Risk-Based Inspection Technique - Offshore Structures: Design, Construction and Maintenance - page 633

A review of the data in the context of when the damage was discovered is
informative and demonstrates trends in failure rates.
A study of platforms installed between 1966 and 1977 in the southern sector
of the North Sea suggests that the platforms may have been operated for
40 years before damage occurred. However, a major inspection campaign
undertaken in the 1980s discovered damage and it is unknown when the
members actually cracked.
Structures that were installed from 1971 to 1980 in deep water in central and
northern sectors of the North Sea continue to sustain through-thickness damage.
A proposed explanation for this is that, while fabrication defects resulted in
cracking in the first few years of life that was discovered, assumptions and errors
made during design yielded structural defects and stresses that are resulting in
ongoing fatigue damage. Structures from this period were designed without
the detailed knowledge available at present, particularly analysis software and
fatigue design factors.
Structures installed from 1981 to 1985 suggest that an unforeseen problem,
such as a fabrication defect, will be exposed at an early stage of the structure

s
operational life, before it settles into a period when little or no damage occurs
unexpectedly. However, with improved analytical techniques and detailing,
long-term fatigue performance has improved dramatically and has resulted in
no damage being noted after initial fabrication-defect problems have shaken out.
No gross damage has been noted in structures installed between 1986 and
1995; the explanation appears to be good fabrication and design practices
and the use of NDT. However, newer structures have seen less service life
than older structures.

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BIBLIOGRAPHY

API, 1996. Base Resource Document — Risk-Based Inspection — Preliminary Draft, API 581. American
Petroleum Institute, Washington DC.
API RP2A-SIM, 2006. Structural Integrity Management of Fixed Offshore Structures. American
Petroleum Institute, Washington DC.
API RP2A-WSD, 2000. Recommended Practice for Planning, Designing, and Constructing Fixed
Offshore Platforms, Twentieth ed. American Petroleum Institute, Washington DC. Supple-
ment 1.
Bea, R.G., Puskar, F.J., Smith, C., Spencer, J.S., 1988. Development of AIM (Assessment, Inspec-
tion, Maintenance) Programs for Fixed and Mobile Platforms. In: Offshore Technology Confer-
ence, OTC Paper 5703 Houston, TX.
DeFranco, S., O Connor, P., Andy, Tallin, Roy, R., Puskar, F., 1999. Development of a Risk Based

Underwater Inspection (RBUI) Process for Prioritizing Inspections of Large Numbers of
Platforms. In: Offshore Technology Conference, OTC10846, Houston, TX.
Det Norske Veritas (USA) Inc. (DNV), 2002. Risk Based Underwater Inspection

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— Phase II, for

Amoco, Houston. Document Number 221-8823, 8 February 2002.
El-Reedy, M.A., 2003. Life-Cycle Cost Design of Deteriorating Offshore Structures. In: Offshore
Mediterranian Conference Proceeding, OMC2003, Ravenna, Italy.