Collision-accidental limit states-based safety studies for a LNG-fuelled containership

Highlights

• Design of a hypothetical 9,000 TEU LNG-fuelled containership.

• Selection of a total of 12 collision scenarios with varying loading conditions and collision speed.

• Structural crashworthiness analysis using nonlinear finite element methods.

• Accidental limit states based safety studies.

• Discussion for the applicability of the existing IMO IGF code for LNG tank designs to LNG-fuelled ships in collisions.

Abstract

Liquefied natural gas (LNG) is an increasingly popular fuel alternative of ships for reducing greenhouse gas emissions. The interest in LNG fuel propulsion systems is rapidly growing, yet certain important safety design and engineering issues remain poorly constrained. The safety of LNG-fuelled ships must be assessed in terms of the potential structural damage owing to collision accidents and resulting LNG spills, especially for ship types that store the LNG storage tank within the cargo hold. In this paper, accidental limit state (ALS) based safety assessment for LNG-fuelled containership structures using nonlinear finite element methods is studied at the most unfavourable scenario of collisions between a striking ship's bow and a struck ship's LNG fuel tank, where the struck ship is in full load condition at a standstill, while the striking ship of the same size as the struck ship has different loading conditions in the ballast load condition, 50% partial load condition and full load condition, with varying collision speed at 0.5, 3, 6 and 9 knots. A hypothetical 9,000 TEU (Twenty-foot Equivalent Unit) LNG-fuelled containership was designed in accordance with the requirements for the international gas fuel transport standards, accommodating a membrane-type LNG fuel tank located amidship. It is found from the present study that inner side hull structures of the struck ship can be damaged in ship-ship collisions, and the current industrial guidelines for LNG fuel tank designs are required to amend to apply for LNG-fuelled ships.

Introduction

The International Maritime Organization (IMO) enforces marine environmental regulations to reduce the emission of air pollutants, such as sulphur oxides (SO_X), nitrogen oxides (NO_X) and carbon dioxide (CO₂) from ship operations (IMO, 2019a). A 0.5% (or 5,000 ppm) global cap on SO_X was imposed in 2020 and regulations relevant to tier-III reductions of NO_X emissions in all seas worldwide aim to reach 80% compared with tier I (IMO, 2019b). Alternative energy sources are therefore required, such as natural gas (NG), liquefied NG (LNG), liquefied petroleum gas (LPG), biofuel, methanol, hydrogen, and ammonia.

LNG has received the most attention as an alternative fuel with the potential to reduce NO_X emissions by up to 80%, completely remove SO_X and particulate matter (PM) emissions and reduce CO₂ emissions by at least 20%. The number of ships using LNG as fuel is therefore rapidly increasing (see Fig. 1), as is the application of LNG-fuelled systems in large commercial ships, such as containerships and crude oil tankers, as well as small ships that navigate coastal areas. Fig. 2 shows an example of an ultra large LNG-fuelled ship of 23,000 TEU under construction with a Gaztransport & Technigaz (GTT) Mark Ⅲ-type LNG membrane tank at a shipyard.

Although LNG is an ecologically friendly source of energy, it is a hazardous fuel type associated with its cryogenic and flammable characteristics. For example, an unexpected LNG leakage can critically damage ship structures by brittle fracture at cryogenic conditions, leading to fires and explosions with ignition sources (ISO, 2015). Among several types of marine accidents in the shipping industry, collisions are the most frequent type (Logistics Middle East, 2016, Seatrade Maritime News, 2014, The Local, 2018). For containerships alone, a total of 866 maritime accidents such as collisions, grounding and other contact events were reported during 1990–2012, and 44% of them were owing to collisions (Pagiaziti, 2015).

LNG fuel tanks of small-sized ships are usually positioned on the upper deck or in an open space. However, LNG fuel tanks of large-sized ships, especially containerships are located under the deck or inside the hull space to maximize the efficiency of cargo transport. It is obvious that LNG-fuelled ships in collisions may be at a higher risk than traditional ships because the former type can involve brittle fracture at cryogenic conditions due to leaked LNG and fires or explosions with ignition sources. As such, safety design and engineering in collisions is essential to prevent and control undesirable LNG leaks, involving accidental limit states with structural crashworthiness analysis.

In this paper, the structural safety of a hypothetical LNG-fuelled containership in ship-to-ship collisions is studied. The applicability of the current industrial guidelines for LNG fuel tank designs and arrangements is investigated and the need to improve current design codes is discussed. The highlights of the present study are as follows:

• a hypothetical 9,000 TEU containership is designed with a membrane-type LNG fuel tank located amidship in accordance with the IMO International Code of Safety for Ships Using Gases or Other Low-flashpoint fuels (IGF) code, which is adopted for typical LNG-fuelled ship designs (IMO, 2006, IMO, 2019a);

• The struck ship is in the full load condition at standstill, while the striking ship with the same as the struck ship has different loading conditions in the full load condition, 50% partial load condition and ballast load condition.

• A total of 12 collision scenarios are considered with varying the collision speed and loading condition, where the collision angle between the striking and struck ship is 90° while the collision speed is varied at 0.5, 3, 6 and 9 knots.

• Nonlinear finite element methods using LS-DYNA (2019a, 2019b) are used for the structural crashworthiness analysis;

• Based on the computational results, structural damage characteristics are discussed in association with ALS design to ensure that the main safety functions are not impaired during the accident or within a certain time after the accident, while safety criteria for ALS structural design are based on limiting accidental consequences such as structural damage and environmental pollution (Paik, 2018, 2020, 2022) and

• Applicability of the existing IMO IGF code for LNG tank designs that have been adopted for diesel oil-fuelled ships is discussed with the focus on safety design and engineering for LNG-fuelled ships in collisions.