This article continues the series of research on Mechanical & Industrial Design for Aalto Explorer: Work Environment and Sea Soil.
2. WORK ENVIRONMENT
To better understand the work environment and collect important data to build an optimized requirement list.
After reviewing academic articles and similar projects, we pinpoint a list of fundamental properties that should be carefully researched before starting the project: temperature, salinity, pressure and drag forces caused by the ocean currents.
Temperature: Average 6°C, Range 5°C – 10°C. It’s a crucial parameter in material selection.
Salinity: Average 35ppt, Range 30ppt – 40 ppt. It’s an important parameter in material selection in terms of abrasion and corrosion.
Pressure: It is the main effort parameter. To calculate work pressure, a fundamental physic hydrostatic formula is employed: P=ρgH, in which g = gravity and H = 30 meters (as determined in the project panorama). At the temperature of T= 3.98°C, it is at its maximum density of ρ = 1.000 x 103 kg m-3; above and below that temperature, the density is lower. The addition of salt also increases the density, so that at normal oceanic salinities of some 35 ppt, the densities are near 1.024 x 103 kg m-3.
Pmax= 1.024 × 103 Kg/m3 × 9.82 m/s × 30m = 301.67KPa
*1,2 as the safety factor.
Pproj* = Pmax ×1,2 = 362,004Kpa
Pproj is the pressure that will be used in the simulations.
Drag force is an indispensable parameter in the motion system design.
More details on Drag force:
Another interesting force in everyday life is the force of drag on an object when it is moving in a fluid (either a gas or a liquid). You feel the drag force when you move your hand through water. You might also feel it if you move your hand during a strong wind. The faster you move your hand, the harder it is to move. You feel a smaller drag force when you tilt your hand so only the side goes through the air—you have decreased the area of your hand that faces the direction of motion. Like friction, the drag force always opposes the motion of an object. Unlike simple friction, the drag force is proportional to some function of the velocity of the object in that fluid. This functionality is complicated and depends upon the shape of the object, its size, its velocity, and the fluid it is in. (Source: https://opentextbc.ca/physicstestbook2/chapter/drag-forces/)
FD=1/2 ρ u2 CD A
FD = Drag force;
u = the flow velocity relative to reference area
A = reference area
ρ = mass density object
CD = the drag coefficient
CD depends on the object’s geometry,: Rectangle: CD = 2.05; Long cylinder: CD = 0.82
Since u is the relative velocity between the fluid and the object, it’s important to know the velocity behaviour of the ocean current. The research on this issue didn’t go as planned because we couldn’t find an estimation for current velocity for the depth work.
3. SEA SOIL
There appears to be a lack of useful data on sea soil composition. We found no significant research on the general physical properties of the seabed soil. There were some research papers that discussed seabed properties (e.g. Kim, H.-W, Hong, S, Choi, J.S, and Yeu, T.-K in Dynamic analysis of underwater tracked vehicle on extremely soft soil by using euler parameters). However, the main focus of these paper was something else than just the seabed itself. Most of the ROV vehicles developed seem to be propeller- or jet-powered and do not crawl at all on the seabed. This means that they are completely independent of the seabed conditions. To add to this difficulty, it is unclear how much we can use the data gathered from tracked vehicles operating on land.
Source: Sand underwater on shallow seabed with natural light through water surface, Moorea lagoon, Pacific ocean, French Polynesia
It could have been possible to incorporate the usable data from the research conducted on tracked vehicles operating on dry land instead of underwater. The differences are however too significant because the underwater vehicles encounter lifting force from the displaced water and it’s obvious that the seabed is not the same as land soil.
Seabed soil in general is softer than land soil. It is affected by the fluid motion of water, resulting in minor movements of the topmost layers of the seabed. The presence of water in the soil layers can decrease the cohesion of the soil particles, meaning that the soil particles underwater move easier in relation to each other compared to on dry land. The problem is that the opposite can definitely happen, depending on the soil type underwater. This “softening’’ can affect the vehicle if the motors are not programmed to prevent abrupt accelerations. In case of these abrupt accelerations, the soil layers can start sliding and moving in relation to each other, disrupting the rovers’ movement, especially on slopes.
Of course this is not always the case. The seabed varies very much depending on the geological history of the area. In Finland there are lakes that have bedrock as their seabed but there are also lakes with several meters of extremely soft mud. It is therefore very difficult to say anything about the seabed in general because it varies so much. These differences also reflect on the general appearance of the seabed. Softer seabeds do not carry weight as well as harder surfaces, which means that small rocks or stones will eventually “sink’’ into the seabed. On more firm seabeds, these stay on the top of the seabed, potentially causing trouble for the rovers’ movement. The harder the surface, the greater the obstacles and vice versa. This also causes difficulties in selecting the suitable propulsion system but more importantly, the components for the tracked propulsion.
The seabed composition also affects the slopes on the seabed. With less cohesive soils, the slopes are generally more gentle than on more cohesive soils or bedrock bottom because of smaller angle of repose. For instance, wet excavated clay has an angle of repose of only 15 degrees, whereas wet sand has an angle of repose of 45 degrees.
General rule of thumb seems to be:
- Harder soil: narrower tracks and more ground clearance (because floatation isn’t an issue)
- Softer soil: wider tracks with less ground clearance needed (but not too wide so that there still is a reasonable amount of ground pressure)
More interesting of the next steps will be updated in the following articles. Don’t forget to sign up to our newsletters or join our pioneer group for further updates!