176 Steels:MetallurgyandApplicationsgravity structures. Therefore they require good foundations and a thorough inves-
tigation of the seabed conditions to guard against long-term settlement and tilting.
However, in spite of satisfactory performance, construction in reinforced concrete
has remained relatively rare, apparently for economic reasons.Design considerations
In the UK, the design and construction of offshore structures must comply with
Guidance Notes 22 prepared by the Department of Energy. However, the Depart-
ment of Energy has authorized the following organizations to issue Certificates
of Fitness for offshore structures, in a similar manner to that for ships:
9 American Bureau of Shipping
9 Bureau Veritas
9 Det Norske Veritas
9 Germanischer Lloyd
9 Offshore Certification Bureau
9 Lloyd's Register of Shipping
The Guidance Notes illustrate very clearly the complex loading situation in
offshore rigs by specifying the need to take account of the following loads in the
primary structure:
9 Dead loads- the weight of the structure and other fixed items
9 Imposed or variable loads - loads from drilling operations, consumable stores,
crew members, berthing and landing loads
9 Hydrostatic loads - acting in a direction normal to the contact surface
9 Environmental loads - wind, wave and current which act horizontally and, in
extreme conditions, exert large overturning and sliding forces on fixed instal-
lations
Structural members in the splash zone are also subjected to major impact loads
termed slamming. Whereas hydrostatic loads can be determined to a high degree
of accuracy, environmental loads are random and the extreme magnitudes have
to be predicted on a probability basis.
The most common type of steel platform is the welded tubular frame construc-
tion which is illustrated in Figure 2.19. In such platforms, the most critical areas
are the nodes which represent the intersection of a chord (large-diameter member)
and a brace (small-diameter member). Nodes can be constructed in various
geometries, including T, K and T-K configurations, and they constitute areas
of high stress concentrations. As such, they have a very marked effect on fatigue
behaviour and therefore very careful consideration is given to the design and
quality of welds in these locations. The first indication that fatigue could pose a
major problem in offshore structures occurred in December 1965 when Sea Gem,
a jack-up barge, collapsed in the North Sea, killing 13 people. It was shown that
this failure was due to fatigue cracking which was probably initiated from brittle
fracture, originating in welded components. Detailed consideration has therefore
been given to the fatigue performance of nodal joints and design S-N curves