DEVELOPMENT OF A SOLID MODEL/FINITE ELEMENT MODEL OF A TYPE 23 FRIGATE Answers to Interview Questions Made by: Sulaiman Dawood Barry Submission Date: 25th January 2010
Question 1:Consider the Type 23 frigate; what are five most importantfeatures from creating a computer based model of a ship.Explain your choice.Answer:In order for the ship to float, the weight of the ship must beequal to the displaced volume of the sea water. Since a warshipis made of steel, the density of which is greater than water,therefore a solid ship would have greater weight than displacedwater volume and hence it would not be able to float. Tocounter this, the ship’s hull is made hollow. And beams areused to support the load exerted by water and the structureabove it.The center of gravity (CoG) should be in the midplane,therefore the ship is made to be symmetrical about the it.Otherwise, the center of floatation (buoyancy) might not be inline with CG, thus giving it a reason to be overturned.The length to breath ratio is kept large so that it resemblesfrigate and it is also made streamlined in order to reduce the
drag hence resulting in an increased speed, fuel efficiency andbetter maneuvering than oil tankers which smaller length tobreadth ratio.The hull is covered from the top and made water tight, so thatno water seeps inside.
Question 2:Develop a simple Solid Model of type 23 Frigate using Pro/E orSolidWorks, reflecting your answer to Question 1. Describe thesteps taken during the creation of the model.Answer 2:STEPS TAKEN DURING MODELING OF HULLFirst the top plane is selected as the water plane parallel to it 4planes are drawn. 1. The 1st Plane DTM1 is made 2 units away from the top plane, 2. DTM4 is also 10 units far from DTM1, 3. DTM2 is 40 units from DTM1, and 4. DTM3 is 40 units away from DTM2
On each water plane, waterlines are drawn that are assumed tobe similar to Type 23 frigate. The water line drawn at the topplane which resembles the submerged portion of the hull, it isdrawn as smooth as possible using lines and tangent arcs sothat it forms a continuous profile with no roughness inbetween. Thus making it streamlined. The final shape looks likethe following. Figure 1: Sketched portion on the top planeNow by using the use command in sketch mode, the sketchfrom the top plane is imported in each plane and then it isundergoes changes to give the final result as shown in thediagram below. Figure 2: Isometric view of final Sketches of the hull
Figure 3: Orthographic view of final Sketches of the hullTo make the solid hull from swept Blend, we perform thefollowing stepsSketch Tool Select Front Plane Click Sketch button in thereference windowThis enters into the sketch mode. Now perform the followingsteps to make the reference planes for the sweptSelect Sketch from the menu bar References Select twoextreme planes, i.e. the Top Plane and DTM3 Click solvecommand in the references dialogue box.After this step a straight line is drawn to connect these planes.We then exit the sketch mode.From Insert menu in the Menu Bar Swept Blend On thedashboard click Sections Selected Sections
Select Top Section and Click Insert, it becomes orange in color.Repeat this step for each section, from top to bottom withsequenceAfter this accept by clicking the green tick mark and you will getthe following image: Figure 4: Solid Model of Half HullTo complete the solid hull, select the ship and then mirror itabout the front plane using mirror . Figure 5: Symmetric Model of HullUsing the shell tool , remove the material from the inside andwe now only we have to add ribs to the inside of the hull.
Figure 6: Hollow HullTo add the ribs, we have to perform the following tasksMake parallel planes on at the location at which the ribs aresupposed to be added using Datum Plane option. Using the RibTool , click sketcher button and then click on to the plane onwhich the ribs are to be added.Main Menu Sketch Reference Select the cursor with aan X mark Select wireframe then on the drawingboard click hull to make it reference line of the hull, also selectthe two other planes to completely close the rib. Click solvebutton to solve it the reference planes and the select a line andmake it diagonal as show in the diagram to make the rib.
Figure 7: Making Rib ModelOn the dashboard click check button to exit and make sure thatthe direction of the arrow is down to make it solid. Finallyspecify the thickness of the rib. Figure 8: First Rib of the HullFollow the same procedure to add the other rib and thenmirror both the ribs about the midplane to make it on bothsides.
Figure 9: Ship’s Hull Complete formThis Completes the Hull Part of the body
MODELING OF DECKEach part is first made as half plane then mirrored as in theprevious sectionsThe sketch of the top-half plane is copied and pasted in thesketcher mode of the new part file. The sketch is extruded tosome about thickness of 10 units .The helipad’s half-sketch is shown and then selecting thematerial removal option from the extrude menu. It is thenextruded 5 units below. Figure 10: HelipadRadar Tower is made using blend protusion with smooth optionselected. A rectangle is drawn and then right click and selectthe toggle section option. Make a smaller rectangle centered atthe top portion of the previous rectangle. Your screen shouldlook then like this.
Select the check option to exit the sketch mode and enter theextrusion depth of 100 units.The cannon is made using revolve protusion option. First asketch is made ahead of the radar tower as shown and then it isgiven a revolved protusion of 180 degrees. The gun portion which is at an angle is made by selecting theplane of the tower and moving it near the cannon. A smallrectangular section is made and then extruded to complete thecanon.Finally the half-deck is mirrored to complete it.
Question 3:Using the model develop for question 2, highlight the regionsthat you would consider to be of interest when developing afinite element model of the vessel. These may be due togeometrical difficulties when meshing, stress concentrations,mesh refinement vs element budget, material models etc.MESHINGAmong the whole ship structure, the hull is given the mostimportance and hence it is given the highest priority ogf mostaccurate meshing.First the hull part file is opened and to generate a mesh, go tothe Application and open Mechanica. In the window opened,select structural and click okAssign materials and assign steel to the model. Now go to theAutoGEM in the main menu and select control in that menu.We will create dense meshing near sharp corners and with amaximum size of 2 units whereas the whole length of the wholeship is 800 units from fore to aft. Now we select the surfaces ofthe ribs and the edge lines being made when the two surfacesare intersecting at the bottom. Click ok and the surface will looklike the following.
The element budget is considered like this, First an auto mesh iscreated and the results are computed. Next the degrees ofpolynomial is increased to see if the results converge. If thehighest degree polynomial fails to do so, then the element sizeis reduced and the calcucatins are performed again. Thisprocedure is carried out until the previous result convergewithin acceptable range of accuray.