Ocean nutrient enhancer "takumi" engineering and model experiment for upending of long slender steel riser

Abstract

The ocean nutrient enhancer (ONE) is composed of a spar type floating structure from which is suspended a 175 m long compliant steel riser. The ONE was installed at the center of Sagami-Bay in May 2003 for the purpose of discharging density controlled water, which is adjusted by mixing the up-welled deep ocean water (DOW) and surface water, into the euphotic surface layer, in order to investigate the effect of DOW for developing a new fishing ground. The Marino-Forum 21's ONE project was started in 2000. During the conceptual design stage, two types of ONE were studied to reduce the installation cost and site construction duration. One was called "fix riser type" and the other was "pin riser type". In the selection of the concept, the most important technical aspects were riser fatigue strength and strength during upending. At first, a ridged connection type was examined since the upending of an entire ONE was problematic because of its heavy top. Although the possibility of upending was confirmed by simulation, the concept was not selected. Further investigation indicated that the riser needed to have a very high fatigue strength and construction cost was very expensive. The "pin riser type" was finally selected. One of the most difficult technical challenges was that the riser is installed by the upending method. The entire length of the riser is prefabricated at the yard and towed to the site by hanging from a spar buoy and with two additional barges supporting horizontally. At the site, the riser is set to an upright position by the free fall upending method. There was the possibility that a very large bending stress may occur during free fall upending due to the riser's large diameter and weight. The model basin study was performed to establish and confirm the analytical method to simulate suitable riser behavior during towing, installation, operating and especially during free fall upending