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Tow (or transportation)

Last updated on: July 28, 2008

This article is an introduction and a brief how-to guide for performing a simple tow (or transportation) analysis.

Egret Jacket

Table of contents

Why tow?

Due to logistics, quality control and prohibitive costs, offshore structures are typically fabricated onshore, and then towed—on a cargo barge—to their intended service locations, usually offshore. Transportation and arising stresses out of it therefore become important aspects in ensuring pre-service compliance for an offshore structure.

Importance of a tow analysis

  • To assess and design the structure for transport stresses.
  • To provide additional temporary members (sea-fastenings) for support during transport.
  • To strengthen structure to suit transport analysis, orient member spanning to be beneficial and economical during transport.

Factors affecting tow

  • Sea-state.
  • Barge size (larger barge—more stable, lower stresses; smaller barge—higher stresses).
  • Weight of the structure.
  • Overall COG (center of gravity) of the structure.
  • Overall COR (center of rotation) of the transport barge.

Barge criteria

Based on their size, they can broadly be classified as large barges and small barges. Barges longer than 76m, and wider than 23m are classified as large barges—as a general guideline. Small barges are those that do not exceed 76m in length and 23m in width.

Typical large barges:

Barge type Length (m) Width (m) Depth (m)
PEGAH 91.3 27.4 6.1
SELCO GIANT 2 91.3 27.4 6.1
LB1-SB282 (NPCC) 130.5 30.5 8.0

Typical small barges:

Barge type Length (m) Width (m) Depth (m)
CB (MDL 578) 64 22 4.0
CB (MDL 750) 60 18 4.0

Computer model

Jacket loadout model A four point support Jacket loadout model is used for in a tow analysis.

The computer model used will be similar to a Loadout model, with additional tie-downs, and sea-fasteners as required.

tow.gif

Where:

F = Component of gravity plus inertia.
G1 = Center of gravity of jacket.
G2 = Center of gravity of the tow.
M = Meta-center of the tow.
A = Areas of potential impact.

Transportation forces

Transportation forces consist of the following:

  • Self weight of the modeled structure.
  • Non-modeled pre-installed item loads.
  • Equipment dry loads.
  • Inertia loads—due to motions of the transport barge.

Inertia forces are generated when platform components (jacket, deck) are transported offshore on barges or self-floating. They depend upon the weight, geometry and support conditions of the structure (by barge or by buoyancy) and also on the environmental conditions—waves, winds and currents—encountered during transportation. Types of motions a floating structure may experience are shown schematically in the figure below.

motion.gif

Barge motion data

Depending upon barge size, and in the absence of motion data available from a full blown barge motion study along the intended transport route, the following motion data (from Noble Denton criteria) is normally used for calculating inertia loads.

Large barge Small barge
Motion Amplitude Period Amplitude Period
Roll 20° 10s 25° 10s
Pitch 12.5° 10s 15° 10s
Heave 5m 10s 5m 10s

Load combinations

1. Simulating Head seas 100%Pitch + 100%Heave
2. Simulating Beam seas 100%Roll + 100%Heave
3. Simulating Quartering seas 50%Pitch + 50%Roll + 100%Heave

In addition, effective horizontal shear force due to barge inclinations, corresponding to the max. pitch/roll angle, may be included in the cases above. Wind load, in general, may not be considered in the above cases.

Analysis

The transportation analysis is a two step process.

  1. Static analysis: Analyze the structure for it’s static (dead) loads and get a solution—we’ll call it a static solution.
  2. Tow (dynamic) analysis: Generate inertia loads corresponding to motion characteristics. (Motion characteristics are either actually furnished by a naval architect following barge motion analysis, or in the absence of a barge motion analysis, industry-wide acceptable recommendations by notables such as of Noble Denton may be followed.)

(In tropical seas, it is noted that Noble Denton criteria is slightly conservative and is only used in the event barge motion characteristics data is not available.)

  • Ensure that inertia loads thus generated are of proper magnitude as expected and comply with the motion characteristics.
  • Get a solution for inertia loads generated using motion characteristics. We’ll call this solution a dynamic solution).
  • Post and Postvue: Combine the two solutions: static + dynamic to obtain a combined solution which will be used for performing code–check.

Tow analysis (SACS) input file using motion cards:

  1. * AS PER NOBLE DENTON CRITERIA
  2. * PITCH ANGLE = 12.5d; PERIOD = 10s; SURGE = SIN(12.5) = 0.216
  3. * ROLL ANGLE = 20d ; PERIOD = 10s; SWAY = SIN(20) = 0.34
  4. TOWOPT MNECLD WP -12.389 0.000 -8.500 XYZ
  5. POSITION
  6. * HEAD SEA CONDITION (100% PITCH AND 100% HEAVE)
  7. MOTION 21 +12.5 10. +.216 +0.2
  8. MOTION 22 -12.5 10. -.216 +0.2
  9. MOTION 23 +12.5 10. +.216 -0.2
  10. MOTION 24 -12.5 10. -.216 -0.2
  11. * BEAM SEA CONDITION (100% ROLL AND 100% HEAVE)
  12. MOTION 25 +20. 10. +0.34 +0.2
  13. MOTION 26 -20. 10. -0.34 +0.2
  14. MOTION 27 +20. 10. +0.34 -0.2
  15. MOTION 28 -20. 10. -0.34 -0.2
  16. * SIMULATING QUARTERING SEAS (50% ROLL, 50% PITCH AND
  17. * 100% HEAVE)
  18. MOTION 29 +10. 10.+6.25 10. +.108+.17 +0.2
  19. MOTION 30 -10. 10.+6.25 10. +.108-.17 +0.2
  20. MOTION 31 -10. 10.-6.25 10. -.108-.17 +0.2
  21. MOTION 32 -10. 10.-6.25 10. -.108-.17 -0.2
  22. MOTION 33 -10. 10.+6.25 10. +.108-.17 -0.2
  23. MOTION 34 +10. 10.+6.25 10. +.108+.17 -0.2
  24. MOTION 35 +10. 10.-6.25 10. -.108+.17 -0.2
  25. MOTION 36 +10. 10.-6.25 10. -.108+.17 +0.2
  26. END
  27. Download this example: /inputfiles/trans-motion.txt

Tow analysis (SACS) input file using acceleration cards:

  1. * AS PER NOBLE DENTON CRITERIA
  2. * PITCH ANGLE = 12.5d; PERIOD = 10s; SURGE = SIN(12.5)= 0.216
  3. * ROLL ANGLE = 20d ; PERIOD = 10s; SWAY = SIN(20) = 0.34
  4. TOWOPT MN CG -12.389 -8.500 XYZ
  5. LCFAC 1.10 2
  6. ACCL 0.00 0.00 0.00 1.00 0.00 0.00
  7. ACCL 0.00 0.00 0.00 0.00 1.00 0.00
  8. ACCL 0.00 0.00 0.00 0.00 0.00 1.00
  9. ACCL 1.00 0.00 0.00 0.00 0.00 0.00
  10. ACCL 0.00 1.00 0.00 0.00 0.00 0.00
  11. ACCL 0.00 0.00 1.00 0.00 0.00 0.00
  12. END
  13. Download this example: /inputfiles/trans-accl.txt

Appendix: List of barges

Please see a list of barges and launch barges for 40m width and above.

Discussion

This article does not cover many aspects and variables involved in a typical detailed design. However, the author is of the opinion that it provides a general guideline in understanding and performing a tow analysis. Readers are welcome to suggest errors, omissions and obvious inclusions—if any—in this article. You may write to the author here.

Disclaimer: Please be aware that the author bears no responsibility with regards to the content and use of this article.

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