applications ➤
Finite Element Models (FEMs) were also used to investigate dynamic stresses in different components of the vessels. Rigid body models with mass and inertia
properties can predict global motions and accelerations of the rigid bodies, and the reaction forces between these rigid bodies. Te simulations also revealed the extent of the offloader linkage motion, which is useful information to ensure those motions remain within the design limits for a prescribed sea-state. Te everyday wear and tear on FPSOs
must also be modelled based on the results of hydrodynamic analysis. Such strength and fatigue verifications of the mooring components are achieved using FEMs, taking into account the manufacturing process, under extreme load cases – nearing the chain minimum breaking load.
A FPSO has various systems for handling and
separating the different hydrocarbons, as well as mooring systems and a system for the dynamic
OFFSHORE
PLATFORMS MUST SURVIVE IN DEEPER WATERS
positioning. Tese have to be designed against severe sea states because, should the weather and sea conditions exceed the design operating conditions, the vessel has to be free to leave the site in order to avoid possible damage to the structures, and to the pressure pipelines. Tese pipelines, through the wellheads located on the seabed, transfer the hydrocarbons from the oil pool to the FPSO. Similarly, when the oil field is exhausted, the FPSO has to be disconnected to be relocated in a new oilfield. Terefore it is necessary for the FPSO to be
equipped with a disconnectable transfer system (DTS). One of the main components of a DTS is the multibore quick connector disconnector
A fully 3D FE model for a riser spool (max principal stress range mapped)
coupler (QCDC), which contains multiple production and injection lines and valves. Designing a multibore QCDC is a complex
engineering task involving advanced knowledge in designing pressure vessels, as well as structural systems. For example, the riser’s pressures are in the range of 520 bar, and resulting buoy axial load is in the range of 20,000 kN. A variety of design standards also have to be taken into consideration, since they apply to the different components of the system. Te connector has to be designed against
normal operating conditions, extreme operating conditions, offshore pressure test conditions, and hydrostatic test conditions. A fatigue analysis must be performed as well, to evaluate the impact of the variations of the axial load transferred by the risers and by the mooring lines.
FEM models were largely used with shell-type
models for the structural components – upper and lower part of the connector, upper spool connecting the QCDs to the rotary table, and lower spool connecting the QCDC to the riser buoy – and full 3D models for pressure components. Tis type of connector is unique, and trial and
error procedures cannot be applied. Efficiency and risk have to be assessed upfront, during the design phase. Shop tests are applicable at the end, but they are just a means of confirming that the design was correct. In other words, only a simulation-based approach can efficiently lead to the correct sizing of the structure and its components, as well as evaluating the different what-if scenarios to deliver the required robustness.
Extreme environments Conditions from normal up to the extreme must also be considered in simulating the dynamic response of a semi-submersible platform and its on-deck facilities. Key features such as the water, the moving floor, and the finite element structure of the platform, cranes and anchor cables, must all be included in the simulation. For example, the deflection of a crane boom,
as well as the fluctuating stresses induced in the crane structure by the motion of the semi- submersible platform, have been taken into account in Figure 1. Te results show the influence of a flexible
mooring system on the dynamic response of the semi-submersible platforms. Compared to an unmoored platform, the flexible anchor cables cause a moderate increase in the pitch and heave of the platform, and a significant increase in the surge of the platform. Te calculation of fluctuating stresses induced in the structural
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0
Clamp stress of a clamping mechanism mounted on a rotating collar operated by a couple of hydraulic cylinders
28 SCIENTIFIC COMPUTING WORLD
Example of a QC / DC structure model with Von Mises stress mapped, related to a particular load case (ULS B1)
10 20 Time [s]
Figure 1: Deflection and fluctuating stresses in crane structure caused by to strong wave action
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EnginSoft
Stress [MPa]
EnginSoft ESI Group
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