Equalisation of christmas trees

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In the design process for all wells, consideration should be given to the repressurisation of the Christmas tree after it has been closed in. Several approaches may be taken depending on the circumstances and the design of the well.

In the case of the well being shut in and depressurised with full pressure below the closed SCSSSV, there has to be some means of equalising the pressure across the sub-surface valve before it can be opened.

Repressurisation scenarios

There are several scenarios possible for the repressurisation of a Christmas tree:

 1.Using the equalisation feature of the sub-surface valve (if fitted).

2.Repressurisation of the string above the SCSSSV from another well via the Production Manifold (or the Kill Manifold, bearing in mind the directions on kill systems and the kill philosophy).

3.Pumping into the string above the SCSSSV a fluid which is compatible with the produced hydrocarbons (e.g. diesel) and pressurising this fluid until the flapper opens.

4.Using a supply of inert gas at sufficient pressure for same.

5.Using a combination of the methods described under steps 2 and step 4; if the other well cannot supply sufficient pressure this deficiency can be made up by an additional supply of inert gas.

Inter-well connection features

Any inter-well connection should have the following features:

 1.Minimum line diameter of 2" (50 mm).

2.For low pressure, normal temperature (non-gas) applications, any quick connecting rigid piped system or flexible hoses are acceptable.

3.For high pressure, high/low temperature gas and H2S applications any suitable metal to metal seal system may be used.

4.In all cases where jointed or flexible hoses are used there should be a documented and auditable means of determining whether the system is certified for use i.e. pressure test certificates or insuring authority approval for use.

5.In all cases where either jointed rigid line or flexible hoses are used suitable anchoring devices should be used to restrain the line in the event of a failure. This is a mandatory safety requirement.

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