EN3501 Engineering Analysis Assignment

Task(s)–content
A conductor-supported platform uses the structural strength of the well conductors to
support the platform topsides and process equipment. In this case, 4 conductors are used to support a 15,000kg topsides and process equipment as shown in figure 1 and 2 simplified layouts of the design below. Each conductor also contains steel casing. Each set of the concentric tubes are cemented at various depths below the seabed. All dimensions, weights and material specifications for the wells are shown in Figure 1 and 2.
Since risers are exposed to environmental loads (waves, wind and currents) in the offshore locations, they will vibrate axially, transversely and torsionally at low frequency. To design a safe structure, it is important to know its natural frequency.

As a part of this coursework,
(i) You are required to model the conductor/riser structure shown by discretizing
(modelling) it in a series of masses and springs. You have to calculate the natural
frequencies of the structure by modelling it as both;
a) single degree of freedom and
b) two degrees of the freedom
(ii) You have to find the equivalent stiffness of each structural component such as pipe and topside. State the difference in the value of natural frequencies calculated using two different degree of freedom systems. Consider the longitudinal vibration only

Task(s)–format
An electronic copy of the deliverables outlined below should be submitted to the coursework drop box on Moodle - in one document with the coursework front cover as the first page.
Your task is to prepare a report analysing the proposed model and its suitability for this application. The report should include:
1. Calculations for equivalent stiffness and masses with appropriate assumptions (Mathcad may be suitable).
2. A justification for choice of discretising structural mass and stiffness. Please also discuss suitability of your method, using reference, where relevant and appropriate.
3. Discuss the difference in the value of natural frequencies calculated using two different degree of freedom system (one and two DOF). Explain the reasons behind such differences in values of natural frequencies.

Assumptions:
1. The stiffness of topside (with mass of 15,000 kg) can be considered 10 x the stiffness of the conductor pipe.
2. The length of all pipes below the mudline should be considered equivalent to 6 x OD of conductor pipe. Full 150m length below mudline is not considered in any calculations as the pipes below fixity (at 6 x OD of conductor) do not experience oscillations or affect the wave induced oscillations.