A method of transporting a rigid tie-in connection 10 across a body of water 22 for installation underwater. The method comprises: supporting the tie-in connection 10 in the water by buoyancy acting on a frame 12 that supports the tie-in connection 10 until installation; lowering the buoyancy-supported frame 12 and the tie-in connection 10 into a sub-surface transit configuration; and in the transit configuration, towing the buoyancy-supported frame 12 and the tie-in connection 10 behind a towing vessel 38 toward an installation site. A system for implementing the method comprises: a towable frame 12 arranged to support the tie-in connection 10 until installation; and external buoyancy 30 attached to the frame 12 by an extensible suspension link 14 arranged to suspend the frame 12 from the buoyancy 30 in water in use, the link being extensible to lower the frame 12 from a raised surface-tow configuration into a lowered sub-surface transit configuration.
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