Offshore Marine Technology: 1st Quarter 2017
It’s been a tough couple of years for heavy-lift vessel owners and operators, with offshore oil and gas platform installation projects grinding to a halt. According to industry analyst Douglas-Westwood (DW), the number of fixed offshore assets installed in 2017 will be approximately 45% fewer than those recorded in 2014.
However, DW has identified a couple of lifelines for heavy-lift specialists. The first is the decommissioning market, which will surely depend on vessels with the lifting power to safely dismantle large offshore platforms.The second is the offshore wind farm sector, especially with the arrival of more powerful, next-generation turbines. This consideration would also appear to have informed the development of the SOUL heavy-lift jack-up concept, which has been jointly formulated by Dutch marine consultancy SeaOwls and Norwegian shipbuilder Ulstein. It is the developers’ hope that the SOUL concept enables renewable energy contractors to install next-gen wind turbines – featuring capacities of 10-12MW apiece – within the typical timeframe for current 6-8MW turbine installations. The scalable nature of the SOUL concept permits a variety of customised jack-up sizes, to enable the carriage of three to six 10-12MW turbines.
Additionally, its design will enable it to fulfil a range of oil and gas-related offshore decommissioning tasks – including, for instance, the transportation of large monopoles and jackets and transformer substations.
Customers will also be able to specify varying crane sizes, variable loads and deck layouts, Ulstein and SeaOwls confirm. Presenting the concept to the public in Q1 2017, however, the companies provided the SOUL 8800 as an example of a potential model.The SOUL 8800 would feature: a length of 164m; a beam of 104m; a hull depth of 13m; a deck area spanning 9,000m²; and a variable deck load (VDL) of 16,000tonnes. Boasting approximately 36MW of power, the vessel would rely on a DP2 system for positioning. Its VDL would enable the SOUL 8800 to carry six 10MW-capacity or 12 6MW-capacity wind turbine generators. This variation of the SOUL concept would accommodate 90-130 persons.
A lighter version has also been proposed, in the form of the SOUL 5250; this would measure, roughly, 120m x 90m, and would feature a VDL of 9,000tonnes, enabling the carriage of three 10MW and six 7MW turbines.
According to Erik Snijders, founder and MD of SeaOwls, the idea of entrusting heavy-duty turbine installation to a floating, ‘legless’ vessel was dismissed as impractical, compared to an “optimal jack-up design” – namely, “a square platform with the legs spaced out as much as possible”. Snijders adds: “Rotating the platform by 45degs provided a natural bow shape with two legs and the crane on vessel centre line.”Another notable feature of the SOUL jack-up concept is that it will not carry ballast water. Instead, to optimise stability, the design team has ensured that cargo is carried in the vessel’s centre, so that the jack-up can retain its balance at all stages during the installation process. Extra stability is conferred via the SOUL’s larger-than-usual beam.
Considerations regarding weight, and therefore cost reduction, proved critical to the design process. Both companies have conceived the SOUL on the basis that “the ratio of variable deck load [VDL] over light ship weight [LSW] determines the cost-effectiveness of a jack-up vessel”. Basically, the size of the VDL reflects the vessel’s cargo-carrying capacity, hence its profitability: the ability to carry more turbine-related items (or oil and gas-related equipment) on the deck makes the vessel a more cost-effective option for charterers. A vessel that can carry more equipment needs to embark on fewer journeys between shore and offshore site to provide effective installation support, thus reducing both project times and fuel consumption rates.
At the same time, incorporating more steelwork into the jack-up’s structure not only increases the overall weight (or LSW) of the unit, but directly affects its VDL in a negative way, meaning that the heavier and more ‘built-up’ the jack-up is, fewer items of turbine equipment can be carried. Developing the SOUL has therefore entailed a fine balancing act between these two considerations.
As a rough guide, with conventional jack-up vessels, approximately 35% of its overall weight will be dedicated to VDL – meaning that the wind turbine equipment/parts it carries on deck will not exceed 35% of total unit weight. VDL increases somewhat when one factors in a self-propelled jack-up barge, where the ‘VDL over LSW’ ratio increases to approximately 40%.It is Ulstein’s and SeaOwls’ aim, however, to realise a VDL rating of 45% with the SOUL concept, which would effectively make the unit one of the most competitive and cost-effective assets on the market.
Overall lightness would be achieved by a cruciform structural layout, which, Ulstein says, would make the completed SOUL more than 10% lighter than conventional jack-up designs.Bram Lambregts, deputy MD at Ulstein Design & Solutions, adds: “This seemingly simple twist in the design allowed us to make a huge improvement when it comes to operational aspects too. With the main crane around the stern leg, optimal main deck reach and over-the-side lifting capabilities are created – and, as the hull now houses much larger leg footings, bearing pressures on the seabed are reduced, while the wake of the spud cans does not interfere with the inflow to the propulsion thrusters.”
