Hydrodynamic Research Topics for DP Semi Submersibles
Authors: Cozijn J.L. , Buchner B. , van Dijk R.R.T. 
Published: October 2008

The study of the behavior of dynamically positioned semisubmersibles in the time domain can deliver important information on the positioning accuracy and thruster loading of the structure. It is not sufficient to only consider a static equilibrium of environmental and thruster forces. Dynamic effects in the stationkeeping performance of the semi submersible may play an important role.

The environmental loads contain both mean and low frequency varying components. Results from calculations and regular wave tests show that the use of diffraction analysis to calculate the wave drift forces on a semi submersible will lead to an underestimation of the loads.

The total thrust force delivered by the azimuthing thrusters on a semi submersible is influenced by a large number of variables. Interaction effects, such as thruster-hull, thruster-current and thruster-thruster interactions, are the result of complex physical phenomena. Results of model tests show, that the thrust loss due to thruster-hull interaction effects can be up to 40% of the open water thrust.

Other examples of dynamic effects in the stationkeeping behavior of a DP semi submersible include filtering of measured position signals, the application of wind feed forward, the thruster response times and the use of forbidden sectors for the azimuthing thrusters. It is possible to include these effects in DP model tests.

Introduction

Dynamically positioned semi submersibles are being used for deep water drilling in areas such as the Gulf of Mexico, Campos Basin and West of Africa. There are a number of hydrodynamic aspects, which can be of importance in the design of the DP system. These aspects include environmental loads as well as the response of the DP system and thrusters.

The design of a DP system often involves a static footprint approach, which can be very useful, especially in the early design stage. However, this approach only considers the mean environmental loads and thruster forces, while in practice dynamic aspects may also play an important role, e.g. vessel drift off in severe environmental conditions will typically occur after passing of a large wave group.

In the present paper the dynamic behavior of dynamically positioned semi submersibles is discussed. For the dynamic behavior of a semi submersible three different aspects can be distinguished.The environmental loads on a semi submersible may vary in time. The total environmental loads show mean and low frequency varying contributions, as well as wave frequency contributions. The mean and low frequency varying environmental loads will lead to a mean position error and low frequency structure motions to which the DP system will respond. The wave frequency variations only lead to first order (oscillatory) structure motions to which the DP system must not respond.The delivered thrust force of an azimuthing thruster may differ considerably from its open water thrust in bollard pull condition. Thruster-hull, thruster-current and thruster-thruster interaction effects are caused by complex physical phenomena.Filtering of the measured position causes phase lag in the determination of the low frequency position of the structure, which serves as input for the closed loop DP controller. Furthermore, the performance of the hardware of the DP system may lag behind the required total thrust forces. The resulting phase lag changes the effect of the adjusted P(I)D control coefficients and thus the behavior of the system.

Time domain computer simulations and DP model tests offer the possibility for assessing the dynamic behavior of the system. Results of this approach include positioning accuracy and thruster loading in