A systematic approach to ship safety

New software developed by naval architects and marine engineering consultants Safety at Sea to address the International Maritime Organization's challenging regulations on a ship's safe return offers potential for aspects of offshore vessel design.
As the hunger for far-flung cruises and remote offshore exploration has intensified, administrations at IMO have recognized that Search & Rescue alone will be unlikely to meet all possible accident scenarios.

For this reason, 1 July 2010 saw entry into force of new regulations, amending SOLAS and demanding more emphasis on the prevention of a casualty from occurring in the first place. They also demand that future ships should be designed for improved survivability so that, in the event of a casualty, persons can stay safely on board for ultimate rescue.

The rules incorporate criteria for a casualty threshold, defined as the amount of damage a ship can sustain according to its design and still be safe for those onboard. Catching the eye are new overall assessments for essential systems' availability.

Alternative design and arrangement have long been incorporated in SOLAS, but the latest developments augment this thinking to include the provision of safe areas onboard and operability standards for the essential systems to be maintained so that a ship can sustain essential functions and support people until assistance is available or the damage can be recovered after a casualty. This will require redundancy of propulsion and other essential systems; onboard safety centres, from where safety systems can be controlled, operated and monitored; and the new requirements offering a means of verification. The ship's crew may not be able to move a ship to a port or place of refuge for reasons of stability, for example, and so the new requirements define systems operability to maintain the safety of people or systems onboard.

Initially driving the IMO's Maritime Safety Committee was concern over larger passenger ships, working on the principle that ‘a ship is its own best lifeboat'. This approach envisages that passengers and crew should normally be able to evacuate to a safe area onboard and stay there. In addition, essential systems such as propulsion and steering must be sustainable.

Offshore equivalence
However, as the number of large accommodation vessels set to work in increasingly remote locations for offshore construction has increased, so too has impetus grown for the rule changes to be transposed for use within the IMO's guideline Code of Safety for Special Purpose Ships. The Code, revised in 2008, applies to vessels of more than 500gt and carrying more than 12 special personnel. It entered into force in May 2010. Under SOLAS provisions, any vessel carrying more than 12 personnel not crew or special personnel automatically becomes a passenger ship.

The general aspirations of IMO were translated into specific ship design guidelines as part of detailed contributory work undertaken in the EU-funded SAFEDOR project, which included a new methodology that is leading on to the development of software codes to provide the engineering verification results. Participants included Safety at Sea (SaS) and the University of Strathclyde's Ship Stability Research Centre. Subsequently, SaS has continued the development of the codes, with the new software being marketed as I-SYS.

The essential systems are identified by SOLAS; the software, in line with the overall assessment framework, allows the designer to assess if a given essential system is critical or not and, if so, that it is operable when the ship is subject to a number of damage scenarios. Critical systems are identified in the overall assessment of essential systems to have a possibility to fail to operate adequately as a consequence of one or more fire casualty case, each not exceeding the fire casualty threshold, or as a consequence of one or more flooding case, each not exceeding a single watertight compartment.

The resulting package has already been used in a number of projects in the passenger ship sector, forming the basis for systems capabilities post-damages on behalf of Daewoo Shipbuilding & Marine Engineering (April-July 2009) and Samsung Heavy Industries (two projects, one running January-May 2010, and the other from June-August 2010).

Also proceeding has been initial work in the offshore sector. SaS has used a comparable methodology to work on requirements as they relate to fire protection, vessel subdivisions and systems redundancy.

According to Safety at Sea's safety engineering director Dr Luis Guarin, the new software codes offer key opportunities in relation to offshore construction, field development support vessels and other offshore craft, if they have large accommodation blocks. A further potential application includes I-SYS being used as a support tool for the assessment of dynamic positioning systems.

‘We have carried out two initial jobs for Samsung Heavy Industries (SHI),' Guarin notes. ‘One of these focused on risk assessment of fire protection arrangements in some areas below the main deck for a field development support vessel under construction for Saipem. This included advanced fire engineering calculations and escape route analysis (by simulations). Alternative design evaluations were also made of fire protection arrangements in machinery spaces (as per IMO MSC/Circ.1002).

‘In the second project, which focused on an offshore construction vessel, we carried out evacuation/escape route analyses on behalf of Ulstein Sea of Solutions (whose client is Heerema), as part of analyses required by Norwegian Maritime Directorate (NMD) regulations.'

Edwin van Leuwen, Ulstein Sea of Solutions project manager, explains: ‘Due to the type of operations performed by these types of heavylift/pipelaying ships, they always have a large number of people onboard and are therefore classed special purpose ships and follow cruise ship rules for SOLAS to a large extent. On previous projects we did escape analyses ourselves based on rule calculation methods. The potential of the SaS escape route analysis program looked far more efficient than our own calculation method. We challenged SaS with a very tight schedule as it was and still is a short track project, yet they delivered on time whilst providing us with some suggestions to improve the arrangements, in order to avoid congestion of people. After the first draft report based on SOLAS requirements, additional requirements from NMD were analysed. The analysis showed compliance of the arrangement with both SOLAS and NMD requirements.'

‘Results of the overall assessment are given in a form of listings of systems affected by analysed damage scenarios,' says Dr Guarin. ‘Additionally, for each damage scenario (or set of scenarios) a list of unique "restoring solutions" can be generated. These solutions provide guidance for eliminating system vulnerabilities.

‘In detail, I-SYS uses a set of logical (Boolean) expressions to define physical systems' dependencies, including equipment, piping, pumps, switchboards, cabling and tanks, and a set of Binary Decision Diagrams to solve input equations. A fire or flooding casualty occurring within a space or group of spaces affects directly or indirectly the systems and functions embedded within the ship environment. Casualty scenarios include flooding to single WT compartments, fire casualty threshold and the loss of single main vertical fire zones due to fire.'

Future capability
According to Dr Guarin, work is continuing to extend the use of the I-SYS package. ‘So far, in general, the assessment has been focused on evaluating systems' design. Watertight subdivision is optimized separately. It is our intention to modify and use the software to evaluate the impact of subdivision on the SRTP capabilities. This requires an interface linking automatically the systems' components and the watertight subdivision.'

The use of I-SYS has already proved far reaching for ship designers in general, he adds. ‘Changes have been made to electric distribution/piping systems topology. So far, we have been able to identify mainly gaps in the electric distribution system and piping systems (for example, lack of redundancy in piping or cabling – various systems, lack of bypass and/or isolation valves for technical water).

‘In future, a major part of development work will be to enhance the user interface. Modelling of systems and post-processing of results is currently rather time consuming but, based on the experience so far, we are writing the specification requirements for a graphical user interface that will allow importing watertight subdivision and A-class bulkheads geometric information into the systems' definition environment. The new interface will also allow more efficient post-processing of results.' OE

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