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MATERIALS IN OFFSHORE CONSTRUCTION

Classification of steel by codes

RECOMMENDED PRACTICE FOR PLANNING, DESIGNING AND CONSTRUCTING FIXED OFFSHORE

PLATFORMS - WORKING STRESS DESIGN. RP-2AWSD - AMERICAN PETROLEUM INSITUTE

STEEL GROUPS - API RP- 2A WSD

Steel are grouped according to the strength level and welding characteristics.

Group I - steels with specified minimum yield strength (SMYS) of 280MPa or less, carbonequivalent is 0.4% or less

Group II - 280Mpa < SMYS < 360MPa, carbon equivalent upto 0.45% and higher, requiresthe use of low hydrogen welding process

Group III- High strength steels, SMYS is > 360MPa; special welding procedures required. Steel structures - strength is not only the criteria Should have good superior low temperature toughness for the base metal and the welded

joints to avoid brittle failure. For this, besides charpy impact properties, the material requires

good CTOD - (crack-tip-opening-displacement ) properties. CTOD is one of a family of fracture

mechanics tests that measures the resistance of a material to growing of a crack.

Fixed offshore structures - medium grade structural steels, with yield strengths typically in therange of 350MPa. These steels are well documented and covered by existing codes and

standards. In recent years, there has been an increasing interest in the use of higher strength steels for

these installations.

Benefits from an increase in the strength to weight ratio and the associated savings in the costof materials.

As a result, significant parts of several platforms (jacket and topsides) have been constructedfrom 400 -450MPa steel and installed in the North Sea.

However, to date, fatigue sensitive components (e.g. tubular joints) have generally beenfabricated from medium strength steel because of better knowledge on these steels with

regard to fatigue performance and the lack of increased performance of high strength steels in

this area.

The principal application of very high strength steels in the offshore has been in the fabricationof jack-ups.

Steels with nominal yield strengths in the range 500 - 800MPa are normally used in thefabrication of legs, rack and pinions and spud cans.

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Jack-ups, used primarily for drilling, have many years of satisfactory experience in use, operatingin a variety of water depths, but are normally brought into dry dock for inspection at 5year

intervals, to determine whether any damage or cracking can be found and to get it repaired.

High strength steels have also been used in tethering attachments for floating structures in TLPs(tension leg platforms) and for mooring lines with semi-submersible module offshore drilling

units (MODUs).

High strength steel SMYS > 350MPa is used in offshore structures.

ALUMINIUM OR ALUMINUM

They are used in hulls, deckhouses and hatch covers of commercial ships. They are also used in

equipment items such as ladders, railings, gratings, windows and doors. Passenger vessels utilize

large quantities of aluminium in superstructure and equipment. Small and high speed boats in

particular are constructed of aluminium using Aluminium alloys. The strength of Aluminium alloys

are approaching to that of mild steel. This leads to a great advantage as "equal-strength structures

can be designed with a weight saving of 55 to 67%. Aluminium's specific weight is 1/2.5 of steel.

However, the cost of aluminum is approximately twice that of steel. But maintenance costs work out

to be 1.5 times the cost of aluminium, over a period of ten years. Further, aluminium does not need

any protective coating and therefore cost over the life of the structure is lower. In particular, alloys -

5xxx series alloys for marine applications have weld yield strengths of 100 to 200MPa. They are also

used in in pressure vessels in liquid natural gas (LNG) transport ships. These pressure vessels

mounted on the hull of a ship are usually spheres and can be of substantial dimension ~ 36m

diameter with a wall thickness of 5cm. Insulations are provided so that the low temperature do not

reach the ship's hull.

Other non-ferrous metals

Few non-ferrous metals also have increased application in marine environment. For example,

Cupronickel - Copper, Nickel (69% Copper, 30% Nickel), K-Monel-Nickel, copper (Nickel 65%, Cu 30%

+ others including Al and Titanium), Nickel Copper 400 ( Nickel 66%, Cu 31.5% + others, excludes Al

and Titanium) and Bronze (Cu 90, Zn 10). Cupronickel are widely used for condenser applications

such as tubes, tube sheets and manifolds (pipe with one inlet and many outlets or one outlet and

many inlets). Bronze is widely used for valves, pumps and heat exchangers and in applications which

do not required high strength. Other non-metallic materials like fibre glass, glass (tempered), wood,

concrete and ferrocement are also popular. Fibre glass is the most prominent nonmetallic material

for ocean applications. It is essentially used for fabricating small boats and buoys. Fibre glass is a

composite material consisting of reinforcing materials of fibrous nature and a bonding material.

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Reinforcing material gives strength to the structure and consists of glass fibres, or carbon graphite,

nylon, silica or metals such as steel, aluminium, boron and tungsten; Bonding material - epoxies,

polyesters, phenolics and silicones. Most common is glass fibre with an epoxy or polyester binder. Its

strength depends on the manufacturing process and performance characteristics vary with the

manufacturer. Fiberglass polyester mat is used widely in the production of small boats and buoys.

Advantages of FRP are the absence of maintenance and its durability under a variety of operating

conditions. Fibreglass, owing to its internal damping characteristics, may heat up when subjected to

fast-changing stress cycles; it reaches its fatigue strength at 10million cycles and the ratio of fatigue

strength to tensile strength is below 0.25. Fibreglass is inclined to lose strength by the absorption of

water when immersed over long periods of time and by exposure to ultraviolet rays. Coatings are

advisable when structures made of this material are exposed to long periods in water and exposure

to rays. Absorption of water results in substantial decrease in compressive strength and ultraviolet

rays impart brittleness to fibreglass. They tend to laminate upon application of heat. An important

caution is that most fiberglass resins will burn.

Concrete has popular applications in marine environment due to its excellent compressive strength,

and resistance to attack by sea water. Its low tensile strength leads to design with minimum tension,

bending and shear stresses. Reinforcements including prestressing or ferrocement may be desirable

in such cases. Ferrocement consists of wire mesh or similar reinforcement that gives the cementconsiderable stability and permits some tensile stress. Ferrocement has been used to construct

barges, boats etc. Pressure vessels for LNG storage are made of prestressed concrete. The major

disadvantage is that concrete suffers some deterioration during freezing and thawing. One of the

oldest material used in the ocean is wood. For many years, it was the only material used for ship

building. It is extensively used for pilings, docks and similar application. Wood laminates, available as

commercial brands are also being used as structural members. Buoyancy materials have specific

gravity considerably lower than that of water. When integrally included in an underwater structure

they provide buoyancy. For example, small submarines, oil well drill pipe, deep-sea buoys. Buoyancy

material, most commonly are wood (specific gravity 0.5 ) and gasoline (specific gravity 0.7).

Desirable characteristics required for buoyancy materials are that they do not compress and do not

absorb water. Syntactic foams cater to the needs of buoyancy materials. These are hollow glass

spheres, which have a high compressive and shear strength, dispersed in a plastic matrix. Most

efficient syntactic foams use glass spheres of extremely small diameter called microballoons with an

epoxy resin binder. They have low water absorption and can be easily be handled with woodworking

tools.