**2.2 The use of dynamic positioning systems in drilling operations**

Drilling operations take place over a wellhead. The primary purpose of the DP system is to maintain the position of the drilling vessel so that the riser/stack angle containing the drill string is close to zero, compensating for currents or tidal flow if necessary [4]. This angle is the one measured between the riser (on the top) and the wellhead or lower marine riser package (LMRP) [3]. This function is known as riser angle or riser follow mode. The DPO monitors the riser difference angle through sensors located around the LMRP. A watch circle system is created so the DPO can monitor the movements of the vessel. When the rig is moving, different levels of alarm are set to ensure the safety of the operations at all times [4].

The main risk in any DP operation is losing position (which is known in drilling operations as an excursion) during operations. Therefore, the DPO should react in a short time to correct or mitigate the consequences of this loss [5].

To maintain the position of the drilling riser, the system consists of a closed-loop control function that receives information from different sensors that measure wind, currents, heading and position. It sends a command to the propulsion units to counter rest the forces that, according to the information, tend to take the vessel out of position.

The desired position is input by the DPO, who supervises the operation in the Human Machine Interface (HMI), also known as the DP console. The DPO operator is a certified officer of the watch who has followed a training and certification scheme to cover this board position [6].

Finding which variables and in which way and measure they affect an incident having a human cause can help focus on the riskiest situations and improve the safety of drilling operations. From the results obtained, it would be possible to propose operational limits to improve the safety of drilling operations.

DP drilling incidents have been the object of different academic research. In 2011, Haibo Chen [7] published a paper where he introduced the safety of DP operations based on a barrier model. Previously the same research team had already published an article about the safety of such units [8].

The most interesting approach to the human factors in DP incidents has been proposed by Chae [9], while formal safety assessment was applied to them [10]. Dong [11] focused his research on the incidents that had taken place during offshore loading operations. Overgard [12] also researched the human element during DP incidents.
