Towing - all about the preparation
HR Wallingford and its Australia Ship Simulation Centre in Fremantle have shed interesting light on the degree of planning involved in the towing and positioning of the largest offshore floating facility ever built.
The towage industry has been handling the transport of large vessels and odd-shaped structures since the dawn of ocean towing. Perhaps the most noteworthy period was in the 1960s and ‘70s when records were successively broken with the enormous offshore oil and gas production platforms towed from manufacturing sites in Norwegian fjords and Scottish lochs to their permanent locations in the North Sea.
Extensive planning was naturally involved in these early days but state-of-the-art technology now allows detailed simulation of such operations where tug masters and pilots can simulate various scenarios and evaluate the effects of elements such as environmental changes and unplanned events.
UK-based HR Wallingford is an independent civil engineering and environmental hydraulics research institution providing practical solutions to water-related challenges. It is a truly global organisation and includes the Australia Ship Simulation Centre in Freemantle and these resources were used recently in planning the month-long towing of Shell’s Prelude FLNG facility from its builder’s yard in South Korea 5,800km to its permanent location at a remote gas field 475km off the coast of Western Australia.
Catchy comparisons are used to describe the formidable structure that is the Shell Prelude FLNG facility. At 488m in length it displaces the same amount of water as six of the world’s largest aircraft carriers and consumes 50m litres of cold water every hour to help cool the natural gas but it is the sheer dimensions of the vessel that required careful planning for the tow to site.
The tow was entrusted to Singapore-based POSH Terasea with PACC Offshore Services Holdings in charge of the operation with three ocean-going tugs employed for the journey and a fourth accompanying and in reserve during the voyage.
The Australia Ship Simulation Centre created a detailed navigation simulation of Prelude which was used to prepare the tow masters, tug masters and pilots, allowing them to familiarise themselves with how the FLNG manoeuvres at sea. HR Wallingford’s involvement included real time navigation simulation for Prelude’s departure from the Geoje Shipyard in Korea, along the Busan Channel and the positioning and mooring once on site.
Also included were berthing and departure simulations of the LNG, LPG and condensate offtake tankers that will moor alongside the FLNG along with the provision of ongoing pilot and tug master training. A bespoke web-based decision support tool was also supplied to assist with operations planning. Up to six integrated simulators at the Ship Simulation Centre were used to simulate Prelude and the tugs for the departure operation and positioning during connection of the mooring lines once at the installation site.
Actual wind, wave and tidal conditions were recorded and then modelled allowing the crew to accurately test the capability and power of the tugs in advance. The simulated positioning element was used to prepare for the real-life operation in which the tugs were attached to the 700m long towing wires weighing approximately 30t tonnes each.
Captain Roy Lewisson, master of Deep Orient, the vessel that connected Prelude to its 16 mooring lines, and who took part in the simulator training said: "Being able to accurately test the maneuvering beforehand was a real advantage. Never before in oil and gas history have we had the chance to practice in the simulator before we get on the water."
Dr Mark McBride, HR Wallingford’s Ships Group manager, said: "There was a need to assess many aspects of this unique offshore floating facility, which included the maneuvering issues associated with the arrival and departure of the offtake LNG carriers. For this we used real time navigation simulation, so that we could identify the limiting conditions for safe maneuvering, as well as the tug requirements, and for developing appropriate maneuvering strategies."
By Peter Barker
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