Analysis of postural stability for Chinese crews working on moving ocean platforms

Moving ocean platforms, such as ship, fishing vessels and offshore platforms etc, suffer seriously from sea loads as wind, wave, currents and other environmental factors. Platforms’ motions induced from various sea loads have significant effects on crews aboard. Severe motions of the platform can make crew lose balance or cause motion induced interruptions (MIIs). This paper studies postural stability of crews on moving platforms based on Chinese anthropometric standards. The relationships between height of center of mass (COM), moment of inertia and human’s height, mass of human body are analysed. The postural stability indicator, tipping coefficient is obtained of human body on moving platform by applying Chinese anthropometric standards. Then the stability domains of human bodies with translational is derived considering the physical parameters of Chinese adults. The work can be helpful for estimation of MII probability and safety of crews on moving ocean platform.


Introduction
Floating platforms are widely used in marine transportation, fishery, exploitation of ocean resources and oceanographic survey. Due to the loads of wind, wave and current of ocean environment, motions of ocean platforms in six degree of freedom (sway, surge, heave, pitch, roll and yaw) occur frequently. These motions have significant effects on the health and safety of crews aboard. Motions of ocean platforms have various negative effects, such as Motion Induced Interruptions (MIIs), Motion Induced Fatigue (MIF) and Motion Sickness (MS) to human (Wertheim,1998) [1] . MIIs were defined as the occasions when sailors on ships with strong motion are easily to become unstable or lose balance and have to stop their work in order to avoid further damage (Graham,1990) [2] . In China, there are huge number of workers as sailors, fishermen, and offshore crews working on moving floating ocean platforms. All the people mentioned above are facing serious health and safe hazards from these motions.
There is plenty of research work on human motion on moving platforms. Graham (1990) [2] made the definition of MII and pointed out it had obvious effects on comfort and performance of human aboard. He developed the model for MII with simplified physical parameters of human body. It provided a useful way to quantify human performance on ships and predicted the number of motioninduced interruptions. Crossland used the model to compare the number of MIIs between experiment and estimation, and found they agreed fairly well (Crossland, 1998) [3] . Haward, Lewis and Griffin(2009) [4] studied the whole human vibration under the motion of platforms and analysed the relationship between response and factors such as exciting frequency, amplitude, direction and so on. Nawayseh and Griffin((2006) [5] studied the effect of frequency, magnitude and direction of oscillation on the postural stability of standing people. Complex models as articulated inverse pendulum model were also adopted to study the control of human body (Wedge and Langlois,2003) [6] . Yao et al studied the human response on ocean platform based on the multi-body dynamics model [7] . An review of the state of art on postural of human body on moving platform was given (Erica Nocerino,2010) [8] .Based on existing research some guide rules were given in ABS for habitability of crew on moving platform or offshore installation (ABS, 2010) [9] .
Most of the existing MII models neglect the actual physical parameters of human body, especially the variation of people from different countries. In order to evaluate human performance accurately, this paper studies the stability domains of human on moving ocean platform. Physical parameters of Chinese crews are considered based on national standard of Chinese adult anthropometry. Then, stability domains of human could be obtained by solving equations of the MII model.

Human parameters based on Chinese standard
The three-dimension human body model and anatomical planes are shown in Figure 1(a). In this paper only the motion in coronal plane is studied and the coordinate frames of human body O-XY is shown in Figure 2  Basic planes and axes in human body model are shown in Fig 1(a). In this paper only the motion in coronal plane is studied (Fig 1(a)); and the coordinate frames of human body O-XY is shown in Fig  2(b), and the origin point locates on the gravity centre of human body. Width of feet, dimension and area of feet occupying et al can also affect the postural stability obviously. In this paper only natural standing posture is considered, which is standing up straight with feet about shoulder width apart. According to convention of ergonomics, the width of feet or shoulder width W (shown in Figure 2) is a function of the height of human body H as

W=0.222H
(1) As to Chinese, several national standards on human factors have been carried out such as GB 10000-1988[16], GB/T 12985-1991[17] and GB/T 17245-2004[13]. In the latter standard the values of average height of COM, moment of inertia along with standard abbreviation are given. Moreover, the standard also provided function reflecting the relationship between height of COM, inertia of moment with mass, height and so on based on Chinese anthropometry data. The relationship between height of mass centre and body mass, height is: Where m is the mass of human body, H is the height of human body, 0 B ǃ 1 B and 2 B are regressive coefficients. The coefficients for male and female are shown in Table 1. According to equation (2), values of the height of COM calculated by Graham's model [2] and the present model are shown in Figure 3. Figure 3(a) shows that height of COM increases with the height of human body when the mass is constant value (60Kg). Figure 3  According to Chinese adult's anthropometry standard (GB/T 17245-2004), the moment of inertia is related to mass and height as below: Where 0 C , 1 C and 2 C are the regressive coefficients. The coefficients of equation (3) can be found in Chinese adult's anthropometry standard (GB/T 17245-2004) [13] . For the moment of inertia with respect to COM on coronal plane the coefficients are shown in Table 2.

Human body model for MIIs estimation
The motion equation of human body can be obtained as: (8) In this situation, the human body will tip to the left direction. Similarly, when human body tips about point O', the motion equation is: Human parameters in these equations have been calculated and can be applied in the MII prediction. If ocean platforms only have translational motion, then equation (8) In equation (10) and (11), we can find that the ratio h w is an essential factor to affect the stability of human, which is defined as tipping coefficient e. It is obvious that probability of crews suffering from MII on ocean platform would increase with tipping coefficient. Based on equation (2), (12) and (13) the tipping coefficient is: The relationship between tipping coefficient, height and mass is shown in Figure 6.

Stability domains
Assuming the coefficient of friction is sufficiently large and there is no slipping between human and deck, crews will move together with ocean platform. So the human body would have the same motion parameters with ocean platform. These motion parameters as x a , y a ,and θ and θ can be regarded as the excitation that crews suffer from motions of ocean platform. In this paper, the model of human body considered has the height of 1700mm and the mass of 60kg.

Under the excitation of translational acceleration
When the rotational motion is neglected, only translational motions with horizontal acceleration x a and vertical acceleration y a are considered. According to equation (10) and (11), two oblique straight lines could be obtained, as line 1 and line 2 in Figure 6. The value of intersection points C 1 , C 2 are g h w ± on x axis. This is to say the stability domain is closely related to the tipping coefficient. If the tipping coefficient increases, the domains would be larger, as is shown by the dash lines. So large tipping coefficient is helpful for stability of human body.
When the vertical acceleration equals -g the human body will be in the condition of weight loss and the contact loss between feet and floor as point D in fig 6. It is unstable whatever the horizon acceleration.

Conclusions
In this paper, the postural stability for Chinese crew working on moving ocean platform is studied. Some human parameters as the height of COM and moment of inertia are calculated based on the standard of Chinese adults' anthropometric parameters. Effects on tipping coefficients of these parameters are investigated. Stability domain was obtained with excitation of translating accelerations. The work can be used as a guide to estimation the possibility of MII and to build an index to describe the stability degree of crews on moving platform.