Click here to load reader

Deterministic Fatigue Analysis

  • View
    102

  • Download
    4

Embed Size (px)

DESCRIPTION

good one

Text of Deterministic Fatigue Analysis

  • DR361.WP5 UNCONTROLLED COPY. DOCUMENT VIEWED ON THE NETWORK TAKES PRECEDENCE.

    Engineering & Construction Sector

    DESIGN REFERENCE

    OFFSHORE STRUCTURESDETERMINISTIC FATIGUE ANALYSIS

    DR 361 Rev 0 L SEPTEMBER 1991

    John Brown Engineers & Constructors Limited20 Eastbourne Terrace, London W2 6LE

  • DR361.WP5 UNCONTROLLED COPY. DOCUMENT VIEWED ON THE NETWORK TAKES PRECEDENCE.

    Engineering & Construction Sector

    DESIGN REFERENCE DR 361/0 L

    OFFSHORE STRUCTURESDETERMINISTIC FATIGUE ANALYSIS

    CONTENTS

    1. OBJECTIVE

    2. DEFINITION

    3. OUTLINE OF THE METHOD

    4. COMPUTER MODEL4.1 STRUCTURAL MODEL4.2 TOP BRACING LEVEL4.3 PILE SLEEVES AND FOUNDATION

    5. WAVE FORCE ANALYSIS5.1 WAVE FORCE COEFFICIENTS5.2 MARINE GROWTH5.3 WAVE THEORY5.4 WAVE FORCES AT ELEMENT LEVEL5.5 SURGE, TIDE, CURENT AND WATERDEPTH5.6 WAVE HEIGHT DISTRIBUTION5.7 ASSOCIATED PERIODS5.8 SELECTION OF WAVES FOR THE ANALYSIS5.9 SELECTION OF FATIGUE DESIGN WAVE FOR THE FOUNDATION

    6. DYNAMIC AMPLIFICATION FACTORS6.1 CHOICE OF NATURAL PERIOD6.2 CHOICE OF DAMPING COEFFICIENT6.3 OBSERVATION ON NATURAL PERIODS6.4 THE PERIOD RANGE6.5 IGNORING DYNAMIC AMPLIFICATION

    7. RELATED PROCEDURES

    8. REFERENCES

    FIGURESTABLES

    APPENDIX A:HYDRODYNAMIC COEFFICIENTS FOR CONDUCTOR GUIDES

    APPENDIX B: WAVE FORCE ANALYSIS - TWO COMPONENT REPRESENTATION

  • DR361.WP5 UNCONTROLLED COPY. DOCUMENT VIEWED ON THE NETWORK TAKES PRECEDENCE.

    Engineering & Construction Sector

    DESIGN REFERENCE DR 361/0 L

    OFFSHORE STRUCTURESDETERMINISTIC FATIGUE ANALYSIS

    REV 0 ISSUED SEPTEMBER 1991

    PREPARED BY: OFFSHORE STRUCTURES

    APPROVED BY: ..........................K LOGENDRA,ASSOCIATE DIRECTOR,OFFSHORE STRUCTURES

    AUTHORISED BY: ..........................H. THIRKELL,DIRECTOR OF ENGINEERING

  • DR361.WP5 UNCONTROLLED COPY. DOCUMENT VIEWED ON THE NETWORK TAKES PRECEDENCE.

    Engineering & Construction Sector

    DESIGN REFERENCE DR 361/0 L

    OFFSHORE STRUCTURES PAGE 1 OF 14DETERMINISTIC FATIGUE ANALYSIS

    1 OBJECTIVES

    The two objectives of this Design Reference on the deterministic fatigue analysis are

    - to capture current design practices as applied to recent John Brown projects

    - to clarify certain elements in these practices

    This is achieved by taking the Bruce deterministic fatigue analysis (Ref. 1) as a basis forthis Design Reference.

    2 DEFINITION

    Fatigue analyses are carried out to establish the adequacy of tubular joints and otherstructural details in jacket structures against wave loading.

    The deterministic fatigue analysis is characterised by analysing the jacket for a series ofdeterministic waves thus taking full account of the non-linear part in the drag-fluid loading.

    This analysis method is only able to address dynamics in an elementary fashion. Therefore,for jackets with natural periods greater than four seconds, a spectral analysis is essential.

    3 OUTLINE OF THE METHOD

    The deterministic fatigue analysis of a jacket structure is closely linked to a normal designwave jacket analysis. It repeats the same analysis for a series of waveheights andassociated periods from a number of directions.

    An outline of the procedure is given in Fig.1. In principle it consists of two parts: part 1 isindependent of the local joint geometry and establishes the member end forces andmoments and their annual frequency of occurrence. It is also in this part that dynamicamplification will be addressed.

    Part 2 is related to the local joint geometry. It requires the selection of appropriate stressconcentration factors (SCFs) and SN curves information on which has been collected inDR363.

    The descriptions in Chapters 4,5 and 6 on modelling, wave force analysis and dynamicamplification should be self-explanatory. An exception has to be made for Section 5.7 onassociated periods; their selection may be Client specific and a particular warning is raisedfor the part of the curve representing the relationship between wave height and wave periodwhich is to be associated with dynamic amplification.

    4 COMPUTER MODEL

    The analysis will be performed on a 3D structural model which is based on the in-place

  • DR361.WP5 UNCONTROLLED COPY. DOCUMENT VIEWED ON THE NETWORK TAKES PRECEDENCE.

    Engineering & Construction Sector

    DESIGN REFERENCE DR 361/0 L

    OFFSHORE STRUCTURES PAGE 2 OF 14DETERMINISTIC FATIGUE ANALYSIS

    analysis model. The procedure adopted for the analysis is shown in Figure 1.

    The computer model will have to address both the structural and the hydrodynamiccharacteristics of the platform.

    4.1 STRUCTURAL MODEL

    The structural model consists of the following elements:

    a) Jacket, appurtenances and deckb) Pile Clustersc) Foundations

    All structural members of the jacket will be included in the models and member sizes willbe based on uncorroded sections for stiffness analysis purposes only.

    Non-structural members, such as risers and conductors, will be modelled as equivalenttubes for the evaluation of wave loads but are generally excluded from the stiffnessanalysis. It is good practice to model one or two platform conductors in detail for theevaluation of fatigue life of these components. Because of the distances betweenconductors the effect of shielding will be small and can in practice be ignored.

    4.2 TOP-BRACING LEVELS

    Specific attention should be given to the fatigue strength of the bracing level above thewater and the first bracing level below the water particularly for drilling platforms.

    Nowadays the top bracing is often taken so high (i.e. 10m above MSL) to eliminate shipimpact on these members. As a consequnce the frequency of impact of these braces for afatigue analysis will also be small. On the other hand for older installation this level canwell be between 6-10 m above MSL and therefore this level will see regular wave loadingas follows:

    - wave slamming (see DR365)- direct wave loading- buoyancy

    The effects should be properly modelled in the fatigue analysis unless it can bedemonstrated by inspection and simplified analysis that fatigue at this level is of noconcern.

    In order to examine the first plan level below MSL for its fatigue strength it isrecommended to make a rather detailed model of this bracing level (particularly of theconductor guide framing) so that the vertical loading and its effect on the structure will befully assessed.

    4.3 PILE SLEEVES AND FOUNDATION

  • DR361.WP5 UNCONTROLLED COPY. DOCUMENT VIEWED ON THE NETWORK TAKES PRECEDENCE.

    Engineering & Construction Sector

    DESIGN REFERENCE DR 361/0 L

    OFFSHORE STRUCTURES PAGE 3 OF 14DETERMINISTIC FATIGUE ANALYSIS

    A pile cluster model, including sleeves and shear plates, will be included in the stiffnessmodel as a separate component using the substructuring technique in 'ASASH'.

    The piled foundations will be represented in the fatigue analysis by an equivalent beamspring model at each pile location. The model will be developed for the centre of damagewave, which will be calculated using an S-Curve approach as explained in Section 5.9.

    A short tubular representing the pile section properties will be included as well andbetween the top of packer can and scourline will facilitate the computation of pile fatiguedamage in the fatigue analysis.

    5 WAVE FORCE ANALYSIS

    The wave force analysis will be carried out using the ASASWAVE program.

    5.1 WAVE FORCE COEFFICIENTS

    The model used in the ASASWAVE load generation analysis will be adapted from themain structural stiffness model with diameters and/or CD and CM values adjusted to reflectthe waveload on the structure as explained below.

    Various values of CD and CM have been used in the past but in the future the FourthEdition of the Guidance Notes will be adopted as follows:

    CD = 0.6 for members without marine growthCD = 0.7 for members with marine growthCM = 2.0

    Anodes can be taken into account by increasing CD by 5%.

    For members supporting conductor guides CD and CM values can be calculated using themethod described in Appendix A.

    5.2 MARINE GROWTH

    To allow for the extra wave loading caused by the presence of marine growth on jacketmembers, conductors, risers, and caissons, their effective radii will be increased inaccordance with the Clients specification. The increase in Cd due to marine growth willhave been incorporated in the analysis if the force coefficients mentioned in Sect.5.1 aboveare used.

    5.3 WAVE THEORY

  • DR361.WP5 UNCONTROLLED COPY. DOCUMENT VIEWED ON THE NETWORK TAKES PRECEDENCE.

    Engineering & Construction Sector

    DESIGN REFERENCE DR 361/0 L

    OFFSHORE STRUCTURES PAGE 4 OF 14DETERMINISTIC FATIGUE ANALYSIS

    Wave theories will be chosen in accordance with the criteria shown in Figure 2.

    5.4 WAVE FORCES AT ELEMENT LEVEL

    Forces will be calculated at a minimum of three points along each member and then appliedto the member.

    The process is repeated for ten time-steps equally spaced at 36 phase angles. In this waywave loads are available for discrete time-steps as the wave passes through the structure.

    For each wave of frequency w, the program scans the time history to locate the maximumand minimum for each force component and determines the amplitude (a) and phase angle() for an equivalent sinusoidal force which best fits the element loads. This sinusoidalforce is represented by a vector of real and imaginary components.

    F(t) = aei(wt+)

    By this method the amplitude and phase of an individual applied force, displacement ormember stress can be retained more correctly than relating them to suc

Search related