order of importance) the following: (1 ) total vertical penetration or settlement 

 into the seafloor; (2) differential vertical motions (differential settlement) or 

 rotation; (3) lateral motion (skidding); (4) soil behavior in the vicinity of the 

 installation (such as excess pore pressure and location of soil strain resulting 

 in installation movement); and (5) dislocation of soil mass (such as scour, fill, 

 or mass movement — slope instability) in the vicinity of the installation. 

 Applicable monitoring techniques are in use on land for all of these. 



These techniques can, and have been, modified for use on the seafloor 

 for submerged installations. For observing immediate, large-scale movements 

 of an installation shortly after deployment, simply visual (direct or by closed- 

 circuit television) observations by divers, submersible, or some remote 

 observation system [CURV (Cable-Controlled Underwater Research Vehicle), 

 for example] have been successfully employed. Similar visual methods can 

 be employed for monitoring smaller movements (or other behavior phenomena) 

 over longer periods of time if some form of referencing foundation position is 

 added. 



Another technique for monitoring smaller movements involves the 

 usage of mechanical and fluid measuring systems such as shown in Figures 1 

 and 2. The mechanical system references movement to a vertically stable 

 reference rod (isolated from surface movements) while the fluid system relates 

 movement to a constant-elevation fluid interface. NCEL (Naval Civil Engineer- 

 ing Laboratory) divers have monitored the performance of several model 

 foundations which employ mechanical and fluid referencing techniques 

 (Figures 1 and 2) in up to 130 feet of water. The fluid system concept has 

 also been utilized to measure differential vertical movement of a structure. 

 These measurements were made by attaching the reference stand to one end 

 of a structure and locating the sighting tube (Figure 2) at the opposite end. 

 The accuracy of measurements for the mechanical and fluid referencing sys- 

 tems is typically in the order of 0.125 inch. 



The periodic monitoring of installations in deeper water could be 

 accomplished by employing the same measuring systems and a small submer- 

 sible; however, it is typically more economical to use some sort of automated 

 data collection system. The LOBSTER (Long-Term Ocean Bottom Settlement 

 Test for Engineering Research) employs such a method. This device (Figures 3 

 and 4) uses the same mechanical reference system as shown in Figure 1 ; how- 

 ever, data are automatically taken (rate is variable from once every 7 seconds 

 to once per hour) from three sensors which measure total settlement (accuracy 

 about 0.02 inch) and footing tilt (differential settlement) in two perpendicular 

 planes (accuracy about 0.5 degree). The LOBSTER is deployable in water 

 depths to 6,000 feet for durations of up to 1 year. All data are stored inter- 

 nally on digital tapes which are recovered at the end of the deployment. 



