This document discusses methods for improving welds to extend the lifetime and integrity of offshore structures. It describes techniques like grinding, hammer peening, and needle peening that can be applied to weld toes to improve fatigue life. The document also provides an example of using burr grinding to address corrosion damage on an offshore brace. It analyzes inspection findings that identified more fatigue cracks than predicted, indicating higher crack potential and poor welding quality for the drilling unit.

1. Weld-impr-example_sect_1_5 1
LIFE TIME EXTENSION AND
INTEGRITY OF OFFSHORE STRUCTURES
5_WELD IMPROVEMENT METHODS
Why and How to grind
by use of a burr grinder?
• Crack growth due to fatigue loads – example
• Why do grinding improve the fatigue life?
• Corrosion and Crack growth
• Procedure for grinding
• Where to perform grinding by a burr grinder?
• Filings and protection
Weld Improvement Example in Offshore Oil & Gas

2. 1
Weld-impr-example_sect_1_5 2
Class improvement *1.3,
Fatigue life *2.2
Ref- IIW, Commission XIII
Post Weld Treatment
Burr grinding TIG Dressing
Re-melting the material at the weld toe.
• Improved profile
• Remove weld toe flaws
• Plate thickness > 6mm
Class improvement *1.3,
Fatigue life *2.2

3. Weld-impr-example_sect_1_5 3
Hammer Peening
Compressive residual stress at the weld
toe.
• Improved profile
• Compress weld toe flaws
• Plate thickness > 4mm
Class improvement *1.3,
Fatigue life *2.2
Needle Peening
Compressive residual stress at the weld
toe.
• Improved profile
• Compress weld toe flaws
• Plate thickness > 4mm
Class improvement *1.3,
Fatigue life *2.2
Effect of grinding
• Remove smaller weld defects i.e. Longer crack initiation period
• Smother transitions i.e. reduce stress concentration that reduce the
crack growth rate.
• A deep groove will increase the stress concentration i.e. increased
crack growth rate
• Grinding increase the fatigue life with a factor of 2 to 6
• A ground grove of 1.5 – 2mm give capacity as a weld without
grinding.
Corrosion and fatigue crack growth
• Corrosion results in a shorter crack initiation period
• Corrosion results in an increased crack growth rate
• Corrosion reduce the fatigue life estimate with a factor of 2
-2,0
-1,0
0,0
1,0
2,0
3,0
4,0
5,0
20 220 420 620 820
AcceptableGrooveDepth
Initial Fatigue Life Estimate
Acceptable Groove Depth as function of initial
fatigue life estimate and condition at groove
No Shape
Haz
No gr.
Corrosion, No Shape
Haz
No gr.
Air, No Shape
Haz
No gr.
Air, Shape
Haz
Grind
Air, Shape
Haz
No gr.
Shape
Haz
No gr.
Area
code
Pos.
nr.
Clock
Pos.
No
Shape
Haz
No gr.
Shape
Haz
No gr.
Shape
Haz
Grind
PK3 3 0100 1,4 2,9 4,0
PK3 3 0200 0,7 1,9 2,8
PK3 3 0300 0,8 2,0 2,9
PK3 3 0400 0,5 1,6 2,4
PK3 3 0500 1,4 2,9 4,0
PK3 3 0600 3,0 6,8 8,0
PK3 3 0700 2,1 3,8 5,1
PK3 3 0800 1,2 2,6 3,6
PK3 3 0900 1,2 2,6 3,6
PK3 3 1000 1,5 3,0 4,2
PK3 3 1100 2,1 3,8 5,1
PK3 3 1200 3,0 5,8 7,5
Acceptable groove depth

4. Weld-impr-example_sect_1_5 4
Figure 1 A butt weld given damage due to corrosion. Weld material is grey, corrosion damage is red
and groove shape is yellow.
Case:
External corrosion resulted in a flaw
at the brace in a butt weld. The flaw
is ground by a burr grinder to an
elliptic shaped groove in the direction
of the stress. Wall thickness of the
brace is 32mm and has a diameter of
2.5m. Max groove depth will as far as
possible be located outside the weld
and HAZ area.

5. Weld-impr-example_sect_1_5 5
Ok, However
only grinding of
weld toe was
required. (Also
weld toe between
the string of
welding.)
Not as wanted as
grind disk and flap
disk has been
used. Needle
peening was
required.
Figur 7.5 Slipt spor etter fjerning av Samgong-anodeNormal quality, removal of anode.
FEM- model of weld detail of Ring stiffener
55 mm
20 mm
Grinding for weld improvement, has effect on
the Stress Intensity Magnification Factor, Mk
The Mk function estimated base on detailed analysis
da/dN = C⋅(Y·Mk · ∆σ·√πa )m
0,6
0,8
1,0
1,2
1,4
1,6
1,8
2,0
2,2
2,4
2,6
2,8
3,0
3,2
0,0 0,1 0,2 0,3 0,4 0,5
a/t
Mk
Membrane stress, rho = 1 mm
Membrane stress, rho = 10 mm
Bending stress, rho = 1 mm
Bending stress, rho = 10 mm
The Magnification Mk factor also for toe grinded welds Validation of the probabilistic fracture
mechanic model
• Deterministic validation. The FM model applied v.s. a
reference FM model.
• Probabilistic FM model v.s. probabilistic SN-model
• Sensitivity sign check
• Standard checks in structural reliability i.e.
- absolute sensitivity value check
- importance factor check
- simulation v.s. FORM/SORM

6. Weld-impr-example_sect_1_5 6
The Challenge of Quality - Workmanship
• Case Veslefrikk B
– Detailed engineering and analyses
– Detailed follow up activity at site
– The Integrity engineers at site
• Case Drilling Unit
– Detailed engineering and analyses
– Normal follow up activity at site
– No Integrity Engineer, Site and Inspection Engineers
Case Drilling Unit – North Sea,
Detailed FEM , FLS-analyses in 1999 and 2002:
• Reconstructed in year 2000
•Detailed As-Is and probabilistic fatigue assessment in 2004
•RS2009 included extended inspection scope, twice of normal
scope
•IS2011 NDT scope closely to 150% of normal renewal scope
Drilling Units – after RS2004
• The experience shows that the critical fatigue failure are mainly due to gross
errors in design, fabrication or inspection scheduling.
– The FLS analysis may be insufficient w.r.t. extent or quality
– gross error during design or fabrication
– knowledge not developed based on in-service observations i.e. validate the
prediction model (SRA-model)
– Re-analysis based on incomplete as-is description or re-assessment
process have not included knowledge from fabrication and maintenance
• Successful implementation of SRA for fatigue assessment fulfil the NORSOK
requirements:
To predict and to determine with a reasonable level of confidence the
existence, extent and consequences of deterioration, damages and defects of
the structural components
• We recommend to include an inspection program for verification of the
predictions of the SRA model in addition to the inspections scheduled by the
SRA model
Every inspection of a defined inspection point is given one row in
the spreadsheet. The spreadsheet is based on “positive” reporting.
i.e. both “finding” and “no-finding” are recorded.
Number of items (rows in spreadsheet): 1124 + 282
Number of findings: 204 + 83 (43 hot spot)
Number of cracks: 100 + 83 (43 hot spot)
Number of cracks not finally repaired: 47 (24 hot spot)
All 0-30%
prob fat
30-70%
prob fat
70-100%
prob fat
# of cracks: 83(43) 42(17) 31(18) 10(8)
# not finally
repaired:
47(24) 24(12) 20(10) 3(2)
Table 4.1 Number of cracks detected up to now in year 2009. The
numbers in brackets are number of different hot spot given detection
of crack.
Drilling Units – offshore RS2009
Year detected Potentially
fabrication
defect
Potentially propagating
cracks
Most likely
propagating cracks
Per cent belief of
crack to be
propagating
< 30%
Between
30-70% >70%
1980 4 6 3
1981 1 1
1982 2 1
1990 2 1
1994 1
1998 2 15 10
2003 2 1
2004 22 19 7
2009 42(17) 31(18) 10(8)
Total 72(47) 76(63) 35(33)
Classification of cracks. The number in brackets represents number of hot spot given
detection of crack and not the actual number of cracks.
Drilling Units – offshore RS2009
# of Hot Spots
within group
Range of Fatigue life in
years
Cumulative list
# of Hot Spots
0 1-5 0
2 5-10 2
13 10-15 15
21 15-20 36
36 20-30 72
25 30-40 97
38 40-60 135
62 60-100 197
91 100-300 288
121 300-∞ 409
Summary of the calculated fatigue life estimates.
* - high SCF included due to lack of information for hot spots that
potentially have low fatigue lives.
Drilling Units – offshore RS2009

7. Weld-impr-example_sect_1_5 7
Inspection scheduling – Principle
The theory predicts approximately 3-15 per cent of the scheduled
inspection to detect small fatigue cracks. The probability for detection of a
through thickness crack is less than 0.5 per cent. The answer is given by
the future inspections. Significant deviation between prediction and
observation motivate for a re-assessment.
•For RS2009, internally a total 200 NDT inspections done
•Findings are 10 or 42 fatigue cracks detected
•Percent detection of crack; 10/200 = 5%, 42/200 =
21%
•Inspection result indicate the fatigue crack potential to
be higher than assumed in the inspection scheduling.
•Inspection result indicate very poor welding quality, re-
welding and improvement will be proposed.
Present fatigue crack growth potential
According reporting in April 2009 the following cracks were identified as the most critical:
Item Description Fatigue life in
years
Used SCF
1 SC1-10-06, end of stiffener, ground out by burr
grinding to a depth of 15mm, wt=24mm
0,18, (52) 4,3 (*1,1)
2 SK3-5-0600 – weld repair given initial large root
failure, latest repair in June 2009 reports root
failure of 3-4mm.
0,21 (32,5) 3,7 (*1,1)
3 SK3-5-1200 – weld repair given initial large root
failure, latest repair in June 2009 reports root
failure of 3-4mm.
0,34 (52,2) 3,7 (*1,1)
4 SK3-5-0300- end of stiffener, ground out by burr
grinding to a depth of 9mm, wt=23mm
0,39 (39,9) 3,35 (*1,1)
Table 1 The critical item and fatigue life estimates for present condition. Fatigue life and
SCF in brackets represent the values without any crack or groove present.
Remaining fatigue capacity of brace after leakage
Use of BS7910 for flaw assessment
Procedure applied:
- Use of quality class
- Model of through thickness crack in a plate
- A half pipe model given the crack at the edge
The analyses done is not a detailed assessment including estimation of crack size given
final cracking. However, the analyses give an indication of time from leakage to loss of
brace. Based on the result in table 1 and 2 we select the SCF = 2.5 and SCF = 3.5 as
upper and lower boundary for a reasonable time estimate. The fatigue life estimate is
than between 0,26 and 1,07 year.
NB!! We have limited the assessment to one crack even if section SK3-5 has critical hot
spot at three clock positions.
Conclusions – 10.09.2009
• Ref. conclusion after RS2004
• Additions:
– The internal inspection for RS2004 was not managed by skilled
personnel – most likely the poor welding quality would have been
focused in 2004 if the inspection team of 2009 had been used
– Modification of cracked area and other high stress area , will require very
god weld and detailed design quality. Ongoing activity and proposal to be
delivered DNV the following weeks.
.
Case Drilling Unit, North Sea, Status March 2013
• Extended and frequent NDT inspection
• No findings compromise the safety against larges disaster
• Number of findings larger than acceptable level of 3-15 % finding and 0.5%
fatigue failure (TTC - Through Thickness Crack)
• 21% and 5% during offshore survey RS2009
• 10% and 1% totally for RS2009, (1633 NDT inspections )
• 2% and 1% for IS2011. ( 107 NDT inspections )
• Des.2012: Ca. 40 NDT for SC4 og PC4 => 50% og 22% byAS2012
• Mars2013: 153 NDT, 7 (14) find => 5% (9%) og 0% by AS2013

8. Weld-impr-example_sect_1_5 8
Compensating measures:
•Install stress gouges
•New structural analyses delivered from 2 independent vendors
•Inspection of critical area after storm periods
•Start of design for re-construction
Case Drilling Unit, North Sea, Status March 2013 Preliminary results from the stress gouge measurement after 1
winter season
• Measurements indicate 20% longer fatigue life estimate than the analyses
from vendor 1
• The fatigue load effect in the braces are high but within acceptable limits
• The fatigue load effect is locally within the column base tank too high.
Reconstruction is required.
Centre line
Clock
position
Mean
RatioS
95% Confidence,
Upper limit
RatioAWSG
95% Confidence,
Lower limit
RatioAWAA
0600 0,97 <0,95, 2,03> <0,72, 0,8>
0300 1,16 <1,04, 2,64> <0,71, 1,11>
0900 0,56 <0,65, 1,05> <0,43, 0,5>
1200 0,44 <0,34, 1,04> <0,24, 0,44>
SCF=1,53, Traditional design
R=2000mm,
SCF=1,08
SCF=1,2, behind the TIP,
Factor for fatigue life = 2,3
Weld improvement factor = 4
Total factor = 9.2
End of external doubling plate, (#72)
Fatigue crack observed after 5 year

9. Weld-impr-example_sect_1_5 9
Alternative Follow-Up Regime for Structure:
• Level 0 : Standard “DNV” Follow-Up Scheme - IIP
• Level 1 : Based on detailed in-service FLS analyses
• Level 2 : RBI analyses including inspection history
• Level 3 : Extended by As-Is description and improved
follow-up regime for inspection, repair, modifications and
weld improvement
Validation Studies – The Benefit of Refinements
• Studies for Philips Petroleum, bp, TFE, PSA-N and HSE-UK in the period 1995-1999,
• 3 published reports by HSE-UK and more than 10 papers
• Detailed assessment of more than 500 fatigue cracks
Main Conclusion
• Traditional follow-up regime – level 0 and level 1
– 20-40% of the detected cracks are in accordance with the initial fatigue analyses
– Crack growth does mainly occur at details given poor fabrication quality
– All units given comparable amount of inspections
• AIMS based approach – level 3 and partially level 2
– The calculations are based on As-Is description
– 70-80% of the fatigue crack growth are according the calculated crack growth
potential
– The scope of inspection varies from 30 to 200% of a traditional follow up scope
Competance at Site
Unit 1 – Level 3
• Extensive and detailed follow up regime at site by competent
personnel
• Upon 10 years operation
– 73 fatigue cracks of 1 at weld improved area i.e. hot spot given ultra high
fatigue loads
Unit 2 – Level 2
• Traditional site team for inspection, repair and modification
• Upon 5-years operation
– 180 fatigue cracks at 143 different locations
– All cracks at hot spot given high fatigue loads
– Main reason: Poor workmanship for weld improvement and local modification
Page 52
Integrity Management
vs.
Inspection Management
Inspection
Integrity Engineering and Inspection
working together on-site, on-shore and off-shore.
Integrity Eng.
Condition
Assessment –
The process of
What to be done
Perform
inspection –
The process of
How to do