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Case Study

Stress and thermal analysis of an LNG marine loading arm

Transferring Liquid Natural Gas (LNG) from shore to ship and vice-versa at a temperature of -163 degrees Celsius presents a number of problems due to thermal shock and ice build-up on the transfer equipment. To help assess the behaviour of a new LNG loading arm design during cool-down, operation and subsequent warm-up Woodfield Systems Ltd (now part of Aker Solutions) employed LUSAS Consultancy Services. By using LUSAS Analyst it was possible to prove to the client Abu Dhabi Gas Liquefaction Co. Ltd (ADGAS) that its request for halving the cool-down period could be achieved without any detrimental effect on key components.

 

Overview

The 16 inch nominal diameter loading arm is a new design to replace existing LNG loading arms at the ADGAS Das Island Site in the United Arab Emirates. It consists of a base riser contained within a pedestal fixed to the jetty and a system of articulated pipes attached to a counterbalance supporting structure. Swivel joints allow for movement of the arm in all planes and freely follow the motion of any tanker when connected. These joints are required to carry high loads and must not allow any product leakage. Before LNG can be transferred the loading arm components must undergo a cooling process to prevent thermal shock. Nitrogen and/or LNG vapour is pumped through the system initially, before LNG is introduced. To fully investigate the effect of this cooling process a number of different analyses have been carried out including Steady-state Thermal, Steady-state Semi-coupled Thermo-mechanical, and Transient Semi-coupled Thermo-mechanical using four separate and very detailed finite element models.

Loading arm assembly

A 3D model of the whole pipeline assembly modelled the global behaviour of the system. This highly complex modelling and analysis process simultaneously analysed the effect of thermal and structural loads on the product flow-line. Results obtained confirmed the amount of shrinkage and stresses in the components as a result of the cooling and highlighted the importance of using localised models for more detailed analysis.

Loading arm assembly (jetty pedestal not included)

Jetty pedestal

A 3D model of the pedestal, top-box, riser, swivels and luff and slew bearings investigated the cooling effects of the riser pipe on surrounding components. A conservative steady state thermal analysis was used due to the convection and conduction effects involved.

Support brackets

A 3D model of a support bracket investigated the extent of cooling and thermal stresses induced as a result of the relatively rigid circumferential restraint. Whilst less prone to suffer from thermal shock (as the plates are thin) the brackets were susceptible to developing a thermal gradient caused by the plates acting as warming fins on the pipe. As a result the brackets were subject to further design and modelling to improve their performance.

Jetty pedestal

Swivels

A 2D axisymmetric model of a swivel joint investigated its performance under a variety of transient thermal loadings. Thermal loadings give temperature profiles and thermal stresses that vary in time. As a result, graphs of time temperature histories and plots of von-Mises equivalent stresses and relative axial displacements for selected nodes on the swivel easily showed the different stages of the cooling regimes considered and helped assess whether the seals could maintain product sealing during cool-down and loading.

Reducing cooling-down time

The 2D transient thermal analysis of the swivel showed that the male part cooled much quicker that the female part due to their relative masses and configuration. To evaluate different cooling options maximum von-Mises Equivalent stresses on the most highly stressed part of the swivel were plotted for comparison and are shown on the accompanying graph.

Cool-down graph and corresponding stress relationship

Red Curve : An analysis assuming the introduction of LNG liquid with no prior cool-down period shows why prior cooling is necessary. Stresses exceed the allowable for the material chosen.

Blue Curve : A cool-down period of 2 hours (as used on the existing installed loading arms) using a mixture of nitrogen and LNG vapours produces the least stress on the swivel when pure LNG is introduced.

Green Curve : An investigative 1 hour cool-down period of 15 min Nitrogen/LNG mix followed by 45 min LNG vapour cools the system faster than the the 2 hour regime. However, this causes a corresponding increase in stress levels resulting in the peak level at the introduction of LNG being approximately 20% higher.

Pink Curve : A 1 hour cool-down using only LNG vapour produces the most favourable result. The system cools faster and whilst the stresses are the highest initially, the peak stress at the introduction of LNG is only slightly higher than that caused by the 2 hour regime. This is the recommended option.

Benefits

By employing LUSAS as an independent consultant Woodfield Systems were not only able to confirm or adjust their designs to ensure that they met the best possible design conditions but were also able to provide complete confidence to their client that the designs used would provide trouble free operation, improve vessel turn-around and give longevity of service.

 


 

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