless soil. Tandem anchor arrangements have been found that result in a tandem system capacity that exceeds twice the capacity of a single anchor. These methods are operationally feasible and offer the potential for anchor weight savings.

INTRODUCTION
The Navy has been involved in an ocean test program of typical Navy and commercial drag embedment anchors. 1,2,3,4 An important aspect of this program has been the determination of effective methods of using the Navy's large inventory of STOCKLESS anchors. Tandem (piggyback) and parallel anchoring methods had been considered during this program and several methods have been found suitable with STOCKLESS anchors.
The Navy's expanding fleet mooring requirements and the capacity limitations of the STOCKLESS anchor has made it necessary to extend the test program to evaluating multiple high efficiency anchors. The overall effort consists of laboratory tests in cohesionless (sand) and cohesive (soft clay) soil, and then prototype field tests of the most promising anchor combinations.
This paper describes the laboratory test program in cohesionless soil and provides test results describing the effectiveness of the multiple anchor methods.

TEST APPARATUS AND PROCEDURES
Two model anchors were fabricated for this test program; both were identical 1/20 scale models of a 6,000 lb STATO anchor. A holed plate was attached to the anchor shank to enable a shank-shackle attachment which is subsequently described in the tandem anchor section. As with the STATO prototype the fluke angle could be adjusted, however, for the multiple anchor tests the fluke angle was set at 32°. The test chain between anchors models the effective diameter of 2-inch welded chain. The models can be seen in Figures 1 and 2.
The anchors were instrumented so that load and drag distance could be recorded continuously. The sand used was a dry poorly graded medium dense sand that was vibrated to a maximum relative density of65%. The soil was enclosed in a 9.5' × 2.5' × 1.3' deep box. The anchors Were dragged through the sand with a wire attached to a capstan driven by a variable speed motor.

SINGLE ANCHOR TESTS
Prior to multiple anchor tests the single anchor's (Figure 1) holding capacity was defined so that subsequent holding capacities of the tandem anchors could be compared to the standard holding capacity. The average amount of anchor roll was 4° and this amount of roll is considered minimal. All subsequent holding capacities are compared to the single anchor capacity using the parameter S, where S is the ratio of anchor capacity to the standard anchor capacity. The standard capacity for the single anchor with wire is 15.9 lbs.
Three-Year Experience in the Numerical Prediction of the Slow-Drift Motion of Moored Tankers
Authors: Molin Bernard 
Published: October 2008

ABSTRACT
A numerical model is described which allows to simulate the slow-drift motion of moored tankers, in waves, wind, and current. Emphasis is put on the physical problems for which a better knowledge is required in order to improve the model. Some numerical applications are presented.

INTRODUCTION
For the past years IFP has been carrying out a thorough research program on wave-drift forces and slow-drift motion of moored tankers. In a previous OTC paper I it has been described how wave drift forces in regular waves could be obtained from potential theory, through diffraction-radiation codes. In irregular waves the drift force varies according to the wave envelope squared, and induces slow-drift motion of moored structures. Wind and current also play an appreciable role, and in some cases may cause dynamic instabilities of the system 1.
So far the only tools for designs of SPM systems have been model testing and rules of thumb based on previous experience and the calculation of mean wind and current loads 2. In most cases model testing remains the only alternative for the final design of a system however the need is felt for more handy and less expensive tools for preliminary designs or parametric studies.
In the recent past different attempts at solving this problem numerically have been proposed. Due to the many occurring non-linearities, simulation models are usually preferred ',4, eventhough they present some disadvantages regarding the statistical interpretation of the results. Models dealing directly with the statistics 5 or working in the frequency domain are so far restricted to academic cases.
The main difficulty in elaborating a simulation model for slow-drift motion consists in formulating properly the hydrodynamic exciting loads and damping terms. Since one is dealing with a resonant system the latter ones are as important as the former ones, even- though they have been kind of overlooked in many instances. The first paragraph consists in a short presentation of the simulation model. In the second one some considerations are given on the problem of properly formulating the hydrodynamic loads and damping. They aim more at pointing out where the problems are than at providing solutions. Last the third paragraph presents some results obtained with the model, and discusses their validity.

PRESENTATION OF THE MODEL
Figure 1 schematically illustrates the problem that is considered: a tanker moored by a hawser to a buoy anchored in 0, and exposed to waves, wind, and current coming from different directions in the horizontal plane. The wave-system, defined by its power spectrum S(w), is assumed to be mono directional, while the wind and current may be time varying.
The position of the tanker is referenced by the horizontal coordinates (xG,YG) of its center of gravity G, its heading, together with its vertical coordinate ZG.
Interpretation Aspects of Offshore VSP Data
Authors: Poster C.K. 
Published: October 2008

ABSTRACT
A data set, comprising a vertical seismic profile (VSP), logs, and a seismic section, is discussed primarily in terms of stratigraphic interpretation of seismic events. The logs are presented in their original forms, as well as in interpreted forms suitable for comparison to seismic data.
The VSP data are processed for comparison to the seismic section. A simple estimate of the frequency content of the seismic signal with depth is made.
The importance of using a full suite of logs in the interpretation of seismic data is illustrated.

INTRODUCTION
The vertical seismic profile provides for the accurate depth determination of upgoing and downgoing seismic events; it thus allows a study of the seismic wavelet and permits a close comparison of the character of reflected events with formation properties as indicated in the well logs. The correlation of the VSP data and the log data is of interest because it includes, to the extent not possible in the laboratory, the in situ character of the formation, such as degree and type of saturants, pressure, formation thickness, etc.
For the seismic data, the reliability of the correlation is dependent on the data quality and the degree to which the upgoing reflections can be confidently extracted from the vertical sections. For the logs, data quality is again important, as are selection and processing of the logs. With a comprehensive data set of good quality, detailed interpretation of the VSP data and, therefore, of the surface seismic data can be enhanced for formation evaluation.
In this paper, this correlation is carried out for a data set obtained offshore in a sand-shale sequence. The nearly 12,000-foot-deep well was cased in three sections: to 990 feet, to 5720 feet, and then to total depth. The seismic data were acquired with a single 200-cubic-inch airgun operated at 2000 psi from the rig. The downhole receiver was the Schlumberger Well Seismic tool. A suite of logs was acquired in the well before casing was set. All these data were acquired as part of a standard commercial operation with no special procedures used.

PRODUCTION OF THE VSP
Typically, 4 airgun shots were acquired and stacked (or averaged) at each of 75 levels in the well. The levels were 150 feet apart, and the data were acquired at I-ms intervals. After data editing and filtering, a velocity filter was applied to separate upgoing and downgoing events. Then, a predictive deconvolution operator was applied to each level to suppress multiples, and that was followed with a wavelet deconvolution process to produce a zerophase wavelet. In each of these deconvolution steps, we have referred to the downgoing wave data set at a particular level to compute the filter for processing the level's corresponding upgoing wave data set. Figure 1 summarizes the processing of the data.
Analysis of Riser Measurements in the North Sea
Authors: Verbeek P.H.J. 
Published: October 2008

ABSTRACT
A programme of riser measurements has provided a variety of riser field data, collected at several locations in the North Sea. Data were obtained by means of instrumented riser joints, installed in both a 24" and a 16" drilling riser. Bending moments in the riser, riser extensions, motions of the semi-submersible and wave heights were recorded simultaneously.
The measurement programme was aimed at verifying predictions from theoretical riser models in the typical regions of riser response, viz. the wave active zone at the top, the boundary layer at the bottom and the riser main section in between. In this paper attention is focussed on the data obtained for riser response in the wave-active zone. Measured results are compared with theoretical predictions based on both analytical solution methods and numerical time-domain simulation techniques. In particular, the relationship between the standard 'deviation of riser bending response and significant wave height, and the probabilistic nature of riser response are discussed.
It is concluded that standard deviations of measured bending moment response are in-line with theoretical results. The probability distributions of measured response were found to be non-Gaussian and consistent with theoretical predictions based on Morison-type wave loading.

INTRODUCTION
Marine riser analysis is based on physicomathematical models, which involve descriptions of the sea, the riser structure and the interaction between the two. These models are developed as design tools for future production riser systems. In order to verify the theoretical riser model, use is made of riser data obtained under full-scale field conditions. These data were provided by a programme of riser measurements in the North Sea. In this programme riser responses were measured by means of instrumented riser joints (IRJ): first, intermittently from 1977 until 1979, by means of an IRJ installed in the 24" drilling riser of the Sedco-700 and later, from 1979, by means of an IRJ installed in the 16" drilling riser of the semi-submersible Stadrill; see Fig. 1. Simultaneously, responses established by the IRJ's, top tensionings, wave heights and floater motions were measured.
The analysis of the measured riser data was performed to verify the riser model in the three characteristic regions of response which have been identified along marine risers 1; viz., a wave-active zone at the top, a boundary layer at the bottom and a main section in between. This paper is restricted to the measurements in the wave-active zone. Attention is focussed on those response functions which are of prime importance in the assessment of the design parameters expected fatigue damage and expected extreme response; viz., standard deviations and probability distributions of response. Both standard deviations and probability distributions of measured bending moment are compared with predictions obtained from the theoretical riser model. The measured probability distributions are also used to study the non-linear behaviour of riser response in the wave active zone.