Mechanical lifting failures - OGP

RADD – Mechanical lifting failures
current may increase the excursion (in one direction). At 1000 m depth the excursion is
found to increase 10 to 25 metres for an average current velocity of 0.25 m/s and up to
200 m for a current of 1.0 m/s.
The effect of currents may be included if one dominant current direction can be
identified. This may be applicable for rig operations for shorter periods, for example
during drilling, completion and intervention/construction above subsea wells. However,
for a dropped object assessment on a fixed platform, seasonal changes in current
directions may be difficult to incorporate.
When establishing a "safe distance" away from activities the effect of currents should
be included. A conservative object excursion should be determined, including
consideration of the drift of the objects before sinking, uncertainties in the navigation of
anchor handling vessel, etc.
3.5 Kinetic energy
A dropped object from a crane and hitting the topsides will have a kinetic (impact)
energy E k given by:
E k = m.g.h
where: E k = kinetic energy at impact (J)
m = mass of the object (kg)
g = gravitation acceleration (9.81 m/s 2 )
h = height from release point to point of impact (m)
The maximum impact force depends on the object itself and the orientation when
hitting, and can be found from structural collapse calculations. The impact resistance of
structures can be found from deterministic structural strength calculations.
The kinetic energy of a dropped object on subsea installations depends on the velocity
through the water, the shape of the object and the mass in water. After approximately 50
- 100 metres, a sinking object will usually have reached its terminal velocity.
The terminal velocity is found when the object is in balance with respect to gravitation
forces, displaced volume and flow resistance. When the object has reached this
balance, it falls with a constant velocity, its terminal velocity. This can be expressed by
the following equation:
where: m = mass of the object (kg)
g = gravitation acceleration (9.81 m/s 2 )
V = volume of the object (the volume of the displaced water) (m 3 )
ρ water = density of water (typically 1025 kg/m 3 for the North Sea)
C D = drag coefficient of the object
A = projected area of the object in the flow-direction (m 2 )
v T = terminal velocity through the water (m/s)
The kinetic energy of the object, E T , at the terminal velocity is:
Combining these to equations gives the following expression for the terminal energy:
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