Splash Zone Lifting Analysis of Subsea Structures

2010 ◽  
Author(s):  
Arunjyoti Sarkar ◽  
Ove T. Gudmestad
Keyword(s):  
Added Mass ◽  
Double Pendulum ◽  
Hydrodynamic Coefficients ◽  
Splash Zone ◽  
Sea State ◽  
Flat Surfaces ◽  
Analysis Methodology ◽  
Loading Case ◽  
Hoisting System ◽  
Water Particle Kinematics

The lifting analysis of a subsea structure determines the maximum allowable design sea state in which the structure can be installed safely. Normally, such analysis on the structure at the splash zone governs the expected largest forces in the hoisting system and in turn the allowable sea state since the water particle kinematics is larger in the splash zone. In this paper, the DNV Recommended Practice for Modelling and Analysis of Marine Operation (DNV-RP-H103, April 2009) is discussed with emphasis on the hydrodynamic coefficients and analysis methodology for the splash zone lifting analysis. An approach is suggested here to take into account the free surface proximity effect on added mass of flat surfaces in the absence of test results. Discussions on the following points are also included: • For structures which show restricted sea state due to large double pendulum motion and consequently high dynamic tension in the crane wire, a solution could be obtained by lowering the sling angles. • For inertia dominated structures, the drag coefficients should be chosen with caution unless experimental results are available since the drag may induce unrealistic damping in the system. • For the structural design of large subsea structures, the design DAF for submerged condition should be chosen from a preliminary lifting analysis result. The current industrial practice of using DAF = 2 with respect to the static submerged weight could be increased following the analysis result to optimise the use of the crane capacity by achieving a higher design sea state. • For lifting analysis of structures with large added mass / submerged weight, modelling of winch speed may represent a worse loading case as compared to the case with zero winch speed in the splash zone. • For the splash zone analysis, correct modelling of the stiffness of the crane structure along with the wire is important. The assumption that the crane structure is rigid may lead to unrealistic analysis results. Experimental programmes to obtain further information on the amplitude dependent characters of the hydrodynamic coefficients, the stiffness and the damping of the Crane, the wires etc are furthermore recommended.

Dynamic Forces and Limiting Sea States for Installation of GRP Protection Covers

2017 ◽  
Author(s):  
Frøydis Solaas ◽  
Peter Christian Sandvik ◽  
Csaba Pâkozdi ◽  
Timothy Kendon ◽  
Kjell Larsen ◽  
...  
Keyword(s):  
Time Domain ◽  
Linear Time ◽  
Added Mass ◽  
Alternative Methods ◽  
Splash Zone ◽  
Cfd Simulations ◽  
Sea State ◽  
Glass Fiber Reinforced ◽  
Dynamic Forces ◽  
Theory Solution

This paper describes a study aimed at finding and demonstrating a feasible method to reduce the uncertainties in calculation of dynamic forces and limiting sea states for installation of protection covers produced from glass fiber reinforced polyester (GRP). Uncertainties arise in the choice of hydrodynamic coefficients and the applied analysis method e.g. the Simplified Method, as suggested in DNV-OS-H206, versus time-domain simulations. The maximum limiting sea state for water entry and lowering through the splash zone has been assessed stepwise by use of alternative methods. Firstly, the hydrodynamic force coefficients for a fully submerged, selected GRP cover were estimated manually, by use of simplified data in DNVGL-“Recommended practice for modelling and analysis of marine operations”, DNVGL-RP-H103. The estimated hydrodynamic added mass was compared with the potential theory solution obtained by use of WAMIT. WAMIT calculations are also performed to obtain added mass and potential damping for the cover with different draughts at the selected installation angle. Viscous damping and added mass will be dependent on amplitude of oscillation and is studied by CFD simulations. A fully submerged cover is oscillated harmonically with different amplitudes at a selected period. The obtained added mass and damping coefficients were used in a numerical model including installation ship, lifting gear and GRP cover, in the non-linear time domain simulation program SIMO. The lowering through the splash zone were finally performed in some selected wave conditions to illustrate how a realistic limiting sea-state for the lowering through the splash zone may be estimated.

Lifting Analysis of Subsea Framework Structures

2014 ◽  
Author(s):  
Tor E. Søfteland ◽  
Odd V. Skrunes ◽  
Daniel Karunakaran
Keyword(s):  
Structural Integrity ◽  
Maximum Load ◽  
Hydrodynamic Forces ◽  
Sea Surface ◽  
Hydrodynamic Coefficients ◽  
Splash Zone ◽  
Sea State ◽  
Dynamic Forces

Installation of subsea structures and equipment involves a lifting operation where the objects are exposed to large hydrodynamic forces when entering the oscillating sea-surface. During deployment, as the structure is lifted through the splash zone, snap forces due to slack or overload due to dynamic forces contribute to the maximum load experienced by the structure over the course of its design lifetime. This paper presents a method of how to verify the structural integrity of a subsea framework including determination of the maximum allowable sea-state in which the structure is safely installed. As well as describing an overall methodology for a subsea lifting analysis, hydrodynamic coefficients for cylinders in the splash zone is provided.

Strength Performance of Steel Catenary Riser Using Short Term and Long Term Analysis Methodologies

2016 ◽  
Author(s):  
Feng Wang ◽  
Roger Burke ◽  
Anil Sablok ◽  
Kristoffer H. Aronsen ◽  
Oddgeir Dalane
Keyword(s):  
Bending Moment ◽  
Current Profile ◽  
Short Term ◽  
Sea State ◽  
Strength Performance ◽  
Steel Catenary Riser ◽  
Analysis Methodology ◽  
Riser Design ◽  
Term Analysis

Strength performance of a steel catenary riser tied back to a Spar is presented based on long term and short term analysis methodologies. The focus of the study is on response in the riser touch down zone, which is found to be the critical region based on short term analysis results. Short term riser response in design storms is computed based on multiple realizations of computed vessel motions with various return periods. Long term riser response is based on vessel motions for a set of 45,000 sea states, each lasting three hours. The metocean criteria for each sea state is computed based on fifty six years of hindcast wind and wave data. A randomly selected current profile is used in the long term riser analysis for each sea state. Weibull fitting is used to compute the extreme riser response from the response of the 45,000 sea states. Long term analysis results in the touch down zone, including maximum bending moment, minimum effective tension, and maximum utilization using DNV-OS-F201, are compared against those from the short term analysis. The comparison indicates that the short term analysis methodology normally followed in riser design is conservative compared to the more accurate, but computationally more expensive, long term analysis methods. The study also investigates the important role that current plays in the strength performance of the riser in the touch down zone.

Reynolds Number Effects on Hydrodynamic Coefficients for Pure In-Line and Pure Cross-Flow Forced Vortex Induced Vibrations

2009 ◽  
Author(s):  
Jamison L. Szwalek ◽  
Carl M. Larsen
Keyword(s):  
Reynolds Number ◽  
Prediction Models ◽  
Current Knowledge ◽  
Cross Flow ◽  
Reynolds Numbers ◽  
Added Mass ◽  
Hydrodynamic Coefficients ◽  
Vortex Induced Vibrations ◽  
Reynolds Number Effects ◽  
Sinusoidal Oscillations

In-line vibrations have been noted to have an important contribution to the fatigue of free spanning pipelines. Still, in-line contributions are not usually accounted for in current VIV prediction models. The present study seeks to broaden the current knowledge regarding in-line vibrations by expanding the work of Aronsen (2007) to include possible Reynolds number effects on pure in-line forced, sinusoidal oscillations for four Reynolds numbers ranging from 9,000 to 36,200. Similar tests were performed for pure cross-flow forced motion as well, mostly to confirm findings from previous research. When experimental uncertainties are accounted for, there appears to be little dependence on Reynolds number for all three hydrodynamic coefficients considered: the force in phase with velocity, the force in phase with acceleration, and the mean drag coefficient. However, trends can still be observed for the in-line added mass in the first instability region and for the transition between the two instability regions for in-line oscillations, and also between the low and high cross-flow added mass regimes. For Re = 9,000, the hydrodynamic coefficients do not appear to be stable and can be regarded as highly Reynolds number dependent.

Importance of Added Mass for the Interaction Between IL and CF Vibrations of Free Spanning Pipelines

2011 ◽  
Author(s):  
Ida M. Aglen ◽  
Carl M. Larsen
Keyword(s):  
Cross Sections ◽  
Amplitude Ratio ◽  
Large Diameter ◽  
Cross Flow ◽  
Added Mass ◽  
Flexible Beam ◽  
Hydrodynamic Coefficients ◽  
Cross Sectional ◽  
Mass Coefficient ◽  
The Stability

The importance of cross-flow (CF) response generated by vortex induced vibrations (VIV) of free spanning pipelines has long been recognised. The significance of in-line (IL) vibrations has recently been understood and hence also been subjected to research. The combined effect of CF and IL vibrations is, however, still not fully described. This paper highlights the CF-IL interaction with focus on the transition zone from pure IL to CF dominated response, giving special attention to how the added mass influences the interaction. Results from extensive flexible beam tests connected to the Ormen Lange (OL) development have been used as a basis for this study. Trajectories for cross sectional motions from the flexible beam test were identified, and then used as forced motions of a large diameter rigid cylinder exposed to uniform flow. Non-dimensional parameters like Reynolds number (Re), amplitude ratio and reduced frequency were identical for the two tests. Hence, forces found from the forced motion test could be used to find hydrodynamic coefficients valid for the flexible beam case. This paper discusses the results from the flexible beam tests with a relatively short length to diameter ratio (L/D) of 145. Modal analyses by Nielsen et al. (2002) show that the first mode dominates in both directions for these particular tests, even though the IL response frequency is twice the CF frequency. In this paper the added mass variations along the OL flexible beam is studied. Forces acting on 4 different cross sections along the beam are measured for 7 different prototype velocities. For each test the hydrodynamic coefficients are calculated, and the results show how the added mass changes along the beam for increasing velocities, and thereby creates resonance for both IL and CF response. The stability of the added mass coefficient throughout the time series is also evaluated.

scholarly journals Hydrodynamic Coefficients of Simplified Subsea Structures

2018 ◽  
Author(s):  
Fredrik Mentzoni ◽  
Mia Abrahamsen-Prsic ◽  
Trygve Kristiansen
Keyword(s):  
Cross Section ◽  
Significant Interaction ◽  
Hydrodynamic Interaction ◽  
Forced Oscillation ◽  
Large Range ◽  
Circular Cross Section ◽  
Added Mass ◽  
Hydrodynamic Coefficients ◽  
Force Estimation ◽  
Damping Coefficients

Simplified two-dimensional models, representing components of complex subsea structures, are experimentally investigated. Individual as well as combinations of components in different configurations are tested, in order to study the effect of hydrodynamic interaction. The components include porous plates and cylindrical pipes with circular cross-section. Hydrodynamic added mass and damping coefficients, relevant for force estimation during lifting operations, are presented. The coefficients are obtained based on forced oscillation tests for a large range of Keulegan–Carpenter (KC) numbers and forcing periods, and compared to numerical source panel results for the low KC limit, as well as recommendations given by DNV GL, where relevant. Coefficients for all configurations are found to be highly amplitude dependent. Significant interaction effects are found for the assembled structures, causing either reduced or increased total added mass and damping coefficients compared to the super-position of the coefficients for individual members.

Control Oriented Maneuver Model of Prismatic Planing Hull

2021 ◽  
Author(s):  
Li P. Sung ◽  
Brian Fabien
Keyword(s):  
Test Data ◽  
Potential Flow ◽  
Low Cost ◽  
Added Mass ◽  
Cost Models ◽  
Computational Resource ◽  
Initial Design ◽  
Calm Water ◽  
Loading Case ◽  
Turning Maneuver

Roll yaw coupled dynamics are not well characterized and existing low cost models are limited in range of applicability. We utilize an interpolation based approach to address a wider range of conditions with reduced computational requirements. By interpolating test data to estimate hydrodynamic forces and empirically modeling roll damping and added mass, we establish a 4DOF maneuvering model for prismatic planing hulls in calm water. It is validated against relevant tests and show significant computational resource savings in comparison with potential flow based methods. Simulation of an extreme turning maneuver and an asymmetrical loading case demonstrates its potential for use in initial design, control and evaluation.

Calculation of In-Line Vortex Induced Vibrations of Free Spanning Pipelines

2007 ◽  
Author(s):  
Carl M. Larsen ◽  
Rune Yttervik ◽  
Kristoffer Aronsen
Keyword(s):  
Cross Flow ◽  
Added Mass ◽  
Response Analysis ◽  
Hydrodynamic Coefficients ◽  
Response Model ◽  
Analysis Method ◽  
Vortex Induced Vibrations ◽  
Long Time ◽  
General Response

Pure in-line (IL) vibrations will in many cases contribute significantly to fatigue damage for free spanning pipelines. This might be the case even if IL amplitudes are smaller than cross-flow (CF). While CF response has been subjected to research for a long time, little attention has so far been given to the pure IL VIV case. The hydrodynamic coefficients needed for response calculation have in fact not been available until recently, but results from forced IL oscillations have improved this situation. Data for added mass and force in IL direction has been used to establish a general response model along the same lines as for traditional CF response analysis. This has made it possible to calculate stresses from IL VIV in free spanning pipelines, and include the influence from interaction with the seafloor at the span shoulders. A brief presentation of the analysis method is given, but the main part of the paper gives results from a case study that illustrates important effects and the significance of IL response as compared to CF.

HYDRODYNAMIC COEFFICIENTS AND FORCES ON MULTIHULLS IN SHALLOW WATER WITH CONSTANT OR VARIABLE DEPTH

Transport ◽  
2008 ◽  
Vol 23 (3) ◽  
pp. 245-252 ◽  
Author(s):  
Noureddine Dabssi ◽  
Mohamed Chagdali ◽  
Alain Hémon
Keyword(s):  
Boundary Element Method ◽  
Shallow Water ◽  
Numerical Method ◽  
Boundary Element ◽  
Added Mass ◽  
Hydrodynamic Coefficients ◽  
Variable Depth ◽  
Harmonic Oscillations ◽  
Hull Forms ◽  
Hydrodynamic Reaction

Numerical and hydro dynamical procedures are developed to compute bidimensional hydrodynamic coefficients and forces on multihulls associated with harmonic oscillations in shallow water with constant or variable depth. The forces are composed of two parts and include the sum of incident and diffracted forces and hydrodynamic reaction. The latter one is used to determinate the hydrodynamic coefficients (added mass and damping). The numerical method used is the Boundary Element Method. We can compute flow around multihulls sections. An application to cylindrical, right triangular and rectangular hull forms is presented.

Wave Measurements Using an Air Pressure Gauge

2008 ◽  
Author(s):  
Leonardo Brantes Bacellar Mendes ◽  
Luis Manoel Paiva Nunes ◽  
Dio´rgenes Penteado ◽  
Jose´ Antonio Moreira Lima
Keyword(s):  
Sea Water ◽  
Pressure Gauge ◽  
Air Pressure ◽  
Production Costs ◽  
Sea State ◽  
Coastal Site ◽  
Wave Measurements ◽  
Analysis Methodology ◽  
Level Data ◽  
Water Level Data

An equipment for wave and tide measurements using an air pressure gauge was developed and tested in a Brazilian coastal site (Ilha Grande Bay Maritime Terminal, Lat 23° 03’ 22” S/ Lon 44° 14’ 07” W) for 18 months. The main properties of this equipment were the low production costs and the improved maintenance procedures required by a dry sensor fixed over the sea water level. Data were analyzed using a standard Fast Fourier Transform (FFT) algorithm and the results were very good. As an example of the adequate spectral resolution attained with the analysis methodology, results from a bimodal sea state will be presented, the first peak with period of 5.43 seconds and the second one with a period of 8.30 seconds.

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