OIL COMPANIES INTERNATIONAL MARINE FORUM
Summary of the Results of the
MARIN Study
to Validate the Adequacy of
SPM Mooring Equipment
Recommendations
Tandem Single Point Mooring Study 2005
May 2007
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responsible operation of oil tankers and terminals, promoting continuous
improvements in standards of design and operation
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Oil Companies International Marine Forum
OIL COMPANIES INTERNATIONAL MARINE FORUM
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2
Table of Contents
Page
1.0 Introduction
4
2.0 Study Assumptions
4
3.0 Study Sensitivity Results
7
4.0 Summary
19
5.0 Study Conclusion
20
3
1.0 Introduction
Shipboard equipment recommended in the 3rd Edition of OCIMF’s Recommendations
for Equipment Employed in the Mooring of Ships at Single Point Moorings has been
increasingly used for mooring conventional tankers in tandem to FPSOs and FSOs.
Industry concerns regarding the suitability of such equipment for tandem mooring
arrangements resulted in OCIMF members commissioning the Maritime Research
Institute Netherlands (MARIN) to conduct a series of numeric modelling simulations.
The study made use of the "Shuttle" numeric modelling application specifically
designed for tandem mooring arrangements and was supervised by a small MARIN
and OCIMF-member steering committee. The objective of the study was to "test"
those concerns using the "Shuttle" application to simulate a range of tandem mooring
configurations and environmental conditions.
The simulation scenarios were selected in order to isolate and identify the impact and
magnitude of the main components affecting peak hawser loads in tandem mooring
arrangements.
The results and conclusions derived from this study were considered informative and
appropriate for summary in this information paper with reference in the renamed
Recommendations for Equipment Employed in the Bow Mooring of Conventional
Tankers at Single Point Moorings 4th Edition. They should be treated with caution
as they represent simulations based only on pre-defined equipment in specific
environmental conditions.
2.0 Study Assumptions
The study was subject to constraints. The following is a brief description of the main
assumptions.
Conventional Tankers
Two typical conventional tanker models equipped with bow mooring equipment fitted
as recommended in the 3rd Edition were used. Assumed deadweights were 150,000
and 300,000 tonnes.
CALM Buoy
Some simulations were conducted using conventional tankers moored to a typical
deep water Cantenary Anchor Leg Mooring (CALM) buoy. This provided a useful
comparison.
The CALM buoy mooring system selected represents a typical deep water mooring
system (714 metres) consisting of 3 bundles of 3 mooring lines the angles between
the bundles being 120 degrees. Each calm buoy mooring line consisted of a steel
wire and chain combination 1070 metres in length.
4
Figure 1 - CALM buoy anchor pattern
FPSO
A barge shaped FPSO of 300,000 tonnes DWT with typical deck equipment blocks
was selected.
FPSO Mooring Arrangements
Two FPSO mooring arrangements were selected.
Figure 2 - Turret moored FPSO anchor pattern
The turret mooring system selected represents a typical deep water mooring system
(714 metres) consisting of an external turret moored by 12 lines in four bundles of
three lines each.
5
Figure 3 - Spread moored FPSO anchor pattern
The spread mooring system selected represents a typical deep water mooring
system (714 metres) consisting of 12 lines in four bundles of three lines each.
Each mooring line (turret and spread) is made up of a combination of chain and wire
of total length 1299 metres and anchor radius 1028 metres.
Some simulations were repeated with shallow water mooring systems (65 metres).
Hawser
A grommet nylon hawser with 5 metre length chafe chains at each end was
assumed. During the simulations different total hawser lengths of 80, 110 and 160
metres were tested and in one simulation the hawser material was changed to a
stiffer material (polyester). Several simulations were repeated utilising twin hawsers
to separate chain stoppers.
Environmental Conditions
The FPSO, and/or tanker, was always heading north at the commencement of each
simulation. For most of the simulations one of two extreme offloading conditions was
assumed. It is important to understand that these extreme operating environmental
conditions were selected in order to compare results and understand the effect of
altering one component including environmental criteria. They were not intended to
be reflective of maximum operating criteria for a particular mooring arrangement and
the resultant hawser loadings should not be viewed out of context.
Environmental events of "wind squalls" and "internal waves, currents and solitons"
were also simulated with a number of different sensitivities.
Condition E1 - (significant environmental criteria from same direction)
Wind
Current
Waves
15 metres/second (m/s) from north
1.5 m/s from north
3.5 m significant wave height (Hs)
10 s dominant wave period (Tp) from north
6
Condition E2 - (significant environmental criteria from different directions)
Wind
Current
Waves
15 m/s from west
1.5 m/s from north
3.5 m Hs
10 s Tp from west
Variation on the above conditions were tested to simulate the hawser load sensitivity
to:
a)
b)
c)
d)
no current - to encourage fishtailing
150 and 120 degree angles between current and wind
lower significant wave heights (2.5 m Hs)
longer period waves (16 s Tp)
Internal Waves, Currents and Solitons
For tandem moored FPSO/tanker simulations it was assumed that the tanker would
encounter a 3 knot current stern first and the FPSO would not react to the current
(current forces set to zero). The velocity of the approaching current was varied as
was the application of tug forces.
Squalls
The simulation used actual squall wind measurements both in terms of direction and
speed provided by an OCIMF member company. A weak current was applied to
some of the simulations and the sensitivity of using a single pull back tug was tested.
Tugs
The "shuttle" numeric modelling application has the ability to simulate the interactions
of up to two tugs on the moored tanker by the following:
a)
b)
constant pulling force in a constant direction
vector tug(s) (either push or pull) in auto pilot mode which in the case of one
vector tug controls the load on the hawser or, in the case of 2 or more vector
tugs, controls the load on the hawser to minimise the heading difference
between the FPSO and the tanker.
3.0 Study Sensitivity Results
SPM Type
In steady environmental conditions, the calculated and most probable maximum
hawser loads were, as expected, significantly higher for the two FPSO mooring
arrangements compared to the CALM buoy. The sensitivity simulations indicated the
differences to range between 20 to 35%. The differences between the two FPSO
mooring arrangements were more complex and are described separately.
7
Hawser Length
A standard nylon hawser length, excluding chafe chain(s), was selected at 100
metres. However several simulation runs were repeated with shorter (70 m) or
longer (150 m) hawsers.
hawser load [metric tonnes]
250
simulation A7: buoy with 300,000 DWT loaded tanker, environment E1
simulation A9: Repetition of A7 with 70m hawser
200
150
100
50
0
0.9
0.92
0.94
0.96
0.98
1
time [hours]
1.02
1.04
1.06
1.08
1.1
Figure 4 - Loaded VLCC on CALM buoy
The graph in Figure 4 represents the hawser loads predicted for a 110 metre hawser
(blue) and a 70 metre hawser (red) for a loaded VLCC moored to a CALM buoy in
steady severe environmental conditions from the same direction (E1). The same
simulation comparison using environmental conditions E2 (different directions)
produced similar results.
800
hawser load [metric tonnes]
700
simulation B8: ballast 300,000 DWT FPSO with 300,000 DWT loaded tanker, environment E2
simulation B10: Repetition of B8 with 150m hawser
600
500
400
300
200
100
0
0.5
1
1.5
2
time [hours]
2.5
3
3.5
Figure 5 - Loaded VLCC tandem moored to ballasted spread moored FPSO
Figure 5 is a hawser length comparison graph representing the predicted hawser
loads for 110 metre hawser (blue) and a 150 metre hawser (red) for a ballasted
VLCC moored to a spread moored FPSO.
8
600
simulation C8: ballast 300,000 DWT turret moored FPSO with 300,000 DWT loaded tanker, environment E2
simulation C10: Repetition of C8 with 150 m hawser
hawser load [metric tonnes]
500
400
300
200
100
0
0.5
1
1.5
2
time [hours]
2.5
3
Figure 6 - Ballasted VLCC tandem moored to loaded turret moored FPSO
Figure 6 represents the same situation as Figure 5 except that the FPSO was turret
moored and hence free to rotate. The differences between peak hawser loads are
less pronounced than calculated for the spread moored FPSO configuration.
The main observation from these results is that increasing the length of the hawser
and thus decreasing the stiffness of the mooring arrangement and can result in
significantly lower peak hawser loads. Operators should be aware that increasing
the hawser length can increase the tendency to fishtail.
Twin Hawsers
Several single hawser simulations were repeated using twin hawsers made secure to
separate chain stoppers. The simulation assumes that the twin hawsers are evenly
matched in terms of rope properties. The intention was to understand the value of
twin hawsers in load sharing and the impact on individual peak hawser loads.
hawser loads simulation B8/B12: Ballast FPSO and loaded 300kDWT tanker in environment E2
600
load hawser simulation B8
load hawser 1 simulation B12
load hawser 2 simulation B12
hawser load [metric tonnes]
500
400
300
200
100
0
0.5
1
1.5
2
time [hours]
2.5
3
Figure 7 - Loaded VLCC tandem moored to a ballasted spread moored FPSO
9
hawser loads simulation C8/C12: Ballast FPSO (turret) and loaded 300kDWT tanker in environment E2
hawser load [metric tonnes]
600
load hawser simulation C8
load hawser 1 simulation C12
load hawser 2 simulation C12
500
400
300
200
100
0
0.5
1
1.5
2
time [hours]
2.5
3
Figure 8 - Loaded VLCC tandem moored to a ballasted turret moored FPSO
Both Figures 7 and 8 represent different FPSO mooring configuration simulations
under the same environmental conditions (E2) with the blue line indicating the
hawser loads in a single hawser system and the red and black lines indicating the
hawser load in each of a twin hawser system. It is of interest to note that the peak
hawser loads for the Turret Moored FPSO (Figure 8) were, as would be expected,
lower than those on the Spread Moored FPSO (Figure 7). The consistent
observation from all twin hawser comparison simulations, including those with tug
assistance, is that twin hawser systems reduce the peak load on an individual
hawser.
Tugs
Simulations were repeated with combinations of tug assistance. The Shuttle tool is
capable of simulating a constant force in a fixed direction or single or multiple vector
tugs. Vector tugs can be simulated in auto pilot mode to attempt to maintain the
hawser load below a specified peak load or if two or more vector tugs are used to
maintain the hawser load and minimise the heading difference between the FPSO
and the tandem moored tanker. For Turret Moored FPSO tandem simulations a
single "pull back" 60-tonne bollard pull vector tug was made fast at the tanker stern.
For Spread Moored FPSO tandem simulations combinations of 1 and 2 vector tugs
(1 ASD tug acting as push/pull at the bow and the other pulling at the stern) were
assumed.
The results of the simulations indicated that in most circumstances the use of 1 or
more tugs resulted in a slight reduction of peak hawser loads however in some of the
simulations higher peak hawser loads were experienced with tugs when compared to
the equivalent simulation without tugs. Figure 9 is an example of the use of tugs
actually increasing the peak hawser load.
10
600
hawser load [metric tonnes]
500
simulation B4: ballast 300,000 DWT spread moored FPSO with 150,000 DWT loaded tanker, environment E2
simulation B14a: Repetition of B4 with 2 tugs
400
300
200
100
0
0.5
1
1.5
2
time [hours]
2.5
3
Figure 9 - Loaded Suezmax tandem moored to a ballasted spread moored FPSO
700
simulation B8: Deepwater ballast FPSO with 300,000 DWT loaded tanker, environment E2
simulation B16a: Repetition of B8 with 2-60 tons tugs
hawser load [metric tonnes]
600
500
400
300
200
100
0
0.5
1
1.5
2
time [hours]
2.5
3
Figure 10 - Loaded VLCC tandem moored to a ballasted spread moored FPSO
700
hawser load [metric tonnes]
Shallow 600
Water
simulation C8: Deepwater ballast turret FPSO with 300,000 DWT loaded tanker, environment E2
simulation C16: Repetition of C8 with 1-60 tons tug
500
To compare the effect of the water depth (and hence stiffness of the FPSO mooring
400
arrangement
on mooring hawser loads) a number of simulations were repeated at a
mooring 300
depth of 65m utilising typical shallow water mooring systems.
200
100
0
0.5
1
1.5
2
time [hours]
2.5
3
Figure 11 - Loaded VLCC tandem moored to a ballasted turret moored FPSO
Shallow Water
To compare the effect of the water depth (and hence stiffness of the FPSO mooring
arrangement on mooring hawser loads) a number of simulations were repeated at a
mooring depth of 65 metres utilising typical shallow water mooring systems.
11
hawser load [metric tonnes]
200
150
100
50
simulation A8: Deepwater CALM buoy with 300,000 DWT loaded tanker, environment E2
simulation A11: Repetition of A8 with shallow water mooring system
0
0.5
1
1.5
2
time [hours]
2.5
3
Figure 12 - Loaded VLCC moored to a CALM buoy
800
simulation B8: Deepwater ballast FPSO with 300,000 DWT loaded tanker, environment E2
simulation B17: Repetition of B8 with shallow water mooring system
hawser load [metric tonnes]
700
600
500
400
300
200
100
0
0.5
1
1.5
2
time [hours]
2.5
3
Figure 13 - Loaded VLCC tandem moored to a ballasted Spread Moored FPSO
800
hawser load [metric tonnes]
700
simulation C8: Deepwater ballast turret FPSO with 300,000 DWT loaded tanker, environment E2
simulation C17: Repetition of C8 with shallow water mooring system
600
500
400
300
200
100
0
0.5
1
1.5
2
time [hours]
2.5
3
Figure 14 - Loaded VLCC tandem moored to a ballasted turret moored FPSO
In all cases the maximum hawser load increases due to the change from deep to
shallow water mooring systems.
Wind Squalls
The effect of wind squalls was simulated using offshore wind squall data (West
Africa) collected and provided by an OCIMF member.
12
wind squall
25
wind speed [m/s]
20
15
10
5
0
0
2000
4000
6000
8000
10000
12000
14000
time [s]
Figure 15 - Wind squall plot of wind speed against time
wind squall
360
wind direction [deg]
300
240
180
120
60
0
0
2000
4000
6000
8000
10000
12000
14000
time [s]
Figure 16 - Wind Squall plot of wind direction against time
For the purposes of the simulations the CALM Buoy and Turret Moored FPSO
simulations assumed the use of a single "pull back" tug of 60-tonnes constant bollard
pull. The Spread Moored FPSO simulation assumed two tugs.
13
simulation A12: Wind squall, CALM buoy and 300 kDWT ballast tanker with 1 constant force (60 tons) tug
hawser load [metric tonnes]
120
100
80
60
40
20
0
0.5
1
1.5
2
2.5
3
3.5
time [hours]
Figure 17 - Wind squall simulation of a ballasted VLCC moored to a CALM Buoy
hawser load [metric tonnes]
simulation B25a: Spread FPSO and 300kDWT ballast tanker with 2 60 tons tugs in wind squall
150
100
50
0
0
0.5
1
1.5
2
time [hours]
2.5
3
3.5
Figure 18 - Wind Squall simulation of a ballasted VLCC tandem moored to a spread
moored FPSO
simulation C18: Wind squall, Turret moored FPSO and 300 kDWT ballast tanker with 1 constant force (60 tons) tug
120
hawser load [metric tonnes]
100
80
60
40
20
0
0.5
1
1.5
2
2.5
3
3.5
time [hours]
Figure 19 - Wind Squall simulation of a ballasted VLCC tandem moored to a turret
moored FPSO
All three ballasted VLCC squall simulations predicted peak mooring hawser loads
well bellow 200 tonnes. However, the main concern highlighted by wind squall
simulations was the difficulty in controlling the position of the VLCC relative to a
Spread Moored FPSO during the event. Figures 20 and 21 show the "birds eye" plot
of the relative positions during the simulations. Figure 21 simulation with one tug in
"pull back" mode results in a collision.
14
Figure 20 - Wind squall simulation - Two tugs attending a ballasted VLCC tandem
moored to a spread moored FPSO
Figure 21 - Wind squall simulation - One tug attending a ballasted VLCC tandem
moored to a spread moored FPSO
Internal Waves, Currents and Solitons
The effect of a moving current line on mooring configurations was modelled by
assuming a current front approaches the stern of the moored tanker and imparts
current forces increasing in magnitude depending on an assumed current front
approach speed. The simulation confirmed the expectation that a tug assisted
Spread Moored FPSO tandem operation could easily get into handling difficulties
during severe events. However the simulation of a tug assisted tanker tandem
moored to a Turret Moored FPSO demonstrated that severe moving current events
could be handled safely with predicted peak hawser loads remaining less than 200
metric tonnes.
15
hawser load [metric tonnes]
simulation C20: Turret moored FPSO and 300kDWT loaded tanker in riptide with 1 constant force (60 tons) tug
150
100
50
0
0
0.2
0.4
0.6
0.8
1
time [hours]
1.2
1.4
1.6
1.8
Figure 22 - Moving Current simulation - One tug attending a loaded VLCC tandem
moored to a turret moored FPSO
Waves
Some simulations were repeated with lower significant wave heights (2.5 m Hs as
opposed to the simulation standard of 3.5 m Hs) and at a longer wave period (16
seconds as opposed to 10 seconds).
These simulations demonstrated a significant reduction in peak hawser loads with
reduction of significant wave height and with increase in wave period. On average a
reduction of 1 m Hs resulted in a reduction of 43% on peak hawser load. When
combining increased wave period with a reduction in significant wave height a further
reduction of 35% of average peak hawser load was recorded. Figures 23, 24 and 25
provide an example of the effects reducing significant wave height and wave period.
simulation C16: Repetition of C8 with 1-60 tons tug
hawser load [metric tonnes]
500
400
300
200
100
0
0.5
1
1.5
2
time [hours]
2.5
3
3.5
Figure 23 - Tug assisted loaded VLCC tandem moored to a ballasted FPSO in E2
environment (Hs 3.5 metres and Tp of 10 seconds)
16
simulation C21: Repetition of C8 with Hs=2.5m Tp=10s, 1-60 tons tug
hawser load [metric tonnes]
300
250
200
150
100
50
0
0.5
1
1.5
2
time [hours]
2.5
3
3.5
Figure 24 - Same situation as Figure 23 with wave height reduced to 2.5 metres Hs
simulation C22: Repetition of C8 with Hs=2.5m Tp=16s, 1-60 tons tug
hawser load [metric tonnes]
200
150
100
50
0
0.5
1
1.5
2
time [hours]
2.5
3
3.5
Figure 25 - Same situation as Figure 23 with wave height reduced to 2.5 metres Hs
and wave period increased to 16 seconds Ts
Angle between Wind and Current
The standard environment (E2) used in many of the simulations assumed a 90
degree angle between wind/waves and current. Some additional sensitivity runs
were repeated assuming 120 and 150 degree angles. Increases in peak hawser
loads up to 24% were noted. Because of these very high hawser loads further
simulations were run reducing the current to 1.5 knots, reducing the wave height to 3
m Hs and using twin hawsers. From inspection of Figures 26 and 27, it can be seen
that a combination of reduction of operating criteria and use of twin hawsers can
significantly reduce peak hawser loads.
17
simulation C23: Repetition of C8 150 deg between wind and current, 1-60 tons tug
hawser load [metric tonnes]
600
500
400
300
200
100
0
0.5
1
1.5
2
time [hours]
2.5
3
hawser load [metric tonnes]
Figure 26 - Tug assisted loaded VLCC tandem moored to a ballasted turret moored
FPSO in E2 environment with angle between wind/waves and current increased to
150 degrees. Note the very high peak hawser load of over 600 metric tonnes.
hawser loads simulation C30/C31: Ballast FPSO (turret) and loaded 300kDWT tanker. reduced (1.5 knots) current in C31
300
load hawser 1 simulation C30
load hawser 2 simulation C30
250
load hawser 1 simulation C31
load hawser 2 simulation C31
200
150
100
50
0
0.5
1
1.5
2
time [hours]
2.5
3
Figure 27 - Tug assisted loaded VLCC tandem moored to a ballasted FPSO in
modified E2 environment with twin hawsers.
Note in Figure 27 the blue and red hawser loads represent twin hawsers when the
angle between the wind/waves and current increased to 150 degrees and the wave
height reduced to 3.0 m Hs. The green and black hawser loads represent the same
situation but with the current reduced to 1.5 knots (from 3 knots).
Current
Current obviously induces an additional component to the hawser load but it will also
tend to damp the dynamic behaviour of the mooring system. Some simulations were
repeated with zero current.
18
Figure 28 - Left - ballasted 150,000 DWT tanker moored to a CALM buoy (E1
environment) Right - same situation except with zero current
Figure 28 clearly demonstrates the fishtailing effect induced by wind when not
dampened by current. The increase in peak hawser load was recorded at about 5%
with no current.
Similar simulations were repeated for tankers tandem moored to both Spread
Moored and Turret Moored FPSOs and similarly the increase in peak hawser load
with no current was minimal and not exceeding 5%.
Fairleads
The simulations also provided calculated forces on the tanker fairleads. As expected
the peak force imparted on the fairlead was normally less than that imparted on the
hawser. However there were instances when the hawser angle to the fairlead
approaches 90° that this was reversed and the peak force imparted on the fairlead
was marginally greater than that imposed on the hawser. Forces on both bow chain
stopper and fairlead should be equal when the hawser angle to the fairlead is 60°.
4.0 Summary
This study was undertaken to assist OCIMF members in developing revised
recommendations for SPM mooring equipment in Recommendations for Equipment
Employed in the Bow Mooring of Conventional Tankers at Single Point Moorings 4th
Edition.
Based on the results of this study some of the most important conclusions are:
•
Peak hawser loads in the same environmental conditions can be expected to
be higher for FPSO/FSO tandem mooring systems than standard CALM buoy
SPM systems. In some cases the increase is significant.
•
Peak tandem hawser loads in the same environmental conditions with current
and wind/wave forces from different directions were higher for Spread Moored
FPSOs than for Turret Moored FPSOs.
•
The use of twin hawsers can lead to a reduction in peak hawser loads.
•
The applied wind squall and moving current results produced lower than
expected peak loads (not exceeding 200 metric tonnes for a single hawser
system), however for tankers tandem moored to a spread moored FPSO the
19
tendency for dangerous situations to develop (collision) was increased.
•
The use of tugs in the simulations did improve the relative headings between
the FPSO and the tanker and reduce fishtailing but did not consistently
reduce peak hawser loads. In a minority of simulations peak hawser loads
were higher using tugs.
•
The wave conditions are the most significant single environmental criteria with
regard to peak hawser loads. In a number of tandem situations, reducing the
significant wave height by 1 metre resulted in peak hawser loads decreasing
by 43%.
•
Increasing the length of the hawser generally reduced peak hawser loads.
5.0 Study Conclusion
In preparing the 4th Edition of Recommendations for Equipment Employed in the Bow
Mooring of Conventional Tankers at Single Point Moorings, OCIMF members faced
some challenging decisions. The equipment guidance previously provided for
conventional tankers (Recommendations for Equipment Employed in the Mooring of
Ships at Single Point Moorings 3rd Edition 1993) recommended ship equipment levels
which included twin chain stoppers and fairleads rated at 200 tonnes SWL for
mooring VLCCs up to 350,000 DWT at SPMs. The 3rd Edition also recommended
that terminal operators provide twin hawsers for mooring tankers over 150,000 DWT
to SPMs. The increasing use of the same shipboard equipment to moor in tandem to
FPSOs/FSOs, sometimes with a single hawser, posed some questions that required
further investigation.
These simulations focused on the hawser loads in tandem mooring situations. The
results are sufficiently conclusive for OCIMF members to state that the risk of
imparting loads in excess of the recommended SWL on tanker chain stoppers and
fairleads when moored in tandem to an FPSO/FSO is increased when compared to
CALM buoy moorings. This risk is further compounded by the use of single hawser
systems with some FPSO/FSO tandem mooring arrangements and the added station
keeping difficulties associated with tandem mooring a tanker to a fixed azimuth
spread moored FPSO/FSO.
In reviewing the results of this study, OCIMF members decided to retain existing
recommendations for mooring equipment ratings on existing conventional tankers
and to upgrade recommendations for new tankers delivered during or after 2009.
The results of this study indicated to OCIMF members that there is sufficient margin
for terminal operators to safely moor conventional tankers equipped in accordance
with 3rd Edition recommendations in tandem to FPSOs/FSOs provided there is an
awareness of the limitations of ship's equipment, the impact of the chosen terminal
mooring system and limiting environmental operating criteria. The onus, therefore,
for understanding terminal operating limitations and ensuring the safety of the
complete mooring system, including ship’s equipment fitted in compliance with the
recommendations in the 4th Edition of Recommendations for Equipment Employed in
the Bow Mooring of Conventional Tankers at Single Point Moorings, rests firmly with
the terminal operator. Terminal operators, therefore, are strongly encouraged to
understand and establish environmental limits and procedures that ensure that the
SWL of the mooring equipment fitted to ships calling at these terminals is not
exceeded.
20