fig4 adaptor frame 66 is shown in the retracted position the extended position of the adaptor frame 66 is shown in phantom in fig4 c and fig4 d when in the retracted position the adaptor frame 66 is supported by deck support brackets 38 and not to any significant degree by transport trolley 32 it will be appreciated that the retracted position of adaptor frame 66 is that used during drilling operations when in the retracted position the reactive force of the drilling riser tensioner system 30 is transmitted to the deck structure 64 via deck support brackets 38 the supports of transport trolley 32 e g hilman rollers 54 and support arms 88 are not exposed to the dynamic loads of heave compensation imposed by tensioner system 30 fig5 is similar to fig4 but with adaptor frame 66 in the extended position as shown in fig5 the drt support insert 66 may be lifted relative to the transporter 32 by four hydraulic cylinders 60 two on each side of the insert the geometric shape of the support insert and the transporter may be such that overlap between the two parts provides guidance as the support insert rises limiting lateral loads on the hydraulic cylinders extending adapter frame 66 results in lifting the drilling riser assembly sufficiently to clear the wellhead on the seafloor to which is was connected this permits the drilling riser assembly to be moved horizontally within the well bay without disconnecting either the drilling bop 26 or the drilling riser tensioner system 30 moreover the drilling riser itself may remain in the sea in certain embodiments a dummy wellhead may be provided on the seafloor for landing and securing the lower end of.