- Email: [email protected]
Chapter 4 Limit-state based strength design
63
Chapter 4 Limit-state based Strength Design 4.1
Introduction
This chapter presents limit-state strength criteria for pipeline design. The limit-state based strength design became crucially important when usage factors in wallthickness design are raised from those given by traditional design codes. The discussion of a limit state design approach in this chapter is based on new Guides/Rules and a review of recent design projects and Joint Industry Projects (SUPERB, DEEPIPE, etc.). See Bai and Damsleth (1997). The limit state checks conducted in the strength design are: 9
Out of roundness for serviceability
9
Bursting due to intemal pressure, longitudinal force and bending
9
Buckling/collapse due to pressure, longitudinal force and bending
9
Fracture of welds due to bending/tension
9
Low-cycle fatigue due to shutdowns
9
Ratcheting due to reeling and shutdowns
9
Accumulated plastic strain
The allowable strains, equivalent stresses and bending moments are determined for the following operating scenarios. 9
Empty condition
9
Water filled condition
9
Pressure test condition
9
Operational conditions
64
Chapter 4
The strength criteria will be applicable for the following design situations" 9
Pipeline in-place behavior
9
Trawl pullover response
9
Free-spanning pipelines
9
Pipeline dynamic free-span
The pipeline route is divided into two zones: Zone 1 is the zone where no frequent human activity is anticipated along the pipeline route. For operating phases Zone 1 is classified as "Normal Safety Class" Zone 2 is the parts of the pipeline/riser in the near platform (manned) zone or in areas with frequent human activity. The extent of zone 2 is 500 m from the maximum facility excursion or determined based on risk analyses. For operating phases Zone 2 is classified as "High Safety Class" For temporary (construction) phases both zones are classified as "Low Safety Class", when the pipeline does not contain any hydrocarbons.
4.2
Out of Roundness Serviceability Limit
The pipeline out of roundness is related to the maximum and minimum pipe diameters (Dmax and Dmin) measured from different positions around the sectional circumference and is defined according to the following equation: f0 =
Dmax - Drm D
(4.1)
The out of roundness during the fabrication process is not to be more than 1.5%. The out of roundness of the pipe may increase where the pipe is subject to reverse bending and the effect of this on subsequent straining is to be considered. For a typical pipeline the following scenarios will influence the out of roundness:
-
The out of roundness may increase during the installation process where the pipe is subject to reverse inelastic bending;
-
Cyclic bending may occur as a consequence of shutdowns during operation if global buckling is allowed to relieve temperature and pressure induced compressive forces.
Out of roundness due to point loads is to be checked. Critical point loads may arise at freespan shoulders, artificial supports and support settlement. The accumulative out of roundness through life cycle is not to exceed 4%. This out of roundness requirement may be relaxed if:
Limit-state based Strength Design -
The effect of out-of-roundness on moment capacity and strain criteria is included;
-
The pigging requirements and repair systems, are met, and;
-
Cyclic loads induced out-of-roundness have been considered.
65
Finite element analysis may be performed to calculate the increase in out of roundness during the life cycle of a pipeline. The analysis is to include fabrication tolerances and all loads applied through the pipelines life-cycle such as point loads, bending against a surface, axial load and repeated pressure, temperature and bending cycles.
4.3
Bursting
4.3.1
Hoop Stress vs. Equivalent Stress Criteria
An analytical study by Stewart (1994) and a finite element analysis have demonstrated that for a pipe under combined internal pressure and bending: 9
If a pipeline section is in a displacement controlled situation, then a hoop stress criterion provides a good control of bursting.
9
If a pipeline section is in a load controlled situation then an equivalent stress criterion may be applied to ensure sufficient burst strength for pipes under combined internal pressure and axial loads (the influence of bending is yet to be investigated).
For pipelines in operation, it is generally conservative to apply the equivalent stress criteria to control bursting since the dominating load is internal pressure combined with bending. The bursting failure mode is governed by the tensile hoop stress. To ensure structural strength against bursting, the hoop stress is to fulfil the following conditions: yielding limit state, (Yh ~ rls.SMYS, where rls is usage factor for SMYS (Specified Minimum Yield Stress) bursting limit state, C~h < rlu.SMTS, where flu is usage factor for SMTS (Specified Minimum Tensile Stress) For load-controlled situations, special consideration shall be made to bursting. It has been chosen to use an equivalent and longitudinal stress criterion according to the results from the analytical study and the finite element analysis.
4.3.2
Bursting Strength Criteria for Pipeline
The hoop stress due to pressure containment is not to exceed the following criteria:
(Pi - Pe ) D - t 2t
< ~TS min[SMYS(T),O.S7 * SMTS(T)]
(4.2)
Chapter 4
66
where:
pi =
internal pressure
pe =
external pressure
D=
nominal outside diameter of pipe
t=
m i n i m u m wall thickness
S M Y S (T) = specified m i n i m u m yield stress at temperature T S M T S (T) = specified m i n i m u m tensile stress at temperature T Temperature derating is to be accounted for at elevated temperatures (above 50~ The following usage factors in Table 4.1 apply to the hoop stress equation: Table 4.1 Usage Factors for Hoop Stress Criteria.
Material
Usage
Quality
Factor
Low
Normal
High
Class B, C 1)
rls
0.85
0.80
0.70
Class A2)
~is
0.83
0.77
0.67
Safety Class
Class ,
,
Note: 1) In order to apply these higher usage factors, the following additional requirements with respect to linepipe manufacturing must be fulfilled: SMYS < (mean - 2*Standard Deviations) of yield stress; SMTS < (mean - 3*Standard Deviations) of tensile stress; tfab=2* Standard Deviations of the wall-thickness.
2) After Table 2 in Section 6.4 oflSO 13623 (1997).
Stress Criteria
For internal-over pressure situations, the allowable equivalent stress and allowable longitudinal stress are rI*SMYS(T) and the usage factor rl given in Table 4.2 is after Table 3 in Section 6.4 of ISO13623 (1997). Table 4.2 Equivalent stress design factor. Load Combinations
design factor rl
Construction and environmental loads
1.0
Functional and environmental loads
0.9
Functional, environmental and accidental loads
1.0
However, m o m e n t criteria, in Chapter 4.4 are considered to have better accuracy for strength design.
67
Limit-state based Strength Design
4.4
Local Buckling/Collapse
This section is based on Hauch and Bai (1999).
Local Buckling For pipelines subjected to combined pressure, longitudinal force and bending, local buckling may occur. The failure mode may be yielding of the cross section or buckling on the compressive side of the pipe. The criteria given in this guideline may be used to calculate the maximum allowable bending moment for a given scenario. It shall be noted that the maximum allowable bending moment given in this guideline does not take fracture into account and that fracture criteria therefore may reduce the bending capacity of the pipe. This particularly applies for high-tension/high-pressure load conditions.
Load Versus Displacement Controlled Situations The local buckling check can be separated into a check for load controlled situations (bending moment) and one for displacement controlled situations (strain level). Due to the relation between applied bending moment and maximum strain in a pipe, a higher allowable strength for a given target safety level can be achieved by using a strain-based criterion than the bending moment criterion. Consequently the bending moment criterion can, conservatively be used for both load and displacement controlled situations. In this guideline only the bending moment criterion is given.
Local Buckling and Accumulated Out-of-Roundness Increased out-of-roundness due to installation and cyclic operating loads may aggravate local buckling and is to be considered. It is recommended that out-of-roundness due to through life loads be simulated using finite element analysis.
Maximum Allowable Bending Moment The allowable bending moment for local buckling under load controlled situations can be expressed as: YcF
P
=rl~M J l - ( 1 - a : ~ ] P co 2 o~F,-~0~p,
...... ,~,,
,~
~v
~"~' J
~~"~,)J
.ll-O-~--e-~
where: Mallowable---- Allowable bending moment
= Plastic moment Pl
= Limit pressure
p
= Pressure acting on the pipe
El
= Limit longitudinal force
F
= Longitudinal force acting on the pipe
a
= Correction factor
(4.3)
68
9
Chapter 4
Yc
= Condition load factor
~R
= Strength usage factor
Correction Factor: o~ = 0.25 P_L for external overpressure
(4.4)
a = 0.25 P___Lfor internal overpressure
(4.5)
F,
F~
If possible, the correction factor should be verified by finite element analyses.
9
Plastic (Limit) Moment:
The limit m o m e n t m a y be given as"
0001 / 9
t
(4.6)
Limit Longitudinal Force for Compression and Tension"
The limit longitudinal force m a y be estimated as: Ft = 0.5-(SMYS+ SMTS ). A
9
(4.7)
Limit Pressure for External Overpressure Condition:
The limit external pressure 'p~' is to be calculated based on"
(
~
p3 -PetP~ - P~ +papPfo'7 p` + pe'p~ =0
(4.8)
P el= (i_v 2)'
(4.9)
where:
pp= rlI.,SMYS
2t 1) D
f0
= Initial out-of-roundness 2~, (Dmax-Dmin)/D
E
= Y o u n g ' s Module
a9
= Poisson's ratio
(4.10)
Guidance note: 1.
llfab is 0.925 for pipes fabricated by the UO process, 0.85 for pipes fabricated by the UOE process and 1 for
seamless or annealed pipes. 2.
Out-of-roundness caused during the construction phase is to be included, but not flattening due to external water pressure or bending in as-laid position.
Limit-state based Strength Design 9
69
Limit Pressure for Internal Overpressure Condition:
The limit pressure will be equal to the bursting pressure given by: (4.11)
Pt = 0.5(SMTS + S M Y S ) 2~ Z iJ
where: SMYS = Specified Minimum Yield Strength in hoop direction SMTS = Specified Minimum Tensile Strength in hoop direction
9
Load and Usage Factors:
Load factor 7c and usage factor 'qR are listed in Table 4.3. Table 4.3 Load and usage factors. Safety Classes Safety factors ~
~
Low
Normal
High
.
Uneven seabed
1.07
1.07
1.07
Pressure test
0.93
0.93
0.93
Stiff supported
0.82
0.82
0.82
Otherwise
1.00
1.00
1.00
rlRP
Pressure
0.95
0.93
0.90
rlRF
Longitudinal force
0.90
0.85
0.80
fIRM
Moment
0.80
0.73
0.65
Yc
Guidance notes: -
Load Condition Factors may be combined e.g. Load Condition Factor for pressure test of pipelines resting on uneven seabed, 1.07x0.93 = 1.00 Safety class is low for temporary phases. For the operating phase, safety class is normal and high for area classified as zone 1 and zone 2 respectively.
For displacement-controlled situations the following strain capacity check is given to ensure structural strength against local buckling: 0.8
~/ F " ]/c " ~/sncf " ]/ O _"~"F,c "~" Y E " ~ E,c
~t" P e _< 1
eM,c
Pc
Y~
YR
(4.12)
where: ~F,c~---
characteristic functional longitudinal strain
EE,c----
characteristic environmental longitudinal strain
EM,c =
characteristic buckling strain capacity
Ysncf=
strain concentration factor accounting for increased strain in the field joints due to coating stiffness discontinuities
70
Chapter 4
~F----
functional load factor
YE---
environmental load factor
~D-"
dynamic load factor
~C-"
condition load factor
yR= ~=
strength resistance factor strain capacity resistance factor
Comparing to the equation established in DNV'96, Section 5 C305, two additional safety factors (Ysncf and YD) are included. These additional safety factors are:
9
~tsncffor Strain Concentration Factor (SNCF).
9
~/D to take account for dynamic amplifications during a snap-through dynamic buckling (NystrCm et al 1997).
4.5
Fracture
4.5.1
PD6493 Assessment
Fracture of the welds due to a tensile strain is normally evaluated in accordance with PD 6493 (1991). This assessment method uses a curve (Failure Assessment Diagram) which combines the two potential failure modes: brittle fracture and plastic collapse. Maximum weld flaws, described in Statoil R-SF-260, Pipeline Welding Specification, are to be used as the basic input for the calculations. The flaw has been assumed as maximum allowable defect due to lack of fusion between passes. The defects and material are assumed as below: Type Depth (a) Length (2c) CTOD Material
: Surface flaw due to lack of fusion : 3 mm :50 mm : 0.20 mm (at operating temperature) : As for Parent material
Surface flaw is chosen as the worst case scenario from acceptable flaws specified in the weld specifications. The partial safety factors recommended by PD 6493, are as below:
9
For levels 2 and 3, no additional safety factors are required where worst case estimates are taken for stress level, flaw size and toughness, and all partial coefficients should be taken as unity. (Appendix A.1 of PD6493).
Limit-state based Strength Design
71
PD6493 FAD (Failure Assessment Diagram) gives critical stress for the given defect and material. It is necessary to convert the critical stress to critical strain and for this the RambergOsgood relationship is used as defined below: o3 e=--{l+ (o-),-1} E 7 o-0.7
(4.13)
where: o-0.7 = 430 MPa for X65 at 20~ n
= 26 for X65 at 20~
The allowable strain criterion used in this report is conservative due to: The stress-strain curves used in converting stress to strain are based on the lowest yield stress and lowest ultimate stress. 9
PD6493 has been derived for load-controlled situations, but is here applied to both load and displacement-controlled situations.
9
The flow stress is in PD6493 defined as the average of yield and tensile stress.
Corrosion in girth welds can significantly reduce the critical tensile strain of the girth welds if a flaw is assumed to be in the surface of the corroded weld. Fracture mechanics assessment of existing pipelines has shown that the critical strain can be between 0.1% for heavily corroded pipes and 0.5% for pipes with shallow corrosion defects. However, we shall not assume the combination of corrosion and cracks in the girth welds, although some pitting could occur in the HAZ (Heat Affect Zone). If corrosion takes place, it will occur over a certain number of years after entering into service, when the maximum strain load became lower due to reduced operating pressure and temperature, and "shakedown" of peak stress/strain levels in a number of shut downs. In the technical report "Update of laying criteria for pipelines", Denys and Lefevre, it is stated that the failure of welds under displacement controlled situations is highly dependent on the weld matching (in particular), and on the ratio of yield to tensile strength. They gave an allowable strain of 0.61• = 0.50 % (a safety factor of 1.5 is applied to the critical strain) for defect length t/1.2 and depth 3 mm, assuming that the weld is matched and the ratio of yield to tensile strength is 0.87. They also reported that the results are very sensitive to weld matching (over-matching will increase the allowable strain considerably, and undermatching will reduce it). The Dutch code NEN 3650 states that, normally, a tensile strain of 0.5% will not pose any problems for material and welding in accordance with their specifications. If it can be demonstrated that the ductility of the material is greater, higher strains can be tolerated accordingly.
Chapter 4
72
It was stated by Canadian Standards Associations that the pipeline industry has used a longitudinal tensile strain limit of 0.5%. This limit prevents fracture initiation and plastic collapse from Circumferential weld flaws small enough to be accepted by the specification or that may have been missed by inspection. Zimmerman et al. (1992) and Price (1990) reports that the 0.5% tensile strain limit is a subjective limitation, chosen to coincide with the API yield strength specifications and does not reflect an objective failure limit. The Troll Phase I project applied an allowable strain level of 0.4 % for a 36" gas export line, which was approved by NPD, (Koets and Guijt (1996)).
4.5.2
Plastic Collapse Assessment
It has been observed that all fracture mechanics calculations based on PD6493 lead to Sr=l. Where Sr is defined as:
Sr =
cr o"
(4.14)
f
The flow stress of is according to PD6493 defined as the average of yield stress ~y and ultimate tensile stress ~u of the weld material. For a flat plate with surface flaw under tension, equations for the net section stress CYnfrom PD6493 lead to: O'n
--
~ cr
tCl
(4.15)
where: ~cr
: critical stress
a
: defect depth
c
: half-width of the defect
t
: wall-thickness of the plate
The applied PD6493 assessment criteria can then be re-expressed as: O'cr=
I 1 - (t/ / {1 + t}l O'Y"~" O'u 2
(4.16)
The above PD6493 plastic collapse equation may be valid provided that brittle fracture is not a relevant failure mode, e.g.: 9
The defect depth (a) is less that 3 mm and the length (2c) is less than t (or 25 mm)
9
The material CTOD is more than e.g. 0.2 mm at operating temperature
Limit-state based Strength Design
73
The PD6493 plastic collapse equation can be applied to calculate allowable defect depth (a) and length (2c) for a given critical stress. In addition to PD6493 plastic collapse equation, the following plastic collapse equations are available from literature, see Bai (1993)and Denys (1992): 9
CEGB R6 approach (A.G. Miller's equation)
9
Willoughby's equation
9
the net section yielding collapse solution
9
the CSA Z184 equation
9
Denys's equation
Comparing with other available equations, PD6493 seems to give conservative and reasonable predictions. The PD6493 suggests that the safety factor for CYcrin Eq. (4.16) is 1.1. The readers are suggested to define safety factors based on the structural reliability principles described in Chapters 13 through 15. Chen et al. (2000) discussed formulae for plastic collapse and fracture of pipe with girth weld defects. A study of fracture criteria, conducted as part of the DEEPIPE JIP, was summarized by Igland et al. (2000).
4.6 4.6.1
Fatigue General
Pipeline components such as risers, unsupported free spans, welds, J-lay collars, buckle arrestors, riser touchdown points and flex-joints, should be assessed for fatigue. Potential cyclic loading that can cause fatigue damage includes vortex-induced-vibrations (VIV), waveinduced hydrodynamic loads, platform movements and cyclic pressure and thermal expansion loads. The fatigue life of the component is defined as the time it takes to develop a throughwall-thickness crack of the component. For high cycle fatigue assessment, fatigue strength is to be calculated based on laboratory tests (S-N curves) or fracture mechanics. If no detailed information is available, the F2 curve may be applied as the S-N curves for pipeline high cycle fatigue. Low cycle fatigue of girth welds may be checked based on Ae-N curves. The fracture mechanics approach calculates the crack growth using Paris' equation and final fracture using a recognized failure assessment diagrams (see Chapter 4.5). It may be applied to develop cracked S-N curves that are for pipes containing initial defects. If a fracture mechanics crack growth analysis is employed, the design fatigue life should be at least 10 times the service life for all components. The initial flaw size should be the maximum acceptable flaw specified for the non-destructive testing during pipe welding in question.
Chapter 4
74
4.6.2
Fatigue Assessment based on S-N Curves
The S-N curves to be used for fatigue life calculation are defined by the following formula: log N = log a - m. log Act where N is the allowable stress cycle numbers; a and m are parameters defining the curves, which are dependant on the material and structural detail. Ao is the stress range including the effect of stress concentration. For the pipe wall thickness in excess of 22 mm, the S-N curve is to take the following form: m t log N = l o g a - - - , l o g ~ 4 22
m. log Act
where t is the nominal wall thickness of the pipe. The fatigue damage may be based on the accumulation law by Palmgren-Miner:
Me ni Ofat --Z-~i where: Dfat =
accumulated fatigue damage
rl
= allowable damage ratio, to be taken as 0.1
Ni
=
ni
= number of stress cycles with stress range in block i
number of cycles to failure at the i th stress range defined by S-N curve
A cut-off (threshold) stress range So may be specified below which no significant crack growth or fatigue damage occurs. For adequately cathodic protected joints exposed to seawater, So is thecut-off level at 2x108 cycles, see Equation (4.17).
1 S~=/2"108/-mc
(4.17)
Stress ranges S smaller than So may be ignored when calculating the accumulated fatigue damage.
4.6.3
Fatigue Assessment based on AE-N Curves
The number of strain cycles to failure may be assessed according to the American Welding Society (AWS) Standards At-N curves, where N is a function of the range of cyclic bending strains At. The At-N curves are expressed as below: AE :
0 . 0 5 5 N -~
for Ae > 0.002
(4.18)
for Ae < 0.002
(4.19)
and Ae = 0.016N -~
The strain range At is the total amplitude of strain variations; i.e. the maximum less the minimum strains occurring in the pipe body near the weld during steady cyclic bending loads. A study of low-cycle fatigue conducted as part of the DEEPIPE JIP was summarized by Igland et al. (2000).
Limit-state based Strength Design
4.7
75
Ratcheting
Ratcheting is described in general terms as signifying incremental plastic deformation uhder cyclic loads in pipelines subject to high pressure and high temperatures (HP/HT). The effect of ratcheting on out of roundness, local buckling and fracture is to be considered. Two types of ratcheting are to be evaluated and the acceptance criteria are as below: 1. Ratcheting in hoop strain (the pipe expands radially) as a result of strain reversal for pipes operated at high internal pressure and high temperature. The accumulative hoop strain limit is 0.5%. 2. Ratcheting in curvature or ovalisation due to cyclic bending and external pressure. The accumulative ovalisation is not to exceed a critical value corresponding to local buckling under monotonic bending, or serviceability. The accumulative ovalisation is to be accounted for in the check of local buckling and out-of-roundness. A simplified code check of ratcheting is that the equivalent plastic strain is not to exceed 0.1%, based on elastic-perfectly-plastic material and assuming that the reference for zero strain is the as-built state after hydro-testing. In case the simplified code check is violated, a finite element analysis may be applied to determine if ratcheting is a critical failure mode and quantify the amount of deformation induced by ratcheting.
4.8
Dynamic Strength Criteria
Stress criteria (i.e. allowable moments, allowable stresses etc.), or strain criteria should be specified for the dynamic stresses or strain expected during vortex induced vibrations (VIV). At the maximum amplitude of vibrations, the strength criteria defined in this Chapter should be satisfied.
4.9
Accumulated Plastic Strain
If the yield limit is exceeded, the pipe steel will accumulate plastic strain. Accumulated plastic strain may reduce the ductility and toughness of the pipe material. Special strain aging and toughness testing must then be carried out. Accumulated plastic strain is defined as the sum of plastic strain increments irrespective of sign and direction. The plastic strain increments are to be calculated from the point where the material stress-strain curve deviates from a linear relationship, and the accumulated plastic strain are to be calculated from the time of fabrication to the end of lifetime. Limiting accumulated plastic strain is to ensure that the material properties of the pipe will not become sub-standard. This is especially relevant for the fracture toughness. Accumulated plastic strain may also increase the hardness of the material and thus increase its susceptibility to stress corrosion cracking in the presence of H2S. Stress corrosion cracking is also related to the stress level in the material. If the material yield limit is exceeded, the stress level will necessarily be very high. Plastic deformation of the pipe will also impose high residual stress in the material that may promote stress corrosion cracking.
76
Chapter 4
The general requirement of the accumulated plastic strain is that it should be based on strain aging and toughness testing of the pipe material. It is stated that due to material considerations a permanent/plastic strain up to 2% is allowable without any testing. In practice, this is valid also for the operational case. If the pipeline is to be exposed to more than 2% accumulated plastic strain, as is often the case for reeling installation method, the material should be strain aging tested. However, recent testing of modern pipeline steel has shown that plastic strain up to 5% or even 10% can be acceptable. In order to have an extra safety margin, it is also desirable to have a certain ratio between the yield stress and the ultimate tensile stress. A requirement to this ratio is given in DNV'81, paragraph 5.2.6.2, where the yield stress is determined not to exceed 85% of the ultimate stress. Accumulated plastic strain will increase the yield stress of the material and also increase the yield/ultimate stresses ratio.
4.10 Strain Concentration at Field Joints Due to Coatings It is necessary to evaluate effects of the concrete coating on strain concentrations at field joints. It is found reasonable to assume that the SNCF (Strain Concentration Factor) is 1.2. This value is mainly selected due to an allowable strain as high as 0.4% from the fracture criterion and the technical information from Ness and Verley (1996).
4.11 References 1. Bai, Y. and Damsleth, P.A. (1997) "Limit-state Based Design of Offshore Pipelines", Proc. of OMAE'97. 2. Chen, M.J., Dong, G., Jakobsen, R.A. and Bai, Y. (2000) "Assessment of Pipeline Girth Weld Defects" Proc. of ISOPE'2000. 3. Denys, R.M., (1992) "A Plastic Collapse-based Procedure for girth weld defect Acceptance" Int. Conf. on Pipeline Reliability, June 2-5, 1992, Calgary. 4. Hauch S. and Bai Y., (1999). "Bending Moment Capacity of Pipes", OMAE'99. 5. Igland, R.T., Saerik, S., Bai, Y., Berge, S., Collberg, L., Gotoh, K., Mainuon, P. and Thaulow, C. (2000) "Deepwater Pipelines and Flowlines", Proc. of OTC'2000. 6. ISO 13623 (1997) "Petroleum and Natural Gas Industries; Pipeline Transportation Systems", International Standard Organisation. 7. Koets, O.J. and Guijt J. (1996) "Troll Phase I, The Lessons Learnt", OPT'96. 8. NEN (1992), NEN 3650, "Requirements for Steel Pipeline Transportation System", 1992. 9. Ness, O.B. and Verley, R., (1996) "Strain Concentrations in Pipeline With Concrete Coating", Journal of Offshore Mechanics and Arctic Engineering, Vol. 118. 10. NystrCm P., TOmes K., Bai Y. and Damsleth P., (1997). "Dynamic Buckling and Cyclic Behavior of HP/HT Pipelines", Proc. of ISOPE'97.
Limit-state based Strength Design
77
11. PD 6493, (1991) "Guidance on Methods for Assessing the Acceptability of Flows in Fusion Welded Structures". 12. Price, P. and St. J., (1990). "Canadian Standards Association Limit-states Task ForceState of Practice Review for Pipelines and Representative References". 13. Statoil Technical Specification, R-SF-260, (1991)"Pipeline Welding Specification". 14. Stewart, G. et al., (1994). "An Analytical Model to Predict the Burst Capacity of Pipelines" Proc. of OMAE'94. 15. Zimmerman, et al., (1992). "Development of Limit-states Guideline for the Pipeline Industry", OMAE '92.
Chapter 4 Limit-state based Strength Design 4.1
Introduction
This chapter presents limit-state strength criteria for pipeline design. The limit-state based strength design became crucially important when usage factors in wallthickness design are raised from those given by traditional design codes. The discussion of a limit state design approach in this chapter is based on new Guides/Rules and a review of recent design projects and Joint Industry Projects (SUPERB, DEEPIPE, etc.). See Bai and Damsleth (1997). The limit state checks conducted in the strength design are: 9
Out of roundness for serviceability
9
Bursting due to intemal pressure, longitudinal force and bending
9
Buckling/collapse due to pressure, longitudinal force and bending
9
Fracture of welds due to bending/tension
9
Low-cycle fatigue due to shutdowns
9
Ratcheting due to reeling and shutdowns
9
Accumulated plastic strain
The allowable strains, equivalent stresses and bending moments are determined for the following operating scenarios. 9
Empty condition
9
Water filled condition
9
Pressure test condition
9
Operational conditions
64
Chapter 4
The strength criteria will be applicable for the following design situations" 9
Pipeline in-place behavior
9
Trawl pullover response
9
Free-spanning pipelines
9
Pipeline dynamic free-span
The pipeline route is divided into two zones: Zone 1 is the zone where no frequent human activity is anticipated along the pipeline route. For operating phases Zone 1 is classified as "Normal Safety Class" Zone 2 is the parts of the pipeline/riser in the near platform (manned) zone or in areas with frequent human activity. The extent of zone 2 is 500 m from the maximum facility excursion or determined based on risk analyses. For operating phases Zone 2 is classified as "High Safety Class" For temporary (construction) phases both zones are classified as "Low Safety Class", when the pipeline does not contain any hydrocarbons.
4.2
Out of Roundness Serviceability Limit
The pipeline out of roundness is related to the maximum and minimum pipe diameters (Dmax and Dmin) measured from different positions around the sectional circumference and is defined according to the following equation: f0 =
Dmax - Drm D
(4.1)
The out of roundness during the fabrication process is not to be more than 1.5%. The out of roundness of the pipe may increase where the pipe is subject to reverse bending and the effect of this on subsequent straining is to be considered. For a typical pipeline the following scenarios will influence the out of roundness:
-
The out of roundness may increase during the installation process where the pipe is subject to reverse inelastic bending;
-
Cyclic bending may occur as a consequence of shutdowns during operation if global buckling is allowed to relieve temperature and pressure induced compressive forces.
Out of roundness due to point loads is to be checked. Critical point loads may arise at freespan shoulders, artificial supports and support settlement. The accumulative out of roundness through life cycle is not to exceed 4%. This out of roundness requirement may be relaxed if:
Limit-state based Strength Design -
The effect of out-of-roundness on moment capacity and strain criteria is included;
-
The pigging requirements and repair systems, are met, and;
-
Cyclic loads induced out-of-roundness have been considered.
65
Finite element analysis may be performed to calculate the increase in out of roundness during the life cycle of a pipeline. The analysis is to include fabrication tolerances and all loads applied through the pipelines life-cycle such as point loads, bending against a surface, axial load and repeated pressure, temperature and bending cycles.
4.3
Bursting
4.3.1
Hoop Stress vs. Equivalent Stress Criteria
An analytical study by Stewart (1994) and a finite element analysis have demonstrated that for a pipe under combined internal pressure and bending: 9
If a pipeline section is in a displacement controlled situation, then a hoop stress criterion provides a good control of bursting.
9
If a pipeline section is in a load controlled situation then an equivalent stress criterion may be applied to ensure sufficient burst strength for pipes under combined internal pressure and axial loads (the influence of bending is yet to be investigated).
For pipelines in operation, it is generally conservative to apply the equivalent stress criteria to control bursting since the dominating load is internal pressure combined with bending. The bursting failure mode is governed by the tensile hoop stress. To ensure structural strength against bursting, the hoop stress is to fulfil the following conditions: yielding limit state, (Yh ~ rls.SMYS, where rls is usage factor for SMYS (Specified Minimum Yield Stress) bursting limit state, C~h < rlu.SMTS, where flu is usage factor for SMTS (Specified Minimum Tensile Stress) For load-controlled situations, special consideration shall be made to bursting. It has been chosen to use an equivalent and longitudinal stress criterion according to the results from the analytical study and the finite element analysis.
4.3.2
Bursting Strength Criteria for Pipeline
The hoop stress due to pressure containment is not to exceed the following criteria:
(Pi - Pe ) D - t 2t
< ~TS min[SMYS(T),O.S7 * SMTS(T)]
(4.2)
Chapter 4
66
where:
pi =
internal pressure
pe =
external pressure
D=
nominal outside diameter of pipe
t=
m i n i m u m wall thickness
S M Y S (T) = specified m i n i m u m yield stress at temperature T S M T S (T) = specified m i n i m u m tensile stress at temperature T Temperature derating is to be accounted for at elevated temperatures (above 50~ The following usage factors in Table 4.1 apply to the hoop stress equation: Table 4.1 Usage Factors for Hoop Stress Criteria.
Material
Usage
Quality
Factor
Low
Normal
High
Class B, C 1)
rls
0.85
0.80
0.70
Class A2)
~is
0.83
0.77
0.67
Safety Class
Class ,
,
Note: 1) In order to apply these higher usage factors, the following additional requirements with respect to linepipe manufacturing must be fulfilled: SMYS < (mean - 2*Standard Deviations) of yield stress; SMTS < (mean - 3*Standard Deviations) of tensile stress; tfab=2* Standard Deviations of the wall-thickness.
2) After Table 2 in Section 6.4 oflSO 13623 (1997).
Stress Criteria
For internal-over pressure situations, the allowable equivalent stress and allowable longitudinal stress are rI*SMYS(T) and the usage factor rl given in Table 4.2 is after Table 3 in Section 6.4 of ISO13623 (1997). Table 4.2 Equivalent stress design factor. Load Combinations
design factor rl
Construction and environmental loads
1.0
Functional and environmental loads
0.9
Functional, environmental and accidental loads
1.0
However, m o m e n t criteria, in Chapter 4.4 are considered to have better accuracy for strength design.
67
Limit-state based Strength Design
4.4
Local Buckling/Collapse
This section is based on Hauch and Bai (1999).
Local Buckling For pipelines subjected to combined pressure, longitudinal force and bending, local buckling may occur. The failure mode may be yielding of the cross section or buckling on the compressive side of the pipe. The criteria given in this guideline may be used to calculate the maximum allowable bending moment for a given scenario. It shall be noted that the maximum allowable bending moment given in this guideline does not take fracture into account and that fracture criteria therefore may reduce the bending capacity of the pipe. This particularly applies for high-tension/high-pressure load conditions.
Load Versus Displacement Controlled Situations The local buckling check can be separated into a check for load controlled situations (bending moment) and one for displacement controlled situations (strain level). Due to the relation between applied bending moment and maximum strain in a pipe, a higher allowable strength for a given target safety level can be achieved by using a strain-based criterion than the bending moment criterion. Consequently the bending moment criterion can, conservatively be used for both load and displacement controlled situations. In this guideline only the bending moment criterion is given.
Local Buckling and Accumulated Out-of-Roundness Increased out-of-roundness due to installation and cyclic operating loads may aggravate local buckling and is to be considered. It is recommended that out-of-roundness due to through life loads be simulated using finite element analysis.
Maximum Allowable Bending Moment The allowable bending moment for local buckling under load controlled situations can be expressed as: YcF
P
=rl~M J l - ( 1 - a : ~ ] P co 2 o~F,-~0~p,
...... ,~,,
,~
~v
~"~' J
~~"~,)J
.ll-O-~--e-~
where: Mallowable---- Allowable bending moment
= Plastic moment Pl
= Limit pressure
p
= Pressure acting on the pipe
El
= Limit longitudinal force
F
= Longitudinal force acting on the pipe
a
= Correction factor
(4.3)
68
9
Chapter 4
Yc
= Condition load factor
~R
= Strength usage factor
Correction Factor: o~ = 0.25 P_L for external overpressure
(4.4)
a = 0.25 P___Lfor internal overpressure
(4.5)
F,
F~
If possible, the correction factor should be verified by finite element analyses.
9
Plastic (Limit) Moment:
The limit m o m e n t m a y be given as"
0001 / 9
t
(4.6)
Limit Longitudinal Force for Compression and Tension"
The limit longitudinal force m a y be estimated as: Ft = 0.5-(SMYS+ SMTS ). A
9
(4.7)
Limit Pressure for External Overpressure Condition:
The limit external pressure 'p~' is to be calculated based on"
(
~
p3 -PetP~ - P~ +papPfo'7 p` + pe'p~ =0
(4.8)
P el= (i_v 2)'
(4.9)
where:
pp= rlI.,SMYS
2t 1) D
f0
= Initial out-of-roundness 2~, (Dmax-Dmin)/D
E
= Y o u n g ' s Module
a9
= Poisson's ratio
(4.10)
Guidance note: 1.
llfab is 0.925 for pipes fabricated by the UO process, 0.85 for pipes fabricated by the UOE process and 1 for
seamless or annealed pipes. 2.
Out-of-roundness caused during the construction phase is to be included, but not flattening due to external water pressure or bending in as-laid position.
Limit-state based Strength Design 9
69
Limit Pressure for Internal Overpressure Condition:
The limit pressure will be equal to the bursting pressure given by: (4.11)
Pt = 0.5(SMTS + S M Y S ) 2~ Z iJ
where: SMYS = Specified Minimum Yield Strength in hoop direction SMTS = Specified Minimum Tensile Strength in hoop direction
9
Load and Usage Factors:
Load factor 7c and usage factor 'qR are listed in Table 4.3. Table 4.3 Load and usage factors. Safety Classes Safety factors ~
~
Low
Normal
High
.
Uneven seabed
1.07
1.07
1.07
Pressure test
0.93
0.93
0.93
Stiff supported
0.82
0.82
0.82
Otherwise
1.00
1.00
1.00
rlRP
Pressure
0.95
0.93
0.90
rlRF
Longitudinal force
0.90
0.85
0.80
fIRM
Moment
0.80
0.73
0.65
Yc
Guidance notes: -
Load Condition Factors may be combined e.g. Load Condition Factor for pressure test of pipelines resting on uneven seabed, 1.07x0.93 = 1.00 Safety class is low for temporary phases. For the operating phase, safety class is normal and high for area classified as zone 1 and zone 2 respectively.
For displacement-controlled situations the following strain capacity check is given to ensure structural strength against local buckling: 0.8
~/ F " ]/c " ~/sncf " ]/ O _"~"F,c "~" Y E " ~ E,c
~t" P e _< 1
eM,c
Pc
Y~
YR
(4.12)
where: ~F,c~---
characteristic functional longitudinal strain
EE,c----
characteristic environmental longitudinal strain
EM,c =
characteristic buckling strain capacity
Ysncf=
strain concentration factor accounting for increased strain in the field joints due to coating stiffness discontinuities
70
Chapter 4
~F----
functional load factor
YE---
environmental load factor
~D-"
dynamic load factor
~C-"
condition load factor
yR= ~=
strength resistance factor strain capacity resistance factor
Comparing to the equation established in DNV'96, Section 5 C305, two additional safety factors (Ysncf and YD) are included. These additional safety factors are:
9
~tsncffor Strain Concentration Factor (SNCF).
9
~/D to take account for dynamic amplifications during a snap-through dynamic buckling (NystrCm et al 1997).
4.5
Fracture
4.5.1
PD6493 Assessment
Fracture of the welds due to a tensile strain is normally evaluated in accordance with PD 6493 (1991). This assessment method uses a curve (Failure Assessment Diagram) which combines the two potential failure modes: brittle fracture and plastic collapse. Maximum weld flaws, described in Statoil R-SF-260, Pipeline Welding Specification, are to be used as the basic input for the calculations. The flaw has been assumed as maximum allowable defect due to lack of fusion between passes. The defects and material are assumed as below: Type Depth (a) Length (2c) CTOD Material
: Surface flaw due to lack of fusion : 3 mm :50 mm : 0.20 mm (at operating temperature) : As for Parent material
Surface flaw is chosen as the worst case scenario from acceptable flaws specified in the weld specifications. The partial safety factors recommended by PD 6493, are as below:
9
For levels 2 and 3, no additional safety factors are required where worst case estimates are taken for stress level, flaw size and toughness, and all partial coefficients should be taken as unity. (Appendix A.1 of PD6493).
Limit-state based Strength Design
71
PD6493 FAD (Failure Assessment Diagram) gives critical stress for the given defect and material. It is necessary to convert the critical stress to critical strain and for this the RambergOsgood relationship is used as defined below: o3 e=--{l+ (o-),-1} E 7 o-0.7
(4.13)
where: o-0.7 = 430 MPa for X65 at 20~ n
= 26 for X65 at 20~
The allowable strain criterion used in this report is conservative due to: The stress-strain curves used in converting stress to strain are based on the lowest yield stress and lowest ultimate stress. 9
PD6493 has been derived for load-controlled situations, but is here applied to both load and displacement-controlled situations.
9
The flow stress is in PD6493 defined as the average of yield and tensile stress.
Corrosion in girth welds can significantly reduce the critical tensile strain of the girth welds if a flaw is assumed to be in the surface of the corroded weld. Fracture mechanics assessment of existing pipelines has shown that the critical strain can be between 0.1% for heavily corroded pipes and 0.5% for pipes with shallow corrosion defects. However, we shall not assume the combination of corrosion and cracks in the girth welds, although some pitting could occur in the HAZ (Heat Affect Zone). If corrosion takes place, it will occur over a certain number of years after entering into service, when the maximum strain load became lower due to reduced operating pressure and temperature, and "shakedown" of peak stress/strain levels in a number of shut downs. In the technical report "Update of laying criteria for pipelines", Denys and Lefevre, it is stated that the failure of welds under displacement controlled situations is highly dependent on the weld matching (in particular), and on the ratio of yield to tensile strength. They gave an allowable strain of 0.61• = 0.50 % (a safety factor of 1.5 is applied to the critical strain) for defect length t/1.2 and depth 3 mm, assuming that the weld is matched and the ratio of yield to tensile strength is 0.87. They also reported that the results are very sensitive to weld matching (over-matching will increase the allowable strain considerably, and undermatching will reduce it). The Dutch code NEN 3650 states that, normally, a tensile strain of 0.5% will not pose any problems for material and welding in accordance with their specifications. If it can be demonstrated that the ductility of the material is greater, higher strains can be tolerated accordingly.
Chapter 4
72
It was stated by Canadian Standards Associations that the pipeline industry has used a longitudinal tensile strain limit of 0.5%. This limit prevents fracture initiation and plastic collapse from Circumferential weld flaws small enough to be accepted by the specification or that may have been missed by inspection. Zimmerman et al. (1992) and Price (1990) reports that the 0.5% tensile strain limit is a subjective limitation, chosen to coincide with the API yield strength specifications and does not reflect an objective failure limit. The Troll Phase I project applied an allowable strain level of 0.4 % for a 36" gas export line, which was approved by NPD, (Koets and Guijt (1996)).
4.5.2
Plastic Collapse Assessment
It has been observed that all fracture mechanics calculations based on PD6493 lead to Sr=l. Where Sr is defined as:
Sr =
cr o"
(4.14)
f
The flow stress of is according to PD6493 defined as the average of yield stress ~y and ultimate tensile stress ~u of the weld material. For a flat plate with surface flaw under tension, equations for the net section stress CYnfrom PD6493 lead to: O'n
--
~ cr
tCl
(4.15)
where: ~cr
: critical stress
a
: defect depth
c
: half-width of the defect
t
: wall-thickness of the plate
The applied PD6493 assessment criteria can then be re-expressed as: O'cr=
I 1 - (t/ / {1 + t}l O'Y"~" O'u 2
(4.16)
The above PD6493 plastic collapse equation may be valid provided that brittle fracture is not a relevant failure mode, e.g.: 9
The defect depth (a) is less that 3 mm and the length (2c) is less than t (or 25 mm)
9
The material CTOD is more than e.g. 0.2 mm at operating temperature
Limit-state based Strength Design
73
The PD6493 plastic collapse equation can be applied to calculate allowable defect depth (a) and length (2c) for a given critical stress. In addition to PD6493 plastic collapse equation, the following plastic collapse equations are available from literature, see Bai (1993)and Denys (1992): 9
CEGB R6 approach (A.G. Miller's equation)
9
Willoughby's equation
9
the net section yielding collapse solution
9
the CSA Z184 equation
9
Denys's equation
Comparing with other available equations, PD6493 seems to give conservative and reasonable predictions. The PD6493 suggests that the safety factor for CYcrin Eq. (4.16) is 1.1. The readers are suggested to define safety factors based on the structural reliability principles described in Chapters 13 through 15. Chen et al. (2000) discussed formulae for plastic collapse and fracture of pipe with girth weld defects. A study of fracture criteria, conducted as part of the DEEPIPE JIP, was summarized by Igland et al. (2000).
4.6 4.6.1
Fatigue General
Pipeline components such as risers, unsupported free spans, welds, J-lay collars, buckle arrestors, riser touchdown points and flex-joints, should be assessed for fatigue. Potential cyclic loading that can cause fatigue damage includes vortex-induced-vibrations (VIV), waveinduced hydrodynamic loads, platform movements and cyclic pressure and thermal expansion loads. The fatigue life of the component is defined as the time it takes to develop a throughwall-thickness crack of the component. For high cycle fatigue assessment, fatigue strength is to be calculated based on laboratory tests (S-N curves) or fracture mechanics. If no detailed information is available, the F2 curve may be applied as the S-N curves for pipeline high cycle fatigue. Low cycle fatigue of girth welds may be checked based on Ae-N curves. The fracture mechanics approach calculates the crack growth using Paris' equation and final fracture using a recognized failure assessment diagrams (see Chapter 4.5). It may be applied to develop cracked S-N curves that are for pipes containing initial defects. If a fracture mechanics crack growth analysis is employed, the design fatigue life should be at least 10 times the service life for all components. The initial flaw size should be the maximum acceptable flaw specified for the non-destructive testing during pipe welding in question.
Chapter 4
74
4.6.2
Fatigue Assessment based on S-N Curves
The S-N curves to be used for fatigue life calculation are defined by the following formula: log N = log a - m. log Act where N is the allowable stress cycle numbers; a and m are parameters defining the curves, which are dependant on the material and structural detail. Ao is the stress range including the effect of stress concentration. For the pipe wall thickness in excess of 22 mm, the S-N curve is to take the following form: m t log N = l o g a - - - , l o g ~ 4 22
m. log Act
where t is the nominal wall thickness of the pipe. The fatigue damage may be based on the accumulation law by Palmgren-Miner:
Me ni Ofat --Z-~i where: Dfat =
accumulated fatigue damage
rl
= allowable damage ratio, to be taken as 0.1
Ni
=
ni
= number of stress cycles with stress range in block i
number of cycles to failure at the i th stress range defined by S-N curve
A cut-off (threshold) stress range So may be specified below which no significant crack growth or fatigue damage occurs. For adequately cathodic protected joints exposed to seawater, So is thecut-off level at 2x108 cycles, see Equation (4.17).
1 S~=/2"108/-mc
(4.17)
Stress ranges S smaller than So may be ignored when calculating the accumulated fatigue damage.
4.6.3
Fatigue Assessment based on AE-N Curves
The number of strain cycles to failure may be assessed according to the American Welding Society (AWS) Standards At-N curves, where N is a function of the range of cyclic bending strains At. The At-N curves are expressed as below: AE :
0 . 0 5 5 N -~
for Ae > 0.002
(4.18)
for Ae < 0.002
(4.19)
and Ae = 0.016N -~
The strain range At is the total amplitude of strain variations; i.e. the maximum less the minimum strains occurring in the pipe body near the weld during steady cyclic bending loads. A study of low-cycle fatigue conducted as part of the DEEPIPE JIP was summarized by Igland et al. (2000).
Limit-state based Strength Design
4.7
75
Ratcheting
Ratcheting is described in general terms as signifying incremental plastic deformation uhder cyclic loads in pipelines subject to high pressure and high temperatures (HP/HT). The effect of ratcheting on out of roundness, local buckling and fracture is to be considered. Two types of ratcheting are to be evaluated and the acceptance criteria are as below: 1. Ratcheting in hoop strain (the pipe expands radially) as a result of strain reversal for pipes operated at high internal pressure and high temperature. The accumulative hoop strain limit is 0.5%. 2. Ratcheting in curvature or ovalisation due to cyclic bending and external pressure. The accumulative ovalisation is not to exceed a critical value corresponding to local buckling under monotonic bending, or serviceability. The accumulative ovalisation is to be accounted for in the check of local buckling and out-of-roundness. A simplified code check of ratcheting is that the equivalent plastic strain is not to exceed 0.1%, based on elastic-perfectly-plastic material and assuming that the reference for zero strain is the as-built state after hydro-testing. In case the simplified code check is violated, a finite element analysis may be applied to determine if ratcheting is a critical failure mode and quantify the amount of deformation induced by ratcheting.
4.8
Dynamic Strength Criteria
Stress criteria (i.e. allowable moments, allowable stresses etc.), or strain criteria should be specified for the dynamic stresses or strain expected during vortex induced vibrations (VIV). At the maximum amplitude of vibrations, the strength criteria defined in this Chapter should be satisfied.
4.9
Accumulated Plastic Strain
If the yield limit is exceeded, the pipe steel will accumulate plastic strain. Accumulated plastic strain may reduce the ductility and toughness of the pipe material. Special strain aging and toughness testing must then be carried out. Accumulated plastic strain is defined as the sum of plastic strain increments irrespective of sign and direction. The plastic strain increments are to be calculated from the point where the material stress-strain curve deviates from a linear relationship, and the accumulated plastic strain are to be calculated from the time of fabrication to the end of lifetime. Limiting accumulated plastic strain is to ensure that the material properties of the pipe will not become sub-standard. This is especially relevant for the fracture toughness. Accumulated plastic strain may also increase the hardness of the material and thus increase its susceptibility to stress corrosion cracking in the presence of H2S. Stress corrosion cracking is also related to the stress level in the material. If the material yield limit is exceeded, the stress level will necessarily be very high. Plastic deformation of the pipe will also impose high residual stress in the material that may promote stress corrosion cracking.
76
Chapter 4
The general requirement of the accumulated plastic strain is that it should be based on strain aging and toughness testing of the pipe material. It is stated that due to material considerations a permanent/plastic strain up to 2% is allowable without any testing. In practice, this is valid also for the operational case. If the pipeline is to be exposed to more than 2% accumulated plastic strain, as is often the case for reeling installation method, the material should be strain aging tested. However, recent testing of modern pipeline steel has shown that plastic strain up to 5% or even 10% can be acceptable. In order to have an extra safety margin, it is also desirable to have a certain ratio between the yield stress and the ultimate tensile stress. A requirement to this ratio is given in DNV'81, paragraph 5.2.6.2, where the yield stress is determined not to exceed 85% of the ultimate stress. Accumulated plastic strain will increase the yield stress of the material and also increase the yield/ultimate stresses ratio.
4.10 Strain Concentration at Field Joints Due to Coatings It is necessary to evaluate effects of the concrete coating on strain concentrations at field joints. It is found reasonable to assume that the SNCF (Strain Concentration Factor) is 1.2. This value is mainly selected due to an allowable strain as high as 0.4% from the fracture criterion and the technical information from Ness and Verley (1996).
4.11 References 1. Bai, Y. and Damsleth, P.A. (1997) "Limit-state Based Design of Offshore Pipelines", Proc. of OMAE'97. 2. Chen, M.J., Dong, G., Jakobsen, R.A. and Bai, Y. (2000) "Assessment of Pipeline Girth Weld Defects" Proc. of ISOPE'2000. 3. Denys, R.M., (1992) "A Plastic Collapse-based Procedure for girth weld defect Acceptance" Int. Conf. on Pipeline Reliability, June 2-5, 1992, Calgary. 4. Hauch S. and Bai Y., (1999). "Bending Moment Capacity of Pipes", OMAE'99. 5. Igland, R.T., Saerik, S., Bai, Y., Berge, S., Collberg, L., Gotoh, K., Mainuon, P. and Thaulow, C. (2000) "Deepwater Pipelines and Flowlines", Proc. of OTC'2000. 6. ISO 13623 (1997) "Petroleum and Natural Gas Industries; Pipeline Transportation Systems", International Standard Organisation. 7. Koets, O.J. and Guijt J. (1996) "Troll Phase I, The Lessons Learnt", OPT'96. 8. NEN (1992), NEN 3650, "Requirements for Steel Pipeline Transportation System", 1992. 9. Ness, O.B. and Verley, R., (1996) "Strain Concentrations in Pipeline With Concrete Coating", Journal of Offshore Mechanics and Arctic Engineering, Vol. 118. 10. NystrCm P., TOmes K., Bai Y. and Damsleth P., (1997). "Dynamic Buckling and Cyclic Behavior of HP/HT Pipelines", Proc. of ISOPE'97.
Limit-state based Strength Design
77
11. PD 6493, (1991) "Guidance on Methods for Assessing the Acceptability of Flows in Fusion Welded Structures". 12. Price, P. and St. J., (1990). "Canadian Standards Association Limit-states Task ForceState of Practice Review for Pipelines and Representative References". 13. Statoil Technical Specification, R-SF-260, (1991)"Pipeline Welding Specification". 14. Stewart, G. et al., (1994). "An Analytical Model to Predict the Burst Capacity of Pipelines" Proc. of OMAE'94. 15. Zimmerman, et al., (1992). "Development of Limit-states Guideline for the Pipeline Industry", OMAE '92.