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DNV-OS-C105 Structural Design of TLPS (LRFD method)

Sec.7: Fatigue Limit States (FLS)

SECTION 7
Fatigue Limit States (FLS)

Sec.7
A. Introduction

Sec.7
A 100   General

Industry accepted fatigue S-N curves different from the DNV standards may be considered for acceptance. Fatigue design for composite tendon is given in DNV-OS-C501.
Improved fatigue performance (comparing to what is defined in DNV-RP-C203) of base material may be accounted for in the design, provided that the fatigue performance and fracture mechanic properties of the same, are documented through testing.

Sec.7 A
104   Careful design of details as well as stringent quality requirements for fabrication are essential in achieving acceptable fatigue strength. It is to be ensured that the design assumptions made concerning these parameters are achievable in practice.

Sec.7 A
105   The results of fatigue analyses shall be fully considered when the in-service inspection plans are developed for the platform.

Sec.7 A
106   Structures that are susceptible to low cycle/ high stress fatigue should be analysed to assess damage accumulation during rare events that may be of extended duration. Therefore single event fatigue damage for the hull structure and tendons to be considered for units that are to operate in tropical regions where hurricanes, cyclones etc. can be present. The API RP 2T can be used for further guidance.

Sec.7
B. Hull

Sec.7
B 100   General

Sec.7
C. Deck

Sec.7
C 100   General

Sec.7
D. Tendons

Sec.7
D 100   General

Guidance note:
Fracture toughness testing is performed to establish material properties that in turn can be used to calculate critical flaw sizes. The most common testing is CTOD (Crack Tip Opening Displacement) testing which in most cases are done using 3-point bend specimens. Testing should be performed for both base material and fusion line locations. As a minimum 3 tests should be performed per location and the lowest value of the 3 test results should be used in fracture toughness assessments. Further guidance on fracture toughness testing and assessments can be found in DNV-OS-F101, Section 12, subsection G and Appendix B, A800.

CTOD tests performed in bending may give very conservative results. One way to reduce the conservatism is to perform the testing in tension (SENT specimens). Test performed like this will give a testing condition (constraint) close to that associated with a defect in a girth welded pipe loaded in tension. For SENT testing a minimum of 6 specimens per location will be required.

In case of materials with good fracture toughness properties (typically CTOD values above 0.25 mm), CTOD - Resistance or J - Resistance testing should be performed to establish the tearing resistance of the material.

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To be able to size flaw heights, an ultrasonically based NDT system (UT) must be utilised. The detection ability of an ultrasonically based NDT system shall be deemed sufficient if the probability of detecting a flaw of the smallest allowable height determined during an Engineering Critical Assessment (ECA) is 90% at a 95% confidence level and the probability of under-sizing a defect is less than 5%.

Guidance note:
In general, ultrasonic systems shall be qualified and the performance of the system shall be documented. If such documentation does not exist, fracture mechanics assessments are recommended to be carried out assuming an initial flaw size of 3 x 25 mm (height x length) for Automated Ultrasonic Testing (AUT) or 9 x 50 mm for Manual Ultrasonic Testing.

With the AUT sensitivity level set at 50% of the echo from a 3 mm flat bottom hole, a flaw satisfying the Probability Of Detection (POD) of 90% at 95% confidence level has in most cases been found to be approx. 3mm in height. This is the basis for suggesting an initial flaw size of 3 x 25 mm for the ECA, provided no other data are available. Refer also DNV OS-F101, Appendix E, Section H related to AUT system qualification.

For manual UT, typical POD curves has been established in the Nordtest NDE Programme. With an echo of 20% of the echo from a 3 mm side drilled hole, flaw size corresponding to POD of 90% at 95% confidence level has been found to be approx. 9 mm in height. This is the basis for suggesting an initial flaw size of 9 x 50 mm to be used for the ECA, provided no other data are available.

If UT is based on smaller reference reflectors and/or better sensitivities than those stated above, a reduction of the assumed initial flaw sizes may be made, if properly justified Unless the system can be qualified with the criteria as follows, an initial flaw size of 3 x 25 mm (height x length) should be assumed for Automatic Ultrasonic Testing (AUT) or 9 x 50 mm (height x length) should be assumed for Manual Ultrasonic Testing.

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Sec.7
E. Foundation

Sec.7
E 100   General