Some Observations on the Heave Behavior of Spar Platforms

The spar platform is receiving considerable attention from the offshore petroleum industry, especially following the successful introduction of the first production spar in the Gulf of Mexico by Oryx and CNG in 1996. This paper presents a collection of observations regarding important heave characteristics of spar platforms that have been gleaned from wave-basin model tests, numerical simulations, and comparisons of the two. These observations are pertinent to the heave performance of “classic” (uniform-cylinder) spars. Excessive spar heave could be problematic for production and export risers. Spar-system characteristics affecting heave include draft, heave natural period, heave-damping augmentation, and presence/absence of a vertical tether/tendon. Tether requirements for shallow-draft spars have been found to be much less onerous than once thought; steel tethers yielding a heave natural period of 12 s have been found to be satisfactory. Heave-damping augmentation using multiple “plates” or “disks,” following the work of Troesch at the University of Michigan, has also been found to be quite attractive.