Machine for forming trenches for pipelines or the like in the sea bed

In underseas trenching apparatus for forming trenches for pipelines and the like in the sea bed, comprising a releasably-engaging carriage having motive means for releasable engagement on the pipeline and movement along and in an arc around the longitudinal axis of the pipeline, means for driving said motive means along the pipeline by engagement of the motive means with the pipeline, a trenching unit on said apparatus, and projecting beyond the carriage, power means on said apparatus for operating said trenching unit and said means for driving said motive means, said trenching unit and power means comprising a major part of the weight of the apparatus, buoyancy tanks on the apparatus for supporting a major part of the weight of the apparatus, the improvement in which said trenching unit and said power means are integrally-connected to said buoyancy tanks for simultaneous movement therewith, means connecting said carriage to said trenching unit and said power means for causing the same to move along with the carriage on the pipeline and around said pipeline, said means connecting comprising means permitting relative vertical movement between said carriage and said major part of the weight of said apparatus so that a major part of the weight is not imposed on said pipeline, means connecting said bouyancy tanks to each other for altering their bouyancy in relation to the longitudinal axis of the pipeline and said carriage for causing the trenching unit to be adjusted in an arc about the pipeline upon which the carriage is supported and relieving the major part of the weight from off the pipeline.