A guide line attachment for releasably attaching a guide line to a guide post top or other portion of a sub-sea installation has three relatively movable spring loaded concentric sleeves, the relative movement of which can be used to releasably lock the attachment to the guide post top or other portion. Relative movement of the sleeves also activates a latch to releasably lock the sleeves to each other. The attachment may fit inside or outside a guide post top and may be pre-set so that it will or will not automatically relatch on release. The guide post tops have internal or external grooves for engaging the attachment locking means, and specially designed tools may be used to effect remote release sub-sea. The guide line attachment may be used for all stages of the drilling and production of a sub-sea oil well and avoids the present need to cut and replace guide lines at different stages in the life of a sub-sea well.
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