briefing
Designing to reduce failures D
uring the decade 2000-2010, Lloyd’s Register’s Technical
Investigation
Department (TID) carried out more than 750 investigations, of which over 80 per cent were marine related. Fifty cases (representing a sample of more than 5 per cent) have been reanalysed in further detail, looking again at the causes of failure, and drawing conclusions to benefit the marine and wider industrial communities.
Some of the main findings reported by John
Maguire, structural engineering manager in the TID, were that: • by ship type, almost 80 per cent of the investigations related to five categories: oil and product tankers (20 per cent), passenger and cruise ships (18 per cent), roro vessels and ferries (14 per cent), LNG carriers (14 per cent) and container ships (12 per cent) • by component type, 80 per cent of the investigations related to six categories: engines including turbochargers and alternators (20 per cent), hull structures and hull vibration (16 per cent), rudders and other appendages (12 per cent), shafting and alignment (12 per cent), propellers and pods (10 per cent), and auxiliary machinery (10 per cent).
The most significant finding, LR notes, is that over 63 per cent of the failures could have been anticipated and avoided at the design stage. Most would agree that it is always better to avoid failures in the first place, rather than having to recover from them: a lesson that
many industries have learned.
It is clear that ship safety and performance rely increasingly on complex items of technical design. The quality of the design team obviously has
a major influence; well trained and
experienced personnel, validated design tools and sound management processes all play a significant role. Ensuring that key components are reviewed early in the design process is crucial in helping to minimise problems during subsequent construction, operation and maintenance. An independent technical review during the design process can specifically pinpoint and assess the most critical elements of a ship design, helping to identify technical risks early and find the most effective ways to avoid or mitigate them. Such reviews are perhaps more common in other industries (such as process, aerospace and automotive) and less common in the marine sector, which could benefit from more emphasis on independent technical reviews.
An analysis of the past decade of technical investigations by Lloyd’s Register shows that almost two- thirds of the problems examined could have been avoided at the design stage
by Doug Woodyard
As a specific example, LR suggests, consider an LNG carrier, which presents a number of vibration sources, such as propellers, main engines and cargo machinery. These sources – if inadvertently tuned to any significant hull, panel, structure or machinery natural frequency – may lead to large and damaging responses. Wherever possible, shipyards typically avoid possible resonances by deliberate frequency separation. Where not possible, they mitigate the problem by reducing the excitation source energy, damping down the response or showing that the vibration level – even at resonance – is actually at a low enough level not to cause concern. A useful tool for such a resonance check is the Campbell diagram, which plots ship or engine speed along one axis, structural and machinery response frequencies along the other, and excitation frequencies and multiples (orders) of the engine frequency across the diagram. Possible areas of concern that can be shown up by the diagram are: • at normal operating speed, where the second order engine excitation coincides with the hull girder modes • at manoeuvring speeds, where the second, fifth and sixth order excitations all coincide with the hull girder modes.
Once these risks have been identified they can be addressed. The second order engine excitation can be eliminated by adopting standard engine balancing devices,
such as
chain-driven compensators. Since they occur at relatively low engine speeds, and therefore at low powers, the manoeuvring speed resonances can be shown by calculation to lead to relatively low levels of resonant response.
The Campbell diagram is a useful tool and
a graphic method of assessing vibration risk for a range of ship types, LR explains. While not telling the whole story, it is a valuable means of communication between shipowner, operator, yard, designer and classification society when considering vibration issues. As such, the diagram can form an important part of an independent technical review. • Lloyd Register’s technical
investigation The fractured crankshaft of a general cargo vessel's main engine 14 I Marine Propulsion I February/March 2012
department has undertaken failure investigation, troubleshooting, analysis, measurement and research since 1947. In the early decades its work was mainly carried out for other parts of the LR group but now over 70 per cent of its activity is for external clients, such as shipowners, operators, yards and component suppliers. Case studies are published for the wider marine community, including in LR’s publication Technical Matters. MP
www.mpropulsion.com
Techno Fysica
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