spe-186122-ms application of steel lazy wave riser

SPE-186122-MS

Application of Steel Lazy Wave Riser Solution in Deepwater and Comparison to Other Riser Types Rohit Shankaran, 2H Offshore Engineering Ltd. Hugh Howells, 2H Offshore Engineering Ltd. Marcelo Lopes, 2H Offshore Engineering Ltd.

Copyright 2017, Society of Petroleum Engineers This paper was prepared for presentation at the SPE Offshore Europe Conference & Exhibition held in Banff, Aberdeen, United Kingdom, 5-8 September 2017. 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

In the current low oil price market, innovative low cost solutions are necessary for development of

new fields and life extension of existing fields. For deep water applications, Steel Catenary Risers

(SCRs) and Steel Lazy Wave Risers (SLWRs) provide low cost alternatives to flexible risers and offer

flexibility during design and life extension for Floating Production Systems (FPS) in deepwater.

The majority of deepwater fields in offshore West Africa and the North Sea have traditionally been

developed using flexible risers. SLWRs, a variation of the steel catenary riser (SCR) with added

buoyancy near the touchdown point at the seabed, have recently been deployed in the GoM and Brazil

for deepwater applications. Due to simplicity of design, good track record and qualified suppliers,

fabrication and installation methods for SCRs, SLWRs have become a logical extension of the SCR.

Buoyancy installed on the SLWR helps reduce top tension and decouple vessel motions from the riser

touchdown zone, which ensures that the required strength and fatigue performance can be achieved.

SLWR and SCR configurations are developed for mild environment, deepwater applications

representive of offshore West Africa and in severe environment moderate water depths, representive of

the North Sea. Riser configurations, hang-off loads, strength and fatigue responses are compared for the

configurations. The advantages and disadvantages of SCR and SLWRs are discussed and compared to

other riser types. Costs for flexibles, SCR’s and SLWR’s are also compared.

Introduction

More than 90% of deepwater fields in the North Sea and 50% of the fields in West Africa have

been developed using flexible risers. In relatively shallow water depths, low pressures and

temperatures, good strength and fatigue performance make flexible risers very attractive. However, for

deepwater application, flexible risers are limited by qualified size, cost and realiability.

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2 SPE-186122-MS

SCRs and SLWRs have gained track record for deepwater application with a wide range of FPS’s.

SCRs have been used in up to 1200m water depth in West Africa and 2500m water depth in the Gulf of

Mexico. SLWRs, a varation of the SCR configuration, are getting an increasingly strong track record

with applications in deepwater Gulf of Mexico and offshore Brazil where they have been used in up to

2000m water depth. SLWRs are a variation of the SCR with added buoyancy close the riser touchdown

region. SLWRs help decouple vessel motions in the touch down region from those induced by waves

and vessel motions at the surface, resulting in better strength and fatigue performance compared to an

SCR and they offer a potential low cost alternative to flexible risers.

For a deepwater application, typical of offshore West Africa, SLWR/SCR configurations are

compared for 8” and 10” risers in 2500m water depth. Hang-off loads, strength and fatigue responses are

compared for the SCR and SLWR configurations. For moderate water depths typical of North Sea

environments, SLWR configurations for a range of pipe sizes are compared in ~800m water depth.

Strength and fatigue responses for a typical 24” riser in harsh North Sea environements are discussed.

The design considerations, advantages and disadvantages of SLWR and SCRs are also evaluated and

costs for SCRs and SLWRs are compared to those for flexible risers.

SCR/SLWR Deepwater Application SCR and SLWR configurations are developed for 8” OD and 10” OD risers in 2,500m water depth.

The configurations and hang-off loads for the two riser types are compared in Table 1. Extreme stresses

along the length of the SCR and SLWR are compared in Figure 1. The total lengths of the risers for the

SLWR configurations are 20-25% more than for the SCR’s. The longer riser along with the buoyancy

for SLWRs means more material and installation costs compared to SCRs. The hang-off tension for the

SLWRs are 20% lower for the nominal offset and 50% lower for extreme offsets compared to the SCR

which is beneficial in terms of turret loading and cost.

The SLWR has lower stresses compared to the SCR, particularly at the hang-off and the TDP. In

severe environments, if the SCR motions at the hang-off and TDP get too high, SLWRs should be

considered. The stress distribution in the SLWR can be varied by changing amount and distribution of

buoyancy.

Riser Distance to TDP

(m) Riser Length

(m)

Buoyacy Section Length

(m)

Distributed Buoyancy

Upthrust (te)

Nominal Hang-Off Load

(te)

8” SCR 1,500 3,100 350 N/A 205

8” SLWR 1,810 3,700 (1)

350 80 160

10” SCR 1,480 3,100 470 N/A 460

10” SLWR 1,770 3,900 (2)

470 165 355

Table 1 – SCR/SLWR Configuration – 2,500m Water Depth


SPE-186122-MS 3

Figure 1 – Maximum von Mises Stress Along SCR/SLWR

The first order fatigue response of a 10” SCR and SLWR are compared in

. The critical location for fatigue response is at the riser hang-off and near the touchdown point on the

seabed for both riser types. The SLWR has better fatigue response compared to the SCR at both these

regions. The fatigue response at the hang-off can be improved, as required, by using a tapered extension

to the FlexJoint, using a thicker wall pipe at the hang-off region or specifying higher quality welds in the

fatigue critical region. Further optimisation of the riser configuration by varying the hang-off angle and

buoyancy distribution can also help improve fatigue response.

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MAXIMUM VON MISESS STRESS ALONG THE RISER

1YR CURRENT, 10YR WAVE

SCR SLWR

Yield Stress Limit : 450MPa

TDP

TDP

Hog bend

1.0E+00

1.0E+02

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Distance along the Riser (m)

FIRST ORDER FATIGUE LIFE ALONG THE RISER

SCR/SLWR CONFIGURATION

SLWR SCR

Minimum Fatigue Life Requirement 25yrs


4 SPE-186122-MS

Figure 2 – First Order Fatigue Life Along SCR/SLWR

SCR and SLWRs can be configured for use with a wide range of environments and vessels. In deeper

water and harsh environments, deep-draft production systems are preferred due to their improved heave

response. In West Africa and Brazil they have been used with FPSOs and in Gulf of Mexico they have

been used with Spars, TLPs and Semi-submersibles. For both an SCR and SLWR, VIV suppression

strakes are typically required to mitigate against VIV fatigue.

SCRs are typically recommended for systems where the environments are less harsh, vessel motions

are lower and where top tension is not a concern. SLWRs are generally preferred over SCRs for deeper

water depth when riser hang-off tensions need to be controlled and in severe environments for improved

strength and fatigue response.

When compared to flexible risers, some of the benefits of SCRs and SLWRs are lower cost,

application in deeper water depth, high pressure and high temperatures application and feasibility of

larger riser diameters.

Application of SLWR in the North Sea Moderate water depths between 400m to 1300m, low pressures and temperatures and harsh

environements are characteric of the North Sea oil and gas developments. Potential SLWR

configurations for pipe sizes ranging from 6” to 30” OD, in 800m water depth are shown in Figure 3.

The horizontal length between the hang-off and the touchdown increases with pipe size as more

buoyancy is needed to accommodate bending in the critical sag bend and hog bend sections. The sag and

hog bend sections are also higher in the water column as the pipe size increases. The buoyancy upthrust

and distribution length selected are based on being able to accommodate vessel motions and offsets

ranges in storm conditions and internal fluid variability.

1.0E+00

1.0E+02

1.0E+04

1.0E+06

1.0E+08

1.0E+10

1.0E+12

1.0E+14

1.0E+16

1.0E+18

-100 400 900 1,400 1,900 2,400 2,900 3,400 3,900

Fa

tig

ue

Lif

e (

ye

ars

)

Distance along the Riser (m)

FIRST ORDER FATIGUE LIFE ALONG THE RISER

SCR/SLWR CONFIGURATION

SLWR SCR

Minimum Fatigue Life Requirement 25yrs


SPE-186122-MS 5

Figure 3 – SLWR Configurations in 800m water Depth

To examinine the potential limits for SLWR risers in the North Sea a 24” SLWR configuration in 850m

water depth in a North Sea environments is developed typical of what may be used for a gas export riser.

In order to accommodate the range of vessel offsets, the selected riser configuration has a large hang-off

angle of 18degrees, 620m of distributed buoyancy giving an upthrust of 420Te and a large horizontal

distance between hang-off and touchdown point of approximately two times the water depth. The

strength response of the 24” riser is shown in Figure 4. The riser configuration is optimised such that, the

maximum stress peaks in the SLWR are similar in the sag, buoyancy and touchdown sections of the

riser. The dynamic effect due to the vessel motions coming from wave loading increases maximum

stresses by 20% while stresses at the riser touchdown remain almost unchanged.

Figure 4 – 24” SLWR Strength Response

The fatigue response along the length of the riser for the 24” SLWR is shown in Figure 5. The critical

locations for fatigue are at the riser touch down point and the top of the riser, below the hang-off point

Plan length 1.25 – 2.5 x water depth

0.0

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0.4

0.6

0.8

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1.2

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0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600

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ield

Str

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Length along the Riser (m)

24" SLWR - 10% Near Offset - 100Yr Return

Static von Mises Stress Dynamic von Mises Stress

Stress Limit

TDP Buoyancy Catenary = 620m

18deg


6 SPE-186122-MS

which are in line with the fatigue response for the 8” and 10” risers discussed above. In order to get the

design fatigue response, high quality welds can be specified at the fatigue critical regions. Other

alternatives discussed above to improve fatigue response should be considered.

Figure 5 – 24” SLWR First Order Fatigue Response

Cost Assessment Fabrication and installation activities are major factors in the costs for SCR and SLWRs and account

for around 50% of total riser delivery costs with material and engineering costs making up the remaining

50%. Hence, installation method plays a key role in the costing. Reel-lay, J-lay and S-lay are available

options for installation. Riser less than 16” OD can generally be reel laid and is typically the cheapest

option for installation. Reel-lay operations however require access to a spoolbase which can be a

challenge in some regions such as West Africa. For larger sizes, if feasible, J-lay is the preferred option,

as it the cheaper than S-lay.

Installation of strakes and buoyancy modules add to installation costs as they have the potential to

become a bottle neck during installation. Hence their design and coverage should be optimised to reduce

installation time and cost. This is less of a concern for S-lay as these activities can be conducted in

parallel with welding. Since welding accounts for most of the installation time and costs, specification of

high quality welds also increase installation costs as they increase the risk of weld cut-outs. Hence high

quality welds should be minimised or avoided offshore.

For flexible risers, about 65% of total delivery costs come from material costs which is significantly

higher than it is for SCRs and SLWRs. In current market conditions, installation costs are generally

lower, which when combined with an optimised design can help bring down overall costs. Overall, when

comparing costs for similar size risers, SCR and SLWR risers are 20-30% cheaper than flexible risers.

The larger available diameters for SLWRs also provide even further potential savings compared to

multiple smaller flexible risers offering benefits for applications such as gas export. Hybrid risers are

roughly twice the cost of SCR/SLWR and are not considered to be feasible in the current low price oil

market.

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0 300 600 900 1200 1500 1800 2100 2400 2700

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(m

)

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Fa

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e (

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Length along Riser (m)

24" SLWR - First Order Fatigue Life Along The Riser

DNV C1 Curve DNV E Curve

Target Fatigue Life of 200 Years

Buoyancy Catenary = 620m

18deg


SPE-186122-MS 7

Conclusions The majority of fields in the North Sea and offshore West Africa in moderate to deep water have

been developed using flexible risers. Low pressures and temperatures combined with good strength and

fatigue performance make flexible risers a very attractive solution. However, for deepwater applications,

flexible risers are limited by qualified size, cost and reliability. SCRs and SLWRs offer a potential low

cost alternative to flexible risers and have track record for deepwater high pressure and temperature

applications.

The suspended lengths for the SLWRs are more than those for the SCR’sleading to more material and

installation costs compared to SCRs. The hang-off tensions for the SLWRs are lower than for SCRs,

which is beneficial in terms of turret loading and cost. SLWRs generally have better strength response

compared to SCRs, particularly at the hang-off and the TDP, which combined with lower top tension,

makes them particularly suitable for production systems with high vessel motions and harsh

envrionments in deep water.

In typical North Sea conditions, it is demonstrated that large diameter SLWRs are feasibile in terms of

strength and fatigue response. Some technology challenges exist in terms of fatigue response and

hydrotest, which can be overcome through use of qualified solutions.

Fabrication and installation costs account for about half of the SCR/SLWR delivery costs. Hence,

design and coverage of strakes and buoyancy modules should be optimised to reduce installation time

and cost. Overall, SLWRs are expected to be cheaper than equivalent flexible risers and hybrid risers. A

further more significant cost benefit over flexible and hybrid risers comes from being able to use large

diameter SLWRs instead of multiple smaller risers leading to even more significant cost savings.

References [1] American Petroleum Institute – “Recommended Practice for Design of Risers for Floating Production Systems and

TLPs”. API-RP-2RD, 1st Edition with ERRATA; June 2009.

[2] Det Norske Veritas (DNV) - “Fatigue Strength Analysis of Offshore Steel Structures”. DNV-GL-RP-0005, June

2014.

[3] American Petroleum Institute – “Specification for Line Pipe”. API Specification 5L, Forty-Fifth Edition with Errata;

April 2015.

[4] Izquierdo, A., Aggarwal, R., Quintanilla, H., García, E., López, V., Richard, G., Marques, E., Dell´Erba, E., and Di

Vito, L., “Qualification of Weldable X65 Grade Riser Sections with Upset Ends to Improve Fatigue Performance of

Deepwater Steel Catenary Risers”, ISOPE, 2008

Definitions API American Petroleum Institute

CoG Centre of Gravity

DNV Det Norske Veritas

FOF First Order Fatigue

FPSO Floating Production Storage and Offloading Vessel

Hmax Theoretical Maximum Wave Height

Hs Significant Wave Height

MSL Mean Sea Level

SCF Stress Concentration Factor

SCR Steel Catenary Riser


8 SPE-186122-MS

SLWR Steel Lazy Wave Riser(s)

TDP Touch Down Point

TDZ Touch Down Zone

Tp Spectral Peak Period

VIV Vortex Induced Vibration


spe-186122-ms application of steel lazy wave riser

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