multi-axial fatigue strength assessment of a turret in a fpso

Multi-axial fatigue strengthassessment of a turret in a FPSO

Rick Donk

Faculty 3ME; Section Ship and Offshore Structures

Delft University of Technology

Supervisor

Dr.ir. J.H. den Besten, TU DelftDr.ir. J.J. van der Cammen, Bluewater Energy Services

In partial fulfillment of the requirements for the degree of

Master of Science in Offshore Dredging Engineering

October 21, 2019

Acknowledgements

This thesis will present the final graduation research assignment, where mul-

tiaxial fatigue strength of welded details within the turret of a FPSO are

assessed. This assignment is carried out for Bluewater Energy services, a

company that is specialized in turret designs.

Turret structures within FPSO’s are subjected to a variety of repeated loads

during their service live, therefore fatigue is a governing limit state in turret

structural design. Due to the complexity of these structures and their load-

ing, current fatigue design approaches can be non conservative for welded

details subjected to a multiaxial stress state. Within this research these

welded joints are identified and selected for proper multiaxial fatigue evalu-

ation.

During the last few months a lot of knowledge is gained on this topic, there-

fore I would also like to thank Dr.ir. J.H. den Besten, Ir P.S. van Lieshout

and Prof.dr.ir. M.L. Kaminski for their guidance and supervision.

Furthermore, I would like to thank all Bluewater Energy Services engineers

who help me during this assignment, the section head of the structural de-

partment Ir. J. van Nielen who made this graduation research possible and

especially Dr.ir. J.J. van der Cammen who was my daily supervisor for his

feedback and support during this process.

Last but not least I would like to thank my family who supported me during

this process. This process wasn’t always smooth sailing, but together with

the help of my family I’ve pushed the tides to be in my favor.

Rick Donk 21-10-2019

Abstract

Keywords: Multiaxial fatigue, welded joints, out-of-phase, welded struc-

tures, FPSO, turret, maximum load path, Path-Dependent maximum range,

non-conservative, arc-welded joints, service life, cycle counting.

Turret structures within FPSO’s are subjected to a variety of repeated loads,

due to the harsh environment they operate in during their service live, mean-

ing that fatigue is a governing limit state in turret structural design. Turret

structural components are connected by arc-welded joints which are consid-

ered particularly susceptible to fatigue damage. Due to the complexity of

the structure and its loading, welded details may be subjected to a multi-

axial stress state. Currently fatigue design of turret structures is predomi-

nantly based on a uniaxial fatigue criteria assuming governing mode I (i.e

normal stresses). This design approach can be non-conservative for welded

details subjected to a multiaxial stress state, especially when these are non-

proportional (i.e. out of phase). The estimation of multiaxial fatigue live for

details subjected to a multiaxial stress state is still an extremely complex

task. There is still a discrepancy in obtained multiaxial fatigue live between

different design rules (i.e. as presented in ISSC), meaning that future work

on this topic is required.

Due to the size and complexity of turret structures, identification of welded

details (i.e. in the order of hundreds) subjected to a multiaxial (non)-

proportional stress state is a rather complicated and laborious task. This

thesis proposes a new screening method that identifies sensitive locations

where multiaxiality occurs, either geometry or loading induced. Component

stresses (i.e. Mode I and III) are determined from finite element models using

a mesh-insensitive structural stress method. The stress state (i.e. multiax-

iality and proportionality) of these stress components is determined using

the parameters of an ellipse that encloses the stress data (i.e. component

stresses) in 2-dimensional stress space. Making this a practical and efficient

method to identify sensitive locations within the turret where multiaxiality

occurs.

For the multiaxial fatigue damage calculations of welded details subjected

to a non-proportional multiaxial stress state, the accumulative Moment of

Load Path (MLP) concept is used. Within this concept the multiaxial fa-

tigue damage for any given non-proportional load path is assumed to consist

of two parts. The first part can be considered damage due to the effective

stress range Δσe (i.e. stress due to direct path), and the second is the ”load-

path non-proportionality” fatigue damage due to excursion of the reference

load path. By implementing the MLP-based method as part of the path-

dependent maximum range (PRMD) cycle counting procedure, half cycles

and their corresponding MLP-based equivalent stresses ranges are computed.

Given the MLP-based stress distributions from PDMR cycle counting, the

well know Palmgren-Minor rule is used to determine the accumulation of

fatigue damage considering a proper fatigue resistance curve.

By implementing both the screening and the proposed multiaxial fatigue

damage method onto a relatively simple Tube-to-Flange connection, a com-

parison study is used to determine whether the screening method is capable

of identifying sensitive locations that may be susceptible to multiaxial fa-

tigue. For five considered load scenarios, the screening method showed to

give relatively similar results with respect to the actual fatigue damage cal-

culation, making it a suitable structural screening method. Using the same

Tube-to-Flange connection the multiaxial fatigue damage is calculated based

on DNVGL and compared with those calculated using the MLP-based con-

cept. For uniaxial proportional loading scenarios similar fatigue damages are

observed, however for the non-proportional load scenarios the MLP-based

method gives significantly higher fatigue damage results w.r.t DNVGL.

Performing structural screening on complex structural systems like the turret

considering each load signal during its service live would be very computa-

tionally expensive. Therefore five load scenarios are considered to be suffi-

cient to perform the structural screening. For this thesis a certain domain

of the turret of the Aoka Mizu vessel is screened using the defined screening

load scenarios. Within the scope (i.e. evaluated domain) details resulting in

relative high fatigue damage due to high levels of non-proportionality are not

encountered. Even Though details exist with high non-proportionality fac-

tors the stress ranges of these details are usually relatively small compared

to details subjected to a dominant uni-axial stress state.

Contents

1 Introduction 1

1.1 FPSO fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Problem description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.3 Scope of Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.4 Thesis Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2 Literature review 10

2.1 Multi-axial fatigue according to codes guidelines and regulations . . . . 11

2.1.1 Multiaxial fatigue from Eurocode 3 . . . . . . . . . . . . . . . . . 11

2.1.2 Multiaxial fatigue from IIW . . . . . . . . . . . . . . . . . . . . . 12

2.1.3 Multiaxial fatigue from DNVGL-C203 . . . . . . . . . . . . . . . 12

2.2 Multi-axial fatigue according to structural stress concept . . . . . . . . . 13

2.2.1 Stress-based calculation methods . . . . . . . . . . . . . . . . . . 13

2.2.1.1 Nominal stress . . . . . . . . . . . . . . . . . . . . . . . 13

2.2.1.2 Hot spot stress . . . . . . . . . . . . . . . . . . . . . . . 14

2.2.1.3 Traction based structural stress . . . . . . . . . . . . . 15

2.2.2 Load path stress based representation . . . . . . . . . . . . . . . 18

2.3 Fatigue Life time Estimation . . . . . . . . . . . . . . . . . . . . . . . . 19

2.3.1 Cycle counting . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2.3.2 Fatigue damage accumulation . . . . . . . . . . . . . . . . . . . . 19

3 Structural Screening 21

3.1 Structural stress analysis of weld seams . . . . . . . . . . . . . . . . . . 22

3.1.1 Structural stress tube-to-flange connection . . . . . . . . . . . . . 23

v

CONTENTS

3.1.1.1 finite element model of the tube-to-flange connection . 23

3.1.1.2 Global nodal forces and moments from weld seams . . . 24

3.1.1.3 Local nodal forces and moments at intersection . . . . . 25

3.1.1.4 Line force and moments at intersection . . . . . . . . . 28

3.2 Proportionality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

3.2.1 Proportional loading . . . . . . . . . . . . . . . . . . . . . . . . . 29

3.2.2 Non-proportional loading . . . . . . . . . . . . . . . . . . . . . . 30

3.3 Level of stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

3.3.1 Minimum volume enclosed ellipse . . . . . . . . . . . . . . . . . . 32

3.3.2 Formulation of the level of stress . . . . . . . . . . . . . . . . . . 33

3.4 Screening tube-to-flange model . . . . . . . . . . . . . . . . . . . . . . . 34

3.5 Screening turret structure of the AM FPSO . . . . . . . . . . . . . . . . 37

3.5.1 Finite element modelling of the turret structure . . . . . . . . . . 37

3.5.2 Loads considered for turret screening . . . . . . . . . . . . . . . . 38

3.5.3 Intersections screened within the specified turret domain . . . . . 40

3.5.4 Results Multiaxial Fatigue screening of the Turret . . . . . . . . 41

4 Multiaxial fatigue damage of welded details 44

4.1 Path Dependent Maximum Range cycle counting . . . . . . . . . . . . . 45

4.1.1 PDMR cycle counting procedure . . . . . . . . . . . . . . . . . . 45

4.2 Non-proportional multiaxial loading . . . . . . . . . . . . . . . . . . . . 50

4.2.1 Moment of load path (MLP) concept . . . . . . . . . . . . . . . . 50

4.3 Fatigue resistance curves . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

4.3.1 Test specimen Setup and Loading . . . . . . . . . . . . . . . . . 53

4.3.2 Data correlation pure Mode I and Mode III . . . . . . . . . . . . 54

4.3.3 Data correlation with mean stress correction . . . . . . . . . . . 56

4.3.4 Data correlation for all loading scenarios . . . . . . . . . . . . . . 58

4.4 Results Multiaxial fatigue damage . . . . . . . . . . . . . . . . . . . . . 59

4.4.1 Multiaxial fatigue damage tube-to-flange model . . . . . . . . . . 60

4.4.2 Multiaxial fatigue damage selected turret details . . . . . . . . . 62

vi

CONTENTS

5 Screening versus Multiaxial fatigue damage 64

5.1 Validation of the structural screening tool . . . . . . . . . . . . . . . . . 65

5.2 Fatigue damage comparison study DNVGL vs MLP-based . . . . . . . . 66

5.2.1 Multiaxial fatigue damage calculations based on DNVGL . . . . 66

5.2.2 Comparison study results and conclusions . . . . . . . . . . . . . 69

6 Conclusions 72

6.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

6.2 Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

A Proposal final research assignment BES 78

B Verification study 80

B.1 Mesh sensitivity check according to the structural stress method . . . . 80

B.2 PDMR cycle counting algorithm verification . . . . . . . . . . . . . . . . 81

B.3 Fatigue resistance curve for the SLP-based equivalent stress . . . . . . . 83

C Fatigue loading used for structural screening 85

C.0.1 Fatigue loads based on worst case loading direction . . . . . . . . 85

C.0.1.1 Histogram of the buoy and turret fatigue loads . . . . . 86

C.0.1.2 Screening loads . . . . . . . . . . . . . . . . . . . . . . . 87

References 90

vii

List of Figures

1.1 The Aoka Mizu FPSO as designed for the Lancaster field. This Turret

system will serve as case study throughout this thesis. . . . . . . . . . . 2

1.2 High level flowchart of the work which will be performed in this thesis. . 7

1.3 Flow-diagram for the evaluation of welded structural details subjected to

a multiaxial stress state. . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.1 Principle stress direction given by the principal angle ϕ . . . . . . . . . 12

2.2 Local stress increase at a structural detail (bracket) . . . . . . . . . . . 14

2.3 Through-thickness structural stress definition: (a) Local stresses from

FE-model; (b) Structural stress or far-field stress; (c) Self equilibrating

stress. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.4 Definition of three traction-based structural stress components at the

hypothetical crack plane . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.5 The structural calculation procedure for an arbitrarily curved weld using

shell/plate element models . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.6 Representation of the load path stresses . . . . . . . . . . . . . . . . . . 18

3.1 Section view of the finite element model of the tube-to-flange, consisting

of a tube welded to a flange by means of a fillet weld. . . . . . . . . . . 23

3.2 Finite element representation of a intersection composed of two elemental

subsets {green and red} along the intersection nodes . . . . . . . . . . . 24

3.3 (a) Nodes sorted based on node numbering from Ansys (b) Nodes sorted

using written Matlab code . . . . . . . . . . . . . . . . . . . . . . . . . . 26

3.4 local coordinate for each intersection node individually . . . . . . . . . . 28

viii

LIST OF FIGURES

3.5 Multi-axial stress state due to multiaxial loading . . . . . . . . . . . . . 29

3.6 Load case 1: Proportionality visualization . . . . . . . . . . . . . . . . . 30

3.7 Load case 2: Proportionality visualization . . . . . . . . . . . . . . . . . 30

3.8 MVEE fit over Mode I and III stresses, plotted in a 2 dimensional stress

space . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

3.9 Tube-to-Flange screening results. . . . . . . . . . . . . . . . . . . . . . . 36

3.10 Section view of the finite element model of the turret structure which is

mainly built using shell-181 elements. . . . . . . . . . . . . . . . . . . . 37

3.11 Load scenarios with (a) Combined envelope mooring loads where a phase

shift of 900 is applied to the mooring horizontal moment (b) Combined en-

velope mooring loads where the mooring horizontal moment has a higher

frequency. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40


space . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41


space given loading from selected sea state . . . . . . . . . . . . . . . . . 43

4.1 Illustration definitions: Turning point, Projected Turning point and Vir-

tual path . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

4.2 PDMR cycle counting for remaining load paths (a - c), (b) second counted

half cycle (d) third counted half cycle. . . . . . . . . . . . . . . . . . . . 49

4.3 MLP based non-proportional damage factor illustration . . . . . . . . . 51

4.4 flange-to-tube joint tested by Yousefi et al. [1] . . . . . . . . . . . . . . . 54

4.5 Experimental results of constant amplitude test; R = -1 for pure Mode I

(pink) and pure Mode III (green) . . . . . . . . . . . . . . . . . . . . . . 55

4.6 Experimental results of constant amplitude test taking load ratio into

account . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

4.7 Fatigue resistance SN curve for steel, MLP-based equivalent stress . . . 59

4.8 Multiaxial fatigue results for the 5 load scenarios . . . . . . . . . . . . . 61

4.9 Normalized damage comparison given 5 loading scenarios . . . . . . . . 63

ix

LIST OF FIGURES

5.1 Comparison results where the blue dots represent the estimated normal-

ized damage using the screening tool and the red dots the normalized

multiaxial fatigue damage both for load scenario 4 and 5 . . . . . . . . . 65

5.2 DNVGL detail classification as E for stress direction normal to the weld 67

5.3 Signed principle stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

5.4 Signed principle stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

5.5 Normalized damage comparison given 5 loading scenarios . . . . . . . . 71

B.1 Tube-to-flange model used for the verification of mesh sensitivity . . . . 80

B.2 Mesh sensitivity study using the mesh insensitivity method . . . . . . . 81

B.3 SAE ”Grapple skidder” variable loading . . . . . . . . . . . . . . . . . . 82

B.4 Comparison of ASTM rainflow and PDMR cycle counting results using

SAE “Grapple skidder” load history . . . . . . . . . . . . . . . . . . . . 82

B.5 Fatigue resistance SN curve for steel, MLP-based equivalent stress . . . 84

C.1 Load histogram from BES fatigue report . . . . . . . . . . . . . . . . . . 86

x

List of Tables

3.1 Finite element model propertiest tube-to-flange . . . . . . . . . . . . . . 24

3.2 Finite element properties of the turret model . . . . . . . . . . . . . . . 38

3.3 Maximum mooring envelope load amplitudes . . . . . . . . . . . . . . . 39


3.5 Screening properties of the three selected details . . . . . . . . . . . . . 43

4.1 PDMR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

4.2 Fatigue resistance curve parameters of figure 4.5 . . . . . . . . . . . . . 55

4.3 Mode I and Mode III including correction factor . . . . . . . . . . . . . 57

4.4 Fatigue resistance for VA data . . . . . . . . . . . . . . . . . . . . . . . 59


B.1 Fatigue resistance for VA data . . . . . . . . . . . . . . . . . . . . . . . 84

C.1 Maximum mooring envelope load amplitudes . . . . . . . . . . . . . . . 87

xi

Chapter 1

Introduction

1.1 FPSO fundamentals

A turret moored FPSO is composed of a turret system and a mooring system that

connects the turret column to the seabed. Two types of turret systems are commonly

used for FPSOs - the internal turret system where the turret is mounted within the

FPSOs hull, and an external turret system where the turret is mounted on an extended

structure cantilevered off the vessel bow. Since all the anchor chains of the mooring

system are attached to the turret column, such a system is called a single point mooring

system (SPM). This in contrary to the spread moored system, where the vessel is moored

by anchor legs from the bow and stern of the vessel, usually in a four-group arrangement.

The turret system contains a bearing system that allows the vessel to rotate freely

around the fixed geostatic part of the turret (also known as weathervane). The 360-

degree weathervaning feature of a turret moored FPSO significantly reduces the loads

on the mooring system. Also, vessel motions, particularly rolling motions, are typically

reduced thus allowing more operating uptime during inclement weather conditions. This

weathervaning ability is very important for the offloading operation as the headings of

the FPSO and the export tanker are both into the predominant sea or winds, thus

creating safer approaches and alignments during offloading operations. As the risers are

contained within the turret structure, offloading operations are simplified as the FPSO

hull is uncluttered with risers or exposed mooring lines.

1

1.1 FPSO fundamentals

For offshore areas of the world subjected to harsh environments and where seasonal

cyclonic weather systems are predominant but with characteristically mild environments

throughout the remainder of the year disconnectable turret are preferred. While the

turret enables the vessel to freely weathervane in normal to severe conditions, this

type of internal system allow the vessel to disconnect to avoid typhoons, hurricanes,

icebergs, and other extreme dangerous conditions. The Aoka Mizo vessel is equipped

with a disconnetable turret as shown in Figure 1.1. It should be noted that even thought

this FPSO is equipped with a disconnectable turret, it doesn’t mean that it currently

opperates in a area which might be subjected to seasonal cyclonic weather.

Figure 1.1: The Aoka Mizu FPSO as designed for the Lancaster field. This Turretsystem will serve as case study throughout this thesis.

FPSOs can be a conversion of an oil tanker or can be a vessel built specially for

the application. Although there has been a shift toward newbuild FPSO, especially for

developments in harsh environments, very large crude carrier tanker conversions remain

the basis for projects in areas where benign environmental conditions (mild sea waves

and swells) are predominant, such as off west Africa, southeast Asia, Australia, and

Brazil. Providing flexibility and mobility, tanker conversions in some cases offer quicker

production of first oil. There are, however, certain drawbacks to converting old tankers,

the most important of which is the restriction on the weather conditions and water

2

1.2 Problem description

depth. For these conditions, the demands for integrating the turret into the hull can

become quite elaborate and is therefore usually not economically feasible. With the

development of turret mooring and new-build ship-shaped hulls the number of FPSOs

operating in very deep water and harsh weather conditions has grown substantially.


FPSOs are being recognized as one of the most economical systems to exploit marginal

and (ultra) deep-water areas. Due to the world’s increasing demand for energy, oil and

gas companies continue to move into new and increasingly harsh and remote environ-

ment to meet this demand. FPSOs are therefore a important way to accommodate this

need in a cost effective, flexible way. Due to the fact that these FPSOs and in particular

the turret mooring systems have to operate in harsher environment they increase in size

and complexity. The emphasis therefore lies on the optimization of design, building

and operations in order to achieve high levels of integrity in term of safety, health and

environmental factors, and life-cycle capital (CAPEX) and operational (OPEX) expen-

ditures.

Presently turret mooring system design, construction and operational practices are

largely influenced by high-cycle fatigue as a primary degradation parameter. Empirical

(inspection) practices are deployed as the key instrument to identify and mitigate sys-

tem anomalies and unanticipated defects. Current inspection, maintenance and repair

are time consuming and quite expensive.

Due to harsher operating environments the loading on the turret becomes higher and

more complex. Conventional calculation method using high contingencies may no longer

be economically justifiable. BES currently started a in-house FPSO integrity project

’AMON’, which stands for Aoka Mizu monitoring. The intension is to gain more insight

in the loading and structural response, based on actual measured data from the FPSO.

Timely identifying deviations from design conditions allows to take action in case for ex-

ample the fatigue life is being consumed too fast. Furthermore BES also participates in

the 4D fatigue JIP which focuses more on the fatigue capacity of welded joints subjected

3


to multiaxial (non)-proportional loading. This assignment also mainly focusses on the

fatigue capacity of welded joints subjected to multiaxial (non)-proportional loading, but

taking into account more realistic loading conditions rather then just theoretical loading

conditions.

Fatigue design and evaluation of welded joints are typically carried out by weld clas-

sification approach in which a family of parallel nominal stress based S-N curves are

used according to joint types and loading modes [2]. Therefore the accuracy of the

stress range is very important for assessing the fatigue life. The form of the S-N curve

indicates that a small change in the estimate of the stress range results in a much larger

change in the life because of the equation for the S-N curve, S = CN−1/m. For example

if the stress range were increased by only 20%, the computed life would be reduced by

42% for m= 3. Therefore, an accurate estimate of stress range is required for fatigue

evaluation of a given welded structure. As discussed before this method makes use of

several empirical S-N curves that are associated with detail categories and on corrective

factors. The selection of a detail class for a welded joint type and loading mode is

often subjective and, in many common situations, even skilled engineers might have a

hard time choosing a suitable detail class. This is especially true when the geometry

of the structure is complex or when the stress state is not reducible to a simple main

component. Moreover it must be added that the real structures can develop cracks in

locations different to those indicated in the details present in the standards, resulting

in several limitations.

The structural members of the TMS are Generally evaluated using finite element anal-

ysis, due to it’s complexity. From these analysis it is usually difficult to evaluate what

is ”nominal stress” to be used together with the S-N curves, as some of the local stress

due to a detail is accounted for in the S-N curve. In many cases it may therefore be

more convenient to use an alternative / extrapolation-based hot spot stress approach.

For this approach the hot spot is determined using finite elements calculations at prede-

fined reference points and extrapolated to the fatigue hot spot. The fatigue capacity is

described by the D-curve, which can be considered as the hot spot S-N curve. Accord-

ing to [3], the stress at the fatigue hot spot consists of a nominal stress times a stress

4

1.3 Scope of Work

concentration factor (SCF). With FEA model widely used nowadays, this method gains

a lot of attention, making it a widely used method in several design codes. Even though

this method is considered a widely used method, the hot spot stress calculated by using

different extrapolation procedures and element types varies a lot and besides this the

method is also mesh insensitive [4].

To this very moment, the fundamental references in the design of ship and offshore

structures and the inherent fatigue resistance are directed from uni-axial and constant

amplitude testing ref. Furthermore fatigue design of TMS is predominantly based on

uni-axial fatigue criteria assuming a governing ”Mode I” normal stress. These criteria

are then used in combination with damage accumulation hypothesis (e.g. Miner’s rule)

and cycle counting method (e.g. rain flow counting) or Rayleigh distributed to deter-

mine the fatigue life. Nonetheless, during real-life conditions, structures are subjected

to multi-axial, variable-amplitude loading including non-proportional characteristics for

specific details. Unfortunately, the usability of current multi-axial practices are re-

strained due to limited validation efforts and finite academic scope in testing, which

can be reverted to the general engineering perception that uni-axial loading is the pre-

dominant factor [5]. Recent research has shown that conventional uni-axial methods

significantly overestimate the fatigue lifetime, and lifetime predictions of multi-axial

methods show significant differences [1].

Although all of the above potentially pose relevant and significant problems using cur-

rent evaluation methods, this research will mainly address the latter (weld details sub-

jected to multiaxial non-proportional loading).

1.3 Scope of Work

Within BES the structural department is responsible for the structural design and struc-

tural integrity of structures like the turret within a FPSO. Due to the increasing size

of the turret structures, conventional methods may be too conservative or in case of

details subjected to a multiaxial non-proportional stress state the conventional method

may result in higher fatigue then actually would be the case.

5

1.3 Scope of Work

The focus of this thesis will mainly be on welded structural details which are subjected

to a multiaxial (non)-proportional stress state. Conventional methods can’t properly

estimate the fatigue life of these details. Within design codes a attempt has been made

to account for the fatigue of details subjected to a multiaxial stress state. The meth-

ods from the design codes will be reviewed, determining if these would produce reliable

results or if a more advance methods is required to determine the fatigue life of these

details.

In addition to the design codes, a literature study is performed on newly developed

methods that are not covered by the design codes ”yet”. Its aim is to provide an

understanding of the challenges that still need to be overcome. Also several fatigue

experiments are well documented and used for both validation and further research.

The basis of this research is formed by the development of a multiaxial fatigue impor-

tance screening tool, which would allow engineers to quickly screen complex structures,

selecting only those details subjected to a multiaxial stress state. The structural screen-

ing can be performed using the semi-automated post-processing tool, This tool is to be

developed within this assignment. Validation of the developed screening tool, will be

done using a multiaxial fatigue damage criteria which is capable of properly estimating

the fatigue damage of welded details subjected to a multiaxial stress state. This would

be done in Phase I of this assignment, as shown in flow-diagram 1.3

As described above a multiaxial fatigue damage criteria should be selected which prop-

erly calculates the fatigue life of details subjected to multiaxial stress state. For this

a multiaxial fatigue damage criteria from literature is used. Since this is still under

development, usually only the mathematical description can be found in papers. Com-

mercial programs are not yet available, and therefore development of a algorithm should

also be done in this assignment. This work has been done in phase II, as shown in the

flow-diagram 1.3

Validation of the screening tool and the algorithm developed for multiaxial fatigue cal-

culation is done using a simple tube to flange model from which data is available in

literature. Furthermore a comparative study is performed to show difference between

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1.4 Thesis Structure

the results from the screening tool and that of the actual fatigue damage. Also a compar-

ative study has been performed to show the differences between conventional methods

and newly proposed methods.

Validation using measured loading data and actual multiaxial fatigue testing may be

optional, in case data and machine’s are available.

A flow chart is presented in figure 1.2, which gives a better overview of the different

steps which will be involved in this assignment.

Figure 1.2: High level flowchart of the work which will be performed in this thesis.


The structure of this thesis follows similar steps required for the analysis of welded

structural details subjected to a multiaxial (non)-proportional stress state, at least that

was intended. Some effort will be made now to describe the outline of this thesis; how

it will attempt to achieve the objectives described in section 1.3. Two main parts can

be distinguish namely: Phase I ”Structural screening” and Phase II ”Multiaxial fatigue

damage calculation for welded joints”. The flow diagram shown in 1.3 represents the

steps required to perform a structural screening, the results from the screening are later

compared to the considered multiaxial fatigue damage method.

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Figure 1.3: Flow-diagram for the evaluation of welded structural details subjected to amultiaxial stress state.

The layout of the thesis is setup as follows:

� Chapter 2 focuses on the literature study. Over the last few decades intensive

effort have been made to develop multiaxial fatigue approaches which are able to

deal with difficulties such as (random) variable amplitude (VA) loading and non-

proportionality. With is available in engineering codes and guideline’s along with

that what is available in open literates will be discussed briefly in this chapter.

Special emphasis will be on the methods used for this thesis.

� Chapter 3 focuses on Phase I: ”Structural screening” following the flow-diagram

presented in figure 1.3. The section start with a small introduction, explaining

what will be achieved in more details for Phase I. It will then be followed by

some subsection explaining the fundamentals of multiaxial fatigue. Further the

developed screening method will be described extensively. In order to properly

understand and explain the developed screening method, a ”Tube to Flange”

model is used. All the steps, as shown in the flow-diagram figure 1.3, will be

explained in details. This same procedure will be applied to the turret structure,

the findings and the results are then presented in the last sections of this chapter.

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� Chapter 4 This chapter is a follow-up on the previous chapter. In this chapter

the focus will be more on the multiaxial fatigue ”Damage” of welded components.

The same ”Tube to Flange” model is used in this chapter, to explain the con-

sidered/ developed methods. This will also provide a in-depth overview of the

cycle counting method, which is programmed enabling to count cycles in a stress

space, which accounts for multiaxial fatigue. The developed algorithm will the

be used to determine the fatigue damage of the selected details from section 3.

The development of the required SN-curves is also explained, The results are also

presented in the end.

� Chapter 5 will introduce a review, which is performed to validate the screening

method with actual fatigue damage (basically a combination of section 2

and 3). The validation in the sense of structural response and Model validation

will be described briefly. This chapter can be seen as a result section, presenting a

lot of comparative study results, enabling the author to draw enhance the drawn

conclusion and recommendation described in section 6.

� Chapter 6 will present conclusion and recommendation of this study. Some time

will also be spend on some short of reflection (looking back), where the set of goals

are evaluated.

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multi-axial fatigue strength assessment of a turret in a fpso

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