Design and analysis of slender marine structures – Risers and pipelines

TECHNISCHE UIUVERSITEIT Laboratodwn or Scheepshydromechanîca Archief Mekelweg 2, 2628 CD Deift Tel.: 015- 786873-Fax: 015-781836

WEGEMT SCHOOL

DESIGN AND ANALYSIS OF SLENDER MARINE STUCTURES

- RISERS AND PIPELINES

CONTRIBUTION BY PROFESSOR D GARETH OWEN

HERIOT- WA lT UNIVERSITY

MARINE RISERS

CONTENTS

CHAPTER 1 INTRODUCTION

CHAPTER 2 RIGID TENSIONED MARINE RISERS 2.1 Description of System

2.3 Riser Components

2.4

Design Considerations

2.5

Inspection and Maintenance

2.6

Static and Dynamic Analysis of Vertical Tensioned Marine Risers

2.7 Model Tests of Tensioned Risers

CHAPTER 3 FLEXIBLE RISERS

3.1 Description of Systems and Components 3.2 Riser Configurations

3.3

Design Considerations for Flexible Marine Risers

3.4

Installation and Inspection

3.5 Dynamic Analysis of Flexible Marine Risers

3.6

Mechanical Behaviour of Flexible Marine Risers

3.7

Experimental Studies

3.8

Future Prospects

1. INTRODUCTION

A marine riser is essentially a conductor pipe connected between a fixed offshore platform, a floating platform or a vessel and the welihead at the seabed. The. purpose of

the marine riser is to, provide a conductor for the

conveyance of fluids or drilling hardware between the

platform and the well. In the first sense (viz conductor

for the conveyance of fluids) the conductor is

categorised as a "production riser", while in the second

sense it is categorized as a "drilling riser". There are essentially three kinds of risers

- the rigid integral riser

- the rigid non-integral riser

- the flexible (dynamic) riser

The integral riser design utilizes a structural pipe to support flow-carrying tubulars. The riser is fabricated in 15m joints with mechanical connectors in both ends of

the structural pipe. When the connector is made up, all

the tubular flow paths are made, up simultaneously. This is the concept applied in modern drilling risers.

The non-integral rigid riser design consists of a central

structural pipe supporting vertically spaced guide

funnels or other means of guiding tubulars. Flowline

tubulars are individually run down the riser through the

funnels and latched to the template or manifold. The

structural pipé must be installed and tensioned before

the f lowline tubulars are run. The concept has been

employed successfully in the Argyll and Buchan Fields in

the North Sea.

A flexible dynamic riser design consists of a continuous

flexible pipe or a bundle of flexible pipes, with 'a

specific structure adapted to the production

requirements, connected at the surface (together with the possibility for quick disconnection) and on the seabed.

Flexible risers have some significant technical and

economic advantages, probably the most important one

being their capability to survive a loo .yèar storm

without disconnection: this saves time and miminimises

loss of production.

The original use of marine risers for oil and gas

developments goes back to the early days of the industry.

As oil and gas developments moved offshore, drilling

activities required the use of a conductor between the

drill floor and the sea-bed to act as a conduit for'the

drill-pipe and also to provide an annular space for the

return of the drilling mud. When drilling from fixed

platforms the problem is relatively easily solved by

employing rigid riser pipe that is clamped at intervals

to structural members of the platform along their

vertical run up to the surface. The problem is a more complex one when drilling from a floating or compliant

offshore structure. Since the risers in such circumstances cannot be supported along their- length and--.

only supported at their upper ends at the drilling rig, a number of issues, have to be addressed. In the first

place the riser needs to be maintained under sufficiently

high

tension

at their top to prevent buckling due to self weight of their very slender geometry. In addition, to accommodate the motions of the rig (particularly those in

heave), slip joints and flexible jumpers have to be

provided at the upper

end

of the

riser.

Also the

required tension has to be maintained within acceptable limits by means of a tension compensation system. These

features will be described in more detail at a later

stage in these notes. To prevent overstressing at the

sea-bed a ball-joint is provided. The design of such

rigid tensioned risers was initially dominated by static

considerations. However, in recent years (say the past

two decades) greater attention has needed to be given to the dynamic behaviour of risers. This has been mainly

due to the fact that risers have been increasingly

exposed to more severe conditions (deeper waters, severe wave climates) and this has meant that dynamic effects

have tended to be more important than hitherto.

Furthermore, in the case of production risers, the

operational performance of the floating facility is

critically dependent

on the performance of the riser

system. Thus it had become increasingly important to

predict reliably the maximum riser stresses and

deflections, so that riser designs are compatible with

the anticipated operating and environmental conditions.

As an attractive alternative to the rigid tensioned

production riser, the flexible riser concept has been

developed in recent years. The flexible riser can

accommodate large motions of the surface platform without

the need for the special hardware associated with

tensioned systems (eg flexible jumpers, slip joints, tension compensation, ball-joints etc).

Palmer (1985) has listed the advantages of the flexible riser in comparison with rigid risers, as

follows:-A flexible riser system can accommodate large

motions of the floater.

A flexible riser is generally lighter than a rigid riser, and does not require large axial

tension to keep the bending stress to an

acceptable level. This makes it possible to

eliminate the weight and space needed by riser

tensioning and heave compensation devices.

Since there is less tension in a flexible

riser, its lower end can be simpler, and it

does not need to be so strongly anchored as a

There is more freedom of choice in where to

attach a flexible riser to the floater compared with a rigid system. It does not need to run

through a moonpool, and thus the moonpool is left free for other activities such as well

workovers.

-Because a flexible riser is light and can be

deformed, it can be installed (and if necessary

replaced) by light equipment, based on DP

(dynamically positioned) vessels with minimal

weather dependence and interference with other

vessels.

(vi) The flexibility of the system makes it easier

to alter the configuration and tie in

additional wells during the operating life of

the field.

'While flexible risers are a relatively recent

development as compared to rigid risers, their history

can still be traced as far back as the early 1940's when, during World War II, a long flexible pipe reinforced with braided steel wire was laid across the English Channel to

provide the allies' beach head in France with fuel, as

part of the "Pluto Operation" ' (Chen (1990)).

Flexible risers have been extensively used in connection

with SPM offshore loading and off loading operations since

the late 1950's. Although these hoses should not be confused with flexible marine risers they exhibit many

features that are similar in relation

to both their

construction and design. 'In the early 1970's flexible

marine risers were progressively introduced into the

offshore oil and gas industry. In 1972, flexible pipes produced by COFLEXIP were depolyed as f lowlines in the Emeraude Field (Congo). The next significant mile stone

in the development of flexible pipes was their

application in

1975 as risers in the Poleng Field in

Indonesia. This was followed with the successful

introduction of flexible risers in 1978 in the Enchora

Field in Brazil and later in 1979 at Chevron's Casablanca

Field in the Spanish Mediterranean. Into the 1980's,

with the fall in oil price and the discovery and

exploitation of oil in deeper waters, the past decade has

seen rapid developments and innovative designs for

flexible pipes and pipe systems. Today, flexible pipes

have been used in various parts of the world and have

become more and more important in the offshore oil

industry (from Chen (1990))'. Flexible marine risers, are

progressively replacing rigid tensioned riser systems for

all operations with the exception of those associated

with drilling activities from a floating rig.

(iv)

2 RIGID TENSIONED MARINE RISERS

2.1 Description of System

In fact the term 'rigid' is somewhat ambiguous in that such risers have a considerable degree of movement due to

their stiffness over the length.

As has already been mentioned,

marine risers can be

categorized as either being 'drilling risers' or

'production risers'. Rigid tensioned marine risers can

be used for both areas of application, whereas flexible

risers can only be used as production risers. The

diagram (Figure 2.1) shows schematically the location of the major components of a tensioned riser used within a

drilling riser system. These components comprise

- a connector and flexible joint at the base

- a riser structure in segmented joints

- a tensioning system

- flow connectors at the vessels

Rigid tensioned risers can be further distinguished as

being either 'integral' or 'non-integral' in relation to the structural layout of f lowlines. Such a distinction only really applies to production risers. The nature,

advantages and disadvantages of 'integral' and

'non-integral' riser types is indicated below.

Integral Riser

-

This is built and installed in sections or

joints similar to a drilling riser

- Individual f lowlines cannot be installed or

removed separately

- Internal: tubular f lowlines in this case are

continued wholly within the riser pipe itself

- External: in this case f lowlines are arranged

externally to the central structural pipe that

supports them similar to choke and kill lines

in a drilling riser

Advantages of Integral Riser

These are:

- speed of deployment and retrieval

- they behave well under severe environments

- Internal variety: these can contain and

quickly monitor a leak in a flowing tubular

through the pressure increase in the riser bore

- External variety: these are less expensive in

variety due to difficulties with internal configurations

Non-Integral

Riser

This consists of a central structural pipe supporting

arms with guide funnels which support f lowlines laterally

preventing them from buckling. With regard to

installation, the main structural riser is deployed first

in standard lengths and tensioned, then the f lowlines are run one at a time through the guide funnels.

The advantages of a non-integral riser include the

ability to pull individual f lowlines without disturbing

the entire riser.

Amongst its disadvantages are its relatively slow

installation and retrieval. Also,

in the case of a

weather-related shut-down, the operation to pull the

riser must be commenced well in advance thus

substantially increasing the downtime as compared to an

integral riser.

2.2 Sources of Failure of Tensioned Riser Systems

In general the riser can fail under the following

circumstances:

-When the material at a particular section fails due to overloading or due to load cycles giving rise to a fatigue failure

Local or global buckling, caused by compression

in the riser

Failure due to excessive internal or external

pressure causing 'bursting' (radial outward

stresses) or 'crushing' (radial inward

stresses)

Types of Riser System Failure Modes (see Figure 2.2)

Figure 2.2 refers to the wide variety of failure modes

that can occur in a tensioned riser system. These seven types of failure mode are briefly described below.

1. Distortion of the cylinder cross-section

which can be caused by

-- the existence of a double curvature

bend radii, producing high bending R2(la)

(buckling)

with small moments at

- a lack of buoyancy in the riser producing

collapse of the lower section and excessive

tensioning devices which do nt respond quickly enough for vessel heave may cause the failure

illustrated in ic

- on raising the riser after disconnection,

failure could occur due to insufficient weight

existing in the lower section to

maintain

tension and thereby reduce bending Cid)

Ball Joint Damage may occur due to incorrect

monitoring of th base angle, or vessel wrongly

positioned for current conditions (2 and 3 diagram)

Drill String Fatigue Failure (due tO bending stress reversals) or damage to the BOP causing blowout may occur due to the same reasons as cited in 2. above

(2 and 3 diagram)

4, Riser pipe and/or Conductor Pipe ailures due to

- vertical loads being transmitted through

peripheral shear action at the conductor. The

conductor length is determined by the shear

resistance of the seabed, or

= large bending movements being induced on the

conduôtor column due to the horizontal

component of the riser tension and the weight

of the BOP and horizontal deflection (4) The Emergency Disconnect Fails to Function (5)

Ecçe5sve stress may occur in the riser connection

tO the supplementary buoy.

In most cases of

a

productioñ system with a srf ace vessel and a

submerged buoy, there is a phase difference in

response due to wave loading This may introduce

excessive stress and fatigue conditions at the, buoy

and in the riser segment above the buoy (6)

The conductor pipe may fail and the BOP would

consequently collapse oñto the seabed. This could

ccur due to either

4

above or due tO resonant

response of the BOP mass on the elastic column

support, excited by the sea state forming function

2.3 RISER COMPONENTS

The components of a riser system must be strong enough to

withstand high tension and bending moments1 and have

enough flexibility to resist fatigue, yet be as light as practicable to minimize tensioning and f loatation

requirements.

Some detailed information on important riser components

is given below (as presented in Sheffield (1980)).

Riser Joints

A 'riser joint' is constructed of seamless pipe with

mechanical connectors welded on the ends. Kill/choke

lines are attached to the riser by extended flanges of

the connector. The riser can be run in a manner similar

to driilpipe: by stabbing one stalk at a time into the

string and tightening the connector. Ball Joints

'Ball joints' or 'flexible joints' allow limited angular

motion of the riser. In some cases, these flexible

joints may be a series of ball joints. 'Pressure

compensated' ball joints should be used to decrease the torque required to deflect the joint. The forces acting

on the joint push the inner ball against the outer

casing, causing the joint to bind. To decrease the

required torque hydraulic fluid is injected to spread

apart and lubricate the moving parts. With the large

area involved, relatively small pressures are required.

Slip Joints

A 'slip joint' comprises two concentric cylinders or

barrels that telescope. The outer barrel is attached to

the marine riser, and the riser is held in tension by

wire ropes from the outer barrel to the tensioner.

Buoyancy Modules

Buoyancy or flotation modules can be attached to the

riser to decrease the tension required at the surface. These modules may be thin-walled air cans or fabricated

2.4 DESIGN CONSIDERATIONS

Riser Tensioning

The preferred riser tension can be defined for. any given

situation as the tension that will minimize.:. the

probability of damaging the riser yet cause minimal wear

to the tensioners. Adeqúate tension must be held on thé

riser to insure riser integrity and to keep the riser

system (including the ball joint) straight enough to

drill (in

the case of a

drj)ling riser) without the.

drilipipe's rubbing and damaging the riser.

The marine risér is analogous to a horizontal steel beam

that is supported at both ends and loaded between the

supports. Ït will sag, but the sag will be less if we

pull on both ends of the beam A riser is similar, but it is nearly vertical and the loading consists of forces

resulting from waves, current, riser weight and the

weight of mud (or transported fluid) in the riser. As

the vessel offset increases, the weight of the internal

fluid and riser weight becomes more important in

influencing bending momements. The riser will sag in a

manner similar to the horizontal beam, and tensioning

likewise will decrease the amount of sagging.

It is convenient to define three modes of operation for

the riser as

follows:-Operational - where the bail joint angle is

les than 4°.

Ñon-operationaì. - when the ball joint angle is greater than 40 The ball joint must be kept 'of.f step' as long as the riser is connected, usually until the vesSel or rig has reached an

of fset of 10% of the water depth.

Disconnected - when the riser is disconnected

from the wélihead, and concern for riser

tension is replaced by concern for tenSiöning

and paying out of the guidelines if the vessel

is movéd.

Non-Operational but Connectéd Mode.

Figure 2.3 is a typical example of maximum/minimum stress

behaviour in a riser when the ball joint is on or near

the stop. When 'on stop', maximum and minimum Stresses

will occur at or near the connection between the riser

and the ball joint. When the ball jòint is 'off stop',

maximum stresses my occur at another position in the

riser In Figure 2 3, the ball joint is just 'off stop'

when the maximum and minimum stresses at the ball joint

become coincidental. For this case,. the ball joint

disengages at about 100 kips (100,000lbs = 45359kg )

Increasing the tension to 150 or 190 kips has little

effect on the riser stress, but decreasing the tension to 50 kips could lead to riser failure. The-minimum tension to be allowed for the conditions stated in Figure 2.3 is

loo kips.

Riser tension cannot be held constant in floating

drilling (or production) operations,

and the applied

tension will fluctuate about a mean setting. This

f luctuation is considered by many in the industry to be

about 15% of the mean tension.

As 100 kips

is the

minimum allowable tension, then the tension should be set above 100/0.85, or 118 kips. At a setting of 118 kips, the tension is expected to fluctuate between 100 and 136

kips. This is the 'calculated' tension settings to

minimize the riser stresses for 10% offset and the

conditions listed in the figure. Lower offsets will

require lower tension settings using the same criterion;

however, tensioners are not designed to change tension

rapidly if something should happen. Thus tension

settings for field operations should be based on the

possibility of a failure of systems that will affect

riser stresses.

Operational Mode

Four degrees is the maximum ball joint angle that can be

tolerated without damaging the riser and the BOPs during drilling and other operations. The four-degree angle is

based on experience combined with experimental data of

researchers. In practice, this operating angle will

coincide with vessel or rig offsets of three to four

percent of water depth.

2.5 INSPECTION AND MAINTENANCE

Each riser component, especially riser joints, should

receive periodic inspection and maintenance. Visual

inspections shoúld be made each time the riser is run. Resilient seals should be inspected and replaced when

necessary. Each entire joint should be inspected,

particularly at the sealing areas. Damaged joints should

be sent ashore for additional inspection and repair.

Welds should, as is usual, receive particular attention

2.6 STATIC AND DYNMIIC ANALYSIS OF VERTICAL TENSIONED

MARINE RISERS

The approach here follows that adopted by Pateland Lyons

(1990). The reader is also referred to many of the

publications on this subject, including Gardner and Kotch

(1976), Sparks (1979), Patel et al (1984), Mclver and

Lunn (1983), McNaxnara et al (1986) and Wang (1983).

If the riser is simply considered as a beam column then

the governing differential equation used for internal

deflection is:

I

EI

d2x

J I- dy2 T(x) d2x dy2

wdx

dy

=

f

(1)

where EI is the riser stiffness, T is the axial tension in the riser pipe wall, w is the weight per unit length of riser and contents and f is the lateral (viz normal)

force per unit length. The co-ordinate system used is

shown in Figure 2.4 with x measured from the bottom of

the riser and positive upwards while y denotes the

horizontal riser deflection from the vertical through the

riser bore.

In order to take account of the effects of internal and

external pressure in the governing equation for static

deflection, reference will be made to the static forces

acting on a pipe element in Figure 2.5 (from Patel and

Lyons (1990)). For simplicity the analysis is restricted

to two dimensions.

The static forces acting on the element (Figure 2.5) are

as follows)

An axial tension and shear force within the pipe

wall material.

A horizontal force due to the resultant of external

and internal hydrostatic pressures,

called (F +

F.)

xo

A vertical force due to the resultant of external

and internal hydrostatic pressure (Fr0 + F,, )

A drag force due to sjeady current. The velocity

vector is resolved into components normal and

d2 uy

tangential tò the elenent.,

wIth only the normal

component assumed to exert a distributed force of N

per unit length.

5. The zeiqht of the. element ₍ acting vertically.

downwards.

Summing up the components Of forçe in the y direction

for the elemêht in Figure 2.5 yields the

equation:-(T + (IT) sin (O ±dO)

- T sin O - (V + dV) CQS (O + dO) + V cos

O

+(F +F:)-W

NcosOrdO =0

yo y' R

similarly for the x direction we get

(T + dT) cos (O + dO) - T cos

O + (V ± dV) sin (O + dO)

-V sin O

+ (F+ F.)

+ N sin O r dO

= O

ese equations can be simplified (for small dO) and cömbined to

give:-TdO - dV + (F .± F

W) cos O

yi yo R

= (Ç +

F.) sin O

N r do. = 0

(4)

The analysis now continues With

establishing expressions for the terms

(F ,

F ,

F , F ₎

The derivation

for this is given in Patel and

xo xl yo - yl - -- -- -

-Lyons (1990). When expressions

these terms are substituted into

Equation

4,weget

-T ± p A

p. A] dû - dV + {

(cos. O - sin O dO): (Y A - y

A.)

- y

A COS O - N: } rdû 0 (5)

with WR= y A r dO where

is the weight per unit volume

of the pipe

Using the small deflection equation

dV d2

lEI

dy

dy2 I. dy2

and assuming that the angles between pipe elements and the

vertical are

always small as well as

assuming that

all deflections are small (for a

vertical riser), the following expression can be obtained.

EI d2x dy2 dy 2 N

{l+1

J2}_L12 dx

- (T+p A

-pA.).d2x

dy2

-(y A

_{s s}

-y A +yA) dx

_{o o i}

i - =N

₍₇₎ dy

For a flexible

riser

the approximations leading to Equation 7 are not valid (small deflections, small angles with vertical etc) and

the

governing equation is consequently more complex.

This turns out to

be:-(T + p A

p. A.)

d2y { 1

+

_r

si

J2

}'

o o dx dx2 +

(y A

- y.

A.

- y

A )

dV

dx

= 0

(8)

The term (T+ pA;

p. A.) is sometimes called the effective tension (T).

J

The complexity of the governing equation (even for static analysis of a vertically tensioned riser) means that in

practice solution techniques involve either a finite

element or finite difference approach Such approaches are commonplace and a number of purpose-designed computer

programs are available for the static and dynamic

analysis of both tensioned and flexible risers Examples

of such Software will be given during the SchOoL

he reader is referred to the literature for more

detailed information on dynamic analysis and the

influence of other effects on riser dynamic behaviour,

such as vortex shedding The latter effect is

particularly important in influencing the response of

2.7 MODEL TESTS OF TENSIONED RISERS

Model tests of tensioned marine risers have been

carried

out by a number of researchers. The purpose of these

model tests has been to confirm the results of numerIcal simulations and to identify features of dynamic behaviour

that would not appear to be adequately modelled in

numerical simulations A brief account will be given here of model tests carried out by members of the

Heriot-Watt University Marine Riser Group under G C Hartnup.

The majority of these tests were conducted as part of the

SERC/MTD Compliant Systems Programme over the period

1983-1985, although this group had been involved in model

testing riser systems prior to 1983.

One of the riser systems tested during this programme of research was the Buchan riser. This riser had been in

service for some time prior to the time that the model

tests were conducted. Furthermore, a certain amount of full size data was available Tests were carried out at

the BMT

Ltd No 3 Tank at Feithain, Middlesex, which

facility has since been decomissioned.

Tests were carried out with the top of the rIser fixed in position, and with simulated vessel motion. The water

depth simulated was 116m. Tests were car:ied out in

conditions representing both regular and random waves,

both with and without current, current being represented

by carriage motion in the towing tank.

Tests were carried out at a scale of 1/14.5.

Figure 2.6 Shows details of the model Buchan riser. The

central riser was strain-gauged along its length, as was one of the satellite risers. The motions of the riser

were measured by a purpose-designed TV monitoring system, developed by G C Hartnup. One important aspect was the feature of attempting to model as accurately as possible

during the tests the tensioner system. It was found

during, the tests that comparison with predicted bending movements along the central portion of the riser were not

satisfaCtory. It was concluded that this was probably

due to the fact that the tensioner system js not normally

'modelled' in computer simulations.

Typical results obtaixed f rotn the model tests are given

in Figure 2 7 which shows the amplitudes of in-line

bending movements, in-line displacements and transverse displacements for a wave only situation corresponding to

a regular wave of height 442m (full scale) an4 period

i! I

Figure 2.1

24" Riser System with Diverter for Drilling Hole for 20" Casing

-DERR)C( FLOOR BELL NIPPLE FLOW LINE

BAG TYPE PREVENTER

HYDROLIC VALVE OPERATOR

BLEED-OFF LINE

VALVE

HEAVE COMPENSATOR LINZ

INNER BARREL OF TELCOPING JOINT

-SEA LEVEL

OUTER BARREL OF TELESCOPING JOINT

-RISER COUPLING

FLEXI8LE JOINT

-.

GUIOE FRAME-HYDRAULIC LATCH

PERMANENT GUIDE BASE TEMPORARY GUIDE BASE

50 70

From Sheffield (1990)

TENSIOÑ REOUI RED TO

DISENGAGE

BALI. JOINT MEAÑ MAXIMUM TENSION TENSION

F

--J-

-I

O

-

---::- r

110' 130 - 150 170 APPLIED TENSION - KIFS -

-;Figure2.3

Typical ciirve ofstress vs applied tension of risers -nonoperationa.l mode

O MAXIMUM RISERSTRESS 3-=-- MINIMUM RISERSTRESS

BALL IN1STREES

RISER DIA. -16 IN. WATER DEPTH 500 FT WAVE HEIGHT. 20 PT- -

-AVERAGE CURRENT, 0.33 KNOTS MUD WEIGHT. 10 PPG VESSEL OFFSET-10% o 190 .4

'r

y

Figtre

.4

The conventional vertical riser coordinate system notation, with x measured from the bottom of the riser.

From Patel and Lyons (1990)

y

o

X

Figure 2.5

Riser element describing the static behavidur of a marine riser of arbitary geometry

¡ , TENSION WIRES EXPORT RISER SATELLITE RISER

289m

S1R41H 6AUGE PUSIT1OIIS ON

_52mm

1R0T SMUUTE

RISER. (BU CHAN RISE'

only) 5 10 11 13. TS0 :u

Figure 2.6 Test Point Positions on Risers

¿ 2SaJJ 9t 'd

EXPORT RISER TEST PO!ITS..

SPIDER HEIGI{FS LED POSITIONS _{STRAIII 6AtJG.}

(from base of riser) POSITIONS

BOSlrnm 7701mm 7701mm

- - 7670mm

7202mm 6675mm 6576mm

69 mm

6151mm 5525mm 5100mm 5100mm SOC9riim 1457(4mm 14O14Bmm

35mm

3523mm 3L92n'm 2966mm 2199mm 2129mm 216rnrn 1673mm 1023mm 1023mm 873mrr;

(114th spider on HHL RISER o[y

Omm.

\\\\\\\\\\\ \\\\\SEABEO

lOSimm x 10mm 0/0

LR6ER O[t1ETER STUB (on LUCHAN RISER only)

.ot ¿N nzf (lacvj sa. 'a. su lea I.I4T IN PT (MC% W.1 sia 'e. aie tú

-Figure 2.7 Envelopes for 14.5 waves of 7.62 secs

-I5 -Ii -S i Ì ¿N-t.fl f9OJC rOI4T1 (XI? flZT) IS -13 -t. IS 1»4Ddt iCU 14t3 (XX? P!!?)

i.

3 -4 a ¿ I ts iaa4vV5L DI$JiJ2fI4TS -IS -i -4 i ¿ a 12 -17 ai -21 -IS i 12 21 -IS i Í2 24 LLJ4NTS _{IN-4.114 D;Lvr3}

(I1

WC31

-

Nl. JIZZfl /.IY(.D44l

1D4TIII FiAT IAJ5 5SA5Z) sa.

3.. FLEXIBLE RISERS

3.1 Description of Sytems and ComponentS

A nnnber of factors will Influence the design of a

flexible riser system These factors affect the

requirements relating to the integrity of the riser and would consequently influence the detailed design of the 'structure' of the riser and the selection of the various materials involved in the

construction

of the riser.

Predominant amongst the design önsjderations are:

- the ability of the riser tO withstand the range

of loads that it will be subjected to during

its working life (both

internally

and

: externaly applied).

. that the riser cönfiquration i

appropriate to

its service conditions, . which will be

influenced by the location of the floating

surface facility (or 'rig') in relation to the

position of the subsea welihead or 'Christmas

Tree' as well as the water depth and the

environmental wave/current climaté prevailing

at the locätion of the facility.

the ability of the

internal

and external

coatings to resist attack by corrosion and

deterioration This aspect is complex since in

addition to the natural fluids experienced in

oil production, many chemical additives may be

present in practice.

- _{the long-term integrity of the flexible riser.}

In relation to this, aspects that need to be

considered are:

wear (particularly jn relation to unbonded

flexible pipe)

steel corrosion

deterioration and physical parameters (eg.

pressure and temperaturé) of fluids

fatigue characteristics öf steel and 'plastic

materials'

- effective reliable and cost-effective

manufacturing

= certification requirements

- specifiç client specifiçatipn requirements

Types of. Construction

There are generally two types of flexible pipes: the

bonded and the unbonded. The bonded type comprises

layers of fabriç, elastomers and steel "bonded" together using adhesives and chemical agents Elastomeric

materials are used mainly to embed steel components,

reducing steel to. Steel contact. . Vulcanisation is

applied to cure elastomers, complete steel embedrnent, and

ensure final bonding making a "homogeneous structure".

the unbonded type consists of several individual layers which remain separated allowing internal movement between

components. An. intermediate thermoplastic sheath and

lubricating medium may be used to reduce internal

friction and contact between steel layers. The structure remains unbonded and heterogeneous following manufacture.

Flexible pipes have been made principally by föur major

manufacturers, Coflexip of France, Dunlop of the UK,

Pag-o-Flex of Germany, and Furukawa of Japan Nowadays Coflexip have a

dominant share of the market so the

description that follows of the unbonded type of

construction refers exclusively to the types manufactured

by Cóf.lexip..

Coflexip Unbonded. Pipe

There are two basic designs of unbonded flexible risers manufactured by Coflexip; these are 'rough bore' and

'smooth bore' (see Moore (1989)).

Rough Bore Structures

The rough bore construçtion is used whenever there is a danger to the polymeric liner from permeating

gas, for instance, in multiphase oil and gas

production, gas export, gas lift, gas injection and

annulüs control. A typical construction is shown in

Figure. 3.1 and each layer has the following

functions:.

- the inner interlocked steèl carcass (or the

interlock tube), prevents the collapse of the

thermoplastic layer.

- the intermediate thermoplastic sheath, ensures

that the pipe remains leak proof during

service.

a spiralled steel layer of "zeta." wires,

sustains the radial loads generated by the

internal pressure.

- an additional intermediate thermoplastic layer,

serves as. anti-friction layer.

- double crosswound tensilé armours, provides

- an external thermoplastic sheath, protects the

metallic layers of the pipe against external

corrosion or abrasion and binds the underlying

armours.

Smooth Bore Structures

These are used in applications

which do not

involve gas diffusion through the

pressure-containing plastic layer. A typical structure

is shown in Figure 3.2. In essence it is

similar to that of the rough bore

structure

except that the interlocked steel carcass is

omitted.

Dunlop Bonded Pipe

In contrast to the Coflexip pipe already described,

Dunlop pipe is a construction with all layers "bonded"

together by elastomeric material. Figure 3.3

illustrates structural components of the pipe. As can be

seen, the pipe construction comprises the following

elements:

- Stainless steel strip wound interlock tube

- Duralon elastomeriC liner

- Textile reinforced primary carcass - Duratite hydraulic transfer layer - 2 or 4 brass coated reinforced cables

- Textile reinforced secondary carcass - Nylon fabric reinforced cover

The functions of the layers are described in detail

below:

a: Stainless Steel Interlock tube

This liner, often referred to by the manufacture as the

steel carcass, the flex, or the interlock, provides the necessary hoop strength to resist point and distributed

loads, external pressure and radial forces caused by

tension in the reinforcement cables. The interlock tube

also helps prevent rupture of the elastoineric liner in

the event of rapid internal depressurisation.

Furthermore, the interlock acts as a building mandrel

during pipe fabrication.

b. Duralon elastomeric liner

This layer is resistant to sour crude, gas and chemicals

and provides a barrier preventing gases from permeating

STEEP S RISER (Fiqure 3.6)

This has been developed as a safe solution to the problem of over-crowding on the sea-floor for production systems

where the subsea equipment is located directly beneath

the floating facility.

T1e steep S system differs from the lazy S system in that

the flexible riser itself is used to anchor the

subsurface buoy, and the riser terminates at the riser

bore.

This type of riser has been operating in. the Far East and

in Brazil, and is now being adopted for systems in the

North Sea It was the design selected by Sun Oil for its

Balmoral development.

LAZY WAVE RISER (Figure 3.7)

The lazy wave riser is the result of applying the

distributed buoyancy concept to the lazy S configuration

The inidwater arch, dead weight and connecting tie are al].

eliminated. By determining the most appropriate

distribution of buoyancy modules for each individual

case, it is possible to gain precise control over the

loads exerted on the floating facility, and installation procedures are thereby simplified.

This geometry is best applied to production schemes

involving satellite wellS and/pr floating export systems.

STEEP WAVE RISIR (Figure 3.8)

The steep wave is a straightfòrward extension of the

classic 'steep s' configuration There is no midwater

arch, and the associated buoyancy tanks are replaced by smaller buoyancy modules strategically distributed along the flexible riser itself Installation techniques can be simplified because there is no large single element

These features make the steep wave system particularly

Manufacturing Inspection

As with all supplied products, the reliability of the manufactured article is critically dependant on the

quality of inspection that would occur during and

immediately following manufacture considerable efforts have been devoted to inspection during the manufacturing process to ensure that the as-delivered product meets the

c. Intermedi ate, textile reinforced layers

These layers are spirally applied after extrusion of the Duralon liner Their principal functions are to act as hydraulic transfer layers and to bond the pipe wall into

a homogeneous Strudture during vulcanisation.

d1 Reinforcement cable laye

These are the man internal pressure ôontaining elements

within the pipe construction They are spirally applied

to the pipe in paired, balanced layers to ensure

efficient load sharing.

é. Duratite hydraulic transfer layers

The hlburatite'! iS a specially developed rubber compound

applied between the cable layers This compound

encapsulates and protects the cable wires and also acts

as a hydraulic transfer layer assisting in the load

sharing between dable layers.

f. Cover

The elastomeric cover provides prótection to the internal

layers of the pipe It should withstand long term

exposure to sea water and should also resist abrasion and

marine growth.

After the whole pipe has been formed of the above

structural layers, it is heat-and-chemical treated, ie

vulcanised, to consolidate and to bond the constituent

materials It should be noted here, that there is an

important structural element which is not bonded to the

elastomeric layers - the 'interlock tube' Fluid is

deliberately allowed to pass to the Duralon liner in

order to tränsf er pressure to the load bearing

3.2

RISER CONFIGURATIONS

Thêre are a variety of possible configurations for

flexible risers, the selection of which

depends on a

number of factors pertinent to the field location añd the specific application These factors will be discussed in this section Among a wide range of possibilities, the

most frequently encountered configurations can be

classified-under five main types (see Qin (1987)).

FREE-HANGING RISER (Figure 3o4)

This is most suitable for temporary production or for

permanent field development in average weather

conditions As the simplest configuration, it requires a

minimum investment in equipment The flexible riser runs

in a catenary òonfiguration from the upper connection

point on the floater straight down to the sea-bed where it Öan be connected to any type of subsea equipment.

Möst early flexible risers were of the free hanging riser

type This kind of riser has seen service in various

parts of the World, notably in Indonesia, the

Mediterranean and Brazil, in water depths of 300m and

even deeper.

LAZY S RISER (Figure 3.5)

The lazy S riser is well adapted for all types öf field development and is particularly recommended for the most

severe environments. This system s used to best

advantage in production schemes involving satellite

wells, or whenever the subsea equipment is located at

son distance fröm the floater.

The syctem comprises a flexible pipe

running

down from

the upper connection on the floater via a subsurface buoy

and inldwater arch, in a double catenary. The first

catenary, suspended between the floater and the buoy,

absorbs most of the motion induced by current and waves, and the second catenary runs directly from the inidwater

arch to the sea bottoni for connection to the subsea

equipment. The subsurface buoy is tensioned by means of

achain and dead weight.

The lazy S system is usually installed in diverless

operations from the laying vessel. This type of riser is currently operating with success in the Mediterranean and

STEEP S RISER (Figure 3.6)

-This has been developed asa safe solution to the problem of over-crowding on the sea-floor for production systems

where the subsea equipment is located directly beneath

the floating facility.

The steep S System differS fröm the lazy S system in that

the flexible riser itself is used to anchor the

subsurface buoy, and the riser terminates at the riser

bore.

ThiS type of riser has been operating in the Far East and

in Brazil, and is now being adopte4 for Systems in the

North Sea It was thé design selected by Sun Oil for its

Baimöral development.

LAZY WAVE RISER (Figure 3.7)

The lazy wave riser is the result of applying the

distributed buoyancy concept to the lazy S configuration. The midwater arch, dead weight and connecting tie are all

eliminated By determining the most appropriate

distribution of buoyancy modules for each individual

case, it is possible to gain precise control over the

loads exerted on the floating facility, and installation

procedures are thereby simplified.

This geometry is best applied to production schemes

invoiving satellite wells and/or floating export systems.

STEEP WAVE RISER (Figure 3.8)

The steep wave iS

a straightforward extension of the

classic 'steep s' configuration There is no midwater

arch, and the associated buoyancy tanks are replaced by smaller buoyancy modules strategically distributed along the flexible riser itself Installation techniques can

be simplified because there is no large single element

These features make the steep wave system particularly

3.3 DESIGN CONSIDERATIONS FOR FLEXIBLE MARINE RISERS

The design of a. flexible marine riser system will reqiire cònsideration of a number of factors in relation to its

-- functional suitability, and its

long term integrity

In addition, aspects related to its ease of manufacture and the cost of the product will need to be assessed.

The "functional suitability" of the flexible riser system

relates to the need to ensure that the. system in its

entirety (from its interface at the sea-béd connection

an4 its interface near the surface vessel) will retain

structural integrity from internal and external loadings

that it will be subjected tb during operation In

practice,

the motions of the upper end point

(at the

vessel) and the as-laid configuration of the system will

greatly influence the magnitudes of local loads in

tension (or compression), bending and torsion. The

resulting stresses in the plastic and metallic materials

will be influenced by the precise structure of the

f lexible riser as a component, and whether or not the

component is of the 'unbonded' or 'bonded' variety. In

view of the complexities of the component design, the

prediction of the mechanic1 èrformance of a particular

riser component is extremely difficult, and great

reliance will need to be placed on the results of

component tests in which segments of the riser are

súbject tO loadings characteristics of those that would be encountered in actual service conditions.

The nature of the transported fluid will also have a

bearing on the design of the riser system. Although the temperatures of the fluids would not be sufficiently high

as to cause concern with respect to the metallic

materials, the temperature restrictions of the plastic

materials provide a basic design criterion which must be closely adhered to, especially regarding the long-term

plastic age.ing effect at high temperatures.

Dynamic effects will need to be taken into consideration,

both in relation to dynamic local loads resulting from

the qlobal dynamic response of the system, as well, as

locally occurring effects such as the influence of

impulse loads caused by the occurrence. of slugs in the

fluid flow.

The assessment of the 'long-term' integrity of the riser

will need to take account of the fatigue performance of

the metallic reinforcement. In this context, the.

estimation of the magnitude and number of stress dycles

in the steel at critical locations will be a crucial

element of the design process. Other areas of concern

that would need to be addressed are the wear of the steel

(particularly in the case of unbonded types of

construction), and the corrosion of steel. Whereas f or

steels, the assessment of 'long-term' integrity is

relatively straight forward and well understood, this is

not the situation for the plastic materials. This

assessment would involve a comprehensive understanding of the behaviour of the plastic materials. This would

include an assessment of the long term deterioration of the plastic components under the service conditions, and this will be influenced by the loading (both internally

and externally applied), the nature of the transported

fluid and its temperature and pressure, its resistance to

gas permeation, blistering, ageing etc.

OTHER COMPONENTS

Apart from the basic pipe structures there is, of course, a considerable amount of ancillary equipment used in a

flexible riser system. The short term integrity and ong

term integrity assessment of the systems must evidently include attention tö these items as, in many instances, these could turn out to be critical arêas from the design

point of view.

The 'end fittings' provide the important function of

ensuring that the riser loads (in bending, torsion and

tension) are satisfactorily resisted whilst ensuring that a comprehensive sealing system is achieved both radially

and axially. A detailed discussion of the design

requirements for terminations is given by MacFarlane

(1989). Each manufacturer has tackled the problems

associated with fittings in Specific ways. The adequacy

of terminátions must be determined through careful

detailed design, prototype as well as through in-service experience.

In addition tO 'end-fittings', another important

ancillary component in flexible riser systems is the

'bending stiffener'. This is located normally at the

bottom and top connections. The purpose of the

'stiffener' is to provide additional resistance to

overbending of the riser at critical points (such as the

ends of the riser).

As has already been described, buoyancy is normally

required in a riser system either in the form of Subsea arches (subsea buoyancy unit) or discrète buoyancy units to provide distributed buoyancy over certain sections of the system. These buoyancy sub-systems require careful design and the material for their construction needs to be selected appropriately so as to ensure that they have a long-term resistance to water ingress. Most commonly,

syntactic foam is used which is coated with polyurethane

3.4 INSTALLATiON AND INSPECTION

Section 3.2 has described, the rangé Of riser

configurations that can be adapted in practice

Eventually each donfiguratiön requires its Own

installation procedure, although therè are many featùres common to each procedure. By way of illustratiòn we will concentrate on the installation procedure for the riser system in a lazy-S configuration (see Figure 3 9) The

account here follows that given by Johnston (1989).

Johnston describes the procedure as involving the following sequence of operations

Pre-installation survey

Install RSD (Riser Stability Device)

Attach RRC (Riser Release Connector) to riser Establish pull-in wire

Deploy riser

uli-in RRC to FPF (Floating Production Facility) Connect RR

Lay öut upper catenary Deploy gravity base Attach arch

Figure 3.9 (stage 1) shows the operation of pulling in

the RRC (viz connecting the two halves of the RRC)-. This

task is a common requirement for all riser systems

installation. The lower half of the RRC is first

attached to the riser upper flange on the installation

vessel. The pull-in wire is etablished between the

FPF-based pull-in winch and the lower half of the RRC. The

riser upper flange with the RRC attached is overboarded

from the installation vessel using the vessel crane By

paying out on the riser installation reel and taking up

on the FPF-based pull-in winch, the lower RRC is transferred until the riser is hanging vertically from

the FPF

The RRC lower half and the riser are then

pulled up and mated with the RRC upper half, which is

fixed to the FPF. Once the RRC halves are mated, the

hydraulic clamp within the RRC is operated to lock them

together.

Service. Inspection

As with all offshore equipment and systems, the role of

in-service inspection is particularly important in relation to identifying those areas that have failed or

are likely to fail in the near future. Gross failure (as noted by MacFarlane (1989)) should be signalled weil in

advance by defOrmation of the metal components and

inspection should concentrate on this. As a consequence

of this routine inspection should aim to identify major displacement and deformations in the riser rather than

hope to achieve an indication

that a small break or

3.5 DYNAMIC ANALYSIS OF FLEXIBLE NARINE RISERS

In determining the dynamic behaviour, òne usually starts

with the initial configuration of the systems derived

from a static analysis. The dynamic analysis takes into account the time-varying effects of waves, vessel motion,

the inertia of the system and relative velocities, and

yields the time histories of the responses, for example,

histories of tension and bend radii or curvatures

However, due to the compliant nature of the system, the complexity of the nonlinear hydrodynamiC load conditions and motion phenomena, and the nonlinear properties of the

pipe itself, the problem of determining the static and

dynamic behaviour of a flexible riser turns out to be a very difficu,t and còmplicated 9ne..

The reader is here referred to publications by a number

of authors

on the

subject of dynamic analysis. In

particular, reference should be made to Qin (1987) and

Owen and Qin (1986) Patel and Seyed (1989) consider the

problem of identifying the dynamic excitation forces and

resonant motiOns arising from Slug f lOw through the

riser A wide range of computer software exists for

analysis the dynamic behaviour of flexible marine risers

and these can be used by designers of such systems A

fairly comprehensive review of the current status of the development of dynamic analysis programs is given by

3.6 MECHANICAL BEHAVIOUR OF FLEXIBLE MARINE RISERS

This refers to the examination of the response

of the

riser as a whole and its individual structural

components

to various applied loads

The published literature in

this area is much more limited than

that available in the

area

of

global

dynamic

responSe.

Individual

manufacturers

have

presented

siiiplified

methods

for

analyzing component response for their specifiC

products,

The

complexity

of

the

composite

construction

of

a

flexible riser component means that it

is not readily

amenable to analytical treatment.

Computer models of

component mechanical behaviour would need to be supported

by

experimental

results

from

prototype

tests.

Unfortunately,

the amount of

published infOrmation in

this context is sadly very limited.

The reader is here

referred

to

Chen

(1990),

whose

thesis

concerns

the

mechanical behaviour of a bonded flexible

pipe, typical

of that used for offshore applications.

Two general approaches can be identified

for the study of

the mechanical behaviour of flexible pipes of

composite

construction, these are:

Simplified or analytical approach employing existing

cable theory and the theory of the

mechanics of

composite materials, and

Finite element modelling

With regard to the second approach

(b),

there are

a

number of difficulties to be addressed

For example, the

effect of

large deformations,

incompressibility of the

elastomer layers, and the material heterogenity as

well

as non-linerarity,

all present problems

for utilizing

general purpose finite element packages.

It would be difficult, if not

mpossib1e, to establish a

general model capable of dealing with all types of pipes

Special

treatment

is. required

to simulate

particular

features of different

pipes, such as the 'smooth bore' of

"Coflexip" pipe, the interlock tube of the "Dunlop" pipe

and the corrugated steel liner of "Pag-o-Flex" pipe.

The

key to success

is to

obtain a comprehensive understanding

of the mechanisms involved in progressive deformation and

ultimate failure, the ways in which the several metallic

and non-metallic layers interact and a sound knowledge of

the material behaviour for each component involved in the

3.7 EXPERIMENTAL. STUDIES

Experimental work provides valuable information f Or

validating computer simulatjons and assists the design of

prototype systems. Suôh validation of computer

Simulations is required both for global dynamic response

as well as in simulations developed to determine the

mechanical response characteristics of the riser. In

bth these situations there are a number of difficulties

in developing a reliable analytical model and numerOus

assumptions have invariably to be made.

Ïn the case of validating the computer simulations for

global dynamic response of flexible riser systems, one

can use both full-scale offshore field tests and model

tests. Full scale tests f flexible risers are of great

interest in the determination of their loadings and

motions under realistic Sea environments. Such an

investigation was conducted by O'Brien et al (1984). The

purpose of this particular project was to verify the

behaviour, of a flexible pipe from seabed to a Surface

buoy in a water depth of 106m under severe weather

conditions typical of the North Sea area. The system

consisted of a 135m long 10 inch inner diameter 4500 psi

flexible pipe, filled with water under pressure and

suspended between a surfaçe buoy and a subsea floater

submerged in a catenary form. The data obtained from

this investigation enabled the effective lifetime of a

flexible pipe to be estimated. Data was also available to correlate with results frOm computer programmes that

predicted the overall system response. However, full

scale tests are very limited, largely because of the

costs involved in carrying them out. Also, full scale

tests have no control over the environmental conditions

and the other test conditions.

Model tests, on the other hand, can be an effective way

of investigating the dynamic responses of flexible

risers, thereby validating various mathematical models.

One of the major difficulties in model testing: flexible

riser Systems

is _the problem of scaling. Ideally,

hydroelastic modelling, with proper scaling of elastic,

dynamic and hydrodynainic factors, is required. Such

scaling is, of course, never achievable in practice and

one has to concentrate on providing correct. dimensional

similitude for those non-dimensionless groups that have

the greatest bearing' in influencing system dynamic

response.

Qin (1987) provides extensive details of model tests that

were carried out at Heriot'-Watt University, Edinburgh,

United Kingdom, and the techniques involved in these

tests and some of the more important test results will be

As already mentioned, there are considerable difficulties

involved in predicting the mechanical behaviour of a

flexible pipe. This problem is particularly acute in relation to the determination of the bending stiffness of

a pipe. Furthermore, in view of the need to establish

service lives for flexible pipes

and the problem of

estimating these theoretically for such a complex

component as a flexible pipe,

one has to make use of

prototype tests of specimens that are subjected to

realistic loading conditions over a large number of load

cycles.

A good

review of the methods involved in

component prototype testing is given by Cocks (1989), for the Dunlop bonded type of construction of flexible pipe. Each manufacturer, of course, needs to develop their own

specific types of tests to suit the needs of their

particular product. In general such prototype tests

would include the

following:-- hydrostatic burst tests - tensile tests

- hydrostatic collapse tests - bending stiffness tests

- in-plane and rotational fatigue tests

- multi-load dynamic tests

3.8 FUTURE PROSPECTS

The preceeding sectIons have aIady illustrated the many challenges that still remain in relation to the future application and utilization of flexible riser systems by the offshore oil and gas industry The concept of the flexible pipe has now 'come of age', despite theear1y

reluctance of some operators to adopt such a product

The advantages of. using flexible risers very definitely. outweigh its disadvantages The generally high cost of producing flexible pipe will clearly reduce as improved methods of manufacture are introduced There is still a need to obtain more information from prototype testing of

specimen lengths Unfortunately, much of this

information is not available to the purchaser and the

designer needs to lean heavily on the assurances given by

the product manufacturer on matters such as long term

reliability Operators are now requiring much longer

'life times' for the products supplied to them, and the attention on product safety js intensifying. There are

still considerable discrepancies between the predictions

of mechanical performance and those observed during

prötotype testiñg. In reality, there are probably

problems with both approaches. Further work is needed to

brïdge the gap.

Ïn relation to the area of materials, there is a

continuing need to research the suitability of individual materials for particular applications As has been seen,

each application has to be addressed on its own

particular merits and materials have to be selected

appropriately. One of the challenges today is to provide

hïgh temperature plastics at reasonable cost. Future

development of materials will probably be driven by

deepwater developments. With depths up to 100m being considered for future offshore developments, riser weight will be one of the crucial factors in future designs.

, TYPICAL" ROUGH BORE" DYNAMIC

AIsÈks-riifcrcí-EXTERNAL ThERMOPLASTiC

DOUBLE CROSSWOUNDTENSILE ARMORS -. '-.

t

ZETA SPIRAL₄ INMEDIAT ., LThERMCPLAST1C j . CAR

PRODUCTION: CRUQEJGAS INJECTION: GAS EXPORTGAS... TEST,KJU. ...

HTT.

Figure 3.1

Rough bore Coflexip pipe construction

Figure' .2

A Stainless steel strip wound interlock tube

B Duralon elastomeric liner

C Textile anti-extrusion layer

D puratite hydrailic transfer layer

High tensile steel strand retnforcement

F Textile anti chaffing layer

G Nylon fabric ±eihfdrced cover

Figure 33

Dunlop bond. pipe const-ruö:iOfl

Figure 3.4

Figure 3.

Free-hanging flexible riser

Lazy-S flexible riser

configuration

configuration

Figure 3.6

Figure 3.7

Lazy wave flexible riser

conf:igurat ion

Figure 3.8

Steep wave flexible riser

MSL

F.PF

..\\,/i\\V

3. Overboard riser lower end flange

Installation

vessel

MSL

Seabed

2. Deploy mid - water arch

FPF Winch wire Mooring line Gravity base ,RSD MSL 4. System as installed RRC Mid-woter arch F1eibTe Riser lower riser end flange - RSD Seabed \SV AV /'\ Figure 3.9

Installation sequence for lazy-S flexible riser system

(from Johnston (1989))

FPF

Installation

vessel

Pull -in

winch. _{PiilI -in Installation}

wire vessel MSL FPF

/

SV

/L

RRC/ upper _RRC half lower half RSO Seabed I. Pull-in RRC

REFERENCES

:-'

Z: ''

Chen (1990) Chen z : "The Mechanical Behaviour and

Fatigue Analysis of Flexible Pipes", Doctoral Thesis,

Heriot-Watt University, June 1990.

Cocks (1989) Cocks P J : "Testing and structural

integrity of flexible pipes", Engineering Structures, Vol

11, No 4, October 1989.

Gardner and Kotch (1976) Gardner T W and Kotch M A :

"Dynamic Analysis of riser and caissons by the finite

element method", OTC 1976, Paper No OTC 2651 (1976).

Johnston (1989)

Johnston D R A

: "Installation

aspects of flexible riser systems," Engineering

Structures, vol 11, No 4, October 1989.

Kodaissi et al (1990) Kodaissi E, Le Marchiand E and

Narzul p : "State of the art on the dynamical programs

for the flexible risers systems", 9th International

Offshore Mechanics and Arctic Engineering Symposium,

OMAE, 1990.

MacFarlane (1989) MacFarlane C : "Flexible riser

pipes: problems and unknowns", Engineering Structures,

Vol 11, No 4 October 1989.

Mclver and Lunn (1983) Mclver D B and Lunn T S

"Improvements to frequency domain riser programs", OTC

1983, Paper No OTC 4550, Houston, May 1983.

McNamarra et al (1986) McNamarra J F, O'Brien P J and

Giiroy S G : "Non-linear analysis of flexible risers

using hybrid finite elements", Proceedings of OMAE 1986,

Tokyo, Japan 1986.

Moore (1989) Moore F : "Materials for flexible riser

systems: problems and solutions", Engineering Structures,

vol 11, No 4, October 1989.

O'Brien et al (1984) O'Brien E G, Joubert P,

Kristoffersen B R, Delecolle D and Quin R : "Sea test of

large high pressure flexible pipe", Offshore Technology Conference, OTC 4379, Houston, Texas, May 1984.

Owen and Qin (1986) Owen D G and Qin K : "Model

tests arid analysis of flexible riser systems",

Prdceedins of the 1986 Offshore

Mechanics and Arctic Engineering Symposium, OMAE, Tokyo, Japan 1986.

Palmer (1985) Palmer A C : "Flexible Risers", The

Way Forward for Floating Production Systems, IBC

.P::a11d Ly.ç.n. (1990) Patel .14 H aith Lyons. G J. :

"Marine risers and Pipelines", Chapter 5 in "Marine

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