Main Thesis Report

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PROBABILISTIC CALCULATION FOR

FATIGUE LIFE OF THE STEEL

CATENARY RISER

A THESIS

IN PARTIAL FULFILLMENT OF THE

REQUIREMENTS FOR THE DEGREE OF MASTER OF

SCIENCE

SUBMITTED TO

THE DEPARTMENT OF NAVAL ARCHITECTURE &

MARINE ENGINEERING OF STRATHCLYDE

UNIVERSITY

BY

RONGRITH PICHAIYONGWONGDEE

AUGUST 2011

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ii

This thesis is the result of the author's original research. It has been

composed by the author and has not been previously submitted for

examination which has led to the award of a degree.

The copyright of this thesis belongs to the author under the terms of the

United Kingdom Copyright Acts as qualified by University of

Strathclyde Regulation 3.50. Due acknowledgement must always be

made of the use of any material contained in, or derived from, this thesis.

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iii

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iv

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v

Abstract

In the past decade, the free-hanging SCR is an alternative riser system since the oil reservoirs are found at the depth greater than 1,000 meters where the flexible riser application is limited by the extreme hydrostatic pressure. In this circumstance, the SCR can overcome the difficulty by adding extra pipe thickness. Also, extra benefits of this SCR riser system are inexpensive, simpler installation and easier maintenance which allow companies operate the deepwater field with less complexity system. However, the failure of riser can be occurred and its possibility is greatly associated with the random nature of environmental loads e.g. waves, winds and currents because these environmental loads have immense influence on the vessel’s motions. Therefore, evaluating these factors is an essential criterion in the riser design to estimate the fatigue life. In the past, the riser design was based on the deterministic calculations which the loads are based on common sea states. It was merely possible to utilize all wave and wind data in the calculations because the calculations were limited by its complexity, requirement for huge data storage and long simulation time. However, with the improved capability of today computer, the detail engineering simulation can be done to present accurate and meaningful answers. In this study, the optimal design of the steel catenary riser (SCR) will be examined, even though; the riser fatigue life will be calculated by using probabilistic approach.

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vi

List of Figures

Figure 1 World map of the deepwater oil exploration and production. ... 4

Figure 2 Segments and nodes in riser model. ... 19

Figure 3 Wall tension and pipe pressure force. ... 19

Figure 4 Frame of reference of pipe stress calculation. ... 20

Figure 5 Free-body diagrams for FPSO and riser system. ... 22

Figure 6 Wave force RAOs for surge and heave motions ... 23

Figure 7 Estimate riser profile in static condition. ... 25

Figure 8 Nodes displacement in X-axis. ... 27

Figure 9 Nodes displacement in Y-axis. ... 27

Figure 10 Tensile stress profiles for each riser segment. ... 29

Figure 11 SN curve (API Class X). ... 36

Figure 12 Block diagram represents the approach for project (1). ... 37

Figure 15 Location of Montara field, Timor Sea. ... 40

Figure 16 Deep water areas in the Timor Sea (water depth > 500 Meters). ... 40

Figure 17: Existing development fields in the Timor Sea... 41

Figure 18 Swells from south Indian Ocean to Timor Sea. ... 43

Figure 19 Annual wave rose diagrams measured at Jabiru field. ... 44

Figure 20 Monthly wave rose diagrams measured in Jabiru field. ... 45

Figure 21 Montara FPSO. ... 47

Figure 22 Montara FPSO specifications. ... 48

Figure 23 FPSO model (Side view). ... 49

Figure 24 FPSO model (Front view). ... 49

Figure 25 Plot of maximum von Mises stress and allowable pipe stress. ... 51

Figure 26 Plot of riser utilization (API RP 2RD). ... 51

Figure 27 Bending radius profile. ... 53

Figure 28 Riser curvature profile. ... 53

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vii

Figure 30 Shape configurations of the steel catenary riser. ... 57

Figure 31 Mean von Mises combined stress (vary outside diameter). ... 58

Figure 32 Mean axial stress profile (vary outside diameter). ... 59

Figure 33 Mean bending stress profile (vary outside diameter). ... 59

Figure 34 Mean hoop stress profile (vary outside diameter). ... 60

Figure 35 Riser shapes for various length of riser. ... 61

Figure 36 Mean von Mises stress profiles (vary length of riser). ... 62

Figure 37 Mean axial stress profiles (vary length of riser). ... 62

Figure 38 Mean bending stress profiles (vary length of riser). ... 63

Figure 39 Mean hoop stress profiles (vary length of riser). ... 63

Figure 40 Riser shape configurations when vary the initial offset of FPSO. ... 64

Figure 41 Mean von Mises combined stress (vary initial offset). ... 65

Figure 42 Mean axial stress profile (vary initial offset). ... 66

Figure 43 Mean bending stress profile (vary initial offset). ... 66

Figure 44 Mean hoop stress profile (vary initial offset). ... 67

Figure 45 FPSO heave motion (vary FPSO size). ... 68

Figure 46 Mean von Mises combined stress (vary FPSO size). ... 69

Figure 47 Mean axial stress profile (vary FPSO size). ... 69

Figure 48 Mean bending stress profile (vary FPSO size). ... 70

Figure 49 Mean hoop stress profile (vary FPSO size). ... 70

Figure 50 Wave scatter diagrams prepared in 4 directions. ... 76

Figure 51 Cumulative probability distribution of maximum stress. ... 78

Figure 52 Cumulative probability distribution of stress range... 79

Figure 53 Cumulative probability distribution of the fatigue life. ... 80

Figure 54 Probability distribution of the fatigue life. ... 80

Figure 55 Displacement RAOs (Amplitude, 0 degree wave direction) ... 85

Figure 56 Displacement RAOs (Phase, 0 degree wave direction) ... 85

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viii

Figure 69 Wave load RAOs (Operating draft, 0 degree wave direction)... 96

Figure 70 Wave load RAOs (Phase, 0 degree wave direction) ... 96

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ix

List of Tables

Table 1: Stress definition according to API 2RD... 30

Table 2 Design matrix for rigid risers ... 32

Table 3 S-N curve parameter for API Class-X ... 35

Table 4 Pipeline specifications ... 50

Table 5 Simulation parameters used in the base case ... 56

Table 6 Fatigue damage sensitivity (vary outside diameter) ... 58

Table 7 Cases for riser length sensitivity analysis ... 61

Table 8 Fatigue damage sensitivity analysis (vary initial offset) ... 65

Table 9 Fatigue damage sensitivity (vary vessel length) ... 68

Table 10 Fatigue damage sensitivity (vary simulation time) ... 71

Table 11 Fatigue damage sensitivity (vary length per segment)... 72

Table 12 Omnidirectional wave scatter diagram prepared for 20 years period ... 74

Table 13 Directional wave statistics prepared for 20 years period ... 75

Table 14 Occurrence matrix of directional wave for fatigue analysis (315⁰, 45⁰) .... 77

Table 15 Occurrence matrix of directional wave for fatigue analysis (45⁰, 135⁰) .... 77

Table 16 Occurrence matrix of directional wave for fatigue analysis (135⁰, 225⁰) .. 77

Table 17 Occurrence matrix of directional wave for fatigue analysis (225⁰, 315⁰) .. 77

Table 18 Displacement RAOs (Relative angle = 0 degree) ... 92

Table 23 Displacement RAOs (Relative angle =150 degree) ... 94

Table 25 Wave load RAOs (Relative angle = 0 degree) ... 103

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x Table 30 Wave load RAOs (Relative angle = 150 degree) ... 105 Table 31 Wave load RAOs (Relative angle = 180 degree) ... 106

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Content

Abstract ... v List of Figures ... vi List of Tables ... ix Content ... 1 1. Introduction ... 3

2. Aims of the Project ... 7

3. Critical Review ... 8

4. Motions of FPSO ... 12

4.1. Displacement RAOs ... 12

4.2. Wave Load RAOs ... 12

4.3. Wave and Wind Drag force... 13

4.4. Stiffness, Damping and Added Mass Loads ... 14

5. Motions of Riser ... 17 5.1. Tension Force ... 17 5.2. Bending Moment ... 20 5.3. Pipe Stress ... 20 6. Example Calculations ... 22 6.1. Static Analysis ... 22 6.2. Dynamic Analysis ... 25

7. Recommended Practices for the Design ... 30

7.1. Stress Element ... 30

7.2. von Mises stress ... 31

7.3. Allowable Stress ... 32

7.4. Collapse Pressure ... 33

7.5. Fatigue Life of Riser ... 34

7.6. Rainfall Counting ... 35

7.7. S-N Curve ... 35

7.8. Fatigue Life ... 36

8. Case Studies ... 37

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2

8.2. General Information about the Timor Sea ... 39

8.3. Waves Statistics in the Timor Sea ... 41

8.4. FPSO Simulation Model ... 46

8.5. Riser Simulation Model ... 49

9. Sensitivity Analysis ... 56

9.1. Sensitivity Analysis: Outside Diameter ... 57

9.2. Sensitivity Analysis: Riser Length ... 60

9.3. Sensitivity Analysis: FPSO Initial Position ... 64

9.4. Sensitivity Analysis: FPSO Size ... 67

9.5. Sensitivity Analysis: Simulation Time ... 71

9.6. Sensitivity Analysis: Length of Segment ... 72

10. Probabilistic Fatigue Life ... 73

10.1. Waves for Fatigue Calculation ... 73

10.2. Cumulative Stress Probability Distributions ... 78

10.3. Fatigue Life Probability Distributions ... 79

11. Discussion ... 81

12. Conclusion ... 82

13. Recommendations ... 83

Reference ... 84

Appendix I: Plots of Displacement RAOs ... 85

Appendix II: Tables of Displacement RAOs ... 92

Appendix III: Plots of Wave Load RAOs ... 96

Appendix VI: Tables of Wave Load RAOs ... 103

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3

1. Introduction

Over the next decades, the world’s energy needs will double whereas the existing hydrocarbon supply will significantly depleted since they were consumed vastly to support the world economic growth in the twentieth century. To survive in the future, the offshore industries will be more diverse to develop new technologies and to explore new assets in more challenging environment. The company corporation, sharing know-hows and the best practices across different disciplines will be emphasized in order to quickly invent new technologies greatly demanded in the future. One of the promising areas for the future oil explorations is the deepwater where water depth is much greater than depth along the continental shelf. The deepwater normally range from few hundreds meter to about 6,000 meter for very deep areas. Because of the greater depth and mostly situated in the harsh environment, the exploration and production of hydrocarbon in the deepwater required advanced engineering and the multibillions investment to develop the projects meanwhile risk involved especially in the exploration stage is very high. In last 10 years, oil companies and drilling companies further out the sea to reach the last remaining oil reserves which are believed laid under the deepwater. A deepwater drilling used to be very dangerous and expensive activities in 20th century, but they seem suited in the current years. Also, the great energy demand from developing countries leads to the higher oil price which makes deepwater project become feasible and contributed to a renewed interest in further offshore explorations. For example, Gulf of Mexico is the deepwater region where deepwater drilling is very intense. According to the US Minerals Management Service (US MMS), there are 31 rigs drilling deepwater wells in the Gulf of Mexico in 2008 – compared with only 3 rigs operating in 1992. Seven gigantic deepwater projects come on stream in the US in 2008 including Thunder Horse field which is the largest field in the region. As well the deepwater exploration and production have continued in other regions in several corners of the world including Brazil, Angola, Nigeria, Australia, India, Indonesia, Australia and etc. However, most of deepwater oil has been found at the

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great exte Africa. Ex says that t the future consumpti Fi Surface fa have been 5 years, 6 floating p tensioned preferable refer to th SPAR and height in t Africa due significant nt in the go xxon Mobil the deepwat the deepwa ion. igure 1 Wor acilities and n tripled in n 66 potential roduction p led platfor e types for he distinctiv d TLP types the hurrican e to the adv tly for marg

olden triang , as claims ter explorat ater oil will

rld map of t d subsea tieb numbers of l deepwater platforms in rm (TLP), s the ultra-d ve requirem s are being ne season, w vantages of ginal oil fiel

gle area wh to be the la tion has beg l become m the deepwat backs in the producing r platforms nclude the semisubmer eepwater. T ent in the d chosen as c while FPSO f the platfor lds. hich made u argest comp gun in only more signific

ter oil explo e Gulf of M platforms in will be in storage and rsibles (SEM The differen different env common opt Os are very rm mobility up of Mexic pany in deep half of the cant propor oration and p Mexico, Wes n last 5 yea nstalled in m d offloadin MI) and SP nt types of vironment. tion becaus engaging i y saving the co, Brazil an pwater prod know field rtion of the production. st Africa an ars. And, in major regio g vessels ( PAR which f floating p In Gulf of e the extrem in Brazil an e project inv 4 nd West ductions, ds and in total oil . nd Brazil the next ons. The FPSOs), h are the platforms Mexico, me wave nd South vestment

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5 As water depth increase, so do the drilling and completion cost as well. Therefore, the next challenges are to minimize the cost of drilling, increase well productivity and developing technologies suitable for the deepwater environment. The techniques such as multilateral well, smart well completion, and extended well were developed by universities and famous research centers to make drilling and completion simpler and cheaper. Another importance issue is the platform stability in order to survive in unpleasant sea conditions. Generally speaking, TLPs have a practical limit of a 1,500 meter water depth, so for the deeper water the choices will be SPAR, FPSO or SEMI. The production riser is another main challenge for subsea engineers. In the deepwater, the hydrostatic pressure and temperature are tremendous obstacles for the oil production because they can causes of riser integrity problems such as riser collapse, cracking and fatigue damage. Hence, several types of riser are constructed to suit with the different the water depth, floating platforms and sea environment. The first and simplest riser system used in deepwater is the steel catenary riser system which the riser is manufactured from the steel tube painted with the anti-corrosion chemicals. The Steel Catenary Riser (SCR) has been extensively used in deepwater operations because the cost of material and installation is significantly less than using flexible riser. Therefore, SCR has been vigorously demanded by the deep water development especially where the spool base for flexible riser is not available in that region. Besides, the SCR system is remarkable for its reliability, simplicity and robustness which make it as the first choice for the high pressure-temperature in deepwater applications.

The Steel Catenary Riser is a simple riser system made of continuous rigid pipe. It must be installed from a floating structure and gently laid to the seabed. At the top end, the riser is connected with a flexible joint which is an equipment to allow small angular movement which makes the riser be less restricted to avoid excessive bending moment occurred at the outer rim. At the bottom end, the riser gradually touches the sea bed. The touchdown is known as a critical part on riser because it is where usually subjected with the maximum bending stress so as to the potential of crack and leak are high. The riser failure must be avoid in all means because

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6 consequences are extremely destructed. For the business, it brings in the complicated repairing program and reduction of companies’ revenue. For environment, it causes seriously environmental strains by the spilling hydrocarbon to the nature. Hence, industrial standards for different riser applications are established and enforced to the new deepwater projects. In addition, engineers have been developing a better, more accurate and reliable methods for designing the riser. The study will investigate possibility and perform the preliminary engineering design to make some recommendation for the PTTEP deepwater project in Timor Sea.

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7

2. Aims of the Project

The objective is to discover the optimal design configurations for the riser and also to estimate the fatigue life by using a probabilistic approach. A gas export riser connected with the internal turret FPSO will be modeled to investigate the stress on riser which the stress will be induced by the FPSO motions.

The industry practices for the riser design (API 2RD) are applied in the study to ensure the feasibility of being carried out. The important requirements specified in API 2RD such as the allowable stress, allowable deflection, hydrostatic collapse and fatigue life will be strictly followed. In addition, the study will investigate on how key design parameters such as riser diameter, wall thickness, riser length and FPSO’ size can affect the overall of riser design. The principle stress such as tensile, bending and hoop stresses will examine along the length to identify the critical segment. Afterwards, the fatigue damage will be calculated by using the S-N curve and rainfall half-cycle methods and combined fatigue damage of all sea states by the Minor’s rule. Lastly, a distribution curve of stress and fatigue life will be established.

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8

3. Critical Review

The steel catenary riser has been adopted for many deepwater development projects developed on floating hosts in the North Sea and Gulf of Mexico (GoM). The experience in these area show that the fatigue problem is the most challenging issue for the riser design particularly true for a large diameter riser.

The loading fatigue damage is directly related to the combined effect of various parameters such as environment condition, fluid density and water depth. Also, the riser design is very sensitive to motion characteristics of the host platforms. In ultra-deepwater, the combined mass of the mooring lines, risers and umbilical have a great proportion to the total mass and drag force of the system. So, in the past, the riser and vessel motions will be analyzed by the uncoupled method where the FPSO motions will be calculated separately from riser. The results of FPSO motion then will be applied as initial conditions for the analysis of the riser motions. However, this method seems associated with huge error in hydrodynamics damping force and the resonant responses of the system. As described in the motion studies performed by J. Xu in 2006, he suggests that the restoring stiffness of mooring and riser, mass and viscous damping will change the roll and pitch frequency as well as the slowly varying drift motions.

In the riser design, the characteristics of the floater have strongly link with the dynamic of the riser. The main interfaces are such as hang-off locations, flexible joint, stiffness of the mooring system and maximum heel, yaw and pitch of the vessel in survival conditions. These factors are not exhaustive, and a numbers of piece of information must be collected and exchanged along with the designing phase of riser and floater. The riser design is the result of compromises between tension at the top, maximum bending stress at the touchdown point and risk of the collision with nearby structures due to lateral displacement. In most case, the tension needs to be minimized in order to reduce the hang off load, limit impact to the mooring system and minimize horizontal load at the touchdown point to prevent slipping of flow line.

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9 However, the tension is limited by the demand of size and length of riser as well as the lateral tension required for suppressing the bending stress at the touchdown point. If the analyses are well understood and easy to perform, the riser configuration will be easy to verify. Also, the fabrication and installation method should be done without difficulty. However, at every step of calculations, there are uncertainties involved which have significant impact on the riser configuration such as actual pipe thickness, installation tolerance for subsea equipment, uncertainties in the measurement of the position of the floater, uncertainties of the water depth, sea condition variations and etc. The consequence of all these uncertainties should be analyzed carefully especially the sections included special characteristic; for example, different wall thickness, steel grade and welding method.

In the dynamic condition, several parameters can affect to the riser motions such as the first and second order motions of the vessel, length of riser and sea conditions which they could make the touchdown point shift vertically and horizontally. During the movement, the riser is subjected to additional axial stress making the touch down region highly sensitive to the fatigue failure damage. Also, the platform motions will cause changes in the departure and curvature of the riser, which leads to significant excessive bending stress. Different methods and tools to analyses the riser’s behavior were invented to combat these challenges and to perform the riser design in the time frame of a project.

The extreme analysis of riser, where wave loads based on maximum wave height, is another essential study for the riser design. However, the calculation of extreme wave height is subjected to a large numbers of errors. First, errors arise in the data collection due to malfunction or inaccuracy of the either equipment or method of measurement. Second, the long-term distribution to describe wave characteristics maybe not selected appropriately. The criteria for selection are theoretically various and still unclear. In other means, no reason is to select one particular distribution to describe the wave nature over another. Most of the time, it is often based on the judgment of engineers. Last is an uncertainty from insufficient collected wave data. Because the prediction has extrapolated 20, 50 or 100 years exceed from the service

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10 life, but usually only a few years data collection can be gathered when starting the design. Therefore, it is clear that a high degree of uncertainty is surely expected in the calculation results. In addition, the accuracy results require the time domain analysis of wave frequencies as well as quasi static offset of the system. In such analyses, they are CPU time consuming process which is in general not applicable in the earlier design stage. Therefore, some shortcuts are necessary to be applied; for example, the vessel motions induced by low frequencies wave and the mean environment loads are defined as a static initial offset of the vessel.

Another necessary analysis for riser is the fatigue analysis, especially the case of the large production riser. There are two places on riser prone to the fatigue failure. First is the touchdown point where the bending stress is the highest. Second is the first welding seam below the flexible joint, where the maximum axial stress is occurred. Since there are several uncertainties involved in the calculations, several methods come up in order to achieve higher accuracy for the results. The recent attempts to manage the uncertainty contained in the calculation of probabilistic fatigue life is introduced by Wirsching (Wirsching, 1984) and recently emphasized by Tapan (Tapan K Sen, 2008). However, the study did not embrace the effect of low frequency excursion (slow drift motion), torsional stress and VIV. Also, the random nature of waves which have immense effect on the stress of riser is limited to single directions because the limitation of software used at that time (Virtual Orcaflex 2001). Tapan simulated a typical FPSO operating in the West Africa where water depth is around 1,200 meters. Sea states data are compressed to a number of equivalent wave height and time crossing periods (Hes and Tez) in order to reduce the numbers of cases and time for simulations. Waves are categorized by the wave

height which the equivalent wave height (Hes) is calculated by using weighted

average with H6 for any waves having the same time crossing period (Tez). This

approximation is due the fact that stress is proportional to square of wave height

(H2), and fatigue damage is approximately proportional to the third power of the

stress range (S3). Tapan simulated the stress time history at a node near the

touchdown point to analyses the stress ranges and return periods by using traditional rainfall counting method. After that, the fatigue life is done by using Monte Carlo

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11 Simulation with the probability assigned to input parameters e.g. wall thickness, eccentricity, stress range and stress concentration factor. In his study, the non-directional wave data was utilized to avoid large number of load cases. However, the lack of wave direction data may result in an unacceptable high uncertainty of the simulation results.

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12

4. Motions of FPSO

4.1. Displacement RAOs

The FPSO motion can be described by a displacement RAOs (Response Amplitude Operators). The RAOs consist of pair of numbers to define responses in 6 DOFs (surge, sway, heave, roll, pitch and yaw) of a particular wave period in a certain wave direction. The RAOs are composed of the response amplitude (R) which defines the response when the FPSO exposed to 1 meter wave height. Another component is the phase difference to define a lagging or leading phase of FPSO relative to the approaching wave. The RAOs are strongly related to the shape, size and draught of FPSO normally obtained from the hydrodynamics experiment or the simulation. The RAOs can be expressed mathematically by using the following equation. cos( - ) x R a w t  _{... (1)} where x = vessel displacement (m) R = RAO Amplitude (m) a = wave amplitude (meter) ω = wave frequency (rad/s) t = time (second)

φ = phase difference between wave and FPSO responses (rad)

4.2. Wave Load RAOs

Force and moment can be represented by wave load RAOs in the same manner as the displacement RAOs. In the simulator, the force and moment from wave load RAOs will be combined with other loads to describe the motion by using the Newton’s law of motions. Because the wave load RAO consists of force and moment, their unit are Newton and Newton-meter per wave height respectively.

cos(

)

F

Force R a

 

 

t



... (2)

cos(

)

M

Moment R a

 

 

t



... (3)

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13

F M

where

R = wave load RAOs (N) R = wave moment RAOs (N) a = wave amplitude (meter) ω = wave frequency (rad/s) t = time (seconds)

φ = phase difference between wave and FPSO responses

4.3. Wave and Wind Drag force

Hydrodynamic drag is an important force component for modeling the FPSO motion especially the slow drift motion. Drag forces can be modeled with a relation of the relative velocity, yaw rate and roll rate.

Drag force due to Relative Velocity

Drag force due to water or wind flowing toward the buff body will be calculated by substituting the velocity relative, drag coefficient and projection area into the equations which express drag forces in surge, sway and yaw directions.

2 2 2 1 2 1 2 1 2

Surge w Surge Surge Sway w Sway Sway

Yaw w Yaw Yaw

F C A V F C A V M C A V       ... (4) 3 w 2 where

ρ = density of water or air (kg/m )

C = drag coefficient in the direction respected to vessel heading A = projection area in surge, sway and yaw (m )

V = relative velocity of the water or air past the vessel (m/s)

Drag force due to Yaw Rate

For wind drag, the yaw rate term is insignificant and will be omitted from the calculations; but it is still influential for wave drag to describe the motions of FPSO. The drag due to yaw rate can be expressed by the following formula.

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14 1 2 1 2 1 2 Surge w Surge Sway w Sway Yaw w Yaw F K F K M K             ... (5) 3 w where F = drag force (N)

ρ = density of sea water (kg/m ) ω = yaw rate (rad/s)

K = damping coefficient (s/m)

Drag force due to Roll Rate

As similar as the yaw rate term, the row moment is modeled by using the equation defined in the following.

Roll L Q

M



K V



K V V

... (6)

L Q

where

M = moment due to roll rate (N m) V = angular velocity component (rad/s) K = linear roll damping coefficient K = quadratic roll damping coefficient

Roll 

4.4. Stiffness, Damping and Added Mass Loads

The stiffness, damping coefficient and added mass and are important hydrodynamic variables for evaluating the FPSO motions. These parameters refer to the forces which are described in the following.

Stiffness Load

Force due to stiffness occurs when the vessel is offset from the equilibrium position. The stiffness (heave, roll and pitch) can be represented by the stiffness matrix and it

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15 is a function of vessel type, draught and water plain area shown by the following equations. Heave Heave Roll Roll Pitch Pitch F O M K O M O      _               ... (7) where

F = hydrodynamic stiffness force (N) M = hydrodynamic stiffness moment (N-m) K= hydrodynamic stiffness (N/m)

O = offset from equilibrium position (m)

Damping Load

The damping load is equal to -D*V, where D is the specified damping matrix and V is the vector of FPSO velocity relative to the stationary. The damping loads are calculated by using the following matrix equation.

X X Y Y Z Z X X Y Y Z Z F V F V F V D M M M



                                          ... (8) where

F = hydrodynamic dampling force (N) M = hydrodynamic damping moment (N-m) D = damping coefficient (s/m)

V = velocity (m/s)

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16

Added Mass Load

The added mass load is calculated as similar as the damping loads, but the added mass matrix is used instead of the damping matrix.

X X Y Y Z Z ADD X X Y Y Z Z F V F V F V M M M M



                                                ... (9) ADD 2 2 where

F = hydrodynamic dampling force (N) M = hydrodynamic damping moment (N-m)

M = added mass coefficient (kg)

V = linear acceleration (m/s ) ω = angular acceleration (rad/s )

 

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17

5. Motions of Riser

In the simulation, a riser is divided into a series of line segments which are then modeled by straight-massless model where each segment will have node at each end. The segments will represent axial and torsional stresses occurred in the riser. On the other hand, properties such as mass, weight and buoyancy will all lump to the nodes. A line segment is divided into two halves and the properties such as mass, weight, buoyancy and drag coefficient of each half‐segment will be lumped to the node at the segment end. Forces and moments are calculated in 4 categories and applied at the nodes.

1. Tension Forces 2. Bend Moments 3. Circumferential force

4. Shear Forces (neglected due to insignificant magnitude)

5.1. Tension Force

The tension of each segment is calculated by using the linear stiffness assumption. In this case, the linear axial stiffness represents the axial spring and damper at the center of each segment. A mathematic expression for the tensile force is described below.

(

)

e w o o i i

T T

 

PA PA



... (10)

 

e w i o 2 i Where T = effective tension kN T = wall tension kN P = internal pressure psi P = external pressure psi

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18 The wall tension (Tw) is tension on the riser measured at the pipe’s circumference. It consists of three main components. First is the elongation tension in axial direction, the second is the tension due to the line pressure effect and the last is due to the damping effect. The wall tension can be described by the following equation.

w o o i i

EA.e

T = EA

2 (P A -PA )

o

dL

L

dt









_{ }









... (11) Where EA = axial stiffness (kN/m)

ε = total mean axial strain = (L - λL0) / (λL0) L = instantaneous length of segment (m) λ = expansion factor of segment

L0 = upstretched length of segment (m) ν = poisson ratio

Pi, Po = internal pressure /external pressure (psi) Ai, Ao = internal / external cross sectional areas (mm2) e = damping coefficient of the riser

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To unders tension, co either side forces on hold in th illustrates F stand the fo onsider the e are to rep them are ca e contents e the tension Fi Figure 2 Seg ormula and forces acti present a len alculated as exposed to n and pressu igure 3 Wal gments and d the differe ng axially a ngth of pipe if the lengt the interna ure forces. ll tension an nodes in ri ence betwe at the mid‐p e plus its co th of pipe re l and extern nd pipe pres ser model. een effectiv point of a s ontents. Mo epresented h nal pressure ssure force. ve tension a segment. Th ore importa

had end cap e. The figur 19 and wall he nodes antly, the ps which re below

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5.2. B

In case of which me moment c Where EI = bendi D = bendi C = curvat t = time (s

5.3. P

The stress outside dia which can shown in centerline normal to Consider a (R, C, Z) w

Bending

f linear ben eans the x-an be calcu ing stiffnes ng damping ture of segm sec)

Pipe Str

s calculatio ameters are n be either s the followi where Oz the pipe ax Figur at point P, where R is

Momen

nding, the f - and y- b lated by usi M s (N. m2) g value for a ment (m)

ress

on will be a e given. The steel or titan ng diagram is the direc xis in the cro

re 4 Frame o which can radically ou

nt

formula is bending stif ing the belo

M om ent E a segment ( applied by e cylinder is nium riser. m, the frame ction along oss-sectiona of reference be identifie utwards, C i based on t ffness are ow equation d EI C D N-sec) a simple c s assumed t When cons e of referenc the pipe ax al plane. e of pipe stre ed in the pip is in the circ the isotropi identical. T n. C dt ... cylinder wh to be made sider a cros ce has an o xis. In addit ess calculat pe section. cumferentia c bending Therefore, ...

hich the ins of uniform s-section of origin locate tion, Ox and tion. A local set al direction 20 stiffness bending . (12) side and material f pipe as ed in the d Oy are t of axes and Z is

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21 parallel to the axial direction. Regarding to these axes, the stress component at P is 3x3 matrixes which will be given by

RR RC RZ RC CC CZ RZ CZ ZZ



... (13)

The diagonal entries of the matrix RR, CC, ZZ are the principle stress for radial, hoop and axial stresses respectively. The other 6 off-diagonal components are the shear stresses in 3 dimensions. However, the diagonal stress components are considered as insignificant for the stress and fatigue damage; therefore they will be disregarded in this study.

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6. Ex

6.1. S

The static condition. length is e means, the forces e.g. the calcula simulation

xample

Static An

c analysis In this exa equally divi e segment p . wave and ations. Neve n to obtain b Figure

e Calc

nalysis

is perform ample, the st ided to 4 se properties a wind drag f ertheless, th better accur 5 Free-bod

ulatio

med to obta teel riser is egments by are specifie force will b hese forces acy and reli

dy diagrams

ns

ain the ris deployed to y using the ed into the e e ignored in will be take iability. for FPSO a er position o 1000 met lumped ma each node. n this stage en into calcu

and riser sys

n in the st ter water de ass method. The hydrod in order to ulations in O stem. 22 tationary pth. The . By this dynamic simplify Orcaflex

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Figure 6 Wave for

rce RAOs ffor surge andd heave mootions

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6.2. D

The dyna occurred i from wav design crit after mult stress is a presents h and stress Fig

Dynamic

amic analys in the riser ves and FPS terion for r tiplied with always und how the calc when 1 m w gure 7 Estim

c Analys

sis is perfo in dynamic SO. In the iser becaus h the safety der the reco culations are wave of 8 s mate riser pr

is

ormed to in c situations dynamic an e the riser s y factor. He ommendatio e done for t econd retur rofile in stat nvestigate s which gen nalysis, the stress will b ence, the de ons in all c the node dis rn period ap tic condition the motion nerated by t von Mises be limited b esign must conditions. splacement, pproaches at n. n, force an the excitatio s stress is a by its yield be ensure In the exa elongation t the bow of 25 nd stress on force a critical strength that the ample, it , tension f FPSO.

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Figure 8 N Figure 9 N Nodes displ Nodes displ lacement in lacement in X-axis. Y-axis. 27

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Figuree 10 Tensilee stress proffiles for eachh riser segmment.

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30

7. Recommended Practices for the Design

The API 2RD design practices are widely used in the offshore industry to provide the guideline for the safe and practical design. The critical design parameters are such as maximum stress, bending moment, hydrostatic collapse and the fatigue life which will be explained more in this chapter.

7.1. Stress Element

Three principle stresses will be investigated at the critical sections along the length of riser to ensure that the principle stress is under the allowable quantity. For plain cylinder riser, the principle stress will be classified as one of the following.

Table 1: Stress definition according to API 2RD

Primary

Any normal or shear stress that is necessary to have static equilibrium of the imposed forces and moments. Thus, if a primary substantially exceeds the yield strength, either failure or gross structural yielding will occur.

Membrane

p is the average value across the thickness of solid section excluding the effects of discontinuities and stress concentrations. For example, the general primary

membrane a loaded in pure tension is the tension divided by the cross- sectional area. p may include bending as in the case of simple pipe loaded by a bending moment.

Bending

b is the portion of primary stress proportional to from centroid of a cross section, excluding the effects of discontinuities and stress concentrations.

Secondary

q is any normal or shear stress that develops as a result of material restraint. This type of stress is self-limiting which means that local yielding can relieve the conditions that cause the stress, and a single application of load will not cause failure.

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31

7.2. von Mises stress

For plain round pipe, where transverse shear and torsion are negligible, three principal stress components of primary membrane stress will be equivalent to von Mises stress. The equation is showed in the equation below.

2 2 2 1 ( ) ( ) ( ) 2 e pr p p pz pz pr           ... (14) 2 e 2 pr 2 pθ 2 pz where

σ = von Mises equivalent stress, N/mm σ = principle stress in radial direction, N/mm σ =principle stress in hoop direction, N/mm σ = principle stress in axial direction, N/mm

For a thick walled pipe, the principle stress will be derived from the following equations o o i i o i -(P D +P D ) D +D ( ) 2 ( ) 2 pr o p i o i pz o D P P P t T M D t A I 



       ... (15) 2 i 2 o o i 2 where P = internal pressure, N/mm P = external pressure, N/mm D ,D = outside,inside diameter, mm t = pipe wall thickness, mm

A = crosssection area, mm T = wall tension, N

M = bending moment in pipe, N-mm I_{= moment of inertia, mm}4

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32

7.3. Allowable Stress

Regarding to the API 2RD for a plain pipe, the von Mises stress should be less than the allowable stress defined by the product of the design case factor (Cf) and the allowable stress (σa). ( p)e Cfa ... (16) a a y y a a where σ = C .σ = allowable stress

σ = material minimum yield stregth 2 C = allowable stress factor, C = 3

design case factor

f

C 

Table 2 Design matrix for rigid risers

Design Case Load Category Environmental Conditions Pressure Cf

1* Operating Maximum operating Design 1.0

2 Extreme Extreme Design 1.2

3 Extreme Maximum operating Extreme 1.2

4 Extreme Maximum operating Design 1.2

5 Temporary Temporary Associated 1.2

6 Test Maximum operating Test 1.35

7 Survival Survival Associated 1.5

8 Survival Extreme Associated 1.5

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33

7.4. Collapse Pressure

Riser design must be ensured that it will not be collapsed during any operations. Therefore, the riser should be able to withstand hydrostatic pressure at any time during installation, production as well as workover. The effect of variations in pipe thickness, ovality, eccentricity and residual stress from pipe manufacturing should be included in the pipe collapse pressure. The allowable collapse pressure and collapse pressure for round pipe can be calculated by using the following formula.

a f c

P  D P ... (17)

f c

where

D = collaspe design factor (0.75 for seamless or ERW pipe)

P = collaspe pressure (psi)

2 2 ( ) e y c e y P P P P P   ... (18) max min where

D,t = nominal pipe outside diameter and wall thickness (mm)

D = maximum outside diameter of pipe (mm)

D =minimum outside diameter of pipe (mm) E,υ = modulus of elasticity and Poisson's ratio (N/ 2

2 y 2 2 e 3 i m ) σ = specified minimum yield stress (N/m )

A = cross secional area of pipe (m ) a = cross sectional area of wall (mm ) T = effective tension (N)

G = unit weight of water (kg/m ) H = water depth (m) P = i













i a e i 1/2 2 r y a y a y y r

nternal pressure (psi) P = net external pressure = GH-P S = mean axial stress = (T -PA)/a -P

Y = reduce yield stress = σ 1-3(S /2σ ) - S /2σ

P = yield pressure with simultaneous tension =2Y t/D P_e

 

 

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34 For a pipe designed to meet the external collapse criteria outlined above, collapse will be initiated at a lower pressure by accidents e.g. impact or excessive bending due to tensioner failure. The maximum allowable collapse pressure will be done using the formula below to ensure the pressure differential will be less than the predicted propagation pressure.

d p p P  D P _...(19) p 2.4 p y where

D = collaspe propagation design factor = 0.72 P = collasse propagation pressure=24σ (t/D)

7.5. Fatigue Life of Riser

The fatigue damage in riser comprises of several contributions such as vessel motions, direct wave loads, transportation and VIV. Damage due to vessel motions can be further split into that due to the wave-frequency and the slowly-varying motions. The former refers to the small in the stress magnitude but comparatively rapid in the return periods. Whereas the latter can be perceived as an enormous in magnitude, but less frequent in term of return period. In the API practices, the design fatigue life should be at least 3 times greater than the service life (SF ≥ 3). This recommendation will be applied for any locations that the safety and pollution risk are low and the regular inspection is possible. On the other hand, for locations where the riser cannot be inspected regularly or the safety and pollution risk are highly concerned, the design fatigue life is recommended to be at least 10 times the service life (SF ≥ 10). In this study, the environment in the Timor Sea is considered as a tolerable condition which the regular inspection and maintenance can be performed regularly. Therefore, the design factor of 3 will be used to evaluate the riser design in this project.

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35 where

Design factor = 3.0

Service life = 15-20 years

7.6. Rainfall Counting

The rainfall counting technique is used to analyze the stress time history and present it in form the stress range and the time crossing period. With this technique, the fatigue damage created by different sea states can be integrated by using Minor’s rule which is very effective way to analyze fatigue damage in complicate structures.

7.7. S-N Curve

An S-N curve defines cycles to failure of structures subjected to cyclic loadings. The S-N curve can be derived from either direct experimental or follow the API recommended numbers. Generally, the API Class X is a recommended code for the riser design and it can be expressed by the following equation.

(

)

m

N A SCF

 





 ... (21)

where

N = cycle to failure (cycles) S = stress range, (MPa) m = empirical numbers

Table 3 S-N curve parameter for API Class-X APIClass-X

A _2.50*1013

m 3.74

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7.8. F

From the Minor’s ru is shown b where D = fatigu FL = fatig n = numb N = numb

Fatigue L

S-N curve, ule. Therefo below. ue damage gue life (yea bers of stres bers of stres Figure

Life

, the annua ore, fatigue (%/year) ar) s cycle in 1 ss cycle to f 11 SN curv al fatigue da life is the r D F L = year failure ve (API Clas amage will reverse of to n N 



... 1 = D



... ss X). be accumu otal damage ... ... ulated by u e in one yea ... ... 36 using the ar which . (22) . (23)

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8. Cas

8.1. O

The objec design an constructio Timor Sea to investi sensitivity Next, the stress at th away +/- parameter nodes wil individual of wave h the wave b excitation find the m

se Stud

Overview

ctives of the d estimate on the FPS a, West Aus gate the st y analysis w extreme w he critical s 10% of th rs, such as ll be chan l wave data eight. The p bins. After force at th mean fatigue Figure 12 B

dies

w of the

e study are fatigue life SO model in stralia. The tress profile will be perfo wave load w segment. In he water d FPSO size ged to obs a will be cla probability that, 4 wav he FPSO. L e life of riser Block diagr

Project

e to identify e of the ste n Orcaflex most likely es and ide rmed to see will be gene n this case, depth from e, drifting d serve the i assified into based on th ve trains fro Lastly, the f r. ram represen y the critica eel catenary to represen y sea states entify the c ek the optim erated to ob the FPSO the mean distance, sim impacts to o bins which he statistical m 4 directio fatigue life nts the appr al section, y riser. The nt a typical will be use critical poin mized config bserve the t is assumed position. In mulation tim the fatigu h are create l wave data ons will be distribution roach for pr seek the op e study sta l FPSO use ed in the sim nt on the r gurations. tensile and d to be able n addition, me and nu ue damage. ed for certa a will be ass simulated t n will be pl roject (1). 37 ptimized arts from ed in the mulation rise and bending e to drift several umber of Lastly, ain range signed to to create lotted to

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Figure 13 B Figure 14 B Block diagr Block diagr ram represen ram represen nts the appr nts the appr roach for pr roach for pr roject (2). roject (3). 38

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39

8.2. General Information about the Timor Sea

The Timor Sea is the relatively shallow sea bounded from the north by the Timor Island, from the south by Australia and from the west by the Indian Ocean. Beneath, considerable oil and gas reserves are laid. Nowadays, numbers of offshore production platforms and drilling rigs are in operations in the shallow water depth areas and also trench in the deepwater regions.

Montara field is an oil development filed operated by PTTEP Australasia in the shelf region of the Timor Sea. This field is situated 250 km southwest of the Timor Island and 685 km west of the Darwin city in Australia. The metocean data have been collected extensively in Montara and Jabiru field which is another field nearby. The measuring data provides the fundamental information about wind, wave and current which are essential to evaluate the riser design.

For the Timor Sea, key oceanographic features are listed below.

 The Pacific-Indian ocean flow likely generates persistent west to west-south currents.

 The monsoons are the controlling factor of metocean in the Timor Sea for the short return period wind and wave. Tide is a dominant factor to control the oceanic current.

 The Coriolis’ effect is comparatively weak due to the low latitude and the tropical cyclones are likely immature. However, small but intense tropical cyclones could control the long return period waves and winds.

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Figu Fig ure 16 Deep gure 15 Loc p water area cation of Mo as in the Tim ontara field

mor Sea (wa

, Timor Sea ater depth > a. > 500 Meter 40 s).

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8.3. W

No specifi most appr (away to t data recor from Octo will be use Figure

Waves St

fic wave me opriate mea the northea rded in 2, 3, ober 1983 to ed in the stu 17: Existin

tatistics

easurements asured wave ast ~75 Km , and 4 hour o January 1 udy is availa g developm

in the T

s existed in e data for th m). Most of rly intervals 993, but the able only fr ment fields in

Timor Se

the Montar he Montara the data ta s. The meas e full year o rom Decemb n the Timor

ea

ra area. The Field is the aken are om surement is of direction ber 1995 – D r Sea. e best availa e data in Jab mnidirection done over nal wave dat

December 1 41 able and biru field nal wave 10 years ta which 1996.

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42

Wave Climate

The ambient wave climate for the Montara field is composed of separated sea and swell waves, with a wind-sea/swell separation of 9 seconds (0.111 Hz) found from the plotted Jabiru wave spectra. The combination using the square root of the sum of the square wave height results in the total waves.

2 2

total sea swell

Hs  Hs Hs ... (24)

Sea Waves

Sea waves are waves locally generated by wind. As such, the sea wave climate is very closely allied with the summer westerlies and winter easterlies. Transient variations to these persistent seasonal regimes are caused by the various storm types, which occasionally affect the region. As a result of the very long fetched storm, sea waves may have periods ranging from 2 or 4 seconds to as long as 6 or 8 seconds.

Swell Waves

Surface wind waves which are generated by remote storms (i.e. 400 - 7000 km away) and propagate to a site independently of the storm, are known as swell. In the Southern Hemisphere, swell results predominantly from storms in the Southern Ocean or the southern portion of the Indian Ocean. After generation, swell may propagate towards the equator, gradually dispersing and decreasing in amplitude before arriving at the Timor Sea from the southwest. Since longer period swell suffer less dissipation, periods of long-travelled swell are usually greater than 14 seconds commonly ranging up to 20 seconds and occasionally exceeding 22 seconds. Shorter period swell (6 to 10 seconds), may result from tropical cyclones, and from winter easterlies over the Arafura Sea and eastern portions of the Timor Sea.

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Maximu

Within an maximum significant mean wav been empl

Seasona

The month in the wav monsoona shifting to Figure

um Single

ny sea stat m individual t wave, wit ve period. T loyed in this

al Variabi

hly variatio ve rose dia al wind dire o predomin 18 Swells f

e Waves

te character l wave hei th correspo The formula s study.

lity

on in total w agrams. The ections, with nantly easte from south I rized by a ights (EHma onding perio ations of G max max EH ET  wave height e wind wav h westerly s erly from A Indian Ocea particular ax) may be ods ~10% Goda (1985) 1.86 1.15 Hs Tm   ... t, period and ves or sea seas prevail April to ear an to Timor significant e up to twi longer than ) for non-cy ... d direction waves will ling from D rly Novemb r Sea. t wave hei ice as high n the signif yclonic wav ... are shown closely fo December to ber, before 43 ight, the h as the ficant or ves have . (25) in detail llow the o March, shifting

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back to th present in that gener predomina west). Som The mon correspond he west. A v n the summe rally form ant swell di me shorter p nths with ding to the Figure 19 very small er months, to the eas irection rem period swel the smalle calmest mo Annual wav easterly win possibly at t of the M mains from ll will occas est waves onths for win

ve rose diag nd wave co ttributable t Montara Fie the southw sionally app are Marc nd. grams meas omponent m to distant tr eld. Throug west (and to proach from ch, Octobe sured at Jabi may occasio ropical distu ghout the y a lesser de m the east in er and No iru field. 44 onally be urbances year, the egree the n winter. ovember

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Figure 20 MMonthly waave rose diaagrams meassured in Jabbiru field.

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46

Non-Cyclonic Storm Waves

The summer and winter monsoonal and trade wind surges also generate the strongest non-cyclonic storm waves, which could be coupled with the perennial west-southwest background swell component. It could result in the maximum non-cyclonic total sea states. These non-non-cyclonic total sea states are the controlling storm type for the shorter return periods less than 5 years. Applying a minimum total significant wave height threshold of ≥ 2.7 m (annual), ≥ 2.7 m (summer) and ≥ 2.5 m (winter) to the measured ambient wave database in the Montara Field and excluding any tropical cyclone events resulted in the annual extreme events. These extreme wave events are then subjected to the Conditional Weibull extreme analysis technique. The corresponding parameters such as the extreme significant wave heights (Hs), return period mean wave periods (Tm), spectral peak periods (Tp) and average zero crossing periods (Tz) are derived from the storm peak correlations and shown in the table below.

Table 1 Return period of non-cyclonic winds, waves and currents in Montana field

8.4. FPSO Simulation Model

The FPSO model is constructed based on information derived from an existing Montara FPSO and the specifications are described in the following table. For the hydrodynamic parameters such as the displacement and wave load RAO will be adopted from the typical ship-shaped FPSO. These detail information of FPSO’s hydrodynamic parameters will be provided in the appendix.

1 2 5 10 25 Significant Wave Height Hs m 3.52 3.82 4.15 4.37 4.62 Spectral Peak Wave Period Tp s 9.66 10.07 10.49 10.76 11.06 Spectral Mean Wave Period Tm s 7.4 7.71 8.04 8.25 8.48 Average Zero Crossing Period Tz s 6.75 7.04 7.33 7.52 7.73 Maximum Single Wave Height EHmax m 6.55 7.11 7.73 8.12 8.59 Period of Maximum Wave THmax s 8.52 8.87 9.25 9.48 9.75 Return Periods (Yrs) Non‐Cyclonic Annual Return

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Figgure 21 Montara FPSOO.

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Figure 22 Montara FPPSO specififications.

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8.5. R

The assum riser and s (ERW) or

Riser Sim

mption for t steel tubula r double- su Figure 2 Figure 2

mulation

the riser is ar will be m ubmerged a 23 FPSO m 24 FPSO mo

n Model

that X70 c manufacture arc welded model (Side v odel (Front carbon steel d by either (DSAW). T view). view). l will be us the electric The materia sed to fabri c-resistance al specificat 49 icate the e welded tions are

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50 assumed to conform with the established industry specifications for the minimum tensile strength, service temperature, fatigue resistance, internal corrosion and wear resistance.

Table 4 Pipeline specifications

Steel grade X70

Outer diameter (13') 0.3302 (m)

Inner diameter (11') 0.2794 (m)

Mass per unit length 0.173 (te/m)

SMYS 70 Kips

Bending stiffness 4.60E+04 (kN.m^2)

Axial stiffness 4.66E+06 (kN.m^2)

Poisson ratio 0.293

To justify he pipe specifications above, calculation is made according to the API practices. The allowable stress in pipe, fracture toughness requirement, riser deflection, and collapse pressure and collapse propagation will be checked with the pipe properties. Below is the simulated case when 1 meter wave with about 6-8 second return period is approaching the FPSO.

Allowable Stress in Plain Pipe

The figure below illustrates combined stress profiles from top to bottom of riser. The minimum, maximum and mean of von Mises stress are plotted in the graph in blue, green and black respectively. The red line is the limiting stress for steel pipe which is greater than the combined stress for every pipe section. The Riser API 2RP Utilization, reported as percentage of pipe stress over the allowable stress, is averaged at 0.4 with in a range of +/- 0.2. So, it proves that the purposed riser can withstand the typical sea condition.

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Fig Minimum The minim fracture a mechanics loaded me temperatu gure 25 Plot Fig m Fracture mum fractu at the expe s based con embers such ure is anoth t of maximu gure 26 Plot Toughness ure toughne ected stress nsiderations h as the con her critical um von Mis t of riser uti s ess of mate s level ove s are appro nnections an factor influ ses stress an ilization (AP erial should er anticipat opriate and nd welded s uenced stee nd allowable PI RP 2RD) d be sufficie ted service more impo seams. In ad l behavior e pipe stress ). ent to avoi life. The ortance for ddition, low to be more 51 s. id brittle fracture r highly-w service e brittle.

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52 Therefore, the careful testing for steel toughness should be performed to compare the results between different testing methods. The testing procedures such as Charpy test, CTOD, drop weight tear test are standard test for steel used in the pipeline industry.

Riser Maximum Deflection

The maximum riser deflection is specified to prevent the excessive high bending stress in riser which may cause riser leakage and failure. Even when the riser stress is under the manufacture’s recommended, the larger riser deflection is needed to be controlled to prevent multiple risers from interfering and crashing. Therefore, the riser system may include additional equipment such as tensioners, flexible connections and telescopic joints to provide bending and rotating abilities of the riser. These tools should be designed at the worse condition in the extreme case analysis.

Two figures illustrated below indicated smoothly change of the curvature and bending radius of riser. The maximum bending stress occurred at 2000 meter where the riser is gently approaches the sea bed. The maximum curvature this particular point is averaged at 0.0006 rad/m which is the relatively low when compare with other stress component. Therefore, the planned riser configuration with this trajectory seems to be applicable in the real operations.

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Pipe Colla The criteri exceed th operations by severa stress in apse Pressu

ion for colla e ability of s during the al factors su material. S Figure Figure ure apse pressu f riser to w e service lif uch as abil So, these va e 27 Bendin e 28 Riser cu ure is that th withstand th fe. Theoreti ity, eccentr ariables res ng radius pro urvature pro he external h e hydrostat ically, the c ricity, aniso sult in trem ofile. ofile. hydrostatic tic pressure collapse res otropy as w mendous di pressure sh e experience istance is in well as the ifficulties to 53 hould not ed in all nfluence residual o obtain

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precise es pressure o pressure P pressure. timation of of riser is g Pa and the sa f the collaps given by Pc afety factor se pressure which is th r. The below . However, he multiplic w is exampl in the prac cation of th le calculatio ctical way, he allowable on for pipe 54 collapse e design collapse

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Figure 29 PPipe collapsse pressure aand the net hydrostatic pressure.

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56

9. Sensitivity Analysis

The sensitivity analyses are performed to investigate the relationship between key design parameters and the riser internal stress. Several parameters such as the outside diameter, riser’s length, initial position, FPSO size, simulation time and the length per segment will be examined in this study. The study excludes wave drift motion effect in order to avoid model complexity and enormous time required for simulation, but the study will focus more on the effect due to the heave and pitch oscillating motions because they are believed as the primary factors influence to the fatigue life.

Table 5 Simulation parameters used in the base case

Base case

Steel grade X70

SMYS 482E+3 kips

Wave height 1 m

Water period 6 sec

Wave direction Bow

Water depth (d) 1000 m

Horizontal departure (X) 3000 m

Riser length (L) 3300 m

Half span (l) 1756 m

Internal pressure at z =0 2500 psi

Fluid in riser Gas

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9.1. S

The purpo of riser w damage. In 13.0 to 1 concentrat at the riser of the rise is highly e From the r design bec occurred i Although still a pre which is m

Sensitivit

ose of this s which is ab n this study 5.0 inches. tion. In add r top in whi er. Next is th elevated bec results, 1 in cause it pro in common a thicker pi eferable cho much greate Figure 3

ty Analy

study is to id ble to resis y, the OD is From the dition, two c ich the tensi he area arou cause the ris nch wall thi ovides suffic

sea states ( ipe will resu oice becaus er than expe 30 Shape co

ysis: Out

dentify the st collapse set up in 5 results bel critical secti ile stress is und the tou ser is lifted ckness (see cient streng (allowable h ult in a less e its can p cted field p onfiguration

tside Dia

critical sect pressure a different ca low, the sm ions are ide

extremely h chdown po off from th e case 1) is t gth to withst hoop stress fatigue dam provide suff roduction li ns of the ste

ameter

tion and fin s well as m ases with th maller OD ntified on th high due th int where th e seabed. the appropr tand the ma = 347 MPa mage, a 1 in ficient fatig ife (20 year eel catenary nd the optim minimizing e OD increa is the large he riser. Th e suspended he bending riated choic aximum hoo a, using SF ch thicknes gue life (11 rs). y riser. 57 mum size g fatigue ase from er stress he first is d weight moment e for the op stress = 0.72). ss riser is 1 years)

(68)

F Case n Case Case Case Case Case Table 6 Figure 31 M no. OD 1 _13. 2 _13. 3 _14. 4 _14. 5 _15. Fatigue dam ean von Mi (in) ID ( 00 11.0 50 11.0 00 11.0 50 11.0 00 11.0 mage sensit ises combin in) t (in 00 1.0 00 1.2 00 1.5 00 1.7 00 2.0 tivity (vary ned stress (v n) Dama 00 8 25 5 50 2 75 2 00 1 outside diam vary outside ge per year .98E-03 .22E-03 .86E-03 .14E-03 .11E-03 meter) e diameter). r Fatigue L 111 191 349 467 899 58 Life (yr) 1 1 9 7 9

(69)

Figure 3

Figure 33

32 Mean axi

Mean bend

ial stress pro

ding stress p ofile (vary o profile (vary outside diam y outside di meter). ameter). 59