OTC, Houston, USA: DNV has developed a risk-based approach which analyses the risk of accidental spills from LNG transport. Managing the overall risks by preventing collisions with sufficient energy to release LNG is the only acceptable approach to running the marine operations at LNG terminals safely. Alternative approaches are currently based on worst-case scenarios which focus on the hole size produced by a low-probability collision.
Historically, transporting LNG by sea has an excellent safety record, with no significant loss of cargo event, even though there have been some serious accidents(e.g. groundings) in an operational history that includes over 80,000 loaded transits during 40 years of operation.
“Despite this, the potential impact of accidents involving LNG vessels is still a source of debate, especially fronted by US stakeholders,” says Robin Pitblado, DNV Energy's director of HSE services.
He explains that the current approach focuses on a potential collision and the hazard effect related to a given hole size. “However, DNV’s work concludes that using hole size to determine hazard ranges is not a good way to manage risk and protect the public. The focus instead needs to be on manag ingport activities to prevent situations where actual collision energies cause damage to the inner LNG tank. The controls that should be in place include appropriate carrier operations, potential speed restrictions, tug pilot and escort controls, effective port management and regulatory controls. These measures and controls can ensure to a predetermined level of confidence (i.e. risk) that the energy produced by any collision will be less than the threshold required for the tank to leak,” he says.
Enhanced collision analyses
The risk-based approach of controlling the energy aspects in marine operations is based on the results of a Joint Industry Project in which DNV, with industry support, performed comprehensive Finite Element Method (FEM) analyses to map the external and internal damage to carriers that would result from a wide range of collision scenarios. The simulations included two LNG vessel types, two bow shapes and several vessel sizes and angles of collision. The joint industry project used real collision data from other types of vessels to map known collision outcomes onto the actual external hulls of membrane and spherical LNG vessels after taking account of the differences between vessels and average vessels. The collision data from the EU’s HARDER project, which are the best collision consequence data currently available, have been used.
There is a complex relationship between the striking angles, ship speeds and absorbed deformation energy of colliding ships. Three sizes of striking vessels were investigated – a length of 90m, a length of 140m and a length of 230m.
Standard to be launched
“This enhanced DNV approach, which includes the new method, meets the stakeholders’, and in particular the local community’s desire for protection as in the same manner as by approaches for onshore terminals. We aim to incorporate this approach into a DNV standard methodology at a later point,” concludes Robin Pitblado.
Contact
Robin Pitblado
Phone: +1 832 331 9319
E-mail: Robin.Pitblado@dnv.com
Date: 07 May 2008