Thanks for your response and clarifying the position.
Then your tax liability will be your annual pension plus the income from the Russian oil rig, ( less the personal allowances and the remaining liable to tax, at the appropriate rates)
The reason for this is because you will be treated as remaining resident in the UK as you will not actually be leaving the UK to take up permanent employment and life abroad but merely working out of the UK, on a rotational basis, and as you will spend more than 90 days a year in the UK, as things stand, then you will remain resident for UK tax purposes.
You should advise HMRC of this work abroad, so they can arrange to issue you with a self assessment tax return after each year end, and on this you can declare your pension, the Russian income and any foreign tax suffered (so you do not suffer double taxation on the same income)
They will then advise you what tax is due, which is then paid by the following 31st Jan, following the tax year end.
For example, first tax return will be issued after 05/04/2014, and this return needs to be back with HMRC by 31st Oct 2014 if filling in a paper copy, or by 31st Jan 2015 if filing online. Then any tax due has to be paid no later than 31st Jan 2015.
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