Installation of subsea risers

A method of installing a wave-configuration subsea riser 32 of unbonded flexible pipe comprises lowering the riser progressively into the sea from an installation vessel 30 while suspending an elongate clump weight 38 in a catenary shape that comprises first 70 and second 72 limbs extending upwardly from a conjoining bottom portion 74. An upper end of the first limb, at a distal end of the clump weight 38, is attached 46 to the riser and an upper end of the second limb 70, at a proximal end of the clump weight 38, is suspended from a winch 66 or crane of the vessel 30. While lowering the riser 32 from the vessel 30, the weight load applied to the riser 32 by the clump weight 38 is controlled by adjusting the relative lengths of the first 70 and second limbs 72 of the clump weight 38.