THE JACKET - A PATH TO DEEPER WATERS

Water depths: 25 - 120 meters Weighs up to 828 tonnes Advantages

Very good resistance to overturning Light and efficient construction

Disadvantages

Many man-hours of we ding !omp icated transportation to site

When power companies "egan to oo# at deeper waters for insta ing wind tur"ines they had to consider a ternative support structures$ %hus& the 'ac#et structure entered the sector and moved the "oundaries$ (nti 200)& other structures such as the monopi e and gravity-"ased structures had on y "een a" e to put wind tur"ines at a water depth of 20 meters *+arrow& (,& 200-.$ +ut the +eatrice demonstrator pro'ect changed that$ Ma#ing a eap from 20 to /5 meters water depth& it strong y suggested that the 'ac#et structure had something to offer in terms of arge depths$ %he concept of 'ac#ets is inherited from the oi and gas industry$ 0ac#ets have "een used for supporting rigs at a depth of more than 100 meters$ The structure 1 'ac#et is made up of three or four main egs& connected to each other "y "racings$ 1 e ements are tu"u ar un i#e onshore attice structures which are usua y made from angu ar profi es$

! ic# here to view arge image Figure 1: Terminology used on the jacket. Illustration: LORC, Al ha !entus

%he "racings and egs are connected up into tu"u ar 'oints 2 a critica component of the construction$ %he tu"u ar 'oints have the shape of the etter 3,4 at the egs& the etter 354 where the "racings cross& and the etter 364 at the top and "ottom$

1t present the tu"u ar 'oints are we ded 2 often referred to as we ded nodes$ +ecause of the many 'oints& there is a arge amount of we ding which is performed manua y$ 7t ta#es a ot of man-hours to comp ete the 'ac#et$ %he we ded nodes are the wea# points in the construction in terms of fatigue$ 8atigue occurs under dynamic oading and eads to materia damage of the stee structure$ 7t is especia y at the we dings that fatigue is most i#e y to occur$ 1 so ution to this is to use cast nodes instead$ !ast nodes decrease the fatigue pro" em of the structure since the num"er of we ding seams is decreased$ 9owever& producing cast nodes is more difficu t than we ding$ 0ac#et egs can have a diameter of more than one meter 2 in some cases up to 1$/ meter 2 so the mo d for the cast node itse f has a significant si:e$ +racings usua y have a diameter of ess than one meter$ %he techni;ue for casting in these dimensions is advanced$ 1t present& cast nodes e<ist in prototypes onshore 2 such as in this concept from the =erman company Weserwind$ The Transition Piece Li#e the monopi e& a 'ac#et needs a transition piece to support the wind tur"ine tower$ %he transition piece a so inc udes the wor#ing p atform 'ust "e ow the wind tur"ine tower$ +ut un i#e the monopi e concept& the transition piece on 'ac#ets does not have to eve the construction$ Leve ing is done at the sea"ed$

! ic# here to view arge image Figure ": Terminology used on and around the transition iece. The T# connects the jacket legs to the to$er. Illustration: LORC, Al ha !entus

! ic# here to view arge image %orkers on the lat&orm o& a transition iece &or the Ormonde o&&shore $ind &arm. #hoto: 'en (arden

%he transition pieces so far insta ed have a wide top& they are typica y > meters ta *pages /?/5& %a isman @nergy 200-.& and weigh 1-0 tonnes$ Secondary steel +esides the egs& "racings& and 'oints& the 'ac#et consists of other e ements& ca ed secondary stee $ Aecondary stee inc udes:

Wor# p atform Ladders and stairs 1ccess systems& i$e$ "oat anding 0-tu"e and ca" es !orrosion protection systems

Aecondary stee typica y weighs appro<imate y 150 tonnes$ The oundation 1t the sea"ed& the structure is often attached into the ground using pi es& "ut gravitation "ases or suction anchors are a so possi"i ities$ %he foundation of a 'ac#et with pi es can "e carried out as either post-pi ed or pre-pi ed$ Post-!iling 7n the more traditiona way of insta ing a 'ac#et& the post-pi ing process& the pi es are driven through s eeves at the "ottom of the 'ac#et egs$ %he pi es themse ves might "e hammered or vi"rated into the sea"ed after the owering of the 'ac#et$

! ic# here to view arge image Figure ": The ost) iling rocess. *ainly used in the oil and gas industry.

%ypica y& the connection "etween the s eeves and the pi es is secured with grouting$ %he gap "etween the s eeve and the pi e is fi ed out with a specia grout materia & which transfers oads from the 'ac#et eg to the pi e$

%he connection can a so "e secured using swaging& a co d forging process& where the diameter of the inner tu"e *the pi e. gets e<panded unti it esta" ishes a safe connection to the s eeve$ %he inner tu"e is e<panded using a die *a specia i:ed too used in manufacturing industries to cut or shape materia using a press. or using high pressure water$ %he swaging process is descri"ed thorough y "y Bi Atates 7ndustries$ %oday post-pi ing is se dom used in wind farms$ Bn y in the +eatrice wind farm& where 'ac#ets have "een used for wind tur"ines for the first time& was the insta ation carried out using postpi ing$ 9owever& in the oi and gas industry post-pi ing is wide y used$ %his is "ecause the oi and gas industry typica y on y re;uires insta ation of a sing e structure& whereas wind farms re;uire insta ation of many simi ar structures$ Pre-!iling Curing the pre-pi ed insta ation of a 'ac#et& a temp ate is used when hammering or vi"rating the pi es into the soi $ Bn y after the pi ing process is the 'ac#et owered to the "ottom of the sea& where the spi#es at the end of the egs fit into the pi es$

! ic# here to view arge image Figure +: #re) iling the jacket. A tem late is needed, ,ut it is &aster &or re eated installations.

%he connection "etween the spi#es *'ac#et egs. and the pi es is made with grouting or swaging& "ut a so other connection concepts& i#e Di e Euic# !oup ing from Leenaars +V& have recent y entered the mar#et$

! ic# here to view arge image At Al ha !entus the jackets $ere re) iled. -otice the s ikes on the ,ottom o& the structure. #hoto: Al ha !entus

Dre-pi ing is considered to "e a faster method than post-pi ing$ With pre-pi ing& sma er vesse s can "e used for the pi ing and the arge vesse s can "e emp oyed very efficient y 2 they use itt e time to set the 'ac#ets into the pre-insta ed pi es$ With post-pi ing& the e<pensive arge vesse s have to spend more time with each 'ac#et$ 7n addition& post-pi ing re;uires s eeves mounted on the 'ac#et as we as so-ca ed mud-mats$ %he mud-mats transfer oads to the sea"ed and act as the supporting foundation whi e the pi es are "eing insta ed$ With pre-pi ing& the pi es can underpin the structure$ 1t +eatrice 2 a postpi ed structure 2 the s eeves a one weighed 1-0 tonnes *Aeide & 200).& so a considera" e amount of stee can "e saved "y using pre-pi ing$ %he cost for the pre-pi ing temp ate needs to "e ta#en into account and compared to the cost for the s eeves& "ut at a reasona" e num"er of insta ations there is a cost "enefit of using a temp ate instead of mounting s eeves on every 'ac#et$ %he hammering of pi es into the sea"ed with hydrau ic hammers is& however& e<treme y noisy$ Fising concerns a"out the hea th of fish and sea mamma s means that new insta ation restrictions are going to "e enforced in @urope in order to mitigate the noise$ 1 third method 2 though not one used in wind farms so far 2 is using suction anchors instead of pi es$ %his was proposed at +eatrice& "ut u timate y %a isman @nergy chose a pi ed so ution$ 1nd fina y& one cou d use a hy"rid construction with gravity "ases at the "ottom 2 simi ar to having concrete feet for the structure$ %his has not yet "een seen on wind farms$ "nstallation 1s descri"ed a"ove& 'ac#ets demand thorough preparation of the sea"ed$ 7n most cases& a temp ate is used to drive down the pi es ahead of insta ation$ %he temp ate is essentia for driving the pi es at an accurate distance from each other$ (sing post-pi ing ma#es the temp ate unnecessary$ %he 'ac#et has to "e eve within the standard margins of 0$5 degrees$ (sua y& a Femote y Bperated Vehic e *FBV. is used to measure the height of the pi es when they have "een insta ed

with the temp ate$ %hese measures are ta#en into account when producing the 'ac#et so that height deviations from the pi es can "e eve ed out "y manufacturing the 'ac#et egs and spi#es in engths that compensate for the deviations$ %he 0ac#et with the transition piece mounted is then p aced on the pi es$ 8or this procedure a vesse with a heavy ift crane is needed$ %he connections "etween pi es and 'ac#et are then grouted *or swaged.$ %ransporting the finished 'ac#ets to site is itera y a "ig thing$ 1t 1 pha Ventus& three 'ac#ets were transported on one "arge and this is the record$ +eing around 50 meters ta and weighing "etween 500 and 800 tonnes& not every "arge and crane can insta the 'ac#et$ 1t 1 pha Ventus& the gigantic %hia f from 9eerema Marine !ontractors was used$ 1t +eatrice and Brmonde& the Fam"i: from Aca dis did the 'o"$ %he transition piece is insta ed "efore transport to sea and then the tower and tur"ine in se;uence$ +ut at +eatrice a remar#a" e so ution was chosen: the transition piece& tower& and tur"ine with rotors was assem" ed on the ;uay and sai ed to site in one piece$ 9owever& this method has not "een repeated at other sites$ 1 graphica wa #-through of the pre-pi ed insta ation at 1 pha Ventus can "e found here$ %he post-pi ed insta ation at +eatrice is e<p ained in this video: Gotice that the insta ation at +eatrice is considered a"orious and has "een optimi:ed in ater wind farm insta ations$ "#!rove#ents 7n future one might see transition pieces without the wor#ing p atform and the "racings at each corner of the 'ac#et as a resu t of more integrated designs$ %his is suggested "y Fepower and Weserwind with this prototype of a narrow transition piece which on y weighs appro<imate y -5 tonnes$ 1nother possi"i ity is that the 'ac#et and tower are merged into one construction& rendering the transition piece at sea eve superf uous$ %his is the concept from the Cutch company 2-+ @nergy$ %hey operate with the design of a three- egged 'ac#et or attice structure from sea"ed to nace e$

RE$ERE%CES

@i:e de Vries: 3Wind in deep waters: Wind foundations and specia structures4& Fenewa" e @nergy Wor d *200>. %a isman @nergy: 3@nvironmenta statement4& +eatrice wind farm demonstrator pro'ect *200-. Fi##i Atancich: 4=eoAea: Gew techno ogy drives down tur"ine foundation insta ation costs4& Wind @nergy (pdate *2010. Marc Aeide : 3%ragstru#tur und 7nsta ation der F@power 5M in /5m Wassertiefe4& Atah "au )-& 9eft > *200). W$ @$ de Wries: 31ssessment of "ottom-mounted support structure types4& Dro'ect (pWind *200). Bffshore Atandard: 3Cesign of Bffshore Wind %ur"ine Atructures& CGV-BA01014& Cet Gors#e Veritas *2010.