Section 8 Liquefied gas transfer systems
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 8 Liquefied gas transfer systems

Section 8 Liquefied gas transfer systems

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

8.1 General requirements

8.1.1  Application
  1. The Rules contained within this Chapter apply to liquefied gas transfer system(s) installed on board offshore units, for the purpose of transferring liquefied gas between an offshore unit and a commercially trading Liquefied gas tanker.
  2. The Rules and Regulations for the Classification of Offshore Units are applicable to liquefied gas floating production units and liquefied gas floating storage ship and barge type units. Unless a dedicated or novel offloading design is proposed, the gas carriers used for transferring liquefied gas will have been designed in accordance with the IGC Code, Classification Rules and industry guidance. Thus the means provided for discharging liquid gas are to be in compliance with standard marine practices with regard to Class, layout, loadings and support. Consideration is to be given to guidance provided in the SIGTTO publication titled; Manifold Recommendations for Liquefied Gas Carriers.
    1. Where the method of offloading is of a novel design, such as a tandem over the bow arrangement, the design of the liquefied gas transfer system is to be shown to achieve the same level of safety and integrity as a standard marine system.
    2. Where a traditional loading arm offloading arrangement is installed consideration shall be given to the effects of environmental factors such as unit motions and accelerations. Loading arm support columns are to be designed in accordance with the requirements of Pt 3, Ch 7, 2.7 Lifting appliances of these Rules.
    3. Suitable facilities are to be installed to allow periodic maintenance such as the change out of offloading swivels, bearings and PERC overhaul whilst the unit remains on station.
    4. Each type and design of offloading arrangement is to have the ability to be locked in a safe storage position in the event of extreme storms.
  3. Requirements additional to these Rules may be imposed by the National Authority with whom the offshore unit is registered and/or by the Administration within whose territorial jurisdiction the offshore unit is intended to operate.
  4. Requirements for fire safety are not included in these Rules; instead they are subject to the satisfactory requirements of the National Administration.
8.1.2  Surveys
  1. The survey of these items is to be arranged to coincide with hull and machinery surveys. See Periodical Survey Chapter and Section.
8.1.3  Design and operating principles
  1. Where the operation of the unit is to be at a specific location consideration will be given to the metocean data applicable to that area rather than the global ambient conditions stated in Pt 6, Ch 2, 1.9 Ambient reference and operating conditions of these Rules. Safety systems and essential auxiliary machinery are to operate at the angles of inclination given in Pt 5, Ch 1, 2.1 Inclination of unit 2.1.1 of these Rules. Any proposal to deviate from these angles of inclination will be specially considered taking into account the type, size and service conditions of the unit.
  2. Unless agreed otherwise, the unit is to be capable of operation within specified operating conditions that include maximum sea states, wind conditions and those identified in the Rules for Offshore Units. Where the metocean data applicable to the area where the unit will be stationed provides lesser environmental conditions, consistent with the expected usage, these may be accepted. The following information is to be submitted where relevant to the offloading unit type and its design. Design environmental criteria applicable to each mode, including wind speed, wave height and period, or sea state/wave energy spectra (as appropriate), water depth, tide and surge, current speed, minimum air temperature, ice and snow loads. Consideration is to be given to the content of Pt 3, Ch 10, 3.3 Metocean data of these Rules.
  3. Liquefied gas transfer systems are to be designed and installed such that degradation or failure of any liquefied gas transfer systems will not render another essential system inoperable.
  4. Release of liquefied gas due to the failure, leak or rupture of the system must not lead to catastrophic failure of the hull structure.
  5. Liquefied gas transfer systems are to be capable of operating within the normal vibration modes and cyclic loads of the vessel.

8.2 Acceptance criteria

8.2.1 General
  1. These Rules have been developed to achieve a standard of design and construction quality that ensures an acceptable level of safety and assurance of integrity of the installation.
  2. Deviations from the Rules, using risk assessment as a method for justifying Class, must therefore demonstrate that such changes to the design and construction of an installation or its parts do not result in an unacceptable level of safety or integrity of the installation.
8.2.2 Risk assessment and safety analysis
  1. LR will require the Owner/Operator to develop risk acceptance criteria to be achieved by the design and maintained in service, to ensure the safety and integrity of the installation in line with the spirit and intent of Lloyd’s Register’s Rules.
  2. Risk acceptance criteria are subject to approval by LR.
  3. A safety and reliability analysis is to be carried out to demonstrate that the liquefied gas transfer system achieves a suitable level of safety and reliability. It is to be shown that this is at least equivalent to that associated with terminal practises (i.e., EN 1474, SIGTTO, OCIMF, OGP). The analysis is to be carried out in accordance with acceptable National or International standards such as; ISO/IEC Guide 73, ISO 16903, ISO/TC 16901 and OGP Draft 118683 as well as the spirit of the Revised IGC Code.
  4. The analysis is to include identification of the hazards associated with the operation and maintenance of the liquefied gas transfer system under all normal and reasonably foreseeable abnormal conditions, and, in the event of a single failure, the potential effects on the safety of the offshore unit and its occupants, its machinery and equipment, and the environment.
  5. When the analysis is to be carried out in accordance with land-based codes and standards, the acceptance criteria is to be verified as both appropriate and acceptable for the proposed transfer system when installed on the unit. The analysis is also considered the potential effects of any hazards identified as a result of abnormal conditions and is to include arrangements to mitigate any consequence.
  6. The analysis is to consider at least and not limited to the following hazards:
    • low rate gas leakage, e.g. from joints, seals, etc.;
    • high rate gas leakage, e.g. from pipe rupture;
    • corrosion/erosion in gas piping, components and tanks;
    • mechanical failure of liquefied gas transfer system, equipment or components;
    • control/electrical failure of ESD system, ERS and electrical isolation in liquefied gas transfer system, equipment or components;
    • manufacturing defects in equipment and machinery;
    • human error in operation, maintenance, inspection and testing liquefied gas transfer, equipment and components;
    • location of gas-containing tanks, piping, machinery, equipment and components;
    • fire in areas or spaces containing tanks, piping, machinery, equipment and components;
    • fire adjacent to areas or spaces containing liquefied gas transfer system, cargo tanks, piping, machinery, equipment and components;
    • failure of lifting devices due to heavy loads, maximum sea states, wind conditions; and
    • failure of quick coupling system.
  7. In order to facilitate the proper selection and installation of equipment to be used safely in areas where explosive gas atmospheres may occur, an area classification study, in accordance with Pt 7, Ch 2, 2 Classification of hazardous areas is to be carried out.
  8. To ensure that mechanical equipment located in hazardous areas does not represent a source of ignition, an ignition hazard assessment, in accordance with an acceptable National or International Standard such as EN 13463-1, is to be carried out. See Pt 7, Ch 2, 5.1 General 5.1.2.
  9. The assessment process for liquefied gas transfer systems will consider all aspects of the system including offshore unit to ship dynamic interaction and environmental effects.
  10. The transfer system is to be subject to both commissioning and acceptance trials to show compliance with both safety and operational performance criteria. The acceptance trials are to include operational testing and be witnessed by an attending Lloyd’s Register Surveyor. All safety, operational and functional testing is to be demonstrated by the designer/Builder and Owner/Operator to the satisfaction of LR.

8.3 Documentation

8.3.1  Plans and particulars
  1. Plans, together with the relevant information as detailed in this Section, are to be submitted for consideration. Any subsequent modifications are subject to approval before being put into operation.
  2. Any alterations to basic design, construction, materials, manufacturing procedure, equipment, fittings or arrangements of the liquid gas transfer system are to be re-submitted for approval.
  3. A design statement of the liquefied gas transfer systems that details the capability and functionality under defined operating and emergency conditions. The design statement is to be agreed between the designers and Owners/Operators.
8.3.2  Lifting appliances.
  1. Plans and details of all lifting appliances as required by LR’s Code for Lifting Appliances in a Marine Environment or other specified design code to be submitted.
8.3.3  Piping plans.
  1. Arrangements of loading/offloading system to be submitted for appraisal.

8.4 Materials

8.4.1 General
  1. The materials used in the construction are to be manufactured and tested in accordance with the requirements of the Rules for the Manufacture, Testing and Certification of Materials (hereinafter referred to as the Rules for Materials) and of Chapter 6 of the Rules and Regulations for the Construction and Classification of Ships for the Carriage of Liquefied Gases in Bulk (hereinafter referred to as Rules for Ships for Liquefied Gases), as applicable. Materials for which provision is not made in those requirements may be accepted, provided that they comply with an approved specification and such tests as may be considered necessary.
  2. Materials of construction are to be suitable for the intended service, having regard to the substances, process and temperatures involved.
  3. Details of the materials proposed for all types of construction are to be submitted for approval.

8.5 Liquefied gas transfer system

8.5.1 General
  1. Operating requirement(s) associated with liquefied gas transfer are to meet the requirements of Pt 11, Ch 18 Operating Requirements of the Rules for Ships for Liquefied Gases.
  2. Transfer operations, accomplished by other means than transfer hoses and hard arms, will not be discounted but be given special consideration.
  3. All piping, valves and fittings are to be suitable for the design operating and environmental conditions.
  4. The piping is to comply with the requirements for manufacture, testing and certification of Class II piping systems.
8.5.2  Transfer hoses
  1. There are three types of cargo hoses suitable for liquefied gases transfer. These can be:
    • Composite.
    • Rubber.
    • Stainless steel construction.
  2. Liquid and vapour hoses used for liquefied gas transfer should be compatible with the cargo and suitable for the cargo temperature. The design, construction and testing of such hoses are to be to a suitable national standard such as BS ISO 4089 or BS ISO 5842. For hoses carried on board ship refer to the Rules for Ships for Liquefied Gases.
  3. Each transfer hose should be permanently marked with the following information and be compliant with the requirements of EN 1474 and other applicable Regulations, such as IMO’s International Gas Code:
    • Hose serial number;
    • Internal diameter of the hose;
    • Overall weight of complete hose;
    • Date of manufacture;
    • Date of proof pressure testing;
    • Certifying authority stamp;
    • The maximum and minimum allowable working temperature range.
  4. The hose vendor should provide the following documents:
    • Hose certificate.
    • Hose quantity assurance manual.
    • Inspection, test and storage plan.
    • Operating manual.
    • Hose handling manual.
  5. Where required, hoses are to be supported in a suitably dimensioned cradle or saddle arrangement to ensure that the manufacturer’s bend radius criteria are met. These supports may be integral to the load restraint system thus preventing excessive axial and torsional loads on the cargo hose end fittings. The support’s design, fabrication and fixing arrangements should be such to avoid chafing of the hoses and ability to prevent damage to handrails and other unit fixtures and fittings in the event of an emergency separation.
    1. Due to the difference in electrical potential between the unit and loading ship, there is a risk of an incendive arc when the transfer arms are being connected or disconnected. Arrangements shall be made to avoid the risk of arcing from this source by the installation of an insulating flange in the transfer arm or hose.
    2. Care shall be taken that the insulation flanges are not annulled by the use of electrically continuous hydraulic hoses.
    3. The use of a unit-to-loading ship bonding cable is not only considered ineffective but can also be dangerous if it breaks in a flammable atmosphere, such as where the final stage ESD activation includes automatic separation.
  6. When selecting hose size and length, the manufacturer’s recommendations should be followed to determine the maximum flow rate and other operating parameters. The maximum hose size will also be governed by the capabilities of the onboard lifting equipment and manifold construction.
  7. In determining the size and length of the hose(s) to be used, the following , in accordance with the requirements of the SIGTTO Ship to Ship Transfer Guide for Petroleum, Chemicals and Liquefied Gases, shall be considered:
    • Minimum allowable bend radius of the hose;
    • Horizontal distance between the unit and ship;
    • Difference in fore and aft alignment (manifold offset);
    • Distance between the manifold and the ship’s side;
    • Vertical and horizontal unit to ship movement;
    • Any other special characteristics related to the unit;
    • Relative change in freeboard between the unit and ship;
    • Accessibility of flange connections which are to be minimised;
    • Design flow rate for liquid and vapour hoses as established by the manufacturer;
    • Hose handling requirements and limitations of the asset’s equipment;
    • For tandem offloading; the station-keeping accuracy of the loading ship or the maximum allowable elongation of the mooring hawser.
  8. The liquefied gas transfer equipment should be supported by suitable means to prevent excessive loads on manifold fittings, in accordance with OCIMF/SIGTTO manifold guidelines.
  9. Each hose is to be fitted with an emergency release coupling (ERC). The coupling is to be fitted with a valve, each side of the release point, which automatically closes before parting can occur. Manual activation of the coupling is also to be achievable.
  10. Operation of the ERC is to take place on activation of the emergency shutdown (ESD) system. The ERC is also to operate prior to the transfer hoses becoming over-extended. After activation, the resultant movement of the free end of the hose is to be such as to avoid the possibility of impact and sparking.
8.5.3  Hard arm
  1. Where hard arms are considered for use in liquefied gas transfer operations, the following criteria, in accordance with the requirements of the SIGTTO Ship to Ship Transfer Guide for Petroleum, Chemicals and Liquefied Gases, shall be taken into account:
    • Accelerations;
    • Permissible manifold loadings;
    • Arm working envelope;
    • Arm support arrangement;
    • Arm stowage arrangement;
    • The effect of vibration on the arm;
    • Maintenance requirements;
    • Size of the arm;
    • Connectability;
    • Vertical and horizontal unit to ship movement;
    • Allowable flow velocity and pressure loss;
    • Testing requirements.
  2. An electrical insulation of the hard arm extremity shall be supplied according to the requirements of EN 1474-1. This may take the form of an insulating flange installed in the lower end of the outboard arm or within the middle swivel of the triple swivel assembly. The purpose of the flange is to prevent stray currents from causing an arc at the loading ship's flange as the loading arm is connected or disconnected.
  3. The range of the operating envelope of the hard arm is to be determined by the perceived tidal variations and change of the freeboard between the offshore unit and receiving tanker whilst loading or discharge.
  4. The hard arm is to be provided with an emergency release system to provide a means to quickly uncouple the hard arms with minimum spillage in an emergency.
  5. The physical disconnection may be achieved by means of a powered emergency release coupler (PERC). The effect of PERC activation and the resultant behaviour of the free arms are to be demonstrated. Consideration needs to be given to mitigating the effects resulting from unit motions and that the free arms can be controlled without impacting each other. If a manual type of loading arm is proposed (counter-weighted pantograph type), the furthest extent of the area which the released end of loading arm could extend into would need to be established.
  6. The PERC valves shall close as quickly as reasonably possible with the valve closure time being sufficient to avoid unacceptable surge pressure in pipelines. Such valves should close in such a manner as to cut off the flows smoothly. An interlock shall be provided to ensure that both the upstream and downstream valves are closed prior to the emergency release coupling parting thus prevent or minimising loss of liquid.
  7. The powered emergency release coupler shall be equipped with a device or devices to prevent overpressure due to thermal expansion of trapped product between the valves which have been isolated due to the coupler’s activation and resultant closure of the manifold valves due to activation of the ESD system.

8.6 Drain system

8.6.1 General
  1. Once the transfer operation has been completed and the loading ship ‘topped off’, all liquid lines, transfer hoses and hard arms will be in a liquid full condition. To alleviate the possibility of overpressure within these lines, there is to be a means to either drain these lines back to the storage tanks or provide a suitable drain tank arrangement.
  2. It is envisaged that the loading ship will not have the ability or storage capacity to allow the liquid transfer lines to be blown through. Thus the trapped inventory, from the storage tank pump outlet check valve to the manifold valve of the hard arm or transfer hose, will need to be returned to the floating production unit.
  3. Where novel arrangements are used, such as over the stern tandem boom arrangement, the amount of trapped inventory may be considerable. If due to location there is not the ability to drain the trapped liquid back to the storage tanks then a separate collection and storage tank system is to be provided.
  4. Depending on the liquid being transferred, were sufficient high pressure gas can be generated on board the unit this can be used to blow back the trapped liquid back to the storage tank. If there is the ability to remove non-condensable gases from the storage tanks gaseous nitrogen may be used in lieu of high pressure gas. After blowing through, the headers and discharge lines shall be able to remain connected to the storage tank vapour space thus allowing any remaining puddle of liquid to be boiled off.
  5. Where required, such as over the stern tandem systems were their location is remote from the storage tanks, a drain down arrangement, complete with local collection tank, may be required. This may take the form of a collection tank, having the ability, through either pressurisation or pump, to return the drained inventory back to the storage tanks. Thus any liquid remaining in the boom, manifold and header after discharge is complete would to drain back to the collection tank by gravity.
  6. When a separate collection tank is installed it would need to be provided with dedicated set of equipment and systems to service the tank. These are to include; high level and high pressure alarms, a means to empty the collection tank, a relief valve and vent arrangement suitable for the set pressure of the relief valves and vent gas temperature.
  7. Where low points are generated in liquid headers or manifold were liquid may be trapped these are to be fitted with a means to drain them in accordance with Pt 11, Ch 5, 1.2 System requirements 1.2.2.(b).
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