Offshore Structure Loads and Strength - Offshore Structures: Design, Construction and Maintenance - page 76

The total kinetic energy, E, should be taken to be at least:

14 MJ for sideways collision

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11 MJ for bow or stern collisions

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This corresponds to a vessel of 5000 displacement tons with an impact speed of
2 m/sec. A reduced impact energy may be acceptable in cases where the size of
visiting vessels and/or their operations near the installation are restricted. In this
instance, a reduced vessel size and/or reduced impact speed may be considered.
The reduced impact speed, V, in m/sec, may be estimated numerically from
the empirical relation:

V = ½Hs ð m/sec Þ (2.31)

where Hs is the maximum permissible significant wave height in meters for
vessel operations near the installation, per OTI (1988).
The energy-absorbing mechanisms effective during the collision should be
evaluated. Typically, local member denting, elastic and plastic deflection of the
impacted member, global elastic and plastic response of the whole structure and
denting of the ship are the main mechanisms.
In a rigorous impact analysis, the collision actions should be evaluated based on
a dynamic time simulation. The duration of the simulation should be sufficient to
cover all relevant phases of the collision and the energy-dissipation process.

Dropped Objects
When evaluating the impact risk from dropped objects, the nature of all crane
operations in the platform vicinity should be taken into account. If the probabil-
ity of impact is not negligible, relevant accidental design situations should be
defined and evaluated following the requirements. Depending on the conse-
quences for the structural integrity, the need for a rigorous impact analysis
should be determined.
Irrespective of whether a rigorous analysis is required, robustness in relation
to impact loads (vessel collisions and dropped objects) should be incorporated
into the design by indirect means, such as:

Avoiding weak elements in the structure (particularly at joints). ●

Selecting materials with sufficient toughness. ●

Ensuring that critical components are not placed in vulnerable locations.

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2.8 FIRES AND EXPLOSIONS
Hydrocarbon-pool fires on the sea surface can cause heating of, and hence
degradation of the properties of, structural components. Sources of hydrocar-
bons include conductor or riser fracture or spillage from the topside after a pro-
cess vessel rupture, while ignition sources can include radiation from oil
burners and flares.