MAD DOG
The Mad Dog Project will use a polyester mooring system on the drilling and production truss spar. This will be the first use of a permanent polyester mooring system on a Floating Production System (FPS) outside of Brazil and the first time polyester has been used on a spar.
Challenge:
Project overview
The Mad Dog spar is located in Green Canyon Block 782. The water depth is approximately 1384 m at the spar nominal position. The spar topside is designed to produce 80,000 barrels of oil per day and 40 million standard cubic feet of gas per day. The truss spar hull lightship weight is about 19,000 tonnes with a diameter of 39 m, overall length of 169.2 m and hard tank length of 74.7 m. Freeboard once installed is 15.2 m. The spar moonpool is designed for 13 top tension production risers and one dedicated slot for the drilling riser.
Design overview
The mooring system is composed of 11 lines arranged into three groups; two groups with four lines and one group with three lines. The selection of three groups rather than four minimized the number of foundations required below the escarpment where the geology is highly variable making the foundations difficult to design and install. Group 1 required only three mooring lines since directional wind, wave and current were used thus producing less loading on these lines. Suction piles will be used for the mooring line foundations.
The mooring system was designed to API RP 2SK and 2SM. The safety factor on the polyester components was increased by 20% over what 2SM recommends. The 20% additional safety factor on the polyester can be viewed as a material factor to account for any unknowns and is inline with what other operators use. In addition, a two-line failure case was examined to the 10-year return period event with a safety factor of 1.0 on the chain. A system robustness check was performed to the 1000-year return period hurricane for the intact condition with a minimum safety factor of 1.25 required on the chain.
The design life of the off-vessel components of the mooring s 20 years and the system is to have a minimum fatigue life of 2000 years. A 0.6 mm/year corrosion and wear allowance on the chain diameter was used and in no conditions was polyester allowed to touch the seafloor. Tension in the polyester was to remain above 5% of minimum break load (MBL), and if not, cycle counting would need to be performed and limited to 100,000 cycles per 2SM. If this could not be met, testing would be conducted to demonstrate the rope could withstand tension-compression fatigue. For top tension riser stroke limitation, offsets were limited to 5.5% of water depth for the intact case and 6.25 for the damaged condition.
Dynamic and static stiffness- background
Steel chain, wire or spiral strand produce a nearly linear load-extension curve over the range of loading involved in most mooring designs. A polyester mooring tether however has highly non-linear load-extension curve thus complicating the mooring and riser design. In order to get stiffness properties to guide the early stages of the design, data was requester fro the various rope manufacturers and a literature search was conducted. The majority of the stiffness data that exists focuses on dynamic axial stiffness. However, little information is available on static stiffness.
An easy way to express polyester rope axial stiffness is to make non-dimensional per the follow equation:
Kr=EA/MBL
This also allows stiffness to be compared for ropes of different break load since stiffness of a particular rope construction will be approximately proportional to the number of fibers within the rope. For the mad dog mooring system, the preliminary design was indicating a rope with a break load of approximately 2000 tonnes, which is significantly larger than any rope ever manufactured and tested.