The North Sea Concrete Platforms: 20 Years Of Experience
Authors: Fjeld Svein , Hall M.E. , Hoff G.C. , Michel Dominique , Robberstad Lasse , Vegge Aune , Warland T.A. 
Published: October 2008

ABSTRACT
The experiences from in-service inspection of the offshore concrete platforms installed in the North Sea up to 1989 are summarized.
It is remarkable how well the concrete structures have performed in the hostile North Sea environment. No significant signs of material deterioration, corrosion of reinforcement or other material related deficiencies have been observed. In the submerged zone the reinforcement appears well protected against corrosion by the cathodic protection.
Observed deficiencies are mainly caused by falling objects or ramming ships.
The good performance is attributed mainly to good concrete quality, i.e., the low permeability of the concrete in situ.

INTRODUCTION
The following in-service experience summary rovers the 22 GBS platforms installed from 1973 up to and including 1989. The smaller platforms installed after 1989 are too new to demonstrate their real long-term durability properties.
The accumulated age of the platforms is thereby about 300 years, i.e., an average platform age of 15 years.
The basis for the work has been existing data concerning:inspection programmedinstrumented monitoringfindingsevaluation of findingsmaintenancerepairsstructural studies and re-analyses.
The inspection results referred to in this paper are mostly related to findings from subsea inspection. Areas with difficult access such as inside oil storage tanks, ballast compartments etc. have not been subject to inspection.
In normal cases, visual inspection by diver or ROV supplemented by a limited scope of instrumented monitoring is an adequate basis for periodic condition assessment of an offshore concrete structure. Typical annual subsea inspection costs for a platform are in the order of 200 000–300 000 USD.
Extended inspection based on laboratory testing of drilled core samples are carried out after extreme exposures (e.g., ship collisions) and for special investigations. The findings reported in this paper are to a large extent based on the latter type of inspection, both for submerged and atmospheric areas.
This data has been collected and evaluated to focus on the main experiences in terms of overall operative engineering conclusions.
The scope is limited to the structural concrete of the main load bearing elements. Secondary concrete elements of lesser quality have been used for different purposes on the topsides and in the substructure for foundations, embedments, impact protection etc. These concrete elements are not dealt with in this paper.
The concrete structures have been fitted with comprehensive mechanical outfitting to support the topside activities and for temporary use during construction and installation. Also, these systems are beyond the scope of this report and they are only dealt with to the extent they influence the concrete structure.

GENERAL CONDITIONS
It is remarkable how well the concrete structures have performed in the hostile North Sea climate. No significant sign of material deterioration, corrosion of reinforcement or other material-related deficiencies have been observed in the submerged zone.