This paper describes the results of the first phase of the project Forces On Complex Offshore Structures (FOCOS) jointly initiated by Danish Hydraulic Institute and Danish Maritime Institute with the ambitious objective to quantify the hydrodynamic load effects for various complex offshore structures, to assess the validity of current design methods and subsequently enhance them

The main objectives of this phase of the project have been

  • To examine the relative importance of the individual structural components on the total wave force and overturning moment for a 4-legged jacket including conductors and conductor frames (shielding- and blockage effects)

  • To evaluate whether current design practice for jackets is conservative

An extensive model test programme has been carried out In order to determine the hydrodynamic forces on a jacket In 2- and 3-dimensional waves The following configurations have been tested

  • Jacket side (vertical plane frame)

  • Jacket (space frame) with and without Internal members and with varying number of conductors

The recorded maximum total shear forces and overturning moments have been plotted as function of the complexity of the structure In order to evaluate the spatial effects and the shielding and blockage effects

Furthermore, the recorded maxima have been compared to ‘state of practice’ computer calculations where the largest wave In the Irregular sea state as described by the Stokes 5th order wave theory has been stepped through the structure, and base shear and overturning moment has been determined by means of Morison's equation neglecting member Interference effects

The comparison lends support to the use of CD = 0 7 In ‘state of practice’ design calculations for complex jacket configurations However, for structures with limited extension in the water plane current design practice might yield non-conservative hydrodynamic loads


Structural design or reappraisal of an offshore structure relies on a number of assumptions with inherent uncertainties that have to be considered closely In order to establish the specifications which the structure must comply with Environmental conditions at the site are In this context naturally of fundamental Importance and will consequently have a decisive Influence on the design specifications Estimation of the environmental loads is a particularly difficult task involving selection of kinematics mode is and force coefficients

Offshore jackets consist mainly of tubular joined together Current design practice for this type of structures IS to design them to withstand the load from the most probable maximum wave In the sea state that has a recurrence period of 50 or 100 years, see for Instance DIF (1983) or API RP2A (1987) The design wave IS modeled by Stokes 5th order wave theory, Skjeibrela & Hendrickson (1962), or Deans stream function wave theory, Dean (1974) The design wave IS stepped through the structure and the hydrodynamic load is determined by means of Morison's formula, Morison et al (1950)

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