bluuebunny/crossref_metadata_2025 huggingface dataset:
Human and Organization Errors in Reliability of Offshore
Structures
Authors: Bea R. G.
Published: February 2008

This paper addresses human and organization errors (HOE) in the context of quantitative reliability analyses (QRA) that are intended to help improve the quality of offshore structures. A classification of HOE is proposed that addresses individual, organization, equipment/hardware, procedures/software, and environmental considerations. A generic process to address the life-cycle quality characteristics of offshore structures incorporating HOE is discussed. Based on these developments, a probability-based risk analysis is developed that addresses HOE in addition to the structure system aspects. Alternatives for improved management of HOE are discussed including quality assurance and quality control (QA/QC), and design of error-tolerant or “robust” structures. Application of the HOE classification, generic assessment process, QRA formulation, and QA/QC measures are illustrated.
Stability-Based Design of Two-Point Mooring Systems With Hydrodynamic
Memory
Authors: Chung J.-S.
Published: February 2008

A systematic method of determining the design parameter values of two-point mooring (TPM) systems is presented. The conclusions of nonlinear stability analysis of horizontal plane slow motion dynamics of TPM systems with hydrodynamic memory are used to guide selection of the design variables. Specifically, within the stable regions of bifurcation diagram, a set of parameter values is chosen systematically to examine the corresponding TPM system. Then, each TPM system is simulated and the results are evaluated in terms of given criteria such as tension level of the mooring line or offset of the moored vessel. The hydrodynamic memory effect due to the oscillatory motions of moored vessel is included in the formulation. The memory effect influences the stability properties of TPM systems and alters the design chart/bifurcation diagram and particularly the static loss and dynamic loss of stability. The significance of hydrodynamic memory on several TPM systems is illustrated by comparing the statistics of the simulations. The design approach implemented in this work shows that nonlinear stability theory can be used effectively in the actual design process of mooring systems to reduce dramatically the number of simulations required. It is also shown that the often inconclusive nonlinear time simulations can be guided by theoretical nonlinear stability analysis of system dynamics to produce improved design values of system parameters of TPM systems.
Modeling the Effect of Shelter in the Modification of Waves From the
Open Sea to Near-Shore
Authors: Oliveira-Pires H. , Carvalho F. , Pontes T.
Published: February 2008

An efficient method has been developed to compute the wave conditions in sheltered areas from the open sea directional spectrum using a reverse tracing wave ray model with weighted spectral filtering. The results were verified for Porto Cachorro—Azores, surrounded by several islands with an intricate geometry, and for Sines, at the west coast of Portugal. The range of applicability of this type of ray model seems to lie at the transition between the open sea, well represented through large-scale wind-wave models, and the zones close to shore where shallow water wave breaking (not represented in the model) begins to play a dominant and decisive role.
On the Wind and Current Forces Acting on a Very Large Floating
Structure
Authors: Ohmatsu S. , Takai R. , Sato H.
Published: February 2008

This paper deals with the estimation method of wind force and current force acting on a very large floating structure. In this paper, as a very large floating structure, we consider a matlike very large platform which will be used as a floating airport. In order to estimate the fluid drag coefficient for such a huge structure, wind tunnel and towing tank model experiments were conducted paying special attention to the frictional drag coefficient. This paper shows the methods and results of these model experiments and discusses the application methods for wind and current forces on a full-scale structure.
Reliability-Based Criteria for Fixed Steel Offshore Platforms
Authors: Efthymiou M. , van de Graaf J. W. , Tromans P. S. , Hines I. M.
Published: February 2008

The key issue addressed in this paper is the accuracy of structural reliability models for the case of fixed steel offshore structures under extreme storm loading. The emphasis is on engineering accuracy for the purpose of use in decision-making, and more specifically to achieve sufficient accuracy to enable the use of reliability models in deriving design criteria for fixed offshore platforms. These reliability models are used to derive partial load factors for use in conjunction with API LRFD to achieve a target reliability level appropriate for permanently manned installations. These load factors are location-dependent. Further load factors are proposed for the design of new, not normally manned installations.
Ringing and Double Frequency Response of a Tripod
Authors: Nedergaard H. , Tychsen J. , Lyngesen S.
Published: February 2008

The dynamic behavior of a tripod steel jacket installed in 65 m water depth in the North Sea has been investigated. The analysis method is a time-domain simulation of the platform exposed to irregular sea. Similar to monotower platforms, the so-called “ringing” phenomenon caused by higher-order harmonics in the wave loading is detected. Further, the analysis discovered another dynamic feature, “double frequency response,” caused by the interaction between the spatial distribution of the wave load and the shape of first vibrational mode. The dynamic response showed that the top of the platform vibrates notably more in first mode than found by a quasi-static analysis with incorporation of standard dynamic amplification factors. However, the resulting extreme stress level and accumulated fatigue damage are found quite similar and the consequences to human comfort small. The basis for this finding is the fact that first mode contributes very little to the stress level in critical points.
Wave Energy-Driven Resonant Sea-Water Pump
Authors: Czitrom S. P. R.
Published: February 2008

A wave-driven sea-water pump which operates by resonance is described. Oscillations in the resonant and exhaust ducts perform similar to two mass-spring systems coupled by a third spring acting for the compression chamber. Performance of the pump is optimized by means of a variable volume air compression chamber (patents pending) which tunes the system to the incoming wave frequency. Wave tank experiments with an instrumented, 1:20-scale model of the pump are described. Performance was studied under various wave and tuning conditions and compared to a numerical model which was found to describe the system accurately. Successful sea trials at an energetic coastline provide evidence of the system’s viability under demanding conditions.
A Response Surface for Structural Optimization
Authors: Rule W. K.
Published: February 2008

Structural optimization is computationally intensive. Typically, a finite element model must be solved repeatedly during the process. Accordingly, various approximation schemes have been developed to reduce the cost of solves or to eliminate solves entirely during portions of the optimization process. In this paper, a new type of approximation or interpolation function in the form of a global response surface is described. With this function, approximations can be made with a limited amount of calibrating data points. The response surface requires no matrix inversions for coefficient evaluation, and therefore it is relatively efficient and stable. The new response surface is used in conjunction with a modified zero-order Powell search algorithm. The effectiveness of the response surface is evaluated for a reasonably realistic offshore structure application. This planar truss structure is subjected to a deck load, a lateral wave load (considered quasi-static), and member self-weight loads. The objective for this test case was to minimize the structural cost. Nodal coordinates were considered to be the explicit design variables. Response surface results are compared with those obtained by the common optimization algorithms: steepest descent, conjugate directions, and a conventional implementation of Powell’s method. The response surface appears to be effective for the test case considered.
A Reliability-Based Calibration Study for Upheaval Buckling of
Pipelines
Authors: Mørk K. J. , Bjørnsen T. , Venås A. , Thorkildsen F.
Published: February 2008

The present paper describes a reliability-based design procedure against upheaval buckling of rock or soil-covered pipelines. The failure mode considered is “snap-through” buckling. The study is performed using state-of-the-art design methodologies, including an assessment of all known uncertainties related to the load and capacity, measurements, surveys, and confidence in the applied models. A response surface technique is applied within the level III reliability analysis. Target safety levels are discussed for both SLS and ULS conditions, and a case-specific reliability-based calibration study is performed in order to derive a consistent design format.
Wave-Induced Pore Pressure and Effective Stresses in a Porous Seabed With Variable Permeability
Authors: Jeng D. S. , Seymour B. R.
Published: February 2008

An evaluation of wave-induced soil response is particularly important for marine geotechnical engineers involved in the design of foundations for offshore structures. To simplify the mathematical procedure, most theories describing the wave-seabed interaction problem have assumed a porous seabed with uniform permeability, despite strong evidence of variable permeability. This paper presents an analytical solution for the wave-induced soil response in a porous seabed with variable permeability. Verification is available through a reduction to the simple case of uniform permeability. The results indicate that the effect of variable soil permeability on pore pressure and effective stresses is significant.
Power Absorption by Arrays of Interacting Vertical Axisymmetric Wave-Energy Devices
Authors: Mavrakos S. A. , Kalofonos A.
Published: February 2008

The paper deals with the evaluation of the optimum wave-power absorption characteristics of arrays of interacting wave-energy devices. The hydrodynamic interference effects among the devices are exactly accounted for using a method that can solve the problem to any desired accuracy. The method is based on single-body hydrodynamic characteristics that are properly combined through the physical idea of multiple scattering to account for interaction effects. Extensive numerical results for a variety of different array arrangements and individual device geometries are presented and comparisons are made to predictions based on approximate theories, the accuracy of which is critically assessed.
Direct Approximation of the Extreme Value Distribution of Nonhomogeneous Gaussian Random Fields
Authors: Maes M. A. , Breitung K. W.
Published: February 2008

In many reliability problems, particularly in the area of offshore and marine safety analysis, the maxima of stochastic processes and random fields, describing random load responses and resistances, are critically important. In the case of stationary differentiable Gaussian processes, approximations for the distribution of the maxima using upcrossing rates are well known. In this paper, we consider nonstationary differentiable Gaussian random fields. A new technique first applied by Sun (1993) can be used to derive direct approximations for the tail probability. This is an alternative approach based on an expansion of the covariance operator and on more or less geometric concepts. The method does not rely on assumptions regarding the point process of upcrossings. It provides, directly, an asymptotic approximation for the distribution of the maximum of the random field.
Hybrid Design Concept Using High-Strength Cast Steel Inserts for Tubular Joints of Offshore Structures
Authors: Sonsino C. M. , Umbach R.
Published: February 2008

In a joint project of a German working group supported by the ECSC and the Studiengesellschaft fu¨r Stahlanwendung e.V., the fatigue behavior of large-scale hybrid tubular joints with inserts manufactured from the high-strength cast steel GS-12 MnMo 7 4 welded into tubular members formed from the fine-grained steel StE 500 were compared to the behavior of large-scale welded tubular joints. The latter were made from medium-strength fine-grained steel StE 355 and high-strength StE 690. In addition, data from hybrid joints with cast steel inserts of medium-strength GS-8 Mn 7 welded into StE 355 tubulars is available for comparison. The tests were carried out under variable amplitude loading in artificial seawater. The results were evaluated for the failure criteria fatigue life to crack initiation (a = 1 mm) and through crack. With medium-strength (Rp0.2 > 355 N/mm2) hybrid tubulars, where by the use of cast steel inserts the welds were removed into areas of lower stress concentration, fatigue lives higher than a factor of 100 were achieved compared to the welded nodes, even those made from StE 690. However, by the use of high-strength (Rp0.2 > 500 N/mm2) cast steel inserts and tubular members of corresponding strength, the fatigue life to crack initiation was improved by a factor of two despite a thickness reduction compared to the medium-strength design. Post-weld treatments of the welded tubulars without cast steel inserts like shot-peening, TIG-dressing, or their combination resulted only in a slight increase of fatigue life. The results of this investigation do not only show how to improve the fatigue life by a new design using cast steel inserts, but indicate also how to revise design codes from the point of damage calculation (damage sum of 0.50 for welded nodes and 0.25 for cast steel inserts instead of the conventional value of 1.00), as well as consideration of fatigue life to initiation of a technically detectable crack with a defined depth e.g., a = 1 mm.
Nonlinear Effects on Long-Term Distributions of Wave-Induced Loads for Tankers
Authors: Guedes Soares C. , Schellin T. E.
Published: February 2008

A method of long-term formulation of the nonlinear wave-induced vertical load effects on ships was applied to three tanker hulls of different sizes. For large tanker hulls, the nonlinear effect is not significant, and thus linear theories can continue to be used for earlier studies on these kind of ships, contrary to what was shown earlier for containership hulls. However, for smaller tankers, significant nonlinear values were obtained, with both sagging and hogging nonlinear results being larger than the linear ones.
A Multipole Accelerated Desingularized Method for Computing Nonlinear Wave Forces on Bodies
Authors: Scorpio S. M. , Beck R. F.
Published: February 2008

Nonlinear wave forces on offshore structures are investigated. The fluid motion is computed using a Euler-Lagrange time-domain approach. Nonlinear free surface boundary conditions are stepped forward in time using an accurate and stable integration technique. The field equation with mixed boundary conditions that result at each time step are solved at N nodes using a desingularized boundary integral method with multipole acceleration. Multipole accelerated solutions require O(N) computational effort and computer storage, while conventional solvers require O(N2) effort and storage for an iterative solution and O(N3) effort for direct inversion of the influence matrix. These methods are applied to the three-dimensional problem of wave diffraction by a vertical cylinder.
Statistical Extrapolation of Extreme Value Data Based on the Peaks Over Threshold Method
Authors: Naess A.
Published: February 2008

This paper discusses the use of the peaks over threshold method for estimating long return period design values of environmental loads. Attention is focused on the results concerning the type of asymptotic extreme value distribution for use in the extrapolation to required design values obtained by such methods, which in many cases seem to indicate that the Weibull distribution for maxima is the appropriate one. It will be shown by a closer scrutiny of the underlying estimation process that very often such a conclusion cannot in fact be substantiated.
Life-Cycle Reliability Characteristics of Minimum Structures
Authors: Bea R. G. , Brandtzaeg A. , Craig M. J. K.
Published: February 2008

Minimum structures are used primarily because of their low initial investment and fast-track schedule characteristics. These structures generally cost less and take less time to commission than more traditional structures such as four-pile (leg) platforms. A wide variety of configurations of these structures have been proposed and installed in offshore locations around the world (Craig, 1995). This paper details a reliability and risk-based approach that has been used to characterize the life-cycle risk characteristics of three minimum structures and one traditional four-pile structure. The structures were all designed according to current American Petroleum Institute guidelines (1993). Results from the life-cycle (design, construction, operation) analyses of the quality and risk characteristics of the alternative structures are summarized. The occurrences and influences of human errors throughout the life-cycle are included.
Turret Mooring Design Based on Analytical Expressions of Catastrophes of Slow-Motion Dynamics
Authors: Bernitsas M. M. , Garza-Rios L. O.
Published: February 2008

Analytical expressions of the bifurcation boundaries exhibited by turret mooring systems (TMS), and expressions that define the morphogeneses occurring across boundaries are developed. These expressions provide the necessary means for evaluating the stability of a TMS around an equilibrium position, and constructing catastrophe sets in two or three-dimensional parametric design spaces. Sensitivity analyses of the bifurcation boundaries define the effect of any parameter or group of parameters on the dynamical behavior of the system. These expressions allow the designer to select appropriate values for TMS design parameters without resorting to trial and error. A four-line TMS is used to demonstrate this design methodology. The mathematical model consists of the nonlinear, fifth-order, low-speed, large-drift maneuvering equations. Mooring lines are modeled with submerged catenaries, and include nonlinear drag. External excitation consists of time-independent current, wind, and mean wave drift.
An Application of the Peaks Over Threshold Method to Predict Extremes of Significant Wave Height
Authors: Ferreira J. A. , Guedes Soares C.
Published: February 2008

The paper describes an application of the Peaks Over Threshold (POT) method to significant wave height data of Figueira da Foz, Portugal. The method is briefly explained and justified. The exponential distribution is shown to be adequate for modeling the peaks of clustered excesses over a threshold of 6 m. Estimates of return values are given. The exponential character of the data is theoretically justified in the Appendix.
Fracture of Ductile Pipelines Subjected to Bending and Reverse Bending Loads
Authors: Hoo Fatt M. S. , Wang Z. B.
Published: February 2008

Closed-form solutions for the elastic-plastic deformation and fracture responses of an offshore pipeline during reeling and unreeling operations are derived. As an example, an API 5L X52 pipeline of thickness 25.4 mm (1 in.) and outer diameter 406 mm (16 in.) bent over a drum radius of 16.5 m (54 ft) and then straightened back to its original configuration is considered. It was found that the pipeline underwent significant plastic deformation during bending and reverse bending. The plastic zone extended from the top (bottom) of the pipe to 16.46 mm above (below) the neutral axis during bending. Modified J-integral and JR resistance curves were compared to evaluate through-thickness crack growth and stability. The difference between predicted crack growth and experimental results varied from 30 to 140 percent. J-integral-based crack growth predictions were significantly lower and much closer to experimental results than previous predictions based on crack tip opening displacement. Finally, a failure assessment diagram (FAD) for the pipeline was presented. The FAD would enable one to assess the propensity to fracture in pipelines.
Wet Welding as a “Serious” Repair Procedure?
Authors: Szelagowski P.
Published: February 2008

Due to intensive and concentrated research activities during the last 10 to 15 yr, the quality of wet-welded joints has been improved to an extent that this process is currently regarded as a potential alternative to the more costly dry hyperbaric welding processes in comparable water depths. The wet welding process has matured to an interesting alternative repair process due to its high flexibility and versatility and its low investment costs with respect to achieving comparable weldment quality. However, due to the previous bad reputation of the poor weldment quality in former times, related to extremely high hardness, high porosity, high hydrogen contamination, and, in combination with this, high cracking susceptibility, the wet welding process still requires concentrated activities to improve its reputation and credibility, especially in European oil field application. New acceptance creiteria, more detailed information on the achievable weldment quality, and especially the development of life-predicting data for wet-welded components on the one hand, and new design criteria especially related to the process application in wet environment as well as excellent training of diver welders on the other hand, have been required. Advanced testing methods had to be applied, additional design criteria had to be developed, and achievable weldment quality data had to be included in acknowledged and approved standards and recommendations in training and certification standards for diver welders. All these data are now available. These results have been achieved with the financial support of the European Community through the THERMIE PROJECT FUNDING.
Effect of Slow-Drift Loads on Nonlinear Dynamics of Spread Mooring Systems
Authors: Bernitsas M. M. , Kim B.-K.
Published: February 2008

Spread mooring systems (SMS) may experience large-amplitude oscillations in the horizontal plane due to slow-drift loads. In the literature, this phenomenon is attributed to resonance. In this paper, it is shown that this conclusion is only partially correct. This phenomenon is investigated using nonlinear stability and bifurcation analyses which reveal an enhanced picture of the nonlinear dynamics of SMS. Catastrophe sets are developed in a parametric design space to define regions of qualitatively different system dynamics for autonomous SMS, including mean drift forces. Limited time simulations are performed to verify the qualitative conclusions drawn on the nonlinear dynamics of SMS using catastrophe sets. Slowly varying drift forces are studied as an additional excitation on the autonomous SMS and simulations reveal that slow drift may cause resonance or bifurcations with stabilizing or destabilizing morphogeneses. The mathematical model of SMS is based on the slow-motion maneuvering equations in the horizontal plane (surge, sway, yaw), including hydrodynamic forces with terms up to third-order, nonlinear restoring forces from mooring lines, and environmental loads due to current, wind, and wave-drift.
Assessing the European Wave Energy Resource
Authors: Pontes M. T.
Published: February 2008

The European Wave Energy Atlas (WERATLAS), developed within a R&D European project, includes a wide range of annual and seasonal wave-climate and wave-energy statistics for 85 offshore data points distributed along the Atlantic and Mediterranean European coasts. The data used are results of the numerical wind-wave WAM model, implemented at ECMWF, and buoy data for the North Sea, Norwegian Sea, and Barents Sea. A full verification of WAM results against buoy and satellite altimeter data revealed that the accuracy of the results is very good for the North Atlantic, but the hindcasts quality is lower for the Mediterranean, probably due to poorer accuracy of the input wind fields. The patterns of power level and power directional distribution over the Northeastern Atlantic are presented along with the interannual wave and power variability. The wave power level is much lower in the Mediterranean, where it is not possible to find a general pattern for the power level and its directional distribution.
Underwater Wet Repair Welding and Strength Testing on Pipe-Patch Joints
Authors: Wernicke R. , Pohl R.
Published: February 2008

The underwater wet welding method for repairing of submerged structural members has been intensively developed during the last years. It is an economical and especially more time-independent procedure. The shortened repair duration is a major advantage in regions with rough sea climate as the North Sea area. During research projects the weldability and fatigue performance of pipe-patch connections have been tested. These joints are in use for repairing of collision indents. The shape of the actual used repair patch was optimized to the requirements of underwater wet welding at high tensile strength steels as BS4360 Gr 50D. Steel materials in this grade show problems in regard to high hydrogen susceptibility, and therefore cold cracking. The fatigue behavior of the patch-welded pipe structural member has been investigated. First test series were carried out using as-welded joints. In a second part of the project, post-weld-treated connections were tested. The weld seams on these joints were partially ground or hammer peened. The test results have been evaluated by means of the hot-spot concept and then faced with actual code requirements. Extensive strain gage measurements and finite-element calculations have been carried out to provide the stress state in the structural details.
A Predictive Electrochemical Model for Weld Metal Hydrogen Pickup in Underwater Wet Welds
Authors: de Medeiros R. C. , Liu S.
Published: February 2008

Weld metal hydrogen pickup in underwater wet welding is severe due to the presence and dissociation of water surrounding the welding arc. This undesirable behavior can be minimized, however, with the use of oxidizing-type electrodes. The purpose of this investigation has been placed on the fundamental understanding of the effect of hydrogen pickup by the slag on the weld metal diffusible hydrogen content in direct current, shielded metal arc welding (SMAW) for both electrode-positive polarity (DCEP), and electrode-negative polarity (DCEN). To accomplish this purpose, 20 experimental oxidizing electrodes containing systematic ferric oxide (Fe2O3) additions, ranging from 0 to 70 wt. percent, to the flux coating were investigated. The mole fraction ratio of CaO/SiO2 in the fluxes ranged from 0.05 to 0.35, independent of the ferric oxide additions. Underwater, bead-on-plate welds were deposited on ASTM A36 steel coupons at 0.27 m (city) water depth using a gravity feed system. Welding parameters were held constant throughout the experiments. Weld metal diffusible hydrogen content was determined using the mercury displacement method according to current AWS standard. To correlate weld metal hydrogen content with slag chemistry, the slag hydrogen contents were also determined. The measured diffusible hydrogen contents showed that Fe2O3 was effective in reducing weld metal hydrogen content. Higher hydrogen values were always related to lower Fe2O3 contents initially present in the flux, for instance, 71 mL/100g (DCEP − 0 wt. percent Fe2O3) as compared to 31 mL/100g (DCEP − 36 wt. percent Fe2O3). Amazingly, diffusible hydrogen as low as 13 mL/100g was obtained with the use of DCEN polarity along with 53 wt. percent Fe2O3 in the flux coating. X-ray diffraction (XRD) conducted on different slags showed that the lower diffusible hydrogen values were always associated with the presence of fayalite (2FeO·SiO2). Complementing XRD analysis, Mo¨ssbauer spectroscopy analyses carried out on different slags showed that all ferric (Fe3+) oxide initially present in the slags had transformed to ferrous oxide (FeO), free or combined. Chemical analyses showed that weld metal hydrogen pickup was strongly dependent on the solubility of water in the slag systems. The total and diffusible hydrogen content in the weld metal increased monotonically with increasing slag hydrogen content. Finally, variations in weld metal hydrogen as well as slag hydrogen content with both polarity and iron oxide content in the slag were successfully predicted using an electrochemical model that describes the slag/metal interface equilibrium. In this investigation, the slag/metal interface has been identified as responsible in controlling the weld metal hydrogen pickup. The model assumed that hydrogen was present in the slag as (OH)− ions and that FeO displayed ideal solution behavior.
Third-Order Hydrodynamic Loads on an Oscillating Vertical Cylinder in Water
Authors: Markiewicz M. , Łe¸tkowski P. , Mahrenholtz O.
Published: February 2008

The third-harmonic component of the third-order hydrodynamic loads on a vertical circular cylinder oscillating in water is calculated by a conventional perturbation method within the framework of a potential theory. Although the third-order forces are expressed in terms of the first, second, and third-order components of the velocity potential, the latter is not directly required for the calculation. It is replaced by a properly defined linearized radiation potential via Haskind-like theorem. The results of the study are applicable to the analysis of high-frequency resonances of deepwater offshore structures under earthquake excitation or under steep waves (ringing problem).
Hydrodynamic Evaluation of Multi-Layer Wave Filters and Absorbers
Authors: Clauss G. F. , Kuhlmann G. H. , Ku¨hnlein W. L.
Published: February 2008

Wave filters and absorbers are important coastal structures. Their efficiency depends on wave reflection and transmission as well as on energy dissipation. Because of their nonlinear characteristics, wave filters and absorbers must be analyzed and optimized experimentally. In a more general approach, two universally valid coefficients are determined for each individual filter element. Based on these data, arbitrary, multi-layer wave absorbers can be evaluated. Filter hydrodynamics mainly depend on porosity and wave kinematics, i.e., the profiles of horizontal velocity and acceleration. With reference to an initial wave, the related near-field velocity and acceleration at the filter element are expressed as functions of a drag and an inertia term. The corresponding coefficients, which are universally applicable, in connection with the relevant horizontal velocity and acceleration at the filter area are used to calculate the reflection, transmission, energy dissipation, and wave forces on arbitrary filter or absorber components, elements, and systems. The numerical analysis is validated by an experimental program in regular and irregular seas, and transient wave trains. Transient wave trains are efficiently used for: 1) Separate measurements of the initial wave group, as well as its reflection and transmission. Due to the short duration, the incoming wave train is easily separated from the reflected and the transmitted wave. This allows an accurate analysis of the associated flow fields and energies. For a variety of wave filters, the paper presents the evaluation of the foregoing characteristic coefficients, based on experiments with representative transient wave groups. 2) Model tests in extreme wave conditions (freak waves). Tests with breaking transient wave packets prove that the obtained maximum wave force on a structure is much higher than the long-term model test results in irregular waves, for a given spectrum.
Wave-Energy Conversion Through Relative Motion Between Two Single-Mode Oscillating Bodies
Authors: Falnes J.
Published: February 2008

Wave-energy converters (WECs) need a reaction source against which the wave forces can react. As with shore-based WECs, sometimes also floating WECs react against a fixed point on the seabed. Alternatively, for a floating WEC, force reaction may be obtained by utilizing the relative motion between two bodies. A load force for energy conversion is assumed to be applied only to this relative motion. It is assumed that either body oscillates in one mode only (mostly, the heave mode is considered here). The system, if assumed to be linear, is proved to be phenomenologically equivalent to a one-mode, one-body system, for which the wave excitation force equals the force which is necessary to apply between the two bodies in order to ensure that they are oscillating with zero relative motion. It is discussed how this equivalent excitation force and also the intrinsic mechanical impedance of the equivalent system depend on the mechanical impedances for the two separate bodies, including the radiation impedance matrix (which combines radiation resistances and added masses). The equivalent system is applied for discussing optimum performance for maximizing the absorbed wave energy. It is shown that, for an axisymmetric system utilizing heave modes, it is possible to absorb an energy amounting to the incident wave power on a crest length which equals the wavelength divided by 2π, even though the power take-off is applied to the relative motion only. Moreover, it is shown that it is possible to obtain an equivalent excitation force which exceeds the wave excitation force on either body.
Structural Redesign by Large Admissible Perturbations With Static Mode Compensation
Authors: Bernitsas M. M. , Suryatama D.
Published: February 2008

The method of LargE Admissible Perturbations (LEAP) solves redesign problems of complex structures without trial and error or repetitive finite element analyses. Code RESTRUCT (Redesign of STRUCTures) produces an optimal redesign of minimum structural change or minimum weight with modal dynamics and/or static displacement specifications. LEAP allows for large structural changes. First, the general perturbation equations are derived relating the original structure S1 (known) to the objective structure S2 (unknown) which is to satisfy the designer’s specifications. Next, the redesign problem is solved using an incremental prediction-correction scheme. In the past, LEAP produced accurate results even for 100–300 percent changes in redesign for modal objectives without any intermediate FEAs. Accuracy in static redesign, however, was limited to about 50 percent changes in static objectives. In this work, a new static general perturbation equation and the corresponding LEAP algorithm are developed to achieve accuracy for 100–300 percent changes in static performance as well. The new formulation includes the static deflection shape as the zeroth mode in the expansion of static properties in terms of dynamic modes. Systematic numerical applications show that high accuracy is achieved by fewer extracted modes.
Nondestructive Detection of Cracks in Tubular T-Joints Using Vibration Characteristics
Authors: Cheng S. M. , Swamidas A. S. J. , Wallace W. , Wu X. J.
Published: February 2008

This paper presents the results of an experimental investigation of the nondestructive inspection (NDI) of cracks in an intermediate scale tubular T-joint similar to those used in offshore platforms. To simulate the real situation, cracks were generated under fatigue loading. Modal testing and analysis were then carried out to study the vibration characteristics of the tubular T-joints. Strain gages were used as the main transducers to acquire data because strains are more sensitive to the presence of cracks. Strain frequency response functions were particularly analyzed. Three salient phenomena were observed: (a) antiresonance shifts, (b) quasi-static phenomenon, and (c) nonlinearity, all of which are indicative of the presence of cracks, even at a normalized crack size (the ratio of cracking area to load-bearing area) as small as 0.07. Therefore, the method developed in this study renders a more promising NDI technique than the frequency monitoring technique. Finite element analyses were also conducted to confirm the experimental results and to depict the mode shapes.
Statistical Properties of Successive Wave Periods
Authors: Myrhaug D. , Slaattelid O. H.
Published: February 2008

A parametric model of the joint distribution of successive wave periods obtained as best fit to observed wave data is presented. The wave data were obtained from measurements at the Frigg field, central North Sea in 100 m water depth including more than 3 million individual waves. It appears that the parametric model gives a good representation of the main statistical properties of the measured data.
Stochastic Response Analysis of Deepwater Structures in Short-Crested Random Seas
Authors: Teigen P. , Naess A.
Published: February 2008

The paper discusses the problem of estimating the response statistics of moored large-volume offshore structures subjected to short-crested random waves. A general second-order theory is described that makes it possible to carry out the entire analysis in the frequency domain, which is computationally more efficient than time domain analysis, which generally requires considerably more CPU time to reach the same level of accuracy.
On the Joint Distribution of Wave Heights and Periods: The Role of the Spectral Bandwidth
Authors: Rodriguez G. R. , Rubio Royo F. , Pacheco M. , Martinez A.
Published: February 2008

The influence of the spectral bandwidth on the probabilistic structure of the joint distribution of wave heights and periods is analyzed by means of simulated wave records. In particular, the effects of the spectral bandwidth on the asymmetric and bimodal structure of the distributions are examined. Moreover, the adequacy of some theoretical models to describe such features is considered.
The Hydrodynamics of Floating Compound Cylinders
Authors: Williams A. N. , Li W.
Published: February 2008

The hydrodynamics of a freely floating compound cylinder consisting of a surface-piercing cylindrical column resting on a larger, concentric cylindrical base are investigated theoretically. Linear wave theory and small-amplitude structural oscillations are assumed, and an eigenfunction expansion approach is used to obtain semi-analytical expressions for the hydrodynamic excitation and reaction loads on the structure. Numerical results are presented for a range of wave and structural parameters. It is found that for certain parameter combinations, negative surge/sway added mass coefficients are predicted. The numerical results for the hydrodynamic quantities obtained by the eigenfunction approach have been verified with those obtained by the linear radiation/diffraction program UHWAVE. These results have implications for the tow-out analysis of large gravity-base oil production platforms.
Wave-Induced Seabed Response Around a Pipe Laid on a Poro-Elastic Seabed
Authors: Jeng D.-S. , Cheng L.
Published: February 2008

The stability of a pipeline has been extensively studied by coastal and marine geotechnical engineers in recent years. However, most previous investigations have been only concerned with the fluid field around the pipelines, not the wave-induced soil response below the pipelines. In this paper, a two-dimensional finite-element model is employed to investigate the wave-induced seabed response around a pipe laid on a poro-elastic seabed. Unlike conventional investigations, both soil permeability and Young’s modulus are considered to vary with soil depth in the present model. Based on the proposed model, the mechanism of the wave-induced pore pressure and effective stresses are explored.
Mobilization of Fully Plastic Moment Capacity for Pressurized Pipes
Authors: Mohareb M. , Murray D. W.
Published: February 2008

An analytical expression is derived for the prediction of fully plastic moment capacity of pipes subjected to axial loading and internal pressure. The expression is based on the von Mises yield criterion. The expression predicts pipe moment capacities that are in good agreement with full-scale experimental results. A universal nondimensional moment versus effective axial force-pressure interaction diagram is developed for the design of elevated pipe lines.
Evaluating the Effective Friction Factor and Overall Heat Transfer Coefficient During Unsteady Pipeline Operation
Authors: Price G. R. , McBrien R. K. , Rizopoulos S. N. , Golshan H.
Published: February 2008

This paper presents a method to determine the effective friction factor and overall heat transfer coefficient for a high-pressure, natural gas pipeline during fully transient flow conditions. Time-varying SCADA (supervisory control and data acquisition) measurements at the pipeline boundaries (i.e., inlet and outlet) provide boundary conditions for a transient flow model, as well as additional information which is utilized to determine these parameters. The resulting friction factor and overall heat transfer coefficient minimize the least-squared difference between the additional SCADA measurements at the pipeline outlet and the corresponding values predicted from the transient flow model. This concept is referred to as parameter estimation. The transient flow model is based on a numerical solution of the one-dimensional conservation equations (i.e., continuity, momentum, and energy) which are discretized using a highly accurate compact finite-difference scheme. The transient flow model and parameter estimation is incorporated into a computer program that is initially tested on a simple pipeline with steady flow conditions. The predicted outlet pressure and temperature using the estimated friction factor and overall heat transfer coefficient exactly matches the corresponding prescribed values. Subsequently, a portion of the Foothills Pipe Line Ltd. transmission system in Alberta is considered using time-varying SCADA flow measurements. The resulting outlet pressure and temperature from the transient flow model are in good agreement with SCADA measurements for this pipeline section.
The Feasibility of a Central Turret in FPSO Systems
Authors: Fernandes A. C. , Sphaier S. H. , Correa S. H.
Published: February 2008

The use of Stabilizers in FPSO (floating production, storage, and offloading) systems with a turret is investigated under the light of a reviewed, perhaps new, stability analysis specifically for the turret and an order of magnitude analytical approach. This has been made together with model testing results and time-domain nonlinear simulations. It is shown by these various and independent methodologies that stabilizers, never considered so far for applications on FPSOs, are naturally feasible, opening the field for novel designs.
Asymmetric Hydrodynamic Impact and Dynamic Response of Vessels
Authors: Xu L. , Troesch A. W. , Peterson R.
Published: February 2008

Theoretical simulations are compared with drop test experiments of vessel free-fall impact on calm water, with zero and nonzero initial heel angles. The impact force and moment are predicted by using a theory described in Xu et al. (1998). The resulting motions of vessels at asymmetric impact include vertical water entry and transverse rolling. The downward deceleration equation combines the hydrostatic and hydrodynamic forces and the weight of hull, while the rolling equation takes into account the restoring moments due to asymmetry. Another simplified approach of a water entry dynamics and injury model (Gollwitzer and Peterson, 1996) is also applied to evaluate the impact acceleration for symmetric cases. The experiments were conducted at the Coastal Systems Station, Panama City, Florida, using a 2 ft × 8 ft (0.61 m × 2.44 m) prismatic hull model with 20 deg deadrise. The comparisons between the asymmetric impact theory and experiment shows varying degrees of qualitative and quantitative agreement.
Optimization of Catenary Risers
Authors: Larsen C. M. , Hanson T.
Published: February 2008

This paper describes how the design of a catenary riser can be formulated as an optimization problem by using riser costs as the criteria function, design requirements in terms of maximum allowable stress and buckling capacity as constraints, and riser dimensions as free variables. The theory has been implemented in a computer program that can generate an optimized riser design for given design parameters such as water depth, diameter, pressure, and platform excursions. The developed software consists of a conventional program for two-dimensional riser analysis and a set of standard routines to minimize a nonlinear function subjected to general constraints. A case study where design parameters and requirements have been varied is also presented. The importance of buckling versus allowable equivalent stress as the most critical constraint has been investigated for varying water depth. The Conclusion of this work is that optimization is a useful tool for riser design, and that the proposed strategy for selection of design variables and constraints will enable an engineer to identify designs with minimum costs in an efficient way.
Seismic Risk Analysis of Mega-Floating Structure and Dolphin System
Authors: Koike T. , Hiramoto T. , Mori H.
Published: February 2008

The floating type structural system is applicable to large offshore structures including airports, power plants, and multi-purpose facilities. Such a large-scale floating structure should be in service at least 100 yr because of the large capital investments and social utilization, so it also should be designed to be safe and stable against several natural hazards and accidents during its service life. In this report, seismic response and risk analyses of a mega-floating structure supported with many dolphins (MFSD) are carried out to study the mutual interaction and its instability of the floating-dolphin system against earthquake attack. The final goal is to obtain the optimal allocation of dolphins to minimize the displacement of an MFSD.
On-Line Optimization of the Turning Process Using an Inverse Process Neurocontroller
Authors: Azouzi R. , Guillot M.
Published: February 2008

This paper presents a feedback neurocontrol scheme that uses an inverse turning process model to synthesize optimal process inputs. The inverse process neurocontroller is implemented in a multilayer feedforward neural network. On-line adjustments of feed rate and cutting speed parameters are carried out based on a cost/quality performance index, estimated from force and vibration sensor measurements. Both non-adaptive and adaptive neurocontrol schemes are considered. The simulations and experimental investigations presented herein demonstrated the effectiveness of neural networks for controlling and optimizing turning operations. Applied to single point turning of a typical finishing cut, the final dimensions and surface finishes were found to be better by 40 and 80 percent respectively, while productivity was increased by 40 percent over the conditions proposed in machining data handbooks. This approach is also applicable to several other manufacturing processes.