Design of Floating Vessel Drilling Riser
Authors: Fischer William 
Published: December 2007

FISCHER, WILLIAM, STANDARD OIL CO. OF CALIFORNIA, LA HABRA, CALIF. LUDWIG, MILTON, SAN FRANCISCO, CALIF.

An offshore drilling riser, necessary to return drilling fluid to the drilling vessel and to guide the rotating drill pipe, will fail in water depths greater than 200 to 300 ft if it is not partially or completely supported by a tensioning device at the top. As water depth increases, substantial bottom tension is also necessary and the connector at the bottom must be designed for axial tension unless equivalent weights are provided just above the bottom connector. The differential equation for deflection of a riser subjected to static forces is solved and generalized design curves for calculation of required minimum top tension are presented. An analysis of riser response to cyclic wave forces and vessel motion is not included but approximate solutions indicate that dynamic effects, for the optimum riser design of smallest practical diameter in water depths of 60 to 1,000 ft and under moderate sea and vessel motion conditions, are small compared to static effects. Thus, fulfillment of static requirements will lead to a satisfactory design that provides an economic solution to the most difficult problem associated with o fish ore drilling in deep water.

Introduction

The simplified solution for riser design offered in this paper is based on static force considerations only, and these over a range of water depths, sea conditions and vessel motion conditions within which they will control. Within such limitations, dynamic forces will be small by comparison. This approach to riser design is intended to serve as an interim design guide pending publication of such papers as will include both static and dynamic considerations. A typical layout for subsea drilling from a floating vessel is shown on Fig. I. ‘The drilling riser, reduced to bare essentials, is simply a long tubular column to return the drilling fluid to the ship and, secondarily, to guide the rotating drill pipe. This column is axially loaded by its own weight and, with no vertical support at the top, must ultimately buckle and fail when its length becomes sufficiently great, even though there is no lateral ocean current force and the ship is directly over the hole. For example, the critical buckling length in feet for a riser with zero tension at the top is 2.75 (EI/w)(1/3) if the top and bottom connections are angularly flexible, and 3.75 (EI/w)(1/3) if the top connection is angularly flexible and the bottom connection is angularly stiff. With a flexible bottom connector, this critical length is 247 ft for a 16-in. diameter by 0.438-in. wall riser which contains 120 lb/cu ft drilling fluid and free-standing 45-lb/ft casing. Increasing the wall thickness to 0.750 in. would increase the critical length slightly to 270 ft, and increasing the diameter to 24 in. for 0.438-in. wall thickness would increase the critical length to only 310 ft. Practically, there are ocean currents and the vessel does move off location so that bending of an unsupported riser would become excessive at lengths somewhat less than these critical buckling lengths.

JPT

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