Rapidly Installed Breakwater System
Force projection requires moving large quantities of personnel, equipment, and supplies, predominantly via sea lift. This process is termed Logistics Over The Shore (LOTS). Desert Storm/Desert Shield and Provide Comfort operations are recent examples (wartime and peacetime) of U.S. military requirements to deploy and sustain forces. Due to the nature of these operations (wartime scenarios will likely include damaged fixed ports, and peacetime humanitarian relief operations are likely to be conducted in less developed areas of the world with substandard port facilities initially), nearly all foreseeable sustainment efforts will likely require LOTS operations to be successful.
LOTS operations typically involve Roll-on/Roll-off ships, auxiliary crane ships, containerships, and large tankers that anchor offshore in deeper water. When wave conditions permit, smaller watercraft (lighters) ferry the ships' cargo to various offloading points along the shore. The Department of Defense has a requirement that offloading operations continue through a sea state 3 condition (wave heights up to 5 ft). The critical problem to be addressed during LOTS operations is the demonstrated inability to effectively conduct offloading operations when wave climate conditions exceed what is commonly termed sea state 2 (wave heights up to 3 ft). The troublesome condition described above is commonly known as the 'sea state 3 problem,' and exists when significant wave height exceeds 3 ft with wave periods about 6 sec. The military considers the sea state 3 problem to be critical, since these conditions exist a significant percent of the time worldwide. The sea state 3 problem is considered to be a potential "war stopper" for present force projection plans and technology.
The strategy under consideration involves deploying the RIB system prior to the development of a sea state 3 condition so that calmer water will exist in the immediate vicinity of the anchorages below the sea state 3 threshold. With this scenario, crane operators and stevedore crews could continue to function during open-ocean sea state 3 and even greater, since existing lighters can operate effectively in sea state 3 once safely loaded. An efficiently performing RIB system would cause the limiting sea state condition to be determined only by capabilities of the various lighterage and equipment being used during the LOTS operation.
The RIB system consists of a V-shaped structure in plan view, with rigid vertical curtains extending from the surface of the water toward the bottom for a distance sufficient to preclude excessive wave energy from penetrating beneath the structure. When deployed, the tip of the V is oriented into approaching waves, and works by spreading and reflecting incoming waves. Incident waves are deflected at the apex of the "V", providing a sheltered area inside and for some distance in the lee of the structure. Mooring loads are minimized because the structure is designed to deflect incoming wave energy rather than absorbing or reflecting it. Incident waves are 'trained' away from the interior of the V, providing a sheltered area inside the V and in the lee of the structure. Ships and lighterage are moored in the lee of the V for offloading.
Depending on the specific location where the RIB system is to be employed, the length of the legs of the V will vary, but may range between 700 and 1,000 ft. The two legs are joined at the front of the RIB system via a 'nose buoy' which acts as a docking station and allows the interior angle of the legs to vary between 0 and 60 deg. This design allows the legs to be linked together and stream directly behind when the RIB system must be towed to alternate locations. Mooring loads are minimized since the structure is designed to deflect waves rather than absorb them. Since the structure is oriented at an oblique angle relative to approaching waves and is several wavelengths long, the oscillatory nature of the wave forces also results in a smaller net force on the mooring lines. The positive forces associated with the crest are simultaneously reduced by the forces associated with the trough which acts in the opposite direction.
Engineers and scientists at the U.S. Army Research and Development Center, Coastal and Hydraulics Laboratory (CHL) are developing a rapidly installed breakwater (RIB) system specifically designed to address problems associated with the efforts of U.S. armed forces to offload ships during Logistics Over The Shore (LOTS) operations. During such operations, problems arise when seas become sufficiently energetic to limit capabilities of ship-based crane operators and stevedore crews. The RIB system is designed to address this documented deficiency by creating a 'pool' of calmer water where these operations can occur so that crews will be able to continue to function.
For many years, CHL has been involved with the design and deployment of floating breakwaters, primarily for application within bays or estuaries which are semi-protected from large waves. Such structures typically are intended to attenuate waves with heights not exceeding 4 ft and periods not exceeding 4 sec. Extrapolation to an open ocean environment is at least an order of magnitude greater in difficulty. In an oceanic environment, waves with heights up to 10 ft are common during storm conditions, with associated periods up to 10 sec. Previous tests have shown that to be effective, floating breakwaters must have widths on the order of 1/4 of the wavelength being attenuated, and hence, be very massive to be effective. Such structures are simply not feasible for most temporary floating breakwater applications, since it would be necessary to transport large volumes of a massive structure to the site being sheltered. This problem was the driving force behind recent floating breakwater (RIB system) developments at CHL.
During energetic seas, the primary problem occurs in key offshore areas (anchorages) where containerships and roll-on/roll-off (RO/RO) vessels discharge cargo and unit equipment onto much smaller vessels (collectively termed lighterage). To date, research efforts have concentrated on military applications for the RIB system; however, it is anticipated that additional studies will be forthcoming focusing on non-military applications. Potential civil applications include rescue and recovery operations, exposed marine construction operations (e.g., bridge repair, rubble-mound breakwater construction), temporary small vessel/watercraft shelter from energetic seas, and exposed dredging operations.
The U.S. Army Engineer Research and Development Center (ERDC) performed the Rapidly Installed Breakwater System (RIBS) XM2000 Experiment in June-July 2000 at the Naval Air Station (NAS), Pensacola, Florida. An extensive instrumentation was installed on the RIBS in order to measure wind, waves, RIBS motion, and mooring line loads. During the experiment, a first-level data analysis and data quality checking for collected data was executed in order to allow a reliable second-level analysis and additional post-processing.
FY 2001 Accomplishments included the design of RIB XM 2001 (two segments aligned in a single leg) to include: fabrication of two interchangeable RIB segments and connectors; design, procurement, and testing of mooring system for RIB XM 2001; demonstration of RIB employment alternative(s). Completed final design of RIB Nose Section and initial design of RIB employment/deployment system. Completed final design of the Advanced Technology Demonstration (ATD) RIB to be used in FY 2002.
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