Steels_ Metallurgy and Applications, Third Edition

Low-carbon structural steels 175

Table 2.13 KL factors for the determination of

hull girder section modulus in higher-strength steels

Specified minimum yieM stress-Nlmm 2 KL

315 0.78

340 0.74

355 0.72

390 0.68

Notes

1. Intermediate values by interpolation


2. For mild steel, KL = 1.0
   After Lloyd's Register of Shipping, Rules and Regulations for
   the Classification of Ships. 19

in higher-strength steels. For local scantling (thickness) requirements, a higher-

strength steel factor, K, is determined as follows:

235

K -- ~ or 0.66, whichever is the greater

tro

where tro = minimum yield stress in N/mm 2.
For mild steel, K = 1.0.
This would permit the use of steels with a minimum yield stress of up to
355 N/mm 2. However, the rules state that special consideration will also be given
to steels with minimum yield stress values greater than 355 N/mm 2.

Offshore structures

Since the early 1970s, the UK has exploited its natural gas and oil resources and

offshore platforms have become symbols of achievement in terms of design,

materials and construction. One might therefore hold the view that offshore

structures of this type were developed specifically for operation in the North

Sea whereas such platforms were first constructed in the Gulf of Mexico in the

1940s. However, the conditions in the North Sea are considerably more severe

with operating depths of 170 m compared to 7 m in the Gulf and with very

much colder and rougher climatic conditions. Offshore platforms have now been

constructed in large numbers and in 1981 it was reported 2~ that more than 10 000

structures were in operation world-wide.

Although most offshore structures have been constructed in steel, several very

large structures were built in reinforced concrete in the 1970s. This followed

the tradition, established in the late 1880s, for the use of concrete in port and

harbour installations, such as piers and jetties. Such facilities rank amongst the

largest man-made structures and, for example, the Statfjord C platform had a

float-out weight of more than 600 000 tonnes. 21 Concrete structures rely on their

sheer mass to maintain their position on the seabed and for this reason are called