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Advanced ABS Engineering Analysis for LNGC

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The American Bureau of Shipping (ABS) plays a critical role in ensuring the safety and integrity of LNG carriers through rigorous engineering analysis and certification processes. ABS engineering analysis encompasses a comprehensive range of evaluations, including structural integrity assessments, fatigue analysis, and risk management, specifically tailored to the unique challenges posed by the transportation of liquefied natural gas. These analyses are crucial in verifying that LNG carriers meet stringent safety and design standards, taking into account factors such as the cryogenic nature of LNG, potential sloshing effects within tanks, and the extreme environmental conditions to which these vessels are often subjected. ABS utilizes state-of-the-art simulation tools and methodologies to conduct these assessments, ensuring that all aspects of an LNG carrier’s design and operation are optimized for safety, efficiency, and compliance with international regulations. This proactive approach helps to minimize risks and enhance the operational reliability of LNG transportation on a global scale.

The American Bureau of Shipping (ABS) provides essential engineering analysis services for LNG carriers (LNGCs) to ensure that these specialized vessels meet the highest safety and performance standards. Given the complexities involved in the transport of liquefied natural gas, such as the cryogenic handling of the cargo and the structural integrity of the vessels under extreme conditions, ABS employs advanced engineering methodologies to assess and certify LNGC designs. These analyses include detailed evaluations of the ship’s structural strength, thermal and mechanical stress responses, and dynamic behavior under operational conditions. ABS‘s role is critical in facilitating the adaptation of innovative technologies in LNG transportation, such as the development of new materials for cargo containment systems and the implementation of energy-efficient and environmentally friendly solutions. By rigorously applying engineering principles and safety standards, ABS helps ensure that LNGCs operate reliably and safely across global maritime routes, thereby supporting the expanding LNG market and its increasing role in the global energy landscape.

Engineering Analysis Applicable to LNGC

There are several engineering analysis that ABS performs when requested to class a new Liquefied natural gas (LNG) Carrier VesselLNGC. Some of these analyses are mandatory as a condition for classification and some are optional and are carried out at the Shipyard’s and/or Owner’s request.

Safehull

Whenever SAFEHULL is available, it is mandatory for the classification of ship.

When SAFEHULL was first introduced in the Rules on 1993, it was a very innovative approach for the determination of the hull scantlings, as it was replacing the old semi-empirical approach of the previous Rules with much more refined engineering criteria. The main features of SAFEHULL are the followings:

  • SAFEHULL strength criteria are based on a net scantling where the nominal design corrosion values are deducted.
  • North Atlantic wave data are used to conservatively represent the operating environmental design condition.
  • SAFEHULL criteria are developed to define realistic representations of the dynamic load components and the load combinations for strength evaluation. For LNGC, the loading criteria, sloshing forces, Origin, Applicability, Requirement of IMO Gas CodeIMO IGC pressure and membrane strain requirements are incorporated into the SH-LNG system.
  • SAFEHULL consists of two phases. Phase “A” gives the scantlings of the midship section and supplies a first evaluation of the fatigue life of the critical details. Phase “B“, is a more refined finite element analysis, which is carried out to confirm and optimize the results obtained by the Phase “A“.
  • Fatigue evaluation is included for longitudinal members in Phase “A” and for the critical locations of main supporting structures in Phase “B“.

Dynamic Loading Approach (DLA)

The Dynamic Loading Approach (DLA) provides enhanced structural analysis basis to assess the capabilities and sufficiency of a structural design at the explicitly determined wave loads.

The analysis is mainly composed of two phases:

a) SEAKEEPING AND SEA-LOADS ANALYSIS

This analysis is performed to provide load and pressure for the stress analysis. The seakeeping analysis considers the fully loaded and ballast conditions in head-on and oblique seas to determine the maximum torsional moment, vertical and horizontal bending moments as well as the hydrodynamic pressures. The analysis also determines the maximum motions of the vessel, which will be used in the sloshing calculations, as applicable. The seakeeping analysis may be based on the wave conditions for a specific route or for unrestricted service.

b) FINITE ELEMENT METHOD STRESS ANALYSIS

The first step of this analysis is the determination of the internal tank pressure by analyzing the motions of ship at sea and the additional loads due to the sloshing. The structural analysis will be accomplished through the use of a 3-D coarse mesh FEM model of the entire vessel. In fact the structure response calculation using a model of the entire vessel is believed to be more comprehensive and realistic than that used for SAFEHULL phase “B“. The 3-D FEM model is automatically balanced in waves, where no arbitrary boundary conditions are imposed on the model. The balancing procedure takes into account the hydrostatic and hydrodynamic pressures of the waves, the dynamic effects of ship structure due to the motion of the ship and the dynamic effect of the liquid in full and partially filled tanks. The 3-D coarse mesh FEM of the entire vessel, including the cargo tanks structures and their foundations (where applicable), is analyzed for two loading conditions (full loading, ballast), each one with still water, sagging and hogging wave and for maximum vertical acceleration and roll angle. Once the general stresses of the ship have been determined, the highly stressed areas are further examined using zooming criteria.

The results of the DLA analysis cannot be used to reduce the basic scantlings obtained from SAFEHULL criteria.

DLA is not mandatory, when it is performed; a proper notation is entered into the Record. In general, all LNGC owners request this notation.

Spectral Based Fatigue Analysis (SFA)

In addition to fatigue strength criteria in SAFEHULL, more extensive Spectral-based Fatigue Analysis (SFA) techniques can be applied.

The SFA is used to verify the adequacy of the fatigue lives of the critical locations in the entire hull structure.

The screening process is used in the selection of areas for which the Spectral-based Fatigue Analysis will be done.

The SFA (40) notation means that the design fatigue life value is greater than the minimum value of 40 years in the North Atlantic environment condition.

Containment System Strength Assessment

A numerical code ABSDYNA is used to evaluate the impact strength of containment system in the membrane type LNG carrier.

The scheme consists of sloshing impact pressure simulation by a fully nonlinear time domain calculation and sloshing model test results, and a hydro-visco-elastic analysis of containment system with consideration of fluid-structure interaction during the vibratory motion of the containment system due to sloshing impact.

Failure modes of the containment systems are considered for the strength criteria.

Since the structural elements of polymer and plywood insulator show the visco-elastic behavior at the higher loading rate, that is the case in the sloshing impact load, dynamic material properties of insulation system are considered in the evaluation.

In general this analysis is carried out only in case of significant differences with respect of already existing designs, such as large increase of tank/ship dimensions, change of tank shape, change of filling ratios, etc.

Pump Tower and Base Structure Analysis

ABS SLOSH is used for the evaluation of pump tower structural strength including base support.

ABS SLOSH consists of 3-D panel seakeeping calculation program PRECAL, F. E. sloshing calculation code SLOFE and F. E. analysis of pump tower structure.

The stresses on the pump tower structure due to the combined load including sloshing load, gravity load, inertia due to ship motion, thermal load are obtained using PATRAN/NASTRAN.

As the acceptance criteria, cylindrical members and tubular joints are examined according to the American Petroleum Institute code (API-RP-2A-WSD).

Vibration Analysis Including Containment System

The global free and forced vibration including the insulation system is performed with plate element model of hull structure and solid element of the insulation system considering material damping properties.

The natural frequency of the ship with insulation system will be evaluated for the risk of resonance. The plate panel vibration with insulation system will be evaluated under the vibratory influence due to the propulsion system.

Hydrodynamic added mass of the outer hull, internal General Overview of LNG Cargo Tanks (Typical Operations)LNG cargo, temperature effects for material properties are to be considered.

MSC/NASTRAN program is to be utilized for calculation.

Shaft Alignment Calculation

The propulsion shaft alignment is evaluated by using ABS SHAFT program, taking into consideration the expected hull deflection under various operating conditions including the full and ballast conditions with maximum sagging and hogging conditions.

From the DLA model, the deflection is extracted at the locations of bearings of the propeller shafting and the crankshaft in case of slow-speed diesel engine.

The shaft alignment offset will be further defined so as to allow optimization of the alignment design for the predicted hull deflections.

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Май, 04, 2024 50 0
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