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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

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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)

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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.

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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

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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.

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