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Tool & Rubber
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Bio-Shield
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 Prevents Marine Biofouling Permanently
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Shell Development Co., in cooperation with Mark Tool Co., began a program to produce the optimum anti-foulant for offshore structures. Two proven materials -- copper-nickle and SPLASHTRON® -- were selected to work together as a unit. The result was BIO-SHIELD®. Mark Tool Co. manufactures Bio-Shield by license under Shell''s U.S. patent number 4,415,293.
BIO-SHIELD® keeps marine growth from attaching to conductors and platform members in the first place, allowing designers to take full advantage of the cleaner profiles, and eliminating the necessity of any expensive removal operations.
The copper-nickle compound was chosen because of its long history as an effective anti-foulant, its ease of fabrication, and its comparatively low cost. SPLASHTRON was selected as the insulating layer for several reasons:
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It bonds strongly to metal, both steel and copper-nickle
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It has proven to have a very long service life in the harsh "splash zone" environment.
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It is a good electrical insulator
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It provides a bonus in corrosion protection for the metal surface to which it is attached.
Marine life, such as algae, barnacles, and mussels, has been the scourge of shipbuilders for centuries. Marine growth also has become a serious problem for offshore operators and platform designers today. While there has always been advantages to keep marine growth off of ship hulls to maintain speed, save fuel, and keep ships working instead of in drydock, it is vitally important to prevent biofouling on offshore platforms for different reasons. In many areas of the world, marine life - such as barnacles, mussels and algae - don't build up much more than an inch or two. However, in many areas such as offshore California and the North Sea (anywhere waters are relatively cold and thus hold more nutrients), biofouling can build up to as much as 36" thick. At these levels it becomes dangerous since weight increases the strength requirements of the platform.
Depending upon the type of growth, biofoulants can weigh up to 150 pounds per cubic foot. It becomes dangerous due to the added area for wave loading and drastically reduces the fatigue life of a platform. Also, the danger of platform overturn is increased. When biofouling becomes excessive, about 6" thick or more, platforms must be constructed much stronger than normally would be necessary, or the growth must be removed regularly.
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Traditional Approaches To Solving Marine Growth Problems
By far the most economical approach is to prevent biofouling. Shipbuilders found that using copper sheathing not only prevented marine borers in their
wooden hulls, but also kept barnacles and other marine life from attaching. Copper-hull sheathing was used on the winners of the America''s Cup races
for many years, including the renowned Cutty Sark. |
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Traditionally, in the petroleum industry, platform designers have taken two approaches to combat the severe problem. First, some have used much thicker metal and driven pilings deeper into the ocean floor. They design as if the buildup on platform members and conductors would present a solid wall to wave action. This approach is very expensive, and on deeper platforms can cause a weight problem that makes transportation and installation difficult. A second approach is to plan for periodic removal of the marine growth, and to design and budget accordingly. Waterblasting biofoulants from 48 conductors off California costs about $75,ooo per occurrence. And, the growth comes back twice as fast as it originally formed, so subsequent removals must be more frequent.
Conductors coated with BIO-SHIELD® are protected from marine biofouling and corrosion. |
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Typical Applications of BIO-SHIELD
BIO-SHIELD heathing will prevent biofouling on conductors, platform legs and diagonals, J-tubes, and risers... any piping or structure in the area of high buildup. In a typical application, Mark Tool Co. applies a 1/2" thick layer of SPLASHTRON, bonded completely to the pipe; and a 1/16" thick plate of 90-10 copper-nickle is bonded to the outside of the "Splashtron" The alloy is completely insulated from any contact with the steel, and will retain its anti-fouling ability for the projected life of the installation. The first large scale application was on Shell''s EUREKA platform, installed in more than 700 feet of water off California. Sixty 24" OD conductors were coated with BIO-SHIELD. Shell saved 250 tons of steel in the design because it did not have to allow for the effects of wave loading on conductors fouled with 24" or more of marine growth. In addition, hundreds of thousands of dollars will be saved in maintenance expense over the life of the platform (projected to be 40 years) because the biofouling will not have to be removed regularly by. |
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Laboratory Tests
Initially, a series of carefully controlled laboratory tests were conducted to show the bond strength of rubber to metal, the electrical insulating ability of SPLASHTRON, and long-term performance under temperature tests. Test results were:
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The bond test showed a shear bond strength between alloy and rubber of 389.63 psi, well above the estimated 15 psi exerted by normal driving forces.
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Electrical resistance of the SPLASHTRON material was measured at 125 milli on ohms (125 mega ohms).
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After 96 cycles to 80°C (176°F), and cooling to 30°C (86°F) in artificial seawater, a sample of the alloy/rubber, bonded to steel, showed no signs of separation, and no change in the SPLASHTRON or the copper-nickle.
Above: Shows the basic design of how BIOSHIELD is made and A bond test sample used to test the strength and durability of BIOSHIELD |
Field Tests
A 24" OD conductor was coated with SPLASHTRON and copper-nickle, and installed offshore. No special handling was necessary. The pipe was driven with a D-36 hammer capable of exerting 83,ooo ft/lbs force. The test pipe was subjected to 240 blows per foot at final depth. There was no slippage of the alloy/rubber sheath, and no sign of movement or bond tearing. A test piece of 6 5/8" OD pipe was installed on a platform off California in early 1978, just below the waterline where marine growth is heaviest. On the test piece, a band of copper-nickle was centered on a layer of SPLASHTRON. The test piece was inspected by divers after 237 days and no marine growth was found on the copper-nickle, while 3-4" built up on adjacent surfaces. The test pipe has been inspected several times since and there is still no buildup on the copper-nickle.
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