subsea - page 2

7/27/2019 subsea - page 2

1/16

BasicConcepts


2/16

Subsea Engineer s Handbook Sect ion 2

In-Spec Inc. 1999 0 Basic Concepts

Table of ContentsSection 2

Page

1. What is Pressure? 1

2. Basics of Hydraulic Pressure 1

3. Steel Information 2

4. Hardness Table 4

5. H2S And Well Control Equipment 56. Rubber Goods Storage 11

7. Elastomer Operational Limits and HT Elastomers 13


3/16



What is Pressure?

1. A piston with 1 square inch of area supporting 1,000 pounds on top generates 1,000psi as the term defines it.

2. 1/10ththe weight will produce 100 psi, centerexample or

3. 10x more area also reduces the pressure to 100 psi, as in right handexample.


4/16



Basic Steel Information

Several words about steel, which may sound familiar, will be encountered throughoutour discussion of BOPs. Lets take the time to understand them now so we can usethem accurately later. Frequently we will talk about steel strength and hardness.These are both related.

1) StrengthThe strength of steel is defined as the maximum force, in psi, which can beapplied with out destroying the parts usefulness. The steels strength can beexpressed in several ways, any one of which may be regarded as the strength ofthe material. It is measured by literally pulling a sample of the steel apart andmeasuring the force required.

2) StressMore technically, the force applied to the steel sample is called stress. It ismeasured in units such as PSI.

3) StrainNote that the sample just before breaking is longer. The deformation is calledstrain.

Above is a steel sample prepared for the tensile tester which will gripthe sample in the threaded area as and pull to destruction.


5/16



By graphing the applied force to the tensile test sample (stress) on the left-handside of the graph and the resulting deformation of the sample on the bottom of thegraph.

We see a point YP where the sample began to rapidly deform with little or noadditional force applied. The sample held a maximum force at point TS beforebreaking at point BS (breaking strength).

Going b ack to the f i rs t comm ents, the strength of steel is determined by

referr ing ei ther to the po int at which the steel began to :

easi ly deform , cal led Yield Stress, or Yield Point (YP on th e chart)

o r

the poin t at wh ich i t actual ly parted, cal led Tensi le Stress (TS on the

chart) .

The oil field refers more often to yield point when talking about API flanges, drillpipe grades, riser wall material, and H2S service. This is because once thematerial deforms, it is generally no longer suitable for its intended service, Itleaks.


6/16



Hardness Conversion Numbers for Steel

BRINELL

3000 kq. Load10 mm. Ball

ROCKWELL SHORESCLEROSCOPE

TENSILE

STRENGTH

psi(Approx.)Diameter

MillimetersHardnes

sNumber

A Scale B Scale C Scale 15-NScale

2.25 745 84.1 65.3 92.3 91 2.30 712 - - - - -2.35 682 82.2 61.7 91.0 84 -2.40 653 81.2 60.0 90.2 81 -2.45 627 80.5 58.7 89.6 79 2.50 601 79.8 57.3 89.0 77 -2.55 578 79.1 56.0 88.4 75 -2.60 555 78.4 54.7 87.8 73 2980002.65 534 77.8 53.5 87.2 71 2880002.70 514 76.9 52.1 86.5 70 2740002.75 495 76.3 - 51.0 85.9 68 264000

2.80 477 75.6 - 49.6 85.3 66 2520002.85 461 74.9 - 48.5 84.7 65 2420002.90 444 74.2 - 47.1 84.0 63 2300002.95 429 73.4 - 45.7 83.4 61 2190003.00 415 72.8 - 44.5 82.8 59 2120003.05 401 72.0 - 43.1 82.0 58 2020003.10 388 71.4 - 41.8 81.4 56 1930003.15 375 70.6 - 40.4 80.6 54 1840003.20 363 70.0 - 39.1 80.0 52 1770003.25 352 69.3 110.0 37.9 79.3 51 1700003.30 341 68.7 109.0 36.6 78.6 50 1630003.35 331 68.1 108.5 35.5 78.0 48 1580003.40 321 67.5 108.0 34.3 77.3 47 1520003.45 311 66.9 107.5 33.1 76.7 46 1470003.50 302 66.3 107.0 32.1 76.1 45 1430003.55 293 65.7 106.0 30.9 75.5 43 139000

3.60 285 65.3 105.5 29.9 75.0 - 1360003.65 277 64.6 104.5 28.8 74.4 41 1310003.70 269 64.1 104.0 27.6 73.7 40 1280003.75 262 63.6 103.0 26.6 73.1 39 1250003.80 255 63.0 102.0 25.4 72.5 38 1210003.85 248 62.5 101.0 24.2 71.7 37 1180003.90 241 61.8 100.0 22.8 70.9 36 1140003.95 235 61.4 99.0 21.7 70.3 35 1110004.00 229 60.8 98.2 20.5 69.7 34 1090004.05 223 - 97.3 18.8 - - 1040004.10 217 - 96.4 17.5 - 33 1030004.15 212 - 95.5 16.0 - - 1000004.20 207. - 94.6 15.2 - 32 990004.25 201 - 93.8 13.8 - 31 970004.30 197 - 92.8 12.7 - 30 940004.35 192 - 91.9 11.5 - 29 92000

4.40 187 - 90.7 10.0 - - 900004.45 183 - 90.0 9.0 - 28 890004.50 179 - 89.0 8.0 - 27 880004.55 174 - 87.8 6.4 - - 860004.60 170 - 86.8 5.4 - 26 840004.65 167 - 86.0 4.4 - - 830004.70 163 - 85.0 3.3 - 25 820004.80 156 - 82.9 0.9 - - 800004.90 149 - 80.8 - - 23 -5.00 143 - 78.7 - - 22 -


7/16



H2S and Well Control Equipment"The Details"

1) What is H2S service equipment?

Many metals, when under stress, experience brittle failure when exposed tohydrogen sulfide gas. A hydrogen sulfide environment, or frequently called sourservice, causes metals to suddenly fail in a brittle manner at a fraction of theirnormal strength after extended periods of satisfactory performance.

a) Over the years this process of has been given many different names, but it isnow generally known as Sulfide Stress Cracking (SSC).

i) Sulfide because H2S and at least traces of water must be present.ii) Stress because a tensile stress - either residual (from thermal treatment of.

fabrication), applied (service load), or a combination of both - must bepresent.

iii) Cracking because failure occurs by sudden brittle fracture rather than bythe slow ductile yielding normally encountered in metal failure.

2) What causes normally ductile metals to fail in a brittle manner?

a) The ability of a metal to resist brittle fracture is related to its toughness, which

is determined by the metals strength and ductility. In general, as strengthincreases, ductility and therefore, toughness decrease.

b) The inherent toughness of a metal is influenced by many factors - chemicalcomposition, microstructure, thermal treatment, grain size, etc. Any loss oftoughness during metal manufacture, processing, or in service is termedembrittlement. Metals can be embrittled in many different ways (there are atleast nine different ways steels can be embrittled during thermal treatment orin elevated temperature service) and by many different agents, but the chiefembrittling agent is hydrogen.

c) There are two reasons for this. First, the hydrogen atom - the smallest of allatoms - can readily enter into and diffuse through the metal lattice, even atroom temperature. Second, metals are used in such a wide variety ofhydrogen-containing environments - water, moist air, lubricants and otherhydrocarbons, acids and other chemicals, - and atomic hydrogen is formedduring so many operations - pickling, electroplating, welding, corrosionreactions, galvanic coupling, stray and impressed currents - that there is


8/16



ample opportunity for hydrogen to enter the metal. And, as we shall see, verylittle hydrogen is needed to cause extensive damage to the metal.

3) What does hydrogen have to do the effects of H2S?

a) When nascent atomic hydrogen (H+) is generated at a metal surface by any ofthe operations mentioned earlier, most of it quickly combines with otherhydrogen atoms to form molecular hydrogen (H2), that is too large to enter themetal lattice, and bubbles off harmlessly as gas. But some of the hydrogendoes enter the metal, and, if a poison such as H2S is present, much more willenter, since H2S delays the recombination reaction - possibly by severalorders of magnitude. In effect, this increases the amount of atomic hydrogenavailable for entry into the metal.

b) This explains why steel in a mildly alkaline environment containing H2S ismore likely to suffer hydrogen damage than the same steel in an acidenvironment not containing H2S. This is the only effect of H2S on SulfideStress Cracking - to permit the absorption of greater quantities of atomichydrogen than would be absorbed if H2S was not present. Once in the metal,the source of the hydrogen and the presence of H2S in the environment is ofno consequence.

c) Therefore, we can say that Sulfide Stress Cracking results from a form ofhydrogen damage known as hydrogen embrittlement that is made much moresevere by the presence of H2S.

4) How does hydrogen embrittle metals?

a) Hydrogen embrittles metals by reducing the ductility and the fracture stress.Although the manner in which this is accomplished is still controversial, it isknown that metals may crack and fail when exposed to hydrogen. The failuremay occur immediately or may be delayed for hours, days, weeks, or evenyears. For example, a highly stressed carbon steel spring will shatterspontaneously when dropped into a strong acid. In the more usual case, thereis no apparent damage for an extended period of time, then sudden, complete,brittle fracture occurs. This is known as hydrogen-induced, delayed, brittle

fracture.


9/16



5) What is the mechanism of delayed brittle fracture?

a) Delayed brittle fracture occurs in three separate and distinct stages:

Stage 1 - An incubation period during which no hydrogen damage orcracking occurs.

Stage 2 - A period of relatively slow crack growth and complete failure.

Stage 3 - A period of rapid crack growth and complete failure.

b) The delayed failure process is shown schematically on the attached graph, inwhich time-to failure is plotted against stress. If the applied stress is greaterthan the Upper Critical Stress (UCS), failure occurs immediately. If the applied

stress is less than the Lower Critical Stress (LCS), or Threshold Stress, thereis no hydrogen damage and brittle failure does not occur. At intermediatestress levels, given sufficient time, hydrogen damage and brittle failure areinevitable.

c) During Stage 1, hydrogen is entering into and diffusing through the metal.Under the combined influence of a hydrogen concentration gradient (betweenthe hydrogen in the environment and the hydrogen in the metal) and a stressgradient (from the applied stress), the hydrogen accumulates at locally highlystressed regions. When a critical hydrogen concentration is reached, a sub-microscopic crack initiates and propagates to the edge of the hydrogen-rich

zone, then stops.

d) The slow crack growth process of Stage 2 is analogous to the crack initiationprocess of Stage 1. Hydrogen diffuses to the highly stressed region justahead of the existing crack tip, and, when the critical concentration is reached,the crack grows to the edge of the hydrogen-rich zone, then stops. Thisdiscontinuous, stop-and-go crack growth continues until the remaining sectioncan no longer sustain the load, and Stage 3 - rapid crack growth and completefailure - ensue.

e) No hydrogen movement of hydrogen damage occurs during Stage 3, and the

final failure may be either brittle or ductile, depending upon the stress level,the strength and fracture toughness of the metal, and the geometry of thefailed piece.

f) he delayed failure graph shows why hydrogen embrittlement damage may notbe detected by the usual impact and tension tests - the time-to-failure for thesetests is less than the incubation period required for hydrogen damage to occur.


10/16



g) The graph also explains how a metal that has been exposed to a hydrogen-charging environment may be salvaged in an undamaged condition. If thetime of exposure has been less than the incubation period of the metal and

environment, the hydrogen can be removed by baking the metal at anelevated temperature or aging at room temperature, since hydrogen willdiffuse out of a metal as well as into it. Of course, if the incubation period hasbeen exceeded, baking or aging will not heal the initiated crack.

h) The Lower Critical Stress (Threshold Stress) has important practicalsignificance. It is the maximum stress that can be applied to a given metal in agiven environment without hydrogen- induced brittle failure occurring. Athigher stresses (,) failure is inevitable. The threshold stress is affected byhydrogen concentration, stress concentration, metal strength, andtemperature. It can vary from near zero for high strength metals in severe

environment to levels approaching the metals tensile strength for low strengthmetals in mild environments.

i) The delayed failure behavior of metals is affected by the interaction of variousmaterial and environmental factors. Incubation time, crack growth rate andtime-to-failure may all be shortened or lengthened; Upper and Lower CriticalStress values may be increased or decreased.

6) What are the factors that affect the delayed failure behavior of metals in H2Senvironments?

a) The principal material factors affecting delayed failure behavior are strength(hardness), hydrogen content, stress (Load), and temperature; but thermaltreatment, and compositional and processing factors may also have an effect.The principal environment factors are H2S concentration, pH, andtemperature; but corrosivity, galvanic coupling, and cathodic currents may alsohave an effect.

b) Strength (Hardness) is the most important material factor. Time-to-failure andthe Lower Critical (Threshold) Stress both decrease dramatically as metalstrength increases. Since fracture toughness generally decreases as strengthincreases, the critical hydrogen concentration required for crack initiation and

propagation is much less (Possibly 1 ppm or less) for higher strength metalsthan for lower strength metals. Furthermore, since higher strength metals cansustain higher stress before plastic deformation relieves the stresses; higherstress gradients occur in higher strength metals, particularly in the presence ofstress concentrations such as notches, scratches, or threads.


11/16



c) Hydrogen Content - With increasing hydrogen content the stress required forcrack initiation and propagation decreased and the critical hydrogenconcentration is attained more rapidly. Consequently, the Upper and Lower

Critical Stresses and the time-to-failure all decrease as hydrogen contentincreases.

d) Stress (Load) - At stresses higher than the Lower Critical (Threshold) Stress,time-to-failure decreases with increasing stress level. However, the effect ofstress is considerably less than that of strength and hydrogen content. Atstress levels appreciably higher than the Threshold Stress, the effect of stresson time-to-failure is minimal; but at stress levels only slightly higher than theThreshold Stress, the effect of stress is significant and may increase time-to-failure by a factor of 10 to 100. The role of stress appears to be to provide adriving force (stress gradient) for the accumulation of hydrogen at the crack

initiation site. The effect of stress is accentuated by stress concentration suchas notches, scratches, or threads.

e) Temperature - The effect of temperature on delayed failure behavior iscontrolled by its combined effect on hydrogen diffusivity and the metalsfracture toughness, both of which increase with increasing temperature. Thedriving force of the stress gradient is presumably unaffected by temperature

i) Unlike other embrittling mechanisms, hydrogen-induced delayed brittlefailure is most severe at near-room temperatures. As the temperatureincreases, the higher hydrogen diffusivity moves the hydrogen through the

metal faster than the stress gradient can concentrate it at the crackinitiation site, and the metals increased fracture toughness requires ahigher critical hydrogen concentration for crack initiation and propagation.Consequently, the time-to-failure increases with increasing temperature,until a temperature is reached at which delayed failure does not occur. Asthe temperature decreases below-room temperature, the hydrogendiffusivity decreases until a temperature is reached at which the hydrogenis practically immobile, and crack initiation does not occur. However, sincefracture toughness is also decreasing, the metal is more susceptible tobrittle fracture, and, if a crack does initiate, crack propagation occurs muchmore rapidly, and both the Upper and Lower Critical Stresses decrease.

f) Thermal treatment and compositional and processing factors - as they apply to

specific metals - are in themselves a detailed topic.

g) To sum up the discussion to this point, a critical combination of strength,stress, and hydrogen must be present before crack initiation and delayedbrittle failure can occur. If any of these factors is less than its critical value, orif the temperature is not in the susceptible range, delayed failure will not occur.


12/16



h) Since properly processed metals generally do not contain damaging amountsof hydrogen, the environment must supply additional hydrogen for delayedfailure to occur. The ability of an environment to supply atomic hydrogen is

related to its Effective Hydrogen Pressure - a thermodynamic property that isdefined as the pressure that would exist in the environment if it consistedentirely of hydrogen. Those environmental factors that decrease time-to-failure, such as H2S, low pH, impressed currents, etc., increase the EffectiveHydrogen Pressure.

i) H2S Concentration - The presence of H2S is the most important environmentalfactor affecting delayed failure behavior. Aqueous H2S environments haveextremely high Effective Hydrogen Pressure (10 6 - 10 8***atmospheres). H2Sconcentration as low as 1 ppm (.0001%) may be sufficient to cause brittlefailure in high strength steels, but the time-to-failure is quite long. As H2S

concentration increases, the Upper and Lower Critical Stress and time-to-failure all decrease dramatically. At higher H2S concentrations, lower strengthsteels will fail, and failure time is much shorter at all strength levels.

j) pH - In the absence of H2S, the Effective Hydrogen Pressure of low pHaqueous environments is sufficient (1- 4 - 10 6***atmospheres) to causedelayed failure in most steels, but at a pH of about 6 or higher, failure is muchless likely. In the presence of H2S, however, delayed failure can be expectedto occur at a pH as high as 9 to 10.

k) Corrosivity, Galvanic Coupling, and Cathodic Currents - By increasing the

amount of atomic hydrogen liberated at the metal surface, these factors(particularly in the presence of H2S) may increase the Effective HydrogenPressure sufficiently to change an environment incapable of causing delayedfailure to one that can cause delayed failure.

l) Temperature - By increasing the corrosion rate and electric conductivity,higher temperatures increase the amount of atomic hydrogen liberated bycorrosion reactions, galvanic couples, and cathodic currents.

Conclusion

In conclusion, whether a given metal in a given environment will fail in a brittlemanner by hydrogen-induced delayed failure depends upon the complex interactionof a number of factors - the strength, stress, and temperature of the metal, and thepressure of H2S, pH, temperature, and corrosivity of the environment.


13/16




14/16




15/16




16/16



CAMERON

High Temperature BOP Elastomers

Engineering Bulletin EB833 D

There is no industry-accepted method of formally rating BOP packers and seals.The following are estimates based on lab testing and field performance.

Temperature ratings for elastomer components must take into account theenvironmental exposure history, the mechanical loading history, the chemicalenvironment while at temperature, and other factors. Therefore, a single numberrating can be misleading if all conditions are not understood.

PREVENTER/ELASTOMER TYPE CONTINUOUS EXTREMES

ANNULAR BOPs 70oF 180oF 30oF 200oFVBR PACKERS 70oF 180oF 30oF 200oFRAM BOPs* - High Temp. 0oF 250oF -20oF 350oFRAM BOPs* - Std Temp. 0oF 220oF -20oF 250oF

* All ram BOP elastomers except VBR packers

Studies performed by Cameron have determined that for high temperature subseaBOP stacks, the convection from the cold sea water keeps the majority of the ramBOP from ever reaching the flowing temperature of the well. Therefore, for hightemperature subsea stacks, the only elastomers that need to be changed to the hightemperature variety are the ram packers and top seals. It is not necessary tochange the bonnet seals or the connecting rod seals.

The same does not follow for high temperature surface stacks, since the air does not

provide the same level of heat convection. On high temperature surface stacks, theentire BOP, including the operating system, should be assembled with hightemperature elastomers.

Date post:	13-Apr-2018
Category:	Documents
Upload:	lulalala8888
View:	218 times
Download:	0 times

subsea - page 2

Documents