NAME 315/335: Port and Harbour Engineering Chapter: Mooring for Port and Harbour

Chapter: Mooring for Port and Harbour

1. Introduction

A mooring refers to any permanent structure to which a vessel may be secured. Examples
include quays, wharfs, jetties, piers, anchor buoys and mooring buoys. A ship is secured to a
mooring to forestall free movement of the ship on the water. An anchor mooring fixes a
vessel's position relative to a point on the bottom of a waterway without connecting the vessel
to shore.

Mooring analysis of a vessel alongside a berth usually involves a number of non-linear

mooring lines, extending at different vectors in both the horizontal and vertical planes, with
elastic fenders, acted upon by wind, current, and sometimes other forces, which may vary in
time and direction.

Movements of moored ships can be caused by various external influences such as: winds,
currents, waves, tides, passing ships and cargo handling operations. The movements of a
moored ship at a berth can be either horizontal (surge, sway and yaw) or vertical (roll, pitch
and heave). These ship motions are shown in Figure 1. Vertical ship motions are independent
of mooring system. Horizontal motions are dependent on the loading conditions of ship, the
mooring arrangements (i.e. geometry and stiffness of mooring lines and fenders and type of
berth). The ship motion affects the efficiency of cargo handling.

Ships are loaded and unloaded, at port structures like wharfs or quays, piers and jetties, and
sometimes alongside moles or breakwaters. The type of berthing structure depends on

Prepared by: Dr. Md. Mashiur Rahaman, Dept. of NAME, BUET

NAME 315/335: Port and Harbour Engineering Chapter: Mooring for Port and Harbour

purpose of the berth, size of ship, type of cargo (i.e. oil, ore, coal, grains, containers etc). It
also depends upon the environmental conditions. Quays and Wharfs are solid continuous
walls, whereas Relieving Platforms, Jetties and Dolphins are open piled structures. For bulk
cargo, open type construction with approach trestle is selected. For large oil Tankers Dolphin
or island type berthing terminal is chosen. Bollards are the vertical posts on the wharf or quay
to tie the mooring ropes to secure the ship. They are generally manufactured from cast iron.
Spheroidal graphite cast iron increases its load capacity.

The ship needs to remain in stable condition at the berth to facilitate loading unloading
operations. As such, the ship needs to be tied up with ropes to the berthing structure. This
arrangement of ropes is called as mooring. The mooring arrangement for ships at berth;
include the following elements (Figure 2):

 Mooring lines which connect the ship at the berth

 Bollards on shore
 Fenders between the ship and berth

The most important elements with respect to movements of moored ships at berth the
mooring lines and fenders, which are considered as the more flexible elements in mooring
arrangements.

A “mooring system” refers to the various components – an anchor, a rode (typically a rope,
chain, or cable), a buoy, and a pennant.

Prepared by: Dr. Md. Mashiur Rahaman, Dept. of NAME, BUET

NAME 315/335: Port and Harbour Engineering Chapter: Mooring for Port and Harbour

2. Anchor

There are four basic types of permanent anchors used in moorings:

2.1. Deadweight Anchor:

Deadweight anchors are the simplest type of anchor and, in many harbor settings, provide the
greatest reliability. Their holding power is through weight. Once the heavy weight settles on
the bottom, the anchor becomes embedded and the suction effect helps to increase its holding
power. Deadweight anchors can be made of most any heavy object but are typically a large
stone block or concrete. Due to their size and weight, these anchors typically require a barge
to set. Such moorings are better suited to rocky bottoms where other mooring systems do not
hold well. The disadvantages are that they are heavy, bulky, and awkward.

Fig.: Deadweight Anchor

2.2. Mushroom anchors:

Mushroomanchors get their name fromtheir shape, which looks like anupside down
mushroom. Theywork best in mud, sand, silt orother soft ocean bottoms wherethey can be
easily buried. Onceburied, a mushroom mooring istypically thought to be able tohold up to 10
times its weight.In certain conditions, mushroomanchors that are not properly embedded in
the bottom can dislodge and lose holding power. This is commonly referred to as “spin out.”

Additionally, mushroom anchors run the risk of having the chain wrap around the shaft of the
anchor when not properly set. This can reduce the scope of the mooring. The advantage is
that it has up to ten times the holding-power-to-weight ratio compared to a dead weight
mooring; disadvantages include high cost, limited success on rocky or pebbly substrates, and
the long time it takes to reach full holding capacity.

Prepared by: Dr. Md. Mashiur Rahaman, Dept. of NAME, BUET

NAME 315/335: Port and Harbour Engineering Chapter: Mooring for Port and Harbour

Fig. 4: Mushroom Anchor

2.3. Pyramid anchors:

As the name implies, these one-piece cast iron anchors are shaped like a pyramid. Pyramid
anchors are essentially a redesigned version of a mushroom anchor. They are comparable in
terms of holding strength, but their smaller size pyramid shape helps them penetrate the
bottom more rapidly. Pyramid anchors have a shorter anchor shaft which helps to minimize
the potential for chain wrap and also makes then useful in more shallow water settings. These
anchors are used in hard, rocky or sandy bottoms. Sometimes these anchors are call “Dor-
Mor” anchors after one of their manufacturers.

Fig.5 : Pyramid Anchor

2.4. Screw-in or Helix Anchors:

Helix (or screw-in) anchors are made of high tensile steel with an attachment eye at the top
and large threads at the lower end. These anchors come in long (8 ft.) lengths with varying
diameters of threads (10”,14”). To set the anchor, they arescrewed in until the top eye is
essentially flush with the bottomconnected to traditional groundtackle. While screw-in or
helixanchors have been used by theo shore oil industry for well over20 years, they are a
relatively newtechnology in anchoring systemsfor the yachting and smallercommercial vessel

Prepared by: Dr. Md. Mashiur Rahaman, Dept. of NAME, BUET

NAME 315/335: Port and Harbour Engineering Chapter: Mooring for Port and Harbour

markets.Recent industry tests are showingthat these anchors can offersignificantly more
holding power that traditional anchors when set property and in the right conditions. These
anchors may also help eliminate or minimize damage to sensitive benthic resources like eel
grass, shell fish beds and lobster habitat.

Fig.6: Screw-in or Helix Anchor

3. The Rode System

The Rode system, which runs from the anchor to the mooring buoy, has two basic parts – a
ground line and a riding line. The ground line, which lies on the bottom, is traditionally made
up of heavy chain (bottom chain). The riding line is typically a smaller sized chain called
“top chain.” The rode can also be made of line or cable.

The rode system is normally several times longer than the depth of the water (typically 1 1/2
times maximum water depth but varies depending on harbor conditions) to assist in the angle
of forces that are placed on an anchor. The longer the rode, the lower the angle of force will
be on the anchor. As more of the force is pulled horizontally, the holding power of a
traditional anchor improves. A longer rode also increases the swinging circle (scope) of each
mooring. By adding weight to the bottom of the rode, such as using a length of heavy bottom
chain, the angle of force can be decreased further. Heavy bottom chain can scrape the harbor
bottom around the anchor. Newer technologies have been designed to help alleviate this.

Prepared by: Dr. Md. Mashiur Rahaman, Dept. of NAME, BUET

NAME 315/335: Port and Harbour Engineering Chapter: Mooring for Port and Harbour

Fig.7 : Mooring Rode

Shackles and swivels are used to connect the anchor, bottom chain, top chain and mooring
buoy together. Shackles and swivels can become weak links in the mooring system.
Therefore, high quality parts are an important consideration.

Table: Summary of Anchors used for Mooring

Prepared by: Dr. Md. Mashiur Rahaman, Dept. of NAME, BUET

NAME 315/335: Port and Harbour Engineering Chapter: Mooring for Port and Harbour

4. The Mooring Buoy

The mooring buoy has two functions. First, it serves to support the rode and oats it o the
bottom. Second, it absorbs the shock of heavy waves and wind, which helps to increase
holding power. US Coast Guard and many Harbor Ordinances have regulations around the
type and color of mooring buoys. Many buoys are constructed of polyethylene plastic filled
with polyurethane foam and treated with UV inhibitors. However, air inflated net buoys are
also commonly used due to their lower costs.

Fig. 8: Mooring Buoy

5. Pennants:

The pennant ties the boat to the mooring. The pennant should run as straight as possible to
avoid cha ng. The sharper the angle of the pennant the greater pressure on the line which
increases cha ng. Some harbors ordinances require a secondary or “safety” pennant which is
typically longer than the primary pennant and used as a back-up in case the primary fails.

Prepared by: Dr. Md. Mashiur Rahaman, Dept. of NAME, BUET

NAME 315/335: Port and Harbour Engineering Chapter: Mooring for Port and Harbour

A typical mooring scheme

Number Name Purpose

1 Head line Keep forward part of the ship against the dock

2 Forward Breast Line Keep close to pier

3 Forward Spring Prevent from advancing

4 Aft Spring Prevent from moving back

5 Aft Breast line Keep close to pier

6 Stern line Prevent forwards movement

The following sketch shows a vessel berthed starboard side to, but any side may be chosen,
depending on the wind direction.

Prepared by: Dr. Md. Mashiur Rahaman, Dept. of NAME, BUET

NAME 315/335: Port and Harbour Engineering Chapter: Mooring for Port and Harbour

6. Mooring System Layout

6.1. Swing moorings or Single Point Mooring:

Swing moorings also known as simple or single-point moorings, are the simplest and most
common kind of mooring. A swing mooring consists of a single anchor at the bottom of a
waterway with a rode (a rope, cable, or chain) running to a float on the surface. The float
allows a vessel to find the rode and connect to the anchor. These anchors are known as swing
moorings because a vessel attached to this kind of mooring swings in a circle when the
direction of wind or tide changes.

Single point mooring (SPM) is a floating buoy/jetty anchored offshore to allow handling of
liquid cargo such as petroleum products for tanker ships. SPM is mainly used in areas where
a dedicated facility for loading or unloading liquid cargo is not available. Located at a
distance of several kilometers from the shore-facility and connected using sub-sea and sub-oil
pipelines, these single point mooring (SPM) facilities can even handle vessels of massive
capacity such as VLCC.

Single point mooring (SPM) serves as a link between the shore-facilities and the tankers for
loading or off-loading liquid and gas cargo.

Prepared by: Dr. Md. Mashiur Rahaman, Dept. of NAME, BUET

NAME 315/335: Port and Harbour Engineering Chapter: Mooring for Port and Harbour

Some of the major benefits of using SPM are:

• Ability to handle extra large vessels

• Doesn’t require ships to come to the port and thus save fuel and time
• Ships with high drafts can be moored easily
• Large quality of cargo can be easily handled

6.2. Pile moorings

Pile moorings are poles driven into the bottom of the waterway with their tops above the
water. Vessels then tie mooring lines to two or four piles to fix their position between those
piles. This is to avoid the massive damage that would be caused by many vessels anchoring.

Prepared by: Dr. Md. Mashiur Rahaman, Dept. of NAME, BUET

NAME 315/335: Port and Harbour Engineering Chapter: Mooring for Port and Harbour

6.3. Multiple anchor mooring systems use two or more (often three) light weight
temporary-style anchors set in an equilateral arrangement and all chained to a common center
from which a conventional rode extends to a mooring buoy. The advantages are minimized
mass, ease of deployment, high holding-power-to-weight ratio, and availability of temporary-
style anchors.

6.4. Conventional buoy or multi buoy mooring:

In this method the bow of the ship is secured using both her anchors whereas the stern is
secured to buoy around it . In the approach firstly vessel approaches the final berthing
position from forward at an angle of 90 degrees to her final direction of berthing. The
starboard anchor is then let go first at a pre decided spot while the ship is making headway.
Required amount of cable is paid and the astern propulsion too operated simultaneously to
stop the vessel. Once the vessel is stopped in water port anchor is let go and thus vessel
positions her stern along the centerline bifurcating the buoys. For aligning the vessel along
this centerline port cable is paid out and starboard cable heaved in with astern propulsion.
The helm and engines to be carefully used during this maneuver to ensure the stern is
swinging clear of any of the buoys. During unberthing the anchor cables are heaved in to
move the vessel forward and the weight is taken on windward lines while casting off other
lines to prevent swinging of the stern into the other buoys. This manoeuvre requires skill and
efficient operation of ship’s crew as well as of the mooring equipment as often weight of the
lines can be immense.

7. Mediterranean Mooring of Ships

What happens when there is not enough space at the wharf for a number of ships or boats to
be moored? Or what happens when there is a ramp at the stern of the ship (like a car carrier)
that connects it to the hold? The answer is that the ship has to be moored perpendicular to the
wharf with its stern transom (surface) lying parallel to the jetty. Such “end on” to the quay
berthing is called “Mediterranean Mooring”.

Mediterranean mooring, also known as “med mooring”, is a technique for mooring a vessel to
pier at a perpendicular angle. The ship thus occupies less space as it is connected to a fixed
length of pier along the width rather than its length. The disadvantage of Mediterranean
mooring is that it is more likely to result in collisions and that it is not practical in deep water

Prepared by: Dr. Md. Mashiur Rahaman, Dept. of NAME, BUET

NAME 315/335: Port and Harbour Engineering Chapter: Mooring for Port and Harbour

or in regions with large tides.

Mediterranean mooring can be done in two ways. Now the boats or crafts like yacht can be
berthed bow to or stern to with the wharf depending upon the convenience or preference of
the crew but in both cases an anchor is dropped from the opposite end of the one that is
approaching the jetty in such a way so that that the end remains fixed and the anchor can be
hove up and hauled out.

7.1 CASE 1: (Berthing Stern to)

At first the place where the ship’s stern is to be brought alongside is located. A firsthand
knowledge of the shore line is required, like if there is a protrusion or not at the underwater
section or the bed is shallow or shoaled. In that case the stern first approach is aborted and the
bow first approach is tried. For a vessel having normal right handed propeller the wharf is
best kept on the port side and the distance is kept more than seven ships length.

The ship is at first kept parallel to the berth and then it is brought astern. The point of
berthing is now generally abeam. All through the astern movement the rudder is kept to hard
over port so as to get the maximum canting effect. As the ship falls astern, the bow will
naturally cant to starboard due to transverse thrust. The Starboard anchor is dropped in such a
way so that the scope (it’s the ratio of the length of the anchor rode and the vertical distance
from the bow of the vessel to the bottom of the water or the bed) is 7:1 (approx). So if the sea

Prepared by: Dr. Md. Mashiur Rahaman, Dept. of NAME, BUET

NAME 315/335: Port and Harbour Engineering Chapter: Mooring for Port and Harbour

bed is 10 m down from the bow, a distance of 70 m is required. Smaller crafts with shorter
anchor chain will try to make this distance as much as practicable.

The chain is let to fall free so that the anchor digs at the sea bed in the first attempt. It is
ensured that there is no snag in the rode (it’s the length of the anchor line or chain) and that
there is enough length of the chain to reach the berth. The anchor must be dropped away from
the other anchor lines or the lazy lines, which are used to retrieve heavy pre embedded bow
lines substituting the anchor. Otherwise fouling of the anchor chains or these lines can be
quite a cumbersome affair during sailing.

Meanwhile hand fenders are arranged along the sides to protect the hull of the ship/boats on a
windy day from colliding with the other crafts that are docked already. The vessel is slowly
brought astern keeping in mind that the wind is not on- shore. Otherwise the fall cannot be
restricted. Off shore winds are of great help as they dampen the stern board movement and
the vessel will have a tendency to stop without much use of the ahead movements on the

Prepared by: Dr. Md. Mashiur Rahaman, Dept. of NAME, BUET

NAME 315/335: Port and Harbour Engineering Chapter: Mooring for Port and Harbour

telegraph.

When the wind is athwart, the first line ashore will be the one from the windward side. In this
case if the wind pushes the starboard hull the starboard quarter line is to be sent ashore first.
Then the other rope from the opposite end is thrown ashore and ensured that both the lines
are” doubled up” i.e., the lines are made to take a turn on the shore bollard and are sent on
board. This helps the crew while hauling out as only casting one end of the line from on
board the line can be retrieved with the winch without any shore assistance. On a windy day
the first shoreline is sent ashore by a small motorboat or a dingy so that the vessel can be
warped in with the help of its winches. While hauling in if the bow swings corrective counter
measures are taken with short kicks of forward movements and necessary helm or holding the
anchor a bit, as the case may be. The lines ashore are fast in such a way that the angle
between them is wide enough to restrict any swing of the stern due to any surge in the wave
or wind. It is best to set another pair of lines across the stern to the shore.

The boarding ramp is employed for the crew to step out from the stern. Also cars and trucks
loaded in the car carriers can be loaded or unloaded.

CASE 2 (Berthing Bow to):

Docking “Bow to” is much easier though as the pilot or the master has much more control
over his ship as he steers in letting his stern anchor far off the berth so as it holds the stern
during the stay. A line from each quarter is hitched to the chain aft to restrict any side wards
play. Two lines are passed ashore from the port and starboard bow at a wide angle as in the
previous case so as to make fast the bow to the jetty. This type of bow fast approach is done
if there is an underwater shoal below the jetty or there is a protrusion.

In both cases the anchor is snubbed when there is only a boat length difference between the
quay and the bow or stern. Engines are also given accordingly so as to avoid any untoward
collision.

Prepared by: Dr. Md. Mashiur Rahaman, Dept. of NAME, BUET

NAME 315/335: Port and Harbour Engineering Chapter: Mooring for Port and Harbour

8. Dolphin

A dolphin is an isolated marine structure for berthing and mooring of vessels. It is a common
practice to combine dolphins with piers which drastically reduce the size of piers. Dolphins
are generally divided into two types, namely breasting dolphins and mooring dolphins.

Breasting dolphins serves the following purposes:

Prepared by: Dr. Md. Mashiur Rahaman, Dept. of NAME, BUET

NAME 315/335: Port and Harbour Engineering Chapter: Mooring for Port and Harbour

(i) Assist in berthing of vessels by taking up some berthing loads.

(ii) Keep the vessel from pressing against the pier structure.
(iii) Serve as mooring points to restrict the longitudinal movement of the berthing
vessel.

Mooring dolphins, as the name implies, are used for mooring only and for securing the
vessels by using ropes. They are also commonly used near pier structures to control the
transverse movement of berthing vessels.

Fig. : Dolphins

Prepared by: Dr. Md. Mashiur Rahaman, Dept. of NAME, BUET

NAME 315/335: Port and Harbour Engineering Chapter: Mooring for Port and Harbour

Access to a dolphin may be via a pedestrian bridge (mooring dolphins) but is usually by boat.

There is also Berthing dolphins which are at least a minimum of a pair which are designed to
resist the lateral (sidewise, both compression and tension forces) load imposed on them by
the moored vessel due to wind and wave. The vessel may abut directly on the dolphins or
floating spacers called camels may be placed in between the vessel and the dolphins.

Dolphin berthing and mooring systems are commonly used where piers or quays for docking
vessels are not available. Mooring dolphins may also be used in conjunction with short piers
where the length of the vessels docking on the pier normally extend beyond the length of the
pier.

Prepared by: Dr. Md. Mashiur Rahaman, Dept. of NAME, BUET