NUMERICAL MODEL OF " DRILLING SHIP - DRILLSTRING" SYSTEM BEHAVIOR IN HIGH CURRENT
Vadim S. Tikhonov, CNIGRI, Moscow, Russia
Alexander I. Safronov, NTC"Energocosmos" Moscow Russia
Michael Ya. Gelfgat, Nicolay N. Grebtsov and Alexander M. Podrazhansky, Aquatic Co. Moscow Russia
ABSTRACT
When drilling different-purpose boreholes without riser in currents higher than I m/s, serious difficulties are encountered. Sizable curving of the drilLstring (DS) axial line due to the effect of DS hydrodynamic drag hinders drilling operations. The experience of deep stratigraphic boreholes drilling in the Gibraltar Strait and calculations of DS stress-deformation state have shown that to bring about the conditions providing for successful drilling operations and to reduce stresses in the DS, it is necessary not only that conventional means designed to diminish DS drag are used, but also that the ship is offset relative to the borehole into a certain point.
In order to estimate the possibility of reducing and optimum distribution of stresses in DS, we have developed in this report a 3-D numerical model of DS static accounting for geometric non-linearity of DS axial line and conduct constraints on its displacement. A transformation of initial DS equations is proposed providing for significant simplification of the mathematical model. Finite differences are used to approximate the derivatives with respect to DS length; the nonlinear system of difference equations thus obtained is solved by iteration using the method of pass. On the basis of the numerical method, BURN.PAS program has been elaborated.
The efficiency of the model proposed is illustrated by specific examples of drilling boreholes in the Gibraltar Strait. It is noted that if the drilling ship is held right above the borehole, even provided that fairings are used to reduce DS drag from 1.3 to 0.3, in currents higher than I m/s stresses in the DS may exceed the allowable value. When optimum ship offset in set up, the limiting current value may be increased up to 1.8 - 2.0 m/s.
CERTIFICATION OF OFFSHORE MOORING STEEL WIRE ROPES
PER WILLIAM LOHNE, Det Norske Veritas AS
ABSTRACT
The trend to produce oil in increasingly deeper water has led to the development of floating production solutions for the exploitation of the energy resources in these areas. It is a fact that steel wire ropes have been used and are being proposed as line segments i n the majority of the mooring systems of these units/ships. This paper specifics requirements for the materials, design, manufacture and testing of large diameter offshore mooring steel wire ropes and may serve as a technical reference document in contractual matters between the purchaser and the manufacturer. Typical applications covered are permanently moored floating production systems (EPS), offshore loading systems and mobile offshore units.
