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Izadparast A.H.,SOFEC Inc. | Niedzwecki J.M.,Texas A&M University
International Journal of Offshore and Polar Engineering | Year: 2012

The Rayleigh-Stokes model has been widely applied to represent the probability distribution function of crests and troughs of weakly nonlinear random processes. In this study, the parameter estimates for the 3-parameter Rayleigh-Stokes probability distribution function model are obtained from application of 2 moment-based empirical parameter estimation methods, i.e. conventional method of moments and method of linear moments. Monte Carlo simulations are utilized to compare the performance of these parameter estimation approaches in estimating the parameters of the Rayleigh-Stokes distribution, and also to evaluate the uncertainty of the extreme statistics. Additionally, the effect of sample size on the uncertainty of the model statistics is evaluated. Finally, the Rayleigh-Stokes model is utilized to estimate the probability distribution function of disturbed wave crests beneath a mini-TLP, and the model performance is evaluated. © by The International Society of Offshore and Polar Engineers.


Izadparast A.H.,SOFEC Inc. | Niedzwecki J.M.,Texas A&M University
Probabilistic Engineering Mechanics | Year: 2013

The use of multi-parameter distribution functions that incorporate empirically derived parameters to more accurately capture the nature of data being studied is investigated. Improving the accuracy of these models is especially important for predicting the extreme values of the non-linear random variables. This study was motivated by problems commonly encountered in the design of offshore systems where the accurate modeling of the distribution tail is of significant importance. A four-parameter Weibull probability distribution model whose structural form is developed using a quadratic transformation of linear random variables is presented. The parameters of the distribution model are derived using the method of linear moments. For comparison, the model parameters are also derived using the more conventional method of moments. To illustrate the behavior of these models, laboratory data measuring the time series of wave run-up on a vertical column of a TLP structure and wave crests interacting in close proximity with an offshore platform are utilized. Comparisons of the extremal predictions using the four-parameter Weibull model and the three-parameter Rayleigh model verify the ability of the new formulation to better capture the tail of the sample distributions. © 2013 Elsevier Ltd.


Izadparast A.H.,SOFEC Inc. | Niedzwecki J.M.,Texas A&M University
American Society of Mechanical Engineers, Fluids Engineering Division (Publication) FEDSM | Year: 2012

Ocean engineers are routinely faced with design problems for coastal and deepwater structures that must survive a wide range of environmental conditions. One of the most challenging problems in the field of ocean engineering is the accurate characterization and modeling of the interaction of ocean waves with these offshore structures. The random characteristic of ocean environment requires engineers to consider the effects of random variability of the pertinent variables in their predictive models and design processes. Thus, for ocean engineering purposes, one needs to have accurate estimates of the probability distribution of the key random variables that will be used in sensitivity studies, reliability analysis, and risk assessment in the design process. In this study, a family of semi-empirical probability distribution is developed based on the quadratic transformation of linear random variable assuming that the linear random variable follows a Rayleigh distribution law. The estimates of model parameters are obtained from two moment based parameter estimation methods, i.e. method of moments and method of linear moments. The studied semi-empirical distribution can be applied to estimate the probability distribution of a wide range of non-linear random variables in the fields of ocean wave mechanics and wave-structure interaction. As examples, the application of the semi-empirical model in estimation of probability distribution of: a) ocean wave power, b) ocean wave crests interacting with an offshore structure is illustrated. For this purpose, numerically generated timeseries and experimentally measured data sets are utilized. Copyright © 2012 by ASME.


Duggal A.S.,SOFEC Inc. | Izadparast A.H.,SOFEC Inc. | Pothuganti V.K.,SOFEC Inc.
Proceedings of the Annual Offshore Technology Conference | Year: 2015

This paper provides an overview of the challenges associated with the stationkeeping of floating systems in frontier deepwater, and the range of solutions to address them. The paper presents the use of existing technologies, materials and components, and compare and evaluates them to the use of novel materials, new and possibly not field proven technologies, and design approaches. The paper provides a high-level design basis and strategies for the stationkeeping system design, identifying key drivers in the selection of the stationkeeping system. The paper then utilizes an example of an FPSO system moored in a range of water depths to illustrate the current capabilities of conventional technologies and strategies to improve performance. Copyright © (2015) by the Offshore Technology Conference All rights reserved.


Disclosed embodiments relate to systems and methods for mating a wind turbine off-shore to a spar buoy without the use of a crane barge. The system may include a spar buoy, wherein the spar buoy is secured to a foundation, and a wind turbine to be installed on the spar buoy. The system may also include a first truss affixed to the top of the spar buoy and a second truss affixed to the bottom of the wind turbine. The first truss may comprise either stabbings or receptacles configured for mating to the second truss and the second truss may comprise either receptacles or stabbings configured for mating to the first truss.


Patent
Sofec Inc. | Date: 2014-02-11

A fluid swivel (10A) that includes a stationary annular structure (16A, 17A) defining upper and lower annular seal slots (61, 62), and a rotatable outer housing (20A) operatively connected to the stationary annular structure (16A, 17A), the outer housing defining a radial groove (30) that has upper and lower surfaces. Upper and lower seals (40, 41) are positioned between the stationary annular structure (16a, 17a) and the rotatable outer housing (20a) and disposed in the upper and lower seal slots (61, 62). An inner fluid manifold (26A) is disposed in the stationary annular structure (16A, 17A) and arranged to provide fluid of pressure P to the radial groove (30), the fluid exerting a first force (Fv_(1)) longitudinally inwardly on the outer surface of the outer housing (20A), and a second force (Fv_(2)) longitudinally outwardly on the upper and lower surfaces of the radial groove (30), the second force (Fv_(2)) having a greater magnitude than the first force (Fv_(1)) so the outer housing (20A) deflects outwardly, pushing the upper and lower seals (40, 41) into the upper and lower seal slots (61, 62).


A single point mooring includes a yoke-shaped articulated arm assembly, which is affixed to the seabed by a fixed foundation or mooring base assembly, and a mooring support structure, which is retrofitted to a vessel. The articulated arm is connected to the base by a triaxial articulation joint for accommodating yaw, pitch and roll. To moor the vessel, the arm is connected to the mooring support structure with weighted pendants on port and starboard sides. The pendants are suspended at an adjustable elevation with respect to the mooring support structure, thus allowing the mooring system to be used at varying water depths. A combined connection bracket hosts both pendant linkages and umbilicals or hoses for speeding the process of connecting and disconnecting a vessel from the mooring system. A removable anchor, such as a suction pile, fixes the mooring system to the seabed, allows the system to be easily redeployed to another location.


Patent
Sofec Inc. | Date: 2013-10-02

A chain support, hinged on two perpendicular axes to allow chain movement in two perpendicular planes is disclosed. The apparatus provides an improved arrangement to allow chain to be pulled at an angle above the chain support while maintaining straight pull across a stopper mechanism.


Patent
Sofec Inc. | Date: 2015-07-02

A high pressure swivel is disclosed which includes swivel modules each having a stationary inner structure and a rotatable outer structure that can rotate about the inner structure. The inner structure of each module has the same number of vertical passages as the others. The inner structure of each module has a circumferential flange on the top and on a bottom lip so that each swivel unit can be secured to another swivel in its top or bottom with clamps around the flanges. The inner structures are rotatably positioned with respect to each other when assembled so that a vertical passage from a stationary base manifold to an outlet on the rotatable outer structure is formed. Various combinations of standard swivel modules can be assembled to accommodate specific swivel stack requirements for flow capacity and separate or commingled flow paths.


Patent
SOFEC Inc. | Date: 2016-08-12

A fluid swivel including a stationary inner housing assembly and a rotatable outer housing operatively connected to the inner housing assembly. The outer housing includes an annular passage with upper and lower surfaces. Upper and lower seals are positioned between the inner housing assembly and the outer housing in seal recesses. A passage in the inner housing assembly provides fluid of pressure to the annular passage. The fluid exerts a first force axially inward on outer housing outer surfaces, and a larger second force axially outward on the annular passages upper and lower surfaces. The second force is greater than the first force so upper and lower ring portions deflect outward such that the elastic axial deflections of the inner housing assembly match the axial deflections of the outer housing, thereby causing the axial clearances between the components at the upper and lower seals to remain almost the same.

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