Workload Simulation and Comfort Evaluation of Workover Rig Driller

Considering that the method of evaluating comfort by subjective feeling isn’t precise enough, and there are few studies on the comfort of the wo-rkover rig operation, taking the XJ70 workover rig as an example, comprehen-sively considering the impact of operating time and operating cabin layout para-meters, the digital model of the human body sitting posture of Chinese adult males with typical percentiles is established in the AMS biomechanics software, the model of brake handle and seat is imported, and the inverse dynamics simu-lation of the brake handle operation is performed; Taking the muscle activity of relevant muscle groups as the evaluation index, the influence and mapping rela-tionship of sitting height and backrest inclination on the muscle groups during the brake handle operation are studied. The handling comfort evaluation model based on the muscle loads is built, which provides a new way for the evaluation of comfort and has a certain guiding significance for selecting comfortable operating posture and optimizing the adjustable range of seat.


Introduction
Muscle is the source of human body movement.Muscle fatigue is the root cause of uncomfortable feeling.It directly affects the state and health of the workers, and has a certain degree of influence on the reliability, safety, and efficiency of operation.Stud-ies have shown that long-term, overloaded, and monotonous repetitive operations lead to long-term chronic accumulation of human muscle fatigue, which can easily lead to musculoskeletal disease and unsafe accidents [1][2][3] .Therefore, the domestic and for-eign scholars have made in-depth study on the distribution of operating load and han-dling comfort of automobile and bus drivers from the physiological and psychological aspects [4][5][6][7][8] .Wang et al. [9] studied the comfort evaluation of clutch pedal operation by studying the relationship between subjective comfort and biomechanical parame-ters.Pick A J et al [10] studied the changes of surface EMG in eight muscles of the driver's upper limbs during the steering.Reed M p [11] pointed out that the driver adapts to the cab layout parameters and the geometric characteristics of the seat mainly by changing the lower limb posture, while the posture change of the trunk is small, and the seat height, the steering wheel position and the seat cushion angle have a great influence on the driving posture.Rasmussen [12] established the driver's bio-mechanical model by the Anybody, analyzed the driver's discomfort based on muscle fatigue and tangential force, and optimized the seat cushion and backrest angle.In recent years, with the rapid development and popularization of biomechanics and computer simulation technology, many scholars at home and abroad have begun to study the muscle load which is closely related to human comfort.Hwang.B et al. [13] quantitatively analyzed the driver's fatigue based on EMG, and studied the relation-ship between lower limb discomfort and muscle strength; Liu Y H et al. [14] studied the function of driver's shoulder muscles in vehicle steering; Aliah binti et al. [15] studied the effects of the car seat cushion, the backrest inclination and the spring stiff-ness of the pedal on the driver's muscle activity and spinal joint strength; Hu Jing et al. [16] proposed a method of comfort evaluation based on muscle surface EMG.With the improvement of the HSE management system for on-site operations in the oil industry, ergonomics has been widely used.There are scholars in China who have conducted ergonomic evaluation and optimal design of human-machine interface for workover rigs based on subjective experience, joint angles and external environ-ment [17][18] , which lacks the research on operation comfort and is not conducive for us to deeply understand the causes of human comfort, fatigue and even injury.In view of this, according to the ergonomics knowledge and driller operation char-acteristics of workover rigs, taking the XJ70 workover rig as an example, the digital model of the human body sitting posture of Chinese adult males with typical percen-tiles is established in the biomechanical analysis software AMS.The brake handle and seat model are imported to perform inverse dynamics simulation on the operation of the brake handle.With the muscle activity of relevant muscle groups as the evaluation index, the influence and mapping relationship of seat parameters on each muscle group during the brake operation are studied.And the maneuvering comfort evalua-tion model based on muscle load is established, which can quantitatively evaluate the comfort of manipulation in different postures, and provide a theoretical basis for the selection of a reasonable posture and the optimization of adjustable range.

Muscle load simulation model
Based on the established musculoskeletal human-body model, the kinematic and dynamic datas are input, and then the human muscle load can be obtained through inverse dynamics simulation.However, there is a common problem of muscle redundancy in a typical musculoskeletal model (that is, the number of muscles available is more than the joints that need to be driven).In order to solve this problem, an objective function is constructed according to the optimization principle: Where i f is the muscle force of the number of i muscle in the current case, m i f is the maximum muscle force of the muscle, both the unit is N, and n is the number of muscles.According to the Hill three-element muscle model and polynomial criteria, the value of the number p is taken as 3.
With the improvement of the automation on oilfield workover operations, the sitting posture has become the main posture for the driller due to its advantages of safety, comfort, convenience, difficulty in causing fatigue, and suitability for fine work.The survey finds that the main problems in driller operations are poor working conditions and great labor intensity, which lead to the most of drillers are male and there is no obvious age area; and the driller operations based on sitting posture can easily cause fatigue due to the long working time, the small space and the poor environment; among them, brake handle operation has the characteristics of large load, high frequency and long time repeatability.Therefore, a human-machine interaction simulation model for brake handle operation of workover rig is established.According to the main body size of adult males aged 18 to 60 provided in the national standard GB10000-1988, the digital models of sitting adult males with P5(H=1.583m,M=48kg), P50(H=1.678m,M=59kg), and P95(H=1.775m,M=75kg) percentiles are established respectively in the biomechanical simulation software AMS.The specific datas are shown in Table 1.Taking the XJ70 workover rig as an example, the model of brake handle and seat are imported.The kinematic parameters of the driller in the reference case are defined as follows: upright backrest (0°i nclination), 105°between the trunk and thighs, 135°for the thigh and lower leg, and 110°for the lower leg and right foot; the dynamic related parameters are obtained by applying reasonable constraints to the driller's joints and setting reasonable boundary conditions.The specific datas are shown in Table 2.
Finally, the initial state model of the brake handle operation can be obtained, which allows the driller to hold the brake handle with his right hand and change the sitting posture with the parameters of sitting height and backrest inclination.As shown in Figure 1, the original point of the global coordinate system is located at the sitting reference.

Reverse dynamics simulation analysis
It mainly studies the influence of the muscle load and 2 seat parameters of the sitting height and backrest inclination on the muscle load of the typical percentile during brake handle operation.According to the ergonomics knowledge and driller operation characteristics of the workover rig, the main parameters of the simulation experiment are set as follows: the time for the brake handle to the limit position is 0.8s, the range of the sitting height is 0.35 ~0.55m, the range of the backrest inclination is -25°~5°, and the step length is 10.From the perspective of biomechanics, the musculoskeletal human-body system is the basis to realize various functions of the human-body .but it is not realistic to study every muscle.Therefore, the muscles of the driller are divided according to the principle of human-body parts, and then the muscle load is studied.As shown in Figure 2, Left/Right limbs muscle group includes the muscles of shoulder, upper arm, fore arm and hand.The trunk muscle group includes the muscles of neck, chest and back, waist and abdomen, and pelvis.The Left/Right legs muscle group includes the muscles of thigh, lower leg, and foot.For each muscle group, the average muscle activity of the relevant muscle group is calculated as a load index for the muscle activity of the muscle group.The synergy of multiple muscles in the human body makes it easier for people to complete their movements.Studies have shown that the discomfort is more obvious with the increase of muscle load, which increasing the risk of muscle fatigue injury and the possibility of unsafe accidents, and then affecting the operator's physical health and safe operation [22] .Therefore, muscle activity is selected as the evaluation index, and the result is a normalization parameter.The advantage is that it does not consider the difference in the exertion ability between different muscles and the synergism between the muscles, reflects the degree of muscle contraction, and is intuitive.Judging the degree of muscle fatigue, the lower degree of muscle activity can be regarded as reducing uncomfortable feeling.The expression of muscle activity is: max 1, 2,..., Where i A is the muscle activity of the number of i muscle; i F is the current muscle force of the muscle; max i F is the maximum muscle force of the muscle, and the value depends on the product of the physiological cross-sectional area of the muscle and the maximum muscle stress.
Formula ( 2) is the degree of the muscle activity at a certain moment during the brake handle operation.Therefore, to determine the muscle load during a complete brake handle operation process, it is necessary to consider the time factor, and the muscle comfort function is introduced as: 0 dt 1, 2,..., (3) In the formula: i D is the muscle comfort of the number of i during the brake handle operation; t is the brake handle operating time.In accordance with the comfort of the muscles divided as the evaluation criteria of operating comfort, based on the mapping relationship between the muscle comfort of each site and the seat parameters, the muscle comfort of each site is calculated for the overall comfort.Contribution rate, through the weighted overlay can be obtained operating comfort evaluation model, the specific process shown in Figure 3.

Driller's muscle load distribution
The following shows the reverse dynamic simulation results of the brake handle operation in the initial state(sitting height h=0m, that is 0.45m; backrest inclinationα=0°) of the driller.Figure 4 shows the curve over time of the muscle activity in various parts.It can be seen that the variation pattern of muscle activity is consistent among the typical percentile drillers: With the increase of working time, the muscle activity of the trunk and right limbs shows a significant upward trend; the muscle activity values of the left and left legs are similar and both increased with time.The right leg muscle activity gradually decreases with time; among them, the trunk muscle activity value is the largest, followed by the right leg muscle activity value; this change rule is basically consistent with the actual operating conditions, because in the sitting work, the trunk is the main support site, while the right leg is the main force site.At the same time, each muscle group had a higher correlation with the operating time.Therefore, the 50th percentile human-model is selected for in-depth study.In summary, the higher the sitting height, the lower the muscle activity, then the smaller the muscle load, the more comfortable the operation; and the excessively large backrest inclination will cause the driller to bear a large muscle load, which may cause muscle fatigue, and then affect operating comfort.Only by adjusting the sitting height and backrest inclination to the right position and providing a comfortable sitting position can the driller operates brake handle efficiently and comfortably.The following is the independent variables of backrest inclination α and sitting height h respectively take -0.1 ~0.1m, -25 °~5 °, step length is 8, the AMS software in 100 parameter simulation analysis of the driller.The joint distribution of muscle activity (dependent variables) with seat parameter(independent variables) changes is shown in Figure 5.Among them, the initial sitting height is 0.45m, corresponding to 0.00m in the diagram; 0.55m corresponds to 0.10m; 0.35m corresponds to -0.10m; the initial parameter of the backrest inclination is 0°, positive and negative represents the direction of inclination, and backward is negative, toward the former is positive. The

Conclusions
Taking the XJ70 workover rig as an example, a biomechanical simulation model for the brake handle operation is established, and the effects of the typical percentile drillers and seat parameters on the muscle load in each part of muscle group are studied.The results show that: (1) Muscle activity is the evaluation index.Large muscle activity indicats the great muscle load, and the increase of the driller discomfort; The changes on muscle load in all parts of the P5, P50, and P95 percentile drillers are consistent during the brake handle operation.Among them, the muscle load of trunk and right limb is larger.
(2) The change of sitting height has the most significant effect on the trunk and right limb muscle groups with large muscle load.When the sitting height is near 0.45m, the muscle load significantly reduces; and the response of all parts of the muscle group to the parameters of backrest inclination is more significant and larger differences; (3) The influence of seat parameters on the muscle load of drillers is initially obtained, a mathematical model based on this rule to evaluate the comfort of operation is established.which instructes the driller to adjust the seat parameters reasonably from the perspective of reducing muscle load.For P50 percentile drillers, during the brake handle operation, it is recommended to incline about 5 degrees forward and adjust sitting height to 0.33m.The results provide a theoretical basis for the adjustment of seat parameters and the optimization of work posture.It has great significance for protecting the driller's physical health, delaying fatigue and avoiding safety accidents.

Figure 1 .
Figure 1.The initial state model of brake handle operation of the workover rig

Figure 2 .
Figure 2. The whole body muscle group diagram of workover rig driller

Figure 3 .
Figure 3.The whole body muscle group diagram of workover rig driller

Figure 4 .
Figure 4.The whole body muscle group diagram of workover rig driller

Figure 5 .
Figure 5.The whole body muscle group diagram of workover rig drillerIt can be seen from the figure that during the brake handle operation, the muscle activity of the left, right and left leg muscles are more sensitive to changes in the backrest inclination , and the muscle activity increases obviously with the increase of backrest inclination.The trunk and right limb muscles are more affected by sitting height, their muscle activity decrease with the increase of sitting height.It can be seen that the study of muscles at a single site cannot be used as a basis to evaluate operating comfort.Therefore, each muscle group is comprehensively analyzed, and correlation analysis and weighted superposition are performed to obtain the mapping relationship and response surface of seat parameters and operating comfort, as shown in Equation (4) .

Table 1 .
The Parameters Of Musculoskeletal Human-body Model

Table 2 .
Constraints And Boundary Condition