Load Cases

The load cases considered for the FEA analysis are described below. Their values will be summarized in a table at the end of this section. The calculations showing how the values for each load case were obtained can be found in Appendix 1.

1. Self-weight of structure.

2. Product load – the entire product load will act only on the top half of the reel drum as shown in Figure 6.5.1 (left flange removed for clarity).

Figure 6.5.1

48 3. Drum pressure due to reeling tension.

In DNV no. 2.22 Standard, the hoop stress formula in Chapter 2, Section 3, B207 requirements for the C factor for 5 layers and above is 3. However, this will lead to increased pressure values on both the drum and flanges. In the industry, the value for C is usually taken as 1.75. [6] However, in this dissertation both values were considered and the results will be commented in the following chapters.

Figure 6.5.2 Drum Pressure Due to Reeling Tension (left flange removed for clarity)

4. Flange force due to reeling tension.

In DNV no. 2.22 Standard, in Chapter 2, Section 3, B208 the flange “pressure is assumed to be linearly increasing from zero at the top layer to the maximum value of

_' =2 ∙ !"

3 ∙ A ∙ ^ near the barrel surface”. Designers consider the load to be linearly distributed on the spoke and have a triangular shape, with the maximum value near the barrel surface. In reality, the pressure is distributed on the whole flange area which have a circular shape. The spokes are radially extending from the inner rim towards the outer rim and the area from which the pressure is unloaded to the spoke is a circle sector. Since the pressure

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decreases as the corresponding spoke area increases (there is an inverse relationship between them), probably a more accurate distribution of the pressure on the spokes would be a parabolic one, with the maximum value towards the middle of the spoke.

For the purposes of this dissertation, a similar simplifying assumption as the one used by the industry will be considered, thus a triangular load distribution will be modeled on each spoke.

The pressure at the drum surface will be considered linearly distributed on each of the staves and the value will be 3 times greater than the one acting on the flange. [6]

Figure 6.5.3 Flange Pressure Due to Reeling Tension

5. Transverse load on flange under transverse accelerations.

Under transverse accelerations (generated by the ship’s motions), depending on the reeling tension, a portion of the product or the entire product will slide, thus the flange will have to support an additional load.

As the reel tilts under transverse accelerations, if the product is not spooled with a sufficient tension, then the force that keeps the product attached to the drum and prevents it from sliding might be exceeded by the force generated by the mass of the product combined with the

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transverse acceleration. A simple check whether the minimum required friction coefficient (µ=0.1 in DNV 2.22 Chapter 2, Section 3, B513) is exceeded or not (detailed calculations in Appendix 1). In this particular case, the entire product will not slide.

Therefore, a common industry practice is to consider a conical shaped portion of the product to slide and act on the flange. In cross section, the load distribution has a triangular shape, increasing from 0, near the drum surface, and linearly increasing towards a maximum value at the outer layers of the product (Figure 6.5.4). The angle made by the flange and assumed product sliding plane is usually considered to be 30° (Figure 6.5.5).

If the spooling tension cannot generate a drum pressure large enough, then the whole product might slide, thus the entire weight of the product will have to be supported by the flange.

Figure 6.5.5 Proportion in which Sliding Product Pressure Will Be Supported by Flange and Drum

Figure 6.5.4 Flange Pressure Due to Partial Product Slide

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5a. For the purposes of this design exercise, a special, extreme case will be considered when the spooling tension will be taken as 0 and the effects will be analyzed. However, the drum and flange pressures due to spooling tension will then become 0, so it should be interesting to see what actually happens in an extreme case.

6.5.6 Flange Pressure Due to Full Product Slide 6. Forces on the hub under transverse accelerations.

The remaining payload after the product slide assumed in the previous load case will still act as pressure on the drum. Therefore, each of the drum staves will be subjected to a pressure generated by the remaining payload, but this pressure will be acting as friction force, along the local X axis of each stave.

Note: Load cases 5 and 6 are always applied simultaneously.

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7. Forces on the hub under longitudinal accelerations.

Figure 6.5.7 Forces on the Hub Under Longitudinal Accelerations 8. Transverse loads generated by the wind (on the flange).

9. Longitudinal loads generated by the wind (on the drum).

10. Reeling tension

53 Load cases that would be applied to the FEA model

Model Load Cases

LC5 spooling pressure on drum

perpendicular to local x axis of each

stave

500 N/mm 855.77 N/mm

LC6 spooling pressure on flange y

linearly

LC7 flange force from transversal

accelerations y 23.88 N/mm

LC7a

flange force from transversal accelerations when product

slides (S=0)

y 63.58 N/mm

LC8 drum force from transversal accelerations

in line with stave

local x axis 19.56 N/mm

LC9 drum force from longitudinal

accelerations x 252t

LC10 transverse wind load y 800 N/mm2

LC11 longitudinal wind load x 7.78 N/mm

LC12 reeling tension point load 10t

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