This paper presents the development of cooperative control technique applied to vessels equipped with dynamic positioning (DP) system. An illustrative case study is suggested: the launching of subsea equipment using two DP vessels. In this example, the cooperative system controls the relative distance between the DP vessels. One of the advantages of this method is the increase of operation’s safety and operational window, since, among other factors, the tension in the launching cable is reduced by half.
Initially, it was proposed the control of vessels relative positions, trying to keep the movements at the top of the A-frames in counter-phase. This avoids the slackening of the launching cable. For this, an algorithm based on phase estimator (Hilbert transform) associated with a PD control was implemented. The results showed that for regular waves this strategy was effective.
A dynamic mapping was then obtained using simplified 2D simulator, previously validated by comparison with experimental tests. In these maps, two regions are defined — occurrence or non-occurrence of cable slackening — as a function of the distance of the vessels and the depth of the subsea equipment. This map defines the proper set-point for the DP systems for each depth of the subsea equipment. This map is used to define the best relative position for the vessels.
In addition, the hoisting control receives the measurements of the vertical motion at the top of the A-frame, and compensates its motion, trying to maintain a constant lowering velocity. This control was implemented considering errors of 10% and delay of 0.5s in the measurements. The results confirmed that the control is able to eliminate the tension peaks and the occurrence of slackening in the launching cable.
The conclusion is that the appropriate control strategy, considering regular waves, is to combine the control of both position of the vessels and hoisting of the cable. Therefore the position control, coupled with dynamic mapping, defines the “optimal path” to be followed during the line hoisting, trying to keep the vessels as close as possible to the “no slackening” region.