spe-186122-ms application of steel lazy wave riser
Post on 20-May-2022
4 Views
Preview:
TRANSCRIPT
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.
This template is provided to give authors a basic shell for preparing your manuscript for submittal to a meeting or event. Styles have been included to give you a basic idea of how your finalized paper will look before it is published. All manuscripts submitted will be extracted from this template and tagged into an XML format; standardized styles and fonts will be used when laying out the final manuscript. Links will be added to your manuscript for references, tables, and equations. Figures and tables should be placed directly after the first paragraph they are mentioned in. The content of your paper WILL NOT be changed.
Learn more at www.2hoffshore.com
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
Learn more at www.2hoffshore.com
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.
0
100
200
300
400
500
600
-100 400 900 1400 1900 2400 2900 3400 3900 4400
vo
n M
ise
s S
tre
ss (
MP
a)
Arc Length (m)
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
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
Learn more at www.2hoffshore.com
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
Learn more at www.2hoffshore.com
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
0.2
0.4
0.6
0.8
1.0
1.2
0
150
300
450
600
750
900
0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600
Ele
va
tio
n f
rom
Mu
dli
ne
(m
)
vo
n M
ise
s S
tre
ss/Y
ield
Str
ess
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
Learn more at www.2hoffshore.com
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.
0
300
600
900
10
100
1000
10000
0 300 600 900 1200 1500 1800 2100 2400 2700
Ele
va
tio
n f
rom
Mu
dli
ne
(m
)
Un
facto
red
Fa
tig
ue
Lif
e (
Ye
ars
)
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
Learn more at www.2hoffshore.com
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
Learn more at www.2hoffshore.com
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
Learn more at www.2hoffshore.com
top related