Applied

Steel Wire Rope Bending Cycle fatigue

Part Two-Initial Setting of Trigger Action and Trigger Discard Values

The series titled  Wire Rope Integrity Management is to be read in conjunction with the Whole Life management of Steel Wire ropes procedure  and consists of the following elements:

Part One – Bending Cycle Counting

Part Two – Initial Setting of Trigger Action and Trigger Discard Values

Part Three –  Empirical assessment of Steel Wire Rope Service Life using Bending fatigue Calculation

Part Four – Practical example of Bending fatigue Calculator Hardware

Part Five -  Structural Fatigue Cycle Counting


Definitions


Load Modifier

Multiplier applied to bending cycle count depending on applied load on the Wire


Load Collective Modifier

Multiplier applied to bending cycle count depending on Collective of Loads applied to Wire rope


Bending Fatigue Calculator Solution

A Hardware / Software solution as described in Part Four – Practical example of Bending Fatigue Calculator Hardware


Trigger Action Value (TAV)

Section line Bending Cycle Fatigue count to trigger some further action ( NDT, Cut back, determination of ABL etc)


Trigger Discard Value (TDV)

Extended  section line Bending Cycle Fatigue count that leads to discard of the Wire Rope. This is always greater than the TAV.


Initial Assessment of Annual  Bending Fatigue Cycle Count Cfg


Where

Ca  =  Generic annual assessment of Bending fatigue Cycle count

Cdr  = Cycles in Deployment OR Recovery

Cc  = Cycles  due to Compensation

Mlc  = Load Collective Modifier Value


Cycles Due to Deployment recovery in Simple Reeving System


Where

Cdr = Cycles in Deployment OR Recovery

D = Movement of Hook during Deployment / recovery [m]

r = Sheave radius,  this is considered to be 13 times the wire rope diameter


Mdr =  Bending Cycle Plan Multiplier (see MAR/BBE/12/009)

Ml =  Load Multiplier,


Cycles Due to Compensation in Simple Reeving System


Where

Cc = Cycles  due to Compensation

Rc = Movement of Rope due to Compensation [m]

r = Sheave radius,   for generic systems this is considered to be 13 times the wire rope diameter [m]


Mdr =  Bending Cycle Plan Multiplier (see MAR/BBE/12/009)

Ml =  Load Multiplier,


Generic Annual Bending fatigue Cycle Expectations


Key Assumptions

Vessel has 100 % service (365 days per annum)

Vessel is on station 70% of this period  (256 days

Vessel Deploys  / recovers  each 5 days  ( 52 instances)

During each Deployment / Recover greater than 75 % load is seen during Deployment / recovery and in compensation  for  12 Hours following (624  Hours)

For all other periods less than 25% load is experienced   (5496 Hours)

Sea State during these periods is 3m SWH

Hook deployment  is considered 60M

System is considered of type ‘Simple’ with unitary multiplier

Reeving system for Well Enhancer is loosely considered for baseline

Reference should be made to Wire Rope Integrity Management Part One – Bending Cycle Counting and MAR/BBE/10/014 Rope Fatigue Calculation in Well Intervention Guide Wire Ropes



Estimation Bending Fatigue Cycles during Deployment /  Recovery


Where

Cdr =  Cycles in Deployment OR Recovery

D =  Movement of Hook during Deployment / recovery [m]

r =  Sheave radius,  this is considered to be 14 times the wire rope diameter [m]

Mdr =  Bending Cycle Plan Multiplier  which is taken to be 7.5 (see MAR/BBE/12/009)

Ml =  Load Multiplier, this is considered to be 6


For 19mm Rope , per annum

Cdr19 = 860/(0.019x14) x 52

Cdr19 = 168120


For 22mm Rope per annum

Cdr22 = 860/(0.022x14) x 52

Cdr22= 145194


For 26mm Rope Per Annum

Cdr26 = 860/(0.026x14) x 52

Cdr26= 122857


For 48mm Rope per Annum

Cdr48 = 860/(0.048x14) x 52

Cdr48 = 66547


For 72mm Rope per Annum

Cdr72 = 860/(0.072x14) x 52

Cdr72 = 44365



Estimation Bending Fatigue Cycles during Compensation


Where

Cc = Cycles  due to Compensation

Rcp = Periodic Movement of Rope due to Compensation [m]. Assumed Sea state is 3m SWH

r = Sheave radius,   for generic systems this is considered to be 13 times the wire rope diameter [m]


Mdr =  Bending Cycle Plan Multiplier which is taken as 1  (see MAR/BBE/12/009)

Ml =  Load Multiplier,


Where

Rcp  = Periodic Movement of Rope due to Compensation [m

 SWH  = Significant Wave height [m] – assumed to be 3m SWH

r = Sheave radius,   for generic systems this is considered to be 13 times the wire rope diameter [m]

P = Period in Compensation [Hours]

Wp = Wave Period [s] – assumed to be 10


For 19mm Rope , per annum

Cc19 at 25%  = 344 x( 5496 x 1)/(0.019x14)

Cc19 at 25% = 7107609

Cc19 at 75%  = 344 x( 624 x 6)/(0.019x14)

Cc19 at 75% = 4841864

Cc19 = 11949473



For 22mm Rope per annum

Cc22 at 25%  = 344 x( 5496 x 1)/(0.022x14)

Cc22 at 25% = 6138389

Cc22 at 75%  = 344 x( 624 x 6)/(0.022x14)

Cc22 at 75% = 418609

Cc22 = 10320000


For 26mm Rope Per Annum

Cc26 at 25%  = 344 x( 5496 x 1)/(0.026x14)

Cc26 at 25% = 5194021

Cc26 at 75%  = 344 x( 624 x 6)/(0.026x14)

Cc26 at 75% = 3538285

Cc26 = 8732307


For 48mm Rope per Annum

Cc48 at 25%  = 344 x( 5496 x 1)/(0.048x14)

Cc48 at 25% = 2813428

Cc48 at 75%  = 344 x( 624 x 6)/(0.048x14)

Cc48  at 75% = 1916571

Cc48 = 4730000



For 72mm Rope per Annum

Cc72 at 25%  = 344 x( 5496 x 1)/(0.072x14)

Cc72  at 25% = 1875619

Cc72 at 75%  = 344 x( 624 x 6)/(0.072x14)

Cc72 at 75% = 1277714

Cc72 = 3153333



Where

Mlc  = Load Collective Modifier

Srh  = Service reduction [%] at High collective

Srl  = Service reduction [%] at Low  collectives

P%h = % period at High Collective

P%l = % period at Low Collective



Load Collective Modifier =  [(33% x 25%) + (100% x   75%)] / 100% = 83.25%


For 19mm Rope , per annum

Ca19 = 168120 + (11949473/.8325)

Ca19  = 14521841



For 22mm Rope per annum

Ca22 = 145194 + (10320000/.8325)

Ca22  = 1384833


For 26mm Rope Per Annum

Ca26 = 122857 + (8732307/.8325)

Ca26  = 10612114


For 48mm Rope per Annum

Ca48  = 66547 + (4730000/.8325)

Ca48  = 5748228


For 72mm Rope per Annum

Ca72= 44365 + (315333/.8325)

Ca72= 423143



Determining Initial Service Life of Steel Wire Ropes Sl

Along  with a determination of bending cycles is the selection of an Initial  service life for the Steel Wire Rope.


A service life determined through Empirical  periodical evaluation of the condition of the rope subject to Bending fatigue.  The degree of Service life is determined by the Wire rope Usage, typical initial Values are as follows


Setting Maximum Bending Cycles in  Service Life


Where:

Csl  =  Bending Cycle Count for Service Life of Rope  

Ca  = Bending Cycle Count per annum for reeved system

Cg = Sheath Groove modifier

Cb =Sheave Bearing modifier

Cf =Fleeting Angle Modifier

Sl =Service Life [Years],



Effects of Worn or incorrectly   Grooves Cg


Effects of  Sheave bearings  Cb


Effects of  Fleeting Angles  Cf


Setting of Trigger Values


Setting of Trigger Maintenance Values (TMV)

One or more TMV may be assigned to initiate some Planned Maintenance Activity.


TMV is always less than TAV.


Typically this will be one or more of the following:

Cut Back- to relocate area of High Fatigue away from Compensated Zone

Cut Back- to establish residual ABL (ABF)

Cut Back -  to establish residual ABL (ABF) and to establish current condition of rope through Visual inspection

NDT – magnetic anomaly inspection of fatigue area of Rope

NDT – magnetic Anomaly inspection through rope

Visual Inspection – Including opening for inspection  rope core.



Setting of Trigger Action Values (TAV)

The TAV will initiate  an Activity, typically cutback, that will move a section of rope whose local area of Fatigue exceeds the TAV out of the Load Affected Area


The Action may be triggered on a periodic Maintenance base or by some visual inspection, it may also be triggered by a Bending fatigue Cycle count being exceeded.


The value of this count is determined by the Crane Categorisation, for Well Intervention systems all wire ropes which are used  directly in the Intervention process are considered Category D.


Setting of Trigger Discard Values (TDV)

The TDV differs from the  TAV in  a number of key areas

TDV relates to an extended portion of rope  is for discrete zones in the rope.

TDV evaluation is on a Periodic Base, TAV evaluation is an automated continuous process.


This Periodic Base will be set Empirically but may be set initially at


TDV allows for discard  of entire section  only independent of whether CSL has been exceeded in some areas

TDV Values are set empirically but may initially be set at


Removed from the remaining rope and subject to post use inspection in accordance with Whole Life Management of Steel Wire Rope Procedure



Determining Wire Rope  Mean Discard Value

Discard value is applied to the ‘ Active section’ of the rope which is expected to see  load in normal operations.


This should include the portion of the  rope, which through variation in Operation may  be  moved to a position where load is to be applied and which has been previously subject to a significant degree of bending fatigue



The evaluated section of rope is to be considered in 10 or more sections or length not greater than 10m. Within each section an approximated  mean value is to be estimated


The lower 20% of these are to be discarded and the mean value determined of remaining


Calculating Mean Value Xm


X =  A Measured Value in a group

Xm =  The average value of the Group

n = number of measured values