Date 2005
Author Buchner, B., P. Dierx and O. Waals
Address Delft University of Technology Ship Hydromechanics Laboratory Mekelweg 2, 2628 CD Delft
Delft University of Technology
T U Delft
The behavior of tugs in waves assisting LNG carriers during berthing along offshore LNG terminals
by
Bas Buchner, Pieter Dierx and Olaf Waals
Report No. 1466-P 2005
Published in: 24"" International Conference on Offshore Mechanics and Arctic Engineering, OMAE'05
Proceedings of OMAE2005 24th International Conference on Offshore Mechanics and Arctic Engineering (OMAE 2005) June 12-17, 2005, Halkidiki, Greece
OMAE2005-67219
THE BEHAVIOUR OF TUGS IN WAVESASSISTING LNG CARRIERS DURING BERTHING ALONG OFFSHORE LNG TERMINALS
Bas Buchner, Pieter Dierx and Olaf Waals MARIN
PO Box 28 6700 AA Wageningen
The Netherlands
ABSTRACT
For future offshore l>NG terminals lugs are planned to assist L N G carriers during berthing and offloading operations. A model test study was carried out to better understand the tug behaviour in waves and to make a first step in the quantification o f the related weather limits. Scale 1:35 model tests were performed in the two important 'modes' o f a tug during this type o f operation: the "push* mode and the 'puir mode. Realistic weather conditions were used and the tugs were working at the unshielded and shielded sides o f the L N G carrier. Based on the results presented in this paper, it can be concluded that the motions of tugs in waves are significant, even in wave conditions that arc considered to be mild for the berthing and offloading L N G carriers. The resulting push or pull loads may hamper these tug operations significantly. Special measures are necessary to take this behaviour into account in tug design, L N G carrier design and development o f operational procedures and equipment. The paper gives insight in the typical tug behaviour in difterent weather conditions. One should be careful, however, to generalize the present results: with an opfimised tug design and operation the tugs can be used in more severe conditions.
INTRODUCTION
The requirements for the uptime o f new Offshore terminals for L N G import and export are extremely high (95-99%). The Offshore industry' realises that this requires dedicated mooring and offloading systems, so that the offloading o f L N G can proceed in significant waves and swell. Mooring to a dedicated Gravity Based Structure ( G B S ) and side-by-side/tandem mooring to a Floafing Storage and Regassification Unit (FSRU) are considered in this process. This type o f mooring problems has been studied in Büchner et al (2001, 2004) and Van der Valk and Watson (2005). It was recognised as well that operational problems can also affect the operability o f offshore L N G terminals. V a n Doom and Büchner (2004) discussed these issues for o i l offioading terminals and Onassis and Hurdle (2004) studied operational aspects o f offshore L N G terminals.
A s indicated by Onassis and Hurdle (2004). in the design o f new offshore L N G terminals lugs are planned for the assistance o f the berthing and offloading operations o f visiting L N G carriers. This is necessary to assure safe and efficient operations. So far this type o f assisting lugs has mainly been used in sheltered conditions in harbours or other more shielded conditions around terminals, see Figure 1.
' m i ' i i
mmmmi- H a « ' '••
"•: : 1 r
Figure I. A.ssi.sting tugs in a more sheltered condition. Figure 2. Experience with tugs assisting crude carriers
during lightering operations has shown that waves may hamper tug operations significantly (courtesy Capt. Mark Scholma).
However, for new Offshore L N G import or export terminals, these operations should be carried out in a real offshore environment with the related waves. Experience with tugs assisting crude carriers during lightering operations has shown that waves may hamper these tug operations significanfly (see also Figure 2):
• In the pull mode the motions o f the tugs in the waves can cause extreme line loads, resulting in breaking of" the towline (or even the danger o f capsizing o f the tug when large loads are applied transverse to the tug).
• In the push mode the motions o f the tugs in the waves can induce high impact loads in the fender, resulting in large stresses in the side shell o f the L N G carrier. • Green water on the deck can affect the stability o f the
tug as well as the safety o f the crew.
• Excessive motions can influence the capabilifies o f the crew that is working on the tug.
• Tugs may need a significant part o f their power to stay on station themselves.
• Large tug motions and (relafive) wave motions can result in thruster ventilation and reduced thruster efficiency.
Tug operations in waves can consequently be a real bottleneck in the uptime o f a L N G import or export terminal. In Onassis and Hurdle (2005) this problem was addressed briefly, but not quantified in detail. The objectives o f the present paper are to better understand the tug behaviour in waves and to make a first step in the quantification o f the related weather limits. For this purpose a dedicated set o f model tests was carried out.
MODELS AND SET-UP
To better understand the behaviour o f tugs in waves, a pilot study was carried out in the Offshore Basin o f the Maritime Research Insfitute Netheriands ( M A R I N ) . A 1:35 scale model o f an L N G carrier was used, in combinafion with a 35 m long tug. having a bollard pull of 501/500 k N . A third structure, such as a G B S or F S R U , was not modelled in the present tests. The focus was on the local interaction between the tug and the L N G carrier.
L N G C a r r i e r and T u g models
A summary o f the particulars o f both vessels can be found in Table 1.
Table 1. A summary of the particulars of the L N G carrier and tug.
Dcsicnalion Svnibol L N G Carrier Tug Unit
Length Lpp Lpp 274.00 34.80 m
Breadth B 44.20 9.13 m
Depth D 25.00 4.50 m
Draft (even keel) T 11.00 2.73 ni
Displacement A 99,210 550 tonnes
Figure 3. The .scale 1:35 tug model with dummy thrusters and schematic modelling of bow fender characteristics
Towline
The modelled towline o f the tug consisted o f 25 m Dyneema and a 15 m nylon tail. The nylon tail was used to absorb the snap loads in the towline. The line was relatively short because with a short tow line a quick change can be made between push and pull mode, which might be necessary during the operation. Furthermore the short towline allows a more directly controlled operaUon than a long towline. The winch for the towline was assumed to be located at the bow o f tug. The maximum breaking load ( M B L ) o f the total towline was 4500 k N . The load-elongation curve o f the towline is given in Figure 4. In the basin it is modelled with a series o f 3 linear springs with different stiffness, that are blocked at certain elongations.
ftmm - Elongaitofi cwrv«
A picture o f the scale model o f the tug is shown in Figure 3. The model had no active propulsion, but two dummy thrusters.
Figure 4. The load-elongation curve of the towline.
Fender
The bow fender is designed to push against another vessel without damaging the tug and the other vessel. For the present tests a cylindrical rubber fender was modelled with a diameter o f 2 m and a length o f 2 m. The main characteristic o f the fender is its compressibility: Figure 5 shows the force-compression characteristics. For the tests the fender was modelled as a lever arm connected to a spring, as shown on the bow o f the tug in Figure 3.
Figure 5. The force-compression cur> e of the fender.
Set-up
To keep the L N G carrier at its position during the tests, it was moored in a horizontal soft spring system, as shown in Figure 6. Tests were carried out in the 2 important "modes" o f a tug during this type o f operation:
• The 'push' mode, with a fender between the bow o f the tug and the side o f the L N G carrier
• The "pull" mode, with a towing line between the tug and the L N G carrier.
Figure 6. Horizontal soft spring mooring for the L N G carrier.
Because the required mode o f the tug can change quite rapidly during the operation, the lug orientation was kepi the same in both modes: with the bow towards the L N G carrier. In both cases the lug was positioned al approximately 1/3 from the bow. The push and pull modes are shown in Figure 7 and Figure 8.
During the present pilot tests the tug had no active propulsion and no active heading control capability. This was a result o f cost and scale considerations. Even at a large scale o f 1:35 the size o f the tug is small (1 m length) and the thrusters would be ver\ small as well, resulting in a less reliable modelling of the thruster hydrodynamics.
Still a 501/500 k N thrust needed lo be modelled for both the push and pull modes. The push force was generated by using pre-lensioned springs between the lug and the L N G
carrier. Further two soft spring lines were attached between the bow o f the lug and the L N G carrier to avoid loo large yaw/sway motions o f the tug, as no active heading control was present.
Figure 7 shows the schematizalion o f the push mode that has been used during the model tests. A s in reality, the line from the bow lo the L N G carrier is often part o f the towline o f the tug. This was also done for the present tests: the characteristics o f a 15 m nvlon line was used.
Figure 7. Modelling of the push mode.
The pull mode was modelled by a weight attached to the stem o f the tug, representing the pull force o f 500 kN/501. A 25 m Dyneema low line with a 15 m long nylon tail was modelled between the L N G carrier and the bow o f the lug. Figure 8 shows the schematizalion o f the pull mode that has been used durina the model tests.
Environment
A s the mooring o f an L N G carrier along a floating or fi.xed terminal will mainly lake place in wave headings close lo head waves for the L N G carrier, the present tests mainly focused on this type o f wave headings, except for a lower beam sea. Il should be noted that this type o f wave headings, optimum for the L N G carrier mooring, arc resulting in crifical beam wave conditions for the assisfing tugs.
Figure 9 shows the resulting head waves, bow quartering waves and beam waves for the L N G carrier and the lug in pull mode. A l l waves are assumed lo have a J O N S W A P spectral shape with a gamma value o f 3.3.
270 Jeg
22i>les
IXOdeg
Figure 9. Head waves, bow quartering waves and beam waves for the L N G carrier and the tug in pull mode.
Wave conditions around the assumed working limits o f tugs assisting offshore operations were selected. For the bow quartering wave conditions a sea stale with an Hs o f 1.9 m was selected as base case, both with a longer (swell) period and a shorter (wind sea) period. In addition lo this, a large sea stale o f 3.0 m was used as a sensitivity check for the wave height, see Table 2.
Table 2. Bow quartering waves.
Wave Direction fdegl Hs fm| Tp fs] Wave spectrum Gamma Quartering 225 1.9 14.0 JONSWAP 3,3 Quartering 225 1.9 8.3 JONSWAP 3.3 i . i i i . i r l c i i ; ; ; ' 225 JONSWAP 3,3
Depending on the position o f the tug at the L N G carrier the bow quartering waves represented a 'shielded" or "unshielded" location for the tugs. For the head wave condition also both sea states with Hs = 1.9 m were used, see Table 3.
Table 3. Head waves.
Wave Direction Idegl Ms [ml Tp Wave spectmm Gamma Head 180 1.9 14.0 JONSWAP 3.3 Head 180 1 ^> 8.3 JONSWAP 3.3
The beam wave was limited to a typical low swell condition, see Table 4.
Table 4. Beam waves.
Wave Direction
fdegl Hs [m]
Tp Wave spectrum Gamma
Beam 270 0.95 14.0 JONSWAP 3.3
Limitations
The present pilot tests have been designed lo give the best possible representation o f the full scale situation. However, due to limited resources and practical considerations, still a number o f limitations are present: • The tug had no active propulsion, there are only dummy
thrusters placed al the tug. A s a consequence o f this, no thrust degradation efl"ecls due to thruster performance in waves can be studied with the present tests.
• The tug has no active (heading) control, so the effects o f the control o f lug by its master cannot be studied.
• The tests have been performed in waves only, whereas in reality wind and current will affect the operability o f the tug as well.
• A relatively old, rather narrow, hull type was used whereas modem lugs (Azimulhing Stern Drive or V o i g h l -Schneider) have a large beam and fuller hull shape. • There were no bilge keels or other roll reduction devices
on the tug model.
• The possible effects o f the terminal (such as the F S R U or G B S ) were not considered.
• The present tests were carried out with one tug size and towing/fender arrangement, the sensitivity o f the results for these aspects needs to be investigated in future research.
GENERAL OBSERVATIONS OF TUG BEHAVIOUR Qualitative evaluation
The model test results confirmed that the motions o f the assisting tugs in waves are significanU even in wave conditions that are considered to be mild for the berthing and offloading L N G carriers that are assisted by the tugs.
The following visual observations were made:
• Optimum wave headings for the berthing and mooring o f L N G carriers (close to head waves) are in fact critical beam wave conditions for the assisting tugs. This results in large roll motions o f the lugs.
• Slack tow lines and peak loads occur often, especially when the pull tug is in unshielded conditions. Figure 10 shows an example o f a time trace o f the towline load (F Towline) in such a condition.
The present tests were carried out in a water depth o f 98 m.
Figure 10. Time trace of the towline load (F Towline) with typical slack lines events and peak loads (Hs = 1.9 m, Tp = 8 J s, tug in unshielded conditions).
• For lower wave conditions in the push mode, the tug has the tendency to roll around the fender tip instead o f around a lower point (which is the case for a free floating vessel).
• For large wave conditions the fender comes free from the hull regularly and the relative motions o f the fender with respect to the L N G are large. Peak fender loads occur when the tug hits the L N G carrier hull again. The test with the tug in unshielded conditions and bow quartering waves o f Hs = 3.0 m had to be aborted because the model tug was damaging the L N G carrier model with a steel part o f the model fender. This was caused by pitch motions in access o f 12 degrees.
• The roll motions, fender loads and tow loads are influenced by the L N G carrier. If the tug is in shielded condifions, these motions and loads are smaller than in unshielded conditions, in which wave amplification can occur (waves higher than the incoming waves due to the combined incoming waves and waves reflected on the L N G carrier).
• Due to the large roll motions and relative wave motions (wave run up and down at the side o f the tug) the dummy thrusters were coming out o f the water regulariy. In reality this w i l l affect the thruster etficiency considerably due to thruster ventilation. However, modem tug types (Azimulhing Stem Drive or Voighl-Schneider) have their ihmslers deeper in the water below the hull.
Quantification o f tug behaviour
Table 5 presents the maximum motions and loads for a wave condition o f Hs = 1.9 m/Tp = 8.3 s with the tug al different positions around the L N G carrier (push and pull modes). The maximum loads should be related to the 501/500 k N mean bollard pull in the towline or fender.
Table 5. Maximum motions and loads for a wave condition o f H s = 1.9 m/Tp = 8.3 s with the tug at different positions around the L N G carrier (push and pull modes).
Hs= 1.9 m. Tp = 8.3 s Signal Bow quartering, unshielded Head Bow quartering, shielded Signal
Push Pull Push Pull Push Pull
Max Fx Fender (kN) 1820 - 1255 - 730 -Max F Towline (kN) - 1870 - 1300 - 1275 Max Roll tug (deg) 23.3 20.1 26.7 22.0 170 18.2 .Surge range (m) -2.6/4.0 -4.0/8.7 -0.7/0.6 -1.5/2.2 -4.3/3.0 -5.8/4.4 Heave range (m) -1.2/1.4 -1.2/1.3 -1.6/2.0 -2.3/1.8 -0.8/0.9 -I.1/1.1
To give reliable values, the maximum values are most probable maximum ( M P M ) values, derived from the model test using a Weibull 111. Figure II and Figure 12 give an
overview o f the most important results for the push and pull modes in different wave heights (Hs o f 1.9 and 3.0 m) and wave directions. The Figures give the maximum roll angle and towline load (F Towline) for the pull mode or maximum fender loads (Fx Fender) for the push mode.
Ron 1 . ' Ml.
23.3 1820 Ua>
>*OCIm 30.2 3900
mil Ron F> FaniMr IkNl Ma. 26.7 1255
^
d«Qre«fl I (degre«| 26.2 980 730Figure 11. Overview of the most important results for the push mode in different wave heights (Hs of 1.9 and 3.0 m) and wave directions: the most probable maximum roll angle and most probable maximum fender loads (Fx Fender).
23« m 20.1 1870 •ta> Hi-lOin 31.3 2080 iao <Mgr«s RoO Maa H» l 9 m 23.6 1300 c /
p
136 iMvaat Roa m Ma< Ma-t 9m 18.2 1276 Max. 26.0 1620 500 kNFigure 12. Overview of the most important results for the pull mode in different wave heights (Hs of 1.9 and 3.0 m) and wave directions: the most probable maximum roll angle and most probable maximum towline loads (F Towline).
From the results presented in Figure 11. Figure 12 and Table 5 the following can be concluded:
• The roll motions are large for all conditions tested: up to 26.7 degrees for the H s = 1.9 m. This is affecting the working conditions for the crew heavily.
• Already in the Hs = 1.9 m condition the maximum fender load at the L N G carrier hull is 1820 k N when the tug is on the unshielded wave side o f the L N G carrier. Compared
to the bollard pull o f 50 t/500 k N this is a dynamic amplification o f almost 4 times! This can be critical for the hull o f the L N G carrier. Special measures are necessary in the tug fender and L N G side construction to account for this type o f loads.
• Having a certain shielding o f the L N G carrier certainh helps: the maximum lender load reduced from 1820 k N to 1255 k N in head waves and 730 k N in shielded bow-quartering waves.
• For the towline load in the pull mode something similar occurs: a most probable maximum load o f 1870 k N is found in the unshielded Hs = 1.9 m.
• On the other hand this reduces less clearly in head and shielded bow quartering waves (to around 1300 k N and 1275 k N respectively). This is probably due to the fact that in the pull mode the tug is further away from the L N G carrier, which consequently provides less shielding to the tug than in the push mode.
• The surge motion range o f the tug is large: more than 10 m in the pull mode.
SENSITIVITY CHECKS W a v e height sensitivity
In Table 6 a sensitivity check is presented with respect to the wave height. In this Table the maximum motions and loads are given for a wave condition o f Hs = 3.0 ml Tp = 8.3 s with the tug at different positions around the L N G carrier (push and pull modes). Comparison with the results in Hs = 1.9 m shows the following:
• 1.6 increase in significant wave height results in more than a factor 2.1 higher fender load in the push mode with an unshielded tug. It should be noted that this test was aborted due to large loading and the maximum in a full test duration could have been (much) larger.
• For the shielded wave heading and the pull mode the difference is less clear. These loads increase less than linearly with the wave height.
Table 6. Maximum motions and loads for a wave condition of Us = 3.0 ml Tp = 8 J s with the tug at different positions around the L N G carrier (push and pull modes).
Hs = 3.0 m, Tp = 8.3 s Signal
Bow quartering,
un.sliicldcd Bow quartering, shielded
Signal
Push Pull Push Pull
Max Fx Fender
(kN) 3900*)
-
980Max F Towline
(kN) - 2080 - 1520
Max Roll tug
(degrees) 30.2*) 31.3 26.2 26.0
Surge range
(m) -5.3/12.1 -8.3/5.1 -9.1/6.8
Heave range
(m) -1,9/1.9 -1.3/1.5 -1.7/1.9
The unshielded situation is consequently the most critical and sensitive for the tug behaviour when h is close to the L N G carrier. This is due to:
• the veiy non-linear system o f a tug that can become free from the side o f the L N G carrier in large waves
• the strong wave mn up that can occur on the wave side o f the vessel.
W a v e period sensitivity
Table 7 shows the effect o f the wave period for the present tug: instead o f the peak period of 8.3 s a longer swell type wave o f Tp = 14.0 s is used. The comparison with the shorter wave period is not consistent for all headings and tug modes:
• For the push mode in unshielded conditions a clear reduction in the loads can be observed (to approximately half the load level in the shorter period), but for the pull mode the trend is less clear.
• The roll motion o f the tug has reduced significantly, which is due to the fact that the peak period is further away from the natural roll period of the tested tug model (9.9 s). This directly shows that one has to be careful in generalising the present results: tug stability and the related natural period can clearly influence the results. Something similar applies for the absolute tug size compared to the incoming wave length.
Table 7. Maximum motions and loads for a wave condition of Hs = 1.9 m/ Tp = 14.0 s with the tug at different positions around the L N G carrier (push and pull modes).
Hs= 1.9 m,Tp= 14.0 s Signal Bow quartering, unshielded Head Bow quartering, shielded Signal
pu.sll pull pusli pull push Pull
Max Fx Fender (kN) 820
-
670-
720-Max F Towline (kN) - 1640
-
865 - 1800 Max Roll tug (des) 11.6 16.0 12.1 14.6 9.5 12.0 Surge range (m) -1.3/1.4 -3.5/4.1 -0.5/0.5 -1.4/1.2 -1.4/1.3 -4.7/6.2 Heave range -1.3/1.4 -1.3/1.4 -1.5/1.8 -1.7/1.7 -1.1/1,3 -1,4/1.4*) Test aborted due to extreme loading
T u g behaviour in L N G c a r r i e r beam seas
Finally the situation o f L N G carrier beam waves was investigated, resulting in bow or stem waves for the tugs (depending on tug mode and lug position with respect to the L N G carrier). A low wave height (Hs = 0.95 m) was chosen because higher sea states would be unrealisfic for the L N G carrier mooring in swell waves o f T p = 14 s. Table 8 shows the resulting motions and loads.
Table 8. Maximum motions and loads for a beam wave swell condition of Hs = 0.95 ml Tp =14.0 s (push and pull modes).
Signal Unshielded Shielded
Signal
Push Pull Pull
Max Fx Fender
(kN) 970
-
730Max F Towline
(kN)
-
2250-
1175Max Roll tug
(degrees) 4.0 9.9 6.2 4.8
Surge range
(m) -1,5/1.4 -4.0/7.5 -1,4/1.5 -3.2/2.4
Heave range
(m) -1.0/0.9 -0.8/0.9 -0.6/0.8 -0.8/0.9
The results show large surge motions o f the tug in the pull mode. This is a result of the head wave condition for the tug and the long period wave. The resulting maximum towline load o f 2250 k N is a factor of 4.5 higher than the bollard pull of the tug.
LOADING OF THE LNG CARRIER HULL
For tug support o f L N G carrier manoeuvring operations in harbours and at terminals, typically 'tug push points ' are marked on the hull o f the L N G carrier. The fender o f the tug in the push mode should be applied at these points because this load is preferably applied to a rather stiff and strong vertical frame. However, in an offshore situation these loads are higher than the required push force. The point o f application o f this point also varies significantly, as will be shown below. This can result in unwanted point-type loads on the side shell o f the L N G carrier (although the fender is distributing the load over an area o f approximately 4 m" when fully compressed).
Figure 13 shows the displacement o f the fender over the hull o f the L N G carrier in bow quartering waves o f H , = 1.9 m (left) and 3.0 m (right), both in shielded (bottom) and unshielded (top) conditions. These graphs clearly show that the fender is moving over a large part o f the hull. Even in shielded conditions the smallest displacement window is 3 by
1 meter and this window increases rapidly to areas o f more than 25 m" in unshielded conditions. The fender load will consequently also be applied to locations between two vertical frames. The plate work between the two vertical frames is obviously more sensitive to deformation.
Figure 14 shows again the displacement o f the fender Up over the hull o f the L N G carrier, but now the related fender loads are given in the other axis. It can be observed fi-om this Figure that the large loads on the hull are distributed over a large horizontal area. Further the highest fender loads seem to occur higher up the side shell of the L N G carrier.
These aspects certainly have to be taken into account in the decisions about tug operational procedures, the choice o f equipment and the design o f the L N G carrier hull.
Figure 13. Two-dimensional time traces (Y-Z) of the relative motion of the fender tip relative to the L N G carrier hull. The horizontal axis gives the horizontal displacement (range -4 to+8 m), the vertical axis the vertical displacement (range -5 to +3 m). The top figures show the unshielded bow quartering waves of Hs = 1.9 m (left) and Hs = 3.0 m (right). The bottom Tigures show the shielded bow quartering waves of Hs = 1.9 m (left) and Hs = 3.0 m (right).
Honzonlal à a p l a c s m a n t |m] V t i l i c * ! d i s p l K t m a n l | m |
O
Honzonlal dxplaremnnt
•3 Q Vetiic»! dl9plac«m»nl (m|
Figure 14. Plots of the fender impact loads as function of horizontal (left figures) and vertical (right figures) displacement of the tug fender relative to the L N G carrier. The top figures show the unshielded bow quartering waves of Hs = 1.9 m. The bottom figures show the unshielded bow quartering waves of Hs = 3.0 m.
ASSESSMENT OF TUG CREW PERFORMANCE IN WAVES
Beside the limits imposed by the large loads in fenders and towlines, the safety and performance o f the crew is an important factor in the workability o f the tug in waves. The N O R D F O R S K 'Assessment o f ship performance in a seaway'
identifies different types of work or operations at different locations onboard which impose different levels o f motions and accelerations as limiting criteria. Figure 15 shows the fi\ e areas as identified by N O R D F O R S K (1987):
RMS Roll Motions Tug in Push Mod*, Unshielded
90
1. Fore perpendicular 2. Bridge
3. Amidships
4. VA o f ships length from aft 5. A f t perpendicular
Heavy manual work Intellectual work Transit passengers Heavy manual work Light manual work
Figure 15. Definition of positions on the tug.
For tugs assisfing L N G carriers during berthing and departure operations, locations I, 2 and 4 were identified as the working areas. The limiting conditions for these locations are given by N O R D F O R S K in terms o f R M S (Root Mean Square/Standard deviation) values. These are defined in Table 9.
Table 9. The limiting conditions for the 4 locations are given by N O R D F O R S K in terms of R M S (standard deviation) values.
Location Max acceleration Max roll
Location
Vertical Lateral Max roll
1 2 4 0.15g 0.1 Og 0,15g 0.07g 0.05g 0.07g 4.0 deg 3.0 deg 4.0 deg
Applying these limiting conditions to the test results, the following observation were made:
• The maximum R M S value o f vertical accelerations is not exceeded at one o f the three locations in any wave condition.
• The maximum R M S value o f lateral acceleration is not exceeded at location 1 and 4. A t location 2, the bridge, the lateral accelerations are exceeded during bow quartering waves o f Hs= 3.0 m and Tp= 8.3 s in the pull mode, and at beam waves of Hs = 1.9 m and Tp = 8.3 s for both the pull mode as well as the push mode.
• The R M S values for roll are exceeded during most model tests. Figure 16 shows the R M S values for roll in different sea states in unshielded conditions.
oHs=1.9m Tp=8.3s + H5=1.9m Tp= 14.0s • Hs=1.9m Tp=8.3s xHs=1.9m Tp= 14.0s V Hs=3.0m Tp=8.3s • Hs=0.95m Tp=14.0s
Figure 16. R M S values for the roll motion of the tug in push mode in different sea states. ( R M S values for the tug in shielded conditions arc on average 20% lower)
Although these results are clearly a funcUon o f the natural period o f the tug and the application o f roll reduction devices (which were not present in the present tests), it can be concluded that the roll behaviour o f the tug is one o f the critical factors for the operability o f the tugs in waves.
CONCLUSIONS
Based on the results presented in this paper, it can be concluded that the motions o f tugs in waves can be significant, even in wave conditions that are considered to be mild for the berthing and offloading L N G carriers that are assisted by these tugs. The resulting push or pull loads may hamper these tug operations significantly. For the present tug and configuration the following results were found (one should be careful in generalizing them):
• Optimum wave headings for the berthing and mooring o f L N G carriers (close to head waves) are in fact critical beam wave conditions for the assisting tugs. This results in large roll mofions o f the tugs (up to 26.7 deg for an Hs o f 1.9 m).
• Slack tow lines and peak loads occur often, especially when the pull tug is in unshielded condifions. A maximum tow load of 1870 k N is found in the unshielded H s o f I . 9 m .
• For the push mode the fender loads are high as well. In the Hs=1.9 m condition the maximum fender load on the L N G carrier hull is 1820 k N when the tug is on the unshielded wave side o f the L N G carrier. Compared lo the bollard pull o f 500 k N this is a dynamic amplification o f almost 4 fimes. This can be crifical for the hull o f the L N G carrier. Special measures are necessary for the tug fender design and L N G side constmction to account for this type o f loads over a large area o f the side shell. • The roll motions, fender loads and tow loads are
conditions, these motions and loads are smaller than in unshielded conditions, in which wave amplificafion can occur (waves higher than the incoming waves due to the combined incoming waves and waves reflected on the L N G carrier).
• Due to the large roll motions and relative wave motions (wave mn up and down at the side o f the tug) the dumms thrusters o f the model were coming out o f the water regularly. In reality this will affect the thmster efficiency considerably due to thruster ventilation. However, modem tug t>pes (Azimuthing Stem Drive or Voight-schneider) have their thrusters deeper in the water below the hull. • The crew performance can be clearly influenced by the
mofions o f the tugs. Applying the N O R D F O R S K motion criteria it becomes clear that especially the roll motions can be critical. R o l l reduction devices such as bilge keels can improve this situation.
It should be noted that one has to be careful in generalising the present test results: absolute tug size, bollard pull, tug stability ( G M ) and the push/pull arrangements can clearly influence the tug behaviour and related loads. The present test results can be used to validate numerical tools, which can then be applied to study these aspects and propose optimum solutions for the obsei-ved problems.
REFERENCES
N O R D F O R S K , 'Assessment o f ship performance in a seaway", Copenhagen. 1987.
- Büchner B . . van Dijk A . W . V . and de Wilde J.J. (2001), Numerical muUiple-body simulations o f side-by-side mooring to an F P S O , ISOPE 2001, Stavanger.
Van D o o m . J . T . M . and Büchner. B . (2001), Design and operational evaluation o f offloading operations for deep water FPSOs, D O T 2001, Rio de Janeiro.
Büchner, B . , Loots, G . E . , Forristall. G . Z . and Van Iperen, E . J . (2004), Hydrodynamic Aspects O f Gravity Based Structures In Shallow Water. O T C paper 16716, O T C 2004, Houston.
Onassis, J . and Hurdle, D . P . (2004), Manoeuvring Large Tankers Alongside a Floating L N G ( F L N G ) Facility. O M A E 2 0 0 4 , Vancouver.
- Büchner, B . , De Boer.G. and De Wilde. J.J. (2004). The Interaction Effects o f Mooring in Close Proximity o f Other Stmctures, ISOPE 2004, Toulon.
- V a n der V a l k , C . A . C and Watson, A.(2005): Mooring o f L N G carriers to a Weathervaning Floater - Side-by-side or Stern-to-bow, OTC-I7154, O T C 2005. Hou.ston. '
