Result Interpretation

The initial analysis considered loads calculated with a rope factor C=1.75. The models were run under the relevant load combinations and the results compared. For both design methods were recorded failures – utilization factors higher than allowable – for the flange spokes and for the drum staves. Code checks were made based on Eurocode 3 (for LRFD method) and AISC ’89 (for WSD method), which are built-in the FEA program used. For each of the two methods, the structure of the reel was strengthened, by increasing the cross section of the spokes and staves to the next larger standard profile available in BS4 Part 1 1993 standard for structural sections. Eventually, the final weight of the 2 new reels was compared and the results were commented.

The section increase could not be done just for the failing elements, but for all similar elements, as the reel does not have an upright specific position in which it can be stored, thus all spokes could in turn fail, not just the ones supporting the reel weight on the cradles at a specific moment in time; a similar logic was followed in the case of the failing drum staves.

7.1 Results of the analysis 7.1.1 Flange Spokes

The analysis showed that the reel spokes will fail during transport for both design methods. The initial assumption that the weight of the reel will be supported

only by 3 spokes on each flange was considered unrealistic as the utilization in those spokes was almost 4 times the allowable (allowable = 0.85 << 3.38 actual). Furthermore, no sea fastenings were considered, so the length of the supporting cradle was increased, thus providing support for 4 spokes on each flange.

The utilization ratios decreased to a more reasonable value of 1.36 for the LRFD method. For WSD, the utilizations in the same spokes were around 1.05, so almost 30% lower.

Figure 7.1.1 Flange Spokes with Highest Utilization

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As expected, the spokes suffered failures only during transport, under increased vessel accelerations.

The case in which the reel tilts along its longer axis (transversal load situation considered in the model) is more unfavorable that the longitudinal load situation.

The extreme scenario when the product spooling tension was assumed to be 0, thus determining the entire product to slide and all its weight to be supported by the flange, did not show higher utilizations that the case when only a percentage of the product slides. The utilization values were similar

- 0.49 and 0.5 for full product slide and partial product slide, respectively for the LRFD;

- 0.40 and 0.53 for full product slide and partial product slide, respectively for the WSD.

This might be explained by the fact that the spooling pressure on the flange due to reeling tension, although reduced to half of its operational value during transport, still has an important impact on the behavior of the flange spokes.

7.1.2 Drum Staves

Although there were recorded failures of the staves in the Transport Scenario, the failures were small compared to the ones from the Operational Case.

Almost all the drum staves had utilization ratios above 0.85, the highest being up to 1.79 for the LRFD and 1.47 for WSD. Again, a difference of over 20% between the two methods.

Probably an interesting point is that the parts of the staves with highest stressed were located at the connection between the flanges and the drum, highlighted in red in Figure 7.1.2. This aspect could be of significant importance, especially in the case of the reel design where the flanges connected to the drum through bolts.

The drum hoops did not appear to have a utilization factor higher than allowable.

Figure 7.1.2 Drum Staves with Highest Utilization

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7.2 Comments

Overall, the spokes and the drum staves had to be increased up to the where their maximum utilization did not exceed the allowable. Along with the section increase came also an increase in weight.

The new sections and weight of the reel according to the design methods considered are shown in Table 7.2.

Initial Sections

Final Sections

C=1.75 C=3

WSD LRFD LRFD

Spoke UC 305x305x118 UC 305x305x158 UC 305x305x240 UC 305x305x240 Hoop UC 203x203x71 UC 203x203x71 UC 203x203x71 UC 203x203x71 Stave UC 203x203x71 UC 254x254x132 UC 254x254x176 UC 305x305x240

Reel Self-weight 58t 71.26t 88.63t 102.61t

Weight Difference (%) 24.40% 44%

Table 7.2.1 New Sections for the Structural Members

It can be easily observed that by considering a higher value for the rope factor, as per DNV 2.22 requirements, could lead to a dramatic weight increase. Therefore, at least in case of reels transporting lengthy products that have to be spooled in many layers, based on previous experiences that did not point out reel structural failures, a lower value for C=1.75 or less can be considered more appropriate.

With respect to the design method, since LRFD does not provide formulas to estimate hoop stress, nor drum and flange pressures due to reeling tensions, then the input formulas for both methods is provided by WSD method through DNV 2.22. For ultimate limit states, both methods have a similar allowable utilization factor around 0.85 of the yield strength of the material (for LRFD U = 0.87, for WSD U = 0.85).

So, basically, in the particular case of reels, the main difference between the 2 design methods is the way in which the load combinations are built. In other words, the safety factors applied to each of the load types (self-weight loads – G, live loads – Q and environmental loads – E).

For the particular case of reels, if the LRFD is applied as per DNV-OS-H102 Table 5-1 Load factors for ULS

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without considering any of the allowed reduced safety factors (as stated in DNV-OS-C101 Section 2 B402 and B404 and discussed in Chapter 3 Section 3.3 “Comments” of this paper), then the LRFD method proves to be too conservative, thus leading to a heavier overall reel, which in turn could prove to be at least uneconomical in terms of offshore transportation and lifting.

Another grey area for LRFD could be considered the way in which the safety factors for the environmental loads are chosen, especially in the “Load case b)”; if the designer specified accelerations for the reels are chosen according to DNV’s “Rules for Ships” January 2012, Part 5, Chapter 7, Section 2 E400, then accelerations are already similar with the ship’s design accelerations for storm conditions and a further multiplication for the reel’s accelerations could lead to a reel that is designed to survive in weather conditions that the transport vessel will not.

The governing load combinations that were found to dictate the overall strength of the reel were the Operational load cases for both LRFD and WSD. With respect to the safety load factors in LRFD for G and Q, it can be stated that by increasing by 30% an already heavy structure and equipment (so for a 300t reel plus product an addition of 100t plus a 30% increase of the already high spooling pressures on the drum and flanges) could prove too conservative for a structure where these parameters can and are easily and accurately controlled. Therefore, the weight difference highlighted in Table 7.2.1 can now be explained.

WSD appears to be the more economical of the 2 methods. Also, the reels designed according to this method have been in service for long periods of time without known structural failure, so it also seems to be confirmed by reality as suitable for reel design.

Both methods rely on the hoop stress and drum and flange pressure equations in DNV no. 2.22 (equations 5.1 and 5.2 in this paper) in order to determine the loads to be applied on the reel’s structural members. Therefore, choosing the adequate value for the rope factor (C) should be carefully correlated with the specifics of each design (i.e. payload type, etc.). The increase of the rope factor in the latest edition of the DNV 2.22 standard from 1.75 to 3 for winches with more than 5 layers of product “for subsea retrieval operations with the full load from the first layer” [6], which sometimes is applicable to reels if they have to retrieve umbilicals form subsea, might prove to be over conservative, as long as these operations were also done in the past when reels have been designed according to older editions of the DNV 2.22 standard which did not have this requirement and the rope factor could be considered C=1.75.

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