received by the International Bureau on 27 October 2017 (27.10.2017)

1. An obstruction (80A, 80B, 80C, 80D) for positioning In a recess (90) between an adjacent pair of security bulwarks (1a, 1b, 1c, 30, 119a, 119b, 119c, 119d, 119e, 119f) mounted outside a perimeter railing (20) of a vessel or marine Installation, wherein the perimeter railing (20) comprises a handrail (201), the obstruction comprising:

a rear end (81) for positioning towards the perimeter railing;

a front end (83) for positioning away from the perimeter railing;

a top side (82);

an underside (84);

a pair of side faces (86a, 86b) each for abutting a corresponding side face (16, 116) of a respective one of the adjacent pair of security bulwarks (1a, 1b, 1c, 30, 119a, 119b, 119c, 119d, 119e, 119f);

a pair of opposing lateral cheeks (85a, 85b) separating the front end (83) from a respective one of the pair of side faces (86a, 86b);

a connection facility (87, 88) for connecting the obstruction to at least one of the perimeter railing and the adjacent pair of security bulwarks; and

a shoulder (810a, 810b) facing the rear end (81), located between one of the opposing lateral cheeks (85a, 85b) and the respective one of the side faces (86a, 86b), for abutting a feature (2, 3, 5h, 133) of a respective one of the adjacent pair of security bulwarks (1a, 1b, 1c, 30, 119a, 119b, 119c, 119d, 119e, 119f);

wherein the top side (82) slopes downwardly from the rear end towards the front end during use of the obstruction between an adjacent pair of security bulwarks;

the breadth of the obstruction being substantially less than the depth of the obstruction from the front end (83) to the rear end (81).

2. An obstruction according to claim 1, wherein the side face (86a, 86b) is substantially parallel to a sagittal plane (Χ-Χ') of the obstruction, 3. An obstruction according to claim 1, wherein the side face (86a, 86b) subtends an angle (a) substantially equal to 22.5 degrees to a sagittal plane (Χ-Χ') of the obstruction,

4. An obstruction according to any one of claims 1 to 3, wherein the connection facility (87, 88) is located on the side faces (86a, 86b).

5. An obstruction according to any one of the preceding claims, wherein the connection facility (87, 88) comprises a through-hole (88) for receiving a rod, located In a corner (812) between the rear end (81) and the top side (82).

6. An obstruction according to any one of the preceding claims, wherein the rear end (81) comprises a bay (89) to accommodate the handrail (201) of the perimeter railing (20).

7. An obstruction according to any one of the preceding claims, wherein the lateral cheeks (85a, 85b) are both substantially flat, 8. An obstruction according to any one of the preceding claims, wherein the separation between the pair of opposing lateral cheeks (85a, 85b) tapers from the rear end (81) towards the front end (83). 9. An obstruction according to any one of the preceding claims, comprising a rounded corner (823, 834) between the top side (82) and the front end (83) and between the front end (83) and the underside (84). 10. A perimeter protection system (800A, 800B, 800C, 800D) comprising:

a plurality of security bulwarks (1a, 1b, 1c, 30, 119a, 119b, 119c, 119d, 119e, 119f) arranged side-by-slde and adjacent to each other along a perimeter railing (20) of a vessel or marine Installation; and

an obstruction (80A, 80B, 80C, 80D) according to any one of the preceding claims positioned in a recess (90) between an adjacent pair of the plurality of security bulwarks (1a, 1b, 1c, 30, 119a, 119b, 119c, 119d, 119e, 119f). 11. A perimeter protection system (800A, 800C) according to claim 10, wherein the plurality of security bulwarks (1a, 1b, 1c, 30, 119a, 119b, 119c, 119d, 119e, 119f) comprises a security bulwark (1a, 1b, 119a, 119b, 119c) having a side face (16, 116), which security bulwark (1a, 1b, 119a, 119b, 119c) Is located along a substantially straight run of the perimeter railing (20), and wherein the obstruction (80A, 80C) Is according to claim 2 or any one of claims 4 to 9 as dependent on claim 2 and is positioned with its side face (86a, 86b) abutting the side face (16, 116) of the security bulwark (1a, 1b, 119a, 119b, 119c) located along the substantially straight run of the perimeter railing (20). 12. A perimeter protection system (800B, 800D) according to claim 10 or claim 11, wherein the plurality of security bulwarks (1a, 1b, 1c, 30, 119a, 119b, 119c, 119d, 119e, 119f) comprises a security bulwark (30, 119e) having a side face (16, 116), which security bulwark (30, 119e) Is located at a corner (21) of the perimeter railing (20), and wherein the obstruction (80B, 80D) is according to claim 3 or any one of claims 4 to 9 as dependent on claim 3 and Is positioned with Its side face (86a, 86b) abutting the side face (16, 116) of the security bulwark (30, 119e) located at the corner (21) of the perimeter railing (20). 13. A vessel or marine installation comprising a perimeter protection system (800A, 800B, 800C, 800D) according to any one of claims 10 to 12 arranged along the perimeter of the vessel or marine installation. 14. A vessel or marine Installation according to claim 13, further comprising cargo protected by the perimeter protection system (800A, 800B, 800C, 800D). 15. A method of protecting a perimeter of a vessel or marine installation, the method comprising:

positioning a plurality of security bulwarks (1a, 1b, 1c, 30, 119a, 119b, 119c, 119d, 119e, 119f) side-by-side and adjacent to each other along a perimeter railing (20) of the vessel or marine Installation;

providing an obstruction (80A, 80B, 80C, 80D) according to any one of claims 1 to 9; and

positioning the obstruction In a recess (90) between an adjacent pair of the plurality of security bulwarks (1a, 1b, 1c, 30, 119a, 119b, 119c, 119d, 119e, 119f). 16. A method according to claim 15, further comprising: providing a first one (1a, 1b, 119a, 119b, 119c) of the plurality of security bulwarks with a side face (16, 116);

locating the first one (1a, 1b, 119a, 119b, 119c) of the plurality of security bulwarks along a substantially straight run of the perimeter railing (20);

providing the obstruction (80A, 80C) according to claim 2 or any one of claims 4 to 9 as dependent on claim 2; and

positioning the obstruction (80A, 80C) with Its side face (86a, 86b) abutting the side face (16, 116) of the first one (1a, 1b, 119a, 119b, 119c) of the plurality of security bulwarks. 17, A method according to claim 15 or claim 16, further comprising:

providing a second one (30, 119e) of the plurality of security bulwarks with a side face (16, 116);

locating the second one (30, 119e) of the plurality of security bulwarks at a corner (21) of the perimeter railing (20);

providing the obstruction (80B, 80D) according to claim 3 or any one of claims 4 to 9 as dependent on claim 3; and

positioning the obstruction (80B, 80D) with Its side face (86a, 86b) abutting the side face (16, 116) of the second one (30, 119e) of the plurality of security bulwarks.