SPAR MOORING

MAD DOG

Impact on mooring and riser design

API RP 2SM recommends a two-modulus method’ use a higher storm modulus for checking line tensions and a lower drift modulus for checking vessel offsets. Although this may be adequate for the mooring design, it may not be adequate for riser design in extreme events or everyday fatigue sea states. Further more, since the restoring force for a polyester taut line system is primarily derived from the extension in the line, it it important to adequately understand how the stiffness of the tether will behave over all expected condition.

Three main parameters appear to have an affect on the rope stiffness;

  • With increasing mean load the stiffness increases
  • With increasing load range, the stiffness decreases
  • With an increase in rate of loading period, the stiffness tens to decrease.
  •  

    Dynamic and static stiffness testing, results and analysis

    The standard API RP 2SM stiffness tests were conducted first followed by the additional dynamic and static stiffness tests. For the dynamic stiffness, mean load, load range and rate of loading were investigated. Tate of loading was investigated over a range of approximately 20-120 seconds, the range of wave and low frequency motions for the mad dog spar.

    In order to account for variability in test results, and impact of load range and other factors that impact stiffness, both the static and dynamic stiffness were bounded. The lower bound static drift stiffness was determined to be 18.5, and upper bound was 22.0. The dynamic stiffness is given by:

    Kd=(20.072+0.3028xLm)

    Where Lm is the mean load in terms of percent of MBL.

    Multiplying by 0.95 and 1.05 bound the dynamic stiffness.

    Mooring system properties and performance

    The mooring system was analyzed and designed using the lower bound static drift stiffness to determine mean offset to static components of the load. The dynamic offsets were then added by using the lower bound dynamic stiffness curve resulting from loads that were dynamic in nature. Similarly, to determine maximum line loads, the upper bound static drift stiffness was used to determine mean offset to static components of the load. The dynamic offsets were then added by using the upper bound dynamic stiffness curve resulting from loads that were dynamic in nature.

    The final mooring design resulted in 146 mm k4 ground and platform chain. The polyester rope is 270 mm in diameter with a MBL as prototype tested and calculated per API RP 2SM of 1889 tonnes. There is approximately 122 m of chain outboard the fairlead when the spar is in the nominal position and 76 m of ground chain from where the inverse catenary cuts out above the mudline to the lower end of the polyester. The length of polyester varies since the group 2 mooring cluster is down the escarpment, which is approximately 300 m deeper in water depth. Inboard lines in a group have one 13.7 m test insert and outboard lines have two inserts. Group1 and 3 have three segments of polyester per line while group2 has four. The polyester segments and top polyester connector to platform chain will be connected with specially designed H-link connectors. A sub sea mooring connector will also be used between the ground chain and pile chain.

    The mooring system meets all the safety factors set out including the additional checks not required by API RP 2SK. The mooring also satisfies the offset requirements dictated by the riser systems. The 100-year loop/eddy current, intact condition, controls the design for component strength while the 100-yer loop/eddy, damage condition, controls for offsets.          

    The Mad Dog drilling unit is located in 5,000ft to 7,000ft of water in Green Canyon blocks 825, 826 and 782 The Mad Dog drilling unit is located in 5,000ft to 7,000ft of water in Green Canyon blocks 825, 826 and 782
    The deck was designed around the heaviest hook load available (around 8,000t). The deck was designed around the heaviest hook load available (around 8,000t).
    The field is being developed by 12 wells produced with a single-piece truss spar. The field is being developed by 12 wells produced with a single-piece truss spar.
    The fabrication of the spar hull commenced in Finland in July 2002. The fabrication of the spar hull commenced in Finland in July 2002.
    The trus spar took three weeks to travel from Finland to Passagoula, Mississippi, on the Mighty Servant 1. The trus spar took three weeks to travel from Finland to Passagoula, Mississippi, on the Mighty Servant 1.
    Upending the Mad Dog truss spar. Upending the Mad Dog truss spar.
    Mad Dog truss spar hard tank sections. Mad Dog truss spar hard tank sections

     

    BACK