Loads, load combinations, and design codes are determined and subject to the requirements of the AHJ. AHJs include local, state, federal or regional governmental agencies with oversight for marine terminals. There are two situations that may arise as the engineer seeks to satisfy code requirements. For the case wherein the AHJ has a higher requirement that provided in this reference, then the user is compelled to go to the higher standard and follow the directives of the AHJ. For the situation where the AHJ has no code requirements, or has no knowledge of marine engineering, this reference may provide a reasonable set of guidelines and recommendations to follow.
PIANC WG 153 developed recommendations for the design of marine oil and petrochemical terminals of any size. Much of the guidance provided by that WG is directly applicable to small- to mid-scale marine LNG terminals; therefore, this document will only discuss areas where there is a divergence between these types of facilities. The main difference between LNG facilities and oil terminals is in the acceptable risk levels defining criteria such as seismic return events and setbacks to the site perimeter. Generally, LNG facilities are intended to meet a higher level of protection than other petrochemical terminals which corresponds to a lower level of risk acceptance. In many cases, the design intent provided in the PIANC guidance [PIANC WG 153]
is consistent with that for small- to mid-scale LNG terminals, with some adjustment for alternate risk levels.
As discussed above, much of the code development and research performed to date has focused on large international trade terminals that see regular calls of high volume vessels. The exposed volumes during transfer operations may be orders of magnitude greater than those at a small- to mid-scale LNG terminals. However, pressurised transfer systems are more common for small-scale, introducing other risks. Therefore, the difference in risk may be justification for adapting prescriptive requirements that AHJ’s developed for large international trade terminals. Negotiation with the AHJ’s on prescriptive requirements and development of site-specific risk assessments and mitigations may be necessary, especially at locations where the AHJ has limited experience with LNG. These evaluations and designs developed must be based on a rational basis of design and have consistency throughout the project development; therefore, early involvement of the AHJ is highly recommended.
11.1 Design Codes
Design codes selected are dependent on the requirements of the AHJ. LNG facility specific design codes, such as NFPA 59A (2014), Canadian Standards Association (CSA) (2015), EN 1473 (2007), EN 13645 (2002), and ISO 18683 (2015) are available, but some AHJ’s may not be familiar with or have adopted these codes. Additionally, these codes are often geared towards conventional landside construction, with little consideration of marine facility design. In case codes are not available, the designer will be required to develop a basis of analysis which must be approved by the AHJ.
Code selection is critical in the development of a basis of design for the project. Where possible, selected codes should be consistent in their determination of both demand loads as well as member capacity; therefore, codes from the same region should be used together and codes from different regions should not be mixed. As an example, American codes should not be used to develop demand loads if European codes are used to determine section capacity. Additionally, consideration of location of material supply, fabrication, and construction should be considered in selecting the appropriate code. As an example, European code would be inappropriate for use in the Caribbean, where most source materials come from North America. An approach of referencing multiple codes can lead to confusion in both the design and review process; therefore, conciseness of code selection is highly recommended.
Code Order of Precedence
When referencing multiple codes, it is important to set an order of precedence to ensure that areas where the codes overlap do not lead to confusion in the design or review. Order of precedence can generally be listed as follows:
Local/Regional/Federal Regulations – Codes prescribed by the AHJ that must be satisfied.
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Owner or contractor guidance documents – Some companies have internal requirements or guidance specific to marine terminals. Where available, these criteria should be used;
however, smaller companies may not have developed such criteria.
International standards and guidelines – Guidelines, such as this document, are not legal requirements, but often provide sources of information for industry standard design techniques. Where standard and guidance documents are referenced, detail should be provided on what specific sections of the document are to be used.
Note that not all the above categories may be available for a project. All codes selected will need to be reviewed and accepted by the AHJ. Where no specific code or guidance is applicable, technical journals or papers may also be referenced. Finally, a ‘rational basis’ method may be used; however, this may require additional effort determined by the AHJ, such as an independent peer review.
11.2 Loads
Loads can generally be broken into marine structure specific loads (such as mooring and berthing) and those which are not marine structure specific. Most regulatory codes do not have marine specific requirements; therefore, it is often up to the designer to select appropriate design criteria for marine specific loads. Other loads, such as dead, live, and seismic, are often specified by the AHJ based on local standards. The discussion which follows is general in nature as the actual loads required will be based on the selected or required criteria specific to the site location and intent of the AHJ.
Non-Marine Gravity and Lateral
Non-marine specific loads are those that are typical for any structure. Usually the controlling criteria will be selected from local building or highway/transportation system code(s).
Dead and superimposed dead loads – Self-weight of structural elements as well as weight of any permanent fixtures.
Live Loads – Any transient load due to operations. Typical live loads include:
Uniform: averaged live load from personnel or temporary works
Equipment: such as crane lift or oscillation loads
Vehicular: trucks, forklifts, and mobile cranes (including lifts)
Wind on fixed structures – Loads due to wind on permanent structure and fixed equipment.
In areas of hurricanes or typhoons this load will often control the design of the structure lateral force resisting system outside of the mooring/berthing system.
Snow and ice – Snow (gravity) and ice (lateral) loads are common in colder regions.
Earthquake – The hazards to be considered and methods used for evaluation of earthquake loads vary by region in relation to the strength of local seismic events. Some of the primary hazards and methods are discussed below:
Inertial shaking – Shaking of the ground results in movement of the structure due to inertial response. LNG facilities are commonly designed for a multiple levels of events such as a smaller Operational Basis Earthquake (OBE), after which the facility can continue to operate, or a Safe Shutdown Earthquake (SSE), after which the facility can safely stop operations, but there may be significant structural damage. Each event is associated with a different return period and structural performance.
The selected return period event will significantly alter the demand loads for which the structure must be evaluated and retrofit. In many regions, LNG facilities have been historically held to a lower risk acceptance level by use of a longer return period event;
therefore, while a marine oil terminal may be designed for a 475-year return period event, a LNG facility at the same location would be required to satisfy a 2,500 year or longer return period event, as shown in
Table 11.1. As of 2015, no existing small- to mid-scale LNG projects have been fully developed (that the authors are aware of) which have not made use of the 2,500 year (or greater) SSE event; however, consideration of a lesser event may be appropriate if it is examined through the risk assessment process and is agreed to by the AHJ.
For small- to mid-sized facilities, the desired performance (damage) level may be altered while the SSE design event is held at 2,475 years or greater. Thus, for the same event size, the structure may allow for more rotation and permanent displacement. PIANC
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Report 34 (2001) recommends for ‘special’ structures (which can be interpreted to apply to conventional LNG) that the structure remains serviceable (undamaged) following a major event. For small- to mid-sized facilities the performance may be lowered to allow for slight nonlinearity, assuming that this approach is accepted by the AHJ. Strain limits and other parameters associated with this nonlinear approach are presented in and other marine structure specific documents. Strains maybe considered at slight damage (L1 or OLE), heavy damage repairable within months (L2 or CLE), or collapse prevention (L3 or DE).
Soil slope (kinematic) movement – An additional hazard that is significant to marine structures is soil slope (kinematic) movement. Kinematic movement is one or more of liquefaction, slope stability, or flow slide soil failures. Kinematic movement should be considered in combination with inertial response with input from the geotechnical engineer or record.
Reference Document Performance Level Design Return Period Event (years)
NFPA 59A (USA) OBE (operational) 475
SSE (safe shut down) 2,475
CSA Z276 (Canada) OBE (operational) 475
SSE (safe shut down) 2,475
NOM 013 (Mexico landside LNG)
Operational (SOB) Minimum of 2/3 of 2,475 years or 475 years Extreme (SPS) Minimum of 4,975 years or twice SOB DNV-OS-C503 Concrete LNG
(Satellite plants < 200 t) ‘Foundations shall be designed in accordance with recognized civil engineering practice’
ISO 18683
(LNG bunkering supply points) Not Discussed, Risk Analysis Based Selection Table 11.1: Seismic Return Period Event required by Various International Codes
Earth Pressure – Typically applicable to retaining structures.
Marine
Buoyancy – Uplift due to displaced water is typically insignificant, but can be important for floating structures.
Wave – Wave loads may control the design of trestles and/or platforms where significant seismic loads do not occur or where exposure is severe. Development of wave loading should be performed as part of a site-specific metocean study. Extreme and operational wave conditions should be considered. Wave return event selection should also be rationalised against the design life of the facility.
Berthing – Load transferred from the vessel to the structure during berthing (impact) when the vessel is coming to berth. These loads are further discussed in Section 9.2.
Mooring – Load transferred from the vessel to the structure from a combination of other environmental conditions (including wind, wave, current, passing vessels, etc.) while the vessel is moored at berth. These loads are further discussed in Section 9.3.
Ice – Ice loading specific to marine structures can occur when ice builds up against or between piles or when floating ice impacts piles.
Tsunami – Project risk assessments should include determination of appropriate tsunami risk from near field and far field events. Operating procedures need to consider the likely warning mechanisms and timing that may be available in consideration of planned evacuation for personnel and vessels.
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11.3 Load Combinations
Load combinations are highly dependent on regional requirements and input of the AHJ. As there are a large variety of load combinations; this document does not attempt to prescribe specific combinations for use when designing structures.
11.4 Displacement Limits
Displacement conditions can often control the design of marine structures, with pipe allowable displacements being of critical importance when considering extreme events (such as seismic).
Total differential displacement between independent structures should be combined by adding displacements of each structure, assuming the structures are out of phase, unless otherwise shown through analysis.
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