The Front Runner spar was fabricated in J. Ray McDermott's Dubai yard and transported to the Gulf of Mexico by the submersible barge Zhong Ren 3. The Front Runner hull transportation is unique as compared to all previous hull transports. All hulls to date have been transported on purpose-built heavy-lift vessels (HLV). The Zhong Ren 3 is a converted tanker with no propulsion system. The internal framing system of the transport vessel and its relatively low deck strength, combined with a dramatically slower tow speed, required a unique design approach that included the establishment of a limiting seastate and several hold points during the 61-day tow from Dubai to the Gulf of Mexico. This paper presents the rigorous approach followed to evaluate the structural strength and buckling capacity of the transport vessel and the spar hull during tow. Accelerations and hydrodynamic pressures of the vessel-spar system were calculated using WAMIT? [1]. Design load cases were constructed and analyzed with ANSYS? [2]. Code checks were performed following applicable codes. The fatigue damage of the spar hull during tow was also evaluated. Wind-vortex-induced vortex vibrations of the truss members, risers, and pull tubes of the hull; wave slamming on the soft tank overhang; and submergence were also addressed.
The semi-submersible barge Zhong Ren 3 was used for the hull transportation of Murphy's Front Runner truss spar. The vessel is owned by China Ocean Engineering Company. Table 1.1 presents a summary of the vessel's main particulars. A major conversion of this vessel from an oil tanker was completed in January of 2002. This vessel posed the following challenges:
As with the past spars, the transport of the hull was proposed onboard an HLV. Due to scheduling conflicts, there were no HLVs available. Through a worldwide vessel search, the Zhong Ren 3 was found as a suitable vessel to complete this job without major modifications. The longitudinal bulkheads on this vessel line up with the existing skid shoes on the barge hull.
The vessel does not have onboard propulsion and therefore relies on an ocean-going tow vessel for its propulsion. This resulted in longer tow times from Jebel Ali, UAE, to the Gulf of Mexico (see Figure 1.1 for the transport route). The extended tow time increased the metocean design criteria from previous spars, and the metocean criteria could not be met on this vessel. To mitigate this, safe haven stops in Dubai, Suez Canal, and the Straight of Gibraltar were added. These added stops provided preset harbors that allowed the vessel to hide until potential bad weather cleared. This, in turn, decreased the probability of a large wave height. See Table 1.2 for metocean tow criteria. The longer tow time also negatively impacted the fatigue life of the hull.
The vessel is comparable in size to other vessels that have been successfully used for the transport of spars. Nevertheless, the spacing of the transversal frames is greater than the length of the skid shoes.
