The marine growth data of 40 platforms located in western offshore areas of India, collected during the inspections were statistically analysed. A trend was established for the pattern of marine growth thickness and roughness height over depth. The observed marine growth thicknesses were substantially higher than assumed in the original design criteria. A typical 4 legged well platform Jacket was analysed for studying the effect of higher marine growth thickness on design. The increase in base shear, overturning moment and pile loads for various values of marine growth thickness were estimated and compared with those of the original criteria. A new design criteria vas evolved to strike a balance between the increase in capital investment and the frequency of marine growth cleaning. Recommendations were also made for ensuring the safety of existing platforms designed with the original marine growth criteria.
In the western offshore areas of India, including Bombay High and other adjacent fields there are more than a hundred offshore platforms in service in water depths ranging from 40-80 m. During the first few years of development, starting with 1975, in the absence of any available field data, a uniform marine growth thickness of 38 mm was assumed on all tubular members from an elevation of 3 m above the chart datum to the seabed for the calculation of hydrodynamic forces for design. Subsequently, during underwater inspection of the offshore platforms a more specific site data on marine growth was available. The data collected from 40 platforms revealed that the accumulation of marine growth for most of the platforms were much in excess of the design allowance of 38 mm. At localized areas of some platforms thicknesses of the order of 200-240 nm were also noticed.
