Study On The Safety Of Critical Areas Of Bulk Carrier's Hull In Shipbuilding And Operation

Because of strongly increasing development of China economy and continuously enlargement of world trading, transit of bulk cargoes also increases rapidly. Dry bulk carriers transport a high percentage of world trade with the advantage of greatly reducing packaging costs and speeding up of loading and unloading operations. In tonnage terms, about 33% of the world fleets are bulk carriers. They also include some of the world's biggest ships, and in a certain limit they are leading world trade and industry. So, they are often called as "workhorses" of the world merchant fleet.But, during recent decades, high rate of bulk carrier losses with considerable loss of human life startles throughout the shipping world. It is very apparent that many of the bulk carriers lost - often without trace - had suffered from severe structural damage. In some cases ships had simply broken apart like a snapped pencil. Most of the wreckage or loss of bulk carriers has shown that structural failure is one of the major factors leading to the casualties. So, scientific and practically efficient safety measurements should be taken promptly to prevent bulk carrier structural failure.Most of the bulk carrier structural failures are due to the influence of corrosion and fatigue. According to investigation of bulk carrier structural failure, most of failures start from some critical areas. These areas are also stresses concentrated areas of the hull structure. Stress concentration on the ship hull may be increased due to structural discontinuity and crack or fractures in the structure. Moreover, structural discontinuity and stress concentration of these critical areas can cause fatigue and corrosion easily. If critical areas are degraded, it may induce the whole hull structure deterioration. So, critical areas are very important for the safety of the bulk carrier hull structure.Improper techniques during ship building and operating factors including cargo operation and maneuvering at heavy sea, and age of the vessels, etc., influence critical areas damages and induce bulk carrier structure failure.Firstly, for the safety of critical areas, it is particularly necessary to be careful in the course of ship construction stage. Most of the critical areas in the structure are at the places of structural discontinuity joints, weld joints, welded ends and weld connections. Discontinuities of critical areas are mostly related to misalignment during construction. If there are defects at weld components such as misalignment, lack of fusion or penetration, etc., that weld area can cause stress concentration and other structure deterioration will continually happen. Moreover, modern ship building technology is a process of assembling interim product (parts, subassemblies, outfit units and blocks), so, welding work during ship construction is very important for the structures safety.Secondly, during operation, ship structure may be damaged in the loading or discharging condition at the berth. The damage reasons may be different. They may be due to overloading or improper loading distribution, or loading equipments. If the structure is damaged due to loading equipment impact at the hull and especially at hull's critical areas, and not to repair them in time, these damages become failure reasons of the hull. When the ship is maneuvering at sea, ship structure will suffer from various forces by reason of the heavy weather or heavy seas. These forces cause critical areas stress increasing and cause fatigue failure which accelerating ship hull failure. So, it is necessary to inspect, maintain and repair ship structures frequently. However, sometimes, the improper maintenance and repair actions may increase depreciation, shorten ship's life, and even cause structural failure and guide to happen accidents. The quality of this work will decide the critical area's corrosion rate in some way. Moreover, if some cracks or defects are detected, repair them in time and may prevent increasing of damage. At the same time, when ship is implemented overall repair, detail inspection and repair of critical areas are also important. And hence, during operation, critical areas should be paid special attention for the safety of ship hull.Thirdly, corrosion rate of the critical areas is an important considerable factor for the safety of ship hull girders. Ship hull structure is required to be reliable at anytime during operation. Because of the corrosion degradation effect, section modulus of the ship hull will be decreased and lead to structural failure. While hull girder resistance is degrading with time due to corrosion, the demand loading will also be varied with the operation lifetime. Due to variation of resistance and demand loading, the reliability of the ship structure will be varied with time. Reliability is commonly measured in terms of probability of non-failure of the structure during a certain time of service. So, by assessing time-variant reliability of ship hull, we can predict the probability of structural reliability at any time.In this paper, after researching damages of critical areas on ship hull during shipbuilding and operation in theorectically and practically, safety measurements are found to prevent damages.1. During construction of ship hull, deck and bottom longitudinal structure should be paid attention for structural continuity to reduce the effects of stress concentration. One of the most effective ways for the safety of critical areas such as fillet welds; end brackets and cutouts etc, is the prevention of accidental stress raisers by careful machining and fabrication. During welding condition, a good quality join is essential because insufficient and unsmooth welding condition sometimes induces damage of the area Corrosion protection may also be made starting from the construction step.2. During loading, avoiding the significant damages by preventing accidental overloading and/or uneven loading, close supervision is very important. Moreover, when loading high density cargoes, a relevant slow operation rate is advisable initially until the tank top is covered by a protective layer. To avoid the leading of twisting of the ship, efforts should be made to ensure that sizeable concentrations of cargo do not accumulate on opposing sides of the centerline in different holds. Cargo should always be trimmed reasonably level before sailing to reduce the likelihood of shifting.3. To avoid the structural failures caused by heavy sea, it is required to maneuver the ship as possible as in the course where the heavy wave impact is least. It is very important to avoid several holds flooded, and seawater swarm into iron ore. When seawater swarms into holds, ship will bear more weight than the structure can stand, and in addition with other various outside loads, the structure may be easily failure.4. During inspection, maintenance and repair, a special examination should be made after a potentially corrosive cargo such as high sulphur coal has been carried. The appearance of cracks, either caused by fatigue or physical damage, must never be neglected no matter how insignificant they may seem. Fatigue cracks in ships' structures are normally of a self limiting nature and if not repaired, may also initiate catastrophic failure. So, during inspection local thickness measurements need to be taken and any plating or frames found with excessive loss of material should be immediately renewed. Moreover, repair of any cracks found must be carried out by skilled welders under strict control.5. Of course, damage of critical areas are because of the various stresses inside and outside of the hull, and also may be because of over stressing during ship operating. Over stressing on the hull structure under the conditions of violent weather, heavy sea and cargo operation causes structural damages and are also not strange in the shipping field. In some cases critical situation developed so rapidly that there was insufficient time to broadcast an emergency message. Such dangers will be minimized by providing Hull Stress Monitoring System (HSMS) on the ship hull. HSMS with long base sensors and accelerometers will detect stresses and ship's acceleration on the ship hull during loading, discharging and in maneuvering at sea. HSMS will provide online guidance to ship's staff on enhance of preset stress thresholds, so ship hull structure will achieve the safety. And hence this stdudy has further elaborated as a sample of HSMS that has been already prepared to install in Shanghai Maritime University's training vessel" Yu Feng".The main purpose is to prevent ship structure failure effectively based on study of ship critical areas. The significant works are:Analyze the reasons of bulk carrier failure, and clearly describe the influence on critical areas' structural failure of bulk carrier.Discuss the critical areas of the bulk carrier hull structure and structural deterioration of those areas.Discover the possible damages of critical areas during construction especially in prefabrication and assembly. Moreover, welding deficiencies which may be happened during welding work are investigated in the shipyard, and self opinions for weld details are described.Research about the structural damages at each critical area which may be happened during handling cargo, maneuvering ship at sea, and inspection, maintaining and repairing ship. During inspection, maintenance and repair work, corrosion wastage and fatigue crack of critical areas are emphasized and, corresponding requirements for the maintenance and repair of each critical areas are described.Analyze a methodology to assess the time-variant reliability of a bulk carrier hull subjected from corrosion degradation. For the time-variant reliability calculation, a safety margin formulation is presented with a stochastic process of resisting capacity and demand loading. And the reliability function is calculated at discrete points during the ship's life. Sample calculation has shown that whenever corrosion degradation is present, time dependent reliability calculated with the present practice shows more accurate dynamic result towards the end of the ship's life.With the safety measurements investigated for the critical areas on ship hull and time variant reliability calculation method subjected to corrosion, structural failure of the bulk carrier may be overcome. Hence, this study may be as a guidance to construct new ships, inspect, maintain and repair existing ships, and may help for the safety operation of bulk carriers.