Deterioration

Question 1

Broadly speaking, the modes of deterioration may be classified into six groups

Answer 1

1  Gross structural damage.
2  Corrosion and erosion.
3  Fouling defects.
4  Coating defects.
5  Scour.
6  Metal and weld defects

Question 2

Specific types of deterioration and damage within these groups may be categorised as:

Answer 2

 Deformation of the structure caused by impact.
 Loss of concrete matrix through impact or internal flaws.
 Missing bolts.
 Coating damage through abrasion or impact or deterioration.
 Damaged cables or ducts caused by impact or deterioration.
 Unstable foundations through poor geology.
 Missing members caused by accidental damage or failure.
 Debris, which may cause impact damage or create fouling or overload the
corrosion protection system.

Question 3

How is Accidental damage handled

Answer 3

Engineers will try to anticipate all the different modes of failure when they first design a structure, but deterioration due to accidental damage is difficult to design against

In the United Kingdom, Safety Cases have to be submitted to the HSE for evaluation and assessment, in an attempt to prevent accidental damage from being a threat to safety.

Question 4

In what 2 ways does corrosion occur?

Answer 4

Firstly, uniform corrosion is the process whereby metal is removed from all
over the surface as anodic areas continually shift, so that progressive
thinning of the member or pipe wall goes on

Secondly, pitting corrosion is a localised corrosion which takes place in an
otherwise corrosion free material, creating a pit in the surface of the
material.

Question 5

What is fatigue?

Answer 5

Fatigue is the local failure of the material by crack growth caused by cyclic loading.

Question 6

What variables influence fatigue?

Answer 6

  Number of load cycles. 
  Stress or strain amplitude. 
  Mean stress level. 
  Temperature. 
  Environment. 
  Microstructure of the material. 
  Surface condition.

Question 7

What is wear?

Answer 7

In the offshore environment, wear is the thinning of material due to uniform corrosion, erosion or a combination of the two.

Question 8

What is abrasive wear?

Answer 8

In the mechanism for abrasive wear, a hard particle in one surface indents, grooves and then cuts material from the other surface

Question 9

Wear caused by fatigue

Answer 9

When there is relative motion between two surfaces in contact, the state of stress at any given point on or near the surfaces varies with time and this may cause fatigue, the slow growth of cracks

Question 10

Chemical and corrosive wear

Answer 10

Chemical effects are most commonly exemplified by the repeating cycle of the formation, removal and reformation of oxides (Rust films).

Question 11

What is Embrittlement?

Answer 11

In service could come about due to incorrect welding procedures or by the absorption of a gas, generally hydrogen. It has been encountered in natural gas pipelines and could also come about from the absorption of hydrogen produced by an overprotective impressed current corrosion protection system or associated with sour service.

brittle fracture being more likely to occur at low temperatures.

Question 12

Both types of corrosion are accelerated by:

Answer 12

 Erosion.
 Increase in temperature.
 Increase in oxygen content.
 Chemical attack from biological sources.
 Loading, external or residual (stress corrosion).