Ship Collision And Fendering Of Offshore Concrete Structures
Authors: Larsen Carl Martin , Engseth Alf Gunnar 
Published: December 2007

The first part of this paper gives a broad discussion of the risk for collision between offshore platforms and ships. This risk can be reduced either by reducing the probability of collision events or by reducing the consequences when collisions occur. The paper discusses the means that can be applied to accomplish this, and a proposal for how to take collision risk into proposal for how to take collision risk into consideration when designing a platform is given.

The second part describes a fender proposal for protecting a concrete platform of the wellknown Condeep design. The fender will absorb part of the impact energy from the incoming ship part of the impact energy from the incoming ship and reduce the impact force, but an even more important effect is that the force will be distributed over a large area of the column. Both effects improve the ability of the structure to avoid damage. Economic aspects of fendering are also discussed.

The last part of the work gives a case study of a selected ship/platform system. Possible pact velocities are calculated and impact forces pact velocities are calculated and impact forces are found from a study of energy absorption in the ship, fender and platform. By finding the maximum shear stress that the column can withstand without damage, allowable impact velocities can be found and compared to the expected velocities.

The paper concludes that concrete platforms can be damaged by ship collision with moderate velocities and that the proposed fender will significantly increase the safety of the platform.

Introduction

Offshore oil activities involve many types of risks, and collision between ships and platforms has been pointed out as a main one. Reports from the activity in the North Sea indicates that accidents have occured, but that catastrophes so far have been avoided.

To the event "collision between ship and platform" certain probabilities and consequences are platform" certain probabilities and consequences are associated. The probability tells how often a collision with a given impact energy (ship size and velocity) will occur, while the consequence tells what kind of damage will be involved. The probability must be regarded in relation to the probability must be regarded in relation to the consequence. An event that involves large consequences must have a very small probability of occurence, and the consequence of very frequent events must be small. The risk is often defined as the product [probability × consequence], and can be affected by varying either the probability or the consequence. For a platform - ship system the collision probability is reduced by introducing limitations for operations close to the platform, constructing safe mooring systems and having a good radar survey. The consequences are reduced by strengthening the platform or by fendering. A platform designer platform or by fendering. A platform designer most have in mind all these ways of reducing the risk, and the choice of means most be made after a broad discussion.

DISCUSSION OF COLLISION RISK

A collision event is defined by the followingImpact, defined by ship motions and contact points on ship and platform.Ship, characterized by size, strength and flexibility.Fender, characterized by shape and energy absorption performance.Platform, characterized by strength and flexibility.

If all these factors are known, it is possible to find the collision consequences (extent of damage). If all operations in the vicinity of a platform are known, one could also think that the platform are known, one could also think that the probability for a certain collision could be found probability for a certain collision could be found by, for instance, a probability distribution for impact energy during the lifetime of the platform.

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