Subsea dropped objects

Dropped objects (usually from deck cranes) are a major hazard on offshore platforms. If they fall on the deck, they can hurt workers and/or seriously damage equipment (with the potential for a catastrophic event). If the dropped object is heavy and it falls into the sea, it can be traveling quite fast by the time it reaches the seabed, especially in deepwater. Consequently the dropped object can cause substantial damage to subsea equipment—with the potential for causing a serious environmental problem. 

Areas for potential dropped objects (including subsea) and... 

The consequences associated with a subsea-dropped object can be categorized and used in a hazards analysis as follows ... 

Design subsea components so that they are less likely to fail if struck by a dropped object. 

One means of reducing the risk associated with dropped objects is to create a safe zone or quadrant at the seabed. No subsea equipment should be installed in that zone. All lifts to and from the rig or platform should be carried out in the water column above that zone. Then, if the object is dropped, the chance of an important piece of equipment being hit is minimized. 

Subsea pipeline emergency isolation valves for offshore facilities are provided where a risk analysis indicated topside isolation may be considered vulnerable. They should be protected from ship impacts, anchor dragging, flammable liquid spills and heavy objects that may be dropped from the offshore facility. 

Objects dropped overboard, particularly in deep water, can be moving very fast by the time they reach the seabed and can do a considerable amount of damage to subsea equipment and pipelines. One study, for example, calculated terminal... 

When an object is dropped into deep water it does not fall straight down—it will have a cone-like drop profile. Factors that affect the sideways movement of the object include the initial crane movement, the shape, size, weight, and density of the object, Stillwater motion (the lateral motion of objects due to hydrodynamic effects as they travel through water without considering the effect of currents), and subsea currents (which are variable and which change at different depths). Typical values for the radius of the impact zone in deep water are 50 to 100 meters. 

Once the object has been lowered close to the seabed, it is still above all the subsea equipment it can be moved horizontally to its final location. Should it be dropped, then its terminal velocity at the seabed will be low. 

Objects can be dropped either on the deck or overboard. If something lands on the deck, it could injure or kill a worker who happened to be in the vicinity. It could also seriously damage equipment or cause a release of gas or toxic materials. Objects dropped overboard, particularly in deepwater, can be traveling very quickly by the time they reach the seabed and so can do considerable damage to subsea equipment and pipelines. One study calculated terminal velocities for objects dropped in 8000 ft of water to be in the range of 1.8—3.7 m/s (4—6 miles/hour). 

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