Vessel motions
In the PSE Petroleum Structural Engineering® software, vessel dynamic motions are defined according to API 4F (5th edition) Specification for Drilling and Well Servicing Structures.
The inertial forces due to the vessel dynamic motion as well as radial, tangential and translational forces due to the acceleration of masses attached to the drilling structures have a significant influence on design and reliability.
In various production wells, the offshore drilling structures are located on top decks of vessels, semisubmersible or floating hulls. Vessel motion includes roll, pitch and yaw rotations and heave, sway and surge translations.
The PSE software accepts three types of user input in order to estimate the inertial forces induced by the vessel dynamic motions:
– Linear displacements, angular rotations and time periods
– Linear and angular velocities and accelerations
– Linear accelerations at two points in the vessel which are converted to linear and angular accelerations by the program.
High pressure mud piping, electrical cable trays, junction boxes, racking boards, tong counterweights, turning sheaves, deadline anchors, crown accessories, casing stabbing baskets and other outfitting items add weight to the derrick. Weight data is converted to masses applied at the correct locations on the derrick.
The Direct Method (w/r to rotation axis) allows defining the motion by the means of amplitudes-periods pairs or by the means of accelerations according to the vessel rotation axis and each translational axes. The Indirect Method allows the user to define the motion by specifying two accelerations at two different elevations along the X axis (Pitch) and along the Z axis (Roll). The specified accelerations along the X axis allow defining both the pitch (RZ) angular acceleration and the surge (X) acceleration. Similarly, the specified accelerations along the Z axis define the roll (RX) angular acceleration and the sway (Z) acceleration.