Four years of continuous exposure to wear and corrosive atmosphere have not affected the titanium nitride coating on the riser tension compensator pistons used on the Saga Snorre TLP. The 60 hydraulic piston rods were originally manu factured from 174PH stainless steel with a hard chromium coating, but they had proved extremely vulnerable to wear and corrosion. The result was a need for frequent mainte nance or replacement with the additional expense incurred by the consequential well shutdown. Inconel piston rods were tried but the soft surface on metal gave inferior results when compared with the titanium rods with titanium nitride.
Four years of continuous exposure to wear and corrosive atmosphere have failed to affect the titanium nitride coating on the riser tension compensator pistons used on the Saga Snorre TLP.
Although titanium is unaffected by corrosion, it lacks the hardness necessary for such a demanding application. A process developed by Permascand was therefore chosen as the most appropriate method for creating an outer layer of titanium nitride on the pistons. The process converts the outer molecular layer into titanium nitride to give the pistons a hard yellow coating that has proved completely resistant to wear and corrosion.
The success of the first titanium nitride rods also resulted in 20 being delivered for the Heidrun platform last year and Permascand has recently received an order for additional pistons. Because the company's titanium nitride coated components have proved so successful, it anticipates further significant orders from North Sea operators.
Each piston used on the Saga Snorre platform is manufactured from alloyed titanium to measure up to 3.5 meters long with a diameter of 100 mm. In addition to being half the weight of steel, the titanium is completely unaffected by corrosion although, in its original form, it lacks the hardness of other metals.
The unique qualities of titanium were used in the manufacture of riser tether clamp straps that needed to be both strong and yet flexible to allow for the bending of the riser with a design life of 20 years.
The Chemical Surface Reaction Nitriding process developed by Permascand overcomes this. The process can take over 36 hours during which the work piece is subjected to elevated temperatures in a precisely controlled furnace where it is exposed to an atmosphere of nitrogen. This causes the surface to convert into a thin layer of titanium nitride that endows it with a hardness above 65 Rockwell C (>900 HV).
The nature of Permascand's patented nitriding process makes it possible for very large and very intricate components to be treated. Unlike other methods, such as chemical vapor deposition and physical vapor deposition, the Permascand process cannot be regarded as a coating on a substrate. The nitrogen gas converts the outer molecular layer of the work piece into titanium nitride to become an integral part of its structure. Because the process is activated by a gas, it is possible to achieve an even layer throughout the most intricate components. This includes the deepest recesses of a threaded surface or inside fine spray nozzles. This is also accomplished without altering the dimensions and tolerances to which the component was originally machined.
The hardness of the nitride coating is comparable to that of diamond, but because the layer is thin and ductile, it is not vulnerable to the flaking that would occur to a thicker coating if the titanium substrate were deformed.
The Snorre pistons were subjected to an accelerated testing program that has enabled Perma scand to provide them with a 10-year guarantee against wear. This is in addition to the results of DNV tests which have qua* fied the material for a life span of 30 years against corrosion.
Lower maintenance for Snorre
Commenting on his company's change to titanium nitride pistons, Jan Einar Malmin, Manager of the Saga Snorre platform said "our maintenance costs have dropped substantially." This is in marked contrast to when stainless steel pistons were being used and required changing every 12-18 months. The higher cost of the titanium pistons is completely offset by their dramatically longer life span and the elimination of the regular periods of lost production that occurred during their replacement.
Other recent uses of nitrided titanium may have been less spectacular, but are no less significant in the benefits they have afforded. Alcatel recently gave Permascand one of the most demanding applications for its products when it sought water jet nozzles for three of its sub-sea cable plows. The nitride surface proved more than capable of withstanding the high water pressures, particle abrasion, and corrosion that had previously destroyed nozzles made from other metals and ceramics. Major improvements to valve actuators have also been possible when they have been made from nitrided titanium and these now form a regular part of Permascand's output.
Permascand's abilities with the nitriding process are based on the company's long experience in working with titanium. Part of the Akzo Nobel group, the company entered the titanium business in the 1970s with the development of a dimension stable anode for use in the manufacture of chlorine and chloride. It built on this experience to master the techniques for cutting, welding, and forming titanium. Over the years, Permascand has supplied more than 4,000 tons of titanium equipment and components.
Titanium is unaffected by seawater and its high strength-to-weight ratio have enabled Permascand to supply titanium products for numerous applications offshore. These include its widespread use in piping for platform fire fighting extinguishing systems. Because particles of corroded metal can block spray nozzles when stainless steel piping is used, Permascand maintains a wide range of titanium piping for use in such applications.
These stock levels also proved beneficial when the company was recently asked to supply piping for the gas turbine cooling water outlet on the Heidrun TLP. The piping had proved troublesome in the past and the need was for a system that would last for the life of the platform. The piping system contained elbows in dimensions up to 30 in. and these are not normally stocked anywhere in the world. By suggesting a design to Statoil that used segmented elbows, it proved possible to manufacture and supply the piping in less than ten weeks using material held in stock.
In February, Permascand completed another contract that exploited the qualities of titanium. This involved the manufacture of riser tether clamp straps for the Terra Nova FPSO. Situated in 95 meters of water, the tethers needed clamps that are both strong and yet flexible enough to allow for the bending of the riser. They needed a design life of 20 years and the ability to withstand tensioning to 750 kN.
"It took many hours of computation to predict the dynamic forces to which the straps would be subjected but a design was eventually achieved that met the specifications" said Peder Engstr
Each of the nine clamps required nine titanium straps to be made from bent titanium plate. Plate thickness varied 6-7 mm, according to where the strap was to be used. The plates were then plasma arc welded to flanges machined from solid blocks of Grade 5 titanium. It was the first time that plasma-arc welding had been used on Grade 5 titanium this thick, but the method was chosen because of the high quality of the welds it produced and the limited amount of heat it generated within the work piece. Each of the 156 welds was, of necessity, perfect and an epoxy coating was then applied to the outside of the finished straps, while their inner surfaces were coated with Teflon. Bolts to hold the straps in place were manufactured from a titanium alloy that enables them to be pre-tensioned to >500 kN.
Despite its initial high price as a raw material, the long term cost advantages and the success of projects such as these are expected to increase the use of titanium and titanium nitride in the offshore industry. Kenneth Dahlqvist, Business Manager for Permascand predicts that the industry will account for a significant shift in the world's use of the metal. "In the next five years, we expect the offshore industry to use 50% more than it does today. This is because the offshore industry is going deeper into the ocean and therefore needs titanium's advantages - light weight and no corrosion," he said.
For more information contact Kenneth Dahlqvist, Permascand, Tel: +46 691 355 00, Fax: +46 691 330 40, Email: firstname.lastname@example.org