Dynamic Positioning Calculations

The ability of specialized vessels to remain stationary over a fixed point of the sea-bed is required for a number of offshore and salvage operations. Monohulls ranging in size from small supply vessels to medium sized drill vessels have, during the last decade been either converted or newly built for such roles, and recently large tankers have been designed and built for production from marginal fields. A number of semi-submersibles have also been built with station keeping capabilities.

Where such vessels need to remain on a fixed heading and position within a specified tolerance the use of a number of thrusters is required in order to balance the steady environmental effects due to wind, current and wave action. Some small supply vessels utilize only twin CP propellers, twin rudders and a bow tunnel thruster and rely on manual control to maintain station. Many of the monohulls and semi-submersibles utilize a number of fixed or steerable thrusters, which are automatically controlled to allow the vessel to maintain station. In some cases, the thruster systems are supplemented with either an active or passive spread mooring system.

The Dynamic Positioning module provides the ability to design the thruster system of any vessel, which requires a station-keeping capability with three or more thrust units. The program undertakes a static analysis where the thruster system can be designed subject to steady environmental effects. A safety margin can be included to allow for unsteady environmental effects.

The use of the Dynamic Positioning module to design thruster systems for offshore and salvage vessels will provide you with the ability to optimize the design of the thruster configuration. In addition, you will be able to ascertain if the vessel can maintain station subject to given design environmental conditions.

The graphic shows the outline of the system and depicts the required data input, the outputs and/or purpose to which each calculation option can be utilized:

Only the vessel principal particulars are required by the module and no detailed knowledge of the hull form is required. A thruster configuration needs to be defined in terms of the number of fixed and steerable thrusters, number of rudders and their location on the vessel. The thruster configuration can be changed within the program by specifying a particular thruster as inactive thus allowing you to assess different thruster configurations. Thruster-thruster interactions can be allowed for by the specification of barred zones for any steerable thruster. A vessel forward or transverse speed can be considered and external forces due to effects such as tugs and mooring lines can be included. Default environmental coefficients for wind, current and wave drift effects are available in the program for supply boats, drill vessels and tanker hull forms. Alternative coefficients which are specific to the vessel under consideration can be supplied by you for any of the environmental effects.

Three distinct static analyzes can be undertaken which are:

THRUST

This allows you to specify a design environment in terms of wind and current speed, and sea-state. For a selected range of environmental directions the thrust required to balance with the design environment is determined and hence the maximum thrust required by each unit can be determined. The wave direction may be specified relative to the wind direction by giving an offset in degrees. A zero offset assumes that the wind and waves are coincident. The current direction may be specified in a similar way or may be fixed in one direction which will usually be either head on or beam onto the vessel. Rudder effects cannot be included in this option.

ENVIRO

This allows you to specify the maximum available thrust in each unit and determine the maximum environment the vessel can operate in for a range of specified environmental directions. The wind speed and wave height are assumed to be correlated either from a default relationship or as specified by you. The current speed is fixed and the maximum wind speed and wave height can be determined. Rudder effects can be included and operated either dependently or independently.

As with the THRUST calculation, the wind directions are specified, and the wave direction is given relative to this. The current direction may also be given relative to the wind, or may be fixed in a particular direction.

ERN

This allows you to determine the environmental regularity number ERN in accordance with the procedure defined by Det norske Veritas (References 46). The ERN number gives the percentage of time that a vessel will be able to maintain position and heading in a reference geographic location of the North Sea. The reference position is at the weather ship 'M' located at 66 North 2 East off the Norwegian coast, as shown in References 46.

The most critical vessel heading relative to the environment is considered, which has been taken as the environment beam on to the vessel. The current is fixed at 1.5 knots and the maximum wind speed and wave height that the vessel can hold station in is determined with all thrusters operating at up to 100% power where the significant wave height is assumed to be related to the wind speed as specified in References 47. The analysis is repeated with each thruster inactive sequentially and the most effective and least effective thruster is determined. Further analysis is undertaken with each pair of thrusters sequentially made inactive and the most effective and least effective pair are therefore found. The ERN number is obtained for the five conditions, from data given in References 47. This data relates the cumulative probability distribution of observed significant wave height and wind speed at the reference location to the significant wave height and wind speed in which the vessel can hold station.