Numerical and Experimental Investigation of Accidentally Falling Drilling Pipes

Abstract

ABSTRACT The dynamic motion of freely falling drilling pipes in water is treated theoretically by using slender body theory. A computer program was developed and the results can be used to predict the risk of damage on subsea installations. Time series of the motion, velocity and acceleration are obtained and the impact loads can be predicted. Two model tests are presented. The first test was performed in order to investigate the entire history of events from a drop at the platform deck till the seabed is reached. The intention of the second test is to verify the computer program and is limited to the motion in water. Scale effects between model and full sized pipes are discussed, and comments are made with respect to the assumptions in the theoretical model. INTRODUCTION An important problem for drilling operations is the danger caused by accidentally dropping different objects either from a platform deck, or from a supply vessel or during a hoisting operation. The problem is related as much to the safety of under water installations as to ROVs in the vicinity of the platform. Even under water damage on the platform itself have been reported. Accidental drops of drilling pipes are of particular interest because the maximum velocities which can be obtained are high and the corresponding excursions are large. Consequently the impact loads can be large. One of the main goals of this study is to obtain a basic knowledge about the motion of freely falling cylinders through water. A literature survey was performed, but not much was found on the subject. As a final result, a computer program simulating the motion and tracking the path during a drop was developed. In order to confirm the numerical results and investigate the course of events during a drop, two experimental set-ups were suggested. The results from the model tests are described in reference /1/. First a scaled drop from a platform deck was performed, using two different model-scales. The pipe was released from the height of a platform deck, fell through the air, hit the water surface and traveled through the water until it reached the sea bed. The second test was performed in order to get a better description of the travel through the water. The pipe, having a specified angle of attack with the water surface, was released a short distance below the water surface and the path was observed. The model tests highlighted some of the basic features of a freely falling cylinder through water. In order to obtain a usable mathematical model, these features have to be introduced in the equations of motion. Obviously the viscosity plays an important part because the movement in longitudinal direction is very different from the movement in lateral direction. Model test 2 showed a large time difference between a purely longitudinal motion and a purely lateral motion. The viscous forces are more dominant in the latter case.