Next generation vessels to combat the future

Schiedam based naval engineering consultancy GustoMSC is actively engaged in offshore wind since the start of this century and now a world-leading jack-up installation vessel developer with almost 70% market share. We spoke with new concepts project manager Andries Hofman about past experiences and the huge challenges faced with next-generation vessels for future turbines up to 20MW incorporating 250-metre+ rotors.

A clear majority of the installation vessels still in service across European waters were built during 2008 – 2009, said Hofman. GustoMSC developed a substantial share of these semi-standardized vessels, typically 39 – 40m wide and 130 – 140 metres long, 60 – 90-person crew, and 800 – 1,000 tonne cranes. Main specifications were developed for 7 – 8 nacelles and blades of that time popular 3.6MW Siemens workhorse turbines. He added: “An unexpected constraint that emerged already during the development stage was that more powerful 6MW successor turbines came into focus. These were also larger and heavier, reducing the maximum number of stowed units to generally five or six. When this specific vessel generation entered the market, they were initially over dimensioned and therefore slightly more expensive to operate compared to older-generation smaller jack-up’s.”

Because installation vessels are designed for 20 – 25-year operating life, the investment must be recovered during that time-span for a valid business case. To meeting such long-term goal, it is essential that new-design vessels are already during their concept phase dimensioned for anticipated future turbine and/or foundation scaling demands. Main design focus areas include creating sufficient deck space and payload, surplus jacking and crane capacity, and building-in enough vessel structural reserves for accommodating future demands.

Hofman: “It remains remarkable that installation vessels developed over a decade ago for installing 3.6MW turbines remain in active service until today, which is largely thanks to ongoing vessel adaptation, upgrading and optimizing.”

He directly refers to upgrades of initial 6 – 8MW turbines with 150 – 164m range rotors into today’s 8.0 – 9.5MW successor models characterized by only modest intermediate rotor size increments. Most common installation vessel upgrade is fitting larger capacity cranes, especially for handling largest and heaviest monopiles now up to 1400 tonnes.

Taking the topic further, Hofman said that 10- 15MW turbines with up to 220m rotor sizes are expected in the next 1 – 3 years representing new challenges of completely different magnitude. Thirty to fifty-metre rotor diameter increase means for instance 15 – 25m hub height increment. And head mass (nacelle plus rotor) could on average increase by hundreds of tonnes to around 750 – 850 tonnes, based upon commonly used 60 – 65 tonne/MW specific head mass figures.

GustoMSC third-generation self-propelled jack-up installation vessel capable of installing these future giants is in development. These masses and dimensions already represent a main challenge, said Hofman, but this also only one side of the coin: “Because the new installation vessel generation must serve again 20 – 25 years for a sound business case, we had to look much further ahead. This meant trying to anticipate at 20MW class offshore turbines, even though these only barely exist on paper. Our reference is a 20MW turbine concept with 250m+ rotor diameter developed by DTU (Danish Technical University).”

He added that such third-generation vessel will measure about 140m x 50m (L x W), with over a hundred crew. For matching the installation requirements of these future 20MW turbines with corresponding hub heights, GustoMSC developed an innovative telescopic boom crane. It will be able to cope with multiple technical challenges, including an even larger 30 – 45m hub height increase. This in turn requires sufficient crane boom reach well above hub height for accommodating the hook and additional hoisting gear, and it must meet new boundaries regarding nacelle (hoisting) mass. The impressive crane specifications include a maximum 2,500-tonne hoisting capacity of installing very heavy next-generation (monopile) foundations but with retracted crane beam, and nacelles up to 1,250T with fully extended beam length. This must all be achieved without increasing the jack-up size tremendously for accommodating the long crane boom.

He further argues that a large-scale modern jack-up can be viewed functionally as a large crane on legs. This adds to the overall complexity of translating future turbine sizes and masses in main design specifications for the installation vessel. Hofman: “A ‘Leg Encircling Crane’ is mounted around the jack-up’s rear port side (left, facing forward) leg. During hoisting operations, the beam commonly extends across the ship deck over the starboard side to a turbine being installed. The shortest distance in between the Jack-up legs is a measure of vessel stability in general and during hoisting. If a vessel operates in deeper water the hull must be made wider, with as extra benefit offering increased deck loading space.”

Additional design measures are required for maximum stiffness and strength properties of the interface between the box-type structures accommodating the leg’s supports and jacking systems, and the legs themselves. Large-scale jack-up’s destined for deep-water deployment now commonly comprise ‘open’ lattice-steel triangular shaped legs instead of in tubular steel-type for enhanced sailing stability, and minimized wave and sea-current loading when standing. He added: “These operational impacting factors plus a stable leg-ship interface contribute positively to minimizing hull and crane hook movements. This latter phenomenon itself remains a formidable technical challenge for any supersize ship crane operating in highly dynamic marine environments.”

Increased hull length together with reduced hull width on the other hand promotes superior sailing characteristics, improved fuel efficiency, and better manoeuvrability during port access. Another design challenge is that hull ‘own’ mass (displacement light) is supplemented by varying load consisting of fuel, ballast water, drinking water, and consumables. Combined mass and mass variation must be considered for floating and during jacking-up and jacking down, while water depth is a main driver for ‘standing’ and floating Jack-up stability.

A final topic discussed was on the importance of enhanced communication protocol between Jacking Master and marine consultant responsible for windfarm’ geotechnical surveying. For ensuring safe crane operation it is in Hofman’s view essential that the former has received accurate info over maximum allowable soil loading, prior to jacking-up start and before commencing crane operation. “Not complying fully could cause a punch-through during jacking-up and/or hoisting, with total ship loss the worst consequence. There is still no formal certification procedure for Jacking Masters, whereas any car driver requires a driving license before being allowed to enter the roads. This is a main tension and focus area we will continue bringing to the attention of marine-technical and legal parties involved”, he concluded.

The commercial offshore wind era commenced with Denmark’s 160MW Horns Rev I windfarm. Offshore wind entrant A2SEA’s pioneering M/V Ocean Hanne and M/V Ocean Ady spearheaded offshore installation vessel design and shaped later industry thinking on cost-effective turbine and foundation installation methods. The clever concept involved converting two cargo vessels into installation vessels by welding individual external box-type structures to the hull for accommodating the four legs and jacking mechanisms. This conversion was done during 2001 – 2002 prior to installing all 80 turbines and monopiles. Hofman: “During this pioneering phase there was considerable uncertainty on the future viability of offshore wind. A solution whereby vessel conversion modifications could be undone was within this context considered not a bad idea.”

GustoMSC’s first project engineering involvement was with the six-legged MPI Resolution launched in 2003. This world’s first self-elevating self-propelled installation jack-up had a 300-tonne crane (insert 2) and was principally designed like a merchant vessel. “We had already extensive experience with oil & gas platforms and were called in for designing the legs, the ship-leg interface supporting structures and a new in-house design double-action jacking system. Both experiences above all provided increased understanding that offshore turbine and/or foundation installation does require specialized hardware and an integrated overall design approach”, he added.

GustoMSC’s first in-house designed self-propelled jack-up was Wind Lift I for Bard Engineering, a 102-metre long and 36-metre wide vessel for water depths up to 45m and with a 500-tonne crane. Wind Lift I was further the first installation vessel equipped with GustoMSC’s patented double-action jacking system. Hofman: “This solution is much faster compared to a conventional single-action rack solution. It reduces the non-working period in between (turbine) installation positions, and minimizes the high-risk period when the hull is in an intermediate position between floating and fully standing. If a high wave passes underneath the hull in this floating-standing position, the exposed structure could be lifted and thrown back again with considerable leg damage risk.”

The Leg Encircling Crane is a second major Gusto-MSC in-house innovation, which was first applied at the Crowie crane platform during 1960. It was first re-applied at the second-generation Sea Installer (2012), and now a semi-standard wind industry solution offering maximized deck space and superior crane manoeuvrability compared to a deck-mounted crane.

Intermediate exchange of original cranes to higher rated successors has become recurring offshore wind industry practice. It illustrates the complexity and uncertainties surrounding the rapid technology and market development.

In the early years, alternative solutions have been implemented as well. MPI Resolution’s 300-tonne crane capacity proved for instance insufficient soon after commercial deployment. This crane was laid out for 2MW and lightweight 3MW Vestas V90-3.0 MW ‘successor’ nacelles, but could not handle 400 – 600-tonne monopiles. GustoMSC was asked for a remedying solution but by retaining the crane. Hofman: “This problem was solved by a pivot able deck-mounted structure enabling a pile to be brought from horizontal stowage into vertical installation position. The overall solution involved a pile gripping/guiding device, and a control strategy whereby the hull would be lowered synchronous with the pile for preventing crane overloading. We developed a comparable solution for Bard Engineering, this time for handling and ramming the three piles of their standard Tri-pile foundation.”

This article was previously published in the Offshore WIND magazine, issue 1, 2019.