Reliability Analysis of the Behavior of Dropped Objects

ABSTRACT Dropped objects, such as drill pipe, pose a potential hazard to sub-sea structures. A program has been developed to simulate the submerged trajectory of such objects, and verified against experimental data. A study was conducted to determine the design impact velocity of dropped tubulars, to aid in the column design for a floating structure. Initially the maximum attainable velocity was determined. Subsequent multiple drop simulations produced a quadratic surface of maximum impact velocity with respect to the initial drop parameters. The lack of a consistent definition for the 'design' velocity prompted the application of reliability techniques which established a relationship between impact velocity and probability of occurence. INTRODUCTION Small objects such as scaffolding bars may be dropped frequently with little consequence to sub-sea structures and equipment. However, occasional drops of larger objects such as drill pipe, containers and B.O.P. stacks pose potential greater risks, due to the likelihood of significant damage resulting from their impact. Dropped tubulars, such as drill pipe/casing are particularly prone to rotational effects of hydrodynamic loading, and can turn horizontally upon entry into the water column. This can pose a significant threat to the columns and pontoons of floating offshore units where wall thickness may be reduced, as far as is feasible, to maximise topside capacity. A study of the trajectory and maximum velocity of dropped objects aids in the design of column and pontoon configurations for floating structures. However. designing to the maximum impact velocityderived from a combination of conservative assumptions (due to a lack of better guidance) may lead to uneconomic solutions. Analyses employing multiple simulations and reliability techniques allow a more rational approach to be adopted, producing design velocities which can be associated with an acceptable probacility of occurence. DROPPED ELEMENT TRAJECTORY ANALYSIS Any object dropped off a platform, from the deck or during lifting operations, will undergo three distinct phases during its passage to the seabed.Falling in air.Entry into water, body partly submerged.travel through water co1umn. The aerodynamic effects are generally small in comparison with the gravitational force on the body, therefore attention is focused on the motions of thedropped object during b) and c). A computer program has been developed for Dropped ELement Trajectory Analysis "DELTA", and utilised for the assessment of the likely maximum horizontal velocities from dropped drill pipe/casing. The 2-D motion of the dropped object in the global system is modelled by differential equations relating lateral, vertical and translational forces. The dropped object is discretized into 20 segments, and integrations are conducted numerically. The forces and moments, computed in a local system, are updated every time step before transformation into a global system. A full description of the theory utilised in the model is presented by Luo & Davis (1992).