Olasılıklı yaralı stabilite metodunun incelenmesi ve uygulamaları

Abstract

Günümüzde, gemilerin artan hızlan ve boyutlarına ek olarak, deniz trafiğinin de her geçen gün biraz daha yoğunlaşması kaza riskini ve buna bağlı olarak can ve mal kaybı olasılığım arttırmaktadır. Güvenlik kavramı değişik toplumlar arasında çok büyük farklılıklar gösterebilmektedir. Bu sebeple, denizcilik alanındaki tüm güvenlik düzenlemelerinin amacı imzacı ulusların yeterli güvenlik standartlarına uymasını garanti etmektir. Titanik faciasından sonra yolcu gemileri için bölmeleme gereksinimi ortaya atılmış ve bu konuda çalışmalara başlanmıştır. Sonunda IMO A 265(VIII) kararını kabul ederek yolcu gemileri için olasılık hesaplarına dayanan bölmeleme ve yaralı stabilite kurallarını geliştirmiştir. Bu yöntemin kargo gemilerine de uygulanmasında fayda görülmüş ve yapılan çalışmaların ardından MSC 19(58) karan ile kargo gemileri için olasılık hesaplarına dayanan bölmeleme ve yaralı stabilite kuralları uygulamaya konmuştur. Bu çalışmada, IMO 'nun bölmeleme ve yaralı stabilite konusunda getirmiş olduğu kuralların dayandığı olasılık felsefesi irdelendi. Olası yaranın yeri, boyu ve derinliği ve onların dağılım yoğunlukları ve geminin gözönüne alman yaralanmadan kurtulabilme olasılıkları hakkında bilgiler verildi. Daha sonra, örnek olarak seçilen dökme yük ve Ro-Ro kargo gemilerine bu kurallar uygulandı. Hesaplanan bölmeleme indeksinin istenen bölmeleme indeksinden büyük olduğu, başka bir deyişle bu gemilerin olasılıklı yaralı stabilite kriterim sağladığı görüldü. Aynı gemilerin stabilit eleri bilinen klasik yaralı stabilite metoduyla da hesaplandı ve böylelikle iki değişik stabilite yönteminin karşılaştırılması yapıldı. Bu karşılaştırmalarda görüldü ki, bazı yaralanma senaryolarında gemiler kurtulamıyordu. Ro-Ro kargo gemisi için stabiliteyi düzeltici önlem olarak ambarların boyuna su geçirmez bir perde ile bölünmesi düşünüldü ve bu durumundaki stabiliteleri iki yöntemle de incelendi. Ro-Ro kargo gemisi, bölmeleme indeksini büyük oranda sağlarken yapılan klasik yaralı stabilite hesaplamalarında tehlikeli bir yaralanma senaryosuna rastlanmadı. Ayrıca, ana güvertenin iki açılıp kapanabilir su geçirmez kapılar ile enine bölmelenmesi, ana güverte yaralanmalarında Ro-Ro kargo gemisinin stabilite eksikliğini giderebilmesi için önerildi. Bu durumda yapılan olasılıklı yaralı stabilite hesaplarının kriteri sağladığı gözlendi. Fakat, aynı durum için yapılan klasik yaralı stabilite hesaplarında, geminin kurtulamadığı bazı yaralanma senaryolar olduğu tespit edildi. Son olarak KG değişiminin olasılıklı yaralı stabilite üzerindeki etkileri incelendi ve bunun, "ulaşılan bölmeleme indeksi" üzerinde düzenli bir etkiye sahip olmadığı sonucuna varıldı.

At sea today more ships, particularly those in the container service, are capable of speeds previously attained only in passenger ships. Although operating at lesser speed, the new bulk carriers with their great mass are less maneuverable than ships of that type used to be. The density of traffic in the approaches to seaports is increasing. Because of that, the possibilities exist of a large number of persons at sea in a passenger ship being placed in jeopardy or goods in a cargo ship loss, either as a result of externally caused damage or through the loss of integrity of the ship's internal systems. However, safety is a relative concept, both culturally and economically, and is more actively pursued among advanced societies than in those where life expectancy is marginal. The purpose of international safety regulations is to assure an adequate safety standard to which all signatory nations will subscribe in equal degree. Some maritime nations have found it necessary to go beyond the convention in their national regulations. Their positive experience eventually makes it possible to demonstrate progress and to gain improvement of standards. Meanwhile, their shipbuilders and owners invariably point to loss of competitive position as a consequence of national regulations beyond the international. This is a reason that it often takes a disaster to bring about sufficient pressure for change. The relationship between safety regulations and casualty potential is far from precise. Economics, engineering, politics, publicity and philosophy all have a strong influence. The ideal is a subtle, undefined balance among this elements. To achieve equity in an international regulatory maritime safety system is a major undertaking. The variety and complexity of ship design, operation, operating area, voyage length, etc., etc., have necessitated several international conventions related to marine safety; namely, the Safety of Life at Sea (SOLAS) Convention, the International Load Line Convention, and the Tonnage Measurement Convention. An international convention on safety standards is both a blessing and a curse. For this necessity, SOLAS regulations, are based on the probabilistic concept which takes the probability of survival after collision as a measure of ship's safety in the damage condition, referred to as the "attained subdivision index A". This is an objective measure of ship safety and therefore there is no need to supplement this index by any deterministic requirements. New regulations, therefore, XV are primarily based on the probabilistic approach, with only very few deterministic elements which are necessary to make the concept practicable. The philosophy behind the probabilistic concept is that two different ships with same index of subdivision are of equal safety and therefore there is no need for special treatment for specific parts of the ship. The only areas which are given special attention in the new regulation are the forward and bottom regions which are dealt with by special rules concerning subdivision, provided for the cases of ramming and grounding. In order to develop the probabilistic concept of ship subdivision, it is assumed that the ship is damaged. Since the location and size of the damage is random, it is not possible to state which part of ship becomes flooded. However, the probability of flooding a space can be determined if the probability of occurrence of certain damages is known. The probability of flooding a space is equal to the probability of occurrence of all such damages which just open the considered space. A space is a part of the volume of the ship which is bounded by undamaged watertight structural divisions. Next, it is assumed that a particular space is flooded. In addition to some inherent characteristics of the ship, in such a case there are various factors which influence whether the ship can survive such flooding; they include the initial draught and GM, the permeability of the space and the weather conditions, all of which are random at the time when ship is damaged. Provided that the limiting combinations of the aforementioned variables and the probability of their occurrence are known, the probability that the ship will not capsize or sink, with the considered space flooded, can be determined. The probability of survival is determined by the formula for entire probability as the sum of the products for each compartment or group of compartments of the probability that a space is flooded multiplied by the probability that the ship will not capsize or sink with the considered space flooded. Although the ideas outlined above are very simple, their practical application in an exact manner would give rise to several difficulties. For example, for description of the damage, it is necessary to know its longitudinal and vertical location as well as its longitudinal, vertical and transverse extent. Apart from the difficulties in handling such a five-dimensional random variable, it is impossible to determine its probability distribution with the presently available damage statistics. Similar conditions hold for the variables and physical relationships involved in the calculation of the probability that a ship with a flooded space will not capsize or sink. In order to make the concept practicable, extensive simplifications are necessary. Although it is not possible to calculate on such a simplified basis the exact probability of survival, it is possible the develop a useful comparative measure of merits of the longitudinal, transverse and horizontal subdivision of the ship. XM Fundamentally, three probabilities relate to subdivision and damage stability requirements: a) Probability that a ship may be damaged. b) If the ship is damaged, the probability as to location and extend of flooding. c) Probability that the ship may survive such flooding. The probability that a ship may be damaged is relevant to the required degree of ability to survive damage and to the determination of insurance premiums. It is conditional upon navigational conditions, traffic density, visibility, effectiveness and reliability of navigational aids, ship speeds and maneuvering capabilities, and the judgment, competence and dependability of personnel involved. From the purely theoretical viewpoint, evaluation of the effect of each of these factors would permit determination of probability of damage for each ship, or at least for each class of ship. Practically, the available statistical and other necessary information is not sufficient for this purpose. However, some useful deductions are possible. Examination of casualty data confirms that damages due to collisions (and stranding) are more prevalent in harbor approach areas and areas of especially high traffic density such as the English Channel. Ships whose operation is principally or exclusively in such areas are more likely to be damaged. So long as e ship remains undamaged, there is no need whatsoever for any subdivision and damage stability. The need for evaluation of subdivision and damage stability stems from the knowledge that risk of damage does exist, and this leads to consideration of probabilities (b) and (c). Probability (b) is dependent upon the location and extent of hull damage and upon the arrangement of watertight divisions within the ship. Probability (c) is dependent upon buoyancy and stability in the flooded condition. This in turn is subject to the following variable factors: (i) The location and extent of flooding, (ii) The permeability of flooded spaces, (iii) The draft and stability before flooding. The simplest case is to consider the location and length of damage in the longitudinal and horizontal watertight structural divisions. Damage location x and damage length y are random variables. Their distribution density can be derived from the damage statistics. The probability that a compartment or a group of adjacent compartments is opened is expressed by the factor pi as calculated according to regulation 25-5. Consideration of damage length y only would be fully correct in the case of ships with pure transverse subdivision. However, there are very few, if any, such ships- all normally have a double bottom, at least. XVll In the case where the ship has a horizontal subdivision above a waterline, the vertical extent of damage may be limited to the depth of that horizontal subdivision. The probability of not damaging the horizontal subdivision is represented by the factor vi, as calculated according to regulation 25-6. With the simplifying assumption that the damage is rectangular, the damage can be described by the damage location x, the damage length y and the damage penetration z. They are random variables. The distribution density f(x,y,z) can be derived from damage statistics. The probability that a side compartment is opened can be expressed as p; x r. The probability that a center compartment is opened, in addition to the adjacent side compartment, can be expressed as p; x (1 - r). Again, it must be stated that the probability calculated on the basis of the simplifying assumptions is not exact. Nevertheless, it gives a comparative measure of how the probability of opening spaces depends on transverse and longitudinal structural subdivisions, and thus takes account of the most essential influences, whilst neglecting secondary effects. In this study, the regulations on subdivision an damage stability for cargo ships released by IMO were examined. Meanwhile, knowledge about damage location, damage length and damage penetration and their distribution density was given so that probabilistic method can be understood very well. After that, these regulations on subdivision and damage stability for cargo ships were applied to a bulk carrier and a Ro-Ro cargo ships elected as examples. It was calculated that attained subdivision index A was bigger than required subdivision index R, in other words, these ships provided the subdivision and damage stability criteria. The stability of these ships also were calculated by the classical damage stability method. These two stability method were compared each other. In these comparisons, it was seen that ships could not survive in some considered damage scenario. It was considered that holds are divided in two parts by a center longitudinal watertight bulkhead, in order to make sufficient the stability of Ro-Ro cargo ship in some determined damage condition and her stability was examined by two methods. When Ro-Ro cargo ship provided the subdivision and damage stability criteria, any dangerous damage condition was not come across in the classical damage stability calculations. In addition, when the compartment above mean deck was damaged, in order to provide the stability of Ro-Ro cargo ship, it was considered that this compartment was divided into three compartments by transverse collapsible watertight doors. While attained subdivision index was bigger than required subdivision index, there were several damage conditions which did not provide classical damage stability criteria. XMH Finally, affects of different loading conditions on probabilistic damage stability of cargo ships was examined and any systematic relationship between loading condition and attained subdivision index was not found. This result may be explained by the calculation method of survivability factor s.