SHIP ТО SHIP TRANSFER GUIDE (PETROLEUM)
(Third Edition 1997)
For Use with Crude
Oil and Petroleum Products
Chapter 6
Manoeuvring and Mooring
6.1 BASIC BERTHING PRINCIPLES
Berthing and unberthing operations should be conducted
during daylight unless the personnel concerned are suitably experienced in STS
manoeuvring operations.
The person in overall advisory control must be
mutually agreed between the two ships and this should be clearly established by
both masters prior to the start of operations (see Section 1.4).
For some inshore areas the port authority may require
a pilot to be taken. In such circumstances the pilot should advise on all
aspects of navigation and piloting, but the master remains in overall control
and in command of his own ship.
6.2 MANOEUVRING ALONGSIDE WITH TWO SHIPS UNDER
POWER
One ship, (the constant heading ship) normally the
larger, maintains steerage way at slow speed (preferably less than 5 knots) on a constant heading. Normally the manoeuvre will be made with the wind and
sea ahead, however local conditions and knowledge may dictate an alternative heading. The manoeuvring ship then manoeuvres alongside.
It is recommended that the manoeuvring ship approaches
and berths with her port side to the starboard side of the constant heading
ship.
6.2.1 General Advice for Controlling the Two
Ships
Each ship should take the following into account:
• engine controls, steering gear and all navigation and communications equipment should be in full working order;
• proficient helmsmen should he assigned to steer each ship;
• courses requested by the manoeuvring ship, must be followed by the constant heading ship;
• ship's speed should be controlled by adjusting engine revolutions (or propeller pitch). Any adjustment should be limited; for example, to plus or minus 5 RPM rather than using the relatively coarse engine room telegraph system. However normal full operating range must remain readily available;
• at night the deck should be adequately lit and, if possible, the ship's side and fenders should be spot lighted (see Section 9.5);
• the side for mooring should be clear of all over-side obstructions, permanent and otherwise;
• the navigation lights and shapes appropriate to STS transfers referred to in Section 5.2 should be displayed;
• there should be effective radio communications between the bridge and mooring gangs;
and
• there should be effective communications between the masters of each ship.
6.2.2 Advice for Manoeuvring Alongside
Although individual masters will have their own
preference for the method of manoeuvring their ship, the following points are
emphasised for STS transfer operations:
• the masters of both ships should always be prepared to abort a berthing operation if necessary; the decision should be taken in ample time while the situation is still under control;
• at all times each ship is responsible for maintaining a proper lockout;
• generally during manoeuvring the wind and sea should be ahead or nearly ahead; however local conditions or knowledge may indicate an alternative approach;
• the angle of approach adopted by the manoeuvring ship should not be excessive;
• a common method of berthing is for the manoeuvring ship to approach the constant heading ship from the quarter on the side of berthing. On closer approach she should parallel the course of the constant heading ship; about 50 to 100 metres off. The manoeuvring ship should then position itself relative to the constant heading ship. Contact is made by the manoeuvring ship reducing the distance by appropriate rudder and engine movements until fenders touch, (see Figure 6.1);
• the two ships should preferably make parallel contact at the same speed with no astern engine movements being necessary; and
• the effects of ship interaction should be anticipated when manoeuvring at close quarters.
6.2.3 Manoeuvring a Combined Two-Ship System to
Anchor
On completion of mooring, the constant heading ship
will power all future manoeuvres and, if a transfer at anchor is planned, will
proceed to the agreed anchoring position. During this time, the former manoeuvring ship will
have its engines stopped and rudder amidships. It should be emphasised that, for this period, in
order to avoid problems for the manoeuvring ship the constant heading ship
should not use strong astern engine movements.
FIGURE 6.1: RECOMMENDED APPROACH MANOEUVRE
VESSEL A CONSTANT HEADING CONSTANT SPEED [<5 Kn]
VESSEL B
MANOEUVRING VESSEL
Wherever possible lines should be lead through closed
chocks.
Full size mooring bitts and
Wires should be fitted with synthetic tails with a
minimum length of 10m. Tails should be rated at 125% of the MBL of the line to which they are attached.
Additional lines should be readily available to supplement moorings if necessary or in the event of a line failure.
FIGURE 6.2: RECOMMENDED STS MOORING ARRANGEMENT
The constant heading ship should use the anchor on the side opposite that on which the other ship is moored.
Once at anchor, each ship is responsible for keeping
its own anchor watch (see Section 3.6.)
6.2.4 Underway Transfer
Local conditions, such as those where water depths are
too great for anchoring, sometimes demand that cargo transfer be carried out
with the two-ship system under power and making way through the water. As long as adequate sea-room is available, traffic
conditions, weather, sea conditions and forecasts are suitable, then transfers
of this type can he carried out, but large diameter fenders of an acceptable
type should be used. It should be noted that speeds through the water
should be minimal, preferably in the 1 to 2 knot range.
The constant heading ship maintains steerage way at
slow speed on a steady course (approximately 1
to 2 knots) and the (former) manoeuvring
ship keeps its rudder amidships and remains (with engines stopped) as a towed
ship. In order to minimise towing loads on the moorings, the
constant heading ship should alter her engine revolutions sparingly, adjusting
speed very gradually. The chosen course and speed should be agreed by the
two masters and should result in minimum relative movement between the two
ships and minimum turbulence in the gap between the hulls.
Under such circumstances, while the ships are moored
together as a unit, safe navigation and collision avoidance is usually the
responsibility of the constant heading ship but may be under the direction of
the person in overall advisory control aboard the lightening/receiving unit.
As an alternative, and provided conditions are
suitable and a transfer area of suitable size is available, it can be
advantageous to carry out cargo transfer while the two ships are allowed to
drift freely.
The use of the underway transfer system requires a
full navigational watch to be maintained on the bridge of each ship. If deteriorating weather conditions cause the ships to
roll unacceptably, engine movements can be used to bring the ships to a heading
which results in minimum movement.
Constant attention should be paid to mooring lines and
fenders to avoid chafing and undue stress, particularly that caused by changes
in relative freeboard. If at any time mooring lines need to be re- positioned
or adjusted, this should only be done under strictly controlled conditions.
6.3 MANOEUVRES WITH ONE SHIP AT ANCHOR
STS transfer operations involving one ship already positioned at anchor are quite frequent. For such operations, one ship anchors in a pre-determined position using the anchor on the side opposite to that on which the other ship will moor. A berthing operation should only be carried out after the anchoring ship is brought-up to her anchor and is lying on a steady heading with reference to prevailing current and wind conditions.
The type of berthing operation then undertaken by the manoeuvring ship is similar to a normal approach to a jetty. However, a careful watch should be kept on the heading of the anchored ship and the manoeuvring ship should be advised immediately by the anchored ship if she has any tendency to yaw. Where there is a tendency to yaw excessively, a tug should be employed to hold the anchored ship on a steady heading. If no tug is available, postponement of the operation should be considered.
This manoeuvre can be preferred for more constrained transfer areas, especially when tug assistance is available, or if the manoeuvring vessel is fitted with a bow thruster. Where current and wind are not from the same direction or the wind varies in speed or direction the anchored ship can yaw (or lie cross-current), making it difficult for the manoeuvring ship to berth alongside. In these circumstances tug assistance may be advisable to hold the anchored ship on a steady heading during berthing. Berthing should not be attempted when the tidal stream is due to change. It is recommended that the services of an experienced STS superintendent be utilised for this type of operation.
When approaching a ship at anchor some masters
recommend a wider angle of approach than that adopted for manoeuvres underway. A wider angle of approach, especially when tugs are
not available, helps to avoid early ship to ship contact in cases where the
anchored ship might yaw unexpectedly. It is usually recommended that the manoeuvring ship
approaches and berths with her port side to the starboard side of the other
ship. When mooring to an anchored vessel care should be
taken not to pull the anchored vessel quickly towards the mooring vessel
6.4 MOORING PREPARATIONS
The importance of good quality mooring lines,
efficient winches and deck machinery should be recognised (Reference 1). This is especially true on board the manoeuvring ship
whose mooring lines will normally be used. However, this should also be addressed on the constant
heading ship where rope messengers have to be made ready between fairleads and
deck winches. Mooring operations should be managed to ensure expeditious
mooring line handling. Moorings should be arranged and rigged to allow safe,
effective line tending when the ships are secured together.
Care should be taken where possible to avoid
concentrating loads by passing several mooring ropes through the same fairlead
and on to the same mooring bitt.
The mooring plan adopted will depend upon the size of each ship and the difference between their sizes. As a general guide Figure 6.2 illustrates a recommended and proven mooring plan for an STS transfer operation in offshore waters.
It is important when agreeing a mooring plan to ensure
the following:
• deployment of moorings is selected to allow for ship movement and freeboard changes to avoid over-stressing the lines throughout the operation;
• mooring lines are not too long thus allowing unacceptable movement between the ships;
• mooring lines leading in the same direction are of similar material;
• moorings are to be lead through chocks or leads suitable for STS operations (see Section 9.3); and
• wire mooring lines are properly fitted with synthetic tails of adequate length and strength.
A ship's standard complement of mooring lines is
generally suitable for STS transfer operations but ships equipped with steel
wire mooring lines should fit soft rope tails to them. (Tails should be at least 10 metres long and have a Minimum Breaking Load (MBL) of at least 125 per cent of the wire to which they are
fitted. Connection should be by approved mooring joining
shackle.)
Soft rope tails fitted to wire moorings also introduce
the benefit of making the cutting of mooring lines easier in an emergency and,
for this purpose, long-handled firemen's axes or other suitable cutting
equipment should be available at all mooring stations.
It is normal for mooring lines to be deployed from the
manoeuvring ship and for the eyes of these lines to be put on the mooring bitts
on the constant heading ship.
However, depending on the arrangement it may be
necessary to supplement these moorings with lines from the constant heading
vessel.
Strong rope messengers should be readied on both ships
and in addition, rope stoppers should be rigged in way of relevant mooring
bitts. A rope messenger is a light rope of adequate length
and size which is used for hauling mooring lines between ships.
Where possible, heaving lines and rope messengers should be made of buoyant materials. An adequate number of each should be provided and ready for immediate use.