<p>A seal assembly for sealing an annular space between an inner and an outer pipe in a double-walled subsea pipeline which seal assembly comprises an annular member (1) and moveable blocking means (2, 3) and: (a) under normal operating conditions is in a non-sealing position which allows the passage of a gas through said seal assembly; and (b) is actuatable from a non-sealing position to a sealing position in response to the entry of liquid into said annular space.</p>
