Steels_ Metallurgy and Applications, Third Edition

178 Steels: Metallurgy and Applications

As illustrated in Figure 2.19, special structural steel is used for all the main nodes

and also in the transition area to the shafts. This requires the highest grade of

steel, namely BS 4360 Grade 50E Hyzed. As discussed earlier in this chapter,

such steels are made to low sulphur contents and with alloy additions which

modify the shape of the non-metallic inclusions so as to provide high levels of

ductility in the through-thickness direction of the plate. Primary steel is used in

all other structural members, such as jacket legs and topside module supports, and

the steel is BS 4360 Grade 50E Mod. Secondary steel is used in lightly stressed

areas such as module walls, decking, walkways and ladders and the specified

material is BS 4360 Grade 43D.

Particular attention is paid to resistance to brittle fracture and the Guidance

Notes lay down detailed impact and CTOD test requirements for parent plate,

HAZ and weld materials. The background to these requirements has been

published by Harrison and Pisarski 23 and involved a great deal of work on the

correlation of Charpy V, wide plate and fracture toughness data. In the Guidance

Notes, the impact test requirements are based on a minimum design temperature

of-10*C. However, for other design temperatures, the Charpy test temperature

is altered by 0.7~ for each Ioc that the design temperature differs from -10~

McLean and Oehrlein 24 have reported that the weight of steel involved in a

typical tubular frame jacket is of the order of 13 000 tonnes and, together with the

separately installed module support frame (2500 tonnes), supports the work area

above sea level. The lower ends of the four external legs of the jacket each pene-

trate through a bottle unit, weighing 770 tonnes. These units transfer the service

loads from the jacket to the piles which maintain the platform in position on the

seabed. The topside modules, which house the equipment, work area and accom-

modation, weigh a total of 6600 tonnes. As indicated in a later chapter, austenitic

stainless steels are now being used to clad the walls of the topside modules.

Cast steel nodes

In the early 1980s, significant effort was devoted to the development of cast
steel nodes as alternatives to welded fabrications. The main technical incentive
for this development was that castings could provide smooth, generous radii in
these critical areas, compared with weld connections, and so reduce the stress
concentration factor. 25 Typical compositions for these cast steel nodes are given
in Table 2.14.
According to Billingham, 21 cast steel nodes have enjoyed only limited commer-
cial exploitation but in fact they were used at fatigue-prone locations in the Hutton
Field.

Table 2.14 Composition of cast steel nodes

C% Mn% S% Ni% Nb% V% Mo% Cr% Cu%

Type I 0.14 1.30 0.005 0.43 0.025 0.05 0.11 0.09 0.04
Type II 0.15 0.85- 0.01 0.09-1.2
1.70 max.