M^ = (0.15)(10,407)(0.7)(10. 0)^(1. 25)(26) = 3,429 kN-m (2.529 x 10^ ft-lb) 

 *************************************** 



Before the pile is designed or the foundation analysis is performed, a 

 safety factor is usually applied to calculated forces. It seems pertinent to 

 indicate (Bretschneider, 1965) that the design wave is often a large wave, 

 with little probability of being exceeded during the life of the structure. 

 Also, since the experimentally determined values of C^ and Cp show a large 

 scatter, values of C^ and Cp could be chosen so that they would rarely be 

 exceeded. Such an approach is quite conservative. For the recommended choice 

 of C^ and Cp when used with the generalized graphs, the results of Dean 

 and Aagaard (1970) show that predicted peak force deviated from measured force 

 by at most ± 50 percent. 



When the design wave is unlikely to occur, it is recommended that a safety 

 factor of 1.5 be applied to calculated forces and moments and that this 

 nominal force and moment be used as the basis for structural and foundation 

 design for the pile. 



Some design waves may occur frequently. For example, maximum wave height 

 could be limited by the depth at the structure, ^f the design Wave is likely 

 to occur, a larger safety factor, say greater than 2, may be applied to 

 account for the uncertainty in C^ and Cp . 



In addition to the safety factor, changes occurring during the expected 

 life of the pile should be considered in design. Such changes as scour at the 

 base of the pile and added pile roughness due to marine growth may be 

 important. For flow conditions corresponding to supercritical Reynolds 

 numbers (Table 7-5), the drag coefficient Cp will increase with increasing 

 roughness. 



The design procedure presented above is a static procedure; forces are 

 calculated and applied to the structure statically. The dynamic nature of 

 forces from wave action must be considered in the design of some offshore 

 structures. When a structure's natural frequency of oscillation is such that 

 a significant amount of energy in the wave spectrum is available at that 

 frequency, the dynamics of the structure must be considered. In addition, 

 stress reversals in structural members subjected to wave forces may cause 

 failure by fatigue. If fatigue problems are anticipated, the safety factor 

 should be increased or allowable stresses should be decreased. Evaluation of 

 these considerations is beyond the scope of this manual. 



Corrosion and fouling of piles also require consideration in design. 

 Corrosion decreases the strength of structural members. Consequently, 

 corrosion rates over the useful life of an offshore structure must be 

 estimated and the size of structural members increased accordingly. Watkins 

 (1969) provides some guidance in the selection of corrosion rates of steel in 

 seawater. Fouling of a structural member by marine growth increases (1) the 

 roughness and effective diameter of the member and (2) forces on the member. 

 Guidance on selecting a drag coefficient C/j can be obtained from Table 

 7-4. However, the increased diameter must be carried through the entire 

 design procedure to determine forces on a fouled member. 



7-149 



