Construction commencement of ART 100-42 Class infield support Rotortugs® for KT Maritime

December 4, 2014


Construction of the first of three Infield Support Vessels (ISVs) for the Shell Prelude FLNG Project has begun with a steel cutting ceremony held September 12th at ASL Shipyard Pte Ltd in Singapore. These vessels will support the Prelude FLNG project off the northwest coast of Australia and are being constructed for KT Maritime Services Australia Pty Ltd ( a joint venture of KOTUG International B.V. and Teekay Shipping Australia Pty Ltd).

These innovative vessels were designed by Robert Allan Ltd. in close collaboration with KT Maritime and will be the world’s first purpose-built FLNG ISVs. They will be powered by the patented Rotortug® technology featuring three separate azimuth propulsion units (2 forward, 1 aft). The tugs will measure 41.95 m by 16.00 m and develop over 100 tonnes bollard pull both ahead and astern.

The ART 100-42 Class ISV is a modern high performance tug custom designed to meet Shell’s demanding requirements. The initial concept design for this project was developed by Robert Allan Ltd in direct consultation with Shell Shipping (STASCO) in London. The vessels will be tasked with servicing Prelude’s LNG, LPG and condensate export activities, as well as forming an integral part of the facility’s security, emergency response, and evacuation requirements.

Key duties include:

  • escorting, berthing and un-berthing operations (working off either the bow or the stern)
  • FIFI Class 1 capability
  • floating hose handling operations
  • pilot transfer
  • security activities
  • rescue and emergency evacuation capability


The design of this unique vessel type presented some interesting design challenges to meet Shell’s functional requirements.

A Rotortug’s unique ship docking operation performs ship handling off of either the forward or the aft deck with large wrap angles of the towlines. However, this Rotortug had to also be arranged for all the other operational and emergency duties. This created quite a challenge in suitably arranging key equipment such as Fast Response Craft, Dacon scoop and deck crane, deck cargo crash rails, and hose handling equipment while not compromising towing operations. Some innovative solutions were thus required such as an 800 mm diameter vertically retractable towing staple and creative arrangements of hose handling equipment.

High environmental standards required all fuel oil, lube oil, and other pollutants to be located off the shell. This was extremely challenging for a 42 m vessel with fore and aft propulsion systems coupled with the very high fuel oil and fresh water capacity required for a 35 day offshore endurance, with reserve.

Shell also required the vessels to be able to operate in fairly significant environmental conditions. The Robert Allan Ltd. “RAstar” style hull form was thus selected due to its proven seakeeping performance in high sea states. The RAstar hull form provides considerable reduction in motions and accelerations compared to more standard wall-sided hull forms. To confirm the performance, an extensive model testing program was conducted. The hull demonstrated excellent sea keeping ability in all sea states.



To ensure that Shell’s bollard pull requirements were met with the triple drive arrangement Computational Fluid Dynamics (CFD) analysis was used to optimize the arrangement of propulsors for achieving optimal bollard pull in both directions.



The best possible visibility from the bridge is critical for day to day ship handling, hose handling, and pilot transfer operations as well as for emergency operations such as FRC deployment and Dacon scoop rescue. Thus, a detailed wheelhouse visibility study was conducted to ensure the clear sightline requirements were achieved. The Starboard forward quarter view shown below is an example demonstrating excellent visibility to the winch/staple, fenders, and forward side of vessel.



As is typical with all Robert Allan Ltd. designs, Finite Element Analyses (FEA) of all critical structural foundations were conducted to ensure that these are fit for purpose. Below is an image of the forward winch and staple foundations for one of the applied load cases.



The unique requirements of the Shell Prelude FLNG operation required a unique custom design solution that could perform the many critical support role and demanding functional requirements. Robert Allan Ltd. took the design challenges to heart in developing the unique and innovative ART 100-42 ISV design capable of satisfying an extremely demanding set of performance criteria.



For more information on the ART 100-42 Class tugs, or any other high-performance vessel designs developed by Robert Allan Ltd., please contact