2. Dr. Ir. Slameto Wiryolukito (slameto@material.itb.ac.id)
3. Muhammad Abduh ( abduh@reksolindo.co.id)
* Print Version Published in PetroEnergy Magazine Edition Nov-Des 2007
I. Introduction
Engineering integrity is a subsystem of integrity management. As a system of a whole, engineering integrity works together with the overall operations policy, health safety and environment standard, integrity performance standard, and legislative compliance. Engineering integrity becomes the major concern for oil and gas operator as a result of high impact of catastrophic accident in many countries. Both integrity standard API 1160 and ASME B31.8 were originally addressed for industry consensus in United States. These documents also referred as governing documents for pipeline safety regulation US DOT CFR 49 192. This paper will present an overview of engineering integrity, the elements and approaches to build and develop in oil and gas industry. Since integrity management covers multi aspects, there is a need to be more focus and to limit boundaries for engineering integrity, what are the elements, and how can this subsystem affects and interacts with other subsystem.
II. Engineering Integrity as Systematic Approach
Engineering structures, equipments, and components play a vital role in oil and gas industry as such to maintain production target. Any threat to these components will also threaten the performance overall structures. We may understand that engineering integrity is an integrated system, which is every element, affects other element in overall system. Then simply we can understand that integrity of a pressure vessel once depend on the welding quality. Any defect in welding will threaten overall performance of the pressure vessel. These process elements in developing engineering integrity can be described in Figure 1.
Figure 1 - Element of Process in Engineering Integrity Approach
III. Materials Selection
As we know materials properties depend on its microstructure, any treatments that alters microstructure will alter material properties. Phase transformation occur when steels are in process of heat treatment, welding, and other thermal affected treatments. Materials integrity can be sustained if we can maintain as received microstructure against all process subjected to it. A reliability of an equipment or engineering structures (piping, fixed offshore platform, floating structures) started from the process materials selection. Basic control points for material selection should be:
- Mechanical properties (acceptable of mechanical load)
- Corrosion Properties (sweet corrosion or sour corrosion)
- Failure Resistance, crack resistance (pitting, HIC, toughness)
Some of the codes for material selection guideline in oil and gas industry:
- Material Load Design: ASME Boiler and Pressure Vessel Section II, IX, X
- API 5L Specification for Linepipe
- Material Selection: Norsok M-001 Material Selection
- Corrosion Resistance Material/ SSC Resistance Material: NACE MR 0103-2003, MR 0175/ISO 15156, EFC Document No 16
- Plastic or Fiber Reinforced Plastic Material Design Basis: ASME Boiler and Pressure Vessel Section X, PPI TR-3/2004, API 15LR, API RP-15S
Trend of corrosion resistance alloy (CRA) steel in Oil and Gas
A story of CRA Steel was setback in 1970 era when clad steel and duplex stainless steel were first applied by Dutch operator company NAM1. The story of duplex stainless steel in oil and gas flowlines begins with its greater range of application in term of corrosion resistance. By the end of 2002 duplex stainless steel still the dominant material for flowlines. When facing more corrosive or more sour condition, the choice turned to clad steel. Clad steel with clad material AISI 316L or alloy 825 clad then become the material of choice in H2S environment. It was in Indonesia, for the first time martensitic stainless steel applied by Exxon for Arun Field. Since martensitic stainless steel is less expensive in less corrosion environment and the development of low carbon weldable 13Cr MSS, a research predicted that MSS will dominate the choice for CRA flowlines in the future, Figure 2.
Figure 2. CRA Trend in Flowlines
New Materials Conversion
Facing corrosion problem, oil and gas industry get alternative material choice. The use of fiber reinforced plastic pipe is more economically attractive. Some advantages of this material type compared to conventional steel materials:
- Corrosion resistance;
- More flexibility because FRP is coilable and the installation can be faster;
- Low maintenance cost
The advance material conversion to plastic and plastic reinforced pipe somewhat should consider some limitations as:
- Limited range of pressure application;
- Pipeline integrity program for available methods are for steel materials. There should be further development for developing plastic pipe integrity since the tools and equipment of inline inspection, NDT inspection, and defect assessment is designed for steel material;
- Creep failure of these materials reported. BP Amoco experiencing creep in higher temperature and cyclic loading condition;
- Reactivity of resin with H2S reported by Saudi Aramco for their shallow casing liners;
- Compatibility for connection with other component;
- Brittle like crack in plastic pipe;
IV. Fabrication and Construction
Fabrication and construction become crucial issue when as many studies reported that failure associated with welding and handling. Typical fabrication defect of stainless steel shown in Figure 3. To ensure processes in fabrication and construction maintain materials property there is knowledge demand in phase transformation and material thermodynamic. Unwanted phase transformation can occur during welding. The most stable protective passive film over stainless steel is introduced by passivation. During welding metal is reheated to its melting temperature and variably differ along heat affected zone. Free air exposed to this metal region will produce unstable porous and dense passive film. Unexpected oxidizing during welding can be prevented by application of shielding gas.
Pitting during transport, storage, hydrostatic testing and prior to service
The presence of iron contaminants in contact with stainless steel in aerated water environment tends to break passive film. Effect of pitting becomes worst in stagnant water. Longer periods of stagnant water exposure will create severe pitting attack over stainless steel. A case in relation with pipe handling was experienced by VICO Indonesia in Badak-Bontang pipeline project in 1999. The debris formed during pipe transport to storage in open sandy tropical climate lead to the formation of iron oxide (Fe2O3, Fe3O4) and iron sulfide (FeS). A fingerprint inspection had detected significant 40.000 defect features that can be related with the existence of debris. There were 10 sections replaced against ASME B31.G acceptance limit.
Figure 3 . Typical Welding Related Defect
Deadly Failed Coating
TWI UK said this case as “Deadly Corrosion Failure” as the explosion and fire led to death of several personnel. The pipeline was designed according to ASME B31.4. The breakdown of external coating made cathodic protection overload which cause pitting. Pit coalescence causing a large area to cause failure of rupture exposing and ignite methane gas to open air.
Welding Defect Disaster
Another TWI classic case was the rupture of amine absorber column (designed according to ASME Section VIII Section I, ASTM A516) in 1984, Figure 4. A report said that 17 people dead and economic loss over $ 100 million. Investigation report said that root cause of failure was the formation of hard HAZ in welded part of column shell.
Figure 4. Explosion of Amine Absorber Column (TWI Case, 1984)
Poor Workmanship
Writer's Company recently had an opportunity to asses a gas plant facility in seashore environment. Investigation has found that some critical components (elbows, etc) experiencing residual stress by the evidence of magnetic properties change. Possible causes of this residual stress are the weld spatter and impact contact with hammer or bolt with the pipe segment.
Lesson learnt from above case is that there is a demand of multidisciplinary peer review for the integrity of fabrication and construction structures, involving stress analyst, inspection engineer, and material engineer. Since all of these structures were already fulfill their design code, multidiscipline validation process is important to reduce potential failure. Every engineer involved in fabrication and construction has responsibilities to give more attention to the small issues big impact as follows:
- Material handling: keep stainless steel clean from iron dust, stagnant water, sand, clay that tend to form debris, pitting, and crevice corrosion, avoid contact with steel component with different composition which make cathodic potential for corrosion.
- More attention to condition of stainless steel pipe in outdoor material storage, in a time between post-commissioning prior to service.
- Welding for critical components should assure acceptance welded joint for design life;
- Be careful with critical components, any impact contact with other tools or weld spatter can make localized cold deformation phase transformation which leads to residual stress. This could be harmful in case of erosion-corrosion phenomenon.
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