Calibration of Safety Factors for DNV RP-F105 Free Spanning Pipelines

2006 ◽  
Author(s):  
Gudfinnur Sigurdsson ◽  
Kim Mo̸rk ◽  
Olav Fyrileiv
Keyword(s):  
Fatigue Damage ◽  
Structural Reliability ◽  
Uncertainty Modeling ◽  
Submarine Pipeline ◽  
Safety Factors ◽  
Sources Of Uncertainty ◽  
Fatigue Design ◽  
Vortex Induced Vibrations ◽  
Stochastic Parameters ◽  
Pipeline Design

Free spans often become a significant challenge in pipeline design and operation due to uneven seabed or seabed scouring effects. The trend towards deeper waters, harsher environment and installation of pipelines at very uneven seabed often implies a high number of free spans. High costs related to span intervention puts focus on minimizing these costs and still ensure integrity of the pipeline with respect to vortex induced vibrations (VIV) and associated fatigue damage. On the other hand the potential costs related to fatigue failure of a pipeline (recovery costs and economical loss) are enormous. Therefore it is essential to ensure that the probability of failure for free spans is within acceptable limits, e.g. as required by DNV-OS-F101 “Submarine Pipeline Systems”. This paper describes the structural reliability analysis performed to obtain the safety factors for free span fatigue design. Accumulation of fatigue damage due to VIV of free spans is associated with various sources of uncertainty. The important stochastic parameters are described, and the basis for the uncertainty modeling given. The calibration scope defined from a set of different pipeline cases, span scenarios, and environmental conditions is presented from which calibration results and sensitivities will be discussed.

Fatigue Safety Factors for Deepwater Risers

2002 ◽  
Author(s):  
B. Stahl ◽  
H. Banon
Keyword(s):  
Fatigue Life ◽  
Fatigue Damage ◽  
Safety Factor ◽  
Damage Index ◽  
Rational Approach ◽  
Fatigue Reliability ◽  
Safety Factors ◽  
Fatigue Design ◽  
Uncertainty Parameters ◽  
Deepwater Risers

Fatigue life is governed by a number of variables that are highly uncertain. The safety factor on fatigue life is used in a deterministic way to account for the estimated fatigue damage uncertainty. High uncertainties lead to high fatigue safety factors, and vice versa. Evaluation of the uncertainties in the variables governing fatigue design provides a grip on what the safety factor should be. This paper addresses riser fatigue using a fatigue reliability model that is relatively simple but still captures the important elements of the fatigue problem. The bias and uncertainty of stress range are extremely important parameters in design against fatigue. This is due not only to the fact that these parameters are highly uncertain, but also to the fact that they are greatly amplified in the fatigue damage equation by the ‘slope’ m of the S-N curve. The Palmgren-Miner fatigue damage index and the intercept value of the S-N curve are additional important variables in fatigue design. A model for combining wave-induced and vortex-induced vibration (VIV) is introduced together with the best available data and reference to industry work in this technology area. A recently completed joint industry project on riser reliability provides good calibration points for the critical fatigue reliability variables. Reliability and sensitivity studies are performed to demonstrate the effect of the uncertainty parameters. An approach to selecting deterministic fatigue design factors that yield specified reliability targets is developed and illustrated. The study provides a rational approach to selecting safety factors for design of deepwater risers, taking into account both wave and VIV-induced fatigue damage.

Incorporating the Higher Harmonics in VIV Fatigue Predictions

2007 ◽  
Author(s):  
Vikas Jhingran ◽  
J. Kim Vandiver
Keyword(s):  
Fatigue Damage ◽  
Strain Energy ◽  
Oil And Gas ◽  
Oil And Gas Industry ◽  
Dynamic Strain ◽  
Higher Harmonics ◽  
Gas Industry ◽  
Ongoing Research ◽  
Fatigue Design ◽  
Vortex Induced Vibrations

Vortex-Induced Vibrations (VIV) are an important source of fatigue damage for risers in the Oil and Gas industry. Results from resent VIV experiments by Vandiver et al. [1] indicate significant dynamic strain energy at not only the Strouhal frequency, but also its harmonics. In certain regions of the pipe, these higher harmonics accounted for more that half of the measured RMS strain and increased fatigue damage by a factor exceeding twenty. However, the state-of-the-art in VIV prediction only accounts for the vibrations at the Strouhal frequency. Preliminary results from a second set of experiments, described in this paper, confirm the importance of the higher harmonics in fatigue life estimates of pipes. Further, the authors formulate an approach to incorporate the higher harmonics in VIV related fatigue design. Finally, the authors identify the estimation of the higher harmonics, in both location and magnitude, as an important area of ongoing research, the results of which will be required to implement this proposed method.

Safety Factors Calibration for Wall Thickness Design of Ultra Deepwater Pipelines

2013 ◽  
Author(s):  
Eduardo Oazen ◽  
Bruno R. Antunes ◽  
Carlos O. Cardoso ◽  
Rafael F. Solano
Keyword(s):  
Safety Factor ◽  
Wall Thickness ◽  
Structural Reliability ◽  
Limit State ◽  
Large Diameter ◽  
Design Codes ◽  
Safety Factors ◽  
Offshore Pipeline ◽  
Reliability Methods ◽  
Pipeline Design

Wall thickness often presents a considerable influence in offshore pipeline capital expenditure (CAPEX). This influence is enhanced in design of ultra deepwater trunk lines of large diameter, where any wall thickness increase provides a huge impact on project costs. In ultra deepwater scenarios, thicker pipelines may eventually implicate not only in higher costs, but may also compromise the project feasibility due to installation load constraints related to laying vessels availability. One potential way to reduce the pipeline wall thickness is to calibrate fitness-for-purpose safety factors through application of structural reliability methods, instead of utilizing the standardized safety factors presented in international codes. Since mid-nineties, several offshore pipeline design codes have been allowing the calibration of safety factors by structural reliability analysis. The purpose of such an allowance is that structural reliability methods would eliminate some eventual conservatism presented in the safety factors proposed by codes. Although this enables the achievement of optimized safety factors, more than fifteen years have passed and only few pipeline projects have taken advantage of the benefits of safety factor calibration. This paper evaluates which potential benefits are available through safety factor calibration, particularly for wall thickness reduction purposes in ultra deepwater pipeline design. Calibrated safety factors are presented for some scenarios related to ultra deepwater export pipelines, considering “system collapse criteria” limit state. The calibrated safety factors are compared with the standardized safety factors presented by international pipeline design codes. The potential for safety factor reduction by the utilization of linepipes with more stringent manufacturing tolerances and the consideration of the thermal ageing imposed by coating application are also discussed.

Limit State Philosophy in Pipeline Design

1987 ◽  
Vol 109 (1) ◽  
pp. 9-22 ◽  
Author(s):  
C. P. Ellinas ◽  
P. W. J. Raven ◽  
A. C. Walker ◽  
P. Davies
Keyword(s):  
Structural Analysis ◽  
Safety Factor ◽  
Current Knowledge ◽  
Limit State ◽  
Structural Behavior ◽  
Major Conclusion ◽  
Safety Factors ◽  
Suitable Framework ◽  
General Aspects ◽  
Pipeline Design

This paper considers the application of the limit state philosophy of structural analysis to pipeline design. General aspects of the philosophy are discussed and the approach to the evaluation of safety factors is reviewed. The paper further considers the various limit and serviceability states which would be relevant to a pipeline and reviews the various factors which may require consideration, before a code embodying the limit state philosophy could be formulated. A review of the state of current knowledge on various aspects of geometry and material characteristics, loading and structural behavior is presented. It is intended that such a review can be used as the basis for a larger study to provide guidance and data for the evaluation of rational levels of safety factor. The major conclusion reached by the authors is that a limit state philosophy would be valuable in providing a suitable framework, which may highlight the significant aspects of pipeline design and which can most easily accommodate new requirements and results obtained from research.

Experimental Study on Flexible Bare Pipe Arrays Undergoing Vortex-Induced Vibrations

2018 ◽  
Author(s):  
Y. Liu ◽  
C. Shi ◽  
Z. Liu ◽  
J. Wang ◽  
X. Bao
Keyword(s):  
Fatigue Damage ◽  
Dynamic Responses ◽  
Array Configuration ◽  
Damage Rate ◽  
Flexible Pipes ◽  
Vortex Induced Vibrations ◽  
Helical Strakes ◽  
Preliminary Study ◽  
Single Pipe ◽  
Center Distance

Vortex-induced vibration (VIV) excited by current is a major contributor to the fatigue accumulation of marine risers. For deepwater operations, several risers are often arranged together in an array configuration. In this study, a set of four identical flexible pipes of a rectangular arrangement were tested in a water tunnel. By comparing the dynamic responses of a pipe in an array with that of a single isolated pipe, the effects of the current speed and the center-to-center distance between the up-stream and downstream pipes on their dynamic responses were investigated. Fatigue damages accumulated on each pipe in an array was calculated and a factor, termed “fatigue damage amplification factor”, was defined as a ratio between the fatigue damage rate of pipe in an array and the fatigue damage rate of a single pipe at a same current condition. The results showed that for bare pipes (i.e., without helical strakes), the downstream pipes in an array configuration may have larger dynamic responses and fatigue damage rates than those of a single pipe; and, it is not always conservative to assume that the fatigue damage rate estimated for a single pipe can be used to represent the fatigue damage rates of pipes in an array. This preliminary study provided some meaningful results for the design, analysis and operation of marine riser arrays.

Recent Advances in Pipeline Free Span Design: A New Revision of DNV-RP-F105

2016 ◽  
Author(s):  
Knut Vedeld ◽  
Håvar Sollund ◽  
Olav Fyrileiv
Keyword(s):  
Dynamic Response ◽  
Limit State ◽  
Cross Flow ◽  
Ease Of Use ◽  
Direct Result ◽  
Ultimate Limit State ◽  
Vortex Induced Vibrations ◽  
Load Effects ◽  
Pipeline Design ◽  
Wave Induced

Pipeline free span design has evolved from basic avoidance criteria in the DNV ’76 rules [1], to fatigue and ultimate limit state considerations in Guideline no. 14 [2]. Modern multimode, multi-span free span design is predominantly performed according to DNV-RP-F105 [3]. In 2006, the latest revision of DNV-RP-F105 [3] was written as a direct result of extensive research, performed due to significant free span challenges in the Ormen Lange pipeline project. DNV-RP-F105 was at the time, and still is, the only pipeline design code giving contemporary design guidance for vortex induced vibrations (VIV) and direct wave loading design for pipelines in free spans. The last revision of DNV-RP-F105 included a few, but highly important advances, particularly the consideration for multi-mode and multi-span pipeline dynamic response behavior. In the 10 years that have followed, no breakthroughs of similar magnitude have been achieved for pipeline free spans, but a large number of incremental improvements to existing calculation methods, and some novel advances in less critical aspects of VIV understanding have been made. As a result, DNV-RP-F105 has recently been revised to account for these advances, which include improved frequency-domain analyses of wave-induced fatigue, a new response model for cross-flow VIV in low Keulegan-Carpenter (KC) regimes in pure waves, new analytical methods for dynamic response calculations of short spans in harsh conditions, and extensive guidance on how to apply the recommended practice for assessment of fatigue and extreme environmental load effects on curved structural members such as spools, jumpers and manifold flexloops. This paper gives an overview of most of the important changes and updates to the new revision of DNV-RP-F105. Case studies are used to demonstrate the importance and effects of the changes made, and to some extent how the revision of DNV-RP-F105 can enhance its applicability and ease of use.

An Assessment of EOF Current Scatter Diagrams With Respect to Riser VIV Fatigue Damage

2002 ◽  
Author(s):  
Trond Stokka Meling ◽  
Kenneth Johannessen Eik ◽  
Einar Nygaard
Keyword(s):  
Fatigue Damage ◽  
Principal Components ◽  
Empirical Orthogonal Functions ◽  
Time Varying ◽  
Scatter Diagram ◽  
Orthogonal Functions ◽  
Vortex Induced Vibrations ◽  
Deepwater Riser ◽  
Diagram Approach ◽  
Current Modelling

The accuracy of current modelling is critical when considering deepwater riser fatigue damage caused by vortex-induced vibrations (VIV). In the present study the use of empirical orthogonal functions (EOF) to extract the governing characteristics from huge amounts of current measurements has been assessed. The amplitudes of the time varying principal components (PC) have been organized into bins in scatter diagrams. The accuracy of this scatter diagram approach with different numbers of EOF modes involved has been evaluated in terms of riser VIV fatigue damage.

Methods for Reducing a Large Number of Current Profiles to a Small Subset for Riser VIV Analysis

2005 ◽  
Author(s):  
Kostas F. Lambrakos ◽  
Djoni E. Sidarta ◽  
Hugh M. Thompson ◽  
Atle Steen ◽  
Roger W. Burke
Keyword(s):  
Fatigue Damage ◽  
Average Power ◽  
Small Subset ◽  
Large Set ◽  
Computationally Efficient ◽  
Excitation Mode ◽  
Average Value ◽  
Vortex Induced Vibrations ◽  
Simplified Procedure

The paper presents two different approaches to construct subsets of current profiles from a large set of long term current profiles for the purpose of performing calculations for riser fatigue damage from vortex induced vibrations (VIV). The subsets are intended to reproduce the fatigue damage from the full set of current profiles. In the first approach, the full set of profiles is first sorted into bins based on current magnitude, direction and shear in the profile. The profiles within each bin are then reduced to a single constructed profile through one of many possible current averaging schemes. The present study includes two types of constructed profiles; one profile is generated by the average value of the currents for each bin and the other by the average value plus one standard deviation. The second approach is based on first performing a simplified and computationally efficient VIV analysis of the full set of profiles. The profiles are then sorted into bins by the dominant excitation mode, and then a single profile is chosen to represent all the profiles that excite the mode of interest. The chosen profile for the mode of interest has VIV power-in which is close to the average power-in for all the profiles that excite the mode. The number of profiles in the subset is equal to the number of modes that are excited by the full set of profiles. The VIV power-in in this paper is estimated through a simplified procedure that is consistent with the SHEAR7 methodology. Other available codes can also be used for the simplified VIV calculations.

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