A Method for the Determination of the Reaction Forces and Structural Damage Arising in Ship Collisions
Authors: Davies I.L. 
Published: December 2007

Recognizing that the risk of collision between a ship and an offshore structure is high, most codes and regulatory documents require designers to assess the consequent effects on the structure. Existing requirements, which demand that all the ship's kinetic energy before impact be absorbed by the structure, are very conservative. The method outlined considers the construction and mass of both the ship and structure and assesses the variation of contact force with time, using a multidegree of-freedom lumped-mass approach.

Introduction

Codes of Practice and regulations relating to the design and operation of fixed offshore structures (typically Ref. 1) require that consideration must be given to those load cases associated with ship collision incidents. These documents recognize that supply ships must approach close to offshore structures and that a vessel might well be alongside and involved in operations when rapid sea-state changes occur. Such changes could induce movements in the ship that would cause fender capacity to be exceeded and the structure to be damaged. The regulations also recognize that, through errors in judgment, a supply vessel might collide with an offshore structure during power maneuvering.

On the basis of such reasoning, the various regulating documents require demonstration that an offshore structure is inherently strong enough to withstand safely, though perhaps not without damage, impact of a ship of a specified displacement striking the structure at a velocity related to the sea state at which the vessel would be expected to lay off from the offshore structure. Some of the Codes of Practice give only one set of conditions related to Practice give only one set of conditions related to weather conditions - requiring that, in any incident, the damage that might be inflicted must be acceptable without leading to major collapse. Other documents define an "operating" incident (no damage may develop); this often is related to a collision when a supply vessel is maneuvering alongside.

It generally is agreed throughout these regulatory requirements that supply boats will be typically of 2,500-long-ton displacement and that vessels will be expected to lay off when the sea state produces an approach speed, due to wave motion, of between 1.5 and 2 m/s. In this case, some damage can occur, but it must not be so severe that the offshore structure is placed at hazard either by structural collapse or placed at hazard either by structural collapse or excessive flooding of otherwise dry compartments. Where an "operating" impact is specified, the approach velocity of a 2,500-ton supply boat frequently is given as 0.5 m/s, which is a typical maneuvering speed. For this case, it is required that no damage be inflicted on the offshore structure.

In some special cases, these requirements have been augmented by the operators to facilitate the approach of larger ships - a dedicated tanker, for instance. For these situations, additional load cases have been specified. Typically, this has involved the study of the effects arising from a 100,000-ton tanker nosing the structure at 0.5 m/s. In this case, the assessments have indicated that the bow configuration of the ship is most important, and it has been possible to define those bow forms that, in a ramming accident, might cause unacceptably high damage to the caisson structure.

A similar problem can develop as the result of the impact of heavy objects dropped over the side of the structure or ships in the immediate vicinity.

JPT

P. 2006