Compliancy of Stiff Deepwater Jacket Structures

Abstract

Reductions in steel weight of deepwater fixed jackets by 10,000 tons represent savings of many millions of dollars in material costs alone and substantial cost advantages during the transportation, fabrication, and installation phases. This paper shows that such savings are feasible by considering the impact of fluid‐structure action on design force levels. Deepwater jackets in 1,350–1,600 ft were analyzed, and certain configurations indicated that regular extreme wave dynamic base shears can be as much as 40% larger if fluid‐structure response were ignored. Design base shears derived from spectral‐type methods that accounted for more than a single extreme wave also differed substantially when relative velocity effects were included. The impact of differences in dynamic base shears and overturning moment on jacket steel were quantified. Because of the potential effect on force calculations, practical criteria are desirable that establish when the relative velocity form of Morison's equation is desirable.