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Asset Integrity base shear and thus it would be steeper Atkins has applied the above meth-

Overturning moment if it were based on OTM and conse- odology to assessment of jacket struc- hazard curves quently the code implicit return period tures in many global regions. Although 3 100 to failure would reduce to slightly some further work is required to

Jacket below 10,000 for NNS jackets designed establish complete consistency across

Deck 2.5 to ISO 19902 with ? =1.35 and a single all regions, the next revision of the ISO

E

Jacket leg mode of failure. code may well achieve this. + Deck 2

The hazard curves give the capacity

Target required in order to achieve a specifed Note: This is a shortened version of 1.5 return period for failure of the struc- Fraser’s article. Read the full version at tural system. If signifcant additional www.OEDigital.com. topside load is to be added to a plat- 1 form, then an additional check on the Ramsay Fraser is reliability of the jacket and foundations Atkins’ technical 0.5 is required for the still water condition. director for offshore

The plot below shows the annual structures. Having 0 probability of failure for a structural originally managed 100 100010,000100,000 Normalized Overturning Moment OTM/OTM component when it is designed to an Atkins’ Aberdeen

Return Period (Years)

ISO code utilization ratio of 1.0 and offce across all

OTM hazard curve for wave-in-jacket subjected to loads ranging from 100% disciplines, Ramsay and wave-in-deck.

dead load to 100% wave load. The plot now has a global role which sees him hazard curve for WID load on the plat- shows that the code equation for the work on projects with Atkins teams form. The red line represents the haz- L1 condition governs the Pf for high from across the world. He has a BSc ard curve for combined WIJ and WID proportions of wave load, the abnormal and PhD in Engineering from the load on the platform. This curve shows 10,000-year code equation governs for University of Aberdeen. that once the return period is suffcient members with more than 80% of their for WID load to occur, the OTM capac- load being due to wave load and the Acknowledgments ity required increases rapidly in order still water code equation governs for This article is based on technical to achieve a suffcient return period for low proportions of wave load. This developments by Atkins’ offshore failure of the structural system. This is plot illustrates that compliance with structures team and in particular, Mark to be expected due to high WID loads the ISO code for operational conditions Manzocchi. Also, BP UK structures acting at a signifcant lever arm from results in excessive conservatism for team has shared relevant data for this the legs at the lower elevation of the structural components with a low pro- article with Atkins.

jacket. portion of storm load in comparison to In addition, we have benefted

The red dotted hazard curve on the components with higher proportions of from technical discussions with Pat earlier diagram was calculated from storm load. O’Connor, Dr Mike Efthymiou, Dr

Peter Tromans, Dr Richard Gibson,

Dr Graham Stewart and Prof Richard

Annual probability of failure 1.E+00

Jardine.

1. ABAQUS Software, Dassault 1.E-01

Systèmes Simulia Corp., Providence,

RI, USA.

1.E-02 2. ISO 19902, Petroleum and natural 1.E-03 gas industries - Fixed steel offshore structures 2007. 1.E-04 3. Efthymiou, M., van de Graaf,

J.W., Tromans P.S. and Hines, I.M., 1.E-05

Still water

Reliability Based Design for Fixed

Operational 1.E-06

Steel Offshore Structures, in proceed-

Extreme L1 ings of Offshore Mechanics and Arctic

Abnormal 10,000

Annual probability of failure 1.E-07

Engineering (OMAE), Florence, Italy,

L1 100 implicit target 1996. 1.E-08 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 4. Tromans P.S. and Vanderschuren

E100/(E100+Gnom+Qnom)

L., 1995, Response Based Design

Annual probability of failure for a structural component designed to an ISO code

Conditions in the North Sea – utilization ratio of 1.0 and subjected to loads ranging from 100% dead load to

Application of a New Method, Paper 100% wave load.

OTC 7683, Houston, USA, May 1995.

OE | October 2013 oedigital.com 32 028_OE1013_Asset-3-Atkins.indd 32 9/30/13 1:27 PM