Reduced demand for newbuilds has checked LNG’s advance in the offshore vessel sector, but interest in it as a fuel is growing nevertheless
There have been very few orders for straight supply vessels, anchor handlers and subsea ships of late, hence little additional demand for liquefied natural gas (LNG) machinery to power them, but interest in using LNG is growing in related sectors, as recent orders for heavy-lift vessels have demonstrated.
Since the offshore vessel industry adjusted to reduced demand overall, there has been little change in the number of service and supply vessels in the world LNG-fuelled fleet. Platform supply vessels (PSVs) figure prominently in the at-sea LNG-powered service and supply fleet, accounting for 20 of the 33-ship complement. The orderbook, in contrast, shows much more variety, with a portfolio encompassing dredgers, a semi-submersible crane vessel for Heerema and a windfarm installation/decommissioning ship.
Among the most recent orders is one from the Netherlands-based Jumbo, which has signed a letter of intent with China Merchants Industry Holdings for detailed engineering and construction of a dynamic positioning class 2 heavy-lift crane vessel with dual-fuel engines. DEME group’s new installation/decommissioning vessel will be the first of its kind to be fuelled by LNG. The vessel, Orion, is being built at Cosco in China for delivery in 2019 and is a green design with dual-fuel engines, Green Passport and clean design notation.
DEME of Belgium has been a leading advocate of LNG-fuelled dredgers and has investigated the challenges of using gas to power them. Factors to be weighed up include bunker tank size and location, LNG bunker availability, shipboard bunkering connections, crew training and the step load capability of the various gas-burning engine options.
Combined with its cold box encasement, a cylindrical Type C LNG bunker tank occupies about three times as much space as a prism-shaped tank of marine gas oil (MGO) containing the same energy. While Type C pressure vessel tanks offer many advantages, accommodating them has a significant impact on vessel layout. Also, the tank location restrictions imposed by the new International Code of Safety for Ships using Gases or other Low-Flashpoint Fuels (IGF Code) add another level of complexity to the design challenge.
Naval architects working on the DEME dredgers found that dual-fuel diesel engines worked best for the vessels as they have a better step load capability than gas engines. The mix of LNG and MGO bunker tank capacities associated with such propulsion units also offers a measure of redundancy.
The aim is to optimise the use of LNG, assuming that the cost of this fuel is lower than MGO, but to provide sufficient MGO as a backup. The crew can switch to MGO if the dredger has to operate longer than planned before the next LNG bunkering stop.
Harbour tugs are the second most popular type of LNG-fuelled service and supply vessel after PSVs, and there are a growing number of gas-powered tugs in service and on order. Wärtsilä, with its medium-speed dual-fuel engines, and Rolls-Royce, with its gas-only units, have both enjoyed success in providing tug propulsion systems.
An integrated hybrid power module developed by Wärtsilä for a hybrid tug design highlights OEM’s ongoing interest in the sector, even if demand in the offshore vessel sector remains sluggish. The hybrid power module was developed specifically to meet the needs of the Chinese market and secured approval-in-principle recognition by the China Classification Society (CCS).
The Wärtsilä HYTug emphasises environmental sustainability, operational efficiency and lower fuel consumption than is possible with conventional tug designs. The hybrid power module combines engines, an energy storage system using batteries and power electronics optimised to work together through a newly developed energy management system (EMS).
Since they typically operate in or close to harbours and populated areas, tugs are particularly affected by environmental considerations, and the need for regulatory compliance is an increasing concern for tug owners and operators worldwide.