Moving towards a less damaging working environment

25 Apr 2016
The PAC 24 Mk 3 shown here is the  predecessor to the Mk 4 for which the MOD conducted their testing

The PAC 24 Mk 3 shown here is the predecessor to the Mk 4 for which the MOD conducted their testing

Tim Rees of Canada-headquartered suspension seating company, SHOXS, brings us up-to-date in the long-running saga of the implementation of the European Union's Directive 2002/44/EC.

EU Directive 2002/44/EC specifies stringent constraints on acceptable workplace exposure levels to shock and mechanical vibration.  The directive has a particularly important role to play in the maritime industry, where workers may be subjected to extreme shocks on a regular basis as high-speed craft slam into waves.  The health risks associated with mechanical shock in the marine environment are well-known (see Ensign et al, “A Survey of Self-Reported Injuries Among Special Boat Operators,” 2004), and the EU directive is an attempt to provide protection for those in harm's way.

EU directives are similar to EU regulations, but individual member nations must transpose directives into their own regional laws.  This is especially important in light of the legal precedent set by the case Francovitch v Italy, where it was ruled that a European member state could be held financially liable by those suffering from a loss due to that nation's failure to bring an EU directive into law.  Through legislation introduced in 2005 (No. 1093, The Control of Vibration at Work Regulations) and in 2007 (No. 3077, The Merchant Shipping and Fishing Vessels (Control of Vibration at Work) Regulations), the UK has implemented directive 2002/44/EC, and while the steps taken towards compliance have not been dramatic thus far, recent developments suggest that things may be about to change.

Plainly speaking, the exposure limits specified in Article 5 of directive 2002/44/EC are not achievable with the current state-of-the-art.  However, the directive specifies conditions under which member nations may grant derogations (exemptions) from the exposure limits.  In particular, Article 10(3) states that "[...] Such derogations must be accompanied by conditions which guarantee, taking into account the special circumstances, that the resulting risks are reduced to a minimum and that the workers concerned are subject to increased health surveillance. [...]"  Consistent with this language, the UK's legislation No. 1093 states that if exposure levels cannot be brought within the specified limits they must be reduced to as low as reasonably practicable.

Driven by these legislative steps and the increased awareness of health hazards posed by mechanical shocks in the marine environment, shock-mitigating suspension seats have emerged as a key part of the solution to the problem.  What was once considered a novelty is becoming an essential piece of safety equipment, and there is now a wide range of products available on the market.  But the industry's growth has made the decisions faced by employers and fleet operators even more difficult: with so many products to choose from, which one minimizes health risks to ensure legal compliance?  The confusion has been further exacerbated by a barrage of empty, unsubstantiated performance claims presented by a number of seat manufacturers.

The UK's Ministry of Defence (MOD) has made important steps towards understanding the science of shock-mitigation and standardizing its measurement.  Initially the MOD was granted derogations as there was no legislative guidance on how to actually measure or minimize the health risks involved.  But following a period of risk-assessment, the MOD investigated a range of technologies and testing methods.  For their in-service fleet, suspension seats were deemed reasonably practicable, while for design boats the MOD is also considering novel deck and hull designs. In addition, the MOD is moving towards meeting the legislative requirements through enhanced awareness training, specialized physical conditioning, and health monitoring.  But for both in-service and design boats, suspension seats will play a crucial role, and so the MOD engaged in the development of a scientific seat-testing protocol.  Up to that point, sea-trials performance tests had been favoured by industry, but the MOD protocol prescribed a laboratory drop-test.  The prevailing scientific wisdom suggests that the most dangerous impacts to a seated human are in the vertical direction, and the MOD's test procedure focussed on that aspect of shock mitigation.

A minor digression is needed here to emphasize the benefits of using laboratory drop-tests to measure seat performance.  Out on the water, where the sea-state and weather cannot be controlled, the variable impact angles and complex nature of the pressure distribution that forms on a boat's hull with each wave collision makes it impossible to fairly compare rival seats, even within the same boat.  In contrast, the laboratory offers tight environmental controls and highly repeatable conditions facilitating a fair head-to-head comparison of seats.  The moral quagmire associated with exposing human test subjects to harmful shocks is easily circumvented by using sand or lead-pellet bags to represent seat occupants.  Different payload masses and drop heights can be tested to build a clear picture of how a seat performs across the range of target operating conditions.  By using guided drop decks and adjusting the impact piling geometry, deck impact signals can be isolated to the vertical direction and tuned to closely mimic those experienced at sea.  Laboratory testing has financial benefits as well; with no dependence on weather, tests can be performed around the clock, and without exposure to salt water, equipment has a longer life-span. 

Having established their drop-testing protocol, in 2014 the MOD tendered an open call for shock-mitigating seats to be tested, with the aim of determining the best product with which to equip the Royal Navy's fleet of small boats.  Twelve seat manufacturers submitted seventeen products, and the MOD conducted a very arduous testing programme in 2015.  The drop-tests were a critical part of a broader evaluation process, and two competing products were down-selected for sea-trials.  At sea, basic functionality and operability were assessed by experts, and a Human Factors Integration assessment was conducted.  Ultimately the SHOXS 5005 and 5050 were selected as the winners.  The Royal Navy has ordered 60 of BAE's next generation PAC 24 Mk IV boats equipped with SHOXS seats, with deliveries commencing in the first quarter of 2016. 

The relatively simple appearance of shock-mitigating seats has long belied the subtle challenges involved in assessing their performance.  And in Europe and beyond, the dubious performance claims touted by manufacturers are generally not qualified by a scientific method, impeding rational attempts at product comparison.  By running its own tests the MOD has dealt with this problem, but there has also been an international movement towards developing a marine seat testing standard.  For that purpose, a working group was established under ISO 2631 TC108/SC4.  The working group's members include scientists from the UK's MOD, Canada's DND, and the U.S. Navy; academics from Europe, North America, and Japan; and international industry representatives.  The ISO standard largely builds upon the foundations provided by the MOD's testing protocol.  The group met twice in 2015 (in Southampton, UK, and Victoria, Canada) and remains active in sharing and analyzing data.  In addition to the technical aspects of establishing a repeatable laboratory procedure, members of the group are involved in the analytical investigation of appropriate metrics for evaluating a seat's performance.  These metrics include the vibration dose value, which may be familiar to Europeans, and the shock response spectrum, which measures the effects of acceleration on a simple oscillatory system.  Despite its relatively brief existence, the working group has made substantial progress, and it is anticipated that the first publicly-available version of the standard will be released within the next year.

The developments within the ISO working group are exciting, and they will undoubtedly have a lasting effect on the marine shock-mitigating industry.  Ideally, the current confusion associated with comparing the performance of various products will be replaced by a clear sense of exactly what level of protection each seat can offer.  Boat-builders, seat suppliers, and specification writers alike will benefit from a streamlined product evaluation process.  This will enable them to shift their focus to other aspects of the seat that end-users can directly recognize, such as aesthetics, comfort, functionality, and integration with existing equipment.

The shift towards a full adoption of EU Directive 2002/44/EC in the UK and throughout Europe will take time.  But as the marine shock-mitigation industry reaches maturity, fleet operators who acquire cutting-edge technology will protect their employees more effectively and obtain their derogations more readily.  The enhanced crew safety and legal protections offered by compliance will yield long-term financial benefits to operators who keep pace with the new developments.  The outcome of the MOD's ground-breaking tests provides some immediate clarity on seat performance, but other fleet operators may choose to commission their own intensive tests for specific applications.  Empowered by the foundations set by the MOD and the upcoming ISO standard, this will be possible in a scientifically valid way, with the end result being a safer workplace for boat crews and operators.

Tim Rees holds a Ph. D. in Applied Mathematics, with a specialization in numerical methods and fluid mechanics.  He will be available at the Seawork tradeshow at the SHOXS stand PG130, and he will be giving a technical presentation at the Seawork conference.  If you are interested in learning more about shock mitigation and how it can be applied in specific settings, please feel free to contact him via email at:  Updates on the progress of the ISO standard will also be provided on

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