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Abstract – Air filled salvage lift bags have been a problem and danger since their invention due to uncontrolled vertical acceleration as they ascend the water column. This paper presents the design of an automated purge valve to control a lift bag’s ascent. The automated purge valve monitors the changes in pressure to determine the ascent velocity of the lift bag. The resulting velocity is then evaluated by a proportional, integral, derivative (PID) control algorithm that outputs a signal to a purge valve installed on the top of the lift bag. The purge valve opens or closes to control the purge of the expanding air volume allowing for a smooth controlled ascent.

Index Terms— Lift bag, recovery device, purge valve.

I. INTRODUCTION he concept of using buoyant objects such as barrels or bags filled with air to aid in the recovery of sunken vessels and heavy objects from the ocean depths has been

around for centuries. The lift bag, generally used by attaching the bottom of the bag to the payload to be lifted, is filled with air via either an umbilical hose from the surface or a portable compressed air tank brought down by a diver. The lift bag in the cases stated within this paper is a reinforced flexible bag in the shape of a vertically aligned cylinder. There are many types of lift bags however the ones used here are of the open bottom type that are commonly used in the marine salvage industry today. Despite their usefulness lift bags are widely considered somewhat of a last resort due to uncertainties in their principle operation. Specifically, they have one major drawback: the acceleration can create enormous unpredictability in the ascent of a payload as the lift bag reaches extremely high vertical velocities upon reaching the surface. These high velocities can cause the lift bag to literally launch 30 meters into the air, a potentially hazardous working environment to divers and salvage crews. The design of an automated purge valve (Fig. 1) presented in this paper eliminates the salvage lift bag danger for the marine industry. The automated purge valve addresses the problem of vertical acceleration when using air filled floatation recovery devices as they ascend through the water column. The system is programmed to allow for a smooth controlled ascent.

Manuscript received March 29, 2009. J. Farrell is an Ocean Engineering graduate student at the Florida Institute

of Technology, Melbourne, FL 32901 USA (e-mail: jfarrell@fit.edu) S. Wood is a professional engineer and assistant professor in Ocean

Engineering at the Florida Institute of Technology, Melbourne, FL 32901 USA (phone: 321-674-7244; fax: 321-674-7212; e-mail: swood@ fit.edu).

The automated purge valve monitors the changes in pressure to determine the ascent velocity of the lift bag. The resulting velocity is then evaluated by a PID control algorithm which outputs a signal to a purge valve installed on top of the lift bag. The purge valve then opens or closes to control the purge of the expanding air volume. The system is programmed to effectively purge the correct amount of air to compensate for the air expansion and allow for a smooth controlled ascent.

Figure 1, Automated Purge Valve Prototype Photo Courtesy

of Adam Priest, Florida Institute of Technology

The main goal of designing the automated purge valve was to eliminate the vertical acceleration experienced by a lift bag. This was accomplished by limiting the expansion of air through a purge valve installed on the bag itself. By allowing the expanded air to be purged from the bag as it ascends constant buoyancy can be maintained. Having constant buoyancy allows the lift bag to undergo constant vertical velocity. A constant ascent velocity allows for a whole new range of operations which the lift bag was previously not suited for. From deep water recovery of heavy objects (such as anchors and aircraft) using Remotely Operated Vehicles, to much more efficient small salvage operations where a crane would no longer be needed. The automated purge valve will transform the lift bag’s role from that of an auxiliary and unreliable tool to a high tech piece of equipment. This transformation allows the lift bag to play a significant part of an efficient and streamline salvage operation, not to mention the applications outside of the salvage field where controlled ascents are needed. The same principle of controlled ascent or

An Automated Purge Valve for Marine Salvage Joseph Farrell and Dr. Stephen Wood, P.E., Member, IEEE

IEEE Conference Publishing 445 Hoes Lane

Piscataway, NJ 08854 USA

T

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descent can be applied to many different aspects of the marine and maritime industry allowing what was not previously possible to become a standard way of operating.

II. LIFT BAGS IN MARINE SALVAGE The concept of using buoyant objects such as barrels or

bags filled with air to aid in the recovery of heavy objects from the ocean depths has been around for centuries. Fig. 2 shows William Kemps’ specifications for a rigid lifting device, the forerunner of the modern flexible bag [1].

The term lift bag is defined as the main aid in the lifting of a submersed object through the use of Archimedes’ Principle; however a lift bag is not always of the same material or name. Some are referred to as salvage pontoons and are made of a flexible material having a pontoon shape whereas others are rigid and cylindrically shaped. A lift bag in the cases stated here can be thought of as a bag made of reinforced flexible material in the shape of a vertically aligned cylinder. There are many types of lift bags however the ones concerned here are of the open bottom type and are commonly used in the marine salvage industry today.

The concept of a lift bag is straightforward and can be best explained using Archimedes’ Principle. Archimedes’ Principle states that any object, whether wholly or partially submersed in a liquid experiences an upward force equal to the weight of the liquid displaced [2]. The force on an object can be calculated by subtracting the dry weight of the object from the weight of the fluid displaced by that object.

III. CASE STUDY On May 17, 2002 the 155 meter long vessel “USS Spiegel

Grove,” to be sunk as an artificial reef, sank prematurely and settled inverted on the bottom with its bow 11 meters in the air.

Soon after sinking salvage crews were brought in and a salvage plan developed using 60 to 70 lift bags with lifting capacities up to 25 tons each. The lift bags were attached to one side of the vessel and filled with air to add a buoyant force on that side. Once the lift bags were filled with air, tug boats attempted to pull the vessel back to its righted position. As the vessel started to roll, trapped air from inside the hull escaped causing a loss of reserve buoyancy. Consequently the vessel sank completely ending up on her side. The plan had been to attempt to roll the vessel over entirely (Fig. 3). Unfortunately, funds ran out for the project leaving the vessel lying on her side.1

IV. DRAWBACKS OF USING LIFT BAGS Despite the apparent usefulness of using lift bags there is

one major drawback in their use. That drawback is an

1 The vessel remained lying on the her side for nearly three

years until Hurricane Dennis in July of 2005 turned the vessel to the upright position.

Figure 2. William Kemps’ Specifications for a rigid lifting

device of 1835 [1]

Figure 3. Methods used to right the Spiegel Grove [3]

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acceleration that a lift bag will experience during ascent. The acceleration can create enormous unpredictability in payload ascent as the lift bag reaches extremely high vertical velocities upon reaching the surface, which can cause the lift bags to literally launch into the air creating a potentially hazardous working environment to divers and salvage crews (Fig. 4).

As the lift bag ascends through the water column it experiences a decrease in pressure. The decrease in pressure causes the volume of air in the lift bag to increase. That increase in air volume results in an increase in displaced water creating in an increase in buoyant force. The pressure changes through the water column are caused by the weight of the water from the surface to the depth the bag is at. The equation for the pressure at a given depth is

Pabs = ρ*g*h + Patm (1)

where: Pabs = absolute pressure (kPa), Patm = atmospheric pressure at sea level (kPa), ρ = density (kg/m3), g = gravity (9.81 m/s2), h = depth below fluid surface (m)

Pressure increases linearly with depth and is directly proportional to the density of the fluid (salt or fresh water -1025 kg/m3 and 1000 kg/m3 respectively [2]) and the depth.

Understanding the significance of this rapid change in pressure is made apparent when one looks at how the air volume is affected. As the pressure decreases during an ascent through the water column, the amount of air inside a lift bag will approximately double during the last 10 meters of the ascent. This expansion can be calculated simply

Vn=(VO*PO)/Pn (2)

Where: Vn = New Volume (m3), Pn = New Absolute Pressure (kPa), VO = Initial Volume (m3), PO = Initial Absolute Pressure (kPa)

(2) shows that 0.5m3 of air expands from a depth of 30 meters to the surface to 1.8m3 (assuming that none is purged throughout the process). Applying (3) below, derived from Archimedes’ principle, Fig. 5 shows how the buoyant force increases exponentially with this air expansion causing an accelerated ascent over depth decrease.

B = V*(ρH2O - ρair) (3)

Where: B = buoyancy, V = volume of air, ρH2O = density of water (1026 kg/m3), ρair = density of the air (1 kg/m3)

This expansion is significant as the buoyant force increases exponentially from 500 kg-force to 2,000 kg-force (Fig. 5). This increase by a factor of four causes unpredictable acceleration during a lift bag’s ascent.

Due to the unpredictability involved, lift bags are seen mostly as an auxiliary tool for which to aid in marine salvage operations but by no means the principle solution. The problems associated with a lift bag undergoing an accelerated ascent are numerous and can create extremely hazardous working conditions. In extreme cases if a lift bag breaks free from its payload it will accelerate towards the surface extremely quickly. Upon breaching the surface with a high vertical velocity and no payload the lift bag will generally launch through the air on an unpredictable flight path.

Figure 4. A 25 ton capacity lift bag that broke free as it ascended. This lift bag stands 25 ft high and is shown at 100 ft in the air. Photo - courtesy of Resolve Towing and Salvage, Inc.

Figure 5. Buoyancy vs. Depth profile of lift bag ascending

from 30 meters to surface

V. BACKGROUND AND EXISTING TECHNOLOGY Marine salvage has been an industry since vessels began

travelling the world’s oceans. As ocean vessels evolved they became larger and the voyages became longer and likewise the cargos onboard each vessel became more important and valuable. As the importance of trade increased ships’ holds were filled with gold, guns, and whale oil to name some cargos of importance. As ships became more advanced and their capabilities increased, their importance so too became realized. From defending or attacking countries to transporting armies, food, and conquered gold and silver, these ships became a large factor in a nation’s economy and power.

With the importance and value of ships and their cargos rising, so was the importance of recovering ships lost near shore. Early navigation around un-dredged channels and wartime activity as well as the ever present human error and acts-of-God brought about the sinking and grounding of thousands of ships.

Efforts to salvage what was still worth keeping on the vessels eventually brought rise to the marine salvage trade and hence, the salvage master. With the correct tools and personnel an experienced salvage master could save a company (or government) time and money by recovering the cargo and raising the vessel to be refitted for a quick return to

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the sea. This became more and more important as vessels made the transition from wooden to steel decks and became much larger in size. The tools in which a particular salvage master uses today are in essence the same as those used 300 years ago. Of course the cranes and tow boats used today are much more powerful however the methods of raising a sunken vessel have not changed much for the past two hundred years.

Lift bags and pontoons have for centuries been used in one form or another as a means of facilitating the lift of an object in an immersed fluid. The names given to such salvage tools have ranged from ballast baskets to pontoon bags, air bags and lift bags. Having lift potentials from 25 kg to 300 metric tons each, they are partially accepted as a means of recovering sunken objects without the use of heavy cranes and equipment. In the 1700’s, wooden kegs were used to assist in lifting sunken vessels from the depths by filling them with air after being attached to the vessel. Fig. 6 shows the wreck of the Earl Grey which sunk in 1874. Here it can be seen supported by floatation bags to facilitate its towing the 20 miles to Burnham, by the steamer Fly [1].

Modern lift bags are the descendents of a line of lift gear that has dated back centuries. Lift bags of today are very similar to those of the past however they are usually made of nylon and can withstand the puncture of a screwdriver piercing them with over 90 kg. Most are made to withstand the harsh salvage environment which is where they spend most of their working time. Lift bag uses range from deploying equipment or artificial reefs to rolling 155 meter ships onto their side. The materials used are lighter and stronger than those of the past. As can be seen in Fig. 7, some of the lift bags of today come equipped with manual dump valves that are attached to a lanyard for a diver to have some control over the ascent.

A well known lift bag system that is currently used by navies worldwide today is the Mark V Underwater Explosive Ordinance Disposal System, also known as the enclosed mine lift bag. The Mark V is designed to lift ordinance or mines to the surface for disposal. With lift capacities of 1000 kg from a depth of approximately 53 meters, the unit has a self contained air source that can be acoustically activated from a remote location. The system has a time delay function allowing the start ascent time to be delayed up to 90 minutes and with a computer communication protocol linked to the system the decent can be further delayed to 45 days (Fig. 8) [5].

The Mark V’s ascent rate is controlled at no more than 2m/s (4.5 mph) throughout the ascent. The main difference between the Mark V and the automated purge system described in this document is the purpose behind the design. Whereas the Mark V is a system capable of conducting a delayed autonomous ascent, its control rate is not constant and can only be maintained under 2 m/s. This is a fairly high velocity and would be unacceptable by the purge valve’s design standards.

The Mark V is a fairly complex platform and not useable on commercial salvage jobs due to cost. It is a system composed of many parts that must be kept in good condition and well maintained, whereas the salvage environment is a hazardous unpredictable one. Equipment on a salvage project is constantly exposed to conditions that generally push the limits that it was designed for. Whenever lift bags are used during

Figure 6. The wreck of the Earl Grey, 1874, supported by flotation bags being towed to Burnham [1]

Figure 7. Example of modern lift bag [4]

Figure 8. Mark V underwater explosive ordinance disposal system [5]

salvage operations it is not uncommon to have many of them damaged or destroyed from chaffing or puncture against the steel hull of a ship. Many salvage operations are located in areas near the equator where equipment lies in steel containers left baking in the sun for days or months at a time. The conditions wreak havoc on small sensitive electrical equipment and can ruin the small fittings and gaskets used on the Mark V.

The purge system discussed in this paper is designed to be entirely self-contained eventually having only a power button and one recharge cord. All of the electrical equipment will be housed in a hermetically sealed housing with the rechargeable batteries. The housing will be designed to survive the hot,

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salty, oily and ever moving environment which most salvage jobs offer. The idea of the automated purge system is to provide a rugged and easy to use platform which will allow controlled ascents to take place.

VI. TECHNICAL OVERVIEW The lift bag is very useful in recovering sunken vessels or

objects from most depths, but despite their usefulness they are often considered a last resort due to uncertainties in their operation. The most significant downside is that as the bag ascends through the water it accelerates toward the surface until the lift bag is full of air and its terminal velocity reached. This known problem in the salvage world requires the salvage master to be patient and intelligent in these devices use, calculating the proper initial amount of air needed in each lift bag, as well as the number of bags needed for a specific job. Despite careful planning and calculations time and again salvage crews are forced to make second and third attempts to raise objects from the deep.

The most common fault associated with an accelerated ascent is for the lift bag to breach the surface of the water so fast that air escapes from its bottom. This purge causes the lift bag to sink back down to the bottom causing time, money and can potentially injure divers or working crewmembers.

There are different ways to control the ascent of a lift bag one way is to connect umbilical hoses to the lift bags allowing the air to purge out via a valve on the deck of a boat or ship. This method allows manual control of the lift bag ascent using valves positioned on deck. The problem arises when the lift bags, being in the dynamic marine environment, start to spin on their vertical axis, which is common in areas of high current and or seas. The spinning causes the umbilical to become wrapped around the bag and fouls the air vent preventing air to be purged that in turn causes an uncontrolled accelerated ascent. A spinning bag may cause the dump line to tangle and inhibit a diver from purging air as the bag ascends. This method has been widely abandoned except in rare occasions.

The solution to accelerated ascent is a system that allows excess air to purge at a predetermined rate, adapting to air expansion within the bag. Using a microprocessor to sample the pressure at predetermined intervals the change in pressure over each interval (time) can be calculated yielding an ascent velocity. The system responds accordingly by opening or closing a valve altering the amount of air purged.

VII. SIMULATION The task of developing a working automatic purge system

was started by modeling the ascent process. Simulation algorithms were written in MATLAB using known values such as initial valve and lift bag diameters and length, coefficient of drag, deployment depth, optimal ascent velocity, mass to be lifted and a rough estimation of initial coefficients used in a PID control algorithm. The equations associated with the vertical velocity of the lift bag are based on individual force equations that sum to produce a total force. The model calculates the magnitude of velocity change at each point from Newtonian physics. The approach works only with certain assumptions that limit the power of the computer model. The

first assumption is that all fluid flow is assumed to be laminar throughout the ascent. This is not always true for the air escaping the lift bag, however the model is still able to make an accurate ascent profile.

The process of determining the motion of the lift bag starts with the initial amount of air added to the bag. This variable is an estimate of the actual initial air volume. The model first calculates the density of the air inside the bag as with respect to the pressure.

Given the air density at the input depth the buoyancy can be calculated from the initial volume of the air added to the bag. Once the buoyancy is found the program calculates the total force experienced by the lift bag. The total force is the sum of all of the forces acting on the lift bag in the vertical direction: buoyancy, weight and drag. Drag is initially negated since the system is static. From these forces the initial acceleration experienced by the buoyancy is calculated when air is added.

The model integrates acceleration with respect to the time interval specified by the user to find the velocity at the next point in time. Integrating the velocity, the program determines how far the bag has lifted and therefore at what depth the bag is at. This process of integrating the previous values with respect to time allows the program to follow a time based process which observes the ascent of a specific lift bag at each point in time.

With the distance from the bottom known, the depth can be calculated by subtracting the total distance moved from the bottom by the initial depth given. The program finds the pressure difference from the air inside the top of the bag to the water on the outside. This value allows the program to calculate the potential velocity of the air if it were allowed to purge from the top of the bag.

The program next enters a response process where attempts are made to provide feedback on the effects of adjusting the valve based on the error. To find the error the program takes the last vertical velocity measured and subtracts it from the target ascent rate. With the error known, this value can be used to adjust the valve position to obtain its target ascent rate.

In order to simulate the properties of the purge valve’s actual response, an equation was developed to take the PID control response (4) and translate that into a purge area size. By adjusting the purge area size and using the potential purge velocity the program continues to calculate the amount of air purged during that increment of time between system responses.

PID control algorithm:

[PID] = k*(P + I + D) (4)

P = p*e1 I = i*((e1+e0)/t + I0) D = d*(e1 – e0)/dt

Where: P = proportional control response, I integral control response, D derivative control response, k control response gain constant, p proportional constant, i integral constant, d derivative constant, e1 instantaneous error (m/s), e0 previous error measured (m/s), I0 cumulative integral response, dt time interval between system response (s), and t total time (s).

Volume of air purged:

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Vpurge = vpurge*Apurge*dt (5)

Apurge = ([PID])

Where: Apurge = area of valve opening at that point (m2), Aprevious = previous area of valve opening (m2), [PID] = control response from PID output, Vpurge = volume of air purged w.r.t. time (m3), vpurge = velocity of air leaving valve opening (m/s), dt = time interval between system response.

With the known volume of air purged (5) during the time interval, one can then calculate the new buoyancy by subtracting the volume of air purged from the new volume of air which has increased if the bag has ascended since the last time interval. With the new buoyancy the program loops back to find the new forces imposed on the lift bag. Drag starts to become a significant factor as the velocity of the bag increases.

Once the forces are computed the computational process can start over again to calculate and plot the new values of the bag’s ascent with the new conditions. With this data, profiles of different ascents with large or small changes to the initial conditions of the system and environment can be made to optimize the lift performance. This simulation gave initial conditions from which to work with during prototype testing.

The output data obtained from each theoretical ascent test is the key to the model’s power and usefulness in designing the lift system prototype. Each calculation made by the program is saved allowing the data to be analyzed after the simulation runs are finished.

The maximum velocity of the lift bag is determined by analyzing the velocity matrix and finding the highest value. The maximum velocity reflects how well the system is working with the given PID coefficients and is therefore a very useful piece of information when optimizing the control parameters. Average velocity is determined by averaging the entire velocity matrix to show how well the system performed next to the target velocity. This is one of the most important outputs given and is used to adjust the PID control coefficients.

The time it takes for the bag to surface is an important parameter that is obtained along with the minimum depth reached because in many simulation runs the lift bag would make it mostly to the top before plummeting back down to the bottom due to over-purging at various points. Knowing the minimum depth reached allows the PID coefficients to be manipulated (or even the processor code) to overcome over purging during ascent.

The maximum valve area is a useful parameter for designing the valve orifice. The program takes the valve area matrix and analyzes it looking for the largest value. This determines whether or not the prototype valve will be of sufficient size for ascent with the specified input parameters.

An example of initial conditions for simulated ascent consists of the following parameters: 1) Mass: 25 kg, 2) Diameter: 0.6 m, 3) Length: 2 m, 4) Cd: - 0.1, 5) Initial Depth: 12 m, 6) Initial Air Volume: 0.025 m3, 7) Ascent Rate 0.2 m/s, 8) Time Interval: 0.2 s, 9) Max valve area: 7 cm2, 10) P = 60, 11) I = 0.5, 12) D = 5, 13) K = 1. From each simulation run graphs are generated of depth vs. time (Fig. 9) and velocity vs. time (Fig. 10).

The depth vs. time graph gives an overall condition of the controlled ascent vs. the uncontrolled ascent. A good ascent is indicated by a straight line having the slope of the target ascent rate as seen in Fig. 9 which shows a linear ascent from 12 meters to the surface with a target ascent rate of 0.2 m/s.

During the ascent the velocity is constantly being maintained by the purge valve since the velocity vs. time profile is one of the most important indicators of the system’s performance. The velocity vs. time profile allows one to observe how well the system is adhering to its set target value, which is the velocity. The PID coefficients are tuned using the velocity vs. time profile as velocity is the target value’s unit.

The velocity vs. time profile (Fig. 10) shows how the system is properly adjusting the purge rate to achieve a near perfect

Figure 9. Graph showing controlled vs. uncontrolled ascent from 12 meters to surface. The target ascent rate was 0.2 m/s

Figure 10. Velocity vs. Time graph. The parameter set allow this particular simulation to adhere to the target velocity of 0.2 m/s for most of the ascent.

ascent velocity. The figure also indicates a very controlled ascent despite this tendency of the lift bag to accelerate during the last few seconds of an ascent.

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The Fig. 11shows the purge area vs. time profile. This graph represents the efficiency of the lift system showing how often and by how much the valve is open at each increment in time. Using this graph the PID equations can be optimized for a minimum amount of purges or the size of the valve orifice to accommodate larger or smaller values of purge area can be altered. The maximum purge area used for this ascent is very small, much smaller in fact than the designed purge valve size of 7 cm2. During the simulation the valve orifice remained below 0.2 cm2 for most of the ascent. As the air volume started to increase the control system appropriately adjusted to increase the purge area. These tests allow for multiple valve positions to be tested without actually having to build and test them.

VIII. METHODOLOGY The integration of simple electrical and mechanical

components to form a complex system capable of completing a specific task is the heart of the control system. Each component in the system must function properly at all times during an ascent and each component must constantly communicate with the other components for a proper ascent to occur. Fig. 12 shows the system prototype before a test.

The most important piece of hardware on the control system is the purge valve. The purge valve is what allows air to be purged from the top of the lift bag as deemed necessary by the main processor. In order for the system to work properly the valve must be mounted at the very top of the lift bag to allow the maximum amount of air to purge at any time.

The purge valve design incorporates two compression disks with a flange-like arrangement of bolts for installation. One of the disks is permanently mounted to the valve while the other disk is free. Between each of the disks is a bead of gasket material to ensure watertight integrity. The valve is installed from inside the lift bag with the bolts penetrating each of the four bolt holes in the lift bag’s top surface.

The position of the pressure sensor is as important as it is the critical measuring device that allows proper control manipulation of the purge valve through the processor. The pressure sensor cannot be exposed to bubbles from the purge valve yet must still be placed at the top of the lift bag to allow the pressure sensor to be exposed to the surface. This ensures that the processor can “sense” the surface and react accordingly by closing off the purge valve at the end of an ascent. Positioning was solved by a bracket connected to the purge valve flange bolts to hold the pressure sensor.

Before testing, care was taken to ensure the control system was set up properly by ascertaining that the proper values of each required variable (ascent rate, PID coefficients, k value, and sample frequency) were successfully stored in EEPROM and that the valve and pressure sensor are calibrated.

Testing the control system is done once a pre-operation check is made and then the control system is prepared for submersion. The unit is turned off and the mode set to the “ascent” position. The circuit board is plugged into the housing and each battery checked before use. After each switch is checked for correct positioning the power is turned

Figure 11. Purge area vs. time profile

Figure 12. Control system dry test. Before each test the system is run to find possible errors before deploying the system offshore. Photo Courtesy of Adam Priest, Florida Institute of Technology

on. The green LED blinks indicating standby mode. Once confirmed the circuit board housing is closed.

The lift system is ready for immersion. The lift bag is emptied of air and taken to the desired depth where the payload is attached. The system is now ready for a controlled ascent.

Lift bags can be hazardous and unpredictable, consequently, a safety function was added to the software allowing the user to abort and reset the test while underwater. At any point during an ascent test the test can be interrupted by flipping the remote switch back to the standby position causing the valve to open fully so the air is purged allowing the lift bag to sink.

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IX. ASCENT TESTS Despite intense planning some aspects of the device

integrity were insufficient for proper operation and made testing very difficult. The initial test was to be performed in 7 meters of water with a 16 kg galvanized steel shackle. The system was submerged with the shackle to the bottom. Within seconds of immersion to depth a leak inside the circuit housing was identified. The system was brought back to the surface and the housing found to be a quarter full of salt water. Difficulties such as these were solved so by ascent test #10, after hardware and software modifications were made to the system, the PID algorithm was modified and the main processor was upgraded to the PIC 18F4523, which had a built in 12 bit A/D converter, successful tests were made.

Test number 10 was performed using PID coefficients obtained from the model. The input variables and initial conditions can be seen in Table 1.

Table 1: Initial conditions for test # 10 Initial Depth (m) 9.2, Target Ascent Rate (m/s): 0.2 P coefficient: 20, I coefficient: 0.5, D coefficient: 4.0 k value: 1, Sample Frequency (Hz): 1.19 Weight Lifted (out of water) 15 kg Weight material Cinderblock

During this test the lift bag slowly started to ascend with the cinderblock payload. As the lift bag velocity approached 0.2 m/s the valve purged a slight amount of air causing the lift bags ascent to slow. As the system dropped below the target velocity of 0.2 m/s the purge valve closed allowing the lift bag to accumulate speed. The system continued to maintain a fairly constant velocity of 0.7 m/s for the rest of the ascent to the surface. Fig. 13 and Fig. 14 are of the depth and velocity profiles of the ascent.

The modifications had a significant impact on the system’s performance. Despite the average velocity being higher than the target velocity of 0.2 m/s, the ascent was controlled and fairly constant.

Ascent test 11 was conducted to evaluate the response of the system with a high initial velocity. The PID coefficients were kept constant and the initial conditions are shown in Table 2. For this test the lift bag was filled with about twice the initial amount of air required for a proper ascent. The system corrected for the high velocity almost immediately and maintained a constant velocity of 1 m/s for the rest of the ascent. Fig. 15 and Fig. 16 are of the depth and velocity profiles of test number 11.

Table 2: Initial conditions for test # 11 Initial Depth (m) 8.4, Target Ascent Rate (m/s): 0.2 P coefficient: 22, I coefficient: 0.5, D coefficient: 4.0 k value: 1, Sample Frequency (Hz): 1.19 Weight Lifted (out of water) 15 kg Weight material Cinderblock

Figure 13. Depth vs. Time profile of test # 10

Figure 14. Velocity vs. Time profile of test # 10

Figure 15. Depth vs. Time profile of test # 11

Figure 16. Velocity vs. Time profile of test # 11

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Ascent test number 12 was conducted with a reduced P coefficient of 15. The initial conditions are shown in Table 3.

Table 3: Initial conditions for test # 12 Initial Depth (m) 9.4, Target Ascent Rate (m/s): 0.2 P coefficient: 15, I Coefficient: 0.5, D coefficient: 4.0 k value: 1 Sample Frequency (Hz): 1.19 Weight Lifted (out of water) 15 kg Weight material Cinderblock

The result of the reduced P coefficient from 20 to 15 can be seen as the system is less aggressive and allows the velocity to build throughout the ascent. Fig. 17 and Fig. 18 show the depth and velocity profiles of test # 12.

Test number 14 was performed using only the P coefficient. This was to gather information to start tuning the system in the same manner that the model was tuned to optimize a controlled ascent. The sample frequency was increased to 2.38 Hz for quicker system response. Table 4 is of the initial conditions for test 14. Fig. 19 and Fig. 20 are of the depth and velocity profiles of test # 14.

Table 4 Initial condition for test # 14 Initial Depth (m) 7.74, Target Ascent Rate (m/s): 0.2 P coefficient: 1, I Coefficient: 0, D coefficient: 0 k value: 1 Sample Frequency (Hz): 2.38 Weight Lifted (out of water) 15 kg Weight material Cinderblock

As can be seen from the ascent profiles this test was the most successful. The system was able to maintain a linear ascent at a constant velocity of 0.5 m/s. Although the ascent velocity was above the target velocity of 0.2 m/s, the performance of the system showed that with additional PID tuning, the system can approach and maintain a desired ascent velocity.

X. CONCLUSIONS The design of an automated purge valve has been presented

in this paper. In doing so, further knowledge of the capabilities and limitations of a controlled purge system have been achieved. The computer model and purge valve have shown how the controlled ascent of an automated lift system can be achieved. The computer model is a very unique lift bag ascent simulator that allows the profile of an ascent to be estimated. This new tool could aid salvage masters in operations where lift bags are deployed.

It was discovered that the PID algorithm did allow the lift bag to ascend at a controlled rate, but it was not the ideal equation to use and was consequently modified. Using the corrected model revealed that a PID control algorithm that did not add each new signal to the previous one worked best.

The new PID equation did not over purge as much and was much more flexible with initial conditions such as depth and mass of payload. (6) best demonstrates the difference between the initial and corrected PID algorithm.

Figure 17. Depth vs. Time profile of test # 12

Figure 18. Velocity vs. Time profile of test # 12

Figure 19. Depth vs. Time profile of test # 14

Figure 20. Velocity vs. Time profile of test # 14

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Initial Output(x) = (PID(x-1) + PID(x))*k (6) Corrected Output(x) = (PID)*k Where: PID(x) = calculated PID output at sample time PID(x-1) = previous PID output k = gain value

By the time the equation was modified the valve had already been designed and the system tested nine times using the valve that was designed from the data output of a flawed computer model. The high expansion of air calculated by the model required a much larger valve orifice size. This is the reason that the valve has a maximum orifice size of 7 cm2 when rarely does the newer model predict an area of 1 cm2 for a proper ascent. Regardless of the larger than required valve area, testing continued. The last five tests were very successful in maintaining a constant velocity ascent. In addition, the newer model now shows that lower PID values than were previously used achieves a proper ascent. With a newer and better understanding of how the lift bag will react, the automated purge valve should complete a successful controlled ascent at the target velocity within the next series of tests where PID coefficients are fine tuned

As previously discussed, due to a miscalculation the model had been predicting ascents on the magnitude of four to five times faster than would really have been seen. The valve size was consequently designed much larger than needed because of this and allowed over purges to occur throughout many of the ascents. Had the valve orifice been smaller over purges would have not happened allowing for a controlled ascent.

Despite the setbacks experienced, the modification of the model and processor software confirmed that a successful constant ascent at the target velocity is possible. With further tuning of the PID coefficients, testing of the automated purge valve will continue. Once the control system is adjusted properly the control system will perform a controlled ascent at the target ascent value. Controlled ascents will soon be over a range of different weights and in various depths of water with the same PID coefficients found during the final testing phase.

REFERENCES [1] JW Automarine. Forerunners of the Lifting Bag. Retrieved

December 5, 2007 from http://www.jwautomarine.co.uk/ download/lb_man1.pdf

[2] Zubaly, Robert B. Applied Naval Architecture. Cornell Maritime Press., 1996.

[3] “Down and Out, Reef Sinks Wrong Way.” Miami Herald, May 18, 2002.

[4] JW Automarine. Typical Example of Sub Sea Rigging of Parachute Lift Bags. Retrieved December 5, 2007 from http://www.jwautomarine.co.uk/download/lb_man11.pdf

[5] Subsalve. Subsalve Catalog. Retrieved March 14, 2008 from http://www.subsalve.com/catalog.htm