DAMAGE
INCIDENT INVESTIGATION IN THE GULF - Hydraulic Workover Unit Toppled by Waterspout
Hydraulic Workover is a well intervention technique which can
be used to install or remove tubular in- or out of dead wells. "Dead" well means that the well has
zero surface pressure and has a heavy fluid or mud in the wellbore, or is not
capable of sustaining natural flow.
Advantages of Hydraulic
Workover
A HWO unit can cover the same working envelope as a workover
rig but has advantages over conventional rigs or hoists for certain
applications, such as:
• Small and flexible footprint
• Small environmental impact
• It is quickly and easily mobilized to offshore
platforms
• It is quickly and easily mobilized to remote land wells
• Costs efficient
• Perfect control over the workstring (tubular)
• Custom Rig-up configuration / Modular approach
Hydraulic Power
The HWO unit (Hydraulic Workover Unit) utilizes hydraulic
cylinders to lift the tubular in- or out of the well. The use of hydraulic cylinders allows control
over tubular movements and eliminates the use of a large mast construction
which is present on conventional drilling rigs.
Hydraulic cylinders can generate large pull and push forces and
give the HWO unit a small footprint and layout. These properties are especially
beneficial for offshore operations where space is limited and where weather
conditions can be harsh. In general is the running speed of a HWO unit lower
than that of conventional workover rig.
Incident Investigation
While conducting workover operations, a Hydraulic Workover
(HWO) unit was toppled by strong winds caused by a waterspout that touched down
near the platform. The result was the
entire HWO unit collapsing onto the facility, as well as into offshore waters. The collapse resulted in damage to the lower
riser portion of the HWO unit as well as to several other pieces of well
servicing equipment located on the platform. The upper riser portion and the BOP stack of
the HWO unit fell into the water resulting in total loss of the BOP stack. All personnel had been moved from the platform
to the supporting lift boat prior to the collapse, and no fatalities or
injuries resulted from the incident.
An investigation concluded the following factors contributed to
the accident:
•
High wind loading was exerted on the unit as a
result of the waterspout being so near to the facility. The large surface area
presented by the HWO unit, against which the wind could act, and the close
proximity of the waterspout resulted in a high wind loading being exerted on
the unit.
•
The small deck area of the platform caused an
increase in the angle(s) of the guy lines used to support the HWO unit. This increase in angle(s) resulted in
diminished stability of the HWO unit during the high wind conditions caused by
the waterspout.
•
Installation of additional guy lines would have
helped to compensate for the increased angle(s) in the guy lines, thus
increasing stability and support for the HWO unit in the event of increased
wind loading due to the storm, or swaying due to high sea state and possible sway
in the platform.
We recommend the following when installing guy lines to support HWO or related well servicing equipment:
•
Engineering calculations be performed to
determine the correct angle(s) for the guy lines when installing HWO, or
related well service equipment.
•
Installation of additional guy lines when deck
space on the host platform is limited and guy line angle(s) increases.
•
Seasonal weather patterns be taken into
consideration when installing guy lines and other means of support.
•
Guidelines for construction, safety factor, and
storm load for wire rope guy lines found in API Spec 4F, Section 8.1.2 and
8.1.3 be followed when selecting wire rope used for the purpose of guy lines.
