Another week ... hard to take!, page-39

BOP flex joint tolerates 1 degree to the riser ....... ok lets work with that.

Apologies to Hot - more maths approaching

Tan (1 deg) = 0.01745

tan = opp/adj

adj in this case is the length of the riser. approx. 2100m

So
0.01745 = (allowable movement of Stena at surface)/2100

Allowable movement of Stena at Surface = 0.01745 * 2100 = 36m (either side of well dead centre)

Therefore to maintain < 1 degree angle on the flex joint Stena can wander up to 72m from one position to another inside a 36m radius around FS1 well bore.