Energy-Based Methodology for Collision Protection for LNG Carriers
Authors: Deetjen Caroline J. , Pitblado Robin , Hysing Thor 
Published: October 2008

LNG terminal developments are still the subject of major debate associated with potential hazards of marine accidental events. A recent U.S. Government Accountability Office (GAO) study has highlighted significant uncertainties associated with the underlying science. Prior risk studies have used estimates for potential hole size from accidental events that were based on judgment, as there have been no large accidental releases in the historical record. Hole size estimates have varied from 0.75m to 5m.

A major sponsored project has undertaken a review of hole size from a detailed structural calculation methodology. This has used real collision data from the EU HARDER project to map known collision outcomes onto the actual external hull of membrane and spherical LNG vessels. Using this dimension, detailed FEM analysis was carried out to map internal damages that would result from such an external damage. This showed that LNG vessels have great strength as the structures of the hull and stringers and other structures between the outer and inner hulls take and redistribute collision loads. However, once the inner hull starts to fail, the resistance to small increments in size of failure are relatively less than before inner hull failure. Assuming conservatively a consequential LNG tank hole size would correspond to inner hull tear size, there is no specific hole size that characterizes the collision event. More energetic collisions give larger dimension holes. It is concluded that a hole-size approach to LNG carrier accidental events is not a reliable means to protect public exposures.

This paper presents the results of the Finite Element Method (FEM) analysis for the two LNG vessel types and for several potential collision scenarios (size of vessel, speed, angle of collision). A suggested approach is to characterize the energy necessary to start to fail the inner hull and to plan port controls that can be shown to prevent such energetic collisions to a high and specified level of probability. In this manner, the public would be protected more effectively than nominating arbitrary hole sizes that cause events that in many cases cannot prevent public exposures in worst case scenarios. It is argued that this approach is closer in philosophy to NFPA59A than past approaches, and may therefore address some public concerns.

Introduction

The marine transport of LNG has an excellent safety record, with no significant loss of cargo in an accidental event in over 80,000 loaded port transits. This includes some major accidental events, such as the high speed grounding event on an LNG carrier off of Spain (1979), which damaged the external hull and indented the inner hull and cargo tank, but did not result in any loss of containment. This was an extraordinary validation of the strength and design integrity of such vessels. In the absence of any accidents, several recent studies (Pitblado et al 2004, Sandia Hightower 2004, ABS 2004) identified hazards from LNG carriers serving new marine reception terminals by reference to predicted hole sizes. These varied from 0.75m to 5.0m, also with some cases affecting several tanks simultaneously.

Predicted hazard zones from all these studies could, under pessimistic F2 weather conditions (F stability and 2 m/s wind speed) and some normal D5 weather conditions, affect the public in the vicinity of the accidental event (Pitblado et al 2006) as the drifting vapor cloud could reach the shoreline while still flammable (either LFL or ½ LFL concentration). Thermal radiation might also affect public areas, but may be less of a hazard as shelter would be taken.