Fibre Mooring JIPs by DNV

Transcription

1 Fibre Mooring JIPs by DNV FPSO forum, TWI, Cambridge, UK Vidar Åhjem 26 October 2005

2 Joint Industry Projects Improving Fibre-Mooring Design Practices. - DNV, Tension Technology International, Granherne, Marlow Ropes. - BP, Chervon, Shell, Statoil. Managing the Safe Service Life of Fibre Ropes for Mooring. - DNV, Diolen, DSM-Dyneema, Bexco Ropes. - BP, Kerr-McGee, Shell, Statoil. Both projects are conducted with a rope-industry reference group - Bexco Ropes, Marlow Ropes, Quintas&Quintas, ScanRope, Whitehill Manufacturing. Slide 2

3 Joint Industry Projects Through these broadly based Joint Industry Projects it will be possible to - establish uniform standards. - avoid the pitfalls!! Both JIPs will be carried out 2005/2006, with a subsequent confidentiality period. Slide 3

4 Introduction This presentation is given to describe some of all the potentials of utilising materials performance in design with fibre ropes. In addition to better control with performance, all aspects of these JIPs will significantly reduce the costs associated with fibre-rope moorings for offshore use. The two JIPs are separate projects, with separate project- and sponsor groups. Slide 4

5 Improving Fibre-Mooring Design Practices Fibre-rope behaviour is both predictable and repetitive. We want to utilise this materials performance to improve design methods. The elemental behaviour of the fibre rope can be depicted by a simple physical model: Slide 5

6 The project addresses: Correct dynamic stiffness. Correct offset definition. Correct extreme force definition. Installation length. All to be described in a simple test to be used directly in the design analysis. The test shall be developed to be done in-house by the rope manufacturers such that all design data are available in advance of a project. Slide 6

7 Design Improvements STATIC The fibre rope force-elongation curve defines the load limit of the rope. It is not possible to exceed the force-elongation curve. - This may happen with the dual-stiffness model. Current industry mooring design practices extrapolate beyond the fibre force-elongation curve. This extrapolation leads to significant consequences in the mooring design: - Over prediction of mooring-line tensions. - Under prediction of vessel offset. Up to 20% reduction in required MBS is possible by taking into account the actual behaviour of the fibre rope. Slide 7

8 Over estimation of tension Force Discrepancy in Maximum Tension Increased Loading Calculated Maximum Tension True Maximum Tension Max Offset Dynamic Stiffness Elongation Slide 8

9 Under estimation of offset Maximum Tension Calculated Maximum Offset Discrepancy in Maximum Offset Force True Maximum Offset Dynamic Stiffness Slide 9

10 Defining Extreme Tension and Offset according to Materials Performance Maximum offset Maximum tension Force Depending on previous loading: Continued cycles move back due to recovery Cycles increase / decrease due to loading / unloading Elongation Slide 10

11 Comparison of strength requirements Design Approach Dual Stiffness Material Curve Configuration 1 1 System Description 4 x 3 Tautleg 4 x 3 Tautleg Top Chain Diameter mm Length meters MBS kn 17,617 17,617 Polyester Diameter mm Lenght meters MBS kn 17,999 17,999 # of Reels Required 4 4 Pile Chain Diameter mm Length meters MBS kn 16,376 16,376 Tensions - Intact Chain kn 9,613 (1.83) 8,249 (2.14) Polyester kn 9,062 (1.99) 7,696 (2.34) Offset - Maximum Intact %WD 5.1% 5.7% Damage %WD 7.4% 8.0% Design case - SEMI 12 lines m WD. 15% over prediction of tension. Maximum Offset: - 8 % under prediction. - Or, 0.6% WD under prediction. Slide 11

12 Optimizing strength Design Approach Dual Stiffness Material Curve Configuration 1 2 System Description 4 x 3 Tautleg 4 x 3 Tautleg Top Chain Diameter mm Length meters MBS kn 17,617 14,358 Polyester Diameter mm Lenght meters MBS kn 17,999 14,426 # of Reels Required 4 3 Pile Chain Diameter mm Length meters MBS kn 16,376 13,186 Tensions - Intact Chain kn 9,613 (1.83) 7,804 (1.84) Polyester kn 9,062 (1.99) 7,360 (1.96) Offset - Maximum Intact %WD 5.1% 6.1% Damage %WD 7.4% 8.8% MBS: - 20% reduction - Total - 15% due to Over Prediction. - 5% due to line size optimization. Polyester. Reduction in # of segment per mooring line. Offset. - Increase in offsets due to material behaviour & optimization. - 1% WD Increase in Mean % WD Increase in Max. Slide 12

13 Estimated Cost Savings: Cost Benefit Component Cost Polyester $ 6.80 / kg Chain $ 1.25 / kg Off-Vessel Cost Chain $ 3,518,505 $ 2,805,855 Polyester $ 8,315,040 $ 6,787,080 Total $ 11,833,545 $ 9,592,935 Component Cost Savings Chain $ 712,650 Polyester $ 1,527,960 Total Savings $ 2,240,610 Installation Savings # of Polyester Reels Required 4 3 ~ US$ 2 million cost savings on chain & polyester costs. Reduction in # of polyester segments saves ~US$500,000 Cost savings does not include onvessel mooring components Crane Vessel Day Rate $ 495,000 / day AHV Day Rate (2 Vessels) $ 150,000 / day Estimated Time Savings Crane Vessel 2 hrs / line AHV 8 hrs / line Estimated Savings Crane Vessel $ 495,000 AHV $ 600,000 Slide 13

14 Design Improvements DYNAMIC We believe dynamic stiffness depends on - cyclic load range (major) - mean load (major) - cyclic period (minor) - number of cycles (major) - loading rate and -shape (minor) The standardised test shall describe the full range of stiffness to be applied in design analyses. Slide 14

15 Design Improvements DYNAMIC To make an example, we compiled existing DNV test data to plot stiffness shape. To our knowledge, this JIP is first to establish such a stiffness shape as a general design tool. Data was limited. To be validated in the project. Astounding fit between data and shape. Slide 15

16 Summary Improvements DYNAMIC If the rope manufacturer has established the stiffness shape at the time of enquiry, this information will be available prior to - product selection - mooring system design and it will be quoted together with - price - delivery time - shipping details Significant improvement of the ability to tailor design ropes for specific purposes. Input data to Fibre Rope Modeller if this is used to further optimise rope design. Better control with the fatigue design calculations. Slide 16

17 Summary, Design Practice JIP Significant cost saving can be realized in the mooring components by taking into account the behaviour of the fibre material. Vessel offset may be under predicted if the behaviour of the fibre material is not taken into account during the design process. Similarly, the design tension may be over predicted thus exposing connecting hardware to higher-than-necessary fatigue loads. Installation time can be reduced by a reduction in the number of line segments in each mooring line. Due to the predictable behaviour of the fibre rope, detailed length and stiffness information can be available BEFORE a project is initialised. Slide 17

18 Managing the Safe Service Life of Fibre Ropes for Mooring Slide 18

19 The project addresses: How to qualify a rope for 20 years in service. Not have to rely on regular insert retrievals. How to requalify a rope that has seen excessive loading outside the design basis. How to requalify a used rope of unknown service history. - Can a 20-year old rope be used for another 5 years? If an insert shall be tested, what is the right way to do it? - Break testing has limitations as shown in next slide. Slide 19

20 Sudden death Residual Strength [%] Diolen 855T polyester at 20 C Loading rate: 800mN/tex in 100s Fraction of life time used [-] Break test does not show residual life. Sudden death behaviour is a consequence of creep-rupture damage accumulation. Sudden death is not special to polyester. It occurs in steel too in a fatigue failure mode. (Damage accumulation.) For chain a simple break test is normally not acceptable to assess the Safe Service Life. At least some inspection scheme is applied. (Visual, MPI.) Slide 20

21 Time-To-Rupture vs. load/elongation Force (Illustration only) TTR 1 year TTR 10 years TTR 0.1 second TTR 1 week TTR 1 month TTR several hundred years TTR several thousand years Elongation Slide 21

22 ARELIS The relationship between the changes in Time-To-Rupture with changes in force level has been given by the following logarithmic relationship: (5 ± 1) % of strength per log (time). This means that for example, if the sustained force level is increased from 45 % to 50 % of the strength, then the time to rupture is reduced from years to 1000 years. Equally, if the sustained force level is increased from 90 % to 95 % of the strength, then the time to rupture is reduced from, say, 1000 seconds to 100 seconds. By measuring the time at sustained load level without failure, the Assured REsidual Life Span can be easily calculated for any loading pattern. Slide 22

23 ARELIS 1 0,8 normalized tension 0,6 0,4 D 1 1 week Assured Residual Life Span 0, weeks 0 log (time) 1 20 Slide 23

24 Example ARELIS Example mooring design boundary conditions for 20 years design life: a) 100-year condition of 73 % MBS for 40 hours duration. b) 10-year condition of 68 % MBS for 20 hours duration. c) 5-year condition of 55 % MBS for 15 hours duration. d) 1-year condition of 45 % MBS for 48 hours duration. As a result of the JIP, we assume the following to have been validated: - ARELIS slope: 5 % MBS / decade. - Dynamic loads can be conservatively converted to static. Slide 24

25 Summarize the loads as follows: 100-years condition is expected to occur once in 20 years: a) % MBS % MBS. 10-years condition is expected to occur twice in 20 years: b) % MBS % MBS. 5-years condition is expected to occur 4 times in 20 years: c) % MBS % MBS. 1-year condition is expected to occur 20 times in 20 years. However, we assume constant occurrence of the 1-year condition: d) % MBS % MBS. - This is done to simplify in a conservative manner. Slide 25

26 Test load and duration Calculate equivalent durations at test load of 75 % MBS: a) 100-year condition: Load increase in the test: 75 % MBS - 73 % MBS = 2 % MBS. No. of time decades: 2 / 5 = 0.4 decades. test load: 40 hrs = 15.9 hrs. b) 10-year condition: Load increase in the test: No. of time decades: test load: 75 % MBS - 68 % MBS = 7 % MBS. 7 / 5 = 1.4 decades. 40 hrs = 1.6 hrs. c) 5-year condition: Load increase in the test: No. of time decades: test load: 75 % MBS - 55 % MBS = 20 % MBS. 20 / 5 = 4.0 decades. 60 hrs = 0.01 hrs. Slide 26

27 Test load and duration d) 1-year condition: Load increase in the test: No. of time decades: test load: 75 % MBS - 45 % MBS = 30 % MBS. 30 / 5 = 6.0 decades hrs = 0.18 hrs. Provided loads can be summarized, the following time at test load will be equivalent to all service loads over the lifetime: - Equivalent 75 % MBS: 15.9 hrs hrs hrs hrs = 17,69 hrs. If the required safety factor on life is set to 10, the test time will be: - Test 75 % MBS: hrs = 177 hrs. = 7.4 days. Slide 27

28 What does this mean? This means that if the test sample passes the test without failure then the mooring system has been qualified for 20 years in service for this particular mooring-design scenario - based on failure mode accumulated creep damage as published by Rigo Bosman. - No need to take out inserts regularly since condition has been assessed in advance. The other failure mode internal abrasion must also be ensured against for the 20 years in service. - This is done by establishment of what this project has named the Safe Service Diagram. Slide 28

29 Safe Service Diagram The Safe Service Diagram shows the operating range where abrasion will not affect the rope for the 20-years design life. Load range (%MBL) Example Safe Service Diagram Slack Wear 10% min tension benign ARELIS storm Mean load (% MBL) Max tension = 60% This diagram shows in what region the mooring is safe from internal abrasion. - Other damage modes must be treated accordingly if present, such as mechanical damage. Slide 29

30 Inserts If the ARELIS is tested when the rope is new and if the system operates BELOW the expected maximum conditions, then the regular insert retrievals can be made redundant, or, at least the removal intervals significantly prolonged. If the system has operated ABOVE the expected conditions then an insert should be retrieved and the system re qualified with another ARELIS test. Break-test results may be misleading for polyester rope as it is for chain due to the so-called sudden death. Force For condition assessment it is better to use ARELIS testing. TTR 1 year TTR 10 years TTR 0.1 second TTR 1 week TTR 1 month TTR several hundred years TTR several thousand years Elongation Slide 30

31 Commentary Creep damage is a cumulative process, it is not repaired when the high load is taken off. The relationship between tensile and creep yields the following admissions: At moderate forces the service life of the polyester mooring is very long. At extreme forces the service life of the rope is within measurable range such as days, hours, minutes or seconds. Thus, strength may be defined by a time requirement according to ARELIS. Force TTR several thousand years TTR 1 year TTR 10 years TTR several hundred years TTR 0.1 second TTR 1 week TTR 1 month Elongation Slide 31

32 Defining time requirement Force 1 TTR several thousand years TTR 1 year TTR 10 years TTR several hundred years TTR 0.1 second TTR 1 week TTR 1 month Elongation The reason for having a safety factor is to have reserve capacity for the un-known. For example, if a system designed for 100-year event experiences a 500-year event. Thus, it makes more sense to define a required life time at a certain designated load, than defining a break load that corresponds to a minimal time. normalized tension 0,8 0,6 0,4 D 1 1 week Assured Residual Life Span We do know that the unknown load does not act for a minimal time corresponding to the break test. 0, weeks 0 1 log (time) 20 Proposal: Define the required rope strength as the force level at which it survives 24 hours at sustained, high load. RRS 24 = 1000 tons. Slide 32

33 Summary, Safe Service Life JIP Creep-rupture failure mode assessments (ARELIS) can be used as a tool to predict and calculate safe service life. Regular insert retrievals will be redundant. Used rope may be requalified. Required strength may also be defined based on creep-rupture failure mode. Slide 33

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