SPE-175485-MS Use of Stabilisation Chains to Optimise Lazy ... · This paper was prepared for presentation at the SPE Offshore Europe Conference & Exhibition held in Aberdeen, United

SPE-175485-MS

Use of Stabilisation Chains to Optimise Lazy Wave Flexible Risers in Harsh Environments Gurung, Amul; Viana, Pedro; O'Brien, Aoife; Rimmer, Alex; 2H Offshore Engineering

Copyright 2015, Society of Petroleum Engineers This paper was prepared for presentation at the SPE Offshore Europe Conference & Exhibition held in Aberdeen, United Kingdom, 8–11 September 2015. This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

Abstract The paper addresses the use of steel chains in a lazy wave configured flexible riser system to provide an alternative flexible

riser configuration for use in challenging environments including large vessel offsets and motions, and large ranges of riser

internal fluid properties. While the compliant nature of flexible pipe provides excellent fatigue and strength resistance,

flexible risers typically experience larger deflections when compared with rigid risers, which results in greater challenges

managing interference issues with adjacent structures. Different lengths and variable masses of chain are installed at locations

along the hog bend of the flexible riser configuration. The arrangement of the chain masses, length and positioning along the

line are developed to primarily prevent contact with the seabed and the hull of the FPSO when a range of heavy and light

internal fluids are considered. A number of weighted steel chain configurations are evaluated and presented through an

analytical case study in order to demonstrate the benefits of this approach for a typical generic shallow water application

FPSO system. Installation and hardware design aspects are additional requirements that may need to be addressed in further

assessments.

Through the in-place case study, comparisons are made between the performance of the flexible riser system with and

without the weighted steel chains. Global finite element models are developed to simulate the performance of the different

flexible riser configurations when subject to a range of loading scenarios covering large FPSO offsets, harsh environmental

conditions and a range of riser internal fluid densities. Performance criteria of the flexible riser such as tensile loading,

curvature and motion envelopes are presented to show the improvements derived though optimisation of the chains. It is also

demonstrated that the chain section that extends along the seabed helps to reduce the transverse displacement and “lateral

walking” thus reducing the risk of clashing with adjacent structures and changes in line lay azimuth under strong transverse

current loading. The cost effectiveness of the chain weighted flexible is also compared to other solutions considering new and

retro-fit applications. This work demonstrates that an improved and cost effective solution is developed to provide an

acceptable flexible riser dynamic response for the range of operational fluid densities that may be experienced in its

operational lifetime.

2 SPE-175485-MS

Introduction The simplest form of a flexible riser is a free hanging catenary configuration where the riser is hanging freely from the FPSO

vessel to the riser base with no ancillary equipment along the suspended section. The typical types of configuration for a

dynamic flexible riser are given in Figure 1. Wave configuration risers consider regularly distributed buoyancy modules

along a section of the riser, typically in order to improve the extreme and fatigue response of the riser system. This

configuration has enabled flexible pipes to operate at extreme environmental conditions and large vessel motions, by

reducing potential for over-bending and compression in the riser at the touchdown zone, while also limiting riser top loads.

Although a range of wave configurations are largely employed and the particular benefits of lazy wave configuration risers

are understood throughout the industry, the application of steel stabilisation chains in conjunction with the lazy wave

configuration has not been explored extensively. The weight aided wave configuration, initially developed by Wellstream

(GE Oil and Gas) [1][2], and illustrated in Figure 2, is considered as the primary case study for the following optimisation

assessment. The weight aided wave configuration includes attachment of constant linear weight chains to the hog section of a

lazy wave riser configuration. The weighted chains suspended from the buoyancy modules towards the seabed. The objective

of this paper is to perform an analytical assessment to identify the additional benefits of variable chain sections and

arrangement for a flexible lazy wave configuration riser system in harsh environments.

SPE-175485-MS 3

Figure 1 – Typical Flexible Riser Configurations

Simple Catenary Lazy Wave

Steep Wave Pliant Wave

(Tethered Lazy Wave)

Lazy S

(Mid-Water Arch) Chinese Lantern Configuration

Buoyancy Modules

Arch

Tether

Buoyancy

Modules

FPSO FPSO

FPSO FPSO

FPSO FPSO

4 SPE-175485-MS

Figure 2 – Weight Aided Wave Riser Configuration, [1][2]

Stabilisation Chains Application Weighted stabilisation chains offer a unique opportunity to further exploit the benefits provided by a lazy wave configuration

riser system. The stabilisation chains are attached to the hog bend of the lazy wave riser configuration. The stabilisation

chains are fixed with the usage of steel clamps that are attached to the flexible pipe in between buoyancy modules. A number

of different chain sections, each with a constant linear weight throughout its length, are connected in series and will form an

interlinked system with an optional anchor clump weight at the bottom. The optimised length and weight arrangement of the

stabilisation chains is driven by the expected variation in internal fluid densities. For reference, the effect internal fluid

density variation has on a lazy wave configuration without either stabilisation chains or a tether is illustrated in Figure 3. A

heavier chain section is thus used towards the seabed to prevent the riser becoming sufficiently buoyant such that the

probability of clashing between the riser and hull of the FPSO is reduced. The optional anchor for the stabilisation chains

may be installed on the seabed perpendicularly to the riser lay azimuth. A lighter chain is installed towards the riser

connection clamp to reduce the likelihood of clashing between the seabed and the riser. A graphical illustration of the

stabilisation chains application in a lazy wave configuration riser system is given in Figure 4. A generic arrangement of the

stabilisation chains is presented schematically in Figure 5 and through analysis model view in Figure 6.

Weight Aided Wave

Steel Chains with

Top Aramid Rope

FPSO

SPE-175485-MS 5

Figure 3 – Riser Shape Variation due to Internal Fluid

Figure 4 – Schematic Illustration of Stabilisation Chains in a Lazy Wave Riser Configuration

Touch Down

Point Seabed

Buoyancy

Modules

Riser Base

Stabilisation

Chains

FPSO Mean Sea Level

6 SPE-175485-MS

Figure 5 – Schematic View of Stabilisation Chains Connection and Different Sections – Side View

Figure 6 – Stabilisation Chains Generic Illustration – Analysis Model View

Optional Anchor

Clump Weight

Chain

Increase

in Linear

Weight

Seabed

Buoyancy Modules

Chain

Clamp

SPE-175485-MS 7

Methodology Review Finite element models are created in Orcaflex 9.6b [3] to assess the lazy wave riser configurations considering stabilisation

chains. Generic flexible pipe properties are considered for the assessment with an inner diameter of 0.254m (10inches).

Further properties of the flexible pipe considered for the analysis are given in Table 1. The buoyancy section for the lazy

wave riser system begins at 76m curvilinear length from the hang-off location, having 15 buoyancy modules distributed

along 35m of the flexible pipe. Buoyancy modules properties are also given in Table 1. The shape of the riser is developed to

maximize the benefit of the lazy wave riser system during an oil filled case. This proposed configuration allows the riser

system to maintain the allowable bend radius and avoid overbending or compressive forces for various loading conditions

dictated by large vessel offsets and harsh environmental conditions.

Parameter Value

Vertical Hang-off Angle (deg) 10

Outer Diameter (m, inches) 0.356, 14.00

Inner Diameter (m, inches) 0.254, 10.00

Bending Stiffness (kNm2) 124.900

Axial Stiffness (kN) 711,200

Mass per unit length (kg/m) 184

Operating Minimum Bend Radius Allowable (m) 3.7 (1)

Single Buoyancy Module Volume (m3) 1.030

Single Buoyancy Module Mass (kg) 438

Single Buoyancy Module Length (m) 1.0

Note 1/ Typical MBR for the riser type considered.

Table 1 – Flexible Pipe and Buoyancy Module Properties

Three (3) chains are attached to the lazy wave riser configuration at arc lengths of 82m, 90m and 97m from the FPSO hang-

off location. This modified lazy wave configuration will include a clamp creating a link between the steel chains and the

flexible pipe, and an optional anchor at the seabed attached to the steel chains. The mass and length arrangement of the steel

chains for the riser configuration is defined in Table 3. The analysis model developed for the lazy wave riser configuration

inclusive of the stabilisation chains during a still water event is illustrated in Figure 7. The model considers an oil filled case

(900kg/m3 fluid density) as the base case for the lazy wave configuration shape.

Parameter Value

Number of Buoyancy Modules 15

Buoyancy Section Length (m) 35

Riser Catenary Length until First Buoyancy Module (m) 76

Steel Chain Axial Stiffness (kN) 252,000

Steel Chain Bending Stiffness (Nm2) Not Applicable

Riser Catenary Length until Chain 1 Riser Clamp (m) 82



Table 2 – Buoyancy Module and Chain Attachments to Flexible Riser

Chain Location Mass per unit

length (kg/m)

Chain 1 section

length (m)

Chain 2 section

length (m)

Chain 3 section

length (m)

Top (riser side) 50 20 20 15

Middle 100 15 15 20

Bottom 200 10 10 10

Total Length - 45 45 45

Table 3 – Stabilisation Chain Arrangement

8 SPE-175485-MS

Figure 7 – Analysis Model of Lazy Wave Riser System with Stabilisation Chains in Varied Internal Fluid Conditions

Static and dynamic analyses are performed to assess the benefits of the stabilisation chains in the proposed lazy wave

configuration. Riser vertical excursions are extracted for the entire length of the riser system to assess the clearance range.

The performance of the riser system is assessed for a range of loading conditions including FPSO offsets, extreme storm

loading (wave and current) and the range of riser internal fluid densities identified above. The strength of the riser system is

assessed through typical response parameters such as curvature, distributed tension and hang-off loads. Near and transverse

loadcases representing typical extreme operational scenarios are considered to assess the comparison between the two

configurations, with and without stabilisation chains. The analyses are repeated for each riser internal fluid configuration. For

the loadcases considered, it is assumed that the FPSO is a turret moored vessel with weathervaning capabilities and therefore

the vessel heading is aligned with the current and wave loading (head seas). The FPSO is assumed to offset by a maximum of

20% of the water depth of 60m, which represents an extreme operating scenario [4]. The wave and current loading considered

for the following case study is is given in Table 4. Current speed ranges between 1m/s at surface and 0.4m/s at the seabed.

Regular wave height of 7m and 14 seconds wave period is considered for analysis purposes for the following assessment.

It is worth noting that standard lazy waves are seen to improve dynamic response by decoupling the touch down area from

the wave and vessel induced motions at the hang-off. Therefore, primary focus is given on assessing the additional

stabilisation achieved through the use of chains on the riser and subjecting it to varied fluid densities and transverse current

loading, as further discussed along the following sections.

Gas (100kg/m3)

Oil (900kg/m3)

Sea Water (1025kg/m3)

SPE-175485-MS 9

Figure 8 – Illustrations of Analysis Considered for Comparison

Parameter Value

Wave Height (m) 7

Associated Wave Period (s) 14

Current speed (at surface) (m/s) 1.00

Current speed (at 30m below water surface) (m/s) 0.70

Current Speed (at 60m below water surface) 0.40

Table 4 – Environmental Loading Conditions

Wave Loading

Current loading

FPSO Offset FPSO Heading

FPSO Heading Flexible Riser

Case 0 – Still Water Condition (Nominal Case)

Case I – Near Loading Condition

Wave Loading Current Loading

FPSO Offset

FPSO Heading

Flexible Riser

Case II – Transverse Loading Condition

Flexible Riser

10 SPE-175485-MS

Still Water Conditions Still water condition results for both riser configurations are extracted from the finite element analysis. Still water condition

presents the theoretical and reference state of no vessel offset, no current and no wave. The riser shape with the lightest and

heaviest internal fluid, gas and seawater respectively, are shown and compared with the associated shape without stabilisation

chains in Figure 9. The addition of stabilisation chains for the gas filled riser shows that a considerable reduction in vertical

displacement is achieved. This is more relevant for the hog bend section, but the shape variation of the sag bend is also

limited with use of the stabilisation chains. For the heavier seawater filled riser, the sag bend clearance to seabed is reduced

from approximately 12m to 5m. The shape of the hog bend remains as desired. With the stabilisation chains, the variation in

horizontal separation between hang-off and touch down point is reduced when the internal fluid is changed. This

subsequently reduces the riser length required to achieve zero tension at the riser base and hence decreases the requirements

for a hold-back clamp or the seabed space required for flowline routing.

Where specific minimum clearance limitations are required between sag and seabed or hog and surface (or vessel hull) this

can be achieved through refinement of the chain sections linear weight distribution. Furthermore, if exact variations in fluid

density or minimum clearances requirements are unknown, addition of gradually heavier chains at the lower section of the

string will gradually limit the displacement, whilst maintaining some overall displacement allowance to not incur in

excessive interface loads between the chain string and the riser.

The strength of the riser system inclusive of the stabilisation chains is reviewed in terms of bend radius and effective tension

along the length. The effective tension and minimum bend radius incurred by the riser system under the considered internal

fluid densities are given in Figure 10 and Figure 11 respectively. The bend radii are within allowable of 3.7m for both

heaviest and lightest fluid densities. The lowest bend radius is observed at the sag bend for the oil and water filled risers.

Risers under all configurations remain in tension. A reduction in tension is noted at the sag bend. This reduction in tension is

due to the influence on the stabilisation chains.

Figure 9 – Riser Shape in Still Water – With vs. Without Stabilisation Chains - Gas Filled

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ara

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Riser Curvilinear Length (m)

Use Of Stabilisation Chains In Lazy Wave Configuration

RISER SHAPE IN STILL WATER CONDITIONSWith Vs Without Stabilisation Chains for Gas and Water Filled Risers

Water Filled (With Chains) Water Filled (Without Chains)

Gas Filled (With Chains) Gas Filled (Without Chains)

Water Depth (60m)

SPE-175485-MS 11

Figure 10 – Effective Tension along the Riser for Varied Fluid Conditions - With Stabilisation Chains

Figure 11 – Minimum Bend Radius along the Riser for Varied Fluid Conditions - With Stabilisation Chains

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STATIC STILL WATER TENSION RESULTS Riser Configuration With Stabilisation Chains

Water Filled Oil Filled Gas Filled

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STATIC STILL WATER BEND RADIUS RESULTS Riser Configuration With Stabilisation Chains

Water Filled Oil Filled Gas Filled

Minimum Allowable Bend Radius = 3.7m

12 SPE-175485-MS

Dynamic Response The vertical excursion ranges during a dynamic near current and wave loading event for the gas filled riser system is shown

in Figure 12. It is indicated that the probability of the hog bend interfering with the hull of the FPSO is very high for the riser

configuration exclusive of the stabilisation chains, for this particular case study. The hog bend of the riser system without

stabilisation chains reaches up to 3m below the mean water level. With the application of stabilisation chains, there is a large

reduction in the max vertical position of the riser, from about 68m to 35m, but also on the dynamic excursion range

experienced, in particular by the hog bend. The benefit of stabilisation chains is further highlighted near the touch down zone,

allowing the riser to dissipate the axial tension in a shorter distance.

The lateral displacement of the gas filled riser system under a transverse loading event, with and without stabilisation chains,

is shown in Figure 13. During the transverse loading event, the gas filled riser without the stabilisation chains experiences the

largest excursion of 34m away from its nominal vertical plane, reducing by nearly 50% to 16m with the chain string. The

extreme lateral excursion increases the potential of interference against adjacent subsea structures, in particular other risers

and moorings, but also seabed installed equipment. The riser suspended length from the FPSO hang-off to the touch down

region for the transverse condition is decreased from 200m to 150m through the use of stabilisation chains. The likelihood of

trenching decreases when considering the stabilisation chains as the motions near the touch down zone are dampened. This

furthermore reduces the riser length required to achieve zero tension, thus decreasing the requirement of a hold-back clamp

and seabed clearance requirements, as previously mentioned.

During dynamic excitation, care should be taken to avoid snag loading on the riser-chain interface, which could result from

the full chain length being pulled from the subsea clamp. The behaviour of the steel stabilisation chains are assessed for the

gas filled risers, which experience the largest displacement from its nominal position. The velocity of the stabilisation chains

for each individual chain length attached to the flexible riser is illustrated in Figure 14, where it is shown that the velocity

dissipate before reaching the bottom length of the chains. This illustrates that the mass distribution of the stabilisation chain is

successfully preventing any additional snag loading on the flexible riser.

The use of stabilisation chains for heavier internal fluid densities such as oil and seawater likewise result in lower transverse

displacements. The maximum transverse excursions experienced by the riser with the chains are shown in Figure 15. The

solid lines represent the maximum displacement incurred by the riser configuration inclusive of stabilisation chains while the

dotted lines present maximum displacement exclusive of those. The reduction in maximum displacement experienced by

using stabilisation chain for seawater and oil filled risers are approximately 50% and 35% respectively. The largest excursion

experienced by the seawater filled riser occurs at the sag bend – between 60m and 80m curvilinear distance from the hang-off

region.

The benefits of stabilisation chains in limiting vertical displacements for heavier internal fluid densities at near loading

conditions are observed in Figure 16. While the stabilisation chains manage to limit the overall vertical motions of the riser

system, the configuration may increase the likelihood of contact between the sag bend and the seabed, what is typically a

limiting design criteria. Although an overall minimum clearance between the riser and the seabed is below 1m, it is

considered as acceptable for an extreme condition. Adjustment on the chain sections configuration would allow achievement

of a greater clearance between sag and seabed without jeopardizing the minimum clearance between hog and vessel hull. This

is presented as a sensitivity in the following section.

SPE-175485-MS 13

Figure 12 – Vertical Excursion Range - Gas Filled - With/Without Stabilisation Chains

Figure 13 – Lateral Excursion Range - Gas Filled - With/Without Stabilisation Chains

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Use of Stabilisation Chains in Lazy Wave Configurations

VERTICAL EXCURSION RANGE DURING NEAR LOADINGGas Filled Risers: With and Without Stabilisation Chains

Without Stabilisation Chains With Stabilisation Chains

Without Stabilisation Chains

With Stabilisation Chains

Water Level = 60m

Potential Interference Against FPSO Hull

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LATERAL EXCURSION RANGE DURING TRANSVERSE LOADINGGas Filled Risers: With and Without Stabilisation




14 SPE-175485-MS

Figure 14 – Maximum Velocity Induced in Stabilisation Chains - Gas Filled Risers

Figure 15 – Maximum Lateral Excursion - Seawater and Oil Filled - With/Without Stabilisation Chains

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0.2

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1.4

1.6

1.8

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Ma

xim

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Ve

locit

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m/s)

Chain Curvilinear Length from Riser Clamp Attachement (m)


MAXIMUM VELOCITY INCURRED BY CHAIN LENGTHGas Filled Risers During Transverse Loading

Chain 1 Chain 2 Chain 3

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Use Of Stabilisation Chains In Lazy Wave Configurations

MAXIMUM LATERAL EXCURSIONS DURING TRANSVERSE LOADINGWater And Oil Filled Risers: With And Without Stabilisation Chains

Oil Filled - With Stabilisation Water Filled - With Stabilisation

Oil Filled - Without Stabilisation Water Filled - Without Stabilisation

SPE-175485-MS 15

Figure 16 – Vertical Excursion Range - Water Filled - With/Without Stabilisation Chains

Alternative Configuration

To reduce the likelihood of clashing of the riser sag bend with the seabed during near loading condition, an alternative

configuration is presented as a sensitivity on the possible variations of this method. The stabilisation chain configuration is

redesigned whereby the riser clamp attachment of stabilisation chains start at 97m away from the FPSO (7 m away from the

initial base case configuration). The revised stabilisation chain configuration is tabulated in Table 5 and Table 6. The vertical

excursion range incurred by the seawater and gas filled risers in a revised configuration under a near loading event is shown

in Figure 17. The revised configuration increases the minimum clearance between riser and seabed to 3m for the water filled

condition, as illustrated in Figure 18, where the minimum vertical seabed clearance is shown. The gas filled risers are also

found within the required vertical clearance. The transverse clearance for the seawater and gas filled risers are reviewed to

ensure that the revised chain configuration are applicable to transverse loading. The excursion range at transverse direction to

the riser plane is shown in Figure 19 for gas and seawater filled risers. The lateral excursion range remains similar to the

original chain configuration .The maximum velocity recorded by the chains is shown in Figure 20. The velocity decreases as

the chain length increases away from the flexible riser. The final 10m of the chain length incurs zero velocity implying that

the bottom extremity of the chain does not move.

The minimum bend radius and tension ranges for the heaviest and lightest internal fluid combined with the alternative chain

configuration are shown in Figure 21 and Figure 22 respectively. The overall lowest minimum bend radius of 4.8m which

occurs for the water filled riser system is within the minimum bend radius allowable of 3.7m. The effect of stabilisation

chains are observed in the tension plots for both gas and seawater filled risers. Negative tension of approximately 5kN near

the touch down zone is noted for both gas and seawater filled risers during the near load condition. This magnitude of

compression is considered relatively low and likewise has a relatively low likelihood of occurrence.

Minimum bend radius and tension ranges are shown for the heaviest and lightest internal fluid riser systems under transverse

loading in Figure 23 and Figure 24 respectively. The overall minimum bend radius of 7.7m occurs for the gas filled riser

system. The risers under both fluid arrangements are in tension throughout the length.

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VERTICAL EXCURSION RANGE DURING NEAR LOADINGWater Filled Risers: With and Without Stabilisation Chains




Water Level = 60m

16 SPE-175485-MS

Parameter Value




Table 5 – Alternative Chain Configuration Attachments to Flexible Riser

Chain Location Mass per unit

length (kg/m)

Chain 1 section

lengths (m)

Chain 2 section

lengths (m)

Chain 3 section

lengths (m)

Near Flexible Riser 40 15 20 15

Middle 80 15 15 10

Near Seabed 200 10 10 20

Total Length (m) - 40 45 45

Table 6 – Alternative Chain Configuration Arrangement

Figure 17 – Vertical Excursion Range - Water and Gas Filled - with Alternative Stabilisation Chain Configuration

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VERTICAL EXCURSION RANGE DURING NEAR LOADINGGas and Water Filled Risers With Stabilisation Chains - Alternative Configuration

Water Filled Gas Filled

Water Level = 60m

SPE-175485-MS 17

Figure 18 – Min Vertical Clearance - Water and Gas Filled - Basecase Vs. Alternative Stabilisation Chain

Configuration – Near Loading

Figure 19 – Lateral Excursion Range - Water and Gas Filled - with Alternative Stabilisation Chain Configuration

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VERTICAL EXCURSION RANGE DURING NEAR LOADINGGas and Water Filled Risers with Stabilisation Chains - Basecase Vs. Alternative

Gas Filled (Alternative) Water Filled (Alternative)

Gas Filled (Base Case) Water Filled (Base Case)

Water Level = 60m

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LATERAL EXCURSION RANGE DURING TRANSVERSE LOADINGWater And Oil Filled Risers: With Revised Stabilisation Chain Configuration

Water Filled Riser - Maxixmum Gas Filled Riser - Maximum

18 SPE-175485-MS

Figure 20 – Maximum Velocity Induced in Alternative Stabilisation Chain Configuration - Gas Filled Risers

Figure 21 – Minimum Bend Radius - Seawater and Gas Filled - Near Loading

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Chain Curvilinear Length from Riser Clamp Attachment (m)


MAXIMUM VELOCITY INCURRED BY CHAIN LENGTHGas Filled Risers During Transverse Loading

Chain 1 Chain 2 Chain 3

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MINIMUM BEND RADIUS DURING NEAR LOADINGGas And Water Filled Risers With Stabilisation Chains

Gas Filled Water Filled


SPE-175485-MS 19

Figure 22 – Effective Tension along the Riser - Seawater and Gas Filled - Near Loading

Figure 23 – Minimum Bend Radius along the Riser - Seawater and Gas Filled - Transverse Loading

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TENSION RANGE DURING NEAR LOADINGGas And Water Filled Risers With Stabilisation

Water Filled - Minimum Water Filled - Maximum

Gas Filled - Minimum Gas Filled - Maximum

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MINIMUM BEND RADIUS DURING TRANSVERSE LOADINGGas And Water Filled Risers With Stabilisation Chains

Gas Filled Water Filled


20 SPE-175485-MS

Figure 24 – Effective Tension - Water and Gas Filled Risers - Near Loading

Accidental Condition Assessment The consequences of the potential failure of 1 out of the 3 stabilisation chains are investigated. The chain attached nearest to

the FPSO (or furthest from the touch down point) is subtracted from the overall arrangement at the interface with the riser.

The revised model is illustrated in Figure 25, for gas filled fluid condition. The transverse displacement ranges are checked

for a transverse case to study the effect of the loss of a stabilisation chain. The lightest riser (gas filled riser) is selected for the

following sensitivity. The transverse excursions incurred by the lightest risers are shown in Figure 26. The minimum bend

radius incurred due to the loss of 1 chain is likewise shown in Figure 28. The maximum transverse excursion incurred due to

loss of 1 stabilisation chain is less than 2m when compared against riser configuration with all chains intact. The minimum

bend radius likewise shows a similar trend to the configuration with all chains intact. The loss of 1 stabilisation chain is hence

minimal for the riser configuration assessed.

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TENSION RANGE DURING TRANSVERSE LOADINGGas And Water Filled Risers With Stabilisation

Water Filled - Minimum Water Filled - Maximum

Gas Filled - Minimum Gas Filled - Maximum

SPE-175485-MS 21

Figure 25 – Analysis Model View – Comparison between Intact and 1 Failed Chain – Gas Filled

Figure 26 – Lateral Excursion Range during 1 Stabilisation Chain Failed for Gas Filled Riser

-5

0

5

10

15

20

0 20 40 60 80 100 120 140 160 180

La

tera

l E

xcu

rsio

n R

an

ge

(m

)



LATERAL EXCURSION RANGE DURING TRANSVERSE LOADINGGas Filled Risers: 1 Failed Chain

1 Chain Failed All Chains Intact

1 Chain Failed

All Chains Intact

1 Failed Chain

All Chains Intact

22 SPE-175485-MS

Figure 27 – Vertical Excursion Range during 1 Stabilisation Chain Failed for Gas Filled Riser

Figure 28 – Minimum Bend Radius after 1 Stabilisation Chain Failure for Gas Filled Riser

-60

-50

-40

-30

-20

-10

0

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0 20 40 60 80 100 120 140 160 180 200

Ve

rtic

al

Se

ale

ve

l C

lea

ran

ce

Ra

ng

e (

m)



VERTICAL EXCURSION RANGE DURING TRANSVERSE LOADINGGas Filled Risers: 1 Failed Chain


1 Chain Failed

All Chains Intact

Water Level = 60m

0

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25

30

35

40

45

50

0 20 40 60 80 100 120 140 160 180 200

Min

imu

m B

en

d R

ad

ius (

m)



MINIMUM BEND RADIUS DURING TRANSVERSE LOADINGGas Filled Risers With 1 Stabilisation Chain Failed



SPE-175485-MS 23

Survival Current Loading Assessment

The effect of increasing the current loading is studied for the gas filled system under transverse loading. The aim is to assess

the system robustness and response to a greater environmental loading than eventually foreseen during design and likelihood

of riser transverse “walking”. Current loading is thus increased in 20% (or 0.2m/s at surface) intervals reaching up to a

maximum of twice the default current speed used for the analyses. Maximum transverse displacements incurred by the gas

filled riser system under various current loading is shown in Figure 29. The displacements are compared against the riser

without stabilisation chains under the nominal current loading. The step-out distances are larger for higher current speeds for

the gas filled riser system; however, the step-out distances are much lower than the riser system without stabilisation chains.

It is also noted that the excursions experienced by the riser system without stabilisation chains is under the base case current

speed. The chains are likewise assessed for their displacement range – primarily at the seabed. The stabilisation chains are

allowed to move on the seabed, i.e. there is no anchoring system at their base. The velocity induced in chain 1 (attached to

the riser system nearest to the FPSO hang-off) for various current speeds is shown in Figure 30, where current profiles are

presented as a percentage increase over the base case velocity. Chain 1 incurs the largest excursion due to its attachment at

the hog bend of the riser system and hence is selected for this assessment. For current speeds of up to 1.6m/s at surface the

chain bottom does not incur any movement at the seabed. For current speeds greater than 1.6m/s the chain experiences

limited “lateral walking” in the case where no anchoring system is considered, but does not move more than a meter after the

riser lateral excursion pulls the full chain length and another stable configuration is achieved. If required, in order to improve

stabilisation of the flexible riser configuration, and prevent “lateral walking” of the chains, a chain with increased length or

linear mass increase towards its base could be adopted, or even an anchor clump weight.

Figure 29 – Maximum Lateral Displacement Incurred in Gas Filled Riser under Various Current Loading

-5

0

5

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15

20

25

30

35

40

0 50 100 150 200 250

Ma

xim

um

La

tera

l E

xcu

rsio

n (

m)



MAXIMUM LATERAL EXCURSION DURING TRANSVERSE LOADINGGas Filled Risers During Transverse Loading

No Chains Base Case Current Speed +20% +40% +60% +80% +100%

Without Stabilisation Chains (Default Current Speed)


24 SPE-175485-MS

Figure 30 – Maximum Velocities Induced in Chains for Gas Filled Riser under Various Current Speed

Benefits and Challenges The use of stabilisation chains in lazy wave riser systems offers an opportunity to further exploit the advantages of a lazy

wave riser system for a flexible pipe in harsh environments. The addition of chains in the lazy wave riser system improves

the riser system ability to accomodate large variations in fluid products density by mitigating both vertical and transverse

motions of the riser system, and thus potentially allowing additional adjacent subsea structures to be installed. The modified

lazy wave reduces the length of the dynamic seabed section of the riser system and improves aptitude to approximate the riser

seabed connection (hold-back clamp; riser base) and the touch down zone. The modified lazy wave riser system provides an

alternative solution to a standard lazy wave configuration riser and can be tailored to allow configuration to accomodate

challenging field specific conditions.

In addition to the technical advantages, there are also some commercial benefits from the proposed configuration, when

compared with other configurations such as a pliant, steep wave or mid-water arches, which are common alternatives to a

lazy wave. An estimation of the cost effectiveness for the stabilisation chains configuration relative to other configurations is

illustrated in Figure 31. The stabilisation chains can make use of low cost materials, unlike the ancillary equipment necessary

for other configurations. Additionally, installation times and engineering of such stabilisation chains are expected to be less

than the alternative solutions, including pliant wave, but eventually subject to specific field conditions. Overall, the cost for

stabilisation chain configurations are considerably less when compared to the above alternatives. Stabilisation chains and

clamps required for the configuration are likewise relatively easy to fabricate and install.

Figure 31 – Relative Cost Rating for Typical Flexible Riser Configurations

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 5 10 15 20 25 30 35 40 45

Ma

xim

um

Ve

locit

y (

m/s)

Chain Curvilinear Length from Riser Clamp Attachment (m)


MAXIMUM VELOCITY INCURRED BY CHAINGas Filled Risers During Transverse Loading

Base Case Current Speed +20% +40% +60% +80% +100%

Simple

Catenary

Lazy

Wave Pliant Wave Steep Wave Mid-Water

Arch

Buoyancy

Supported

Risers (BSR)

Configuration Cost

Low High

Stabilisation

Chains

SPE-175485-MS 25

Optimisation of the steel chain configuration attached to the lazy wave riser system is the primary challenge of this solution.

It is important for the system to comprise of the required distribution of weight and length of the chain to achieve the best

solution. The steel chain configuration should be designed to accommodate variations due to full range of internal fluids that

the system is expected to be subject to during its operational lifetime. Further assessments should be carried out to identify

potential risks and hazards in selecting steel chains for the configuration. This paper considers a high level global analysis of

the usage of steel chains, where particular evaluation would be driven by field specific conditions. It is necessary to perform a

local analysis of the behaviour of the steel clamps and chains to further verify the design proposed herein, but it is expected

that existing hardware designs can be transferred to this system. Localised overbending of the flexible pipe due to the

inclusion of steel chains in between buoyancy modules is another area of interest that should be explored in more detail,

although connection directly onto an adapted buoyancy module is also feasible. The flexible pipe is also subject to crushing

loads from the chain clamps and hence should be considered when designing such a system. During a typical design life of 25

years for a riser system, steel chains are subject to the same challenges to that of a typical riser system. This includes fatigue,

stresses, corrosion and wear. Careful consideration should be made in designing the steel chains to withstand such challenges

or be retrofitable in case of failures or localised damage.

Conclusion The benefits of stabilisation chains for a dynamic flexible riser system are addressed for harsh environment and large vessel

offsets. The challenge of improving clearances of the riser system is studied, and the solutions of stabilisation chains are

explored. Some concluding remarks from the assessment conducted for the technical paper presented herein are as follows:

For risers operating under various internal fluid densities, stabilisation chains offer a good alternative to generic

lazy wave risers, whereby the lightest and heaviest fluid riser excursions are limited by the stabilisation chains.

The arrangement and selection of the stabilisation chains configuration should be iterated as necessary to allow

for safe design margins on both extreme configurations conditions, heavy and light.

The use of stabilisation chains for a lazy wave riser system shows a reduction in lateral and vertical

displacements

The lightest fluid arrangement, which typically incurs the largest motions, experiences consequently the most

benefits from the use of stabilisation chains.

Advantages of the stabilisation chains can be observed during extreme near and transverse offset loading

conditions. The chains manage to considerably limit excursions in both directions, without incurring in

overloading of the riser system.

Lateral walking for extreme transverse offset loadings is prevented due to the mass distribution of the chain

arrangements.

The chain configuration shows an acceptable response during an accidental event of chain string failure.

References [1] Yanqiu Zhang, Zhimin Tan and Yucheng Hou – “Design Analysis of a Weight Added Wave Configuration of

Flexible Riser in Shallow Water”, ASME 2010 29th International Conference On Ocean, Offshore and Arctic

Engineering, OMAE2010-20360, pp 403-410, Wellstream International Limited, Houston, TX, June 6-11, 2010.

[2] Yanqiu Zhang et al – “Flexible Pipe Support”, Wellstream International Limited, Houston, TX; WO 2009063163

A1, 22 May 2009.

[3] Orcina – “Orcaflex version 9.6b”, Ulverston, Cumbria, UK; 2012.

[4] D. Hanonge, A. Luppi – “Challenges of Flexible Riser Systems in Shallow Waters”, OTC 20578, Technip – Seal

Engineering SA, 2010 Offshore Technology Conference, Houston, TX, 3-6 May 2010.

[5] ISO 19901-1 – “Metocean Design and Operating Conditions”, First Edition, 15 November 2005.

Documents

SPE-175485-MS Use of Stabilisation Chains to Optimise Lazy ... · This paper was prepared for presentation at the SPE Offshore Europe Conference & Exhibition held in Aberdeen, United