Do I pay tax where the rig is or where I live?
Usually both, in a set order. The country whose waters or continental shelf the installation sits on taxes the work done there, because the activity happens inside its taxing area. Your home country taxes your worldwide income if you remain resident there, then gives relief for the host tax on the same income. The order matters: the host return has to be right first, because it fixes the figure the home claim is computed on. What decides the host liability is rarely your job title. It is whether your rotation counts as offshore activity under that country's own rules, and how many of your days fall inside them.
Is it legal that my offshore agency deducts no tax?
It can be, and it can also mean nobody has taken a position on your behalf. Agencies withhold where they are registered as an employer and where local law obliges them to. If the agency sits in one country, the installation in a second and you live in a third, the agency may have no withholding duty at all, which does not remove your filing duty in the country where the work was done. Silence from a payroll department is not an exemption. Ask the agency in writing which country it treats as the place of work, and keep the reply. It is the starting point for both returns.
Does the seafarers rule cover working on a fixed platform?
Often not, and this is the most expensive assumption in offshore work. The treaty article written for ships and aircraft in international traffic exists because a vessel moves between countries. A fixed installation does not move, so many treaties take offshore work out of that article and put it into a special offshore-activities provision, or leave it inside the ordinary business-profits and employment rules. The result is that a supply vessel and a rig standing beside it can be taxed differently for the same rotation. Read the article that applies to the structure you stood on, not the one that applies to the ship that took you there.
Is my transit time taxed differently from days on the installation?
It can be, and that is why the roster is the evidence. Days on the installation are worked inside the host country's offshore area and are normally sourced there. Days spent travelling, standing by onshore or sitting in a home-country port may be sourced somewhere else entirely, depending on where you physically were and which rule the treaty applies. The practical consequence is that a rotation is not one block of income. It is a set of days, each belonging to a place. Keep the vessel's movement record and your own travel record, because reconstructing a year of rotations from memory turns a defensible split into an argued one.
What happens if I come forward with unfiled offshore years?
You establish which country each year belonged to before you file anything. Unfiled offshore years are usually not one problem but two: a host-country return that was never filed because nobody withheld, and a home-country return that omitted the foreign income and the relief that goes with it. Filing one without the other creates a mismatch that invites questions. The work is to fix the sourcing for each year on the records that exist, file the host returns, then amend or file at home claiming relief for what the host country actually took. Voluntary disclosure routes exist in most systems and their terms differ, so the sequence is decided before the first submission.
Which return should I file first when I work offshore?
The host country's, in nearly every case. Your home return claims relief for tax paid abroad, and that relief cannot be computed until the foreign liability is settled. Filing at home first means either estimating the foreign tax and amending later, or claiming nothing and overpaying. There is a second reason to work in that order. Preparing the host return forces the day-by-day sourcing to be settled, and the same schedule then supports the home claim, so both returns tell one story. Where a host filing date falls before the home one, the answer is to file on that schedule and correct it, not to leave it.
How do you avoid double taxation?
You claim relief once, in the right country, in the right order. Usually the source country taxes first, the residence country then gives a credit for that tax against its own charge on the same income, and a treaty caps the source-country rate. Getting the order wrong is what produces a double charge you then have to unwind. The mechanism differs by income type, which is why we map the whole position before filing either return. See how to avoid double taxation.
How many days can I spend in a country before I become tax resident?
It depends on the country, and a day count is only ever the start. Many use a threshold in a tax year, some also look at averages across several years, and some have no day test at all and decide on where your home and life are. Two countries can both conclude you are resident, which is what the treaty tie-breaker exists to settle. Counting days without checking the tie-breaker is how people end up filing as resident nowhere. See the residency tie-breaker.
