Lifting analysis of integrated spool cover

Abstract

Large competition in the offshore industry requires innovative solutions to satisfy the demand. A new concept of spool installations is considered where three large spools are lifted in an integrated protection structure designed to support the spools during the lift and protect them from impact loads associated with fishing activities and dropped objects after installation. By integrating the spool and protection structure, the time needed for the installation will be significantly reduced. Installation of subsea structures and equipment involves a lifting operation where the object is exposed to large hydrodynamic forces when entering the oscillating sea-surface. The largest load during the structures lifetime may occur during the installation and could be snap forces after slack, or overload due to dynamic forces when the structure is lifted from the vessel and into the water, which is why the lifting analysis is considered in this thesis. The purpose of this report is to verify the structural integrity of the new concept, designed as a complex framework structure and determine the maximum allowable sea state in which the structure can be installed safely. A static analysis is performed where all structural parts are checked according to Eurocode 3 ensuring that no parts of the framework (or spools) are overloaded during the lift. The dynamic forces associated with the lift was accounted for using a dynamic amplification factor to magnify the static force and represent loads occurring during lift off from deck until the structure is fully submerged. From the static analysis, the assumed dynamic forces in slings and crane wire are obtained and compared to associated forces from the dynamic analysis. A dynamic analysis is performed by creating a simplified model in a marine dynamics program where the structure was analyzed in different sea states characterized by the JONSWAP spectra. The tension in slings and crane wire are considered to make sure that they are not subjected to slack or larger forces than the structure can withstand which is represented by the dynamic amplification factor defined in the static analysis. Time domain analysis is performed comparing deterministic (extreme) and stochastic (most probable) values to obtain the limiting sea state for different wave headings, wave periods and significant wave heights which characterize the sea state in which the structure will be installed. The forces on the “real structure” can be obtained with a scale factor that is obtained assuming that the hydrodynamic forces are proportional to the largest relative increase in mass or solid projected area with respect to the simplified model. The assumption is appropriate since the simplified model has the same structural properties as the “real structure” with less structural members. According to the dynamic analysis, the structure could be installed in irregular waves characterized by the JONSWAP spectra with significant wave heights equal or less than 2.5 meter (not including uncertainties in weather forecast). The associated max utilized member with respect to design loads had a utilization ratio of 80% (no 100% utilized members) to account for uncertainties in the structural design that must be finalized before the installation of the spools is performed.