Effect of Wave Induced Loading and Line Loading in Fully Stochastic Fatigue Assessment for West Africa FPSO

2004 ◽  
Author(s):  
T. H. Park ◽  
J. H. Lee ◽  
J. W. Cho ◽  
H. S. Shin
Keyword(s):  
West Africa ◽  
Fatigue Damage ◽  
Bending Moment ◽  
Ship Motions ◽  
Piping System ◽  
Site Specific ◽  
Fatigue Assessment ◽  
Loading Effects ◽  
Wave Induced ◽  
Side Shell

The fatigue assessment for a caisson of the interface between a caisson and a FPSO hull has been performed based on the FMS (Fatigue Methodology Specification) [1]. Investigation is focused on the various loading effects including inertia loading due to ship motions from a site specific swell, sea and line loading from the submerged caisson connected to the side shell of FPSO. The fully stochastic method was used for estimating damage levels from wave loadings. The component-based method was used for line loading effects. The two kinds of results from each case were combined the variance and mean period combination. For the inertia loading, it is shown that a hull deforming due to vertical bending moment is the principal effect for a fatigue assessment. For the line loading, it is found that the loading effect due to the submerged piping system connected to the side shell of FPSO is not significant for the fatigue of the side shell supporting structure. In conclusion, the fatigue damage from the site-specific swell is dominant effect among overall fatigue damage components of FPSO in the specific site condition of West Africa.

Effect of Heavy Weather Maneuvering on the Wave-Induced Vertical Bending Moments in Ship Structures

1990 ◽  
Vol 34 (01) ◽  
pp. 60-68 ◽  
Author(s):  
C. Guedes Soares
Keyword(s):  
Wave Height ◽  
Significant Wave Height ◽  
Decision Rules ◽  
Bending Moment ◽  
Ship Motions ◽  
Bending Moments ◽  
Significant Wave ◽  
Course Changes ◽  
Wave Induced ◽  
Vertical Bending Moment

Statistical data are collected so as to quantify the probability of occurrence of voluntary course changes in heavy weather as well as their dependence on significant wave height and on ship heading. Decision rules are established about when and how to change course, on the basis of the analysis of operational data and of interviews with experienced shipmasters. A Monte Carlo simulation is performed so as to determine how an omnidirectional distribution of initial headings is changed by voluntary course changes depending on the significant wave height. Finally, the effect of the nonuniform distribution of headings on the mean wave-induced vertical bending moment is calculated. It is shown that although heavy weather maneuvering eases the ship motions, it can increase the wave-induced bending moments and thus increase the probability of structural failure.

On the Computation of Wave Induced Bending and Torsion Moments

1971 ◽  
Vol 15 (03) ◽  
pp. 217-220
Author(s):  
T. Francis Ogilvie
Keyword(s):  
Boundary Value Problem ◽  
Boundary Value ◽  
Diffraction Problem ◽  
Vertical Plane ◽  
Bending Moment ◽  
Ship Motions ◽  
Wave Loads ◽  
Cross Member ◽  
Wave Induced ◽  
Incident Waves

In the calculation of wave loads on a ship, one must consider the effects of both the incident waves and the diffraction waves (the latter being caused by the presence of the ship in the incident waves). In the ship-motions problem, Khaskind showed how one can do this without having to solve the diffraction-wave boundary-value problem. Khaskind's procedure is here extended to the calculation of structural loads on a ship. Two examples are discussed: (i) bending moment in the vertical plane of a ship in waves and (ii) torsion in the cross member of a catamaran. Many other applications are possible. In each case, it is necessary to solve a boundary-value problem, but it is generally much simpler than the diffraction problem.

Validation of Spectral Fatigue Assessment of a West-Africa FPSO Using Full-Scale Measurements

2021 ◽  
Author(s):  
Alexandru Andoniu ◽  
Jérôme de Lauzon ◽  
Remco Hageman ◽  
Pieter Aalberts ◽  
Didier L'Hostis ◽  
...  
Keyword(s):  
West Africa ◽  
Fatigue Damage ◽  
Wave Energy ◽  
Fatigue Analysis ◽  
Full Scale ◽  
Fatigue Assessment ◽  
Sources Of Uncertainty ◽  
Wave Conditions ◽  
Offshore Industry ◽  
Different Sources

Abstract In order to ensure structural integrity and safe operations, fatigue assessment of structural details is a key aspect of design and verification procedures for FPSOs. Spectral fatigue analysis is widely used in the offshore industry to assess damage induced by wave loading. However, the actual fatigue accumulation endured by units in operations usually differs from predictions due to the assumptions made at the design stage. One of the sources of uncertainty is the representation of the encountered sea states. The objective of this paper is to use in-situ measurements as a reference for evaluating the accuracy of spectral fatigue analysis and to investigate the influence of wave energy description on the fatigue assessment. Structural health monitoring systems have been increasingly used in the last decade in the offshore industry as they constitute a valuable source of information regarding the actual operating conditions, structural response, or encountered environmental conditions. This data can be used to update fatigue assessment in order to determine the remaining service life, understand how the structure is aging, or support for decision making regarding inspections, maintenance, or lifetime extension. The work presented is based on such information gathered during a measurement campaign performed on a spread-moored FPSO in West Africa. Measured strain time histories at several locations on the hull have been used to derive the actual fatigue damage endured by the unit. These damages are compared to the ones determined from spectral fatigue analysis using stress transfer functions obtained from frequency domain hydro-structure computations. Multiple analyses have been performed to evaluate the impact of different sources of statistical wave data and wave energy descriptions on the fatigue assessment. The wave conditions used originate from wave buoy measurements and hindcast data. Overall, the good agreement between full-scale measurements and calculations confirms the suitability of spectral methods for determining fatigue damage. When incomplete information is available, which is often true in the case of wave statistics, assumptions have to be made regarding parameters such as spectrum shape or wave spreading. However, using the full description of wave energy spectra, if available, can be a way of reducing uncertainties and removing unnecessary assumptions in such analysis. The results of this work show how fatigue assessment can be improved by gaining insight into the different sources of uncertainty, notably the sea state representation. With increasing focus on digital solutions, these results show realistic potential for virtual hull monitoring solutions based on accurate numerical models and realistic representation of wave conditions.

Feasibility of Using Hindcast Data for Fatigue Assessment of Permanently Moored Offshore Units in West-Africa

2021 ◽  
Author(s):  
Remco Hageman ◽  
Pieter Aalberts ◽  
Didier L'hostis ◽  
Alain Ledoux ◽  
Brett Forrester ◽  
...  
Keyword(s):  
West Africa ◽  
Fatigue Assessment

Probabilistic Fatigue Assessment of a Mooring Line Based on Station-Keeping Trials in Ice

2021 ◽  
Author(s):  
Chana Sinsabvarodom ◽  
Bernt J. Leira ◽  
Wei Chai ◽  
Arvid Naess
Keyword(s):  
Fatigue Damage ◽  
Probabilistic Models ◽  
Full Scale ◽  
Mooring Line ◽  
Stress Range ◽  
Station Keeping ◽  
Probability Models ◽  
Line System ◽  
Fatigue Assessment ◽  
Rainflow Counting

Abstract The intention of this work is to perform a probabilistic fatigue assessment of a mooring line due to loads associated with the station-keeping of a ship in ice. In March 2017, the company Equinor (Statoil) conducted full-scale tests by means of station-keeping trials (SKT) in drifting ice in the Bay of Bothnia. The vessel Magne Viking was employed in order to represent a supply vessel equipped with a mooring line system, and the realtime loading during the full-scale measurement was recorded. The second vessel Tor Viking was serving as an ice breaker in order to maintain the physical ice management activities with different ice-breaking schemes, i.e. square updrift pattern, round circle pattern, circular updrift pattern and linear updrift pattern. The fatigue degradation corresponding to these different patterns were investigated. The peaks and valleys of the mooring tension are determined using the min peak prominence method. For the purpose of probabilistic fatigue assessment, the Rainflow-counting algorithm is applied to estimate the mooring stress range. Fatigue assessment based both on Rainflow counting and fitted probabilistic models were performed. For the latter, the stress range distributions from the observed data of mooring loads are fitted to various probability models in order to estimate the fatigue damage. It is found that the stress ranges represented by application of the Weibull distribution for the probabilistic fatigue approach provides results of the fatigue damage most similar to the Rainflow counting approach. Among the different scenarios of Ice management schemes, the circular updrift pattern provides the lowest magnitude of the fatigue degradation.

A Novel High Cycle Fatigue Assessment of Small-Bore Side Branches: Tailor-Made Acceptable Vibration Levels Based on the Remaining Life of Existing Structures

2014 ◽  
Author(s):  
Pieter van Beek ◽  
Richard Pijpers ◽  
Kenneth Macdonald ◽  
Johan Maljaars ◽  
Knud Lunde ◽  
...  
Keyword(s):  
Fracture Mechanics ◽  
High Cycle Fatigue ◽  
Side Branch ◽  
International Standards ◽  
Screening Methods ◽  
Piping System ◽  
Cycle Fatigue ◽  
Fatigue Assessment ◽  
Mechanics Model

In the process systems of offshore installations, welded small-bore side branches can prove vulnerable to high-cycle fatigue failure due to vibrations. This is especially the case for welded connections at tie-in points to the main pipe which are often critical details. International standards and guidelines therefore provide maximum acceptable vibration levels to ensure long term safe operation. In some guidelines, however, these acceptable vibration levels are phrased in terms of screening levels and in practice can be unduly conservative. Process pipework might then unjustly be regarded as unsafe based on measured vibrations in the field. This is especially true for offshore systems, which are characterized by low mechanical damping in the structure. This may result in overdesigned piping or over-conservative operational limits in order to keep vibration levels within the acceptable range. Furthermore, the screening methods and any detailed fatigue assessments typically use established stress-life (S-N) based fatigue design methods where uncertainty exists in the very high-cycle regime. This paper describes a novel and advanced tailor-made fatigue assessment method whereby acceptable vibration levels are based on maximum acceptable stress ranges for individual side branches. The acceptable stress ranges for each critical welded connection are based on a fracture mechanics analysis of fatigue crack growth. This method also minimizes the cantilevered (overhung) mass of small-bore side branches, whilst remaining safe for long-term operation. To illustrate the strength of the assessment methodology in practice, this paper describes the application of the procedure to a 2″ side branch connected to a main piping system. A fracture mechanics model and a detailed 3D finite element model are made. By comparing the stress ranges from the fracture mechanics model with the normalized stress ranges obtained from the dynamic FE analysis, maximum acceptable vibration levels for this particular side branch have been derived. The method is validated with experimental modal analysis and strain gauge measurements.

First Ocean Going Ships With Springing and Whipping Included in the Ship Design

2011 ◽  
Author(s):  
Gaute Storhaug ◽  
Erlend Moe ◽  
Ricardo Barreto Portella ◽  
Tomazo Garzia Neto ◽  
Nelson Luiz Coelho Alves ◽  
...  
Keyword(s):  
Fatigue Damage ◽  
Current Model ◽  
Life Time ◽  
Model Tests ◽  
Full Scale ◽  
Development Projects ◽  
Transient Vibration ◽  
Hull Girder ◽  
Fatigue Evaluation ◽  
Wave Induced

It is well known that ships vibrate due to waves. The wave induced vibrations of the hull girder are referred to as springing (resonance) and whipping (transient vibration from impacts). These vibrations contribute to the fatigue damage of fatigue sensitive details. An Ore Carrier of 400 000 dwt is currently being built by DSME, and at time of delivery, it will be the world’s largest bulk (ore) carrier. The scantlings of large ships must be carefully designed with respect to global loading, and when extending the design beyond experience, it is also wise to consider all aspects that may affect operation and the life time costs. The vessel will also enter a long term contract and is therefore to be evaluated for 30 year Brazil-China operation. In order to minimize the risk of fatigue damage, the vessel is designed according to DNV’s class notation CSA-2 requiring direct calculations of the loading and strength. Further it has been requested to include the effect of springing and whipping in the design. Reliable numerical tools for assessing the additional fatigue effect of vibrations are non-existing. DNV has, however, developed an empirical guidance on how the additional effect may be taken into account based on previous development projects related to the effect of vibrations on large ore carriers Due to the size and route of operation of the new design, it has, however, been required by the owner to carry out model tests in both ballast and cargo condition in order to quantify the contribution from vibration. The results from this project have been used for verification and further calibration of DNV’s existing empirical guidance. A test program has been designed for the purpose of evaluating the consequence in head seas for the Brazil to China trade. Full scale measurements from previous development projects of ore carriers and model tests have been utilized to convert the current model tests results into estimated full scale results for the 400 000 dwt vessels. It is further important to carefully consider how the vibrations are to be included in the design verification, and to develop a procedure for taking into account the vibrations which results in reasonable scantlings based on in-service experience with similar designs and trades. This procedure has been developed, and a structural verification has been carried out for the design. The final outcome of the model test was in line with previous experience and in overall agreement with DNV’s empirical guidance, showing a significant contribution from vibrations to the fatigue damage. The springing/whipping vibrations more than doubled the fatigue damage compared to fatigue evaluation of the isolated wave induced loading. The cargo condition vibrated relatively more than experienced on smaller vessels. Various sources to establish the wave conditions for the Brazil to China ore trade were used, and the different sources resulted in significant differences in the predicted fatigue life of the design.

The Effect of Bow Shape on the Springing/Whipping Response of a Large Ocean-Going Vessel: Investigated by an Experimental Method

2007 ◽  
Author(s):  
Gaute Storhaug ◽  
Torgeir Moan
Keyword(s):  
Fatigue Damage ◽  
Transfer Functions ◽  
Vertical Vibration ◽  
Full Scale ◽  
Relative Importance ◽  
High Frequency Response ◽  
Bow Flare ◽  
Excitation Mechanisms ◽  
The Difference ◽  
Wave Induced

Wave induced vibrations often referred to as springing and/or whipping increase the fatigue and extreme loading in ship hull girders. Both effects are disregarded in current ship rules. Various numerical codes exist for predicting the wave induced vibrations, but so far they are not considered reliable. Another means to investigate the importance of the high frequency response, although more resource demanding, is to carry out full scale measurements and/or model tests. Recently, full scale measurements of blunt ships have been carried out by DNV, and in this paper one of these ships was considered and tested in a towing tank to evaluate the additional fatigue damage due to the wave induced vibrations. Different excitation sources may excite the 2-node vertical vibration mode depending on ship design, and it is not straight forward to determine which is more important. The relative importance of the excitation mechanisms are investigated by two approaches in this paper. The first approach separates the whipping from springing to illustrate their relative importance based on basic theory in combination with model test results. The linear and second order transfer functions are utilized in this procedure. The second approach deals with the effect of the bow design on the additional fatigue damage. Three different bows were tested. The first bow design is identical to the real ship. The second bow design is a simplified version of the first one, by removing the bulb and flare. The third bow is fundamentally different from the two former blunt bows. Bow three is sharp pointed with a vertical sharp stem and vertical ship sides. The results indicate that the importance of whipping depends on the sea state, but that it is of similar importance as springing for the sea states that contributes most to the fatigue damage. Moreover, the difference in the additional fatigue damage due to wave induced vibrations for different bow shapes is moderate. This indicates that vessels with pointed bows and without pronounced bow flare, such as LNG vessels, may have a similar contribution from wave induced vibrations. Modern container vessels, which are more slender, but with pronounced bow flares should be further investigated.

Load Combination for Fatigue Analysis of Ship Structures

2004 ◽  
Author(s):  
Yung S. Shin ◽  
Booki Kim ◽  
Alexander J. Fyfe
Keyword(s):  
Bending Moment ◽  
Linear Summation ◽  
Load Combination ◽  
Longitudinal Stiffeners ◽  
Stress Components ◽  
Tank Pressure ◽  
Wave Induced ◽  
Vertical Bending Moment ◽  
Oil Tanker

A methodology for calculating the correlation factors to combine the long-term dynamic stress components of ship structure from various loads in seas is presented. The methodology is based on a theory of a stationary ergodic narrow-banded Gaussian process. The total combined stress in short-tem sea states is expressed by linear summation of the component stresses with the corresponding combination factors. This expression is proven to be mathematically exact when applied to a single random sea. The long-term total stress is similarly expressed by linear summation of component stresses with appropriate combination factors. The stress components considered here are due to wave-induced vertical bending moment, wave-induced horizontal bending moment, external wave pressure and internal tank pressure. For application, the stress combination factors are calculated for longitudinal stiffeners in cargo and ballast tanks of a crude oil tanker at midship section. It is found that the combination factors strongly depend on wave heading and period in the short-term sea states. It is also found that the combination factors are not sensitive to the selected probability of exceedance level of the stress in the long-term sense.

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