LNG transport technologies evolve to meet market demand

Having started with the increased prospects of using LNG to help cut carbon emissions, the shale gas sector continues to evolve in the U.S. While the fuel shows promise, it does require careful handling. Ship shore link (SSL) technology helps minimize the risks associated with liquefying, transporting, and storing liquefied natural gas (LNG).

By David Pendleton September 29, 2015

As the world’s countries search for sustainable, reliable, renewable energy sources, they are also more closely examining a range of fossil fuels to find the cleanest, and least harmful, to cut carbon emissions and improve air quality in the process. In the drive to cut emissions, interest in LNG is growing. It is being looked at, not only as fuel for generating electricity and heat, but also as a fuel for powering marine vessels, road vehicles, and trains. The LNG market is evolving with the recent development of abundant shale gas deposits in the U.S.

The common, global method for transporting natural gas is to liquefy it. In this state, it takes up less volume and can be transported in specialized LNG carrier vessels. When cooled to -259 F, it is transformed to a liquid that occupies only 0.0017 the volume of the gas equivalent. But transferring LNG from export terminal to LNG carrier (LNGC), and then into the import terminal, comes with a host of risks due to the cryogenic characteristics when the gas is in liquid form. For example, the super cold temperatures of an LNG leak can crack a ship’s deck and hull.

The majority of the world’s LNG cargo fleet and terminals are equipped with SSL technology, a system for communicating emergency shutdown (ESD) signals, telephone, and process data required when cargo transfer is undertaken from ship to shore and from shore to ship.

While international regulations require ships and terminals to have two independent links available and present at least one system for compatibility requirements for passing along mutual ESD signals between ship and shore, this system can handle all four types of links from fiber optic to electric (Pyle National, Miyaki, ITT Cannon and SIGTTO) and pneumatic.

SSL systems have been around since 1975, and in use almost universally in the LNG industry since the publication of the Society of International Gas Tanker and Terminal Operators’ (SIGTTO) Recommendations and Guidelines for Linked Ship/Shore Emergency Shut-Down of Liquefied Gas Cargo Transfer in 1987.

The primary role of the SSL-ESD is to ensure secure ESD signalling to mutually shut down ship and shore pumping and transfer systems in the event of an abnormal condition or emergency. The SSL-ESD helps avoid damaging surge pressures that could lead to spillages of cryogenic liquid, which could, in turn, compromise the hull integrity through cold-cracking. The SSL, which should be ATEX, IECEx, and SIL 2 certified, handles the ESD, telephony, process information, and mooring load monitor data.

While a variety of different technologies were used up to the 1990s, an integrated SSL was then introduced that assured full compatibility between the ship and any of the main systems installed at terminals around the world. This cost-effective flexibility was a significant factor in facilitating the LNG trade to change from the liner to the spot market in a decade when the world fleet expanded three-fold.

In service of SSLs

What’s equally important to mission critical technology is having expert support to service it. Having a 24/7 service team to ensure any system downtime is minimized is a necessity. A ship costing tens of thousands of dollars per day to charter with a cargo worth several million dollars, is an expensive asset to have inactive because the SSL interface cannot be connected to transfer the LNG cargo. SSL technology, and importantly, 24/7 service to test, maintain, and repair equipment ensures that will not happen.

Beyond the ship and the shore

To maximize the opportunities offered by natural gas, floating LNG projects are increasingly being considered as the optimal configuration for liquefaction, storage, and regasification, where building a gas processing plant would not be economically viable or practical.

Floating storage and regasification units (FSRUs), in some cases with floating storage units (FSUs), represent the majority of projects currently being executed or in the front end engineering design FEED stage, as they can provide an attractive fast track solution for small markets and emerging economies.

Further development in SSL technology has been focused on FSRUs in particular. These are specialized vessels that convert LNG stored in their tanks to gas for transport to a gas network. For these vessels, the SSL simultaneously transmits the ESD signals for the gas output system and the ESD signals associated with the liquid transfer into the vessel.

A specially designed digital SSL for FSRU applications will safeguard the FSRU during the transfer process because it sends high pressure gas onshore, and when periodically visiting LNGCs, replenish the FSRU. That 20-hour batch operation must not interfere with the high pressure gas sent out.

A digital SSL has underpinned most FSU and FSRU operations. It exchanges any number of process signals between the ships and shore terminals to safely operate the gas or LNG transfer as a whole. In these large scale operations, jetty mooring load monitor signals can be exchanged with FSRU or FSUs.

The process must be managed from a single location and the operators must be able to access and control the shore and FSRU as one process. The SSL should support this comprehensive oversight and control.

In the case of multiple operating locations, additional flexible SSL telecom channels can be configured to support the extra telephones and hotline phones needed. For the increasingly popular side-moored case, mooring lines from the visiting LNGC are secured to quick-release hooks or bollards on the seaward side of the FSRU.

Transfer of mooring load data from jetty hooks to the FSRU, along with data from the FSRU hooks to the LNGC, ensures that the full mooring plan can be displayed, with quantified data helping first officers aboard both ships to maintain a safe moor.

Evolving for the next big market: Small-scale LNG

Across international markets, LNG is being traded as a commodity. Across the international shipping community, it is being used as a fuel. Each one has different business models but require the same thing: The safe, efficient, timely, and cost effective transfer of LNG from a discharge tank to a receiving tank and compatible systems that allows this.

A key issue for both markets is compatibility. Compatibility guarantees the flexibility of vessels and terminals to trade with any other facility, and ensures they can transfer LNG safely, efficiently, and economically. This is a lesson that the large-scale LNG industry has learned over the past 20 years, which has helped revolutionize the trading patterns of that industry.

Small-scale LNG transfer and the use of LNG as a marine fuel on the face of it look identical to the large-scale industry. However, this is a much more cost sensitive market, and there is a need to strike a balance between the implementation of the compatible systems and safe practices of the large-scale industry, and the need to make this new market economically viable.

Some manufacturers have now developed the universal safety link (USL), to strike this balance. The USL is an integrated fiber optic and electrical safety system that is both IECEx approved (IEC System for Certification to Standards Relating to Equipment for Use in Explosive Atmospheres) and ATEX approved, ATEX being the name commonly used to describe the two European Directives for controlling explosive atmosphere.

It uses industry standard connection systems, and distils the company’s vast large-scale terminal, LNGC, FSRU, and FSU expertise into an optimized product for small-scale LNG transfer or bunkering. The system can also interface with pneumatic systems to allow vessels to load from road tankers with pneumatic ESD control only.

With small-scale LNG transfers, which are undertaken at much higher pressures, careful control of the process conditions of the offloading and the receiving tanks is crucial to avoid nuisance ESD trips or discharges from safety valves. The USL system uses the same digital technology to allow a single operator to safely manage the entire transfer process from the bunkering LNGC.

The USL is designed as a rugged, compact, and cost-effective technology that, when connected to a remote counterpart USL, automatically configures itself to display to the operator the process conditions on board the remote receiving tank system.

– David Pendleton is the managing director of Trelleborg Marine Systems UK. He is an experienced senior manager with more than 25 years of experience in technical and business leadership roles. He was one of the founding directors of Mersey Maritime, one of the UK’s leading industry representatives. David has B.Sc in applied physics from Liverpool John Moores University. Edited by Eric R. Eissler, editor-in-chief, Oil & Gas Engineering, eeissler@cfemedia.com.

Original content can be found at Control Engineering.