SHIP TYPES BY PHYSICAL SUPPORT

Ships can also be classified by the means of physical support

while in operation. Three broad classifications that are

frequently used by naval architects are shown at the following

slide reproduced from an “Introduction to Naval Architecture”

by Gillmer and Johnson.

• Aerostatic Support

• Hydrodynamic Support

• Hydrostatic Support

Another type of classification of ships

AEROSTATIC SUPPORT

Aerostatic support is achieved when the vessel rides on a cushion of air

generated by lift fans. These vessels tend to be lighter weight and

higher speed vessels. The two basic types of vessels supported

aerostatically are

• air cushion vehicle (ACV) hovercraft

• surface effect ships (SES)

Air Cushion Vehicles (ACV)

Air Cushion Vehicles (ACVs) or hovercraft continuously force air

under the vessel allowing some of the air to escape around the

perimeter as new air is forced downwards. They are usually

propelled forward by airplane propeller type devices above the

surface of the water with rudders behind the air flow to control

the vessel.

Hovercrafts are very expensive for their size, but have the unique

property of being amphibious. The Navy utilizes some hovercraft

as LCACs (Landing Craft Air Cushion vehicles) because of this

ability. Their use has opened over 75% of the world's coastline to

amphibious assault compared with 5% with conventional landing

craft.

Surface Effect Ship (SES)

The Surface Effect Ship (SES) or Captured Air Bubble (CAB) craft,

are similar to ACV’s in that they use a cushion of air to lift the vessel.

However, the SES has rigid side walls that extend into the water. This

prevents the SES from being amphibious but reduces the air pumping

requirements and makes them more directionally stable. The side

walls also contribute to the hydrostatic or hydrodynamic support of

the craft allowing the SES to carry more payload. They are usually

propelled by water jets or super cavitating propellers.

A supercavitating propeller is a kind of screw propeller so shaped as

to create a steady cavitation space and prevents the cavitation vapor

bubbles from collapsing on the blades where it might damage the

blade.

The vessel utilises a cushion of air trapped between the sidewalls and flexible bow and stern

seals to lift a large part of the hull clear of the water to reduce drag

HYDRODYNAMIC SUPPORT

Hydro is the prefix for water and dynamic indicates movement.

The two basic types of vessels supported hydrodynamically are

• planing vessels

• hydrofoils.

Planing Hulls

Planing hulls are designed to run on top of the water at high

speeds. To achieve this they typically have a very flat stern. The

hull design (shape) does not limit the maximum attainable

speed but does affect the power required for it to get top of the

water.

Planing hulls use the hydrodynamic pressures developed on the hull at

high speeds to support the ship. They are very fast, some capable over

50 knots. In smooth water they ride very comfortably. When moving

through waves, planing vessels ride very roughly, heavily stressing

both the vessel structure and passengers. This was particularly true of

older types which used relatively flat bottom hulls. Modifications to

the basic hull form, such as deep V-shaped sections, have helped to

alleviate this problem somewhat.

Planing hulls require much larger engines for their size than

displacement hulls. These factors above serve to limit the size of

planing vessels. However, these ships are used in a variety of roles

such as pleasure boats, patrol boats, missile boats, and racing boats.

At slow speeds the planing craft acts like a displacement ship and is

supported hydrostatically.

Hydrofoils Hydrofoil craft are supported by underwater foils, like the wings of

an aircraft. At high speeds these underwater surfaces develop lift and

raise the hull out of the water. Bernoulli’s Principle is often used to

explain how a wing develops lift. These vessels are very fast, reaching

speeds of 40 - 60 knots and compared to planing boats, hydrofoils

experience much lower vertical accelerations in moderate sea states

making them more comfortable to ride.

The hydrofoil can become uncomfortable or even dangerous in heavy

sea states due to the foils breaking clear of the water and the hull

impacting the waves. If the seaway becomes too rough the dynamic

support is not used, and the ship becomes a displacement vessel.

The need for the hydrofoils to produce enough upward force to lift

the ship out of the water places practical constraints on the vessel's

size. Therefore, the potential crew and cargo carrying capacity of

these boats is limited. Hydrofoils are also very expensive for their

size in comparison to conventional displacement vessels.

The U.S. Navy formerly used hydrofoils as patrol craft and to carry

anti-ship missiles (Pegasus Class), but does not use them anymore due

to their high acquisition and maintenance costs.

HMCS Bras d'Or (FHE 400) was a hydrofoil that served in the Canadian

Forces from 1968 to 1971 the vessel exceeded 63 knots (117 km/h;

72 mph), making her possibly the fastest warship in the world

HYDROSTATIC SUPPORT

Hydrostatically supported vessels are by far the most common

type of water borne craft. They describe any vessel that is

supported by “Archimedes Principle”.

Word definition of Archimedes Principle

“An object partially or fully submerged in a fluid will experience a

resultant vertical force equal in magnitude to the weight of the volume of

fluid displaced by the object.”

This force is called the “buoyant force” or the “force of buoyancy”.

Archimedes Principle

Archimedes Principle can be written in mathematical format as

follows.

F=ρg ∇

F is the magnitude of the resultant buoyant force

(kg.m/s2 = N)

ρ is the density of the fluid (kg/m3)

g is the acceleration due to gravity (m/s2)

∇ is the volume of fluid displaced by the object in (m 3)

Displacement Ships

Hydrostatically supported ships are referred to as “displacement ships”, since they

float by displacing their own weight in water, according to Archimedes Principle.

These are the oldest form of ships coming in all sizes and being used for such

varied purposes as carrying cargo, launching and recovering aircraft,

transporting people, fishing, and war fighting.

Displacement hulls have the advantage of being a very old and common

type of ship. Therefore, many aspects of their performance and cost

have been well studied. In comparison to other types of vessels the cost

of displacement ships is fairly low with respect to the amount of payload

they can carry. Disadvantages of displacement vessels include their

limited speed and at times, their seakeeping ability (how they respond

to ocean waves).

SWATH

A special displacement ship is the Small Waterplane Area Twin Hull

(SWATH). Most of the underwater volume in the SWATH ship is

concentrated well below the water's surface as shown in the

following figure. This gives them very good seakeeping

characteristics.

They also have a large open deck and are therefore useful in a variety

of applications requiring stable platforms and a large expanse of deck

space. SWATH vessels are currently utilized as cruise ships, ferries,

research vessels, and towed array platforms.

Two major disadvantages of SWATH ships are deep draft and cost.

Additionally, these vessels present the designer with structural

problems differing from other ships, particularly with respect to

transverse bending moments.

Submarines

Submarines are hydrostatically supported but above 3 to 5 knots

depth control can be achieved hydrodynamically due to the lift

created by the submarines planes and body of the hull.

Submarines have typically been used as weapons of war, but lately

have also seen some non-military application. Some submarines

are being designed for the purpose of viewing underwater life

and reefs. Unmanned submersibles have been used for scientific

purposes, such as finding the Titanic, as well as a wide variety of

oceanographic research.

DESCRIPTION OF OFFSHORE

UNITS

As our world continues to expand in population and energy

consuming products are ever growing, we are dependent on

“energy” more than ever. Oil and gas are still our most

important source of energy.

Within the world of oil and gas, crude oil is called “petroleum”.

Petroleum is a combination of the Greek word PETRA and the

Latin word OLEUM which means “Rock oil”. Crude oil actually

comes from the rocks which is entrapped within the rock

formations. According to scientists, oil and gas come from the

remains of the plants and miniscule animals that lived and died in

the sea millions of years ago. As time passed large amounts of

sediment covered the organic material. The pressure and heat on

the organic material transforms this material into oil and gas

during millions of years.

Early Days

In the early years of 1800 whale oil was used for illumination purposes. As

a consequence whales have been hunted to extinction and people were

anxious to find new alternatives.

Around these times an oil well near Pennsylvania came to the surface of

the land. It literally leaked out of the rocks which inspired a man named

Colonel Drake to recover this rock oil and sell it as an inexpensive

substitute for whale oil. After some years of trial and error resulted in the

technique of drilling to collect the oil from its point of origin, initially at a

depth of 21 metres. In 1897 this drilling is extended to depth of 90 metres

in the ocean, the first steps to offshore activities.

The first real offshore oil was found 9 seamiles offshore in a water depth of

as little as 6 metres. From then on over the last 50 years progress has been

revolutionary. Offshore oil and gas developments are now taking place in

over 40 countries hundreds of kilometres from the offshore in ever

increasing depths.

Definition of Offshore

Offshore refers to industrial activities in open sea, starting from the

search of oil and gas to production and transporting them to shore.

In this section a brief overview of fixed and mobile offshore units, such

as dredgers, pipe laying vessels, drilling vessels, oil production, storage

and offloading units and several types of support and transportation

vessels are going to be given.

Stages of Offshore Activities

Seismic Survey Vessel

The decision to drill at a given location is based on the result of seismic and

geological surveys of the structure underlying the sea bed. The purpose of

this vessel is to produce detailed information for oil companies as a basis for

actual production drilling. This information is the result of the evaluated

reflected sound waves in the sea floor. To obtain these results sound waves are

initiated by the vessel by means of air guns, the reflections are collected by a

number of detectors within long cables towed by the vessel

Some seismic survey systems

Jack-ups

A jack-up rig is a type of mobile platform that is able to stand still on the

sea floor, resting on a number of supporting legs. The most popular designs

use 3 independent legs, although some jackups have 4 legs or more.

The jack-up drilling rig is used for exploration drilling approximately 10

metres to maximum 150 metres water depth. The jack-up barge is a

triangularly or rectangular shaped barge that is towed to the work location.

At the location the barge raises its deck alongside the legs with the lower ends

of the legs resting on the seabed.

Usually three legged ones are used for exploration drilling and 4 legged ones

as work barge for construction work.

Long distance transport of jack-ups is by towing with a tug or by heavy lift

transport ships.

Drilling operations take place in the

elevated condition with the platform

standing on the sea bed. This type of

platform is used for drilling operations

in water depths up to about 100 m. Jack-

ups spend part of their life as floating

structures. This is when such platforms

are towed to a new location

by means of ocean-going tugs. In this

mode, the legs are lifted up and extend

upwards over the platform.

Jack-up rig in drilling mode

Jack-up rig in floating mode

Drilling vessels

When geological predictions based on seismic surveys have been established

that a particular offshore area offers promising prospects for finding oil, a

well is drilled to examine these predictions. These drilling operations are

carried out from barges, ships, semi-submersibles or jack-up rigs in medium

to deep water.(150 to 3000 metres water depth)

To maintain position during drilling operations the ships are either anchor

moored in an anchor pattern or rely on dynamic positioning (DP) depending

on the water depth.

A dynamically positioned (DP) vessel uses its propellers, rudders, tunnel

thrusters to stay in position. A control system continuously determines the

required vector based on information from a position reference system,

like radio or hydro-acoustic beacons or GPS.

Drilling ships

Semi-submersible drilling unit

A semi-submersible drilling unit is used for

drilling the exploration and production

wells in 150 to 2500 metres water depth

An important advantage of the semi

submersible type in comparison with the

ship shaped typed drilling vessel is the better

motion behaviour of the unit in harsh

environments which can give an extended

working window.

A crane vessel, crane ship or floating crane is a

ship with a crane specialized in lifting heavy loads. The

largest crane vessels are used for offshore construction.

Conventional monohulls are used, but the largest crane

vessels are often catamaran or semi-submersible types as

they have increased stability. On a sheerleg crane, the

crane is fixed and cannot rotate, and the vessel therefore

is maneuvered to place loads.

Crane vessels

Fixed production platforms

Tension Leg Platform (TLP)

Floating Production Storage and Offloading Vessel (FPSO)

Shuttle Tankers

Pipelaying barges/semisubs/vessels

For the installation of oil and gas pipelines,

• anchor moored or DP flat bottomed barges,

• semi-submersibles or

• ship shaped vessels

are used.

One of the problems of laying pipes on the sea bed lies in the

limited capacity of the pipe to accept bending stresses. As a

result, it is necessary to keep a pipe line under considerable

tension during the laying operation otherwise the pipe might

buckle and collapse. The tension in the pipe line is maintained

using the anchor lines of the pipe laying vessel.

Pipe laying vessels can consist of semi-submersibles or ship-

shaped hulls. Semi-submersibles have the advantage of better

motion characteristics in waves which is beneficial for the

pipe laying operation. On the other hand, ship-shaped vessels

have a higher variable load capacity and a much higher transit

speed.

Pipe laying vessels are usually moored by means of anchor

systems which are continually being relocated as the laying

operation progresses. A new development is a pipe laying vessels

kept in position and deriving the pipe line tension by using a

dynamic positioning (DP) system instead of anchor lines; for

instance pipe laying vessel ’Solitaire’, operated since 1998 by

Allseas Marine Contractors. This figure shows a comparison of

this vessel with the much smaller ’Lorelay’ of this company,

operated since 1986. The considerable increase of size is obvious

here.

After laying the pipe, it has - in many cases - to be buried in

the sea bed or to be covered by gravel stone. Another

possibility is to tow a trencher along the pipe, which acts as a

huge plow. The trencher lifts the pipe, plows a trench and

lowers the pipe trench behind it. The sea current takes care of

filling the trench to cover the pipe.

Platform supply vessel

Multipurpose Support Vessel (MSV)