Why is stabilisation of floating wind turbines holding back their development?

The Mul­tian­nu­al Energy Pro­gramme aims to increase elec­tri­city gen­er­a­tion from off­shore wind to around 59 TWh by 2035, thereby exceed­ing cur­rent onshore wind gen­er­a­tion¹. At sea, whilst float­ing wind tur­bines offer sig­ni­fic­ant poten­tial for France, only a few pilot wind farms have been com­mis­sioned to date. France’s cur­rent wind farms con­sist of tur­bines fixed to the seabed. To bet­ter under­stand the mari­time chal­lenges facing the wind power sec­tor, Luc Pas­tur, an asso­ci­ate pro­fess­or at ENSTA and a spe­cial­ist in flu­id mech­an­ics, answers our questions.

Luc Pas­tur. Float­ing wind power is not yet com­mer­cially viable, as sta­bil­ising the wind tur­bines remains one of the main chal­lenges. Cur­rents, waves and winds cause the struc­ture to move in vari­ous ways: rolling, pitch­ing and heav­ing – the lat­ter being par­tic­u­larly dam­aging. This causes struc­tur­al fatigue, thereby accel­er­at­ing age­ing whilst redu­cing energy pro­duc­tion effi­ciency. Man­u­fac­tur­ers are there­fore seek­ing to min­im­ise these move­ments, par­tic­u­larly to meet the per­form­ance require­ments of operators.

Heave is a ver­tic­al move­ment of the wind tur­bine, which can be caused by adverse sea con­di­tions or winds. A float­ing wind tur­bine is a struc­ture that oscil­lates in the wave field at its own nat­ur­al fre­quency. When the swell has the same fre­quency as the wind tur­bine [editor’s note: i.e. the same num­ber of oscil­la­tions per unit of time], the oscil­lat­ing motion enters res­on­ance: the oscil­la­tions are then amp­li­fied. This becomes prob­lem­at­ic when the motion exceeds a threshold amp­litude defined by the man­u­fac­turer. Heav­ing affects all float­ing wind tur­bines, but semi-sub­mers­ibles are par­tic­u­larly vulnerable.

Yes, one of the pre­ferred solu­tions is the addi­tion of heave plates. These large steel plates, sev­er­al cen­ti­metres thick, are placed beneath the floats. When the wind tur­bine oscil­lates ver­tic­ally, it drags a mass of water with it, which increases the turbine’s iner­tia, mak­ing it harder to set in motion. These plates also help to dampen the oscil­la­tions thanks to the flow pat­terns they gen­er­ate around them.

Two phys­ic­al mech­an­isms are at work. On the one hand, the oscil­lat­ing motion of the plate in the flu­id [editor’s note: sea­wa­ter] induces areas of low pres­sure beneath the plate. This gen­er­ates an ini­tial struc­ture: what is known as form drag. On the oth­er hand, plate-edge vor­tices are cyc­lic­ally released above and below the oscil­lat­ing plate. All these flow struc­tures take with them a por­tion of the kin­et­ic energy drawn from the plate’s motion, which helps to dampen the plate’s motion.

Sci­ent­ists are work­ing to gain a bet­ter under­stand­ing of the flow struc­tures induced by heave plates. By improv­ing our under­stand­ing of the phys­ic­al mech­an­isms at work in damp­ing the oscil­la­tions of off­shore struc­tures, we will be able to devise even more effect­ive devices.

We have, for example, found that damp­ing is optim­al when the plate is per­for­ated with holes cov­er­ing 10 % of its sur­face area. The holes give rise to a new effect dur­ing the plate’s ver­tic­al oscil­la­tion: flu­id jets are pro­duced through the holes. The shear­ing res­ult­ing from these flow struc­tures degen­er­ates into tur­bu­lence and gives rise to a mul­ti­tude of vor­tic­al struc­tures, right down to very small scales. The smal­lest vor­tices are ulti­mately dis­sip­ated vis­cously. This cas­cade of energy, drawn from the energy of the plate’s oscil­lat­ory motion, is likely to con­sti­tute an import­ant pro­cess in the damp­ing mechanisms.

Heave plates are a pass­ive damp­ing tech­nique that does not require an extern­al energy source. Act­ive tech­niques do exist; not­ably, it is pos­sible to coun­ter­bal­ance heave by con­trolling the rota­tion of the blades via the rotor. This relies on an act­ive con­trol sys­tem, which there­fore con­sumes energy, and acts on the tur­bine and blade shafts based on meas­ure­ments taken of the float­ing structure’s move­ment. How­ever, as this tech­nique is more expens­ive and places great­er strain on the rotor, it should prob­ably be regarded as a sup­ple­ment to heave plates.

Bey­ond these tech­no­lo­gic­al hurdles, the devel­op­ment of float­ing wind tur­bines requires care­ful con­sid­er­a­tion of the entire indus­tri­al chain. Pro­duc­tion sites, the cost of mater­i­als, their trans­port, the assembly of the tur­bines, off­shore moor­ing, etc. all these stages of the value chain must be care­fully con­sidered to ensure the com­mer­cial devel­op­ment of the sector.

Interview by Anaïs Maréchal