Appraisal of Accidental Impact Loadings on Steel Piled North Sea Structures
Authors: Donegan Emmet M. 
Published: October 2008

If a steel piled offshore structure is to exhibit regular and controlled resistance to vessel impacts, then the primary structural framework must be configured with this aim in mind. Damage should be properly addressed at the onset of the structural design process; it is not a subsidiary loading check to be applied to a completed structural design.

The first stage in damage design is to outline the strategy for impact resistance. There are two basic stratagems: reserve strength and redundancy. In general a reserve strength strategy suits unmanned platforms and self-floating towers. A redundant framework is best suited to control damage in conventional jacket structures.

The determination of vessel impact loadings is not an exact science. What is apparent is that a likely extreme impact energy exceeds the resistant strength of conventional steel-piled platform legs. Fenders may be used to absorb impact energy and so bridge the gap between impact energy and resistant strength. Alternative options are to reinforce and strengthen the platform legs internally, to take account of energy absorbed by the ship or to provide alternative load paths for the major leg loads.

More research is needed into the probability of ship collisions occurring, and in particular to the elevations at which impacts of a specific energy may occur. In the absence of this information, it may be prudent to design new North Sea steel piled structures and their foundations with the provision to resist storm wave loadings associated with a leg foundering scheme.

Introduction

All structures in the North Sea are exposed to the risk of collision from passing vessels. However the probability of such an event has been theoretically collated to be 1 event in 2000 platform years2,3.

However, a fixed steel production platform is exposed to a greater risk of marine collision, because platform service craft (diving support vessels and supply boats) must regularly approach and be moored alongside the structure. The probability of a service vessel striking a North Sea production platform has been estimated to be 1 event in 50 years per installation This figure is based on real events.

Until recently, ship impact damage has been treated as a post-design exercise. The completed structure design would be checked for impact resistance using two methods of analysis. These methods were objective ('surgical') element removal and 'minor' impact damage assessment.

Although minor collision damage is still a valid design precept (as the majority of impacts will be in this class), it is now generally accepted that the structure must also be assessed for extreme collision damage. In an extreme collision up to 14.0 MJ of impact energy may be released. This often means that structural elements are 'lost' from the ship impact zone. However, the remaining framework must be analyzed under the failed members' end forces and moments arising at the instant of impact. These failure loads constitute 'subjective element removal' and are usually combined with the mean platform loading regime to estimate permanent frame deformations.