Dynamic Anchoring and Anti-Heave Slide: Key Device for using Floating Supports on the High Seas

Abstract

Abstract To extend subsea oilfield exploration and exploration to greater water depths, work supports set on the sea-bottom must progressively be replaced by floating supports. Work efficiency progressively be replaced by floating supports. Work efficiency then requires the limiting and, if possible, elimination of movements imparted to such supports by the action of the elements, in particular drift, roll and heave. Two systems have been developed to meet this objective: 1) Dynamic anchoring which cancels out drift and reduces roll by purposely positioning the support. 2) A surface slide which eliminates the effect of heave. The operating principle of these systems and practical examples are described briefly in this paper along with the experience gained from using both systems together during the past seven years under a wide variety of operating conditions aboard the I.F.P. experimental vessel "Terebel". It is shown in particular that the great flexibility and adaptability, of these systems make it possible to do the most difficult - jobs on the high seas with remarkable safety and efficiency. Introduction The current search for new off-shore oil and gas fields in deeper and deeper waters and the commercial exploitation of these fields have set a trend for the progressive replacement of work-platforms set on the sea-bed by floating supports: ships, semi-submersibles and the like. Increasing water depths and more hostile seas such as the North-Sea or the ice infested Labrador offshore have made it necessary to use less cumbersome and more efficient mooring systems and to drastically improve platform stability, by reducing such motions as surge, sway, yaw, heave, roll and pitch. pitch. Two devices developed during these last years have gone a long way towards meeting these requirements: 1) Dynamic Positioning, which has solved the difficult problem of fast and safe anchoring in deep waters and which problem of fast and safe anchoring in deep waters and which has brought a Feat improvement in stability by allowing the floating support to head permanently towards the on-coming seas, thus reducing considerably the roll motion. 2) The Heave Compensating Device, which has greatly reduced the vertical motions of any piece of machinery being handled from a floating support. As early as 1963, the "Terebel", a 900 metric tons experimental vessel of the Institut Francais du Petrole, was outfitted with dynamic positioning and a heave compensator. This paper describes sonic of the work done by the "Terebel" and how dynamic positioning and heave compensator have made it possible to carry out some operations with relative ease which, otherwise, would have been very difficult, if not impossible.