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7/30/2019 OMAE2012-83919
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FATIGUE LIFE ANALYSIS FOR A STEEL CATENARY RISER
IN ULTRA-DEEP WATERS
Marcos V. RodriguesSURF & Pipeline Section
Caroline FerrazSURF & Pipeline Section
Danilo Machado L. da SilvaSURF & Pipeline Section
Bruna NabucoAdvisory Offshore and Ships
Det Norske Veritas - DNV, Rio de Janeiro, Brazil
ABSTRACTWith new discoveries in the Brazilian Pre-Salt area, the oil
industry is facing huge challenges for exploration in ultra-deep
waters. The riser system, to be used for the oil transportation
from seabed to the production unit, is one of them. The
definition of riser configurations for ultra-deep waters is a real
challenge. Problems have being identified for flexible risers,
hybrid risers and steel catenary risers (SCR) configurations to
comply with rules requirements and criteria in water depths of
2000m.
The objective of this work is to present a study on the
fatigue behavior of a Steel Catenary Riser in 1800m of water
depth. One of the main challenges for SCRs in ultra-deep watersis the fatigue, due to platform 1st order motions, at the touch
down zone (TDZ).
A case study is presented for a Steel Catenary Riser
connected to a semi-submersible platform. The influence of
some design and analysis parameters is studied in order to
evaluate their impact on the SCR fatigue life. The main
parameters to be evaluated in this work are: The mesh
refinement, in the global analysis, at the Touch Down Zone; The
internal fluid density variation along the riser, and; The 1st
order platform motions applied to the top of riser; In addition to
the results of this paper, some highlights are presented for SCR
analysis in similar conditions.
INTRODUCTIONNowadays, the global trend is an increasing need for oil
and gas. As the easily recoverable fields have been already
developed, the trend in the offshore oil and gas industry is going
deeper into the more challenging outlook. The Brazilian pre-salt
reservoirs are a typical example with ultra-deep waters and
highly corrosive fluid requiring highly tailor-made and
optimized design solutions. This unprecedented need for energy
demand, driving the oil & gas industry constantly into deeper
waters and more hostile environments in search for recoverable
resources, generates a need for new pipelines and riser systems,
and the challenge for engineers have always been to come up
with methods and equipment to meet such needs [1].
The discovery of the Brazilian pre-salt fields has brought
many challenges. The oil found in this area is at depths
exceeding 5000m, under an extensive layer of salt. Reaching
this oil and bring it to the platforms are tasks that require
knowledge and technology.
The challenge is to finding solutions, through the processof being cost effective, overcoming specific technical
challenges, which includes: flow assurance and insulation; high
top tensions for fully suspended systems; fatigue and touch
down uncertainties in suspended systems; high costs for hybrid
riser and flexible systems, etc.
Alternative solutions have been proposed by industry for
pre-salt scenarios, including, riser tower and BSR (Buoyancy
Supported Risers). Tremendous efforts have been expended in
the determination of the global, and local, response of these
systems, in order to increase the confidence in optimum
approaches for the specific applications.
This work presents a discussion regarding to a proposed
riser system solution, based on steel catenary risers (SCRs), for
a Brazilian pre-salt scenario. There are many failure modes for
a SCR to fail; however two of them are expected to dictate the
design as greater water depths are expected: (1) the riser local
buckling capacity due to combined axial load, external pressure
and bending; and (2) the riser fatigue.
Proceedings of the ASME 2012 31st International Conference on Ocean, Offshore and Arctic EngineeringOMAE2012
July 1-6, 2012, Rio de Janeiro, Brazi
OMAE2012-8
1 Copyright 2012 by ASME
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The focus here is to present issues related to the SCR
fatigue analysis. The influence of some design and analysis
parameters is studied to evaluate their impact on the SCR
fatigue life.
STEEL CATENARY RISERSThe steel catenary riser is a cost-effective alternative for oil
and gas export and for water injection lines on deepwater fields,
where the large diameter flexible risers present technical andeconomic limitations. Catenary riser is a free-hanging riser with
no intermediate buoys or floating devices [2].
There are several factors that influence the SCR design.
Some of them are the following:
Metocean conditions;
Host vessel offsets and motions, and;
Structural limitations - burst, collapse, buckling, post-buckling;
Construction issues - manufacturability, tolerances,weld procedures, inspection;
Installation method - tensioning capacity of availablevessels;
Operating philosophy - transportation strategy,pigging, corrosion, inspection;
Well characteristics - pressure, temperature, flowrate,heat loss, slugging;
The producing well characteristics determine variations in
line contents and properties over time, which should be defined
for operation in normal and abnormal shutdown conditions. The
designer should take into account the full range of contents for
all stages of installation, commissioning and operation.
Regarding to fatigue issues, the seafloor touchdown area is
a critical area for steel catenary risers. Soil properties, mesh size
and mean floater position are important for prediction of fatigue
damage. Time domain analyses are generally recommended
together with sensitivity studies to support rational conservativeassumptions regarding soil properties. The adequacy of the
mesh applied in the touchdown area should also be confirmed
by sensitivity studies, as it will be presented further in this
work.
FATIGUE ANALYSISFatigue damage verification is an important issue in riser
design, demanding a high number of loading cases to be
analyzed. The random time domain nonlinear analysis is
considered an attractive and reliable tool for fatigue analysis, as
non-linearities are properly modeled and the random behavior
of environmental loading is considered.
The aim of fatigue design is to ensure that the risers have
adequate fatigue life. Calculated fatigue lives also form the
basis for efficient inspection programmes during fabrication and
the operational life of the risers.
Fatigue damage in risers comes from three main sources:
First-order wave loading and associated vessel motions
Second-order/low frequency platform motions
Riser VIV due to current or vessel heave
FATIGUE DAMAGE ASSESSMENT PROCEDURE
Normally, the fatigue assessment methods based on SN-
curves are used during metallic riser fatigue life evaluation. A
typical sequence in fatigue design of a riser is shown in Table 1,
a detailed description can be found in References [3] and [4].
Table 1 Summary of a typical fatigue assessment procedureTask Comment
Define fatigue
loading.
Based on operating limitations including
Wave Frequency, Low Frequency and
possible Vortex Induced Vibration (VIV)
load effects.
Identify locations to
be assessed.
Structural discontinuities, joints (girth pipe
welds, connectors, bolts), anode
attachment welds, repairs, etc.
Global riser fatigue
analysis.
Calculate short-term nominal stress range
distribution at each identified location.
Local joint stress
analysis.
Determination of the hot-spot Stress
Concentration Factors (SCF) from
parametric equations or detailed finite
element analysis.Identify fatigue
strength data.
SN-curve depends on environment,
construction detail and fabrication among
others.
Identify thickness
correction factor
Apply thickness correction factor to
compute resulting fatigue stresses
Fatigue analyses Calculate accumulated fatigue damage
from weighted short-term fatigue damage.
Further actions if
too short fatigue
life.
Improve fatigue capacity using:
- more refined stress analysis
- fracture mechanics analysis.
- change detail geometry
- change system design.
- weld profiling or grinding
- improved inspection/replacementprogramme
GLOBAL FATIGUE ANALYSIS PROCEDURES
The basis for fatigue damage calculations is global load
effect analyses to establish the stress cycle distributions in a
number of stationary short-term environmental conditions. The
general principles for selection of analysis methodology and
verification of simulation model are outlined in DNV-OS-F201.
The short-term fatigue conditions should be selected
carefully to give an adequate representation of the stress cycles
for the lifetime of the riser system. The selection must be based
on a thorough physical knowledge regarding static- and
dynamic behaviour of the riser system with special attention to
FE modeling, hydrodynamic loading, resonance dynamics and
floater motion characteristics. Sensitivity studies should be
performed to support rational conservative assumptions
regarding identified uncertain parameters (e.g. soil properties
for fatigue analysis in the touch-down area of SCRs)
2 Copyright 2012 by ASME
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Fatigue analysis will normally involve global load effect
analyses in a number of low- to moderate sea-states. This is
because the main contribution to the total fatigue damage in
most cases comes from low- to moderate sea-states with high
probability of occurrence rather than a few extreme sea-states.
CASE STUDYFatigue at the TDZ can be a critical parameter for a typical
Steel Catenary Riser in ultra-deep waters.A case study is presented for a typical Steel Catenary Riser
configuration connected to a semi-submersible platform in
1800m water depth. The main idea is to evaluate the impact of
some parameters influence in the SCR fatigue life due to semi-
submersible 1st order motions.
The impact of the following parameters will be evaluated:
Mesh refinement at TDZ;
1st order platform motions applied to the top of riser RAOs phases;
Internal fluid density variation along the riser.Two different types of SCRs were considered in this study,
a 10.75 riser and a 8.625 riser. The main data for both risers
are presented in Table 2 and 3 below:
Table 2 10.75 type I SCR data.Parameter Value Unit
Nominal top angle 20 Degrees
Wall Thickness 0.0226 m
Yield Stress 450000 kN/m2
Modulus of Elasticity of steel 207000 MPa
Poisson Coefficient 0.3 -
Density of steel 77 kN/m3
Drag coefficient 1.2 -
Table 3 8.625type II SCR data.
Parameter Value Unit
Nominal top angle 20 Degrees
Wall Thickness 0.0242 m
Yield Stress 450000 kN/m2
Modulus of Elasticity of steel 207000 MPa
Poisson Coefficient 0.3 -
Density of steel 77 kN/m3
Drag coefficient 1.2 -
Figure 1 presents a typical SCR configuration and the mesh
refinement considered in the Base cases studied are presented in
Tables 4 and 5. Note that the mesh is more refined at Hang off
region and Touch Down Point area, which are the most critical
region for the fatigue evaluation.
Figure 1 Typical SCR configuration.
Table 4 10.75type I SCR Mesh.Parameter Refinement (m)
Flexjoint
Taper Stress Joint
0.70
0.15
Transition 2.00
Suspended length 5.00
Transition 3.00TDZ 2.00
Seabed region 3.00
Table 5 8.625type II SCR Mesh.Parameter Refinement (m)
Flexjoint
Taper Stress Joint
0.55
0.15
Transition 2.00
Suspended length 5.00
Transition 3.00
TDZ 2.00
Seabed region 3.00
The DNV D and E SN curves and SCFs were used in the
studied cases for both types of risers. Half of corrosion
allowance was considered for carbon steel sections as
recommended by DNV OS F201.
Irregular sea states were considered as representative to the
fatigue life prediction and for comparison among different
parameters studied in this work. The semi-submersible 2nd order
motions were also considered in the performed dynamic
simulations. A 3-hours simulation length was also adopted and
the non-linear dynamic program RIFLEX [5] was used for
simulation.
The fatigue for the base case was evaluated and the fatigue
lives along the risers are presented as follows in figures 2 and 3,
for inner and outer wall. X-coordinate zero is the top of riser.
3 Copyright 2012 by ASME
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Figure 2 Fatigue life along type I SCR.
Figure 3 Fatigue life along type II SCR
The first parameter evaluated was the mesh refinement at
TDZ. The Type I SCR was taken for this evaluation. Theoriginal mesh size at TDZ of 2m was refined to 1m. The
simulation time has increased around 50% with this variation.
The impact of this variation in the fatigue life is presented in
Table 6 below.
Table 6 Fatigue life (years) Type I SCR
Difference wrt Base Case mesh at TDZ %
Inner wall 12.9%
Outer wall 12.8%
The second parameter evaluated was the 1st order platform
motions applied to the top of riser. Basically it was modified to
the phases RAO motions applied to the top of both risers. The
impact of this variation is presented in Tables 7 and 8 below.
Table 7 Fatigue life (years) Type II SCRDifference wrt Base Case motions %
Inner wall -1.5%
Outer wall -3.5%
Table 8 Fatigue life (years) Type I SCR
Difference wrt Base Case motions %
Inner wall -0.8%
Outer wall -0.7%
The third and last parameter evaluated was the impact of
internal fluid density variation along the type II riser together
with mesh refinement variation. It was adopted a mesh of 1m at
TDZ together with the internal fluid density variation. In the
Base Case was considered only one internal fluid density along
the riser length. Now it is considered 3 different internal fluid
densities along the riser length varying with the water depth.
The impact in fatigue life is presented in Table 9 below.
Table 9 Fatigue life (years) Type II SCR
Difference wrt Base Case Mesh +
Internal fluid density %Inner wall 353.9%
Outer wall 483.1%
DISCUSSIONSThe first parameter evaluated was the mesh refinement. It
was observed an improvement of fatigue life around 12% when
the mesh at TDZ was refined from 2m to 1m.
The second parameter evaluated was the 1st order platform
motions applied to the top of riser. A small decrease of fatigue
life around 1% to 3% was found for both risers.
The major impact of sensitivity analysis in the fatigue life
was found for third parameter variation, when the mesh was
refined together with the internal fluid density variation along
the Type II riser.To be sure that this great difference was provided by the
internal fluid density variation additional analyses should be
performed without the mesh refinement variation. This study
will be done in the next step of this work.
But considering that the internal fluid variation presents
this impact in fatigue life, this data must be evaluated very
carefully in terms of fatigue since the density can present a
more refined variation along the riser and a variation along the
life of the SCR.
FINAL REMARKSThis work intended to illustrate, using actual design data,
the fatigue sensitivity to variation of some analysis parameters.According to the findings above some aspects can be
considered. The first is that the mesh refinement can have an
important impact in fatigue life. The second is that the motion
phases did not present significant influence in the fatigue life.
And finally, the internal fluid density variation along the riser
can have significant impact in the fatigue life. This should be
4 Copyright 2012 by ASME
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confirmed with more detailed analysis, but it is an important
parameter that must be very well defined in the basis of the
project taking into account the internal density fluid variation
along the riser and variations during the lifetime of the project
as well.
Only few parameters were chosen for the study presented in
this paper, but other parameters, e.g. the sea floor parameter, are
also very important in the fatigue evaluation.
This work presented only a simple demonstration of howthe fatigue life calculation can be impacted by uncertainties in
analysis parameters and there are many different uncertainties
associated with fatigue life predictions of offshore structures,
which includes issues associated with the following parameters:
Load calculation:o Wave heights and periods / Distribution of waves /
Wave theories;
o Hydrodynamic coefficients;o Marine growth;
Stress calculation / Structural analysis;
SN-data:o Natural scatter;
o Corrosion protection;o Selection of SN-curve;
Fabrication tolerances.
Cumulative damage hypothesis.
Therefore, the study presented here just confirms the
fatigue life calculations can be significantly impacted by data
and assumptions considered in the fatigue analysis.
REFERENCES[1] Silva, D.M.L., Souza Jr., H.A., D'Angelo Aguiar, L.A.,
Souza, A.P.F., Challenges on Designing Pipelines for the
Brazilian Pre-Salt Scenarios,Rio Pipeline Conference &
Exposition 2011, Rio de Janeiro, Brazil, 2011.[2] Chakrabarti, S.K., Handbook of Offshore Engineering,
Vol.2, Elsevier, 2006.
[3] DNV-OS-F201, Dynamic Risers, Det Norske Veritas,
October 2010.
[4] DNV-OS-F204, Riser Fatigue, Det Norske Veritas,
October 2010.
[5] RIFLEX, Theory Manual. Version 3.4, Marintek, 2005.
[6] DNV-RP-C203, Fatigue Design of offshore Steel
Structures,Det Norske Veritas, October 2011.
5 Copyright 2012 by ASME