The Dynamics of a Standard Commercial Ship under Loading - Research Paper Example

Name : xxxxxx Tutor : xxxxxxx Title : Forces in ships Institution : xxxxxxx @2016 Abstract This report covers the dynamics of a standard commercial ship under loading. It gives details and in-depth analysis with the help of visual aids as well as diagrams to depict the actually effect of forces such as stress and strain among others on a ship. The report also gives an informed outcomes on the ship`s structure when it is fully loaded and is in motion at the sea. Various technologies that help in advanced detection of existence of the forces such as stress and strain within the ship`s structure is also discussed in detail. The report introduces the reader to the history of ships as well as the basic components of a ship as well as their respective functions. Introduction A ship is water based means of transport that locomotes on the surface of water by means of buoyancy. Ships are large in size although their capacity varies based on their physical sizes as well as their weight carrying capabilities. Ships are commonly used for transportation of various types of objects and subjects but commonly transport Cargo or passengers. Some ships are known to have dual functionality and in such a case may carry both passengers as well as cargo. Some ships are also known for their sole purpose of entertainment, warfare or other economic activities such as fishing or mining within the water bodies especially seas and oceans4. The ships are known to travel on water bodies and are therefore, their channels are mostly lake, seas or oceans. As such, ships are attributed to the success of most scientific researches by renowned scholars as well as victory in the early as well as modern warfare. Colonization in the early 17th to 18th century were all attributed to sea travel capabilities that helped ship the colonists to the lands they colonized5. Shipping industry especially the commercial ships have been marred with a lot of shortcomings as a result of irregular and unanticipated happenings within the sea. As such, several accidents have been recorded in the sea transport industry and are known to be more recurrent as compared to other transport systems. Such accidents occur as a result of wave patterns within the sea, existence of submerge icebergs especially in the arctic regions as well as unbalanced loadings of the ships1. The aspect of effect on loading of ships as a cause of accidents in the sea travel is a dynamic field that has been researched and can be scientifically proved. Loadings on the ship have been known to cause capsizing, fracturing among other undesired possibilities within the sea travel. Various forces are subjected to the ship under consideration as a result of the loads it is subjected. Such loadings can be viewed from the perspective of transverse as well as longitudinal loading. Among the most important types of forces as a result of ship loading are the stress and strain among others as will be discussed in the report. Parts of a typical commercial ship A commercial ship is built in a standard way with some few variations in the shape as well as size as well as other minor functionality based infrastructure. However, a typical commercial ship infrastructure consist of the following parts with their respective functions2. The Bridge – This is an area normally placed in the front part of the ship. It is normally designed and equipped the right equipment to allow for the control and commanding of the ship. Most of the technical crew of the ship under consideration are based in this section. The figure below is a typical example of the bridge of a commercial ship. Figure 1: A typical bridge of a commercial ship Bulbous – This is a structural part of the ship designed in such a way that helps modify the dynamics of water flow around the hull of the ship and as a result enhance motion of the ship. It is normally placed in the lower most part of the ship and its front end takes a uniform semi-cylindrical shape as shown in the figure below. Figure 2: A typical bulbous of a commercial ship Crane – Crane is normally built as an integral part of a ship and is designed to help accomplish such duties as loading and unloading of cargo containers to and from the ship. It is normally built with its own control area within the cranes infrastructure as shown in the picture below. Figure 3: A crane built as an integral part of a commercial ship. Bow thruster – This is a transversal propulsion mechanism that is normally designed, built and mounted to the bow of the ship. This is the part that is responsible for locomotion of the ship. An example of a bow thruster is shown in the picture below. Figure 4: A typical example of a bow thruster mounted on the bow of a ship Forecastle – This is the part that is conventionally located at the bow of the ship and is responsible for effecting the docking process of the ship. A typical example of a forecastle is shown in the picture below. Figure 5: The forecastle of a typical commercial ship Funnel – The funnel acts as the exhaust mechanism for the ship. It is used to get rid of the boiler steam, smoke or any other exhaust gases within the ship. It consists of long chimney raised vertically along the ship mast. Figure 6: A typical funnel of a ship Rudder – The rudder is used to change the direction of the ship. It therefore acts as the steering mechanism for the ship and aids in navigation. A typical rudder is as shown in the picture below. Figure 7: A typical ship rudder Main deck – The main deck is the principal deck of a typical ship. It is where all decking activities within the ship are done. A typical main deck is shown in the picture below. Figure 8: Main deck The hull – This is the water tight part of a ship. A typical hull for a standard commercial ship is as shown in the picture below. Figure 9: The hull of a commercial ship Super structure – this normally consist of an upward extension structure build on top of an existing structure mostly the deck. A typical example of a super structure is shown in the picture below. Figure 10: A typical super structure Bulk head – this is an integral part of the hull of the ship. It forms a skeleton onto which the hull`s infrastructure is built. A typical example of the bulk head is as shown below. Figure 11: The bulk head The holds – This is the part of the ship where the cargo containers or the actual cargo is placed. Its size varies depending on the capacity as well as the type and size of the ship. Figure 12: The hold of a vessel Waterline – this is used a metric measurement method for the depth of the hull into the water surface. It consists of a calibrated surface that indicated the depth of the hull in meters. Figure 13: The waterline on the hull of a typical vessel Effects of ship loading Stress and strain Ship loading typically subjects the ship to effects of forces, stresses as well as strains that are likely to impart some bad effects on the structure and stability of the vessel. From the scientific perspective, stress can be defined as the load subjected to the vessel vertically. Strain on the other hand refers to the permanent deformity to the vessel as a result of excessive stress4. Forces The forces to which the ship is subjected to as a result of its loads can be categorized into two large categories. Static forces. Dynamic forces. Static forces are internal forces as a result from structural weight as well as the weight of the cargo loadings in the ship. The static forces comprise of the hydrostatic pressure as well as the water on the hull. Figure 14: Static forces for the vessel at rest Dynamic forces on the other hand result due to the ships motion at the sea. They are as a result of the action of winds as well as waves. Such dynamic forces can also be attributed to the effect of the machines operating within the ship such as vibrating generators and boilers. Dynamic forces A ship in motion is known to possess up to six degrees of freedom. The degrees of freedom can be categorized into three linear freedoms and three rotational degrees of freedom. However, the various forces that are effected within the ship are constantly varying in both degree as well as frequency. The movements within the stated degrees of motion results in dynamic forces within the ship and this subjects the ship to effects of stresses within its structure4. Linear or translational freedoms Swaying This is the swing motion of the mast or the bow of the vessel under consideration. The motion is normally from side to side as the time travels along the sea tides. Swaying can be attributed to the presences of waves within the path of vessel locomotion5. Figure 15: Top view of the swaying motion of a ship Heaving This is the vertical i.e. up and down motion of the ship along a linear path and is normally common when the ship is progressing along a wave. It results when the ship is hit up by a wave. Figure 16: Heaving of a ship Surging Surging is that movement of the sheep forward as a result of the ship rising and dipping when a wave that is considered strong enough to effect lifting is encountered. It is the forward and aft motion normally along the x axis. Surging normally occurs upwards on the side of the wave and dipping occurs down the other side of the wave. Figure 17: Surging of the ship Rotational freedoms Yawing Yawing is where the bow of the ship undergoes erratic swaying from the side as the vessel traverses through the water. This can also be defined as the rotary motion of the vessel about the vertical axis. Figure 18: Yawing motion of a ship Pitching This is a motion where the ship sways as it climbs to the crest of a wave and subsequently descends into the next through. The diagram below shows a two dimensional representation of pitching6. Figure 19: Pitching of a ship Rolling This is the subsequent motion of the ship from one side to the other as the ship traverses through the wave. The diagram below shows rolling of a ship as it sails in a sea. Figure 20: Rolling of a ship The diagram below summarizes the direction and the various types of motions of the ship as a result of dynamic forces. Figure 21: The various types of ship motion as a result of dynamic forces Stresses on a ship Modern ships are designed and developed by use of steel plating so as to impart enough strength for the ship to withstand all the forces as a result of its loading at all conditions and dynamics of the ship both during the travel or when docked. As discussed earlier, the forces under consideration may be static or dynamic depending on the occurrence and instance of the ship7. As a result, stresses in the ship as a result of forces. These forces create; 1. Local stresses. 2. Longitudinal stresses. 3. Transverse stress. 1. Longitudinal stresses Longitudinal stresses in a ship can be categorized into two. The first being the weight of the ship together with all that it carries acting downwards. The second kind of longitudinal forces is attributed to the vertical component of the hydrostatic pressure. Hogging Hogging occurs when the buoyancy at the mid of the ship exceeds the weight of the ship due to its loading. Hogging may also occur when the wave crests are amidships. The concept of hogging occurs from the perspective of loading a beam supported at its mid and its loading at the end4. The diagrams below demonstrates how hogging as a result of longitudinal stresses occurs in a ship. Figure 22: Hogging as a result of longitudinal forces in a ship Sagging Sagging normally occurs when the total weight of the ship under consideration at its mid exceeds the buoyancy. Sagging also occurs when the wave through at the mid of the ship under consideration sags. Sagging can be explained from the concept of a beam supported at both ends and its loading done at its mid length. Figure 23: Sagging in ships as a result of longitudinal forces 2. Transverse stresses Traverse stress is experienced by the ship when the transverse section amidships undergoes through subjection of static pressure as a result of surrounding water as well as weight due to loading i.e. weight of the structure as well as cargo6. Several parts of the ship are designed to resist the transverse stresses. Such sections of the ship include; The transverse section of the bulkhead. The floor of the ship. The bracket that exists between the beam of the deck and the side frame for the ship. The pillars. 3. Local stresses Local stresses within the vessel under consideration are as a result of several factors. Heavy loads that are concentrated on a particular point on the ship such as the boiler, fuel tank among others results in local stress concentrations on that section3. Dead cargo such as timber and containers also results in local stress concentrations on such points. Hull vibrations are also know for resulting in stress concentration. Dry docking of the ship subjects the lower part of the ship that is in contact with the surface under high concentrations of local stresses. These comes as a result of the loads of the material as well as the loads on the ship. Effect of stresses on ship structure Hull stresses The hull a vessel in a sea may be viewed as a simple beam that is subjected to an even loading and is considered to be under even support. Even loading of the ship is as a result of the weight of the structure while even support is attributed to the forces as a result of buoyancy in water. The magnitude of both shear as well as bending stresses may be determined by utilizing the theory of simple beam loading. The magnitude of these stresses however should be determined by carrying out compound calculation of both bending and shear stresses. In cases where grounding is evident, it may be necessary also to determine torsional stresses within the structure of the ship5. The stresses in the hull may be categorized into three as listed below. Primary structural stresses. Secondary structural stresses. Tertiary structural stresses. Causes of primary structural stresses in a ship Wight distribution As discussed earlier, weight distribution as a result of loading results in effects such as hogging as well as sagging. Hydrostatic pressure All the loadings within the vessel are Bourne by buoyancy as a result of water pressure. The impacts of the pressure results in inwards distortion of the structure and a subsequent resistance by the hull plating as well as the plate stiffening occurs. Racking Wave actions on the ship subjects the ship to cases of uneven water pressure. The uneven pressure with the aid of the shear stresses within the structure significantly causes the uneven pressure. Racking as an effect has the highest impact at the corners and as such, inspection should be performed at such points1. Docking When a vessel is docked especially by use of keel blocks, there exists a high tendency of the vessel to sag on the transverse section. This effect is normally reduced by increasing the number of blocks i.e. keel blocks. Grounding Depending on the mode of landing of the vessel, grounding normally results in a special case loading on the hull. Uneven loading of the hull is normally possible in cases where the vessel is not uniformly landed at one end as a result of uneven buoyancy. In cases where the landing is considered to be narrow, transverse sagging is possible. Potential failure of the ship Hulls The hull of the ships is mostly designed and manufactured by use of steel and wrought iron that are riveted together tightly. However, due to heavy loads as a result of ship loading, the hull is potential subject to failure as a result of brittle fracture. Potentially, the brittle fracture can be as a result of low temperatures in some parts of the sea, high impact loading as well as presence of Sulphur contents within the material structure. Therefore, based on this information, brittle fracture is normally experienced by the ships navigating through the Arctic’s. Impact loading are also possible in cases where ice bergs are present1. New technologies in design of the hulls however have adopted the use of high quality steels designed specifically for ship building and possess desirable properties of ductility and may deform but never break on impact or when subjected to low temperatures. Design flaws In conjunction with the material failures, poor design in ships is also one of the major causes of failures. Poor design of the compartments that are supposed to be water tight may lead to water leakages that may lead to great disasters when not detected and rectified as immediately7. Impacts with icebergs may also lead to formation of crack on the parts under contact and as such leading to leakages. The small cracks that results may enlarge as a result of water pressure and stresses within the structure under consideration. Sinking Sinking is normally a final stage of failure in any vessel in the sea. It is attributed to various failures within the structure as well as loading of the vessel under consideration. Cracks that leads to leaking of water into the water vessel causes buoyancy imbalance and leads to sinking in extreme cases. Loading the vessel beyond its weight bearing capacity is also known to cause sinking of the ship6. Stress and failure detection technologies in ships Several technologies have been developed by various ship manufacturing industries to aid in detection of failures and existence of undesirable stresses within the ship structure1. These technologies are evolving each time and advancement are made depending on emerging technologies. Some of the technologies commonly used to detect failures within the structure of the ship are as described below. Radiography This one of the most analogous technology of crack detection as a result of stresses within the structure of the ship under consideration. A beam of x-ray radiations are passed from a source to a point and is transmitted via an object that is being inspected. A single plane image of the surface under inspection is generated on the other end of the ray and is used to determine and analyze for presence of any cracks within the surface. The techniques is normally used to access and detect any cracks as a result of stresses in inaccessible parts of the ship such as rudder as well as the conveyor belts. The system is very useful as it can provided a permanent record of a fracture and can therefore be used as a reference in future. However, the technology is subjected to safety issues and is a cumbersome technology to use5. Ultrasonic detection This is one of the most preferred technologies for use especially in parts of the ship that is not subject to noise. A high frequency sound wave that is normally of about 1MHz are considered to have capabilities of being transmitted through the solids. Using this technology, it is possible to detect any slight change in structure of the part of the ship under consideration by measuring and developing a 3 dimensional image of the changes in the ultrasonic wave pulses. Thus, deflection or possible failures can be detected precisely using this technology. This technology however has a limited use and can only be used when there is adequate availability of skilled personnel to operate the equipment4. Liquid penetrant technology (LP) This technology is widely used at the hull structure of the ship to detect any cracks on the surface of the hull structure. This techniques however is so manual and still requires visual manual inspection to detect any cracks within the surface. The technology employs the use of dye and developer to give a guide on the existing cracks within the system. Despite the fact that it is an olden technology, its use is still very common in most ships as a result of its ability to detect cracks even at the tiniest possibilities. The technology however has a shortcoming in that it gives no information on the possible cause of the cracks and may require other means of trouble shooting to determine the cause4. Magnetic particle inspection technique (MPI) This techniques has high similarities with the dye penetrant method discussed earlier. However, in magnetic particle inspection techniques, a power magnetic field is subjected to the object under consideration and subsequently followed by application of magnetic ink. In cases where there exists a crack or any form of discontinuity within the structure of the hull, iron fillings gather along such lines of weakness4. The techniques is a simple one although some level of expertise is required in order to achieve more precise detection of fractures. The techniques however requires that the surface under consideration must be thoroughly cleaned before the procedure is done and in most cases may require that removal of paint be done. As a result, the method is used in early stages during the construction of the ship structure2. Strain gauges This is the most precise and accurate technology that gives an exact amount of stress and strain that a surface under consideration is subjected to. The strain gauges are materials made from highly resistive materials and are mounted on to the areas where stress and failure detection is to be carried out. This technology use the strain gauges as the sensor and computer based analysis of the results are possible by use of data logging techniques. This technology is mostly considered as a laboratory based technology although on board use for continuous detection has been effected3. Conclusion Ship industry as one of the most preferred form of transport especially for transport of large cargo is subject to various failures. Its loading, stress development, design as well as navigation should be carefully done and all aspects taken into consideration. The movement of the vessels with respects to the existences, magnitude and direction of waves affects its possible stability as well as determining its failure. The ability of failure and stress detection within the vessels structure at all times is therefore very important in determining its safety. The use of advanced technologies in mass balancing during loading, advanced stress and crack detection as well as precise navigation with respect to waves should be highly prioritized at all times. References 1. SHANK, M. E. (1953). A critical survey of brittle failure in carbon plate steel structures other than ships. Washington, D.C., National Academy of Sciences-National Research Council. 2. DERRETT, D. R., & BARRASS, C. B. (2006). Ship stability for masters and mates. Burlington, Mass, Butterworth-Heinemann. http://site.ebrary.com/id/10138627. 3. ASHBY, M. F. (1999). Materials selection in mechanical design. Oxford, OX, Butterworth-Heinemann. 4. SOFRONAS, A. (2006). Analytical troubleshooting of process machinery and pressure vessels including real-world case studies. Hoboken, N.J., John Wiley & Sons. http://public.eblib.com/choice/publicfullrecord.aspx?p=244305. 5. EYRES, D. J., & BRUCE, G. J. (2012). Ship construction. Burlington, Elsevier Science. http://www.123library.org/book_details/?id=53617. 6. ADAMS, K. M. (1986). The Effect of Cutouts on Strength of GRP (Glass Reinforced Plastic) for Naval Ship Hulls. Ft. Belvoir, Defense Technical Information Center. 7. OFFSHORE TECHNOLOGY CONFERENCE. (1991). 1991 Offshore Technology Conference, May 6-9, Houston, Tx [papers] CORP OTC. (23rd (23rd. Richardson, Tex, Offshore Technology Conference. Read More