oscillation is such that a significant amount of energy in the wave spec- 

 trum 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 the roughness and effective diameter of the member, and 

 increases forces on the member. Guidance on selecting a drag coefficient 

 Cp can be obtained from Table 7-2. However, the increased diameter must 

 be carried through the entire design procedure to determine forces on a 

 fouled member. 



7.317 Calculation of Forces and Moments on Groups of Vertical Cylindrical 

 PileS o To find the maximum horizontal force and the moment around the mud- 

 line for a group of piles supporting a structure, the approach presented 

 in Section 7.312 must be generalized. Figure 7-59 shows an example group 

 of piles subjected to wave action. The design wave concept assumes a two- 

 dimensional (long crested) wave; hence the x-direction is chosen as the 

 direction of wave propagation. Choosing a reference pile located at x = 0, 

 the x- coordinate of each pile in the group may be determined from, 



^« = K ^os«n' (7-49) 



where the subscript n refers to a particular pile, and ly^ and o^ are 

 as defined in Figure 7-59. If the distance between any two adjacent piles 

 is large enough, the forces on a single pile will be unaffected by the 

 presence of the other piles. The problem is simply one of finding the 

 maximum force on a series of piles. 



In Section 7.312, the force variation in a single vertical pile as a 

 function of time was found. If the design wave is assumed to be a wave 

 of permanent form (i.e. it does not change form as it propagates), the 

 variation of force at a particular point with time is the same as the 

 variation of force with distance at an instant in time. By introducing 

 the phase angle 



27rx 27rt 



e = , (7-50) 



L T 



where L is wavelength, the formulas given in Section 7.313 (equations 

 7-18 and 7-19) for a pile located at x = may be written in general form 

 by introducing 9, defined by 2ttx/L - 27rt/T in place of - 2TTt/T. 



7-114 



