New Non-Linear Methods For Estimation Of Collision Resistance Of Mobile Offshore Units
Authors: Pettersen Erik , Johnsen Kaare R. 
Published: October 2008

A new method to estimate the extent of structural collision damage on offshore platforms is presented The method considers collision forces acting on bracings and chords of semisubmersibles and jackups. An iterative procedure is adopted where .a simplified non-linear stiffness representation and calculation of absorbed energy are essential elements.

It is explained how linear beam element programs may be utilized in the calculation procedure.

Examples of application to both jackups and semisubmersibles are presented.

Finally some design principles are proposed in order to obtain improved protection against global structural collapse

Introduction

In recent years the of shore industry has experienced several accidents with mouile offshore units. In order to obtain an acceptable level of safe all kinds of design loads have to be cohsidered, structural responses must be carefully analyzed and adequate criteria have to be estabtished.

It is recognized that collision loads must be taken into account during the design stage. The vessels serving the offshore platforms are incrasing in size and sophistication thus being able to operate under more severe seastates. The effect of a collision incident is increased accordingly. It is mandatory to design chords and bracings of jackups and semisubmersibles to resist the collision force from large supply vessels and other ships serving the area.

However, it is unrealistic to establisr. design criteria which will avoid major damage from all types of collision events.

An offshore structure cannot be designed to withstand the impact of a large merchant ship a.t a high speed. The design collision criteria for offshore structures should be kept at a probability level in line with the general accepted safety level at sea (see. ref.).

Calculating the extent of collision damage is a complex task, especially when large collision energies are involved. The Ilroblem may be approached at various levels of sophistication from simple, semianalytical handcalculations op to general non-linear analyses based on the finite element method. Neither of these extremes are. suitable in practical design. The simple hand calculation is not reliable enough, the general non linear F.E.M. approach is too time consuming and costly.

In the following an intermediate method is presented. The method takes into account the highly nonlinear behaviour of the structures during collision. This is achieved by a combination of stepwice iteration and application of linear space frame programs.

2. General Description Of Proposed Procedure

The problem of_calculating the extent of structural damage caused by a collision incident may be solved as follows:By means of momentum and energy consideration, calculate the energy which must be absorbed during the collision.Apply a collision load to the structure. Calculate by means of non linear methods the resulting damage.Calculate the energy absorbed by each of the elements where damage has occured according to the damage picture obtained in step b.