Occupant safety in free fall lifeboats

Free fall lifeboat is one of the life saving appliance which is using in lots of ships and offshore platforms. It helps people to evacuate structure and reach safer locations as soon as possible. Unlike other conventional lifeboats, it has the ability to quickly leave the place even in adverse weather conditions. For this reason, free fall lifeboats have been used for many years on ships carrying petroleum and petroleum derivatives and oil platforms where emergency evacuation is very important. Structural safety has been a priority issue in the context of safety studies in free fall. In terms of occupant safety, the mathematical evaluations with acceleration on the seat during the launch were deemed sufficient. However, complex biomechanial properties of human and the design necessity of freefall lifeboat that can be released from high launch points have led to the question of this approach over time. In particular, injury reports from field exercises of lifeboats have drawn attention to this issue and have ensured that relevant agencies work on the subject. In order to carry out detailed studies on occupant safety, it is necessary to understand the injury mechanism. The human injury mechanism works differently for each region of the body. For example, the linear or angular acceleration that the body is exposed to is very critical for head injuries, while the compression caused by the seat belt causes critical injuries in the chest area. To assess different type of injuries and compare them, injury scale called Abbreviated Injury Scale (AIS) is used. AIS system identifies the severity of the injury with codes 0-6. While the severity of injury from 0 to 6 increases, the risk of death from this injury is also increasing. In addition to facilitating the first aid and medical field treatments, the AIS scales are being updated over time with an increase in human anatomical knowledge. In occupant safety studies, engineers generally aim to keep occupant injury results within defined AIS injury level for their products. Dummies are measuring devices that can measure loads or loading effects for known types of injuries with installed sensors, such as acceleration, speed, force, or displacement. Before advanced test dummies produced, cadavers were used to make assessment on occupant safety. However, the available cadavers were different from one another in many aspects, such as the physical dimension, age, and biological characteristics. All these would affect the outcome of the occupant safety evaluations. In order to make a standard safety assessment, the need for the development of test dummies had arisen and many test dummies have been developed after that. Critical loads on passengers in free fall lifeboats are due to the acceleration during dive stage. The previous movements of lifeboat, which are ramp, rotation and free fall stages, have important effect on kinematic of dive stage. The boat is exposed to significant acceleration on the X-axis after contacting with the water. The basic loads for the front seat of lifeboat are observed in this time interval. The lifting force that occurs when a free fall lifeboat enters the water has a rectifying moment effect for the boat. The diving boat starts to straighten up with the effect of this force, and stern side of the lifeboat hits the water. This movement controls the acceleration values that occur in the lifeboat Z direction, especially important for the seats located on the aft side of the boat. Weight, center of gravity, hull form, dive angle and dive speed of lifeboat are important variables for these acceleration loads. The difference in these variables can cause significant changes in the loads on the occupant. The acceleration loads on occupant during free fall lifeboat launch can be examined with three main evaluation methods. The first two methods, referred to as Square Root Sum of the Square (SRSS) and Dynamic Response (DR), are based on mathematical evaluation of the acceleration data measured from seat supports during launch. After using defined limits and formulas Combined Acceleration Response (CAR) index for SRSS method and Dynamic Response Index (DRI) for DR method are obtained. If calculated index is smaller than unity, it means that possibility of major injury is unlikely for both methods. The other method to evaluate occupant safety in lifeboats is using test dummies. In the evaluations made with dummies, besides the acceleration loads on human body, the effects of the peripherals such as seats and belt equipment can be examined for occupant safety. The detailed assessment with test dummies reveals what type of injury has occurred: head, neck, chest etc. After understanding root cause of the injury risk, possible design improvements can be studied on interior parts. In this study, occupant safety evaluation was carried out for a specific loading, which is similar to the acceleration data read from the middle seat of a free fall lifeboat, which is launched from 28 meters. CAR index of defined loading was calculated as 0.99 with SRSS formula for emergency. CAE model is prepared to replicate defined load case with test dummies in Hypercrash program and analysis were made with Radioss. %50th Hybrid III dummy, 4-point seat belts and forward facing seat are used in the beginning of the study. After first CAE analysis completed, dummy kinematics and injury curves were investigated in detailed. According to initial finding on dummy movement and injuries, interior design was tried to be improved by several changes such as; seat orientation and design change, additional belt connection and six-point seat belt. For design improvement studies on belt, additional belt connection and six-point seat belt were tried for forward facing seat. Even the values of neck injury could be improved significantly with the six-point belt, chest injuries were caused to worsen in similar proportions. The additional belt connection on the four-point seat belt system is a positive effect on neck injuries, while the impact on the chest is relatively small. The greatest contribution to the injury values were obtained by rotating the seat 180 degrees in Y-axis to the lifeboat backwards. In this way, the acceleration load on passenger was damped by the seat. These types of seats are used in many lifeboats, which are designed to work in high launch points. As a result of the analysis work done with the rearward facing seat, the injury values decrease significantly and the dummy movements became safer. Even overall injury results are improved, it was noticed that the foam in the seat could not damp dummy loading well. This led to necessity of seat design change. Headrest design was changed to decrease loading on head and neck area with damping. In addition to the new headrest design, all seat foam thickness was also increased in order to provide better load damping to the occupant. At the end, the kinematics and injury values of the dummy improved substantially compared to the initial state. Even though the CAE model was not validated with the test, it was understood that the time and resources spent for the SRSS evaluation were considerably more economical and faster than the evaluation with the test dummies. The fact that computed CAR index is very close to the maximum value of 1 and the dummy injury rating is well below the AIS 2 and AIS 3 levels shows the SRSS method has a enough safety margin to estimate this level of injury. Moreover it has been found that parameters such as seat belt, seat design and seat foam, which does not change results of the evaluations made by the SRSS method, have a significant effect on the injury values. According to studies, the risk of injury to the passenger can be reduced with simple interior design modifications for the same loading scenario. The adverse effects of improper interior designs can only be recognized and corrected by evaluation with test dummies. As a result mathematical occupant safety assessments, which are using accelerations measured on the seat, are economic and rapid evaluation methods. On the other hand, there are incomplete aspects in validating the design in terms of occupant safety since the interior design changes could not be assessed by them. In this case, it is considered that CAE or test evaluation with dummies in lifeboats is necessary as the verification method in terms of final product validation.