In-depth | SHIP STABILITY Damaged ship rules debate rolls on


Te end of January saw some of the world’s experts on ship damage assembled at Te Royal Institution of Naval Architects (RINA) in London for the “Damaged Ship” conference. A broad range of papers covering many aspects of ship damage including: fundamental research; design; regulations and their development; operation; and education were presented. Patrick Couser reports.


at the University of Strathclyde presented a historical perspective of the way in which ship stability, and in particular damaged- ship stability is assessed. Prof. Vassalos’ paper highlighted the rapid increase in complexity of the methods used to evaluate and compare damaged ship stability over recent years and gave an insight into possible further developments in the near future. The hydrostatic stability of damaged


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ships has been considered only since quite recent times. Archimedes first established the principles of buoyancy and stability around 250BC. However, it was not until 1928 that, in response to heavy loss of life (notably the Titanic in 1912), the first SOLAS (Safety Of Life At Sea) convention was established [Vassalos]. Since that first SOLAS convention,


developments in damage stability analysis methods and regulations have progressed at an ever-increasing rate, demanding a steep learning curve. Initial damage stability conventions specified relatively straightforward criteria such as minimum residual GZ for prescribed deterministic damage conditions. However, legislative rules governing damaged ship stability have evolved towards a probabilistic approach whereby a much wider range of ship damage scenarios are to be considered. These


sophisticated probabilistic


methods currently consider only the vessel’s survivability from a hydrostatic stability point of view (albeit with some implicit account taken for hydrodynamic effects). Te trend now is also to consider the ship’s operational capability and residual strength aſter damage from a “safe return to port” perspective. Traditionally, rule development has


generally been reactive in response to ship casualty incidents. However, there


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ong-standing ship-damage expert Prof. Dracos Vassalos, director of the Ship Stability Research Centre


is a developing trend to approach ship damage stability from a more first principles approach. Working towards this goal, the dynamic effects, both on stability and structural strength, of sloshing floodwater are now the subject of focused scientific research, both numerical simulation and experimental modelling. In this vein, experimental and numerical


simulation results from researchers at the UK MOD Damaged Ship Research Group at University College London as well as the University of Southampton were presented. These fundamental research programmes will provide analysis methods and verification data contributing to improved damaged ship assessment frameworks and future rule development. Safety as a design objective and ‘risk-based


design’ have been the main focus of SAFEDOR (www.safedor.org). Te four-year, EU-funded project, completed in 2009, has established a framework for risk-based design. The


framework includes an


assessment of the residual structural strength of the vessel as well as its residual hydrostatic stability, with the goal that the casualty vessel can be its own lifeboat, remaining afloat for five days. Tis habitability requirement is aimed at providing sufficient time for the vessel to return to port under its own power or to await the arrival of external assistance. Mr Seungmin Kwon from the Ship


Stability Research Centre at the University of Strathclyde presented a methodology for the assessment of progressive structural failure. Essentially a LEFM (linear elastic fracture mechanics) and Paris law crack-propagation model is proposed to track the residual structural strength of the vessel in the time-domain. Tis provides a first-principles approach to estimate the length of time a damaged vessel can be expected to remain afloat which depends on the initial damage, weather and flooding. A validation study,


Te “Project Genesis” ships, Oasis of the


Seas and Allure of the Seas, as described by Prof Vassalos in his paper, highlight how this design framework has been used to design not only the largest but also the safest vessels ever built. In his paper, Keith Hutchinson, principal


naval architect of Babcock Integrated Technology, gave a very detailed description of the legislative structure of rule development and application. Te structure and procedures of the IMO are described and the paper gives some insight into the difficulties faced by the rule developers. It is also apparent that further clarification is required for the relatively new probabilistic rules so as to enable consistent and accurate interpretation, ensuring equivalent application of those rules across the board. Mr Hutchinson also explained how feedback is provided to the IMO via IACS’


The Naval Architect April 2011


using data from the Prestige incident off the coast of Spain in November 2002 is described. Te International Maritime Organization


(IMO) is working towards a new, holistic risk-assessment philosophy to address ship damage [Vassalos]. Te key theme areas of this new approach are: 1. Prevention and protection: i.e. aiming to prevent casualties from occurring, but also provide sufficient protection so as to limit the consequences that may arise from the damage sustained.


2. Te time-frame in which events occur (this subject was also discussed by several other speakers). How long will the vessel remain habitable aſter an incident and to what extent will its operations be compromised as a consequence of the damage incurred?


3. Te casualty threshold up to which it is safe for passengers and crew to remain onboard while the vessel attempts to return to port or waits for assistance to arrive.


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