Research Into Good Design Practice for Reels

MSc SUBSEA ENGINERING NAME: TEICA FLORIAN TEICA

RESEARCH INTO GOOD DESIGN PRACTICE FOR REELS

MSc Subsea Engineering 2011-2012 University of Aberdeen Student Name: Mircea Florian Teica

Supervisors Academic: Dr. Mohammed Salah-Eldin Imbabi Industry: Marius Popa

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Abstract This dissertation proposes a study of the current design methods relevant to reels (LRFD and WSD) and discusses their particularities, limitations and how a reel could be designed according to each of them. The design method limitations are discussed in light of recent studies and findings in areas and for equipment similar to reels (i.e. winches). In the second part, a reel is designed according to both design methods and the 2 sets of results are compared. Finally, the “Conclusions and Recommendations” chapter summarizes the most important aspects of reel design, the areas where the accuracy of current standards needs improvement and highlights some areas where further studies may improve the current design practices.

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Table of contents

Chapter 1:

Chapter 2:

Chapter 3:

Chapter 4:

Chapter 5:

Introduction

5

1.1 Scope of Work

8

Design Methods

9

2.1 LRFD (Load Resistance Factor Design) Method

9

2.2 WSD (Work Stress Design) Method

10

2.3 Additional Standards

10

2.4 Comments

11

Design by LRFD Method

12

3.1 Load Types

12

3.2 Load Combinations

14

3.3 Comments

16

Design by WSD Method

17

4.1 Load Types

18

4.2 Load Cases

20

Discussion on LRFD and WSD Methods

21

5.1 Load Combination Factors for LRFD Method

21

5.2 Utilization Factors for LRFD and WSD Methods

23

5.3 Hoop Stress and Flange Pressure

25

5.4 Rope Factor (C)

29

5.4.1 “Large Wire Rope Mooring Winch Drum Analysis and Design Criteria” Study

29

5.4.2 “Problems Related to the Design of Multilayer Drums for Synthetic and Hybrid Ropes” Study

30

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5.5 Spooling Tension – Friction Factor Relationship; Friction between Product Layers

Chapter 6:

Chapter 7:

31

5.5.1 “AM16 Improvement in the Design of Winches” Study

34

5.5.2 “Improvement in Winch Design Guide AM11” Study

35

5.6 Comments

36

FEA Analysis

37

6.1 Reel Design

37

6.2 Boundary Conditions

38

6.3 Load Scenarios

42

6.4 Operational Limitations

44

6.5 Load Cases

47

6.6 Load Combinations

54

Results and Interpretation

55

7.1 Results of the Analysis

55

7.1.1 Flange Spokes

55

7.1.2 Drum Staves

56

7.2 Comments

57

Chapter 8: Conclusions and Recommendations

59

Bibliography

61

Appendix 1: Load Cases

63

Appendix 2: Risk Assessment

72

Plagiarism Cover Sheet

74

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Chapter 1: Introduction The need for fast and undisrupted communication and transportation of resources (i.e. electricity, oil, gas, etc.) over large distances, between continents or countries separated by seas could be solved, as technology evolved through the use of pipelines and electric cables. For example, during World War II, after allied forces disembarked in Normandy, their planned advance into German occupied territories could have been granted to a halt if they had ran out of fuels. Since oil tankers would have been easy targets for enemy bombers, a different solution should have been found. Military engineers came up with the solution of building a pipeline that could link oil supply reservoirs on British soil with unloading stations on the French coast, thus ensuring a safe, quick and continuous supply of fuels. But how to build and lay pipelines in a very short period of time, in war conditions and in some of the most unfriendly waters – the English Channel? The answer was to build the pipe onshore, the transport it offshore and lay it to the seabed. Transportation would have been possible by spooling the innovative flexible pipelines onto giant floating “conundrums” [21] that could be tugged behind vessels, so that the pipe could be unspooled as they approached France.

Figure 1.1 Floating Reel Towed by Allied War Ship [21]

The laying of the pipe went according to plan and the idea proved so good, that allied decided to continue “Operation Pluto” and lay a second pipe.[1][2] Making a step forward in time, up to present days, the need for transporting products, energy and information increased exponentially, especially in the Oil and Gas industry. As easily accessible oil reserves have mostly been depleted, industry now focuses on the deep water fields.

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Deep waters mean larger depths, increased distances to be covered, longer pipes and umbilicals to be laid and larger vessels and equipment need for their transportation and installation. Reels have remained the only way of transporting long sections of umbilicals and cables and started to play an increasingly important role, as flexible pipe became more popular due to its advantages over traditional pipe systems: controlled fabrication onshore and increased laying speeds offshore. But what exactly are reels and what makes them so important? Reels are objects around which long, flexible products are winded for storage [1]. Their storage capacity can vary from a few hundred kilograms to more than 300 tones, so the larger ones can be more than 11 meters high and 9 meters wide. They are made up of a horizontal, cylindrical drum, on which the product is spooled and 2 side, vertical flanges that help keeping the product in place. Reels that make the object of this study are the larger ones, used for transporting increased lengths and weights of product offshore, being able to resist many spooling/unspooling, lifting and transportation cycles for an extended period of time. Therefore, they can be described as portable offshore units that must comply with structural and safety regulations.

Figure 1.2, Courtesy of Forsyths [20]

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Even though their role is primarily simple and they can be described as simple as “dumb pieces of metal”, in reality things are a little bit different: often used to carry payloads exceeding 300 tones, their design must be flexible enough to accommodate further improvements or different types of products, with different material characteristics that would require larger spooling tensions or that could be laid at higher speeds. For instance, large reels are not designed specifically for one job or to operate just in a certain location. Often, the rating of a reel can be increased just by adding some extra stiffeners. Or their drum can be fitted with intermediate spacers – partitions – (as shown in figure 1.3) to be able to transport 2 or 3 products at a time, sometimes with different characteristics: weight, rigidity, etc. Furthermore, they need to be able to be operated throughout a long service life: due to their size and weight, they are quite difficult to build and transport and, most important, expensive.

Figure 1.3, Courtesy of Oceaneering [17] Their design must also make best use of the material characteristics; design concepts and features must ensure a final product that is not too heavy or too flexible. Overdesigning has serious implications especially for large pieces of equipment where 1mm of additional wall thickness could mean 1 tone or more when applied to the whole structure. This does not affect only the material price and building costs (i.e. a plate too thick will be more expensive to buy, manufacture and will require and increased force to be bend in the final cylinder shape), but also its service life. The thick plate would add more weight to the reel, which, in turn, will lead to

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reduced payload or increased transport costs on service ships. This means money lost at each trip or, worse, other reels to be used for their more efficient design. All these and other issues could be avoided through a good design. But the problem is that there is no unified standard that could provide accurate and well documented guidance to an engineer. They must consult numerous standards and recommended practice guides in order to produce a design. This means that a coherent design, based on design factors and load cases specifically adapted to reel particularities is almost impossible to achieve. The standards used for reel design are mostly for general use or only marginally related to reels, and this often leads to overly conservative solutions. Furthermore, the third party verifier’s job is even more difficult and most of the times summarizes in just checking calculations and correct application of designer’s assumptions, but cannot refer to an industry generally accepted set of rules that regulate this grey area. In absence of these rules, the interpretation of the numerous existing standards is highly subjective and dependent on the understanding of each engineer.

1.1 Scope of Work So, in light of those written above, the scope of this dissertation is to summarize the design approaches for reels based on 2 design methods (LRFD and WSD), point out the differences, comment on the results and present its conclusions to the public. The final purpose of this study would be to provide a good starting point for further researches that could ultimately lead to the development of a “recommended design practice” for reels.

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Chapter 2: Design methods The most utilized design methods used for reel design are LRFD and WSD. None of them applies directly to reels. However, engineers have designed reels based on these 2 methodologies until now and are likely to do so in the future, so why the need for a new design code?

2.1 LRFD (Load Resistance Factor Design) Method In LRFD method, the safety of the design is obtained by multiplying the loads and dividing the resistances with safety factors. Material resistances are divided with material safety factors (i.e. γm= 1.15 in DNV standards), while loads are multiplied with factors higher than 1. The value of these factors depends on the safety class desired (e.g. high, medium or low). [3][4] The design approach is described in detail in DNV-OS-H102 “Marine Operations, Design and Fabrication” and DNV-OS-C101 “Design of Offshore Steel Structures, General (LRFD Method)”. However, for the purposes of this dissertation, from all load cases described in the 2 standards, only the Ultimate Limit State (ULS) and the Serviceability Limit State (SLS) will be analyzed, particularly the way how the load factors are chosen in the load combinations. SLS represents the normal operational mode for the offshore structure, in this case the reel. The designers must ensure that during normal operation, the structure will not experience loads that will cause high stresses (close to or above yield) or deformations, thus the structure will not become unsuitable to perform its intended job. Usually, a deformations check (actual deformations are compared with allowable ones) is performed to ensure the suitability of the design. ULS checks will ensure that the structure will not collapse under the worst case scenario load combination that could be experienced during its service life. Basically, the ULS dictates the strength requirements a structure must have, directly influencing its design. Thus, choosing too conservative load factors will lead to an overdesigned structure that would do the job, but in an uneconomical manner.

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So, it is of great importance to understand how the structure works, how it is going to be loaded and operated, how to use the material and, in case of reels, the payload’s characteristics in the benefit of the overall design. LRFD method is used as a general design method, described in the 2 DNV standards. They provide a general design methodology applicable to all structures involved in marine operations

2.2 WSD (Work Stress Design) Method WSD is a design method where safety is achieved by limiting equivalent Von Misses stresses to a decreased value of the material’s characteristic strength. DNV’s no. 2.22 “Lifting Appliances” standard is built around WSD method; the maximum allowable stresses should be equal or lower than 85% of the yield strength of the material. In other words, the usage factor of the material is limited to 0.85. [5][6] This standard is specifically built on industry experience and good design practices for all structures that can be defined as lifting appliances: cranes and their components, spreader beams, lifting sets, etc. The part relevant for reel design is the one dedicated to winches. Although winches and reels basically share the same constructive principles, the size difference and the way they work during operation (from a structural perspective) makes them so different. Lifting Appliances 2.22 standard provides a design procedure for winches and is calibrated according to winch operating requirements and particularities. Reel designers can only use to the part referring to drum and flange design.

2.3 Additional Standards Additional design tools are borrowed from Eurocode 3, DNV-RP-C202 “Buckling Strength of Shells” or other recognized and industry accepted design standards in order to cover the necessary strength requirements of the new design.

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2.4 Comments But all these existing standards do not cover the specifics of reels and their transported products. For instance, none of the design methods take into account the increased number of layers the product is spooled onto the drum and the benefic influence the layers have on the overall drum strength; or how spooling tension in the product is transferred to the drum. Recent studies lead to interesting conclusions that could help to improve the current way of designing reels. Further on, in this paper there will be analyzed 2 of the most commonly used reel designs. Also, there will be analyzed and explained the loads action on the reel, the various load combinations identified during lifting, transportation and operation. One of the designs will be chosen and analyzed in an FEA program according to load cases built on LRFD and WSD principles and the 2 sets of results will be compared. Ultimately, the conclusions and recommendations chapter will try to comment on the ways the design of reels could be improved considering recent studies and findings and on the results from the FEA analysis.

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Chapter 3: Design by LRFD Method Design by LRFD method is covered by DNV-OS-H102 “Marine Operations, Design and Fabrication” and DNV-OS-C101 “Design of Offshore Steel Structures, General (LRFD Method)”. Since these standards provide general rules, applicable to all types of offshore structures and installations, only the information relevant to reel design will be selected and discussed.

3.1 Load Types Every structure, regardless of its nature, will be likely to be subject to the combined effect of at least 2 of the following load types:
Permanent loads (G)
Variable functional loads or Live loads (Q)
Deformation loads (D)
Environmental loads (E)
Accidental loads These loads will be combined into load combinations relevant to the function of each individual structure and their effect on the proposed design will be analyzed. The suitability of the design is confirmed as long as it is not prone to failure in any of the load situations considered (the design load effect – Sd – does not exceed the design resistance – Rd). Of course, a design is considered to be efficient in both economical and engineering terms when Sd is just below Rd in ULS or resistance limit state. This ensures a rational and efficient way of using material characteristics in favor of the overall design and avoids the overdesign of the structure.[3][4] The load types presented above will be grouped into load combinations (also called limit states): ULS, SLS, FLS (fatigue limit state) and ALS (accidental limit state). Each load type will be multiplied by a safety factor according to the possibility of that load type to occur during that particular limit state and the impact it will have on the structure. [3][4]

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Although each load type includes many subcategories of loads to be considered, only subcategories considered relevant to reels will be discussed: •

Permanent loads (G) – fixed loads that will not vary over the entire service life of the structure o Self weight of structure o Weight of permanent installed equipment that cannot be removed (i.e. drum partitions) Weight estimate and weight distribution (thus an accurate estimation of the relative position of the center of gravity) is of great importance especially for structures and equipments subject to lifting operations.

•

Variable functional loads (Q) – these loads can vary during the service life of the structure; they can be defined as: o Payload – stored materials, equipment (i.e. umbilicals, pipes, wires, etc.) o Operation forces generated by reeling/unreeling of product Again, the weight of the payload shall be accurately measured for the purposes of lifting operations. The maximum value of the payload shall be considered for dimensioning the structural elements.

•

Deformation loads (D) – not relevant for reels

•

Environmental loads (E) – loads generated by environmental factors, such as: o Wind o Waves, that generate dynamic effects. In the case of reels, wind loads shall be considered during lifting operations onshore.

Combined wind and wave effects will affect the transport vessel and that will translate into vessel motions. These motions will generate inertia forces and should be considered when designing the sea fastening arrangements (including the sea fastening geometry and structural components that will need to handle the load induced stresses), as well as assessing their impact on the reel structure and auxiliary equipment (towers, rollers) and their connections with the ship. [3][4] 13

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•

Accidental loads (A) – due to occurrence of unexpected events o Dropped objects Since reels can be considered pieces of equipment with transportation purposes and rather

simplistic operational function, the accidental loads considered relevant could only cause damages that would endanger their good operation (large deformations of flanges or severe damage to the flange-drum connection caused by large and heavy dropped objects). Sea fastenings should be able to cope with sudden loads associated to minor ship collisions (very unlikely).

3.2 Load Combinations The load types described in 3.1 will be grouped into load combinations. The design loads used in the load combinations are obtained by multiplying the characteristic loads with design load factors. [3][4] Relevant for reels are the following load combinations: 1. Onshore spooling – loads to be considered: o Self weight of reel o Weight of payload o Spooling tension 2. Onshore storage – same as onshore spooling, only spooling tension will vary; because of deformations in the anchoring devices (e.g. steel wires will increase in length due to tension) that will hold the end part of the product after spooling, part of the spooling tension in the last layers of product will decrease. 3. Onshore lifting: o Self weight of reel o Weight of payload o Storage tension o Loads generated by wind

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4. Offshore operation – this load combination is defined as “ULS a)” in DNV-OS-H102; it can also considered as SLS o Self weight of reel o Weight of payload o Spooling/unspooling tension o Dynamic effects 5. Transit survival – this load combination is defined as “ULS b)” in DNV-OS-H102; o Self weight of reel o Weight of payload o Storage tension o Increased dynamic effects compared to ULS a) The most relevant load combinations that will be further considered are “Offshore Operation” and “Transit Survival”. These will determine the overall strength requirements the reel design must comply with. The FLS will not be covered by this dissertation; ALS is not considered to be relevant to reel design in general. [3][4] DNV standards (OS-C101 and OS-H102) provide a table for the load factors to be used for the 2 relevant ULS load combinations: Load factors for ULS Load Categories

Load Condition

G

Q

D

E

A

a

1.3

1.3

1

0.7

N/A

b

1

1

1

1.3

N/A

Table 5-1 in DNV-OS-H102

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3.3 Comments The standard allows for a decrease of the load factors applicable to G and Q loads in load combination a) to a value of 1.2 if these load types are well defined. This lower value is usually adopted by industry in reel design, as the self-weight of reels, spooling tension and payload are clearly stated. [DNV OS-C101, Section 2, B402] Also, according to DNV OS-C101, Section 2, B404, the load factors for the environmental loads in combination b) can be lowered to a more permissive 1.15 value for unmanned structures during extreme environmental loads. Since “ULS b)” will be associated to “Transit survival” load case, it is assumed that the reel will not be in operation during severe weather conditions, therefore can be considered as unmanned. [3] LRFD standards propose a 10-2 annual return probability for ULS combinations. This translates into a level of safety based on the “100 year storm” occurrence. This can be considered somewhat exaggerated, since reels are transported offshore by ships (more rarely on barges), which according to ship design rules, are designed for loads with 10-8 (or 20 years) probability of exceedance, hence much lower than the 10-2 return probability requirements in OS-C101 or OSH102.[3][4]

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Chapter 4: Design by WSD Method Similarly as the LRFD method, the WSD method and standards propose rules and general principles on how to select relevant load types for different structures and equipment, the way to calibrate them using design load factors and, ultimately, these combined loads will generate stresses on the structural elements that would be divided with the material allowable resistance and compared to agreed usage factors. The usage (or utilization) factors are precisely calibrated so that they would allow the design to achieve a certain level of safety according to the role of the structure/equipment will perform during its service life. [5][6] Table E-1 in DNV-OS-C201 gives the basic usage factors for each of the load conditions considered. The analysis must be conducted for the “worst case scenario” generated by the relevant load combinations. Loading conditions

η0

a)

b)

c)

d)

e)

0.60 1)

0.80 1)

1.00

1.00

1.00

1) For units unmanned during extreme environmental conditions, the usage factor η0 may be taken as 0.84 for loading condition b).

Table E1 Basic usage factors η0 [5] DNV-OS-C201 is a design standard similar with DNV-OS-C101, the difference being that one is built on the WSD method (OS-C201) and the other on the LRFD method. They both provide general design rules for offshore steel structures. However, also based on the WSD method is DNV no. 2.22 “Lifting Appliances” standard for certification. DNV 2.22 is written based on general WSD principles, but tailored on the specific requirements and studied behavior of lifting equipment. Relevant to this study is the section related to the design if winches, to which, as previously mentioned, reels could be associated. The advantage of having a dedicated standard is that it is calibrated to the equipment’s needs. For instance, studies were conducted on specific pieces of machinery (e.g. winches), their behavior was observed during their service life in all types of environments, failures were documented, the causes of failure were identified and lessons could have been learned, 17

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experiments were conducted and designs could have been improved. All these activities generated conclusions which were analyzed and compared with the existing general design rules. Once it was understood how specific equipment actually perform in real situations, the overall design could have been improved. The results were further translated into specific design rules. For the specific case of winches, that meant that the usage factor was increased from lower values – 60% – to 85% of yield strength (for both drum and flange) for functional loads. This is a huge improvement, allowing structures to become lighter, carry more payload or operate at higher tensions. [6]

4.1 Loads Types Just like DNV-OS-C101, DNV 2.22 lists load types relevant to the design of lifting appliances that should be used for design/design verification. For reels, the following loads will be selected:
Principal loads
Loads due to motion of the vessel on which the crane is mounted
Loads due to climatic effects 1.

Principal loads – define loads given by self-weight of components, payload and loads due to

pre-stressing. In case of reels, the pre-stressing load can be translated into reeling tension, or the tension that will be applied to the product (e.g. umbilical, wire, etc.) during spooling in order to obtain a tight, well arranged product on the reel drum, that will not become tangled and will be easy to unspool. Reels are not subject to horizontal loads as described in DNV 2.22 standard, as they only rotate around their longitudinal axis. These forces are considered to be the consequence of loads induced by movement of cranes on rails (generated by acceleration and braking), therefore not applicable to reels. [6] Therefore, they will be disregarded from the load combination. 2.

Loads due to motion of vessel – are represented by the inertia forces that act on the

equipment (reel). The inertia forces are generated by the ship’s motions (pitch, roll, etc.). The ship’s motions will be calculated with DNV Ship Rules Part 3, Chapter 1, Section 4 for a

10-8 probability of repeatability; this corresponds to a once in 20 years probability of

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occurrence, and the values are less than the once in 100 years (or 10-2 return probability) defined in LRFD standards.[7] There are 3 types of vessel motions: -

Vertical (V)

-

Longitudinal (L)

-

Transversal (T) Due to the fact that the vessel has a major axis (longitudinal axis), the ship theory

demonstrates that not all motions are in phase (at their maximum intensity). The 3 motions are combined as in the ship rules – DNV Rules for Ships Part 3, Chapter 1, Section 4 C500). The factor for various motions shall account the targeted level of probability). The possible motion combinations are: i.

V

g±av

ii. V + T

g/(±at)

iii. V + L

(g±av)/ al

where: g – acceleration of gravity (g ≈ 9.81m/s2) av – vertical acceleration of ship at – transversal acceleration of ship al – longitudinal acceleration of ship [7] Depending on the ship’s characteristics and weight, the loads due to vessel motions can be calculated and then the most unfavorable can be applied to the design of the reel. 3.

Loads due to climatic effects – loads due to wind (especially) They are particularly important for lifting of reels onshore; offshore, high winds will

generate high waves, thus increased vessel motions, and so the predominant loads will still be those generated vessel motions.

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4.2 Load Combinations DNV 2.22 gives 3 load cases, similar with the ones in DNV-OS-C201. Relevant to the reels, these load cases will be translated into: Reel

DNV2.22

DNV-OS-C201

Case I Operation in

(Crane without wind)

standard conditions

Load condition a)

SG + ψSL (payload + spooling tension) Case II (Crane working with

Load condition b)

wind)

Environmental dynamic operation

maximum combination of SG + ψSL +SW *)

environmental loads and associated functional loads

Case III (Crane subjected to Transit survival

exceptional loadings) SG+SL+SM **)

*)

Load condition c), d), e)

-

in the case of reels, the maximum operational accelerations are assumed to be the vessel

motions during operation (so lower vessel motions calculated based on a 10-4 probability to be exceeded – or 1 day return period) in the case of reels, Ψ is considered to be 1 **)

extreme loading conditions when the reel is not in operation, but inertia due to vessel motions

are extremely high and calculated based on the “20 year storm” scenario (or 10-8 probability) where: SG – self weight of reel; SL – loads due to payload and spooling tension; SW – loads due to wind; SM – loads due to vessel motion. [6] 20

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Chapter 5: Discussion on LRFD and WSD Methods 5.1 Load Combination Factors for LRFD Method If an engineer would want to design a reel base on the LRFD method, then he would have to refer to DNV-OS-C101 for guidance regarding the load cases and load combinations to be considered. The load factors that will need to be applied to each load type can be found in Table 5-1. As previously discussed, the load combinations that would ultimately dictate the strength requirements of the reel would be those corresponding to the Ultimate Limit State (ULS). Load factors for ULS Load Categories

Load Condition

G

Q

D

E

A

a

1.3

1.3

1

0.7

N/A

b

1

1

1

1.3

N/A

Table 5.1 (DNV-OS-H102, Table 5-1) For “Load condition a)”, which corresponds to the “Environmental dynamic operation” case, the load factor 1.3 for G (self-weight of structure) and Q (live loads) may seem to be too high because: •

The overall weight of the reel should be accurately estimated for lifting purposes;

•

G is fixed – the self-weight of the reel is well controlled;

•

The self-weight of the spooled product (i.e. umbilical) cannot be easily monitored and should be the full responsibility of the end user not to exceed the maximum value; however, there are reduced chances to exceed the maximum value, as only an increase in length would generate additional product weight.

•

The spooling tension (T): it is assumed that the maximum value is not exceeded and that systems to prevent the overcome are arranged – i.e. tensioners;

•

The loads developed on the reel drum and flanges during spooling (pressure loads as a function of T) can only be estimated by using DNV 2.22 formulas for hoop stress and flange pressure [6, Chapter 2, Section 3, B207 and B208]. These can be considered conservative, as they already have built-in safety factors (i.e. the rope layer factor – C). 21

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Having in mind that these loads are constant or with quasi-constant effect, the 1.3 load factor may be considered over conservative and a decreased value of 1.0 for the load factor might be applicable. Assuming that this case reflects the operational condition, the 0.7 load factor used for environmental loads (E) is hard to explain as long as accelerations or wind speeds indicated as the upper limit for the operational criteria are expected. For “Load condition b)”, which corresponds to the “Transit survival” condition, the extreme environmental load factor of 1.3 applied to the environmental loads (E) does not seem to be in accordance with the expected accelerations and wind speeds indicated for a 10-8 probability to be exceeded as indicated in DNV Rules for ships Part 3 Chapter 1 Section4. Based on the above assumptions, this paper would suggest the following values for the load factors: Proposed load factors for ULS Load Categories

Load Condition

G

Q

D

E

A

a

1.0

1.0

1.0

1.0

N/A

b

1.0

1.0

1.0

1.3or 1.0

N/A

For “Load condition a)”: •

A 1.0 load factor applicable to the self-weight of the reel and product and spooling tension (G, Q, T respectively)

•

A 1.0 load factor applicable to environmental loads (E) corresponding to the ship’s maximum accelerations in operational condition (i.e. during spooling/unspooling operations)

Note: If there are doubts regarding the values for T and E, the corresponding values for the load factors can be estimated with other reliable methods e.g. CN 30-6 - Structural reliability analyze of marine structures.

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For “Load condition b)”: •

A 1.0 load factor applicable to the self-weight of the reel and product and spooling tension (G, Q, T respectively)

Note: If there are doubts regarding the value for T, the corresponding value for the load factor can be estimated with other reliable methods e.g. CN 30-6 – “Structural reliability analyze of marine structures”. •

A 1.3 load factor applicable for the ship’s dynamic in transit/survival conditions with accelerations at 10-4 probability to be exceed (DNV Ships Rules Part 3, Chapter 1, Section 4, C500)

or •

A 1.0 load factor applicable for the ship’s dynamic in transit/survival conditions with accelerations at 10-8 probability to be exceed (DNV Ships Rules Part 3, Chapter 1, Section 4) Summarizing the analysis listed above: The operational conditions of the reel could analyzed as LRFD “Load condition a)” with

a general 1.0 load factor for every load type, where environmental loads are associated with the ship’s maximum motions for operational conditions. The “Transit survival” conditions could be analyzed as LRFD “Load condition b)” with a load factor of 1.0 for the self-weight of the reel, product and spooling tension and an environmental load factor of 1.3 or 1.0 depending on the probability level of the environmental loads (dynamic of the ship and wind speed).

5.2 Utilization Factors for LRFD and WSD Methods After analyzing the behavior of the proposed structure under all these load scenarios, the stresses resulted must be compared with allowable utilization factors. Confusions appear when dimensioning the reel’s structural elements, since each standard gives different values for the usage factors (however, the differences are not that high) 23

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Load case 1 2 3

DNV 2.22 1 = 0.67 1.5 1
= = 0.75 1.33 1
= = 0.91 1.1
=

DNV-OS-C201 a) 0.6 b) 0.8 c), d), e)

1.0

Usage factors according to table C-1 in DNV 2.22 (permissible stresses for elastic analysis) and E-1 in DNV-OS-C201 But the load combinations proposed are made of a summation of different load types, each multiplied with a safety factor. The functional loads are represented by self-weight loads, payload and spooling tension. This spooling tension will be used to calculate the hoop stress on the drum. The thickness of the drum can then be easily obtained by dividing the hoop stress with the material resistance and comparing it to a maximum usage factor of 85% of yield strength. [6] It can logically be assumed that by adding the Tare and Payload to the equation, combined with the 85% utilization from spooling tension, the overall utilization will be greater than 85%. So, a safe conclusion would be that the entire functional load combination should be limited to an 85% utilization factor. But then both DNV 2.22 and OS-C201 give a U value of 0.67 and 0.6 respectively for this load combination. Which one is to be used? Similarly, for the Dynamic Operation load case, to the functional loads will be added the inertia loads from the vessel motions, and again, the overall utilization will be higher than 85%. Therefore, the utilization from the functional loads will then need to be further lowered so all the 3 loads (SG+SL+SM) will not exceed 0.85 allowable utilization. But then how to comply with proposed 0.75 and 0.8 utilization in DNV 2.22 and OS-C201 respectively? LRFD method only gives load types and load combinations. It does not provide any tools that would enable the calculation of the stresses in the structural elements, therefore enabling their dimensioning. So, for reel design, hoop stress and flange pressure are calculated based on the same formulas found in DNV 2.22 (so similarly to the WDS method), and comparing results with 0.85 allowable utilization factor. 24

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If the engineer then combines his loads according to the LRFD method: •

ULS a): 1.3 x F + 0.7 x E, just by multiplying 0.85 with 1.3 the usage factors will exceed 1 (0.85 x 1.3 = 1.105 > 1). Then how can be fulfilled the safety requirement of 0.86 utilization in DNV-OS-C101? (
=





=



.

= 0.86)

- γM = material factor according to DNV C-101 Section 5, B100  <  ;  =

1  γ 

where: Sd = design load - Rd = resistance factor - fy = yield strength of material [3]

5.3 Hoop Stress and Flange Pressure When trying to establish the reel’s drum wall thickness, the methodology used is the one proposed by DNV 2.22 standard. Basically, the hoop stress in the drum will be calculated by considering the pressure resulting from on-spooling the product stored on the reel (umbilical, risers, etc.). As the product will be spooled onto the reel under a well-controlled tension, the pressure applied onto it will try and squeeze the drum, thus forcing it to expand on longitudinal direction. The outer flanges of the reel will try to restrict the drum from expanding, thus generating longitudinal stresses into the drum’s walls. The simple equilibrium of forces is achieved by considering only 1 layer of product (umbilical, wire, etc.), so the equations need to be calibrated for the effect of multi-layering. Multi-layering of product onto the drum of the reel or winch involves a much more complex array of forces than the simple case of 1 layer, where only tension is to be considered. This is the reason for the rope layer factor (C) present in the DNV 2.22 hoop tension formula:

25

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 =  ∙

  ∙

!"

#$. [5.1]

where: -

S = spooling tension

-

P = pitch of product (wire, etc.)

-

tav = plate thickness

-

C = rope layer factor: o C = 1 for 1 layer o C = 1.75 for 2 layers o C = 3 for more than 5 layers [6] It can be easily observed that the hoop stress value is governed by 2 parameters: spooling

tension (S) and rope factor (C). DNV 2.22 is built on the assumption that the product will be spooled under maximum allowable and uniform tension. Therefore, the wire will experience the same tension all across its cross section and the entire product will be spooled at a constant, well controlled tension. This is logic and perfectly achievable in the case of steel wire (where the exact material characteristics are known and only one material is used for the wire fabrication) spooled onto winches. But what happens when synthetic fiber rope or composite products (like umbilicals), made up from 5 or more different materials, with different properties, prone to internal slippage between components, for which their behavior under tension is not entirely made public by their manufacturers or simply unknown? [8] Furthermore, umbilical storage reels are extremely large structures, built with high tolerances, even at the drive hub. So, based on reel designers and fabricators information, accuracy is not one of reels’ strengths, especially when talking about spooling. If the tension can be more accurately controlled when using a tensioner, in case of spooling when the reel is mounted on rollers (and the rotation of the reel is achieved due to friction between the rollers and the outer edge of the flange) or by hub drive (i.e. when the reel is mounted between 2 massive towers like in figure xxx. for tower driven systems), correct product spooling is often

26

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acknowledged not by measuring the spooling tension employed, but by visual confirmation that the product is properly arranged onto the reel drum. [8] Sometimes, due to product tension limitations (especially umbilicals) or restrictive crushing pressures imposed by product fabricator or because of umbilical end terminations that would make attaching the product to the reel more difficult and not as strong as originally intended, then the spooling tension will no longer be uniform, neither it will have significant values, so the hoop stress generated will be low. [8] So, what degree of accuracy do the DNV 2.22 formulas have in these cases? If the hoop stress is low, then the pressure acting onto the flanges (which is directly dependent on the hoop stress value, as it can be seen in Equation 5.2), will be even lower, especially if considering the industry preferred arrangement (the product layers carefully winded ones on top of the others as shown in Figure 5.1).

Figure 5.1 Product Arrangement on the Reel Drum

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' =

2 ∙ !" ∙  #$[5.2] 3 ∙ )

where: -

Φ = drum diameter

-

tav = average drum thickness of drum barrel [6] A logical question could be why the product crushing pressure is not considered to be a

suitable design parameter? [8] Would the provided formulas still ensure a safe design or should there be a static (based primarily on self-weight of structure and product) design method provided as back-up, just for such situations? DNV current practices and latest recommendations (that are included in the 2011 updated version of the 2.22 standard), based on studies and industry experience, speak of a linear increase of the value for C from 1.75 for 2 layers to 3 for more than 5 layers of product. According to DNV, some winch and crane manufacturers have even confirmed that the new increased value of C=3.0 matched the results from full scale testing of their products. However, DNV allows decreasing the value for the C factor after special considerations. [9] How relevant and what positive influence can this increase of C have on umbilical reels? Did they consider a large number of materials with different characteristics or just various steel wire ropes? Why is this apparent contradiction in conclusions between DNV and other public studies?

28

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5.4 Rope Factor (C) But why is C factor so important? By multiplying the spooling tension with a higher than 1 coefficient, the hoop stress will increase rapidly, thus leading to the need of a thicker drum wall. Basically, since the spooling tension cannot be modified due to proper spooling and winding reasons, a logical and relevantto-the-design value of C is very important. Understanding the reasons behind a certain value for C and how they can be manipulated into the benefit of the design (thus enabling a decrease in the wall thickness of drum) is crucial. It has been observed that rope stiffness has a direct impact on how the loads are distributed on the drum. Tough, very stiff steel wire ropes will push harder into the surface of the drum, thus almost cutting into its shell. A softer, less rigid product like an umbilical will deform more under the spooling tension loads that a steel wire, and will act like a damper. Combined with the increased lateral area, the pressure will be distributed more evenly onto a larger surface, thus lowering the loads at the drum’s surface. [9][10] Another important aspect is related to how much load will experience the first (inner layer) of the product. If the designers can confirm that the first layer of the product will be subject to a reduced load than the next layers, then DNV guidelines allow the use of a reduced value for the C factor.

5.4.1 “Large Wire Rope Mooring Winch Drum Analysis and Design Criteria” Study The “Large Wire Rope Mooring Winch Drum Analysis and Design Criteria” study supports the above DNV conclusions and proves by calculation the direct relationship between lateral modulus of elasticity and load transfer for the reel drum. [9] The study concentrates on investigating how the stiffness of the wire, spooling tension and number of layers influence the load transfer to the drum. Although well known by manufacturers, from previous experience and analysis conducted over the years by design engineers that got repeating results, it was observed that rope characteristics (diameter, stiffness, 29

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etc.), number of layers and spooling tension dictate the impact on the reel barrel, but no study could show in what proportion each of the 3 affect the overall result. Flange and drum design are directly influenced by product stiffness, yet few have thoroughly investigated this aspect, although accurate results and clear conclusions could lead to more economical winch/reel design. The research results showed 2 important aspects: 1. The wire tension decreases towards the middle layers (possibly due to friction? Fig xxx – no explanation provided); 2. High loads on the flange thrust if products with lower stiffness (products that would deform more) are spooled at high tensions [10]

5.4.2 “Problems Related to the Design of Multilayer Drums for Synthetic and Hybrid Ropes” Study Similar results were obtained and recorded in the study conducted at the University of Clausthal by P.Dietz, A. Lohrengel and others on the “Problems Related to the Design of Multilayer Drums for Synthetic and Hybrid Ropes”. The study revealed that winches carrying products with reduced Young’s modulus in transversal direction will experience lower pressures on the drum, but increased loads on the flanges. This happens because of the deformation of the product’s cross section from circular to oval, thus the pressures on the drum will decrease because of increased product footprint on the drum and because the deformation of the product will act as a damper, consuming energy until the forces will no longer be able to deform the product. In the same time, on transversal direction, the cross section of the squeezed product will increase in width, thus generating additional loads on the flanges. [11] In this case, the way flange pressure is calculated in DNV 2.22, based on the hoop stress will no longer match the actual way the loads are distributed on the drum and flange, leading to the possibility that the flanges will be under-designed. Although their study was conducted on fiber rope, the results can be considered relevant, as umbilicals also have a reduced Young’s modulus in transverse direction, being prone to deformation.

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The rope factor gives a generic value, applicable to all types of products. It is not shown how those values were obtained and based on what assumptions. Does it consider the benefic influence of friction? How does the rigidity of the product influence the overall loads that act on the drum?

5.5 Spooling Tension – Friction Factor Relationship; Friction between Product Layers The standards do not provide any information regarding the positive (or negative) influence the friction between product layers has on load transfer to the reel drum. DNV 2.22 provides a 0.1 friction coefficient for the drum; does that cover accurately the friction between the layers of an umbilical? The following equations and logic was developed with the help and under the guidance of my industry supervisor, Mr. Marius Popa. Based on his experience working with reels, by understanding the general forces that act upon the product and stresses that develop within the product during the spooling process and corroborating them with product rigidity and friction between the product layers, we managed to transfer into equations his way of seeing the state of efforts that act on the product during spooling operations. Spooled product (i.e. umbilical)

Drum Center

Figure 5.5.1 Forces in Spooled Product 31

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From Figure 5.5.1 it can be observed that: *+ = , ∙ -. ∙ 

[Eq. 5.5.1]

- = *' = / ∙ (1)

[Eq. 5.5.2]

 ∙ -. + 1 − *+ = 0 ⟹ *+ −  ∙ -. = 1

[Eq. 5.5.3]

If we assume K = 0, then the equations will become: *+ = 0

[Eq. 5.5.4]

Eq. 5.5.2 will remain the same  ∙ -. + 1 = 0 ⟹  ∙ -. = −1

[Eq. 5.5.5]

By substituting Eq. 5.5.5 in Eq. 5.5.2 we obtain: 67

-5 = −/ ∙  ∙ -. ⟹ 68 = −/ ∙ 

[Eq. 5.5.6]

By integrating and rearranging, Eq. 5.5.6 will become:  = 9 ∙ : ;