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DESIGN CONSIDERATIONS FOR SELECTION OF FLEXIBLE RISER CONFIGURATION

N. Ismail, R. Nielsen, and M. Kanarellis

Wellstream Corporation Panama City, Florida

ABSTRACT

A brief review of recent literature on riser

design is presented and a concise description of

the design process is given. Effects of

hydrodynamical design parameters on marine

flexible riser design are then reviewed. Riser

dynamic analyses are described which

substantiate design criteria for the selection of

riser configuration in deep and shallow water.

The results of dynamic analyses using

computerized numerical models are presented in

this paper in graphical form for the selected riser

design cases. Output includes envelopes of riser

coordinates, axial force and time histories of

forces and wave surface profiles. The results

obtained highlight the significance of motion

spatial gradients for the combined flow of waves,

currents and vessel heave motion on the selection

optimum of riser configuration to address design

requirements.

NOMENCLATURE

A = flexible pipe cross-sectional area

H = water pressure head (h-s)

L = wave length

T = wave period

a = wave amplitude

g = acceleration of gravity

h = water depth

K = wave numberL

2π

Nomenclature (continued)

s = elevation above sea bottom

p = fluid pressure

Greek Symbols:

ρ = fluid density

σ = wave frequency L

2π

Subscript:

i = pipe internal properties

o = pipe external properties

INTRODUCTION

Though flexible pipe as a marine product

was introduced to the offshore market in the early

seventies, it was not until 1978 that flexible risers

were specified and installed in the Enchova field

offshore Brazil (Machado, 1980) as part of a

floating production system.

Since 1980, the use of flexible pipe has

spread worldwide and is used in almost every

offshore oil development today as witnessed in

papers by Mahoney (1986) for North Sea

application, Tillinghast (1990) for Gulf of

Mexico, Gulf of Suez applications, Tillinghast

(1987) and Beynet (1982) for the Far East.

This type of dynamic application is

typically used for floating production systems for

high pressure production risers, export risers,

chemical/water/injection lines and gas lift lines.

Currently, the main manufacturers of flexible

pipes are Coflexip, Wellstream and Furukawa.

References and Illustrations at end of paper

PD-Vol. 42, Offshore and Arctic Operations - 1992 ASME 1992

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At the present time, much interest in riser

systems is shown by the operators as evidenced

by papers by Beynet (1982) and Ashcombe

(1990).

RISER CONFIGURATION SELECTION

Industry practice calls for several types of

riser configurations typically used in conjunction

with Floating Production/Loading Systems. The

standard five configurations generally used are:

Free-Hanging Catenary, Lazy-S, Lazy Wave,

Steep-S, and Steep Wave. Figure 1-a illustrates

these typical types of riser configurations. Figure

1-b illustrates a schematic of a new riser

configuration proposed by Wellstream for the

Alcorn Linapacan Field Development Project.

The motivation for and validity of this new riser

configuration is presented in this paper.

The dynamic response of a particular riser

system is directly related to the environmental

loadings due to the combined wave-current field

flow and the dynamic boundary conditions of the

riser top end at the water surface, coupled with

the interaction arising from the structural non-

linear behavior of the riser itself.

To illustrate, design parameters impacting

the suitability of a particular configuration for a

particular water depth, two riser design cases

were studied in deep and shallow water. The

computer analyses were carried out at

Wellstream’s engineering offices using computer

program FLEXRISER-4 developed by Zentech,

UK.

The results of the dynamic analyses for

these several design cases are the essence of this

paper. The output illustrates the critical aspects of

wave and current hydrodynamics as well as

vessel motion response affecting the selection of

the riser configuration.

FLEXIBLE RISER ANALYSIS AND DESIGN

Flexible pipes and risers are critical

components for offshore field developments

because they provide the means of transferring

fluids, or power, between subsea units and a

topside floating platform, or buoys. These risers

accommodate floating platform motion and

hydrodynamic loading by being flexible. In storm

conditions, they undergo large dynamic

deflections and must remain in tension

throughout their response. They are consequently

manufactured to possess high structural axial

stiffness and relatively low structural bending

stiffness. Their global dynamic behavior can be

considered as more mechanical, or force

dependent, than structural. In contrast, behavior

near the end connectors of a system is governed

by local structural stiffness properties.

DESIGN CRITERIA

Efficient design of flexible riser systems is

made possible by using computer-based solution

techniques.

The design criteria of flexible riser systems

is usually based on allowable pipe curvatures and

tensions prescribed by the pipe manufacturer,

clearances between the riser and other structures,

and boundaries during its dynamic response. The

allowable curvatures and tensions are based on

full-scale test procedures and stress analysis

carried out by the manufacturer. These limits

ensure the pipe is not over-stressed when

responding to dynamic loads and vessel motions.

The system is generally designed so the pipe is

tensioned throughout its dynamic response cycle.

Minimum clearances are also specified to avoid

clashing problems between riser and seabed, or

riser and vessel, and between the riser or other

adjacent risers, cables, or mooring systems.

DESIGN PARAMETERS AND PROCEDURES

The main problem in designing flexible

riser systems is the large number of design

parameters. The environmental conditions, vessel

or calm buoy motions and riser properties are

usually well-defined. The main design parameters

are the choice of riser configuration, the length of

riser, the system geometry and the sizing of

buoyancy modules, subsurface buoy or arch. The

choice of riser configuration is usually based on

economic criteria, position of the wells, wave and

current forces, motion response and excursions of

the vessel or surface buoy as well. The design

procedure can be described as consisting of three

stages.

First Stage

The first stage in designing a flexible riser

system is determining an acceptable system

layout. The first stage is based on static analysis.

It is normal to carry out a parametric study

assessing the effect of changing the design

parameters (i.e., system geometry and length) on

the static curvature and tension. Based on the

results of this parametric study, the design selects

a suitable range of system geometries and lengths

satisfying the design criteria. The parametric

study will also assess the static effects of vessel

offset (displacement of the top end) and the

current loading in different directions.

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Second Stage

The second stage in the design procedure is

performing a dynamic analysis of the system to

assess the global dynamic response. A system

layout and length is chosen from stage one and a

series of dynamic load cases are considered.

These load cases combine different wave and

current conditions, vessel or surface buoy

positions, and riser contents in order to prove an

overall assessment of the riser suitability in

operational and survival conditions. The corre-

sponding analyses are then carried out and

dynamic curvatures, tensions and clearances are

checked against the design limits.

The majority of riser dynamic analyses

packages, including FLEXRISER-4, make use of

the “concept of effective tension” (Sparks, 1983).

Sparks addressed the drilling riser case where the

riser is essentially restrained. A catenary riser on

the other hand turns 900 to meet the sea bed. It is

subject to friction and can be subject to

compression due to these conditions. This

concept accounts for the effects of external and

internal hydrostatic pressure acting on the

internal and external surfaces of the pipe wall. It

is the effective tension which controls the

stability of the riser from the point of view of

deflection. The relationship between effective

tension, Teff, and the “true wall” tension, Twall,

that acts on the pipe wall and contributes to stress

in the pipe wall is:

Twall = Teff + (pi ± ρjg Hi) Ai – (ρ0g HO)AO………….(1)

where:

Twall =Wall tension to be used for stress

calculation in flexible pipe wall.

Teff = Effective tension as predicted by the riser

analysis computer program.

The effective tension is independent from

internal and external pressure. Given the effective

tension, as predicted by the riser global analysis

program, the true wall tension may be simply

calculated from the equation (1). Since internal

pressure affects the Twall, it is important to

carefully note the internal pressure conditions in

the pipe under the maximum load cases as well as

the limiting operational conditions when pressure

in the riser may be released or maintained.

Third Stage

The third stage in the design procedure is

performing detailed static and dynamic analyses

of local areas to design particular components.

This state is presented in a separate publication

(Brown, 1989).

Key papers by operators in this regard are

Out (1989), Boef (1990) of SIPM and de Oliveira

(1985) of Conoco. This third stage of design also

includes a question of life expectancy which has

recently been addressed by Claydon, et al (1991).

All of the stated design aspects are important but

the solution to each problem starts with the

selection of an optimum configuration which is

the subject of the remainder of the paper.

HYDRODYNAMIC LOADING ON RISERS

The determination of surface wave

hydrodynamic loads on the marine systems are

based on two major techniques. For large

structures, the scattering of incident waves is

considered and a diffraction theory is employed.

The wave loads on the small members of flexible

pipes are determined by applying the Morrison

equation. This equation separates the

hydrodynamic loads into inertia and drag forces.

Considering the pseudo static design approach

(deterministic), it has been customary to use

irrotational wave models to predict fluid particle

kinematics for a certain design wave. Drag and

inertia force coefficients are then used to relate

the particle kinematics to the hydrodynamic

forces expressed by the Morrison equation.

Further, it is important to determine appropriate

drag and mass coefficient as well as to adopt the

appropriate wave theory representing the design

wave characteristics.

To include currents in wave force

calculations, the oil industry has traditionally

used the technique of linear super-position.

When doing numerical analysis of the

hydrodynamic loading and selection of flexible

risers configuration, one has to recall the fact the

global riser response in a particular water depth is

affected by the spatial and temporal distributions

of the integral properties of waves (mass,

momentum, pressure and energy); therefore,

these distributions of water-wave properties

significantly dictate the suitability of a particular

riser configuration.

To illustrate some of these water wave

properties, consider a progressive wave moving

in the positive horizontal axis (figure 2). For

simplicity, considering the small amplitude wave

theory, the wave kinetic energy concentration

averaged over one wave period at any elevation

above the bottom is given by:

2kscosh 2kh)(cosh 24

2k2g2a KE(s)

σ

ρ= …………...(2)

In order to represent the above relation in

dimensionless form, the ratio of the kinetic

energy at any elevation ‘s’ to the mean free

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surface is given by

2khcosh

2kscosh

KE(h)

KE(s)= …………………………..(3)

Equation (3) is plotted in figure 3 for the

cases of h/L= 0.05 (conventional shallow water

limit) and h/L = 0.5 (conventional deep water

limit). The figure shows in the case of shallow

water waves, the energy concentration is nearly

uniform with depth; in the case of deep water

waves, the energy is concentrated near the

surface.

It should be noted that effects due to wave-

current interaction should always be considered

in the design and analysis because it might cause

a significant change in the magnitude and

distribution of fluid forces. This change of fluid

forces could be dramatic in coastal waters, as

seen by Ismail (1984), not only due to the strong

current shear, but also due to the change in

characteristics of shoaling waves. The parameters

usually sensitive to wave-current interaction

effects include hydrodynamic force coefficients,

current velocity profiles, and relative direction of

waves and currents.

PRESENTATION AND INTERPRETATION

OF RESULTS

In order to illustrate the potential and the

limitations, of adopting specific riser

configurations in a particular water depth, the

results of riser dynamics analyses for two design

cases in deep water and one design case in

shallow water are presented. The computer

analyses were conducted at Wellstream’s

engineering office using the FLEXRISER-4

personal computer program developed by

Zentech, UK.

FREE-HANGING CATENARY RISERS

A Free-Hanging catenary configuration was

considered for riser systems in two design cases.

The first case is in a 600 m water depth and the

second in a 350 m water depth. Table I illustrates

the main design parameters used as input for the

computer analyses. For the first case, figure 4

shows a snapshot of the riser configuration and

the distribution of axial force along the length of

the riser depicting a small amount of compression

near the seabed. In contrast to this case (for the

350 m water depth), figure 5 shows an

appreciable amount of compressive force in the

riser near the seabed. To maintain the integrity of

a flexible riser system, it is imperative that the

riser pipe remains in tension throughout its

operational life. Because of the development of

this excessive amount of compressive force in the

pipe, it can be safely concluded a Free-Hanging

catenary configuration is unsuitable for this

design case in 350 m water depth. A major reason

of the development of axial compression forces is

that the heave motion associated with the calm

buoy is appreciable. To mitigate the effects of

this loss of tension in the riser pipe, which could

adversely affect the service life of the pipe, one

of the following measures may be adopted:

• Disconnection of the riser under strong

wave conditions;

• Increase the weight of the riser pipe section

near the seabed by wrapping with heavy

material;

• Modify the buoy design to reduce the heave

motion;

• Attach a subsurface buoy, near the seabed.

EASY-TOUCH CATENARY RISER

Each of the above remedies to eliminate

compression forces could be viewed as a possible

solution depending on the technical and

economical constraints of the project in general

and the riser system in particular.

The latter concept of adding a subsurface

buoy (= 60 kN) to the free hanging catenary riser

in the 350 m water depth design case to eliminate

compression forces was implemented. The results

of dynamic analyses demonstrating the success of

the concept are shown in figure 6. It is important

to highlight this, if the position of the surface

buoy is to increase above the seabed, the riser

will be approaching the Steep-S riser

configuration, compression forces would still

develop over the lower catenary portion.

Wellstream had applied this modified concept for

the catenary type configuration in the riser design

for Alcorn’s West Linapacan Development

Project in the South China Sea.

The Easy-Touch Catenary was coined as the

name for this modified catenary configuration.

The validity of the newly proposed riser

configuration was tested under various operating

scenarios for the calm buoy and under normal

and maximum environmental conditions.

STEEP WAVE RISERS

Dynamic riser analyses were conducted for

a design of a single oil export system in a 40 m

water depth site. An extensive analysis was

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carried out prior to arriving at the proposed steep

riser configuration (either 6-inch or 8-inch

flexible pipe diameter) resulting in many unac-

ceptable configurations, primarily due to the

riser’s inability to withstand the severe envi-

ronmental conditions imposed by extreme wave

heights of 15 m for the I year-storm and 20 m for

the 100 year-storm. The analyses showed neither

Lazy-S nor Free-Hanging systems were feasible.

Moreover, the need to eliminate the Steep S

configuration, as an alternative, became apparent

when the sensitivity of the riser’s dynamic re-

sponse to the intensity of additional buoyancy

distribution was determined. Thus, the Steep

Wave was left as the only feasible configuration

for the riser of this oil export system. The

distribution of buoyancy along the Steep Wave

riser was determined (figure 7) to prevent any

buckling instability and to reduce any excessive

angle at the riser base. A snapshot of the riser

configuration under the dynamic effects of waves

and currents is shown in figure 7 for both cases of

far and near field. The corresponding axial

tension force along the Steep Wave riser is shown

on figure 7.

CONCLUSIONS

It was evident the most apparent riser

configuration does not necessarily provide the

appropriate solution for the design cases of

flexible marine risers in deep and shallow water

considered in this paper. The deep-water cases

emphasize a solution based on a simple riser

configuration to facilitate modularity and ease of

installation and removal either the standard

Catenary in the 600 m water depth case or, the

Easy-Touch Catenary in the 350 m water depth

case. In the shallow-water case, the design is a

more complex riser configuration due to the

severe environment loads requiring particular

design configuration loads. Both cases represent

new frontiers for use of flexible pipes as marine

nsers.

The sensitivity of the riser dynamic

response, in particular configuration to

environmental data and vessel/floater motion

data, warrants a careful review of design basis

prior to the dynamic analysis and design of

marine risers. The selected configuration then

determines many design parameters, among

others, design life and bending stiffener and

restrictor requirements.

REFERENCES

American Petroleum Institute, 1987, “RPI7B -

Recommended Practice for Flexible Pipe,”

Houston, TX.

Ashcombe, G.T., and Kenison, R. C., BP

Engineering, 1990, “The Problems Associated

with NDT of High Pressure Flexible Pipes,”

Society of Underwater Technology Conference,

Aberdeen.

Beynet, P. A., and Frase, J. R., 1982,

“Flexible Riser for a Floating Storage and

Offloading System,” Proceedings of Offshore

Technology Conference, Paper OTC 4321,

Houston, TX.

Boef, W. I. C., and Out, J. M. M., 1990,

“Analysis of a Flexible Riser Top Connection

with Bend Restrictor,” Proceedings Offshore

Technology Conference, Paper OTC 6436,

Houston, TX.

Brooks, D. A., Kenison, R. C., and BP

International Limited, 1989, “Research &

Development in Riser Systems,” Subsea, London.

Brown, P. A., Soltanahmadi, A. and

Chandwani, R., 1989, “Problems Encountered in

Detailed Design of Flexible Riser Systems,” Int.

Seminar on Flexible Risers, University College,

London.

Claydon, P., Cook, G., Brown, P. A.,

Chandwani, R., Zentech International Ltd.

London, 1991, “A Theoretical Approach to

Prediction of Service Life of Unbonded Flexible

Pipes under Dynamic Loading Conditions,” J.

Marine Structures, preprint.

Hoffman, D., Ismail, N., Nielsen, R., and

Chandwani, R., 1991, “Design of Flexible Marine

Riser in Deep and Shallow Water,” Proceedings

Offshore Technology Conference, Paper OTC

6724, Houston, TX.

Ismail, N. M.,1984, “Wave-Current Models

for Design of Marine Structures,” Journal of the

Waterway, Port, Coastal and Ocean Division,

ASCE, Vol 110, No.4.

Kastelein, H. J., Out, J. M. M., and Birch,

A.D., 1987, “Shell’s Research Efforts in the Field

of High-Pressure Flexible Pipe,” Deepwater

Offshore Technology Conference, Monaco.

Kodaissi, E., Lemerchand, E., and Narzul,

P.,1990, “State of the Art on Dynamic Programs

for Flexible Riser Systems,” ASME Ninth

International Conference on Offshore Mechanics

and Artic Engineering, Houston, TX.

Lopes, A. P., de silva Neto, S. F., Estefgn, S.

F. and Da Silveria, M. P. R.,1990, “Dynamic

Behavior of a Flexible Line Design Installation,”

Proceedings Offshore Technology Conference,

Paper No. OTC 6437, Houston, TX.

6

Machado, Z. L., and Dumay, J. M.,1980,

“Dynamic Production Riser on Enchova Field

Offshore Brazil,” Offshore Brazil

Conference, Latin America Oil Show, Rio de

Janeiro.

Mahoney, T. R., and Bouvard, M. J.,

1986,“Flexible Production Riser System for

Floating

Product Application in the North Sea,”


Paper

OTC 8163, Houston, TX.

Oliveira, J. G., de Goto, Y., and Okamoto, T.,

1985, “Theoretical and Methodological

Approaches to Flexible Pipe Design and

Application,” Proceedings Offshore Technology

Conference, Paper OTC 5021, Houston, TX.

Out, J. M. M., 1989, “On the Prediction of the

Endurance Strength of Flexible Pipe,”


Paper OTC 6165, Houston, TX.

Sparks, C. P.,1983, “The Influence of

Tension, Pressure and Weight on Pipe and Riser

Deformation and Stresses,” 2nd International

Offshore Mech and Artic Engineering

Symposium,Houston, TX.

Tillinghast, W. S.,1990, “The Deepwater

Pipeline System on the Jolliet Project,”



Tillinghast, W. S. and Shah, B. C., 1987

“Laying Flexible Pipelines Over Coral Reefs in

the Geisum Field, Gulf of Suez, Egypt,”



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Design data Deep water Shallow water steep wave

configuration Case A

Free-Hanging

Catenary

Case B

Easy-Touch

Catenary

Flexible Riser Pipe Data:

Internal diameter, M

Outside diameter, M

Axial Stiffness, N

Bending Stiffness, Nm2

Wt in Air, kg/m

0.1016

0.1631

5.45E7

2.00E3

44.0

0.13

0.2

1.28E8

4.E3

62.0

0.15

0.23

2.00E7

5.24E3

63.0

Environmental data:

Water depth, m

Wave height, m

Wave periods, s

Current speed, m/sec

Top, Bottom

625.0

11.8

11.2

1.1,0.6

350.0

17.0

12.0

1.9,0.7

40.0

15.0

13.0

2.3,1.07

Vessel/Floater Excitation:

Surge, m

Heave, m

1.8

2.0

2.2

5.5, -9.5

2.5

2.2

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