1.3 1.3 Force orcess duri during ng tow tow
Forces during tow transportation consist of the following: • Self Self weig weight ht of the the carg cargo o (str (struc uctu ture re,, equi equipm pmen ent, t, piles, piles, etc). etc).
Abstract
• Any non-modeled non-modeled pre-ins pre-installed talled item item loads. loads.
This document serves as a technical commentary and user manual (where applicable) for calculation scripts in this repository. repository.
• Equipmen Equipmentt dry dry weigh weights. ts. • Inerti nertiaa forces forces..
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Sea transportation induces inertial forces on the cargo due to barge’ barge’ss motion charact characteri eristi stics, cs, draft, draft, and MetoMetocean cean design design conditio conditions. ns. This guideli guideline ne and the associassociated script aid in determining these sea-transport forces using simple calculations—witho calculations—without ut having to rely on sophisticated phisticated and often expensive expensive software, software, such that [a] the cargo and its structural framing can be designed to withstand them, and / or [b] if economics favor it, it, choose a barge with benign motion characteristics, characteristics, which usually means a larger barge.
Due to logistics, logistics, quality control and and prohibitive costs, offshore shore struc structur tures es (viz. (viz.,, jack jacket ets, s, tops topsid ides es,, module modules, s, piles piles)) and even vendor equipment are typically fabricated onshore, and then typically towed towed on a cargo barge to their intended intended service location locations, s, usually offshore. The cargo cargo is typically transported transported on an unpropelled (dummy) barge, usually towed towed by pairs of tug boats. Transportation ransportation and arising stresses out of it therefore become important aspects in ensuring pre-service compliance for an offshore structure or the equipment transported.
1.4 Barge Barge moti motion on char charac acte teri rist stic icss Barge motion characteristics are usually determined by a barge motion motion analysis, analysis, taking taking the tow route in to account. In the the absence absence of such such a detaile detailed d analysis, analysis, GL Noble Denton recommends recommends default motion criteria, criteria, which correspond to the nature of transportation and barge dimensions. Since such a generic criteria criteria does not explicitly illustrate illustrate Metocean Metocean characteristi characteristics cs (viz., wave steepness, steepness, signific significant ant wave height height and period, period, et al.) consid considered ered in either deriving or implicitly, implicitly, these may be deemed some what conser vative. For unrestricted (open sea) transporta transportation, tion, the following (commonly used) characteristics have been extracted and reproduced reproduced below from GL Noble Noble Denton’s Denton’s Guide lines for Marine Transportatio Transportations ns. , h α β 1 > 140m and >30m 10s 20° 10° 0.2g 2 > 76m and > 23m 10s 20° 12.5° 0.2g ≤ 76m or ≤ 23m 4 10s 25° 15° 0.2g where, – Barge length length overall overall (m). – Barge Barge width width (m). – Full cycle period period (in seconds). seconds). α – Roll single amplitude angular acceleration acceleration (typically given in degrees). degrees). – Pitch single amplitude amp litude angular acceleration (typiβ – cally given in degrees). degrees). Heave single amplitude amplitude of linear acceleration (typi h – Heave cally given in terms terms of g, or in meters). To convert a heave of 0.2g in to meters, meters, for example, it can be calculate calculated d as h = 0 .2g · ( T2πh )2 (m). – Acceleration Acceleration due to gravity g ravity,, typically typicall y 9.81m/s2. g –
1.1 1.1 Importa mportanc ncee of tow tow ana analys lysis is • To assess assess and design design the structure structure for transpor transportt stresses. • To provide addition additional al temporary temporary members members (seafastenings) for support during transport. • To strengthen strengthen structure to suit suit transport transport analysis, analysis, orient member spanning to be beneficial and economical during transport. 1.2 1.2 Facto actors rs affec affecti ting ng tow tow • SeaSea-st stat ate. e. • Barge Barge size size (la (large rgerr barge— barge—mo more re stab stable, le, low lower stresses; stresses; smaller barge—higher barge—higher stresses). • Weight eight of the the carg cargo o (stru (struct ctur ure, e, equipm equipment ent,, piles piles,, etc). • Overall Overall COG (cente (centerr of gravity) gravity) of the structu structure. re. • Overall Overall COR (cente (centerr of rotation rotation)) of the transp transport ort barge. 1
The other angular acceleration parameter is Yaw; and other linear acceleration parameters are Surge, Sway. To note, a heave of 0.2g corresponds to an approximate value of 5m.
Further, Surge and Sway can both be calculated by 2 multiplying with the term, 2T ·π to obtain them in meters.
( )
1.6 Transport forces on cargo In the following, W is the design weight of the cargo. Lx, Ly, and Lz are lever-arm distances between center of gravity () of the cargo and barge center of rotation ().
1.6.1 Ro and Heave
1.5 Accelerations 1.5.1 Ro θr = (
2π T r
)2 · α
(1)
The angle, α, is taken in radians in the equation above. Correspondingly, θr is in rad . s 2
1.5.2 Pitch θ p = (
2π T p
Vertical force from : 2
) · β
(2)
The angle, β , is taken in radians in the equation above. Correspondingly, θ p is in rad . s
F vr = W ·
2
1.5.3 Heave
Ly cosα + θr · g
(6)
Vertical force from heave corresponding to : gh = (
2π T h
)2 · h
(3) F vhr =
In the equation above, h is in meters, and corresponding gh is in sm .
W g
(7)
· gh · cosα
2
Horizontal force from :
1.5.4 Surge and Sway
Surge and Sway single amplitudes each can be calculated using Pitch and Ro parameters respectively, and therefore, they are often not furnished. Surge (in terms of g) can be calculated as follows: Surge = 1.0 · g · sinβ
F hr = W ·
Lz sinα + θr · g
(8)
Horizontal force from heave corresponding to :
(4)
and Sway (in terms of g) can be calculated as follows: Sway = 1 .0 · g · sinα
F hhr =
(5) 2
W g
· gh · sinα
(9)
1.6.2 Pitch and Heave V erticalforce = F vr ± F vhr
(16)
Horizontalforce = F hr ± F hhr
(17)
1.7.1 Generating inertia forces using
There are at least two ways of generating inertia forces from barge motions in software, as illustrated below: Using motion cards: * AS PER NOBLE DENTON CRITERIA * PITCH ANGLE = 12.5d; PERIOD = 10s; SURGE = SIN(12.5) = 0.216 * R OL L AN GL E TOWOPT
= 2 0d
; P ER IO D = 10 s; S WA Y
MNECLD
WP
= S IN (2 0)
= 0 .3 4
-12.389 0.000 -8.500 XYZ
POSITION * HEAD SEA CONDITION (100% PITCH AND 100% HEAVE)
Vertical force from : F vp = W ·
Lx cosβ + θ p · g
(10)
Vertical force from heave corresponding to : F vhp =
W g
Horizontal force from : F hp = W ·
Lz sinβ + θ p · g
W g
+.216
+0.2
MOTION
22
-12.5 10.
-.216
+0.2
MOTION
23
+12.5 10.
+.216
-0.2
MOTION
24
-12.5 10.
-.216
-0.2
MOTION
25 +20.
10.
+0.34 +0.2
MOTION
26 -20.
10.
-0.34 +0.2
MOTION
27 +20.
10.
+0.34 -0.2
MOTION
28 -20.
10.
-0.34 -0.2
* 100% HEAVE)
(12)
Horizontal force from heave corresponding to : F hhp =
+12.5 10.
* SIMULATING QUARTERING SEAS (50% ROLL, 50% PITCH AND
21
* BEAM SEA CONDITION (100% ROLL AND 100% HEAVE)
(11)
· gh · cosβ
MOTION
· gh · sinβ
(13)
1.7 Load combinations
MOTION
2 9 +10.
10.+6.25
1 0.
+.108+.17
+0.2
MOTION
3 0 -10.
10.+6.25
1 0.
+.108-.17
+0.2
MOTION
3 1 -10.
10.-6.25
1 0.
-.108-.17
+0.2
MOTION
3 2 -10.
10.-6.25
1 0.
-.108-.17
-0.2
MOTION
3 3 -10.
10.+6.25
1 0.
+.108-.17
-0.2
MOTION
3 4 +10.
10.+6.25
1 0.
+.108+.17
-0.2
MOTION
3 5 +10.
10.-6.25
1 0.
-.108+.17
-0.2
MOTION
3 6 +10.
10.-6.25
1 0.
-.108+.17
+0.2
END
Phasing is assumed to combine,as separate load cases, the most severe combinations of the following:
This above method, not only generated inertia forces, but also combines them as per instructions in the input. Using acceleration cards:
• Simulating Beam seas: 100% Roll ± 100% Heave • Simulating Head seas: 100% Pitch ± 100% Heave
* AS PER NOBLE DENTON CRITERIA
• Simulating Quartering seas: 50% Pitch + 50% Roll ± 100% Heave
* PITCH ANGLE = 12.5d; PERIOD = 10s; SURGE = SIN(12.5)= 0.216 * ROLL ANGLE TOWOPT
In addition, effective horizontal shear force due to barge inclinations, corresponding to the max. pitch/roll angle, may be included in the cases above. Typically, wind forces is not considered in the above combinations. and Heave: V erticalforce = F vp ± F vhp
LCFAC
(14)
= 20d
MN
; PERIOD = 10s; SWAY CG
1.10
-12.389
= SIN(20)
= 0.34
-8.500 XYZ
2
ACCL
0.00
0.00
0.00
1.00
0.00
0.00
ACCL
0.00
0.00
0.00
0.00
1.00
0.00
ACCL
0.00
0.00
0.00
0.00
0.00
1.00
ACCL
1.00
0.00
0.00
0.00
0.00
0.00
ACCL
0.00
1.00
0.00
0.00
0.00
0.00
ACCL
0.00
0.00
1.00
0.00
0.00
0.00
END
Horizontalforce = F hp ± F hhp
(15) The accelerations thus generated need to be further suitably factored and combined with the weight of the cargo to get total inertia loads.
and Heave: 3
1.7.2 Generating inertia forces by hand
Transport forces in terms of W and L for commonly used GL Noble Denton motion criteria: Description Vertical force Horizontal force Ly ± 0.0688 ) 1 ± W · ( 0.9397 + 0.1378 · g ± 0.1891 ) W · ( 0.3420 + 0.1378 · Lz g Lx Lz 1 ± W · ( 0.9848 + 0.0689 · g ± 0.1982 ) W · ( 0.1736 + 0.0689 · g ± 0.0349 ) 2 ± W · ( 0.9397 + 0.1378 · Ly W · ( 0.3420 + 0.1378 · Lz ± 0.1891 ) ± 0.0688 ) g g Lx Lz 2 ± W · ( 0.9763 + 0.0861 · g ± 0.1964 ) W · ( 0.2164 + 0.0861 · g ± 0.0436 ) 4 ± W · ( 0.9063 + 0.1723 · Ly W · ( 0.4226 + 0.1723 · Lz ± 0.1824 ) ± 0.0850 ) g g Lx Lz 4 ± W · ( 0.9659 + 0.1034 · g ± 0.1944 ) W · ( 0.2588 + 0.1034 · g ± 0.0521 )
Last updated: August 24, 2012. Type set in XƎTEX.
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