Section 2 Design aspects
Clasification Society 2024 - Version 9.40
Clasifications Register Rules and Regulations - Rules and Regulations for the Classification of Offshore Units, July 2022 - Part 11 Production, Storage and Offloading of Liquefied Gases in Bulk - Chapter 5 Process Pressure Vessels and Liquids, Vapour and Pressure Piping Systems and Offshore Arrangements - Section 2 Design aspects

Section 2 Design aspects

Parent topic: Chapter 5 Process Pressure Vessels and Liquids, Vapour and Pressure Piping Systems and Offshore Arrangements

2.1 Design pressure

2.1.1 The design pressure P o, used to determine minimum scantlings of piping and piping system components, shall be not less than the maximum gauge pressure to which the system may be subjected in service. The minimum design pressure used shall not be less than 1 MPa except for; open ended lines or pressure relief valve discharge lines where it shall be not less than the lower of 0,5 MPa, or 10 times the relief valve set pressure.

2.1.2 The greater of the following design conditions shall be used for piping, piping systems and components, based on the cargoes being carried:
  1. for vapour piping systems or components that may be separated from their relief valves and which may contain some liquid: the saturated vapour pressure at a design temperature of 45°C. Higher or lower values may be used (see Pt 11, Ch 4, 3.3 Functional loads 3.3.2.(b)); or
  2. for systems or components that may be separated from their relief valves and which contain only vapour at all times: the superheated vapour pressure at 45°C. Higher or lower values may be used, see Pt 11, Ch 4, 3.3 Functional loads 3.3.2.(b), assuming an initial condition of saturated vapour in the system at the system operating pressure and temperature; or
  3. the MARVS of the cargo tanks and cargo processing systems; or
  4. the pressure setting of the associated pump or compressor discharge relief valve; or
  5. the maximum total discharge or loading head of the cargo piping system considering all possible pumping arrangements or the relief valve setting on a pipeline system.

2.1.3 Those parts of the liquid piping systems that may be subjected to surge pressures shall be designed to withstand this pressure.

2.1.4 The design pressure of the outer pipe or duct of gas fuel systems shall not be less than the maximum working pressure of the inner gas pipe. Alternatively for gas fuel piping systems with a working pressure greater than 1 MPa, the design pressure of the outer duct shall not be less than the maximum built-up pressure arising in the annular space considering the local instantaneous peak pressure in way of any rupture and the ventilation arrangements.

2.2 Cargo system valve requirements

2.2.1 Every cargo tank and piping system shall be fitted with manually-operated valves for isolation purposes as specified in this Section.

In addition, remotely operated valves shall also be fitted, as appropriate, as part of the emergency shut-down (ESD) system. The purpose of this ESD system is to stop cargo flow or leakage in the event of an emergency when cargo liquid or vapour transfer is in progress.

The ESD system is intended to return the cargo system to a safe static condition so that any remedial action can be taken. Due regard shall be given in the design of the ESD system to avoid the generation of surge pressures within the cargo transfer pipework.

The equipment to be shut down on ESD activation includes; manifold valves during loading or discharge, any pump or compressor etc transferring cargo internally or externally (e.g. to a shuttle tanker) plus cargo tank valves if the MARVS exceeds 0,07 MPa.

2.2.2  Cargo tank connections
  1. All liquid and vapour connections, except for safety relief valves and liquid level gauging devices, shall have shut-off valves located as close to the tank as practicable. These valves shall provide full closure and shall be capable of local manual operation; they may also be capable of remote operation.

    For cargo tanks with a MARVS exceeding 0,07 MPa, the above connections shall also be equipped with remotely controlled ESD valves. These valves shall be located as close to the tank as practicable. A single valve may be substituted for the two separate valves provided the valve complies with the requirements of Pt 11, Ch 18, 4.2 ESD valve requirements and provides full closure of the line.

2.2.3  Cargo offloading connections
  1. The offloading station is to provide a remotely controlled ESD valve prior to the hose connection to prevent liquid and vapour to or from the facility in the event of an incident. In the event that one or more transfer hoses are not used a manual and controlled by permit (or similar method) stop valve is to be provided prior to the hose connection.

    In the event that the vapour return line is closed the ESD system is to be designed to stop all cargo pumping.

    If the cargo tank MARVS exceeds 0,07 MPa an additional manual valve shall be provided for each transfer connection in use, and may be inboard or outboard of the ESD valve to suit the design of the ship unit.

2.2.4 Cargo tank connections for gauging or measuring devices need not be equipped with excess flow valves or ESD valves provided that the devices are constructed so that the outward flow of tank contents cannot exceed that passed by a 1,5 mm diameter circular hole.

2.2.5 All pipelines or components which may be isolated in a liquid full condition shall be protected with relief valves for thermal expansion and evaporation.

2.2.6 All pipelines or components which may be isolated automatically due to a fire with a liquid volume of more than 0,05 m3 entrapped shall be provided with PRVs sized for a fire condition.

2.3 Cargo transfer arrangements

2.3.1 Where cargo transfer is by means of cargo pumps that are not accessible for repair with the tanks in service, at least two separate means shall be provided to transfer cargo from each cargo tank and the design shall be such that failure of one cargo pump or means of transfer will not prevent the cargo transfer by another pump or pumps, or other cargo transfer means.

2.3.2 The procedure for transfer of cargo by gas pressurisation shall preclude lifting of the relief valves during such transfer. Gas pressurisation may be accepted as a means of transfer of cargo for those tanks where the design factor of safety is not reduced under the conditions prevailing during the cargo transfer operation. If the cargo tank relief valves or set pressure are changed for this purpose, as is permitted in accordance with Pt 11, Ch 8, 1.2 Pressure relief systems 1.2.8 and Pt 11, Ch 8, 1.2 Pressure relief systems 1.2.9 the new set pressure is not to exceed P h as is defined in Pt 11, Ch 4, 3.3 Functional loads 3.3.2.

2.3.3  Vapour return connections
  1. Connections for vapour return from the shuttle tanker to the ship unit shall be provided.
2.3.4  Cargo tank vent piping systems
  1. The pressure relief system shall be connected to a vent piping system designed to minimise the possibility of cargo vapour accumulating on the decks, or entering accommodation spaces, service spaces, control stations and machinery spaces, or other spaces where it may create a dangerous condition.
2.3.5  Cargo sampling connections
  1. Connections to cargo piping systems for taking cargo liquid samples shall be clearly marked and shall be designed to minimise the release of cargo vapours.
  2. Liquid sampling systems shall be provided with two valves on the sample inlet. One of these valves shall be of the multi-turn type to avoid accidental opening, and shall be spaced far enough apart to ensure that they can isolate the line if there is blockage, by ice or hydrates for example.
  3. On closed loop systems, the valves on the return pipe shall also comply with Pt 11, Ch 5, 2.3 Cargo transfer arrangements 2.3.5.(b).
  4. The connection to the sample container shall comply with a recognised Standard and be supported so as to be able to support the weight of a sample container. Threaded connections shall be tack-welded, or otherwise locked, to prevent them being unscrewed during the normal connection and disconnection of sample containers. The sample connection shall be fitted with a closure plug or flange to prevent any leakage when the connection is not in use.
  5. Sample connections used only for vapour samples may be fitted with a single valve in accordance with Pt 11, Ch 5, 2.2 Cargo system valve requirements, Pt 11, Ch 5, 4.1 Piping fabrication and joining details and Pt 11, Ch 5, 6.2 Testing requirements, and shall also be fitted with a closure plug or flange.
  6. Sampling operations shall be undertaken as in Pt 11, Ch 18, 2 Storage and transfer.
2.3.6  Cargo filters
  1. It is anticipated that liquefied gas facilities will remove contaminants before liquefaction. In the event that further filtration is anticipated, e.g. cool down during commissioning, the following shall be applied.

    The cargo liquid and vapour systems shall be capable of being fitted with filters to protect against damage by foreign objects. Such filters may be permanent or temporary, and the standards of filtration shall be appropriate to the risk of debris, etc. entering the cargo system. Means shall be provided to indicate that filters are becoming blocked. Means shall be provided to isolate, depressurise and clean the filters safely.

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