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New Technologies for BOP Shearing
20 August 2013
Covey HallGlobal Manager for Consulting ServicesLloyds Register Energy Drilling
AgendaHistoryObservations as 3rd party verifierGTC Shear Study Use of fiber-optic strain gaugesAcknowledgements References
2003 and 2004 Shear StudiesPerformed by WEST Engineering for the MMS
Understanding the Shear FunctionLast resort is to shear pipe and secure the well with the sealing shear ram.
Failure to shear could result in a major safety and/or environmental event.
Improved strength, larger and heavier drill pipe adversely affects the ability to successfully shear and seal.
Actual Shearing or Breaking ActionUpper and Lower Shear Blades crushing the drill pipe and beginning the shearing (or breaking) operation.
Increased Shear Pressure due to Wellbore Pressure and Hydrostatic Head
The areas where mud, seawater and BOP fluid pressures effect a BOPs operation:1 Mud Pressure2 Seawater Pressure or hydrostatic head3 BOP Fluid Pressure plus hydrostatic head4 Seawater Pressure or hydrostatic head
Area 4 and its pressure effects do not exist on BOPs without tailrods.Hydrostatic HeadWellbore Pressure
Observations AS 3RD PARTY Verifier
ObservationsThere is a small but significant percentage of shear tests where the actual test exceeds the OEM calculation method.
Deviation between actual shear pressure tests and theoretical calculations varies between OEMs
Data input requirements vary between OEMsSome use nominal yield, some use measured stress from tensile test
Manufacturer X, 18 3/4 15k6.625" OD, 0.813" wall Z-140Shop conditions - no correction for MASP or Hydrostatic
Observations (contd)Metallurgical advances in drill pipeS-135 grades are now produced with improved ductility
Z-140 and V-150 grades Higher strength, lower ductility, brittle fracture More common in deepwaterWill ductility improve as it did with S-135?
Material Test ReportsWhat material tests produce the most accurate shear predictions?
Shear Examples Low & High DuctilityLow ductility or brittle pipe (left) and high ductility pipe (right).
The high ductility pipe required almost 2,000 psi (over 300K lb) more to shear than the low ductility pipe even though the grades were the same, S-135.
The brittle pipe had cracking on the sides and did not collapse as much as the ductile pipe.
Pipe GeometrySince the 2004 studyLarger OD pipes, thicker wallsLonger internal upsets and tool jointsIncrease in verifications for concentric tubulars, completion strings, wirelines, etc.Maximum fold-over limit in some model BOP models
Observations (contd)Shear test repeatabilitySame grade & dimension, different vendor - may produce different test resultsSame pipe heat variations in measured yield strength, UTS, and percent elongationSame pipe variations in properties along the length of a single jointNew vs. used (premium) pipeVariations in shear test measurementsWall thickness Testing Protocol Shear AND SealSuccessful pressure tests LP and HPShop tests dont replicate pipe stress conditions down hole.
Observations (contd)Subsea accumulator volume - shear pressure available for emergency functions
Evolution of ram geometry
Replaceable blade componentsLow force geometriesPipe Centering
Keeping track of drill pipe on the rig not easy
Sharing of data has improved in recent years
Attention to unusual loading conditionsSide loadBending loadDrill string in tension, compression, or torsion
GTC Shear Study
Lloyds Energy Group Technology Centre (GTC)Sinagapore
Objectives of Singapore GTCProviding a catalyst for accelerating growthEngagement with clients and government via collaborative projectsEnhancing technical edge by focusing on R&D Themes Technology Readiness Level (TRL) 4-6Developing complementary technology to Southampton GTC Providing facilities for internal and client trainingProviding a mechanism for technology and skills transfer
Shear Ram Reliability Study Phase 1 ObjectivesDevelop FEA modeling method using fracture theory and determine what material tests are required to obtain accurate results vs. actual shear tests of same pipe. Evaluate the relationship between material properties and forces applied during BOP shearing scenarios.Evaluate the relationship between thin-wall and thick walled tubulars.Collect and analyze joint industry shearing data for all grades of tubulars, BOP models, and shearing ram configurations. Repeatability/variance of shear pressures for a given set of drill pipe.
Develop or refine shear ram calculations and modelling techniques based on empirical evidence, using appropriate statistical methods, and determine appropriate factors of safety.
Develop more accurate mathematic models for predicting shear capability, and statistical reliability
Project MotivationShear prediction is currently empirical loosely based on DET and OEM test data directly or indirectly. Different formulas for each OEM.Most of the OEMs and LRED currently use variations of the Distortion Energy fracture theory. Anecdotal results show that the theoretical shear pressures are much higher than actual shear tests in majority of cases, previous exception cases noted.For a given grade of pipe, there appears to be a rather large distribution of test results for force required to shear the pipe.Effects of ductility variation for a given nominal yield pipeIndustry is rapidly changing new pipe grades, improving quality within pipe grades, new ram designs. Prediction based on historical data becomes outdated. No method for new situations.
Project GoalsScientific: Understand shearing, verify theoretical modeling with new fracture and plasticity theory.
Practical: Explore variation of shear force within one pipe, within one heat batch, within a pipe grade.
Long term: develop theoretical prediction tool based on standard material testing.
Fracture Theory and FEA ModelingApplication of a different fracture theory in Engineering Mechanics Literature (Mohr- Coulomb Criterion) crack initiation as an extension of plasticity theory.
FEA Modeling element removal when fracture criterion is met simulated crack initiation and propagation.
Critical is the determination of plasticity and fracture coefficients from material testing.
Distortion Energy Theory
The distortion energy theory says that failure occurs due to distortion of a part, not due to volumetric changes in the part (distortion causes yielding, but volumetric changes due not).
Distortion Energy Theory is less conservative than Maximum Shear Strength Theory (Tresca Criterion), but more conservative than the Maximum Normal Stress Theory (von Mises Yield Criterion)
Assumes compressive and tensile yield criteria are equal
F = 0.577 x SY x A
Where:F = Force, lbsSY = material tensile yield strength, psi0.577 converts tensile yield to shearA = cross-sectional area of drill pipe, in2
Does the distortion energy theory accurately model fracture of newer, stronger, less ductile drill pipe?Is there a better way to model increasingly more brittle fracture characteristics, in order to calculate force (hydraulic operating pressure) required to shear a particular tubular?
MIT Research, 2007 - 2012Have applied a different fracture theory to predict material failures in automotive, structural, and naval applicationsThe Mohr-Coulomb fracture theory, aka the Coulomb Criterion
Applied a different methodology to traditional Finite Element Modeling.Traditional FEA does not address initiation of cracks, but only propagation of cracksThis approach takes into account crack initiation, and then incorporates the removal of the element from the model once yield criteria is exceeded
MohrCoulomb Yield (failure) Criterion(aka Coulomb Criterion)
Similar to Maximum Shear Strength Theory (Tresca Criterion), but take also applies to materials for which the compressive strength far exceeds the tensile strength
Mostly used geotechnical and structural engineering to determine shear strength and fracture angleSource: MIT Paper
Coulomb Criterion Applied to Finite Element Modeling (FEA)Research Phase 1
Phase 1 MethodologyCreate advanced material test coupons from pipe Preferably on fish that have already been tested
Data analysis to get material coefficients from material tests.
Plug those material coefficients in FEA modeling of BOP/Pipe shearing
Compare those FEA results against actual shear test results and calculations using existing Distortion Energy Theory techniques
Material Testing to support the MCC
Material Testing results are then mapped and a 3-D best fit is determined
This determines the yield surface as predicted by the Coulomb Criterion
Source: MIT Paper
That material property data is then applied to every element in the FEA model.Source: MIT Paper
Seek minimum adequate input setMaterial constants needed for FEA can be determined based on variable amount of input data from material testing.Determine minimum adequate data input set.
1) MTR data (Yield, UTS, % elongation)2) Tensile curve3) Compression curve4) Tors