Impact Damage And Assessment Of Offshore Tubulars
Authors: Frieze P.A. , Cho S-R. 
Published: October 2008

Two series of experiments are described in which tubulars typical of offshore construction are subjected to dynamic lateral impact and then residual strength tests.

Model material selection end preparation which involves heat treatment are reported together with the lateral impact test procedure, Damage in the from of local dents and overall bows is reported and dimensionless semi-empirical relationship presented which relate this damage to the initial kinetic energy. it is possible to identify the range of parameters for which damage will not occur.

To determine residual strength a selection of damaged and undamaged tubulars was subjected to axial compression s/one and when combined with external pressure. The test rig and procedure are briefly described and results presented. A semi empirical relationship between axial compressive residual strength and extent of damage is presented.

Finally, a method is presented by which full scale measurements of damaged tubulars can be interpreted for use in the residual strength equations.

INTRODUCTION

A significant number of studies have been devoted to the problem of damage to tubular members which comprise offshore structures. These have included experimental [1-7j, analytical [8-14] and numerical [15,1 6] investigations. The objective of the analytical and numerical studies has generally been to reproduce aspects of the experimental results, usually the residual strength behaviour. In this respect, some of the theoretical solutions provide close estimates but rarely consistently. The reasons for this are detailed but include the following:the solution procedure simulates the behaviour after damage without first re-creating the damage process;solution procedures can only try to replicate the test as reported which will not always reflect the experiment to the degree actually required for a simulation.

Apart from some of the test specimens of [4], all the above cited test series have been conducted on the basis that the behaviour they are attempting to simulate occurs statically. Yet they seek to represent the consequences of vessels and dropped objects impacting a tubular forming part of an offshore structure. The minimum design vessel impact speed is usually 2 m/s which locally translates into a strain rate at which at least the yield stress will be enhanced above its static value during the event. The static value is the one generally reported by experimentalists when conducting such tests. For assessment of damage, this oversight can possibly be partially rectified by using a value of yield stress taken from the mill certificate.

However, from a simulation standpoint, the use of a static approach omits a number of important energy balances. For example, as will be demonstrated later, most impacts involve rebounds so that not all of the impact energy is converted to damage as it is during a static simulation of the event. Also, although it may not contribute significantly in the forms of damage under consideration, distorted shapes which occur dynamically can differ from those generated by the equivalent static event. Some of the static test series aimed to achieve dent damage only whereas, as shown later, both dent and overall bow damage occurs during dynamic impacts.