SUMMARY OF FINAL PROJEC INPUT DATAS FOR PIPELINE FREE SPAN FATIGUE ANALYSIS copy

It also introduces the problem of fatigue analysis for pipeline, and state of input datas for this process in VietNam.. State of input datas for pipeline fatigue analysis The problem of

NATIONAL UNIVERSITY OF CIVIL ENGINEERING

SUMMARY OF FINAL PROJECT

INPUT DATAS FOR PIPELINE FREE SPAN FATIGUE ANALYSIS

Student :VŨ VĂN HOAN Student code :8136-47

Hà Nội, June 2007

TABLE OF CONTENTS

1.1 Vietnamese pipeline route systems 5

1.2 Phenomen of pipeline free span fatigue 5

1.3 State of input datas for pipeline fatigue analysis 6

1.4 Domain of project 6

Chapter 2 METHODS OF FATIGUE ANALYSIS FOR PIPELINE FREE SPAN 8

2.1 Culmulative damage rule of Palmgren-Miner 8

2.2 Fatigue curves (S-N curves) 8

2.3 Deterministic method 9

2.5 Method of the standard: DnV-RP-F105 – Free spanning pipeline 10

Chapter 3 INPUT DATAS FOR FATIGUE ANALYSIS OF PIPELINE ROUTES IN

Chapter 4 APPLICATION 12

This Final project is named: “INPUT DATAS FOR PIPELINE FREE SPANNING ANALYSIS IN CONDITIONS OF SEA-ZONES IN VIETNAM”

The report includes 4 chapters, 1 conclusion and 2 appendixes

1 General

2 Methods of fatigue analysis for free span of pipeline

3 Input datas for fatigue analysis for pipeline routes in VietNam

4 Application

Conclusion

1 Wave datas for pipeline route Nam Côn Sơn

2 Results of Fatigue analysis

1 GENERAL

This chapter gives a gereral view about the pipeline routes system and about occurring probabilty of free spans in VietNamese sea-zones It also introduces the problem of fatigue analysis for pipeline, and state of input datas for this process in VietNam

Based on the two informations above, this chapter sets the domain of the studies of the project

1 Vietnamese pipeline route systems

Nowaday, systems of pipeline routes are developping strongly with thousands of kms in all over the world In Vietnam, there are already several pipeline systems, as: gas pipeline route Bach Ho – Ky Van, pipeline systems of Dragon field, Ruby field, Nam Con Son pipeline system… Vietnamese pipeline systems are also being enlarged in scale, in their economic worth In the future, a number of deep water pipeline route systems plans will be realized in the Southern sea

The observations have shown that, with conditions of the Southern sea, the phenomens of pipeline free span are very popular Therefor, the problem of pipeline fatigue analysis is required for safety installation and operation phases

2 Phenomen of pipeline free span fatigue

The fatigue phemomen was first studied by Wöhler in 1850 And step by step, it takes a more and more important part in processes of pipeline design and has been recommended in many mairne structure design standards Recently, the

most wellknown standard of pipeline fatigue in the world is DnV-RP-F105 –

Free spanning pipeline.

Fatigue of pipeline is caused by the three following causes:

- wave direct action;

- in-line vortex shedding;

- cross-flow vortex shedding

This phenomen may take place in many parts of a pipeline, but only some certain types of charges can make a significant damage which must be taken care

of And in all of the parts of a pipeline, fatigue of free spans on the seabed and fatigue of marine risers are the most considerable that can make them destroyed

in installation or operation phase

With free span of pipeline, there are four principal types as the followings:

- Free span induced by uneven seabed

- Free span induced by uneven underscouring

- On-purpose built free span

- Free span caused by exceptional event

It is free span pipeline fatigue, which is the object of the studies in this project

3 State of input datas for pipeline fatigue analysis

The problem of pipeline fatigue is required solved in installation and operation phase and must be based on the code of a certain recognized standard

In Viet Nam, it is lack of documents on this problem, especially documents on the input datas of the solving process

Moreover, in fact, several software are used for pipeline fatigue analysis It

is so important to gives correct inputs datas for correct results

In conclusion, in Vietnam, the input datas for pipeline fatigue analysis have not been systematized Therefor, a document on this problem is neccesery

4 Domain of project

This project takes concentration in these following problems:

1 Methods of pipeline fatigue analysis systematising

2 Input datas for pipeline fatigue analysis systematising

The direct object of the studies is pipeline span

2 METHODS OF FATIGUE ANALYSIS FOR

PIPELINE FREE SPAN

This chapter introduces the Culmulative damage rule of Palmgren-Miner, brefly about fatigue curves (S-N curves), 3 methods of pipeline fatigue analysis: deterministic method, probability method and method of DnV Recommended Practice –RP-F105 In this project, deterministic method, probability method are introduced as theories of base and sets the method of DnV standard as the main one

Result of free span pipeline fatigue analysis is fatigue life time of the span

or allowable span length, based on the culmulative damage rule of Palmgren-Miner Here presents three methods to solve this problem:

- Deterministic method

- Probabilistic method

- Method of the standard DnV-RP-F105 – Free spanning pipeline

1 Culmulative damage rule of Palmgren-Miner

The rule expresses that: with each level of stress range, Si, the cycles number to failure is Ni, if the number of stress cycles is ni, then the corespondant damage is ni/Ni, and the total damage of structure is a sumptation of all the

damages

The structure is failured when the total damage is equal to unit

2 Fatigue curves (S-N curves)

The rule of Palmgren-Miner requires determination of relation between a stress range and a correspondant number cycles to failure All of these relations

of a structure detailed (or a base material, or a type of welding) makes a fatigue curve of it For the pipe structure detaileds, S-N curves have a common form:

or in a logarithm form :

3 Deterministic method

This method base on the asumption: wave is harmonic and current is regular (deterministic)

In each period time of wave and current, there are ni cycles of environmental charges, and the received stress Si is deterministic By choosing a correct fatigue curve, we have the correspondant number cycles to failure Ni.

The fatigue life time of span will be caculated from the total damage

4 Spectrum method

This is the first of the probability methods to analyse fatigue problem, where waves are considered as probabity processes represented by vary sea states Each sea state is given by an energy spectrum

The culmulative damage is culculated by basing on the following

- the Morison equation is linearized

- pipe-soil interaction is linear

- the distribution of stress ranges of pipe is Weibull distribution

The narrow wave spectratra band with is required If that is violated, a stress factor will be used to modify the stress process into a equivalent narrow bandwidth process

5. Method of the standard: DnV-RP-F105 – Free spanning pipeline

The code of this standard bases on the assumption: the distributions of wave and current are three parameters Weibull distributions

The pipe fatigue life includes:

- life time versus vortex-induced in-line vibrations

- life time versus vortex-induced cross-flow vibrations

- life time versus direct wave action

The fatigue analysis process is given in 3 screening criterions, 1 fatigue criterion and 1 ULS critertion

Each screening criterion checks if a marginal fatigue life calculation is needed The fatigue criterion checks if the spanfatigue life time is less than its design life time The ULS criterion check the stress of pipe in extreme environmental conditions

The marginal fatigue-life-times are defined by response models and a force model

Amplitude response models are empirical models providing the maximum steady state VIV (Vortex Induced Vbration) amplitude response as a function of

the basic hydrodymic and structural parameters The VIV In-line life time and VIV cross-flow life time are defined here

The force model base on the wellknown Morison ‘s equation to calculate the fatigue life time versus wave direct action

3 INPUT DATAS FOR FATIGUE ANALYSIS OF

PIPELINE ROUTES IN VIETNAM

All the types of main input datas for pipeline fatigue analysis and their usages are presented in this chapter For applications in pipeline routes in Vietnamese sea-zones conditions, here provides some useful data patterns

From the methods above, this chapter makes a systematization for input datas for pipeline free span fatigue analysis in Vietnamese conditions

The systematised datas are:

- Materials of pipeline and their technic parameters which are required in free span fatigue analysis;

- Fatigue curves system for pipeline and their usages

- Soils and needed parameters for the pipe-soil interation Here also presents a data parttern of seabed soil in sea zones Bach Long and Ca Mau;

- Current and wave datas, required data types and the usages In the lack

of environmental datas situation in Vietnam, this chapter also presents some wave parameters in vary sea-zones of VietNam (shown in Appendix 1), which can be used in free span fatigue analysis,

4 APPLICATION

This chapter gives an example of pipeline free span analysis, clears up the input datas for this process

The example was solved by a programm “PIPELINE FREE SPAN FATIGUE ANALYSIS PROGRAMM”, written by the authour of the project This programm was written in Visual Basic Application for Excel, includes 8

modules of code, bases on the standard DnV-RP-F105.

The received results are clear, logical and acceptable, and are shown in Appendix 2

In the tendance of develop of pipeline systems with more and more large scale, free span fatigue analysis is neccessery and required in many pipeline design standard in the world

This project systematised the methods for pipeline free span analysis and the input datas for this process Also here, this problem is solved better and better

by a programm written by the authour of this project

With many efforts, this project had certain good results but there are still mistakes