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No. 13
Well Cellar Collapse
July 98
Background
An incident recently occurred on a PDO well pulling hoist. The well cellar collapsed under the strain applied
by the hoist mast base beam resulting in the hoist mast to drop 1 meter. The well and mast were immediately
secured.
It was found that the well cellar had collapsed on the side where the hoist beam was resting on the soil. This
allowed the stool pad, which sits between the hoist mast carrier jacking mechanism and the base beam, to
tumble and to drop the mast 1 meter.
Causes of the incident
 The well cellar was of poor construction quality (from 1977): the concrete showed protruding
gravel from the cement, pouring faults, no reinforcement and a thickness of 17 cm. The location
also had soft spots and bumps.
 The foot print of the hoist base beam was too small for the force to be adequately distributed and
the construction of the stool pad and jack was also unstable.
 Location compaction is limited to 20 ton per square meter with a safety factor of two to three.
The force exerted by the base beam was 60 ton per square meter.
Learning Points and Action to Prevent Recurrence
 The maximum load allowed per square meter of locations is unknown. As a rule of thumb, 20
ton per square meter is used with a built in safety factor of two to three depending on compacting
quality of the soil.
 The load distribution near a cellar wall is mainly distributed to the well cellar wall, (dependent
on the quality of compacting of the soil around the well cellar).
 Old well cellars and locations are suspected to be of lower quality due to repairs or oil spill
clean ups. Refills have been carried out without compacting the soil ( new well cellars have 20 cm
wall thickness and are reinforced ).
The foot print area of base beam and its stability were inadequate for the situation, it was
exposed to i.e. heavy hook loads, subsequent jarring ( shock loads ). The stability and footprint
should be addressed in the work proposal to suit the location condition.
 Planning of all well related activities should include a detailed check of the quality and the
strength of the well pad and well cellar.