The Yme oilfield is located approximately 100 km from the Norwegian coastline in the Egersund basin. The original field development in 2008 consisted on a jack-up type platform (MOPU) and a subsea storage tank at 93 m depth. Both are interconnected through a caisson containing all risers, piping and umbilicals which also supports a wellhead platform (WHM). That original MOPU was removed in 2016 [1] and a redevelopment of the field is currently under execution making use of some of the remaining facilities. In order to re-use the existing tank and caisson, advanced structural fatigue analyses including fracture mechanics, probabilistic assessment, motion monitoring and regular NDT inspection concluded with the requirement for a support structure to extend the service life of the old structure, which fatigue life was limited.

A screening process that covered different alternative solutions lead to a design (called Caisson Permanent Support or CPS) with overall dimensions (L.B.H) of approx. 70m × 50m × 40m and a dry-weight of 1300 t (figure 9). It consists on a beam structure composed of steel tubular braces with diameters ranging from 1,1 to 2,7 m that supports a clamp (6 m height and 5,15m diameter) connected to the caisson. In an iterative process between structural, fabrication and installation designers the optimal final concept was selected after detailed analyses to demonstrate that is capable of meeting the requirement of 20 additional years of structural service life for Yme facilities.

The CPS was successfully installed in June 2019 ensuring the integrity of the offshore structures and enabling the further re-development of the Yme field. Given its dimensions and its particular design purpose, the CPS is regarded as a unique project in the industry. This paper describes the challenges and the engineering work required for the design of the CPS.

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