Textbook of Maritime Medicine

Mechanical vibration is a ubiquitous source of pollution on board ships. Vibration is defined as the variation over time in movement or position of a mechanical system, with an amplitude that is alternately larger and smaller than a reference value. The parameters involved in vibration are frequency of vibration (in Hertz/Hz), amplitude or displacement (m), speed (m/s), acceleration and direction of displacement (m/s²).

Some studies consider impedance, which is the dynamic force to which the structure is exposed over speed (Z = F/v  N s/m).

“Jerk” (m/s³) can also be defined as the derivative of acceleration with respect to time.

It is considered that:

• very low frequency vibrations correspond to frequencies of between 0 and 2 Hz
• low frequency vibrations correspond to frequencies of between 2 and 20 Hz
• high frequency vibrations correspond to frequencies of between 20 and 1000 Hz and more.

They can be either continuous or periodic, and can occur randomly or transiently.

On board ships, personnel are subjected to whole-body vibration, in other words exposure of the whole body to vibration on all three axes (horizontal, vertical and lateral). Acceleration is generally between 0.006 and 0.6 m/s² along each axis in response to movements of the ship, and are very variable depending on sea conditions, wind direction and the position of the subject on board (in the centre or nearer the sides).

Given the complexity of the phenomenon, there are no satisfactory threshold exposure values. An evaluation of exposure of individuals to whole-body vibration is given in international standard ISO 2631.

 Table 11.5.1

Relationship between various levels of discomfort and equivalent acceleration (ISO 2631)

To evaluate the severity of vibration exposure, the acceleration equivalent (aeq) is calculated, which is the effective acceleration value measured at the point of entry into the body along the three orthogonal directions. The acceleration signal is weighted for frequency and direction in order to take into account human sensitivity to these parameters. An equivalent value over 8 hours (aeq(8h)) is obtained by multiplying the acceleration equivalent as measured by the square root of the ratio of daily actual exposure/8 hours.

In 2002 Europe adopted a Directive concerning minimum health and safety requirements regarding the exposure of workers to the risks arising from vibration. This Directive defines:

• a level of exposure action value set at aeq(8h) = 0.5 m/s² over 8 hours. If this value is exceeded, employers are asked to assess and monitor the risks, to reduce vibration levels, to inform and train workers and to arrange health surveillance.
• a daily exposure limit value set at aeq(8h) = 1.15 m/s², above which it is considered that regular exposure to vibration presents such a risk to health that vibration levels should immediately be reduced.

11.4.1   Vibration on board ships

Vibration phenomena are caused by the following sources of excitation:

• the propeller (periodic vibration)
• engine and ancillary machinery (periodic vibration)
• effects of the sea (random vibration)

Installation of ever more powerful propulsion systems on ships with high tonnage with a single driveshaft increases the discrepancy between the rigidity of the driveshaft and the flexibility of the ship’s structure. It has emerged that these phenomena have been responsible for worsening of vibration on board ships. If two sources of excitation are close in frequency, a beat phenomenon emerges, and the frequency of this beat is likely to cause a resonant response. The beat frequency is different from each of the component frequencies, is markedly lower than they are.

On ships there are structures that resonate with forced vibration, for example:

- the entire driveshaft, which is liable to respond laterally or longitudinally to excitation from the propeller or propulsion system, and thus in turn excites the structure of the double bottom;

- the entity made up of a diesel propulsion engine and the structure of the double bottom that supports it. This entity responds to excitation in the form of forces and moments caused by the functioning of the engine, and is likely to make the structure of the hull vibrate.

Other “passive” resonators may be excited, such as the deck, partitions, mast, radar equipment etc.

The wake in which the propeller operates is responsible for fluctuation in the effort transmitted by the propeller to the drive shaft. It also causes fluctuations in pressure, and if cavitation occurs it increases the amplitude of fluctuations of pressure on the hull. These fluctuations of pressure are linked to the following:

• variations in propeller thrust: When the propeller provides thrust, the rear of each blade undergoes depression with respect to ambient pressure, and the front undergoes overpressure.
• the number, surface area and thickness of the blades. Pressure fluctuation is a linear function of mean blade thickness and reduces rapidly as the number of blades increases.
• presence of a variable pocket of steam on the surface of the blade and in its wake, as a result of cavitation. Cavitation is responsible for most vibration problems found on board ships, following excessive pressure fluctuations on the rear underside.

Cavitation is equivalent to an increase in blade thickness, and as such causes an increase in pressure fluctuations.

The fundamental frequency of propellers is around 20 Hz for fixed-blade propellers of between 5 and 6 metres in diameter, and 10 Hz for propellers between 8 and 10 metres in diameter.

If the response frequency of the propeller blades is very high (in the audible frequency range), “singing propellers” may be heard. This can be troublesome for the crew.

Engine vibration is caused by alternating piston/crankshaft motion. Excitation caused by free forces and moments within the engine can have an effect on the vibration response of the ship’s structure and even the girder structure, particularly in medium-sized ships with 2-stroke engines.

Engines generally vibrate at between 3 and 30 Hz.

The level of vibration depends, of course, on the type of engine, and particularly the engine speed.

Sea conditions can also cause several types of vibration:

Vibration of the whole ship

The swell causes random very low frequency vibration (less than 2 Hz) of the whole ship, both longitudinally (pitching) and transversely (rolling). The frequency of this vibration is between 0.01 Hz in very calm seas and 1.5 Hz in bad weather. It is generally between 0.1 and 0.3 Hz. Acceleration ranges between 0.005 and 0.8 m/s², sometimes reaching 1 m/s². These values vary depending on sea conditions and the position of the subject on board the vessel [Kingma¹].

This vibration causes seasickness.

Ship girder vibrations caused by sea conditions

These are usually considered to be of two types:

• “Whipping”,  which is caused by the hydrodynamic impact to the front of the ship, and which is a transient phenomenon that causes the ship girder to vibrate.

Whipping usually occurs when the ship is forging ahead and when there is relative movement of the stem that is great enough to cause impact:

• “slamming”,  when impact occurs to a flat part on the bottom of the ship, when it falls back to the sea after emerging.
• “slapping”,  which occurs when there are impacts on the flat surface of the stem when it has not emerged from the water.
• large waves  (“green sea”).
• “Springing”, which is created by excitation caused by variable hydrodynamic force created by the swell, and which is effectively a phenomenon whereby the ship’s girder is made to vibrate freely.

In conclusion: ships are environments in which significant stress is caused by vibration. Such vibration can be of very low frequency, of low frequency, and to a lesser extent high-frequency (range is between 0.01 Hz and 80 Hz, with a maximum of between 3 and 30 Hz). These vibrations can be periodic or random.

Fig 11.5.1 Summary diagram of vibration on board ships

11.4.2   Effects of ship vibration on humans

The human body as exposed to vibration may be reduced to suspended elements (head, thorax, pelvis) linked by shock absorber systems (ligaments, muscles, intervertebral discs).

The physiological and psychological effects of vibrations on humans are caused by significant relative deformation and displacement undergone by organs and tissues at certain frequencies.

The frequency of the sinusoidal movement of a freely oscillating system if it is subject to impetus and not damped is known as the eigenfrequency. The eigenfrequency of the organ corresponds to the maximum transmission of movement applied to it, if the organ is considered to be an undamped system. If the system is damped (which is generally the case in the human body), maximum transmission of movement occurs at a particular frequency which is known as the resonant frequency. By definition, the resonant frequency is lower than the eigenfrequency, but there is generally only a small difference between the two, as the organs of the human body generally are not heavily damped.

Some resonant frequencies for a subject who is exposed to vertical vibrations:

• Head: 20-30 Hz. Visual disturbance is also observed between 60 and 90 Hz, which can be explained by resonance of the eyeballs.
• Thorax: 3-7 Hz. This explains the respiratory problems that are observed at such frequencies.
• Heart: 4-8 Hz. Chest pains have been described, which could correspond to heart-related pain.
• Abdominal and thoracic organs: 4 to 9 Hz
• Spine: 2-6 Hz (5 Hz)
• Pelvis: 4-9 Hz

At frequencies of less than 2 Hz, the body reacts like a single mass. In a seated human body, the first resonant frequencies occur between 3 and 6 Hz; in a standing human, there are two maximum values, at 5-6 Hz and 12 Hz [Subashi²].

Resonance occurs when transverse or front-to-back vibration occurs at frequencies of around 2 Hz. It is caused by flexion in the lumbar and thoracic spine, in the hip joints and by curvature of the head.

The perception threshold for vibration is around 0.01 m/s².

Vibration perception depends on:

• the region and surface area of the body that is in contact with the source of excitation;
• the intensity, frequency and direction of the vibration;
• the subject’s sensitivity;
• the position and posture of the subject and whether he/she is tense or relaxed;
• the dynamic interaction between the body and the structure via which the vibration is transmitted to the human body;
• the distribution, mass and dynamic properties of any clothes and equipment the subject may be wearing/carrying;
• the environment: noise, temperature, lighting, vision;
• the activity engaged in (physical, mental, visual, oral);
• psychological influences.

It is considered that there are three types of condition under which people are exposed to vibration:

• Vibration transmitted to the whole body in all directions;
• Vibration transmitted to the trunk via the lower limbs of a standing person, the pelvis of a seated person or the bed in the case of someone who is lying down. This is the situation that is encountered on board ships.
• Vibration via the extremities, such as the hands, arms or head.

A change in posture can alter distribution of body mass and resonance linked to vibration. This can also mean that vibration transmission shifts to another part of the body. The effects may, for example, be different for someone who is standing than for someone who is sitting. Muscular activity can modify the effects of vibration on the organism [Huang and Griffin³].

Seasickness is triggered by very low frequency vibrations (0-2 Hz). Seasickness can be defined as a reflexive autonomic crisis that is linked to movement and that is triggered or worsened by vestibular hypersensitivity, autonomic nervous system irritability or psychological predisposition.

The conditions that determine whether seasickness will arise are the movements of the ship. The frequency of vibration ranges from 0.01 Hz in a very calm sea to 1.5 Hz in bad weather; acceleration ranges from 0.01 to 0.8g and sometimes 1g. At such frequencies, the body behaves as a single mass and vibration is transmitted in its entirety, both in terms of amplitude and acceleration. Frequency and acceleration vary depending on sea conditions and the tonnage of the ship. Regular repetition and duration of movement should also be taken into account. In addition to the intensity of the movement, the extent to which it oscillates also plays a role. A movement that is regularly repeated is more harmful than a sudden, irregular movement.

The conditions that promote seasickness are well-known, particularly:

• The atmosphere: this can make seasickness develop more quickly
• Vision: Seasickness can be brought on by juxtaposition of fixed and moving images (as in a cabin with a porthole). In most cases there is a conflict between the visual and vestibular systems; when in a ship, vision can fail to catch up with movement detected by the vestibular system.
• Smells: smoke, paint, fuel, perfume, vomit
• Heat
• Confined spaces
• Time of day: Resistance to seasickness is better during the day.
• Position: lying down is best, as the movement component is attenuatedGeneral condition: tiredness and lack of sleep promote seasickness.
• Habituation: this greatly increases resistance
• Psychological condition: excess emotion
• Contagion: when one person is ill, the people around him/her often are as well
• Apprehension and fear: memories of a bad voyage make seasickness more likely on future voyages.

The order, number and intensity of symptoms can vary greatly depending on the individual and on the type, duration and extent of movement.

The initial sign is a feeling of insecurity. Next the following appear:

- subjective signs: vague feeling of illness, anxiety

- objective signs: pallor, cold sweats, sleepiness, unpleasant feeling around the stomach, yawning, balance problems, clumsiness, salivation, bradycardia, tachypnoea with hypocapnia. The following may also be observed: Inhibition of gastric tone and mobility, a drop in central and skin temperature, cold extremities.

This phase of the condition is characterised by nausea, vomiting and weakness which may lead to exhaustion. A recent study has shown that the risk of nausea is at its greatest at around 0.2 Hz [Golding⁴].

There is marked pallor, the nose is narrowed and the blood pressure falls. Hyperglycaemia may be observed, as well as oliguria.

In all cases there is loss of psychological strength, which is significant and often disabling, with significant reduction in performance, spontaneity and reflexes, total lack of activity, submissiveness, and reduction in muscular strength and co-ordination.

Good progress is made within a few hours, if the conditions that caused the seasickness have abated. If the conditions persist, habituation develops relatively quickly, within 72 hours (finding one’s “sea legs”). Some people do not become habituated, and such people are unfit for sea travel.

In exceptional cases, complications arise: examples are dehydration, reduced blood pressure, ketoacidosis.

The pathophysiology of seasickness is fairly well-known. Balance is a complex system, and information is conveyed by three sensory faculties:

- the vestibular system

- the visual system, particularly as concerns tracking

- the body’s proprioception system, particularly involving the feet and lower neck.

Such information is transmitted to nerve centres in the brain, where they are compared with each other and checked against previously stored information for coherence, which leads to a tailored balance response.

It is certain that seasickness originates in the vestibular system. People who are deaf because of inner ear destruction and animals without labyrinths do not experience seasickness. Very low frequency vibration is within the range of frequencies that activates vestibular receptors. This is not vestibular over-stimulation, but rather a problem with central integration of data that disagree with other receptors, and this causes the symptoms.

Ship roll seems to have a particularly marked effect on the semicircular canals, and pitching affects the otoliths.

In general, seasickness is often explained as a conflict between visual information and vestibular and proprioception information. There is a central difficulty in integrating the various sensory messages concerning body movement (this is addressed in Reason and Brand’s “sensory rearrangement theory” ⁵), or there is a conflict between the inner model and the outer reality.

It seems as though the central nervous system commits to memory sensory information that correspond to a particular movement situation. If signals from the various sensory receptors are compatible with this internal model, there is no problem. If there is a discrepancy, there are two consequences:

a/ the internal model is reorganised: A typical example is known as "mal de débarquement" (disembarkment syndrome), which occurs when a subject returns from a period at sea and continues to experience an illusion of ship movement. Symptoms reduce with repeated travel. The occurrence of seasickness reduces greatly with practice. There seems to be a significant correlation between susceptibility to seasickness and mal de débarquement [Gordon^{6, 7}].

Some authors [Hain⁸, Nachum⁹] make the distinction between several degrees of mal de débarquement: « landsickness » between 0 and 48 hours, mal de debarquement (MDD) between 48 hours and one month, persistent mal de debarquement (persistent MDD) if the condition lasts more than one month. The latter condition, which mostly affects women over 40, may last up to ten years. The unpleasantness of the condition can become quite disabling in such cases.

b/ a sequence of autonomic nervous system effects is begun. Symptoms are caused by the length and intensity of the sensory conflict. O’Hanlon and McCauley¹⁰ and Bles et al. ¹¹ have put forward the theory that only vertical movement causes seasickness (the “subjective vertical conflict theory”), which incorporates the principle that it is the internal representation of gravity that is disrupted when sensory conflict arises. Most sensitivity seems to arise at 0.2 Hz. In an interesting study published in 1998, Wertheim¹² showed that although vertical movement (lift) is a necessary condition for seasickness, rolling and pitching also play an important role. In this study, thirteen men and nine women were exposed to different situations: while the group exposed only to rolling had just one case of seasickness, vertical movement alone caused no cases, but the incidence of seasickness climbed to 50% when the three components were combined. It seems to be head movements, directly connected with the combination of rolling and pitching along with lifting movement, that give rise to the notorious sensory conflict.

Current research into seasickness seem to head in two directions: on the one hand, seeking increased understanding of susceptibility to the condition, and on the other seeking to model the circumstances under which it arises.  Although it has long been known that some individuals are susceptible to seasickness, very recent studies have attempted to achieve a better understanding of it. To this end, a questionnaire was developed: the Motion Sickness Susceptibility Questionnaire.  Buyuklu et al. ¹³ report that, in a population in whom this test was administered, 75% of subjects who were defined as being susceptible to motion sickness were prone to seasickness, 90% to carsickness, 10% to airsickness and 5% to simulator sickness, which suggests to me that this test is possibly not ideally suited to a maritime setting. 10% of those who were classified as not susceptible complained of seasickness in extreme conditions. The team also showed that there was a significant difference between men and women, and that women were more sensitive. A very recent (2009) study by Meissner’s German team¹⁴ found, for example, that there was a significant difference between cortisol levels and susceptibility to motion sickness in men (not significant) and women (very significant). The second area of research is modelling to predict occurrence of seasickness in various parts of a ship, using impressive mathematical formulae processed using powerful computers. Concepts introduced include MSI (Motion Sickness Incidence), which is the percentage of passengers who vomit within two hours, the formula for which includes vertical acceleration among other variables, and VI (Vomiting Incidence) which is the probability of vomiting among a passenger population. The shape of the ship, its speed, centre of gravity, its moment of inertia, sea conditions and wave directions, the distribution of passengers and facilities on board, are also entered into the formula. Of course, all this comes under the heading of naval architecture, but we should take note of an interesting article by B Haward et al. ¹⁵ who studied the emergence of various seasickness symptoms in the crew of a supply boat, taking into account susceptibility to seasickness, ship movement, and also fatigue and sleeping problems. The problems became critical when vertical acceleration (again) exceeded 0.6 m/s².

Along with seasickness, Haward et al.¹⁵ have shown that fatigue and sleeping problems are strongly correlated with movement along all three axes, and the strongest correlation is with sleep quality. Hours spent asleep were negatively correlated with amplitude of movement.

Lower back pain is also associated with an increase in amplitude of ship movement. Törner et al. ^{16, 17} have reported that vertical acceleration of ±0,4m/s² and rolling of ±8 degrees on board trawlers is associated with an increase in cerebrospinal fluid pressure. Hoogendoorn's¹⁸ work suggests that twisting movements of the spine are an independent factor responsible for lower back pain. Such movement is common on board as individuals seek to keep their balance as the ship moves, particularly on board small vessels. However, Törner has shown that knee movement acts as a buffer. In spite of this, compression forces may be increased while carrying materials on board, because of increased contraction when attempting to stabilize [Barzgari¹⁹]. Several authors [Törner¹⁷, Wertheim²⁰] have shown that oxygen consumption was increased in subjects standing on board ships while undertaking a lifting task, but that oxygen exchange was reduced, because of overall muscular tension, which increases fatigue levels. Conversely, Drerup²¹ found no abnormalities in intervertebral discs in subjects exposed to whole-body vibration, in comparison with a control group.

At frequencies of between 2 and 20 Hz, the body no longer acts as a single mass, but as a system of suspended masses. At such levels of vibration, the labyrinth is no longer sensitive.

On board ships, vibration of between 2 and 20 Hz is found, and as we have seen this is linked to the propulsion system and propellers. The intensity of this vibration is generally quite low. Depending on the RPM of the engine, this vibration can be amplified and, if this occurs, inconvenience is caused to those trying to write or read. This can also cause partitions to resonate, and can generate unpleasant noise which will increase general fatigue and exacerbate concentration problems. Many authors have attempted to quantify the reduction in performance experienced by people exposed to periodic and random vibration. Vibration makes a task more difficult and cumbersome. Vibration hampers precise movement and accurate prehension with hands and fingers.

Vibration also leads to an increase in reaction time, requiring greater concentration on the task in hand at the expense of attention to secondary tasks, which means that vigilance is reduced.

In a subject who is exposed to this type of vibration, shifting body mass and maintaining posture (particularly in the case of random vibration such as whipping and springing jerks) leads to stimulation of muscular activity which compensates for the effects of vibration. If there are major jerks (acceleration greater than 2 m/s²), there can be trauma to the lower back, in the form of fractures or compression injuries, particularly to L3-L4 [Ayari²²].

Holmlund et al.²³ have shown that impedance increases as a function of frequency, up to an initial maximum in the 4-6 Hz range, which particularly affects the spine. There is a second and third impedance maximum in frequency ranges 8-12 and 50-70 Hz.

Subjects exposed to medium-frequency whole-body vibration have been shown to have a higher incidence of lower back pain [Burdorf²⁴].

Vibration can lead to microscopic trauma of the spine, particularly the lumbar spine, which are particularly troublesome because the spinal column is unbalanced. On board ships, particularly fishing vessels, vibration is a factor that exacerbates problems caused by postural constraints and the difficulty of keeping one’s balance in a moving vessel, as we have seen. The most significant responses were in the 5-8 Hz range.

Low-frequency vibrations, particularly between 4 and 12 Hz, tend to increase respiratory parameters: respiratory frequency, ventilation rate and oxygen uptake [Maikala²⁵]. These increases seem to be linked to general muscular tension caused by vibration: at 10 Hz, there is very significant tension in the muscles in the lower back, chest, abdomen and back.

An increase in heart rate is often observed. Between 4 and 11 Hz, when vibration is of significant intensity, disturbances to heart rhythms have been observed, in the form of extrasystoles and at times tachycardia.

Occasional cases of myocardial infarction in young people with no history of arteriosclerosis or coronary artery disease, have been linked to vibration.

Digestive tract and urinary tract problems have also been observed, which are partly due to changes in peristalsis in visceral smooth muscle.

This all has an effect on general levels of fatigue, which are already raised by various causes (noise, very low frequency vibration, stress etc).

High-frequency vibrations, above 20 Hz, have a purely local impact. The most commonly studied example of this is vibrating tools. Some seafarers (engineers or deck crew members) are likely to use such tools, for rust removal, sanding and cutting. Upper limb conditions arising from such vibration are well-known. High-frequency vibration can cause angioneurotic problems in the hands and fingers, arthritis in the elbows and finger joints, bone disease in the carpal bones (necrosis of the lunate bone or Kienböck’s disease). These diseases are rare in seafarers, but it is nonetheless essential that the field of maritime medicine gain familiarity with them in order to diagnose and prevent them.

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