This was a British series of naval minelaying operations to create and maintain minefields in the waters off the UK, between Iceland and the Færoe islands group, and in the Denmark Strait to the west of Iceland (1940/44).
'SN' was undertaken over a lengthy period in four main groups. The SN1 to SN11 series of operations to create and maintain the shallow anti-ship and deep anti-submarine aspects of the Northern Mine Barrage between Iceland and the Færoe islands group and in the Denmark Strait to the west of Iceland between 1940 and 1944.
The SN12 to SN18 series of operations was undertaken to create and then to maintain the East Coast Mine Barrier in 1942.
The SN21 to SN27 series was a continuation of the development of the Northern Mine Barrage now including the waters off the Outer Hebrides islands group in 1942.
The SN31 and SN32 series was used to create a minefield in the St George’s Channel part of the South-Western Approaches to the UK in 1940/41. Typical of the series was SN13 of 10 August, in which the destroyers Cossack, Duncan, Maori and Sikh departed Scapa Flow to join the OA.196 convoy, which was escorted by the sloop Rochester and corvette Primrose. The destroyers remained with the convoy as far as Cape Wrath, then proceeded to Loch Alsh for SN13, and returned to Scapa Flow on 13 August. The ships for SN13 proper, under the command of Rear Admiral W. F. Wake-Walker, departed Loch Alsh as the minelayers Southern Prince, Port Quebec, Port Napier and Menestheus (1st Minelaying Squadron). The survey ship Scott laid the required navigational buoys. The following SN14 took place on 28/29 August and involved the same minelayers of the 1st Minelaying Squadron as SN13, but in this instance escorted by the destroyers Ashanti, Bedouin and Tartar screened by the light anti-aircraft cruiser Cairo and supplemented later by the destroyer Active.
The SN41 series was used to create a minefield in the North Channel area of the North-Western Approaches to the UK in July and August 1940.
The SN50 to SN222 series was used to maintain and repair the Northern Mine Barrage between 1942 and 1944.