OTC-27597-MS SCR Life Extension through Managed Shape Change · were applied to streamline this...

OTC-27597-MS

SCR Life Extension through Managed Shape Change M.J. Santala, Y. Constantinides, Chevron Energy Technology Company R. Koska, C. Krapf, D.S. Walters, 2H Offshore Inc

Copyright 2017, Offshore Technology Conference This paper was prepared for presentation at the Offshore Technology Conference held in Houston, Texas, USA, 1–4 May 2017. This paper was selected for presentation by an OTC 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 Offshore Technology Conference and are subject to correction by the author(s). The material does not necessarily reflect any position of the Offshore Technology Conference, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Offshore Technology Conference 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 OTC copyright.

Abstract

Steel catenary risers (SCRs) are a cost effective option for deepwater developments. Throughout industry hundreds of SCRs

have been deployed and have proven to be highly reliable. Nevertheless, as the inventory of SCRs across industry ages and

as operators seek to extend the lives of deepwater platforms beyond their originally intended service life, strategies are

needed to ensure that SCRs can continue to operate reliably. Since touchdown point fatigue damage is a strong factor

determining the service life of an SCR, strategies to mitigate fatigue damage in this region may be required for service life

extension. One such strategy commonly adopted is platform relocation in order to shift the SCR touch down point

throughout the life of the field and hence spreading the fatigue damage along the riser touch down region; however, this

typically needs to be planned for in the original concept development. In this paper, two additional strategies based on

modifying the shape of an existing SCR are presented. These strategies reduce the fatigue damage rate in the original

touchdown point (TDP) region by managing the shape of an existing SCR either through adding buoyancy or by changing the

length of the riser such that the original segment of riser pipe in the TDP region is moved to a low stress region. The

proposed techniques also accommodate riser response during extreme environmental events. The techniques have been

examined based on case studies of a production SCR and an export SCR. Trade-offs in placement of buoyancy and the

impact that shape change has on meeting riser strength requirements are presented. It is shown that the potential exists to

roughly double the computed fatigue life in the touchdown region.

Introduction

Steel catenary risers (SCRs) have proven to be a cost effective and reliable riser system for deepwater development. As a

consequence, SCRs have become one of the most common deepwater riser systems. And, although the population of SCRs is

anticipated to continue growing further, within the next ten years a significant amount of SCRs will be reaching the end of

their planned service life. It is anticipated that in many cases there will be a desire to extend the initial service life of the

floating production platforms and their associated risers. Since touchdown point fatigue damage is a strong factor

determining the design service life of an SCR, strategies to mitigate fatigue damage in this region may be required for service

life extension.

This paper describes two methods that may be considered to mitigate accumulation of touchdown point fatigue;

Addition of buoyancy modules to the in-situ riser to create a marginal lazy wave configuration that shifts the TDP further

away from the nominal TDP.

Change in riser length by removal of joints at the top of the riser to shift the TDP further away from the nominal TDP. It

is expected this would be done should the riser need to be removed from its basket by a heavy lift vessel for replacement

of the SCR tapered stress joint or flex joint.

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2 OTC-27597-MS

Because of the nature of Chevron’s SCR portfolio, it was decided to test these methods for two different size risers hosted by

two different Gulf of Mexico floating platforms in the water depth range of 5000 ft to 7000 ft. The two test cases were a 7”

oil production SCR and a 20” oil export SCR. In both cases the hang-off method was a titanium tapered stress joint (TSJ).

Both risers were fully straked and therefore VIV-induced fatigue damage was negligible compared to wave-induced or hull

vortex-induced motion (VIM)-induced fatigue. The riser / host combinations were based on designs which have been

progressed through detailed design but metocean criteria and soil conditions were updated and other minor simplifications

were applied to streamline this feasibility study. Nevertheless, each riser / host had its own Project-specific design basis and

site-specific metocean conditions*. A full fatigue life analysis was considered, along with the critical strength cases that

controlled the original design. We therefore expect the results to be broadly indicative of those that can be expected in the

same water depth range in the Gulf of Mexico. To simplify the discussion here, presented results will focus on the larger

export riser but key performance metrics will also be reported for the production riser.

Overview of Shape Change Assessment Procedure

An overview of the shape change assessment analysis is presented in the flowchart in Figure 1. The procedure began by

adding buoyancy or changing length with the objective of moving the nominal TDP location by a target distance. The

nominal TDP relocation analysis was done quasi-statically with no environmental loading or vessel offset. The general

analysis workflow was the same whether the buoyancy addition or riser length change strategy was tested.

Figure 1 – Basic Shape Change Assessment Analysis

* Metocean conditions at both sites are representative of deepwater Central Gulf of Mexico conditions in RP-2MET [3].

SCR Design Data

and Fatigue Results

Adjust SCR Configuration (add buoyancy

or add/remove riser joints at hang-off)

TDP Location

Shifted by Target

Distance?

YES

NO

Re-evaluate SCR Fatigue Performance

Target Fatigue

Life Improvement

Factor Achieved?

Re-evaluate Extreme Strength

Performance

Adequate Strength

Performance

Maintained?

Target Distance

to Shift TDP

YES

YES

NO

NO

Assessment Complete


OTC-27597-MS 3

Since the strategies outlined here change the configuration of the riser during its life, the damage rates for both the original

and modified configurations needed to be computed. Fatigue life was maximized based on study of fatigue critical locations

(typically the TDP, buoyancy section and TSJ). The optimal time for the change from the original to the modified shape was

found through an interactive procedure based on an initial assumed transition time. The transition time was successively

adjusted until the overall fatigue life was maximized at the most critical location (Figure 2). The resulting overall fatigue life

of the system was compared to the fatigue life of the original SCR configuration to determine the fatigue life improvement

factor for the modified configuration.

Figure 2 – Example Fatigue Damage at Critical Locations (addition of 54 buoyancy modules)

After determining the touchdown point fatigue improvement, the strength performance of the modified SCR configuration

was assessed. The most onerous extreme or survival operating condition identified from the original work was assessed for

the modified configuration.

Modeling Assumptions

The wave-induced fatigue analysis (WIF), vortex-induced vessel motion (VIM) fatigue analysis and extreme storm strength

analysis were carried out using non-linear time domain analysis programs.† The SCRs were modeled using beam elements

based on nominal wall thickness and material properties. Appropriate seabed stiffness was considered for the riser section

that lies on the seabed.

The bare pipe, straked pipe and TSJ sections were modeled with the appropriate physical and hydrodynamic properties

consistent with normal design practice. The SCRs were modeled as fully straked from the TSJ to approximately 500 ft

beyond the nominal TDP. With the high VIV suppression efficiency of the strakes, the original analyses showed that for both

of these SCRs, VIV-induced fatigue damage was negligible compared to wave-induced and VIM-induced fatigue damage.

As such, for this feasibility study, VIV analysis was not included.‡

Actual implementation of the buoyancy addition strategy would entail adding a number discrete buoyancy modules over a

length of the riser. The outside diameter (ODDBM), the length (LDBM) and the space between the modules (SpaceDBM) all have

a very comparable length scale. From a modeling standpoint, an equivalent continuous buoyancy section was developed.

The equivalent length (LECB) and equivalent outer diameter (ODECB) of the equivalent continuous buoyancy section were

determined in order to achieve the same amount of total up-thrust and added mass applied to the riser as the discrete

buoyancy modules (Figure 3).

† Flexcom [5] was used for analysis of the export riser and OrcaFlex [4] was used for the analyses of the production riser.

The choice of program was based on the format of pre-existing files from previous work. ‡ VIV forcing on the buoyancy elements was not thought to be an important driver to fatigue life in the cases studied, but in

an actual design, this contribution would need to be explicitly assessed.

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Chevron ETC - SCR Shape Change Assessment

BIG FOOT 20" SCR - FATIGUE DAMAGE ACCUMULATION300ft TDP Shift, 54 Buoyancy Modules Added 775ft above Nominal TDP

TSJ Buoyancy Original TDP New TDP

- 54 buoyancy modules added for

300ft TDP shift after 99% of original

design life is used- Critical fatigue locations are original

TDP & new TDP, fatigue damage accumulation of 1.0 at a time of 1.93

multiplied by the original design life (fatigue improvement factor of 1.93)

Fatigue Damage Accumulation Reaches Limit of 1.0

Original SCR Configuration

Modified SCR Configuration


4 OTC-27597-MS

Figure 3 – Modeling Discrete Buoyancy Modules with Equivalent Continuous Buoyancy

Fatigue Analysis

Time domain random sea analyses were conducted to assess wave-induced fatigue (WIF) using three hour vessel motion time

traces (which include vessel offsets) and the associated current profiles. Vortex-induced vessel motion (VIM) fatigue

analyses were conducted to assess the riser fatigue damage that occurs due to low frequency motions of the vessel from

currents. The VIM response was simulated in the time domain as a harmonic motion about the mean offset vessel position

with the associated low frequency transverse motion natural period (Figure 4). The VIM analysis simulation time was

selected to be at least 10 times the average transverse motion natural period of the vessel undergoing VIM.

The contributions from WIF-induced fatigue damage (DWIF) and VIM-induced fatigue damage (DVIM) were computed

separately, but the procedures working from the time domain simulations were the same for each process. For each fatigue

environment (wave or VIM current condition), stress time histories were extracted along the length of the riser and stress

histograms were generated using the rainflow cycle counting method. Fatigue damage was then estimated using the design

S-N curves and SCFs with Miner’s rule to sum the damages from the probabilistically weighted environmental design

conditions.

The factored and combined fatigue life includes a 5% allowance for installation damage and was determined as:

𝐹𝑎𝑐𝑡𝑜𝑟𝑒𝑑 𝐶𝑜𝑚𝑏𝑖𝑛𝑒𝑑 𝐿𝑖𝑓𝑒 = 0.95

10 × 𝐷𝑊𝐼𝐹 + 10 × 𝐷𝑉𝐼𝑀

The two risers studied did have a different sensitivity to wave and VIM damage. In their original configurations the ratio of

separately computed WIF-fatigue life over VIM-fatigue life for the export riser was 2.6 whereas for the production riser the

ratio was 0.29; so the export riser was more sensitive to VIM-induced fatigue and the production riser was more sensitive to

WIF-induced fatigue damage.

Figure 4 – Dynamic Hull VIM Analysis Methodology


OTC-27597-MS 5

Shifting TDP through Added Buoyancy or Removing Joints

For the 20” export riser the outside diameter of the discrete buoyance elements modeled was 1.85 m (6.07 ft) with a net uplift

of 17.6 kN (3.95 kips) per module. The overall effect of adding the buoyancy elements was that, while in the straked region

the SCR had a wet weight per unit length of 2.40 kN/m (0.164 kips/ft), in the buoyancy region the average wet weight per

unit length was -2.35 kN/m (-0.161 kips/ft). As part of this feasibility study, the efficacy of adding buoyancy to the export

riser starting either 325 ft or 775 ft from the original nominal TDP was tested.§ As shown in Figure 5, the amount of TDP

shift that was achieved per unit buoyancy element was greater when the buoyancy added closer to nominal TDP. This figure

also shows the TDP shift per (40’ length) riser joint removed.

Figure 5 – Amount of Export Riser TDP Shift Achieved Versus Number of Buoyancy Elements Added or Joints Removed

Mitigating TDP Fatigue through Buoyancy Addition

Figure 6 and Figure 7 show the shape of the export riser near the TDP with varying amounts of buoyancy added 325 ft and

775 ft from the original touch down region. Since the basic premise presented here is that life can be extended by using

buoyancy to shift the TDP, Figure 5 might lead one to believe that adding buoyancy at the closer offset would be the most

effective way to achieve this result. For large amounts of buoyancy this was shown to be true. However, as shown in Figure

8, when less buoyancy was used the fatigue life improvement factor was higher when the buoyancy was placed further from

the original TDP. The reason for this is that the buoyancy modules themselves create a new elevated stress cycle region in

the sag- and hog-bend region that they create. If a modified configuration sag- or hog-bend elevated stress cycle region

overlaps the original TDP region then little or no benefit may be achieved by adding buoyancy. Indeed, as shown in Figure

9, though adding either 10 or 14 modules starting at 325 ft from the original TDP significantly shifts the nominal TDP, it

actually led to a worse fatigue damage accumulation rate in the region of the original TDP.**

Adding the buoyancy at the

further offset was shown to have two benefits, the sag- and hog-bend elevated stress cycle was less severe than when the

buoyancy was added at closer offset, and the sag- and hog-bend elevated stress cycle region was further removed from the

original TDP high stress cycle region.

§ For the smaller production riser, buoyancy elements with an outside diameter of 1.45 m (4.76 ft) were considered to be a

practical size that might be implemented. These were modeled as having 8.7 kN (1.95 kips) of uplift per element which

provides comparatively more buoyancy (ratio of buoyancy uplift to original wet weight) than for the export riser. For the

production riser, buoyancy was added starting at offsets of either 400 ft or 775 ft from the original TDP. **

In these cases where adding buoyancy led to worse performance at the original fatigue critical point the fatigue life

improvement factor does not drop below 1.0 since the optimized solution in this case was simply not to make the change.

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BIG FOOT 20" SCR TDP SHIFTBuoyancy Modules: 1.85m OD, Length & Spacing, Density = 635kg/m3

Buoyancy Added 325ft above Nominal TDP Buoyancy Added 775ft above Nominal TDP

Riser Joints Removed from Hang-off


6 OTC-27597-MS

Figure 6 – Near Seafloor Export Riser Shape with Buoyancy Added 325ft from Nominal TDP

Figure 7 – Near Seafloor Export Riser Shape with Buoyancy Added 775ft from Nominal TDP

Figure 8 – Fatigue Life Improvement Factor Versus Number of Buoyancy Modules Added to Export Riser

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Horizontal Distance from Nominal TDP (ft)


BIG FOOT 20" SCR CONFIGURATIONTDP Shift with Buoyancy Modules Added 325ft above Nominal TDP

Original Configuration 7 Modules (100ft TDP Shift)

10 Modules (150ft TDP Shift) 14 Modules (200ft TDP Shift)


54 Modules (550ft TDP Shift)

Buoyancy Modules

Nominal TDP Location

325ft

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BIG FOOT 20" SCR CONFIGURATIONTDP Shift with Buoyancy Modules Added 775ft above Nominal TDP

Original Configuration 7 Modules (50ft TDP Shift)



Buoyancy Modules


775ft

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BIG FOOT 20" SCR - FATIGUE LIFE IMPROVEMENT FACTORFatigue Life Extension by Adding Buoyancy Modules

Combined Life, Wave Fatigue + VIM Fatigue

Buoyancy Added 325ft Above TDP Buoyancy Added 775ft Above TDP


OTC-27597-MS 7

Figure 9 – Factored Fatigue Lives for Export Riser with Buoyancy Added 325ft from Nominal TDP

Figure 10 – Factored Fatigue Lives for Export Riser with Buoyancy Added 775ft from Nominal TDP

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BIG FOOT 20" SCR - FACTORED FATIGUE LIFE AT TDPAPI X' in Seawater with CP, KDF=2.5, SCF=1.3, Safety Factor=10

Buoyancy Added 325ft above Nominal TDP, Wave Fatigue + VIM Fatigue

Original Configuration 7 Modules Added 10 Modules Added

14 Modules Added 22 Modules Added 32 Modules Added

54 Modules Added

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BIG FOOT 20" SCR - FACTORED FATIGUE LIFE AT TDPAPI X' in Seawater with CP, KDF=2.5, SCF=1.3, Safety Factor=10

Buoyancy Added 775ft above Nominal TDP, Wave Fatigue + VIM Fatigue

Original Configuration 7 Modules Added 14 Modules Added

22 Modules Added 32 Modules Added 54 Modules Added


8 OTC-27597-MS

Mitigating TDP Fatigue through Joint Removal

Removing joints from the top of the riser shifts the TDP but maintains the basic catenary shape (Figure 11) and the general

TDP fatigue damage pattern also remains similar but shifts accordingly (Figure 12). For both the risers tested here, removing

just a single 40 ft joint achieved an improvement factor on TDP fatigue life of nearly 2.0 and with three joints removed, the

fatigue life improvement factor marginally exceeded 2.0 for both risers considered. The improvement factor versus number

of joints removed for the export riser is shown in Figure 13.

Figure 11 – Near Seafloor Export Riser Shape with Riser Joints Removed from Top

Figure 12 – Factored Fatigue Lives for Export Riser Joints Removed from Top

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BIG FOOT 20" SCR CONFIGURATIONTDP Shift by Removing 40ft Riser Joints at Hang-off

Original Configuration 1 Riser Joint Removed 2 Riser Joints Removed 3 Riser Joints Removed


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BIG FOOT 20" SCR - FACTORED FATIGUE LIFE AT TDPAPI X' in Seawater with CP, KDF=2.5, SCF=1.3, Safety Factor=10Riser Joints Removed from Hang-off, Wave Fatigue + VIM Fatigue

Original Configuration 1 Joint Removed 2 Joints Removed 3 Joints Removed


OTC-27597-MS 9

Figure 13 – Fatigue Life Improvement Factor Versus Number of Joints Removed from Export Riser

Verifying Design Strength Requirements of Modified Shape Risers

From previous design work on both of these riser configurations, the governing cases controlling strength had been

identified. For both risers, the governing strength case was a 100-year hurricane case with intact mooring system. For the

export riser, the ratio of peak to yield stress was higher at the TSJ than near the TDP. Adding buoyancy to the export riser

provided a modest reduction in the peak stresses at the TSJ (Figure 14) and removing joints imposed a modest increase in the

peak stresses at the TSJ (Figure 15). Near the TDP, peak stresses in the export riser varied quite modestly based on the

amount of buoyancy added or number of joints removed (Figure 14 and Figure 15).

For the production riser the ratio of peak to yield stress was higher at the TDP than TSJ and adding buoyancy helped reduce

this peak stress quite dramatically; the ratio improved by a factor greater than two when more than 33 buoyancy modules

were added (at either offset). On the other hand, when riser joints were removed from the production riser, peak stresses

increased at the TDP. The increase in peak stress near the TDP in the production riser when two or more joints were

removed was so high that this would not have been a viable option for the particular production riser studied.

As shown in Figure 8 and Figure 14, adding 22 buoyancy modules 325 ft from the nominal TDP of the export riser provides a

fatigue life improvement factor of over 1.5 and gives adequate performance from a strength standpoint. Interestingly,

however, in this case in the near offset case of the 100-year hurricane a secondary touchdown point can form (Figure 16).

Though the appearance of this secondary touchdown region does not necessarily preclude this particular solution, the

implications of designing a riser which takes on such a configuration during extreme events should carefully be considered

before doing so. Of course, such double-touchdown geometries can be avoided either by adding more buoyancy (Figure 17)

or perhaps by applying the same amount of buoyancy at a further offset.

Overall, the strength studies indicate that shifts in TDP sufficient to extend fatigue life by a factor of two can be achieved and

simultaneously meet strength requirements. However, depending on what the critical design point in the original design was

it may be that some options are precluded from use.

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BIG FOOT 20" SCR - FATIGUE LIFE IMPROVEMENT FACTORFatigue Life Extension by Removing 40ft Riser Joints at Hang-off

Combined Life, Wave Fatigue + VIM Fatigue


10 OTC-27597-MS

Figure 14 – Export Riser Peak Stresses in 100-year Hurricane vs Number Buoyancy Modules Added

Figure 15 – Export Riser Peak Stresses in 100-year Hurricane vs Number Joints Removed


OTC-27597-MS 11

Figure 16 – Export Riser 100-year Hurricane Near Offset Sag Bend with 22 Buoyancy Modules at 325ft from Nominal TDP

Figure 17 – Export Riser 100-year Hurricane Near Offset Sag Bend with 54 Buoyancy Modules at 325ft from Nominal TDP

Discussion and Conclusions

The key conclusions and recommendations from the SCR shape change assessment are as follows:

For both risers studied, TDP zone fatigue life could be roughly doubled by adding buoyancy close to the end of the

original riser design life.

o Adding a large number of buoyancy elements starting close to the TDP led to the overall greatest benefit in

fatigue life for both risers considered in this study.

o Generally, a longer buoyancy section results in a larger fatigue life compared to a shorter buoyancy section.

o If the amout of buoyancy added is to be minimized for cost or operational reasons, adding at a further

distance from the TDP may be necessary to avoid having a secondary hog-sag stress region near the orginal

TDP stress region.

o Adding a small number of buoyancy modules near the touchdown point may also cause secondary contact

of the riser with the seabed in some near offset conditions.

o For the risers studied, strength requirements could always be accomodated. Peak stresses did not

universally decrease with the addition of buoyancy so such cases would always need to be checked.


12 OTC-27597-MS

For both risers studied, TDP zone fatigue could also be nearly doubled by removing a single 40 ft joint close to the

end of the original riser design life. However, removal of joints proved more challenging from a strength

standpoint.

o For one of the cases, removing even a single joint did not pass the preliminary strength checks performed

here. For the other case, removing a single joint was acceptable but the strength case design margin was

reduced.

o The potential for removing riser joints is dependent on the specific configuration and original design

margin. The particulars of which case did and did not work here should not be generalized beyond the fact

that removing lengths from a previously optimized design may prove challanging and needs to be checked.

In summary, both methods of shifting the TDP are capable of providing improved fatigue life, but the strength response of

each specific riser needs to be evaluated on a case-by-case basis. The buoyancy approach certainly affords more opportunity

for fine adjustment/optimization in a real project scenario and there are field-proven techniques for adding buoyancy to an in

site riser. However, having two options (buoyancy addition or joint removal) increases the chance of finding a workable

solution for specific cases and further allows the costs and operational implications of the different solutions to be compared.

For example, if a riser flex joint needed to be changed out as part of life extension, the joint removal option may become

much more attractive operationally since the riser would already need to be lifted from the porch in that case. Depending on

the original SCR design, in addition to confirming adequate strength response, several items that would need to be confirmed

include the following:

If buoyancy modules are added to an SCR, then strakes would need to be removed from the buoyant section. Any

resulting changes in VIV response would need to be determined in order to confirm that the riser maintains adequate

fatigue performance including the effects of VIV;

Riser maintains adequate clearance with other risers, umbilicals and mooring lines;

The change in riser shape associated with adding buoyancy does not cause flow assurance problems, such as

slugging;

Hang-off loading on the riser porch remains within allowable limits.

Nomenclature

DVIM Fatigue damage due to vortex-induced hull motion

DWIF Fatigue damage due to wave-induced motions

SCF Stress concentration factor

SCR Steel catenary riser

TDP Touchdown point

TSJ Tapered stress joint

VIM Vortex-induced motion of platform

VIV Vortex-induced vibration (here in context of vortices shed directly by riser)

References

1. API – “Recommended Practice for Design of Risers for Floating Production Systems and TLPs”; API-RP-2RD, 1st Edition; June

1998.

2. API – “Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms – Load and Resistance

Factor Design”; API-RP-2A-LRFD, 2nd Edition; April 1994.

3. API – “Derivation of Metocean Design and Operating Conditions – ANSI/API Recommended Practice 2MET”, API-RP2MET,

First Edition, November, 2014.

4. Orcina – “OrcaFlex Reference Manual”; Version 10.0d; 2016.

5. MCS International – “FLEXCOM Three Dimensional Nonlinear Time Domain Riser Analysis Software”; Version 7.9; April

2009.


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