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tower extends above the free surface and accommodates a deck and a fluid swivel.

               In  deeper  waters,  it  is  often  advantageous  to  introduce  double  articulation,  the

               second one being at a mid-depth.

                   The  articulated  tower  is  used  as  a  single-point  mooring  system  (SPM)  to

               permanently moor storage and production tankers or is utilised as a mooring and

               offloading medium for a shuttle tanker. The tower must survive its lifetime storm

               as well as the operating sea when attached to the tanker. Fatigue is an important

               criterion for this type of system. In intermediate water depths, the structure may

               need to be treated as a flexible structure for the fatigue stress evaluation. In fact,

               the earlier SPMs built for Petrobras offshore Brazil failed in fatigue near the J-tube

               entrance and had to be de-commissioned after only a few years of operation.

                   3.2.2 Compliant Piled Tower (3%). A compliant piled tower is similar to a


               traditional  platform  and  extends  from  surface  to  the  sea  bottom,  and  it  is  fairly
               transparent to waves. However, unlike its predecessor, a compliant piled tower is


               designed to flex with the forces of waves, wind and current. It uses less steel than a
               conventional platform for the same water depth.


                   3.2.3 Guyed Tower (8%). A guyed tower is a slender structure  made up of
               truss members, which rests on the ocean floor and is held in place by a symmetric


               array of catenary guylines. A guyed tower may be applicable in deep hostile waters

               where the loads on the gravity base or jacket-type structures from the environment

               are  prohibitively  high.  The  guylines  typically  have  several  segments.  The  upper

               part is a lead cable, which acts as a stiff spring in moderate seas. The lower portion

               is a heavy chain with clump weights, which are lifted off the bottom during heavy

               seas and behaves as a soft spring making the tower more compliant.

                   Exxon in 1983 installed the first guyed tower named Lena Guyed tower in the

               Mississippi Canyon Block in 1000 ft (300 m) water depth. It resembles a jacket

               structure, but is compliant and is moored over 360" by catenary anchor lines.






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