Procurement News Notice |
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| PNN | 5840 |
| Work Detail | Naval architects at Robert Allan Ltd had to overcome various challenges in designing new LNG-fuelled escort tugs for Statoil’s Melkøya LNG production terminal near Hammerfest in northern Norway, writes Mike Phillips. These tugs were designed for Norwegian shipowner Østensjø Rederi, which won an open tender for the provision of three escort tugs for the terminal. Østensjø Rederi approached Robert Allan with a unique set of design requirements for a series of three high-performance dual-fuel escort tugs. The rarity of such a solution in the present towing industry, and the unparalleled performance requirements for LNG-fuelled escort tugs provided a formidable challenge. Building on its experience, Robert Allan combined new innovations with the latest computational tools and scale testing to deliver a customised RAstar 4000-DF design. Restrictions Aside from the performance requirements specific to this project, the foremost design challenges for any LNG-fuelled tug are space and location, Mr Phillips continues. Space comes at a premium in any tug design, and unfortunately LNG is not compact. In particular, LNG is not nearly as space efficient as diesel to store. Conventional tugs store diesel fuel in tanks of all shapes, making use of tight spaces which can be of limited use for other purposes. Although membrane tank technologies allow for storage of LNG in hull tanks on large vessels, that technology is not yet economically scalable to the smaller tank volumes necessary in tugs. Currently the most suitable LNG storage solution for smaller vessels such as tugs is the double walled IMO Type C insulated tank. Furthermore, the energy density of LNG is approximately half that of diesel. Together, these limitations can reduce the effective volumetric energy density of LNG on board tugs to approximately one-sixth to that of diesel. This creates obvious challenges to achieving suitable range and endurance from an LNG-fuelled tug. When it comes to the allowable locations of LNG storage tanks on board a vessel, there are specific classification society, national authority, and international code (such as IMO's IGF Code) safety-based restrictions. LNG tanks cannot be closer than 760mm to 800mm from the side shell, nor less than the vessel's overall breadth divided by 15 from the bottom shell. Similarly, they must be at least 900mm away from boundaries with machinery spaces of Category A, such as enginerooms. These major location constraints, along with the space limitations, make it difficult to fit significant quantities of LNG on board a typical tug. Requirements The challenges for LNG tug design do not end there. There are specific code and class requirements for safety and gas systems that cover the ventilation of the LNG system's tank hold, tank connection spaces, gas regulation units, engines, double walled piping systems, bunker stations, airlocks, and the like. The areas within the code-specified radii of the inlets and outlets of the ventilation systems for all of these components are considered to be hazardous zones. These must be kept clear of potential sources of ignition, entrances to accommodation, control spaces, ventilation inlets to non-hazardous spaces, and even some other hazardous zones. Some of these zones have radii of up to 4.5m, which makes compliance difficult on a relatively small vessel. Complicating matters even more, this project required the tugs to have an oil recovery ship class notation with tankage for recovered oil, storage locations for containment booms, and a skimmer. Each of these features has associated hazardous zones of their own that need to be considered in conjunction with the LNG system operation. The question becomes where does one find spaces on a 40m long tug that are suitably clear of deck machinery, entrances to accommodations, machinery exhausts, ventilation inlets and outlets, and other hazardous zones? These constraints must be properly considered during the earliest stage of design. Working decks can be compromised by ventilation pipes, thus obstructing critical views, limiting towline lead angles, and creating potential contact points with the attended vessel, not to mention poor aesthetic appeal. Finding the solutions for these new tugs was not easy, but by involving classification society Bureau Veritas from the earliest concept stages of the design through to its completion, it was possible to obtain an approval in principle prior to full class review. |
| Country | Norway , Northern Europe |
| Industry | Oil & Gas |
| Entry Date | 15 Oct 2016 |
| Source | http://www.lngworldshipping.com/news/view,robert-allen-discusses-challenges-of-lng-escorttug-design_44415.htm |
