Guide tube Download the model: Guide_tube.stask Figure 1. SIMA Guide tube 1. Description This example demonstrates pipe-in-pipe functionality in RIFLEX, used here to model a riser passing through a guide tube / bellmouth. A guide tube is a funnel-shaped structure — wide and flared at the entry (the bellmouth) and narrowing to a tube — that guides a riser or pipe and limits its lateral motion as it enters a vessel or structure. The riser is free to slide axially through the guide tube while the tube constrains it laterally, and the two structures interact through contact and friction along their overlap. Capturing this interaction, and the loads it induces, is the purpose of the model. 1.1. Configuration The model is set in a water depth of 315 m. A riser runs from a termination at the seabed up to a hang-off on the vessel, modelled in three sections with an additional buoyancy section that reduces its effective weight in water. The riser cross section is a slender pipe (hydrodynamic diameter ≈ 0.30 m), while the buoyancy section has a much larger effective diameter (≈ 0.60 m). The guide tube itself is modelled as a separate slender structure hanging from the vessel, to which it is fully fixed at the top. Its lower part is a trumpet (bellmouth) whose cross section tapers smoothly from a wide flared mouth (diameter ≈ 3.85 m) down to a narrow tube (diameter ≈ 1.0 m) over a large number of short segments, above which a series of short cylinders and a longer upper cylinder complete the tube up to the vessel. The riser runs up through this funnel, so the flared mouth catches and guides it while the narrow section acts as the actual guide. 1.2. Pipe-in-pipe contact The interaction between the riser and the guide tube is represented by pipe-in-pipe contact elements, which connect specified segments of one line (master) to segments of the other (slave) and allow one pipe to move inside the other. Each contact defines a radial (linear) contact stiffness and a friction model with a friction stiffness and low static/dynamic friction coefficients, together with contact damping, so the riser can slide and rotate within the tube while transmitting lateral contact forces. A number of such contacts are distributed along the overlap between the riser and the bellmouth to represent the sliding contact over the full engagement length. 1.3. Environment and vessel The environment is a severe irregular sea state described by a 3-parameter JONSWAP spectrum with a significant wave height of 12.5 m and a peak period of 14.5 s. The vessel motion is prescribed through a first-order motion transfer function (RAO) on the pltfrm support vessel, which drives both the guide tube (rigidly attached to the vessel) and, through the contact, the riser. 1.4. Analysis setup The static analysis builds the equilibrium configuration in a load sequence of three steps — volume forces (VOLU), vessel displacement (DISP), and activation of the pipe-in-pipe contact (PIPE) — so the riser is first brought into position and the contact with the guide tube is then engaged. The dynamic analysis is a time-domain simulation of 200 s driven by the wave environment and the vessel RAO, from which the contact forces and the riser response through the guide tube are evaluated. Figure 2. Sketch of model, XZ-plane