Offshore spar production system and method for creating a controlled

Abstract

An offshore system is provided of the type that includes riser pipes (30) extending up from the seafloor (44) to a tall and narrow caisson (12) at the sea surface, with the caisson moored by mooring lines (34) extending to the seafloor and anchored thereat, which minimizes bending of the upper portion of the riser pipes when the caisson drifts in severe weather. Although the caisson has a Ballasted lower end and buoyant upper end to keep its axis (20) vertical, a device is provided for applying a horizontal force (54) to a location along the caisson that is vertically spaced from the upper ends of the mooring lines, to tilt the caisson so the axis of the caisson is parallel to portions (82) of the riser pipes lying immediately below the caisson. In one arrangement, a second set of mooring lines (60) is provided, that have upper ends coupled to second locations (64) along the caisson that are vertically spaced from the upper ends of the first mooring lines. Also, a motor driven device (70) is provided for pulling on selected ones of the lines to tilt the caisson. In another arrangement, largely horizontal force transmitting members (132) extend from a lower portion of the caisson to a location (136) along a single set of mooring lines (126). In still another system, thrusters (152, 154) are used to push at locations along the caisson to tilt it.