Wellhead maintenance: Safeguarding against the unknown

This is an extract from a paper first published at Offshore Middle East 2010, SESSION 5: Asset Management & Facility Integrity, October 12 – 14, 2010 at the Qatar International Exhibition Centre, Doha, Qatar.

Introduction:
Oil exploration and production has played a significant part in the financial success of the Middle East since the mid 1900s. The immense size of these oil discoveries has often meant that fields are continuing to produce commercially viable quantities of oil well past their expected field lifetime. Whilst this is highly beneficial from a financial standpoint, it is important that operators are aware of the consequences in terms of the ongoing integrity of the well hardware and the safe operation of the field. The harsh environmental elements of the Middle East region, accompanied with the corrosive and erosive properties of flowing hydrocarbons result in accelerated wear to the wellhead and tree assemblies as well as deterioration of the casing strings.

Coupled with general wear and tear of mechanical assemblies and additional unforeseen damage it is critical that field operators have suitable plans in place for routine well maintenance operations so that problem areas can be identified and remedied quickly and effectively. This greatly reduces the risk of catastrophic failure events and having to shut-in the well at a later date to replace worn or seized mechanical parts – an operation with significant financial implications. Regular maintenance is a much more cost effective approach than waiting for problems to occur and reacting to them.

Common Failure Mechanisms & Remedies
The structural integrity of a wellhead and tree assembly is dependent on a number of external influences and scenarios and the causes of deterioration or failure are often not immediately obvious. Investigations into a small crack or seepage can quickly escalate into a more serious problem with the well construction requiring immediate attention. Regular inspection and maintenance will ensure that a wellhead continues to operate in a safe and efficient manner well past its original field life.

The most common failure mechanisms found during inspections and their associated remedies are detailed below:

Mechanical Assemblies – Master valves and offtake valves are typically the main point of flow control on the Christmas tree and wellhead assembly. Valves and actuators are a common component affected by seizure and breakage caused by a lack of suitable lubrication and long periods of non-operation. Whilst seizures can often be remedied through the use of flushing lubricants, this approach is not 100% effective. Stuck or seized valves will generally need replacing which requires costly well shut-in operations.

This can be complicated further if the valve is stuck in the closed or partially closed position. When this occurs the valve will need to be hot tapped in order to re-open the well bore whilst maintaining pressure control prior to setting plugs using a polished rod lubricator and replacing the valve assembly. Preventative maintenance is key to ensuring the valves remain in an operable condition – monitoring and recording the number of turns to open and close the valve, looking for signs of damage to the valve body and leakage through the seals and also reacting to early indications of seizure ensure that they can be relied upon in the event of an emergency.

Corrosion & Erosion – Wellhead and tree components are continually subjected to corrosion and erosion – whether it is via the external environment with onshore wellheads being sand blasted by wind and offshore surface wellheads sitting in a corrosive salt-water atmosphere, or via the internal environment with hydrocarbons eroding the internal casing/tubing strings over a period of time. The gradual thinning of the metalwork caused by these corrosive environments will eventually lead to cracks or holes, through which hydrocarbons or gas will leak, causing contamination and further speeding up the deterioration of the wellhead materials.

Whilst it is difficult to determine the structural integrity of the casing strings at any given time, regular pressure monitoring of the well bore and casing annuli makes it possible to identify trends in flow characteristics. Changes in pressure over time could mean a transfer of fluids between annuli and therefore a possible breach. In extreme cases it is possible for the hydrocarbons to breach the outermost casing, in which case immediate action is required to locate the point of breach and patch/repair as necessary prior to locating the leak path from the inner tubing and securing the wellhead structural integrity. This has further complications for onshore wells where for near-surface breaches excavation is required to locate the leak origin. However, if the breach is deep inside the well the only option to the operator is to conduct a full workover.

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