A New Formulation for Equivalent Hydrodynamic Modeling of the Jack-Up Legs

2002 ◽  
Author(s):  
M. Daghigh
Keyword(s):  
Type Equation ◽  
Offshore Structures ◽  
Geometrical Model ◽  
Hydrodynamic Coefficients ◽  
Slender Bodies ◽  
Pile Model ◽  
New Formulation ◽  
Jack Up ◽  
Inertia Forces ◽  
Drag Term

Regulations of offshore structures suggest the application of Morison type equation for the estimation of forces induced by wave and current on the slender bodies of Jacket and Jack-up structures. However, common values of hydrodynamic coefficients are rarely defined in two different regulations. Estimation of global responses of Jack-up structure, the simplified geometrical model is used, therefore we will try to modify the DNV formulations in order to estimate the hydrodynamic forces on equivalent pile. Finally, the forces on the real structure and the equivalent pile model are compared and it has been shown that the approximation of the inertia forces has more accuracy comparing to the drag force, due to the nonlinear effect in drag term.

Numerical Study of the Combined Loading Envelope of a Skirted Spudcan Foundation

2014 ◽  
Author(s):  
Ning Cheng ◽  
Mark J. Cassidy ◽  
Yinghui Tian
Keyword(s):  
Clay Soil ◽  
Numerical Study ◽  
Failure Mechanisms ◽  
Small Strain ◽  
Offshore Structures ◽  
Combined Loading ◽  
Finite Element Analyses ◽  
Failure Envelope ◽  
Foundation Type ◽  
Jack Up

Foundations for offshore structures, such as mobile jack-up units, are subjected to large horizontal (H) and moment (M) loads in addition to changing vertical (V) loads. The use of a combined vertical, horizontal and moment (V-H-M) loading envelope to define foundation capacities has become increasingly applied in recent years. However, there is no study on the skirted spudcan, a new alternative foundation type to the conventional spudcan footing for jack-ups. In this study, the combined V-H-M yield envelope of a skirted spudcan foundation in clay soil is investigated with small strain finite element analyses using 3D modeling. The footing’s uniaxial bearing capacities and failure mechanisms are described. The failure envelope for the combined V-H-M loadings is presented. A comparison of the bearing capacities between the spudcan and skirted spudcan of various dimensions is also presented.

An Improved Body-Exact Method to Predict Large Amplitude Ship Roll Responses

2018 ◽  
Author(s):  
Rahul Subramanian ◽  
Naga Venkata Rakesh ◽  
Robert F. Beck
Keyword(s):  
Incident Wave ◽  
Large Amplitude ◽  
Nonlinear Models ◽  
Body Position ◽  
Computational Cost ◽  
Offshore Structures ◽  
The Body ◽  
Surface Boundary ◽  
Strip Theory ◽  
New Formulation

Accurate prediction of the roll response is of significant practical relevance not only for ships but also ship type offshore structures such as FPSOs, FLNGs and FSRUs. This paper presents a new body-exact scheme that is introduced into a nonlinear direct time-domain based strip theory formulation to study the roll response of a vessel subjected to moderately large amplitude incident waves. The free surface boundary conditions are transferred onto a representative incident wave surface at each station. The body boundary condition is satisfied on the instantaneous wetted surface of the body below this surface. This new scheme allows capturing nonlinear higher order fluid loads arising from the radiated and wave diffraction components. The Froude-Krylov and hydrostatic loads are computed on the intersection surface of the exact body position and incident wave field. The key advantage of the methodology is that it improves prediction of nonlinear hydrodynamic loads while keeping the additional computational cost small. Physical model tests have been carried out to validate the computational model. Fairly good agreement is seen. Comparisons of the force components with fully linear and body-nonlinear models help in bringing out the improvements due to the new formulation.

scholarly journals Experimental Pullout Capacity of Screw Piles in Dry Gypseous Soil

2021 ◽  
Vol 318 ◽  
pp. 01002
Author(s):  
Mahdi O. Karkush ◽  
Omar J. Mukhlef
Keyword(s):  
Relative Density ◽  
Offshore Structures ◽  
Pullout Capacity ◽  
Single Screw ◽  
Gypsum Content ◽  
Pullout Testing ◽  
D 20 ◽  
Pile Model ◽  
Supporting Structures ◽  
Gypseous Soil

Screw piles are widely used in supporting structures subjected to pullout forces, such as power towers and offshore structures, and this research investigates their performance in gypseous soil of medium relative density. The bearing capacity and displacement of a single screw pile model inserted in gypseous soil with various diameters (D = 20, 30, and 40) mm are examined in this study. The soil used in the testing had a gypsum content of 40% and the bedding soil had a relative density of 40%. To simulate the pullout testing in the lab, a physical model was manufactured with specific dimensions. Three steel screw piles with helix diameters of 20, 30, and 40 mm are used, with a total length of 500 mm. The helix is continuous over the pile's embedded depth of 400 mm. The results of tests revealed that decreasing the length to diameter (H/D) ratio resulted in a higher pullout capacity of screw piles and a lower corresponding displacement.

scholarly journals New Investigations for Offshore and Onshore structures

2019 ◽  
Vol 1 (1) ◽  
Author(s):  
Levent Yilmaz
Keyword(s):  
State Of The Art ◽  
Offshore Structures ◽  
History Of ◽  
Jack Up

In this research the investigated parameters are given as below: History of offshore and onshore construction…Onshore structures…StorageTanks Breakwaters……Estacada…Offshore structures…Stationary fixed platforms (steel, concrete, combined, arctic)…Floating (Jack-up, semi-submersibles, TLP,…)…Pipe lineThis issues are given as State-of-the-art and written new examined innovative researches.

System Identification of Jack-Up Platform by Spectral Analysis

2010 ◽  
Author(s):  
X. M. Wang ◽  
C. G. Koh ◽  
T. N. Thanh ◽  
J. Zhang
Keyword(s):  
Spectral Analysis ◽  
System Identification ◽  
Time Domain ◽  
Time History ◽  
Offshore Structures ◽  
Wave Load ◽  
History Of ◽  
Structural Responses ◽  
Jack Up ◽  
Numerical Strategy

For the purpose of structural health monitoring (SHM), it is beneficial to develop a robust and accurate numerical strategy so as to identify key parameters of offshore structures. In this regard, it is difficult to use time-domain methods as the time history of wave load is not available unless output-only methods can be developed. Alternatively, spectral analysis widely used in offshore engineering to predict structural responses due to random wave conditions can be used. Thus the power spectral density (PSD) of structural response may be more appropriate than time history of structural responses in defining the objective (fitness) function for system identification of offshore structures. By minimizing PSD differences between measurements and simulations, the proposed numerical strategy is completely carried out in frequency domain, which can avoid inherent problems rising from random phase angles and unknown initial conditions in time domain. A jack-up platform is studied in the numerical study. A search space reduction method (SSRM) incorporating the use of genetic algorithms (GA) as well as a substructure approach are adopted to improve the accuracy and efficiency of identification. As a result, the stiffness parameters of jack-up legs can be well identified even under fairly noisy conditions.

Response Spectrum Estimation for Fixed Offshore Structures With Inundation Effect Included: A Price’s Theorem Approach

2004 ◽  
Vol 126 (4) ◽  
pp. 337-345 ◽  
Author(s):  
Xiang Yuan Zheng ◽  
Chih Young Liaw
Keyword(s):  
Fourier Transforms ◽  
Volterra Series ◽  
Response Spectrum ◽  
Power Spectra ◽  
Memory Systems ◽  
Offshore Structures ◽  
Spectrum Estimation ◽  
Nonlinear Frequency ◽  
Finite Memory ◽  
Jack Up

Instead of a Volterra-series based method, a method based on Price’s theorem and Fourier Transforms is proposed for evaluating the nonlinear response power spectra of fixed offshore structures modeled as finite-memory systems. The numerical computation effort required for spectral estimations using the proposed method is significantly less than that of the Volterra-series method that requires multi-dimensional integrations. Nonlinear frequency-domain analyses of two jack-up platform structures are carried out to study the effects of inundation, cubic and quartic drag terms and polynomialization methods.

1300 Tons Structural Support to Secure the Integrity of an Offshore Platform

2020 ◽  
Author(s):  
Francisco Javier Becerro ◽  
Beatriz Alonso ◽  
Roland De Burgh Daly ◽  
Petter Vabø
Keyword(s):  
Service Life ◽  
Dry Weight ◽  
Offshore Structures ◽  
Screening Process ◽  
Field Development ◽  
Structural Fatigue ◽  
Structural Support ◽  
Original Field ◽  
Jack Up ◽  
Alternative Solutions

Abstract The Yme oilfield is located approximately 100 km from the Norwegian coastline in the Egersund basin. The original field development in 2008 consisted on a jack-up type platform (MOPU) and a subsea storage tank at 93 m depth. Both are interconnected through a caisson containing all risers, piping and umbilicals which also supports a wellhead platform (WHM). That original MOPU was removed in 2016 [1] and a redevelopment of the field is currently under execution making use of some of the remaining facilities. In order to re-use the existing tank and caisson, advanced structural fatigue analyses including fracture mechanics, probabilistic assessment, motion monitoring and regular NDT inspection concluded with the requirement for a support structure to extend the service life of the old structure, which fatigue life was limited. A screening process that covered different alternative solutions lead to a design (called Caisson Permanent Support or CPS) with overall dimensions (L.B.H) of approx. 70m × 50m × 40m and a dry-weight of 1300 t (figure 9). It consists on a beam structure composed of steel tubular braces with diameters ranging from 1,1 to 2,7 m that supports a clamp (6 m height and 5,15m diameter) connected to the caisson. In an iterative process between structural, fabrication and installation designers the optimal final concept was selected after detailed analyses to demonstrate that is capable of meeting the requirement of 20 additional years of structural service life for Yme facilities. The CPS was successfully installed in June 2019 ensuring the integrity of the offshore structures and enabling the further re-development of the Yme field. Given its dimensions and its particular design purpose, the CPS is regarded as a unique project in the industry. This paper describes the challenges and the engineering work required for the design of the CPS.

Dynamic modelling of flexible structures using a local frame formulation

1999 ◽  
Vol 213 (7) ◽  
pp. 645-653 ◽  
Author(s):  
G A Chochia ◽  
P K Chawdhry ◽  
C R Burrows
Keyword(s):  
High Speed ◽  
Flexible Structures ◽  
Dynamic Modelling ◽  
Element Formulation ◽  
Local Frame ◽  
Incremental Method ◽  
Slender Beams ◽  
Simplifying Assumptions ◽  
New Formulation ◽  
Inertia Forces

In this paper a new finite element formulation for slender beams is suggested that is valid for large rotations and deflections. Traditional formulations such as the floating frame approach and the incremental method are limited to small deflections, which restricts the scope of their applicability to small accelerations. In the local frame formulation no simplifying assumptions are made in accounting for the inertia forces. As the formalism remains consistent with large deflections it can be used for accurate evaluation of deformations caused by inertia forces at large accelerations. The paper is concerned with multibeam plane motions. The new formulation is implemented as a computer algebra tool that generates dynamic equations of flexible multibody systems from specifications. A number of case studies related to high-speed machinery are presented.

Stochastic Linearization and its Application in Motion Analysis of Cylindrical Floating Structure With Bilge Boxes

2016 ◽  
Author(s):  
Yan-Lin Shao ◽  
Jikun You ◽  
Einar Bernt Glomnes
Keyword(s):  
Motion Analysis ◽  
Model Test ◽  
Added Mass ◽  
Offshore Structures ◽  
Test Results ◽  
Viscous Effects ◽  
Natural Period ◽  
Stochastic Linearization ◽  
Excitation Force ◽  
Drag Term

To account for the viscous effects of damping devices, for instance, bilge keels or bilge boxes, on the motions of ships and offshore structures, Morison’s equation is often adopted as an empirical but practical approach in the design process. In order to combine the standard engineering panel method with the drag term in Morison’s equation, and remain in the frequency domain, the drag term has to be linearized based on, for instance, stochastic linearization. In this paper, the stochastic linearization scheme is implemented in an in-house code and verified through the comparison with the DNV GL software WADAM. The model test results of a large cylindrical FPSO with bilge box are used to calibrate the drag coefficients in the Morison’s equation. When the linearized drag forces are included, heave motion RAOs correspond better to the model test results. However, the predicted natural periods of heave motions are seen to be smaller than those obtained from model tests. It is suspected that the viscous flow separation around the bilge box increases the added mass of the unit beyond what is predicted by potential flow alone. Discussions are made on the effect of viscous added mass on the heave natural period. It is quite common to only include the damping effects in the motion analysis for large offshore structures and ignore the contribution of the viscous effects on the excitation force. For the considered cylindrical FPSO, this paper demonstrates that the viscous excitation force can be important in survival conditions.

Jack-Up Site Assessment: The Voyage to an ISO

2011 ◽  
Author(s):  
Mike J. R. Hoyle ◽  
John J. Stiff ◽  
Rupert J. Hunt
Keyword(s):  
Offshore Structures ◽  
Site Assessment ◽  
Detailed Review ◽  
Technical Developments ◽  
Ocean Environment ◽  
The Many ◽  
Analytical Tools ◽  
Jack Up ◽  
Safe Working

As a mobile offshore unit, a jack-up can float, self-elevate and then impose proofing loads on the seabed to establish a safe working envelope for subsequent drilling or construction activities. Jack-up site assessment has seen significant advances over the 50 or more years that jack-ups have been deployed offshore. The advances have occurred as a result of both the improvements that have been seen in the analytical tools and also as a result of the many significant strides in our understanding of the physics of the jack-up and the ocean environment. Jack-ups, due to their semi-compliant nature, pose particularly difficult challenges to the designer and operator — more so than most offshore structures. This requires cutting edge technologies to assure that practical solutions can be reached without either too much conservatism or too little safety. These advances have often occurred, or been encouraged, as a result of the engagement of the operators hiring the jack-ups and their technical overseers such as Jan Vugts. Jan was instrumental in initiating the Shell study into jack-up site assessment that resulted in a follow-up JIP which developed the SNAME Recommend Practice. Since then ISO 19905-1 has been under development and Jan has provided both encouragement and also very detailed review comments. This paper charts the voyage from the early days of jack-up site assessment through to the development of the ISO and highlights some of the key technical developments.

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