Maximum of Permissible Horizontal Motions a Floating Drilling Vessel

ABSTRACT Horizontal vessel motions [i.e., drift, sway or surge] result in bending of the drill string members in the Vicinity of the rotary drive bushing at the-vessel and at the blowout preventer close to the seafloor. Allowable horizontal vessel motions are calculated as a function of hookload, using both cumulative fatigue damage and drill pipe strength as criteria for drilling and for pulling stuck pipe. The presently used rule of limiting horizontal vessel motion to 5 percent of water depth is shown-to be too restrictive in some cases and too lenient in others. INTRODUCTION When drilling from a floating vessel} the rotating drill string is subjected to fatigue damage caused by reversing stresses which arise from heave, roll, pitch and horizontal motions of the vessel. In Ref. 1, the authors have examined the effect of drilling vessel roll and pitch on the cumulative fatigue damage of the kelly and the first joint of drill pipe below the kelly. As a continuation of that investigation, the present study covers the effect of horizontal motions of the vessel on the drill string. Horizontal departure from over the borehole may be oscillatory [i.e., sway or surge], static [i.e., drift] or most often, some combination of the two. An accepted limit of horizontal vessel motion of percent of the water depth has been suggested and frequently followed.2 In reality, the allowable motion is strongly influenced by the hookload. Motions greater than 5 percent of water depth are often permissible, and under some conditions, motions should be limited to less than 5 percent of water depth. The purpose of this paper is to suggest maximum permissible motions imposed by drill pipe fatigue damage and pipe strength, as a function of hookload. IDEALIZED SYSTEM Fig. 1 shows an idealized system, highly exaggerated for explicitness. Although pipe bends are shown quite acute, bending stresses were properly defined mathematically. The pipe bending at the rotary drive bushing [labeled RDB in Fig. 1] is relieved in most cases by a gimbaled bushing and largely taken by the kelly, a more durable member. Therefore, the point of most severe damage is in the area of the blowout preventer rams [labeled BOP]. Generally, the drill pipe is run inside a larger pipe called a riser, which provides a return for the drilling mud from the sea floor to the vessel. For the sake of simplicity) the interaction of the drill pipe with the riser was not considered in this investigation. It is believed that the effect of this interaction on drill pipe bending is small. The dimension D in Fig. 1 is the distance from the rotary drive bushing to the blowout preventer. This distance throughout this paper is called water depth which may or may not correspond closely enough to the reality to be used interchangeably. When in doubt, the true distance D should be used and substituted for water depth when using the curves in this paper. The dimension H in Fig. 1 is the horizontal departure of the vessel from over the borehole.