Por último, la tercera etapa trata de los contenidos connotativos

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

SCF 1 .0

7.8. F

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

ulated by u e in one yea

8. Cas

stress at th away +/-

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 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

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

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.

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).

8.3. W

will be use

Figure

Waves St

fic wave me opriate mea the northea rded in 2, 3, ober 1983 to

ed in the stu 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.

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.

Maximu

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

from south I

rized by a ights (EHma

onding perio ations of G

max h westerly s erly from A

Indian Ocea

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

back to th n the summe rally form ant swell di me shorter p nths with

ding to the

Figure 19

very small er months, mains from

ll will occas est waves onths for win

ve rose diag

nd wave co ttributable t Montara Fie

sured at Jabi

may occasio ropical distu ghout the y

a lesser de m the east in

Figure 20 MMonthly waave rose diaagrams meassured in Jabbiru field.

45

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 

Periods Parameter Unit

Figgure 21 Montara FPSOO.

47

Figure 22 Montara FPPSO specififications.

48

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

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.

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

id brittle fracture r highly-w service

e brittle.

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.

Pipe Colla 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

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

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

Figure 29 PPipe collapsse pressure aand the net hydrostatic pressure.

55

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

Density of fluid 0.205 te/m^3

9.1. S

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 cted field p

onfiguration 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).

F

Figure 31 M

no. OD

tivity (vary

ned stress (v n) Dama ge per year .98E-03

e diameter).

r Fatigue L 111

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

9.2. S

amage of 60

n period (20 s; whereas t

iod in realit ser. The is

tic fatigue tion of the 32 in order t

34 Mean hoo

ty Analy

tudy is to fin n which it sing the sam e length vari

ult in case 6 02 years w 0 years). H the random ty can gene sue of ran calculation steel caten to maximiz

op stress pro

ysis: Rise

nd the optim is able to me conditio ies from 3,3

6, the 1.32 which is con However, the e the fatigu

ofile (vary o

er Lengt

mal ration o minimize on as the pr

300 to 3,700

half-span r nsiderably l e model is itions with r stress resu states will all, this st ser using 1 e life.

outside diam

th

of the water annual fati revious stud 0 meters in

ratio offers longer than based on th larger wave ulting in the

be scrutini dy, 9 differ 50 meter in

a minimum n the expect he general s e height and e shorter fat

ized further ests that th d 11” ID sh

60 the half-ge at the

rent riser ncrement

m annual ted field sea state d shorter tigue life r in the he shape hould be

Case n

gure 35 Rise

ble 7 Cases

er shapes for

for riser len

Length (

r various le

ngth sensiti

ngth of rise

vity analysi

age per yea

9.41E-03

r Fatigue L

106

Figure 36

Figure 3

Mean von M

37 Mean ax

Mises stress

xial stress pr

s profiles (v

rofiles (vary

vary length

y length of r

of riser).

riser).

62

Figure 38

Figure 3

8 Mean ben

39 Mean ho

nding stress

oop stress pr

profiles (va

rofiles (vary

ary length o

y length of r f riser).

riser).

63

9.3. S

ndition. In by +10%, + med as conse s. In the ex nd 7.73 seco wave correla ately 100 m years. Ther to the stress wave conditi

the results n being slac ess is domin ffset distanc n case 5. I meters, but it refore, it im

s and fatigu ion where th

shape conf h are derive

show that t cked. This i nated over t ce has signi

In case 5, t significant mplies that ue of the ris he occurren

figurations w

SO Initia

t riser desig ial position f the water wave drift m

ght and wav ed from the

the annual d is because

when vary t

al Positio

gn can surv ns are shift depth. The motion durin ve period a e extrapolat

damage incr the initial g stress. The ct to the fati O offset in

es the fatigu drift effect er, the study lity is consid

the initial of

on

ive from a ted from th ese horizont ng the extrem

are assumed tion of the

reases when position is e simulation igue damag creases by ue life from may have y is simulate

derably low

ffset of FPS

64 extreme he mean tal shifts me wave

Case

s (vary initia

(vary initia

Figure

Figure 4

e 42 Mean a

43 Mean be

axial stress p

ending stres

profile (var

s profile (va

ry initial off

ary initial o fset).

ffset).

66

9.4. S

results in motions w

e extreme w 50 meter. T ment of FPS mum fatigue

sensitivity cause the in

far off the induced hea the larger which make y the top fac O will be tre 2 and 3. Th 50 meters. T hich is appro

e 44 Mean h

analysis, i ncreased si e typical w ave motion

hydrodyna es riser life

cilities and emendously he study sug Therefore, oximately th

hoop stress

ysis: FPS

ion, the FPS ent sizes are ty and deter

increasing F ize shifts th wave freque due to the w amic dampin

much long the field req y suffered f ggests that a

the FPSO he same siz

profile (var

SO Size

SO’ size is e simulated rmine the o

FPSO size he natural f

encies. The wave excita ng force w ger. Howev

quirement.

from the gre a suitable si

size in this e of the Mo

ry initial off

varied from d in this stud

optimal FPS

will enhan frequency e erefore the ation. As w which can m

ver, the size On the othe eat effect of ize of FPSO s study is p ontara FPSO

fset).

m 100 to 35 dy to investi SO size resu

nce the stab

nce the stab

In document EN ~tI~II (página 93-99)