The object of this master thesis is to investigate a drilling riser exposed to currents, waves and vessel motions at different water depths. The aim is to develop an advisory system for vessel position based on the riser response and also to better predict the transition from operation to a potential disconnect. The advisory system will make use of watch circles which define the limits for normal operation, halt in operation and a possible disconnect.
As the most accessible oil reservoirs are exploited, there is a need for drilling operations at ever greater water depths and in harsher environments. The riser, and its interactions with the vessel and the blow-out preventer (BOP), is a crucial element in a drilling operation. In order to ensure a safe and productive operation, the riser top and bottom angles must be within acceptable limits. For excessive angles, a disconnect from the well is carried out. This will imply a major loss of income, and it is therefore highly undesirable to disconnect when not necessary. However, the consequences of not disconnecting when necessary are even more severe.
The riser is exposed to currents, waves and vessel motions which cause it to deflect. The effect from these influences must be investigated in order to predict the riser angles in various conditions and thereby develop watch circles taking these angles into account.
The riser has been modeled and analyzed in RIFLEX, a software program developed at MARINTEK for static and dynamic analysis of slender marine structures. In order to investigate the response in both shallow and deep waters, water depths of 300 and 2000 [m] have been applied. The riser is exposed to unidirectional current, regular waves and vessel motions. The top tension is kept constant for each water depth.
At 300 [m] water depth, the riser was found to be highly affected by vessel offsets, and the control objective for this depth must be to avoid any offset. At 2000 [m] water depth, however, both lower and upper angle will stabilize at acceptable magnitudes for the maximum static vessel offset studied in this thesis. The upper angle will however be excessive during the movement. The lower angle will not immediately respond to a motion at the top end of the riser, and it increases slowly for an increasing vessel offset. Hence, for a drive-off, which is an uncontrolled excursion of the vessel due to a failure in the DP system, the DP operator will have some time before a disconnect has to be carried out. The current affects the riser considerably more in deep waters compared to shallow waters. Also, first-order waves affect the upper angle significantly more than the lower angle, both in shallow and deep water.
The watch circles which have been developed prove that the limits for a halt in the operation and a possible disconnect are larger than the conventional guidance limits. By allowing the top angle to become excessive during a limited amount of time, one can uphold the operation for a longer period of time, and also avoid an unnecessary disconnect.
The main contributions in this thesis are the simulation study of the drilling riser and the proposed guidance limits for operation and disconnect.