Yük gemilerinin dip yapılarının loyd kurallarına göre incelenmesi

Abstract

Günümüze kadar geçen zamanda inşa edilen gemilerin dip yapıları incelendiğinde, enine ve boyuna sistemde inşa edil dikleri görülmektedir. Son yıllardaki gelişmelerden dolayı gemilerin genel olarak boyuna sistemde inşa edilmekte ol duğu gözlemlenmektedir. Bu da, dip yapısının mukavemetinin gemi bünyesindeki önemini arttırmaktadır. Bu çalışmada, dip yapısında yer alan; merkez iç omurgalar, yan iç omurgalar, boş ve dolu döşekler, dip ve iç dip lev ha kaplamaları, vb., Türk, Fransız ve Amerikan Loyd kuru luşlarının vermiş oldukları bağıntıların, gemilerin tek dipli ve çift dipli inşa edilme durumlarına ve ayrıca hem enine hem de boyuna sisteme göre incelenmiş ve aralarında ki benzerlik ve yaklaşımlar ortaya konulmaya çalışılmıştır. Çalışmanın sonunda, ana ölçüleri verilen bir yük gemisinin dip yapısı ve elemanları, bahsedilen kuruluşların vermiş oldukları bağıntılar vasıtasıyla tek-çift dip ve enine-bo yuna inşa durumlarına göre nümerik olarak hesaplanmış ve mukayesenin iyi anlaşılabilmesi için sayısal sonuçlar çizelgelerle birlikte sunulmuştur. Ayrıca, genel olarak dip elemanları için eğilme momenti hesap yöntemi açıklanmış ve bahsedilen elemanlar için moment diyagramları hesaplanmış tır.

An investigation into the bottom structures of existing ships shows that they are constructed either in the transverse system of framing or longitudinal system of framing, depending on the strength requirements of the particular design. Recently, however, the tendency is in favour of the lon gitudinal framing system. This fact is related to the ge neral acknowledgement of the importance of the longitu dinal strength of ship hulls. Under these circustances, the bottom structures assume a special significance due to their considerable contributicn to the longitudinal strength. In this study, the construction of bottom structures of steel cargo ships is examined with respect to the "Rules and Regulations for the Construction- and Classification of Ships"of various classification societies such as the Turkish Lloyd (TL), American Bureau of Shipping (ABS) and Bureau Veritas (BV). The scantlings of the members of the bottom structures are determined from the rules of the classification so cieties mentioned above. The results thus obtained are compared to understand the differences or otherwise amongst the various classification society rules. Further more, the comparisons are extended to include independent results obtained from theoretical procedures for the cal culation of the scantlings of the bottom structures. In the beginning of the study, a notation list is present ed covering the symbols and definitions common to all the classification societies included in this investigation. Other symbols and definitions are presented in the re levant chapters, mainly chapters 2, 3 and 4. Chapter 1 presents an overview of the problem at hand. In the following chapters, the rules for steel cargo -viii- vessels of various classification societies are examined and applied to the bottom structure of a sample ship con sidered to be typical of its kind. The procedure includes four alternatives such that the bottom structure may be single or double bottom as well as transversely or longi tudinally constructed. Therefore, the scantlings are de termined using the rules of the societies exactly for the cases of single and double construction, firstly for a transverse system and then a longitudinal system. The investigation covers the complete bottom structure which includes construction elements such as floor plates, solid and open floors, center keelsons, side keelsons, inner bottom plating, bottom shell plating etc. The main scantlings that are determined include thicknesses, depths, section modulea. etc. and their arrangements and construction details. To summarise, the following cases are considered for the bottom structures of steel cargo ships : 1. Cargo ships constructed with a single bottom, a) built in transverse framing system, b) built in longitudinal framing system 2. Cargo ships constructed with a double bottom a) built in transverse framing system, b) built in longitudinal framing system. At first, a cargo ship built in the transverse system of framing with a single bottom is considered. The minimum scantlings are obtained in accordance with the rules of the three classification societies mentioned above. The formulea given by the societies are made of parameters which are straight forward constants, and others which are variables such as the length (L) or depth (D) of the ship. These formulea determine the minimum scantlings such as the thickness of center keelson, the section mo dulea of floor plates etc.. The parameters that appear in the formulea given by the societies are also compared. The arrangement of the st rength elements and the construction of the single bottom structures are presented in terms of drawings and appro priate illustrations. The significance and contribution -xx- to the strength of the overall structure of each structural element is discussed. The second case of single bottom in a longitudinal system of framing is examined using the same procedures as above. First the minimum scantlings given by the formulea of the classification societies are examined. The results thus obtained, then are used to compare, firstly, the transverse and longitudinal framing systems and secondly, the differences amongst the selected classi fication societies. These comparisons highlight the differences that are pro duced by the changes in the parameters used in the formu lea or sometimes, by the use of a different formula altogether. For example, the defination of the frame spacing in a transverse system varies from that of the longitudinal system, which results in final minimum scantl ings that differ depending on the selected framing system as well as the classification society taken into conside ration. The next stage of this study is the determination of the scantlings of double bottom of steel cargo vessels cons tructed in the transverse and longitudinal framing sys - terns and the comparisons of the results obtained. The same procedures as above are used. One of the most im portant differences here is in the contribution of the double bottom to the overall strength of the ship hull. For ships over 50 meters in length, the double bottom has a primarily positive effect on the strength of the global structure. A double bottom is constructed using open floors consisting of bottom frames and reverse frames, which are positioned between the solid floors. This form of construction, in itself is different from a single bottom in a way apparently imp rowed from the strength point of view. In general, the bottom structure of a steel cargo ship assumes significance in the region of the ship of about length 0.7.L amidships. Therefore, the study is concen - t rated in this area. The bottom structures at the forward and aft sections of the hull are examined seperately. It is seen that, the forward and aft bottom structures constructed with more strength than those in the 0.7.L midship region. The formulea of all classification societies yield higher scantlings at the bow and stern regions. It must be em phasised that the local strength considerations gain im portance at these regions. For example, the engine room bottom structure consists of solid floors with considerably -x- increased scantlings at every frame. For double bottoms, the inner bottom plating and the shell plating thicknesses are given by the rules of the of the classification societies. After the determination of plating scantlings, the bottom structure is determined by the cases of "Design Loads" taken into consideration depending upon the strength requirements of the ship's hull. The design loads are classified according to their origin : 1. External or Sea loads 2. Internal or Cargo loads As a general rule, the external loads to be considered correspond to the following conditions : a) Fully loaded condition at the scantling draught b) Ballast condition In addition, each load condition is determined for two cases : - Head sea condition - Beam sea condition The distribution of the external loads acting on the outer shell and the members is determined by considering two cases : - Ship on a wave crest - Ship at calm water draugth The distribution of the internal loads or cargo loads is determined as specified under the headings : - for cargo tanks, - for other tanks, - for dry cargo with accelaritons calculated at the center of gravity of each compartment. These requirements determine the loads to be taken into -xi- account for the scantlings of primary structural members of holds and tanks. The loading cases considered to be based on the most severe service loading conditions men tioned above, are determined to obtain the load heights at given load densities and the pressures on the shell of the hull. In the relevant chapter of this study, the loading cases are illustrated in appropriate figures as defined by the classification societies. In Chapter 5 of this study, a numerical example is pre sented in detail. The sample ship is a steel cargo vessel whose dimentions used in the determination of the scantl ings are given at the beginning of that chapter. The scantlings of the bottom structure elements are determine ed for each classification society. The results are pre sented in detailed tables together with the relevant formulea so as to allow meaningful comparisons between the two types of framing as well as comparisons amongst the results obtained from the rules of various classifica tion societies. Chapter 6 presents a discussion of the differences in the determination of the minimum scantlings for each st ructural member, including the formulea from which the results are obtained. On a basis of member by member comparison, it may be stated that, the differences are, in general, small. These differences originate from the variations in the parameters such as length (L) instead of beam (B) of ship, used in the formulea of various classification societies. However, a general overview of the scantlings of the total bottom structure shows that an increased scantling of a given structural element is counteracted by a decrease in the scantling of an other element. In other words, the differences that may exist do not indi cate any loss or gain in the overall structural strength of the total construction. The results show that Bureau Veritas (BV) and American Bureau of Shipping (ABS) rules produce very similar scantlings. The Turkish Lloyd (TL) rules yield slightly increased scantlings for the bottom structure. In this study, the scantlings of every element of the bottom structure of a steel cargo vessel are determined by rules of three selected classification societies. The results obtained indicate that the existing differen ces are small although the formulea by which the values are obtained contain different ship properties as para meters. -xxi- Also, the standart definations of the parameters as well as the constants used in the formulea vary depending on the choice of classification society. Despite such varia tion, the conclusion arrived at this study is that the minimum dimensions of the bottom structure elements of a steel cargo ship can be obtained from the "Rules and Regulations" of any one of the selected societies. It must be remembered that the longitudinal strength of the ship hull is important and that the bottom structure pro vides a significant contribution to this end.