Work platform

A stable offshore platform for surface or undersea work is provided by connecting a semisubmersible trailer unit with a workboat. Preferably, two columns of the trailer are joined by a beam member or truss, and this assembly is linked to the workboat via a truss-like trailer tongue with a ball swivel joint. When in transit, the semisubmersible unit is deballasted to a shallow draft condition and towed by the workboat.

BACKGROUND OF THE INVENTION 
Many thousands of vessels form a fleet of marine transportation boats 
serving companies working in offshore waters throughout the world. The 
vast majority of these vessels support offshore oil and gas exploration 
and production activities. Most of these vessels are classed as tugboats, 
supply boats, or crew boats, and have displacement type hulls. A common 
problem with such hulls is that it is difficult for men to perform useful 
work from the decks thereof due to waves which wet the decks and which 
cause excessive roll and pitch motions of the vessels. Weather in the Gulf 
of Mexico is occasionally severe enough to hinder or delay operations, but 
weather in the North Sea is so severe as to have a significant impact on 
operations from such vessels. Transfer of men and equipment to offshore 
platforms, handling of anchor buoys, and deployment of men and equipment 
subsea are examples of operations wherein a safe and stable work platform 
is needed. 
A potential solution to this problem is to build and use larger vessels. 
This slightly improves the situation, but at a significantly increased 
cost, and generally it is not considered an economically feasible 
approach. 
Another potential solution is to build a fleet of small to medium sized, 
column-stabilized, semisubmersible vessels like the larger semisubmersible 
vessels used for drilling operations. These are stable work platforms 
having long natural periods of response in the order of 20 to 40 seconds. 
By contrast, a 200-foot supply boat has natural periods of from 6 to 9 
seconds, which is quite close to the period range of highest wave energy. 
While such semisubmersibles would significantly reduce wave-induced 
motions and deck wetness, it would be an excessive cost of retiring the 
conventional fleet and building a new and more expensive fleet. 
SUMMARY OF THE INVENTION 
The present invention generally pertains to a work platform and methods for 
the use thereof, which platform includes at least two connected 
semisubmersible flotation means, means for pivoting the flotation means, 
and connecting means extending between the flotation means and the 
pivoting means. In a preferred embodiment, the flotation means is two 
connected pontoons, and the pivoting means is a ball swivel joint. 
More generally, the present invention pertains to a stable offshore 
platform and methods for the use thereof for surface or subsea work, which 
platform is a semisubmersible unit attachable to a workboat or other 
support means. In a preferred embodiment, pontoons of the semisubmersible 
unit are connected to the workboat or other support means via a gooseneck 
truss, trailer tongue or the like, and a ball swivel joint or the 
equivalent. When not in use, or in transit, the semi-submersible unit may 
be deballasted to ride high in the water and permit entrance to shallow 
harbors, or it may be hoisted over the workboat or other vessel or support 
means. 
A preferred method for using the work platform involves positioning the 
platform over at least one object in the water, and securing and raising 
the object to the platform. The object may be, for example, a buoy which 
is to be repaired or refurbished, or a spring buoy used to position an 
articulated drilling platform, or the object may be an anchor of a pipelay 
vessel which is to be moved to a different position. Another preferred 
method for using the work platform requires using the platform to support 
underwater operations. The underwater operations may be, for example, 
diving operations, submarine deployment and retrieval, subsea well 
service, and core drilling and bottom sampling. Yet another preferred 
method pertains to utilizing the work platform as a stable base from which 
associated operations requiring stability are directed. Stability is 
required, for example, wherein fire fighting equipment is deployed on the 
work platform or wherein helicopters are landed on the work platform. And 
another preferred method involves utilizing the work platform as a stable 
pulling means. For example, seismic cables or oil skimmers may be pulled 
with the platform.

PREFERRED EMBODIMENT OF THE INVENTION 
The present invention provides a new solution to problems of offshore 
handling and transfer operations in rough weather (see related U.S. patent 
application Ser. No. 197,399 filed Oct. 16, 1980). Generally, the 
invention is a semisubmersible, gooseneck style trailer work platform 
which is towable by a conventional vessel such as a supply boat, or by 
other means. The trailer/work platform is pivotable from a raised or 
hoisted transit mode to a lowered or partially submerged working mode. 
With the floating semisubmersible trailer/work platform, workmen can 
safely climb aboard from a supply boat, rig, or other vessel and do useful 
work on a stable, dry deck. Further, using the present invention, men and 
equipment can be easily and safely transferred subsea or upward to an 
offshore platform or larger floating vessel. 
As shown in FIG. 1, the semisubmersible trailer/work platform 10 includes 
pontoons 11 and 12 which may be in a lowered position for work use or in a 
partially raised position (as shown in phantom) for transit. 
Alternatively, the pontoons may be hoisted over the vessel or other 
support means. Pontoons 11 and 12 are at the ends of two vertical columns 
13 and 14 which in turn are connected by slanted cross beams 15 and 16 
which abut pivoting framework 17. Vertical beam 18 extends downwardly from 
the opposite end of framework 17 to a ball swivel joint 19, or the like, 
which in turn can be mounted on a track or slot 20 so that it can be moved 
forward or aft, or even released aft in an emergency. If the trailer/work 
platform is released, buoyancy 9 in beam 18 provides sufficient flotation 
to prevent the trailer/work platform from overturning. The buoyancy 9 may 
be a water tight compartment, tankage, foam, etc. 
For best motion response with platform 10 in a working mode, ball swivel 
joint 19 is located at the center of pitch of the supply vessel or other 
support means 21 to which it is attached. For storm survival, ball swivel 
joint 19 can be moved aft to minimize the risk of the underside of the 
pivoting means or framework 17 contacting the stern of vessel 21 while the 
vessel is pitching in severe waves. Alternatively, or in addition, the 
trailer/work platform 10 can be deballasted to a light condition as shown 
in phantom in FIG. 1 for storm survival. Low tension tieback ropes, chains 
or cables 22 and 23 are used to ensure that the trailer/work platform does 
not jackknife into contact with the vessel or other support means. 
The trailer/work platform 10 can take many forms, depending on the specific 
application. Thus, it can have an aft deck 24 to support workmen or it can 
be used for offloading to platforms. Crane 25 may be used for offloading 
to platforms, etc. not having cranes, but it generally is preferred to use 
other cranes when available. The trailer/work platform can have various 
other lifting means such as gantry hoist 28 for lifting loads from supply 
vessel deck 26 to work platform 24 or to move equipment and/or personnel 
subsea. Also, the trailer/work platform can support submarine operations 
29 by having special launch/recovery equipment. Although submarines can be 
launched from the platform in various ways, it is preferred to launch from 
a duct 32 between pontoons 11 and 12. This facilitates getting the 
submarine through the wave/air interface, hence the cylindrical duct 32 to 
guide the submarine in and out of the water. Alternatively, an elevator 
may be used to lift and lower the submarine. Although it is prudent to 
keep the trailer/work platform simple, thrusters 30 can be placed in 
pontoons 11 and 12 and extended downward for operation to aid in 
offloading operations, then retracted when not in use. 
Just like other semisubmersibles, the trailer/work platform 10 can be towed 
out in a shallow draft, low ballast condition and then be ballasted down 
at the work site to perform required work. By towing the trailer/work 
platform in a lightened condition, the reduction in tow vessel speed due 
to the trailer is normally small, and shallow harbors can be used. In the 
lightened mode, the trailer/work platform 10 has the performance of a 
displacement type catamaran. Once the trailer/work platform is ballasted 
down with equipment weight and/or by water 31, the favorable long period 
semisubmersible characteristics above mentioned appear. 
A floodable chamber 33 may be attached to pontoons 11 and 12 be beams 34 
and 35 and hinges 36 in order to provide added stability to the 
semisubmersible. The amount of ballast in chamber 33 is adjustable by 
changing water level 37. This chamber provides more righting moment in 
roll of the semisubmersible. Alternatively, a dead weight 40 may be 
suspended by cables 41 and 42 from pontoons 11 and 12 as shown in FIG. 4. 
PRINCI CHARACTERISTICS OF THE TRAILER/WORK PLATFORM 
Characteristics of the trailer/work platform can generally be divided into 
two categories: 
Direct Load Lifting-in which the trailer/work platform serves as a "sea 
crane" to handle lifts or lowerings of structures between its support 
pontoons. Hence, such lifts are called direct lifts (at full capacity), as 
contrasted with lifts by ordinary cranes which must be accomplished "over 
the side", with a derated crane lift capacity. 
Stable Work Platform-in which the trailer/work platform principally serves 
as a stable floating structure upon which, and from which, useful work can 
be safely and effectively performed without waveinduced problems. Thus, in 
use a stable work platform is provided to do above water work as well as 
support subsea work. 
It should be recognized that any offshore operation involves the use of 
both of the above two principal characteristics of the trailer/work 
platform, i.e. a stable work platform and a sea crane, so it is often 
difficult to determine which characteristic is predominant. However, the 
following examples generally require more of the stable work platform 
characteristic: 
(1) Diving and Submarine support-The trailer/work platform can be fully 
outfitted with equipment known to the art to provide support for diving 
(e.g., diving bell) and submarine operations (FIG. 5). The conventional 
way of supporting a submarine is to put a large A-frame with a davit on 
the stern of a supply boat. Sea states of 4-6 feet are the limit of supply 
boats, while the trailer/work platform can handle 12-16 feet seas, or 
higher. 
(2) Subsea Well Service or Core Drilling-The trailer/work platform is 
ideally suited to core drilling, i.e., getting a sample of bottom soil 
(FIG. 6). To perform this task, a small derrick is mounted on the platform 
and drilling operations performed therefrom. The derrick can also be used 
for maintaining or servicing subsea wells. 
(3) Platform Tender-The trailer/work platform can be loaded with providions 
to be brought offshore to a well platform location by a supply boat (FIG. 
7). A winch or other means on the well platform to the well platform. Then 
the supply boat is free to perform other functions and the trailer/work 
platform can be unloaded as needed without interruption of drilling 
operations on the well platform. 
(4) Loading-This use also pertains to delivering equipment, etc., to an 
offshore platform or vessel (FIG. 8). In rough seas it is desirable to 
remove equipment by crane from the stable trailer/work platform rather 
than from a wave-tossed supply vessel. The trailer/work platform can be 
backed up to the platform under an overhead crane. For this purpose, 
propulsion may be provided in the legs of the trailer/work platform. 
(5) Heliport-A trailer/work platform may be employed as a heliport in areas 
which otherwise don't have a landing pad for helicopters (FIG. 9). 
(6) Firefighting Service-Firefighting equipment, e.g., pumps, nozzles, 
etc., can be monted on the trailer/work platform, which then can be backed 
up to a vessel on fire or an oil well fire, etc., and provide a high 
platform so that water may be directed accurately down onto the fire, as 
opposed to the conventional method of arcing the water upward and then 
downward onto the fire (FIG. 10). The pumps can also be used as propulsion 
means for the trailer/work platform. 
(7) Seismic Work-The trailer/work platform also is adaptable to towing 
seismic source guns and streamer cables (FIG. 11). With typical seismic 
operations underway, rough weather can cause problems; hence the motion at 
the stern of the vessel where the seismic cable is towed causes changes in 
tension in the cable, thus interfering with seismic recovery. By 
comparison, the trailer/work platform is very stable in rough weather, 
which makes it excellent for handling seismic equipment. 
(8) Salvage Support-The trailer/work platform is particularly suitable to 
removing people and cargo from a stricken vessel in a safe manner (FIG. 
12). Thus, the platform can be backed up to the stricken vessel, thereby 
keeping a safe distance between the stricken vessel and the rescue/tow 
vessel. Dangerous cargo, e.g. oil, can be lightered from the stricken 
vessel by hoses passing across the platform to a receiving vessel. The 
trailer/work platform also eliminates the necessity for small boats 
working adjacent the stricken vessel to handle divers, workmen, etc. 
(9) Spilled Oil Recovery-The trailer/work platform, due to its stability in 
rough weather, is especially useful for recovering spilled oil (FIG. 13). 
Accordingly, the platform can be used independently, with its own 
propulsion system to tow oil skimmers, etc., once the platform has been 
towed to the oil spill scene by a workboat, etc. A barge may be used in 
conjunction with the trailer/work platform to store recovered oil. 
In the examples which follow, involving typical offshore operations, the 
above two mentioned characteristics also are embodied; however, the sea 
crane characteristic is predominant: 
(1) Installation, Removal and Maintenance of Buoys-Buoys are used, for 
example, in harbors to mark channels or provide warning (FIG. 14) or as 
part of a loading terminal such as a single point mooring buoy. Such buoys 
are in a wave environment for long periods of time and frequent 
maintenance is required. Such maintenance and refurbishing is a difficult 
task because of the difficulty of effecting repairs in situ. Each buoy 
must be removed from its moorings, brought to shore, repaired, etc., then 
brought back to sea. However, the trailer/work platform can be positioned 
above the buoy as a floating crane to lift the buoy up and snug the buoy 
against the stable bottom side of the platform, whereby on-site 
maintenance work can be performed on the buoy without further moving it. 
(2) Placement and Removal of Structures on the Seafloor-Here the principal 
operation is to both deliver and place a structure such as a subsea well 
template at a subsea location without use of a separate derrick vessel for 
placement (FIG. 15). Alternatively, structures (such as anchors, well 
templates, downed airplanes, BOP stacks, small boats, etc.) can be 
recovered from a subsea location by use of the direct lift capability. 
Cyclic wave-induced loads are minimized due to the inherent stability of 
the trailer/work platform. 
(3) Anchor Handling for Pipelay or Other Vessels-One leveling factor for 
laying pipelines in rough water is that of moving anchors head. Although 
the pipelay vessel can handle the waves, conventional anchor handling tugs 
often cannot. Use of the trailer/work platform to capture the buoy and 
move up the anchor makes this operation significantly easier (FIG. 16). 
(4) Pipelay Stinger Support-The trailer/work platform can be combined with 
a pipeline stinger to provide a stronger, easier to deploy, more 
repairable and more transportable stinger than is possible with 
conventional stingers (FIG. 17). 
EXAMPLES 
The present invention was tested at 1/50th scale in a wave tank, using a 
simulated 2000-ton prototype supply vessel and a simulated 350-ton 
trailer/work platform, the trailer/work platform was calm and stable in 12 
and 25-foot waves with wave lengths 20 times the wave height while the bow 
moored vessel was excessively pitching and rolling. Natural periods for 
prototype roll and heave for the trailer/work platform were in the 15 to 
35 second range. For this combination of trailer/work platform and vessel, 
useful work could be done in wave heights to about 20 feet. The 
vessel/trailer/work platform combination tracked well under tow in both 
light and heavy ballast conditions. Based on estimations, light ballast 
condition would only reduce the normal vessel transit speed by no more 
than one knot. 
PRIOR ART 
U.S. Pat. No. 3,323,478 discloses a method of joining two vessels of 
conventional shape using spreader bars and tie lines, with a truss work 
between the vessels supporting a drilling rig. The joined vessels are like 
a hinged catamaran and not like a semisubmersible vessel as used with the 
present invention and as shown in The Technology of Offshore Drilling, 
Completion and Production by ETA Offshore Seminars, Inc., 1976, page 14. 
In the present invention the trailer is at the stern of the towing vessel, 
as opposed to alongside the vessel. Also, the present invention is 
structurally joined to the towing vessel with a single ball swivel joint 
and not the complicated means shown in U.S. Pat. No. 3,323,478. 
The Technology of Offshore Drilling (above cited, page 14) shows a typical 
semisubmersible vessel commonly seen in rough weather areas like the North 
Sea as well as in more moderate weather climates. These vessels are free 
standing, either self-propelled and positioned with thrusters or 
non-self-propelled and moored in position to do work. If the 
semisubmersible is not self-propelled, it is towed to its work location by 
tow vessels using tow lines between the tow vessels and the 
semisubmersible vessel. Thus, the semisubmersible is handled like a 
conventional barge. By comparison with a free standing semisubmersible, 
the semisubmersible of the present invention has one corner supported by a 
structural ball swivel joint on the deck of a tow vessel. So, in effect, 
the semisubmersible of the present invention is like a trailer at the rear 
of the tow vessel and is markedly dissimilar to a free standing 
semisubmersible. 
Manned Submersibles by R. Frank Busby, 1976, page 611, Office of the 
Oceanographer of the U.S. Navy, shows how a spar buoy assembly can be 
connected alongside a floating vessel by way of a single-degree-of-freedom 
hinge (like a door hinge). This assembly then can be used to support 
operations of a subsea nature. By comparison with the present invention, 
(a) the spar buoy assembly is positioned alongside the vessel and not 
behind it; (b) the spar buoy assembly has no submerged pontoon below water 
to permit heavier lifts, (c) the hinge point of the spar buoy assembly is 
well above the vessel deck so that roll motion of the vessel is amplified 
into a sizable transverse motion and rotation of the spar buoy assembly, 
(d) the hinge of the spar buoy assembly is a simple one-degree-of-freedom 
hinge and not a ball swivel hinge having three degrees of freedom as in 
the present invention and (e) there is only one flotation means (a spar 
buoy) whereas a minimum of two flotation means (columns plus pontoons) is 
utilized by the present invention. Hence, there are few similarities 
between the present invention and the spar buoy assembly.