Studlink and Studless Fatigue Curves for Mooring Lines

API TECHNICAL REPORT 2FC-1 FIRST EDITION, JANAURY 2020

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Contents

Page

1. Introduction 1

2. TN and SN Mooring Component Fatigue Curves 1

3. Fatigue Curves for Chain Mooring Lines 4

   1. General 4

   2. Derivation of Studlink Fatigue Curves 5

   3. Derivation of Studless Fatigue Curves 10

   4. Proposed Studlink and Studless Fatigue Curves for Mooring Chain 12

4. Fatigue Life Comparisons Based on API’s 2SK Example 12

   1. General 12

   2. Discussion 13

Annex A (informative) Studlink and Studless Chain Fatigue Test Data 15

Annex B (informative) Example Fatigue Damage Calculations for API’s 2SK Second Edition 22

Bibliography 24

Figures

1. All Studlink ORQ Saltwater Fatigue Test Failures 4

2. Probability Plot for All Studlink ORQ Saltwater Fatigue Test Failures 6

3. Studlink Fatigue Curves for Specific Probabilities of Failure (P) and Number of Links (J) 8

4. NEL Studless and Studlink Fatigue Test Data 10

5. Probability Plot for All Studless Saltwater Fatigue Test Failures 11

Tables

1. Summary of Saltwater Chain Fatigue Failure Data 1

   1)

2. Individual Link Probability of Failure (Pf

   Required for Line Probability of Failure 7

   10

3. Intercepts log (a∗) Required for Fatigue Curves Associated with Line Probabilities 8

4. Safety Factors Required for Line Probability of Failure of 10−4 and Fatigue Curves Based on 2.5 % Probability of Failure 9

5. Comparison of NEL Studless and Studlink Regression Curves 11

6. Proposed Parameters for Studlink and Studless TN Fatigue Curves 12

7. Comparison of Studlink Fatigue Lives for API’s 2SK Example 13

8. Comparison of Studless Fatigue Lives for API’s 2SK Example 13

1. Summary of ORQ Studlink Saltwater Fatigue Test Results 15

2. Summary of R3 and R4 Studless Saltwater Fatigue Test Results 18

1. Present API Fatigue Curve (m = –3.36, log10 (a) = 2.568) 22

2. Proposed Studlink Fatigue Curve (m = − 3.0, log10(a) = 3.0) 23

3. Proposed Studless Fatigue Curve (m = − 3.0, log10(a) = 2.5) 23

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Studlink and Studless Fatigue Curves for Mooring Lines

1. Introduction

   
   

   This report summarizes the derivation of fatigue curves for studless and studlink chain mooring lines for inclusion in API 2SK, Third Edition. The second edition of API 2SK [1] has a single fatigue curve for mooring chain, this curve is a non-unique lower bound to all of the in-water and in-air studlink fatigue test data. API 2SK, Second Edition, does not have separate fatigue curves for studlink and studless chain.

   
   

   The studlink fatigue curve derived in this report is based on the saltwater tension-tension fatigue tests performed by the National Engineering Laboratory, Glasgow, U.K. (NEL) and Exxon Production Research Company (EPR) on oil rig quality (ORQ) studlink chain with bar diameters of 2 in., 3 in., and 4 in. [3, 4]. The studless fatigue curve is based on the saltwater tension-tension fatigue tests performed by NEL on R3 and R4 studless chain with a bar diameter of 3 in. [5]. All publicly available saltwater studlink and studless chain fatigue data are summarized in Table 1, and the fatigue test data are listed in Annex A.

   
   

   Table 1—Summary of Saltwater Chain Fatigue Failure Data

   
   

   Chain Fatigue Failure Data	Diameter (in.)	Slope			Censoring
   		m	−95 %	+95 %	% Failed	Failed	Runout	Tested
   ORQ Studlink, NEL	3 and 4	−2.588	−2.802	−2.374	50	20	20	40
   ORQ Studlink, EPR	2	−3.951	−4.425	−3.653	See NOTE	40	See NOTE	See NOTE
   ORQ Studlink, Shell a	See NOTE	—	—	—	See NOTE	16	See NOTE	See NOTE
   ORQ Studlink, NEL and EPR	2, 3, and 4	−3.054	−3.26	−2.848	See NOTE	60	> 20	> 80
   K4 Studlink, NEL	3 and 4	−1.902	−2.212	−1.591	52	12	11	23
   R3 Studless, NEL	3	−2.419	−2.613	−2.225	52	44	40	84
   R4 Studless, NEL	3	−2.336	−2.574	−2.098	43	31	41	72
   NOTE The diameter of the Shell chain is not known; or the percent failed, number of runout links or number of tested links were not reported.
   a The Shell tests were all performed at the same tension range, consequently they contain no information on the slope (m) of the fatigue curve.

   
   

2. TN and SN Mooring Component Fatigue Curves

API’s component TN curves are specified in terms of the non-dimensional tension range (T), which is calculated by dividing the dimensional double amplitude tension range by a reference breaking strength. For studlink chain, API specifies the reference break strength as the catalog break strength (CBS) of ORQ chain of the same size (reduced by a corrosion allowance). API’s fatigue curve is expressed as shown in Equation (1).

log

10

(N ) = m log

10

(T) + log

10

(a) (1)

where

T is the tension range/CBS of ORQ;

m is the slope of fatigue curve;

log a is the intercept of fatigue curve;

10( S)

N is the number of cycles.

1