The response of deepwater floating structures to strong currents is of significant interest to operators, analysts, and engineering construction organizations. Loop currents and eddies affect the motions and fatigue lives of the platform's risers and mooring systems. The structure's attitude and motion response to current, affect production and impact safe operation during drilling, riser installation, and ROV work.
BMT Scientific Marine Services (BMT) has worked with J. Ray McDermott over the last two years on fast track projects to deploy motion, environmental, and deep current profiling systems on Spars that were well along in their construction cycle. These systems were requested by the Spar's eventual owner, for use as operational tools to support ongoing platform and riser response analysis.
This paper describes the design cycle, overall system concepts, and data collection system for these projects.
Floating structures in deepwater are subject to many environmental forces that impact their safe operation. Wind, waves, and currents, all affect the structure, mooring systems, risers, and subsea operations. The effect of ocean currents on a structure lead to offsets in the location of the facility with respect to the seafloor, cause oscillatory vortex induced motion (VIM) of the hull, and vortex induced vibration (VIV) of risers and moorings.
The monitoring of the currents around a floating structure has become easier with the advent of Acoustic Doppler Current Profilers (ADCP). These instruments allow profiling of the water column up to 1000 meters from a single instrument. Use of ADCPs as a permanent installation on a floating structure requires a design and engineering effort to optimize the performance of the instrument, and allow the routine servicing it requires. Development of the hull infrastructure to support an ADCP installation is best completed during structural design, before steel is cut.
The necessity to monitor the currents around a deepwater floating facility has recently been given a new priority with the publishing of Notice to Lessees (NTL) No. 2005-G02 by the Minerals Management Service. This NTL requires ocean current monitoring for all floating production facilities and mobile offshore drilling units in water depths greater than 1,312 feet (400 meters).
Developing and installing a current profiling system on a structure that is nearing the end of its primary hull construction, presents challenges to the design and construction team. The support carriages for the ADCPs required to monitor the water depth of today's floating facilities can weight up to 4,000 lbs. The structures that support these carriages have a significant impact on the hull structural design.
The engineering process to add current profiling systems to two spars already under construction was completed by the authors and is described in detail in later sections. The structural design and bracket support structure construction work required in the shipyard was completed in record time. The balance of the ADCP carriage support structure was installed on the spar dockside, prior to location towout and upending. The ADCP instruments were deployed by ROV and operated the first time in the field.
