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Buoy is the structure which is floated on sea surface in order to indicate the presenting obstacle such as reef and shallow sea and to show the direction of sea route to ship during sailing. Generally, the conventional material of buoy is steel and it has some problems. Firstly this steel light buoy has safety risk in case of collision between ship and steel buoy. Secondly steel buoy revealed high corrosion environment of salted water and oxide and corrosion of steel can lead to marine pollution. Thirdly it needs too much maintain cost because of its heavy weight. In this study, in order to overcome these problems we changed the buoy material from conventional steel body to polyethylene body. Polymer buoy body was designed with module type part and it can reduce total weight up to 43.12%. To evaluate the strength of that part, the structure analysis simulation was carried out with respect to stress, displacement, and strain. Maximum stress was 1.667 × 107 N/m2 and it was 25% of yielding stress of base material. Maximum displacement and strain were 3.164 mm and 0.00433353 and they are too small value and in safe range with comparing to total length of body. The stability of polymer buoy body was compared with conventional buoy with respect to center of gravity, center of buoyancy, metacenter, oscillation period, and tilt angle by wind, tidal current, and wave. Every value was improved comparing conventional one and we can get more stable buoy. Therefore the new polymer buoy body could prove its safety and stability.

Buoy is the structure which is floated on sea surface and fixed by weight at seabed in order to indicate the presenting obstacle such as reef and shallow sea and to show the direction of sea route to ship during sailing. Generally, the conventional material of buoy is steel. Steel buoy has a lot of advantages such as high strength and easy manufacturing using cutting and welding process to make shape. However, it has some problems. Firstly this steel light buoy has safety risk in case of collision between ship and steel buoy. Secondly steel buoy revealed high corrosion environment of salted water and oxide and corrosion of steel can lead to marine pollution. Thirdly it needs too much maintain cost because of its heavy weight. In order to overcome these problems, many researches have been conducted.

Many countries have recognized the problem and limitation of steel buoy for long time and they have tried to study and make new material buoy which are light and environmental friendly. French Government [

In Korea, government studied and analyzed the trend of buoy material through basic research and design work of low cost and environmental friendly buoy development and suggested the development direction of it [

In this study, we designed and developed the buoy material from conventional steel body to polyethylene body. Polymer buoy body was designed with module type part. In order to evaluate the strength of that part, the structure analysis simulation was carried out with respect to stress, displacement and strain. To apply this new buoy to sea coast, the performance of buoy with polymer float body should be check with respect to stability. The stability of polymer buoy body was compared with conventional buoy in terms of center of gravity, center of buoyancy, metacenter, oscillation period, and tilt angle by wind, and tidal current.

Korean government defined 11 standard light buoys which can be installed differently according to location, sea wave, and water level and they must be designed and fabricated followed by the law of “Standard buoy manufacturing and quality maintenance”.

As shown in

Float has the largest contact area with sea in buoy and its shape is airtight container type. Therefore it is made through cutting, bending, and welding process of steel conventionally. In this study, we changed the float body material from conventional steel body to polyethylene body. Plastic float has a lot of advantages: it is lighter than steel material, it is easy to make using forming process with mold die and suitable to mass production, it is simple to assemble and maintenance, and it can minimize damage in case of collision between buoy and ship.

Design for light weight buoy with plastic float body was carried out using SOLIDWORKS 3D CAD program. The target was Korean Standard LL-26(M) buoy which are most widely used in Korean sea coast.

weight could obtain high weight reduction: float body lessen weight down to 32.8% and ballast was 63.7%. The number of ballast could be decreased because float body weight was reduced. Total weight ratio between steel and polymer buoy is 55.9% and total weight reduction rate was 44.1%.

In order to evaluate and confirm the safety of new designed float body, we simulated structure analysis base on acting load on plastic float body.

Parts | Conventional Buoy (kg) | New Designed Buoy (kg) | Ratio of weight (%) |
---|---|---|---|

Float | 2891 | 948 | 32.8 |

Mast and Steel Structure | 1215 | 1220, | 100.4 |

Ballast | 1637 | 1043 | 63.7 |

Total Weight | 5743 | 3211 | 55.9 |

The material properties of PE float are shown in

The structure analysis was simulated in terms of stress, displacement, and strain. The results of simulation were in ^{2} at hook point which are about 25.44% of yielding stress of material in

In order to evaluate the stability of new designed buoy with polymer float body, we compared the characteristics between conventional buoy and new buoy in terms of center of gravity, center of buoyancy, metacenter, period of motion, and inclination angle. As the input parameter, we assumed extreme natural condition: sea depth was 20 m, sea wave period was 10 sec, sea wave height was 5 m, and tidal current was 5 kts. Final results and calculation was referred to handbook of the navigation beacon [

Center of gravity of new designed buoy could be calculated by weight distribution for all element frames. Total weight of all element was 3211kg as shown in _{f}・m in this study. Center of gravity could be obtained with Equation (1).

where, M_{c} is the first moment of weight and W is the total weight for buoy.

Center of buoyancy is defined as the center of submerged volume of buoy. Submerged volume of buoy is equal to displacement which is water volume pushed by buoy. To compute the center of buoyancy, displacement and first moment of drain water volume should be known. The values of them are in

Material Property | Density (kg/m^{3}) | Yielding Stress (N/m^{2}) | Tensile Stress (N/m^{2}) |
---|---|---|---|

Value | 937 | 1.66714 × 10^{7} | 2.05941 × 10^{7} |

Material Property | Young’s Modulus (N/m^{2}) | Poisson’s Ratio | Shear Modulus (N/m^{2}) |

Value | 7.1 × 10^{8} | 0.439 | 5.94002 × 10^{7} |

Parts | Displace Volume (m^{3}) | Distance of Datum Line to Center of Displacement Volume (m) | First Moment of Displacement (m^{4}) |
---|---|---|---|

Float Cover | 0.33 | 0.343 | 0.113 |

Float Air | 2.604 | 1.183 | 3.08 |

Steel Structure | 0.199 | −1.706 | −0.339 |

Total | 3.13 | - | 2.854 |

is in Equation (2).

where, M_{B} is the first moment of displacement, and V is displacement volume.

We can define BM as the distance from center of buoyancy to height of metacenter. It effect the period of rolling motion and force of restitution of float body. As the BM value is higher, the buoy is more stable. BM is calculated by Equation (3)

where, I_{x} is the moment of inertia for draft of float body and its value is 3.976 m^{4} in this study. BM is 1.1269m as shown in Equation (3).

Also GM is defined as distance from center of gravity to height of metacenter. As the value of GM is greater, the buoy is more stable. GM is calculated by Equation (4) and its value is 2.005 m.

Buoy’s oscillation period (t_{0}) is defined as the back and forth time which float body is vibrated to the center of weight of buoy and it is obtained by Equation (5).

where, k is the radius of gyration for moment of inertia for additional mass and its value is 2.08 m and g is acceleration of gravity.

Generally inclination angle is generated by wind, tidal current. The inclination angle by wind and tidal current occurred when the moment by wind and tidal current is equal to righting moment. In this study the calculated righting moment by wind applied to buoy was 863.27 kg_{f}・m when the wind speed is supposed to 45 m/s and the calculated righting moment by tidal current was 268.434 kg_{f}・m when the speed of tidal current is 5 kts. Inclination angle is like Equation (6) as below.

where, M_{R} is righting moment. Using above values, the inclination angle by wind and tidal current is 7.7˚ and 2.4˚, respectively.

In this study, we changed the buoy material from conventional steel body to polyethylene. Polymer buoy body was designed with module type part and it can reduce total weight up to 44.1%. To evaluate the strength of that part, the structure analysis simulation was carried out with respect to stress, displacement and strain. Maximum stress was 1.66714 × 10^{7} N/m^{2} and it was 25% of yielding stress of base material. Max displacement and strain were 3.16441 mm and 0.00433353 and they are too small value and in safe range with compare to total length of

Stability Parameters | New Designed Polymer Buoy | Conventional Steel Buoy | |
---|---|---|---|

Center of Gravity | KG | 0.175 m | 0.281 m |

Center of Buoyancy | KB | 0.911 m | 0.28 m |

Distance from KB to metacenter | BM | 1.269 m | 0.40 m |

Distance from KG to metacenter | GM | 2.005 m | 0.399 m |

Period of motion | t_{0} | 2.95 sec | 7.84 sec |

Inclination angle by wind | θ | 7.7˚ | 19.69˚ |

Inclination angle by tidal current | θ | 2.4˚ | 19.46˚ |

body. The stability of polymer buoy body was compared with conventional buoy. Every value was improved comparing conventional one and it can get more stable buoy. Therefore the new polymer buoy body could prove its safety and stability.

This research is supported by Future Marine Industry Technology Development Project of Korea Institute of Marine Science & Technology Promotion in 2014 (Project No. 20140162).

Young Whan Park,Tae Wan Kim,Jae Sub Kwak,In Kwan Kim,Ji Eon Park,Kyong Ho Ha, (2016) Design of Korean Standard Modular Buoy Body Using Polyethylene Polymer Material for Ship Safety. Journal of Materials Science and Chemical Engineering,04,65-73. doi: 10.4236/msce.2016.41011