A support structure for supporting a vessel, the support structure comprising: a base support block; an upper support block mounted on an upper surface of the base support block via an intermediate operating layer; a compressible contact layer for contacting the undersurface of the vessel, said compressible contact layer being supported on an upper surface of the upper support block; a height adjustment mechanism mountable in the intermediate operating layer; and a rigid spacer mountable in the intermediate operating layer; wherein the intermediate operating layer is adjustable between: a load bearing configuration in which the rigid spacer is mounted in the intermediate operating layer to transfer a load on the upper support block to the base support block, an intermediate configuration in which the height adjustment mechanism is mounted in the intermediate operating layer and operable to lift the upper support block to introduce a clearance gap that permits insertion and removal of the rigid spacer, the height adjustment mechanism being configured to bear the load on the upper support block in the intermediate configuration and to compress the contact layer to form the clearance gap without substantial change to the position of the vessel despite relative movement of the upper support block, and characterised in that the base support block and upper support block have substantially the same size and weight. a base support block; an upper support block mounted on an upper surface of the base support block via an intermediate operating layer; a compressible contact layer for contacting the undersurface of the vessel, said compressible contact layer being supported on an upper surface of the upper support block; a height adjustment mechanism mountable in the intermediate operating layer; and a rigid spacer mountable in the intermediate operating layer; wherein the intermediate operating layer is adjustable between: a load bearing configuration in which the rigid spacer is mounted in the intermediate operating layer to transfer a load on the upper support block to the base support block, an intermediate configuration in which the height adjustment mechanism is mounted in the intermediate operating layer and operable to lift the upper support block to introduce a clearance gap that permits insertion and removal of the rigid spacer, the height adjustment mechanism being configured to bear the load on the upper support block in the intermediate configuration and to compress the contact layer to form the clearance gap without substantial change to the position of the vessel despite relative movement of the upper support block, and a load bearing configuration in which the rigid spacer is mounted in the intermediate operating layer to transfer a load on the upper support block to the base support block, an intermediate configuration in which the height adjustment mechanism is mounted in the intermediate operating layer and operable to lift the upper support block to introduce a clearance gap that permits insertion and removal of the rigid spacer, the height adjustment mechanism being configured to bear the load on the upper support block in the intermediate configuration and to compress the contact layer to form the clearance gap without substantial change to the position of the vessel despite relative movement of the upper support block, and characterised in that the base support block and upper support block have substantially the same size and weight.