A clamp comprises a first arcuate portion, a second arcuate portion connected to the first arcuate portion via a hinge, and a ratchet mechanism for locking the first and second arcuate portions in a closed position. The first and second arcuate portions each comprise a circumferential groove which tapers in a radial direction from an opening of the groove to a base of the groove. The groove of one of the first and second arcuate portions has an angular extent which exceeds 180 degrees and has an opening with a width which is narrower at opposing ends of the groove than at a position partway between the ends.