If you are on an oil or gas related contract onboard a vessel in the Norwegian sector, you are liable to pay Norwegian tax.

If the vessel is mobbing for the job, your still on the contract, if you travelling to it, your still on the contract, if your demobbing/waiting on weather, getting drunk in the pub in Stavanger - your still on contract? You still pay.

If the vessel finishes its contract and you are babysitting the ROV system alongside or whatever - you are not liable.

They key is if the vessel is working under an oil or gas related contract and is in Norwegian waters - you pay.

You only pay Norwegian tax when in Norwegian waters, not when you are in UK waters.

Norway have a Seafarers Deduction - work over 135 days a year onboard vessel which is REGISTERED TO WORK IN THE NORWEGIAN SECTOR (note the distinction, you dont actually have to spend 135 days in Norwegian sector). This will give you a discount of up to NOK70,000 a year in earnings without having to pay Norwegian tax on it.

Norway has a different tax agreement with different countries, so best to double check what rules apply to your country.

When your vessel is under contract on Telecoms (Cable) operations.
i.e. Not Oil and Gas related.

Before you (a British taxpayer) leaves the UK to take up residence elsewhere, you should file a P85 form with the IRD. This informs the IRD of your  intention to leave the UK as of a certain date, and remain abroad thereafter. You must then remain overseas for a full tax year (April 6th-April 5th) to establish your non-residency for tax purposes.

Leaving the UK on 1st April and staying out of the country continuously for 13 months is fine; leaving on 1st May and returning 11 months later for a holiday is not.

This is what they call 'Temporary visits'

After establishing that critical period of 12 months’ non-residency for tax purposes, you are permitted to return for a visit of not more than 183 days in the subsequent tax year (April 6th - April 5th).

However, if you did that the following year also, you could very well lose your non-resident tax status. This is because the IRD permits an average stay in UK of not more than 90 days per annum aggregated over a four year period.

It is therefore always better to remember the 90-day rule rather than the 183-day rule, just to be on the safe side. (What if you had to be unexpectedly hospitalised in UK, for instance?)

Since the onus is on you to prove how short a period you stayed, not on the IRD to prove how long you stayed, the question often arises of how you can prove when you return and leave again, since UK Immigration Officers don’t normally stamp UK passports upon entry or exit.

A little common sense applies here. First, you could retain the last page of your airline tickets to show which flights you used (assuming you didn’t change your booking - in which case you have to ask your ticketing agent to put a sticker on the back page as well as the front one).

Secondly, you could politely ask the immigration officer upon arrival and/or departure to stamp your passport, explaining (if you think it necessary) that this is to prove your date of arrival/departure for tax purposes. The officer may look at you a little peculiarly, and be slightly exasperated at the extra bother you’re putting him to, but he will do it if asked to. (And by the way, the IRD does not count the day of your arrival nor day of departure in calculating your period of temporary residence in UK.)