Marine fouling is found to influence the loading of an offshore structure in at least five ways;
Increased tube diameters, leading to increased projected area and displaced volume and hence to increased hydrodynamic loading.
Increased drag coefficient, leading to increased hydrodynamic loading.
Increase-d mass and hydrodynamic added mass, leading to a reduced natural frequency and hence to an increased dynamic amplification factor.
Increased structural weight, both in the water and above the water level in air.
Effect upon hydrodynamic instabilities, such as vortex shedding.
These effects are examined separately in detail and together as they affect the overall performance of the structure in relation to codified design criteria.
The direct increase in loading due to marine growth arising from the increase in projected area, displaced volume, drag coefficient and structural weight is shown to affect both the calculation of ultimate design wave and also the stress range calculated for a fatigue life estimation. The increased dynamic amplification factor shown to be a secondary effect, influencing only wave periods near to the structure natural frequencies; that is the middle to smaller ranges of wave height that effect the fatigue life of the structure.
Generalised results are given which show the trend of increased load and the reduction in fatigue life with increased marine growth thickness.
Also, should a programme of cleaning become necessary, criteria are established for the location and amount of cleaning that would be most beneficial.
In the case of hydrodynamic instabilities, the effect of marine growth is found to be unpredictable and could lead to either an increase or decrease of structural reliability.
Finally an indication of the modification of the overall safety of typical structures by marine growth is given using techniques of reliability analysis.
The material presented in this paper forms part of the background material of a report prepared for MaTSU, United Kingdom Department of Energy, entitled 'Appraisal of Marine Fouling on Offshore Structures'. This report examines the potential extent and importance of the marine fouling problem as it is applied to structures in the UK sector of the North Sea. The scope of this paper is to examine the qualitative effect that marine fouling has on fixed steel platforms by application of conventional design techniques to typical structures or parts of structures with greater or lesser fouling thicknesses. Three main design criteria are examined; maximum 'life-time' loading leading to first excursion failure, dynamic resonance of the structure and fatigue life estimation.
Finally, the conclusions are examined in the light of the more rigorous techniques of reliability analysis applied to the design sensitivity of offshore jackets.
For a 'lifetime' design wave calculation, the maximum stress in any part of the structure is calculated for the passage of the 'lifetime' wave.