Reducing Fatigue Risks onboard Oil Tankers and LNG Carriers

An enhanced Fatigue Design Assessment (FDA) Level 2 solution is now available in LR’s ShipRight software. It provides a robust tool which can help owners to reduce their risks. This first release covers oil tankers and membrane type LNG carriers.

Lloyd’s Register has given designers, ship owners and operators an updated tool to help assess new or existing designs and to help further reduce the risk of fatigue damage in ship’s hull structures.

Cracks and fractures at connections between longitudinal stiffeners and transverse frames of ship’s structures, particularly in large oil tankers with higher tensile steel constructions, were a serious concern in the 1980’s and the early 1990’s. To address these concerns in the early 1990’s, Lloyd’s Register introduced ShipRight FDA procedures to promote structural detail design and fatigue performance assessment.

Over the last 20 years, Lloyd’s Register has continuously invested in the development of ShipRight FDA procedures. A new FDA Level 2 module has been incorporated within the ShipRight software which further improves on current technology and makes the assessment tool more robust and versatile. The ShipRight FDA Level 2 module includes the following features:

New interface for the modelling and fatigue result graphics, compatible with Windows 7 64-bit platform.

More accurate calculation of ship motions and hydrodynamic loads. This overcomes the existing limitation which is constrained by parametric methods.
New solver to accurately take into account load component interactions (cargo inertia loads, wave bending moments and wave pressures).

New features based on users’ feedback and comments, such as five input transverse sections to better describe hull form changes.
Extensive tests and validations against Lloyd’s Register’s experience on fatigue design assessments.
Jimmy Tong, Technical Manager of Ship Structure Strength at Lloyd’s Register’s Marine Technology and Engineering Services, said “These upgrades our ShipRight software, the industry’s preferred fatigue design assessment tool, to a higher level of usability and accuracy”.

Fatigue design assessments, particularly for LNG carriers, become more demanding and challenging as ship owners often require long fatigue design life in harsh wave environments, e.g. 40 years fatigue life in North Atlantic wave environments.
“Designers and owners need to have confidence in the structural performance of the latest generation of these large ships in such harsh wave environments,” said Jimmy Tong.

This comprehensive structural analysis puts greater emphasis on the quality of the design details, particularly in the higher risk regions of the hull. Fatigue damage is a direct consequence of cyclic loading, construction standards and trading wave environments, with alignment also playing an important part. The new tool provides a reliable means to identify high-stress locations and to help reduce the risk of structural failure.

Ships complying with the requirements of the ShipRight FDA procedures will be eligible to be assigned the notation ShipRight FDA or ShipRight FDA Plus based on the specified requirements on fatigue design life and associated trading wave environments.

This new release of ShipRight software is focused on oil tankers and LNG carriers, and applicable to longitudinal end-connections. Other ship types will follow in the next stage.