Abstract:
A method and a device for protecting a vessel&#39;s loading space, the hold ( 1 ) of the vessel being provided with cargo pressure tanks ( 12 ), from excess pressure if a gas leakage should occur in the hold ( 1 ), the vessel being provided with a ventilating duct ( 8 ) which is separated in a gastight manner from the remaining rooms of the vessel and extends along the hold ( 1 ) of the vessel.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   The present application is the U.S. national stage application of International Application PCT/NO2004/000225, filed Jul. 21, 2004, which international application was published on Jan. 27, 2005 as International Publication WO 2005/007505. The International Application claims priority of Norwegian Patent Application 20033298, filed Jul. 22, 2003. 
   BACKGROUND OF THE INVENTION 
   This invention relates to a method for protecting a vessel&#39;s loading space from excess pressure. More particularly it concerns a method for protecting a loading space provided with cargo pressure tanks from excess pressure if a leak should occur in a cargo pressure tank or piping belonging thereto. The invention also includes a device for practicing the method. By loading space is meant in this context the ship&#39;s hold and adjacent rooms, for example a valve compartment and a piping room. 
   The shipping of gaseous petroleum products has essentially been carried out by means of the so-called “Liquified Natural Gas Method” (Liquified Natural Gas—LNG). The method comprises the cooling of gas to a liquid form, after which the gas can be transported in ship tanks at atmospheric pressure. Expensive equipment is required at both the point of shipment and the terminal for the treatment of the gas. Since the gas has to be cooled to a relatively low temperature, up to one fifth of the gas is spent on running the cooling and heating processes. Energy consumption like that just for the transport-related processes is expensive and moreover environmentally alarming. 
   Several other ship-based solutions have been proposed, wherein the gas is pressurized and/or cooled in order to achieve a gas density practical for the purpose. Such solutions have not become widely used in practice, but a solution in which a large number of vertical, tubular pressure tanks are placed in a ship&#39;s hold, has drawn considerable attention. The method is referred to as “Pressurised Natural Gas” (PNG). In accordance with such a method the gas is compressed at the point of shipment to an overpressure of a few hundred bars, and the gas is then filled into the cargo pressure tanks located on the ship. The cooling is limited to a simple and cheap removal of the compression heat from the gas, so that the transport temperature will be close to the ambient temperature. 
   By the use of cargo pressure tanks and associated piping, which are subjected to a relatively high pressure during the transport, it is of great importance in terms of safety that gas that might flow out of the cargo pressure tank or the piping by a possible leak, can be dealt with safely, without this involving the risk of damage to the rest of the cargo pressure tanks or the ship. 
   In the planning of a ship&#39;s configuration, possible unintended events that might occur must be analysed, after which the ship is designed with technical solutions arranged to relieve such events. 
   SUMMARY OF THE INVENTION 
   The invention has as its object to indicate a method and a device, which protect, on several levels, the ship&#39;s loading space from undesired excess pressure, while at the same time a possible excess pressure is relieved in a controlled and to the ship safe manner. 
   The object is realized in accordance with the invention through the features defined in the description below and in the following Claims. 
   The ship includes four kinds of rooms: hold, valve compartment, pipe corridor and ventilating duct. In principle, these rooms are separated in a gastight manner from each other and from the rest of the ship&#39;s rooms. The hold is most preferably divided into a number of hold sections. Each hold section comprises a number of cargo pressure tanks and is provided with an adjacent valve compartment. The hold volume enclosing the cargo pressure tanks is normally filled with a non-flammable gas. 
   One or more pipe corridors and ventilating ducts extend preferably longitudinally of the ship. 
   The ship&#39;s loading and unloading pipe is placed in the pipe corridor and is connected to the cargo pressure tanks of each hold section by means of a distribution pipe and a necessary number of shut-off valves and connecting pipes. 
   All valves belonging to the piping for connecting the cargo pressure tanks to the loading and unloading pipe are placed in the valve compartment. Preferably, there are no valves of any kinds placed in the hold. 
   The ventilating duct discharges into a discharge duct whose discharge to the atmosphere is placed at a safe distance from possible ignition sources and personnel. 
   The hold sections, valve compartments and pipe corridors are connected through at least one pressure relief check valve to the ventilating duct. A possible leakage leading to a pressure build-up in the hold, valve compartment or pipe corridor will thereby be ventilated through the ventilating duct. 
   By supplying fresh air to the pipe corridor and/or the valve compartment it is possible to carry out repair work and shut-off work in the pipe corridor, or the valve compartment itself, if a leakage is being ventilated through the ventilating duct. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In what follows there will be described a non-limiting example of a preferred embodiment and method visualized in the accompanying drawings, in which: 
       FIG. 1  shows schematically in a section a greater number of cargo pressure tanks installed in the hold section of a ship; 
       FIG. 2  shows a section I-I of  FIG. 1 ; and 
       FIG. 3  shows a simplified circuit diagram of the hold section, valve compartment, pipe corridors, ventilating duct and piping. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In the drawings the reference numeral  1  indicates the hold of a ship, comprising mutually gastight hold sections  2 . Each hold section  2  is provided with a valve compartment  4 . Along the hold sections  2  extends an elongate pipe corridor  6  and a ventilating duct  8  arranged parallel to the pipe corridor  6 , the ventilating duct  8  being connected to a discharge duct  10 . The valve compartment  4 , pipe corridor  6  and ventilating duct  8  are separated, mutually gastight, from each other and from the hold sections  2 . 
   A number of cargo pressure tanks  12  are placed in each hold section  2 . 
   Each hold section  2  is provided with a first pressure relief check valve  14  and a first rupture disc valve  16 , both valves  14 ,  16  being arranged to open for gas to flow out of the hold section  2  and on into the ventilating duct  8  if the pressure within the hold section  2  exceeds predetermined pressure values. A second rupture disc valve  18  is arranged to open for gas to flow out of the hold section  2  into the atmosphere if the pressure within the hold section  2  exceeds a predetermined pressure. 
   The valve compartment  4  is provided with a second pressure relief check valve  20  and a third rupture disc valve  22 , both valves  20 ,  22  being arranged to open for gas to flow out of the valve compartment  4  into the ventilating duct  8  if the pressure within the valve compartment  4  exceeds predetermined pressures. 
   The pipe corridor  6  is provided with a third pressure relief check valve  24  and a fourth rupture disc valve  26 , both valves  24 ,  26  being arranged to open for gas to flow out of the pipe corridor  6  into the ventilating duct  8  if the pressure within the pipe corridor  6  exceeds predetermined pressures. 
   A loading and unloading pipe  28  extends inside the pipe corridor  6  and is connected through a first shut-off valve  30 , a distribution pipe  32 , a number of second shut-off valves  33  and connection pipes  34  to each cargo pressure tank  12  or groups of cargo pressure tanks  12 . 
   The distribution pipe  32  is connected through a fourth pressure relief valve  36  to a drain pipe  38 . The drain pipe  38  is connected to the discharge duct  10  and is disposed parallel to the loading and unloading pipe  28  in the pipe corridor  6 . If the pressure within the distribution pipe  32  exceeds a predetermined value, the fourth pressure relief valve  36  opens, so that gas is drained in a controlled way through the drain pipe  38  and the discharge duct  10  into the atmosphere. 
   The loading and unloading pipe  28  communicates with the discharge duct  10  through a fifth pressure relief valve  40  and through a first controlled valve  42  connected in series with a second controlled valve  44 . A suction valve  46  is connected between the two controlled valves  42  and  44 . 
   A number of meters, not shown, together with a control system, not shown, are arranged to measure at least pressure and temperature in the cargo-holding portion of the ship and signal if an abnormal cargo condition should arise. The control system not shown is programmed to carry out relieving operations, such as the opening of controlled valves. 
   Should a leak occur in a cargo pressure tank  12  or one of the connecting pipes  34 , causing the pressure within the hold section  2  to rise above a predetermined pressure, the first pressure relief valve  14  will open, so that gas can flow from the hold section  2  through the first pressure relief valve  14  into the ventilating duct  8  and then on into the atmosphere through the discharge duct  10 . 
   If the pressure in the hold section  2  rises further, the first rupture disc valve  16  opens to a greater flow volume from the hold section  2  to the ventilating duct  8 . 
   The second rupture disc valve  18  opens for gas to flow out of the hold section  2  into the atmosphere if the pressure within the hold section  2  should rise even more. 
   By a possible external leak in one of the shut-off valves  30 ,  33  or the portions of the connecting pipes  16  located in the valve compartment  4 , the second pressure relief valve  20  opens for gas to flow through from the valve compartment  4  to the ventilating duct  8  whenever a predetermined pressure is present in the valve compartment  4 . Should the pressure in the valve compartment  4  rise further, the third rupture disc valve  22  will open for gas to flow through from the valve compartment  4  to the ventilating duct  8 . 
   By a possible leak in the loading and unloading pipe  28  and the drain pipe  38  located in the pipe corridor  6 , the third pressure relief valve  24  will open for gas to flow through from the pipe corridor  6  into the ventilating duct  8  when a predetermined pressure is present in the pipe corridor  6 . Should the pressure within the pipe corridor  6  rise further, the fourth rupture disc valve  26  will open for gas to flow through from the pipe corridor  6  into the ventilating duct  8 . 
   The fifth pressure relief valve  40  is arranged to open for gas to flow through from the loading and unloading pipe  28  into the discharge duct  10  at a predetermined pressure within the loading and unloading pipe  28 . 
   During repair work in the corridor  6 , the loading and unloading pipe can be evacuated by opening the first controlled valve  42  and the suction valve  46 .