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LNG World Shipping

LNG World Shipping

Escort tugs in the service of offshore LNG

Tue 13 Mar 2018 by Mike Corkhill

Escort tugs in the service of offshore LNG

Providers of escort tug services for floating LNG terminals in open seas face unique challenges but also play a role in facilitating project realisation 

A growing network of LNG export and import terminals worldwide means an expanding fleet of escort tugs needed to safely shepherd visiting LNGCs to and from their berths and mooring positions, and to serve as emergency responders should the need arise during cargo transfer operations.

The LNG terminal community is now being joined by the inaugural floating LNG production (FLNG) vessels and an increasing number of floating storage and regasification units (FSRUs) in varied locations. These floating LNG facilities provide challenges for escort tug service providers in addition to those encountered servicing traditional, shore-based terminals.

Offshore Cameroon

Among the notable new deployments of escort tugs at offshore LNG facilities is that for the Cameroon export project. The scheme is using Hilli Episeyo, the world’s first conversion of an LNG carrier into an FLNG vessel, and is currently poised to commence commercial operations.

Hilli Episeyo has a storage capacity of 125,000 m3 and will produce 1.2 million tonnes per annum (mta) of LNG under an eight-year charter contract with Société Nationale des Hydrocarbures (SNH) and Perenco Cameroon. Smit Lamnalco is supplying the escort tugs for the operation and is using three 32 m vessels from its existing pool. 

Hilli Episeyo, developing the Kribi field off the country’s southern coast, is moored in relatively shallow water 14 km offshore by means of an external frame turret arrangement which allows it to weathervane. Cargo transfers to loading LNGCs are to be carried out with the two vessels positioned side-by-side (SBS). 

Smit Lamnalco expects the tugs’ escort and operations-monitoring duties to settle into a routine fairly quickly. The relatively benign sea and weather conditions in the area have provided the project with a 90% operability window, while the tugs’ class certificates highlight that their station-keeping, towage and emergency response capabilities measure up to what is required.

Support Craft at Liquefied Gas Facilities: Principles of Emergency Response and Protection - Offshore, a document published by the Society of Gas Tanker and Terminal Operators (SIGTTO) in 2016, provides guidance for tug operators on appropriate responses in the event of an emergency at an offshore gas terminal. Implementation of the recommended actions to be taken by such craft will help to prevent and mitigate accidents. 

Each gas terminal has traditionally specified the performance standards that the specialist assist vessels it uses must comply with as part of its emergency response plan. The variable nature of these requirements had increased over the years as the global network of gas terminals has spread, prompting SIGTTO to develop harmonised guidelines.

Notwithstanding the support offered by generic guidelines, each offshore gas terminal has its own unique set of operational challenges. These include providing the required level of redundancy; arranging for the necessary logistics support; catering for local content requirements; tailoring an appropriate training regime; and organising regular exercises to test response plans. 

If requested, escort tug operators can get involved with a new offshore gas terminal project at an early stage to provide valuable consultancy advice on the optimum scheme layout, operating procedures and tug fleet. Such support can not only assist the floating vessel operator, the gas seller, the LNG buyer and the port authority achieve commercial acceptability for their offering but also help determine the nature of the tug operator service package that is contracted.

Prelude coming up

Another FLNG project due to commence operations in 2018 is Shell’s Prelude initiative 230 km off the coast of Western Australia. The energy major has contracted Perth-based KT Maritime Services Australia, a joint venture between Kotug International and Teekay Shipping Australia, to provide three 42 m, 100-tonne bollard pull tugs to serve as escort tugs for the 488 m floating unit.

Termed infield support vessels (ISVs) by Shell, the tugs have been built by ASL Marine Holdings in Singapore to Robert Allan’s advanced rotor tug ART 100-42 design. The vessels embrace the rotor tug power system technology pioneered by Kotug through a combination of Robert Allan’s RAstar hullform and three separate azimuth propulsion units – two astern and one amidships. 

Poisitioned in open seas where the water depth is 250 m, Prelude will remain on station for its entire working life. Offloading operations, to LNG and LPG carriers and condensate tankers moored alongside in an SBS configuration, are subject to the vagaries of the wind and the waves.

Variables also include the possibilities of beam and quartering seas, side-to-side rotation of the LNG carrier, wave motions and different liquid filling levels in the cargo tanks.

To deal with what can be challenging conditions, Shell and KT Maritime have introduced an amended push-pull method for bringing loading vessels alongside the FLNG unit. With this procedure two of the ISVs, sailing stern first, establish a towline connection, one at the bow and one at the stern of the approaching offloading vessel. After they bring the vessel alongside the FLNG, the tugs move to its side and push it against the FLNG, at which time mooring lines are fixed in place. 

Whereas a conventional azimuthing stern drive (ASD) escort tug would be unable to execute such a manoeuvre in anything but good weather conditions, the ISVs are able to handle the side-on push movement in waves likely to be encountered in exposed locations without the risk of damage to either themselves or the offloading vessel.
Kotug has also proposed an alternative procedure for work in seas of increased wave heights known as ‘rotoring’ or indirect towing. During rotoring the ISVs assist the LNG carrier on a short wire length, staying connected centre forward and centre aft. They are then able to push against the short tow line and do not need to be repositioned to transfer forces onto the LNGC. Kotug points out that rotoring mitigates the risk of damage to the hull of the LNG carrier and is only possible with the unique rotor tug configuration.

The Prelude project promises the LNG industry a steep learning curve. From the outset of commercial operations the performance of the ISVs, the FLNG vessel and offloading gas carriers and tankers will be carefully monitored and measured, to enable experience and expertise to be fed back into the playbook in the drive for continuous improvement.

FSRU action 

Bangladesh stands ready to make an entrance onto the world LNG stage as an importer of considerable potential. Five major receiving terminal projects are currently taking shape, comprising three FSRU-based and two shore-based facilities, while two smaller schemes involving the use of FSRU barges to provide gas to fertiliser producers in the Chittagong area could materialise quickly.

The first two major FSRU projects are both due onstream within the next 12 months; both are 3.75 mta schemes; and in both cases the regas vessels will be stationed near Moheshkhali Island in the Bay of Bengal.

The first Bangladesh FSRU, to be provided by Excelerate Energy, is due to commence operations by mid-2018. Svitzer will provide marine support to the terminal using a fleet of five vessels, including three RAstar 3200 escort tugs designed by Robert Allan and built by Cheoy Lee Shipyard in Hong Kong.

Excelerate will use one of its FSRUs with a bow ‘moonpool’ able to accommodate the terminal’s submerged turret loading buoy. The regas vessel operator has been involved in many FSRU projects worldwide over the past 13 years but reports that the Moheshkhali scheme represents an industry landmark, being the first floating LNG terminal to be fully permitted, constructed, implemented and operated through a single contract and company.    

The second Moheshkhali project, mounted by Summit LNG, is due onstream early in 2019 and PSA Marine of Singapore has been awarded a 15-year contract to provide the required towage services. In its first involvement with the LNG sector, PSA will supply three escort tugs, a crew boat and a supply vessel.  

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