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API Specification for Drilling and Well Servicing Structures (5th edition)
Wind loads, based on the velocity component approach, are defined according to API 4F Specification for Drilling and Well Servicing Structures (5th edition).
The API 4F specifications for wind loads based on the velocity component approach is integrated into the PSE Petroleum Structural Engineering software. Accordingly, drilling structures are classified based on their Structural Safety Level (SSL) and their offshore or onshore location.
The design reference wind velocity Vref value chosen should be a 3-second gust wind measured at an elevation of 10 m (33 ft) in open terrain or water, with an associated return period of 50 or 100 years.
The Petroleum Structural Engineering® software has a tool to generate wind and ice loads on open structures such as drilling structures. It allows generating automated ice loads or wind loads on each element of the structure.
The PSE software automates wind loads applied to members. These loads are calculated based on the projected area, projected pressures or velocity components approaches. The program offers a variety of wind profiles and automates the determination of the shape coefficients (drag factors).
A wind profile in a selected direction provides the wind intensity that generates the wind loads to structural members and surface areas. As many as required wind directions can be defined through different basic loads.
Member selection procedures allow the application of the wind profile to the entire structure or to specific zones and excluding members behind or in front of wind walls. It is possible to apply the API 4F wind loads directly to elements such as equipment, wind walls and other objects attached to the drilling structures.
The shape coefficient (Cs) is automated in the PSE software for various section shapes. The program accounts for the gust factor (Gf) and the reduction factor for shielding (Ksh) for members and appurtenances.
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.
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.
Wave and current loads generated forces applied to submerged structural members in platforms and floating hulls are analyzed through linear and nonlinear kinematics in accordance with the API RP 2A specifications.
The PSE software computes wave and current forces applied on the structural members. The wave kinematics can be established using either Airy’s linear theory or Fenton’s nonlinear theory.
The linear kinematic theory is valid where the wave height is small compared to the water depth. On the other hand, the nonlinear kinematic theory, proposed by J.D. Fenton, solves the motion equations by representing the velocity potential and surface elevation with a Fourier series.
The later method minimizes the error of each parameter governing the wave motion equations and is valid over the entire spectrum.
The PSE software accounts for the following wave profiles and kinematic parameters:
Preview of the wave surface profiles, velocities and accelerations at any point is readily available.
According to commentary C.3.2.1 of the design code API RP-2A-2003, the Doppler effect is accounted for by calculating an apparent period defined as the wave period as seen by an observer moving with the current.
Marine growth increases the cross section diameter and surface roughness of the members and it is defined by a set of elevation-thickness pairs.
In the PSE software, the current profile is described with respect to the sea bed. The current speed is defined by a set of elevation-velocity-angle triplets and the reduction of the current speed in the vicinity of the structure or the blockage factor is accounted for.
In order to combine the current with the wave profile, the current needs to be stretched, or compressed, to the local wave surface. Two stretching methods are available:
The input for the member wave loads consists of the following six parameters:
The member forces, calculated using Morison equation, vary according to the position of the waves with respect to the structure. In order to obtain the maximum forces in the members, the critical position of the wave crest is determined by the program.
Improve productivity for the Analysis, Design and Rehabilitation of drilling structures.