On the basis of the built criteria for the tandem slewing operation of a SSCV, an optimization program is developed on the purpose of advising the operating procedure of tandem lifting, trying to utilize the limits of the SSCV's capability. Dynamic effects of the coupled system in the whole pre-established operating process are investigated through time domain coupled analysis, taking into account environmental loads, thrust allocation of the DP system, righting moment by virtue of ballast water, and the variation of wind load coefficients and mass matrix.


A very large dynamically-positioned SSCV with heavy lifting capability of up to 25,000 ton is being designed. It is capable of placing a 20,000 ton cargo on its main deck via synchronous operation of two fully-revolving cranes. With seamless lifting, relocating and transportation, the delivery of this SSCV will introduce a new mode for installation and decommissioning of very large offshore structures.

The purpose of this paper is to study the tandem slewing operation of the SSCV. The criteria with respect to such an operation are summarized. Three operational schemes are introduced, namely 1) invariant cargo orientation, 2) invariant hoisting angle, and 3) a combination of them. A mathematical model is built for the calculation of static responses in any operating phase, and based on that, an optimization program is developed for the purpose of advising on the step-by-step tandem slewing operation, trying to maximize the SSCV's capabilities. Due to high complexity of this process, dynamic effects are not accounted for at the first stage; however, they are investigated subsequently in a time domain analysis after establishing the proper operating procedure.

For the tandem lifting operation, the swinging motion of the object is subject to its position relative to the SSCV, so a simulation for the whole pre-established process is performed. The wind load in different phases are evaluated using WINDOS software, based on series calibration and correlation of the wind tunnel test results. The alteration of mass matrix and wind load coefficients in the tandem slewing process is taken into account by assigning properties of individual moving part. As the floating status stands still by virtue of the ballast water, the wave excitation force, frequency dependent added mass and damping derived from HydroD software remain unchanged, so the real-time 1st order motion can be captured in each phase. In order to counteract the low frequency environmental loads, the thrust allocation of the DP system is implemented by a PID controller via an external function in Orcaflex software. As a substitution for the ballast water, time-varying dummy moments are exerted on the floater so as to keep it statically upright.

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