Mark point (1) and determine course (1) onto Oil Platform. Go to next point on the shore and go to mind course (3) and distance to it from point (1). Mark point (2) and determine course (2) onto Oil Platform in it. Switch to calculator and resolve triangulation task with known value. Make the projection of Oil Platform point and see coordinate for it. More points would give more precision result...

Another way determine course onto Oil Platform. Then correlate three value for distance: the height of device, the length extended hand this handheld, the height of something on Oil Platform (people, helicopter, safety boats...) . And again math, math and math...