Material Technologies, Subsea Systems & Flow Assurance PDF

PETROLEUM OPEN LEARNING Material Technologies, Subsea Systems & Flow Assurance Part of the Subsea Technology Series Material Technologies, Subsea Systems & Flow Assurance MATERIAL TECHNOLOGIES, SUBSEA SYSTEMS & FLOW ASSURANCE - VERSION 1.2 Designed, Produced and Published by...

PETROLEUM OPEN LEARNING Material Technologies, Subsea Systems & Flow Assurance Part of the Subsea Technology Series Material Technologies, Subsea Systems & Flow Assurance MATERIAL TECHNOLOGIES, SUBSEA SYSTEMS & FLOW ASSURANCE - VERSION 1.2 Designed, Produced and Published by OPITO Ltd. OPITO Ltd - Petroleum Open Learning Minerva House Bruntland Road, Portlethen Aberdeen AB12 4QL Tel: +44 (0)1224 787800 © OPITO 2014 All rights reserved. No part of this publication may be reproduced, stored in a retrieval or information storage system, transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise without the prior permission in writing of the publishers. With thanks to and to Hugh Grant for his subsea expertise 3 Contents Material Technologies, Subsea Systems & Flow Assurance Section 1. Basics of Engineering Materials & Properties......................................... 8 Section 2. Introduction to Materials for Subsea Equipment................................... 11 Section 3. Applications of Materials........................................................................ 22 Section 4. Failure Mechanisms.............................................................................. 25 Section 5. Cathodic Protection & Hydrogen Sulphide............................................ 27 Section 6. Coatings & Surface Protection.............................................................. 29 Section 7. Basics of Metal Fatigue......................................................................... 30 Section 8. The Erosion Process............................................................................. 32 Section 9. The Main Subsea Assemblies............................................................... 35 Section 10. Manifold Design & Insulation............................................................... 38 Section 11. Manifold Facilities & Flow Management.............................................. 41 Section 12. Control System.................................................................................... 45 Section 13. Pipeline End Manifold.......................................................................... 50 Section 14. High Intergrity Pipeline Protection System.......................................... 52 Section 15. Pipeline End Termination..................................................................... 53 Section 16. Corrosion, Erosion & Radio Activity..................................................... 55 Section 17. Subsea Separation.............................................................................. 57 Section 18. Main Sensor Systems......................................................................... 61 Section 19. Well Intervention Vessels.................................................................... 64 Check yourself answers......................................................................................... 68 4  Visual Cues Training targets Test yourself Check yourself Summaries for for you to questions to see answers to you to recap on achieve by the how much you let you see if the major steps end of the Unit understand you have been in your progress thinking along the right lines Training Targets On completion of this module you will have a good understanding of: The materials used in general engineering and in particular those used in the oil industry. The grades of steel, plastic or composites used for making subsea equipment. The materials or processes used to prevent corrosion. The major components that are a necessary part of a field development for sustainable production and flow assurance. The installation, hook-up and operational control of the installed system. Flow Dynamics. Export line inspection systems and safety shut down of the export line. 5  Introduction: Material Technologies, Subsea Systems & Flow Assurance The earliest known source of iron based metals Very much later by the 1800’s high quality iron found on earth came from Meteor strikes and was hot hammered to compress its molecules were found in Egypt and India by ancient people together to form Wrought Iron. This had a much many thousands of years well before the actual higher strength and led to it being used for Iron Age period of about 2,500 years ago. structural engineering purposes. People in certain areas of the world were using When carbon was added to the molten iron heavy stones as tools and weapons. Although it became basic carbon steel that was strong they were unaware of what this iron was or it’s enough to be used for bigger structures, ability to be developed into a basic engineering pressure containing boilers and heavy machinery material. applications. It did hold their interest enough to try some The carbon needed for steel production very experiments with it for example heating it and soon outgrew the supply available from forest observing the particles got very much hotter and charcoal burners. The then demand for domestic stayed unchanged to be heated over again. gas made from coal solved the problem. The residue from coal gas making is called coke; Although a number of countries were involved this is very high carbon content and solved the the earliest success in melting the stones, that problem of the carbon supply. were iron ore, the Eastern Asiatic region was probably the first to develop a fire and air fuelled 100 years later additions of Chromium, combination that generated sufficient heat to Molybdenum and Vanadium elements were smelt the ore and form a blob of raw iron that proportionally added to make new materials that could be pounded into simple tools or weapons. gave engineering what was needed for almost From an engineering viewpoint this was a low any application. strength metal and very much inferior to the Other very important materials are all forms Iron Nickel of the earlier times but for mankind of rubber. In the 1736 Charles Marie De La the discovery must have been a quantum Condamine patented rubber so named for its leap forward and was the basis for intensive ability to erase marks from paper and other development over the centuries. materials. 6 Rubber trees grow in the warm humid parts of the The earliest designs were simply collecting centers world and when the sap or raw latex is processed that gathered the small lines together at a point and formed into a sheet form, it is used for sealing on the seabed near to the wellsite locations and storage tanks and pipes and the manufacture of connected them to a manifold. early hoses. A much larger export line then carried the produced fluids in greater volumes more efficiently to the Latex is also widely used as a binder in the platform or even to a shore terminal. manufacture of everyday consumer items such as pencil erasers and many other things. Xmas trees and the equipment installed within the wellbore today are a very efficient part of a subsea When the Vulcanisation process was invented production layout and can function in water depths in 1839 by Charles Goodyear who patented the of 2,000 meters. Typically they will handle oil or gas addition of Sulphur, Peroxide or Bisphenol this production and separation, gas lifting, water and improved the elasticity and prevented perishing in carbon dioxide re-injection and also play a major sunlight. Vehicle tyres are moulded from rubber with part in the export line inspection equipment and the Carbon Black added for durability. spill protection system valves. Many materials can be added to rubber and moulded Modern subsea systems comprising of Xmas trees, to give lots of capabilities for many engineering manifolds and flow-lines are designed for life of purposes such as resistance to heat and pressure. field in functionality and structural integrity they will Resins and plastics have a huge amount of uses assure the producing group of a daily volume of for industry and again can be engineered to give a crude oil or gas delivery so that the logistics for the sale of refining stock can be maintained. wide variety of properties. The basic material is a by-product of the oil and gas refining process and All the installed systems except for some of the in that form is only suitable for floor coverings and wellbore casing is recoverable and theoretically can coatings to give corrosion resistance. be refurbished and reused. To enhance the engineering capability certain Where practicable the produced fluids or gas will additional substances can be mixed in and melted be sent ashore by pipeline and there it will be under pressure to give the desired result of making processed. it harder or more resistant to heat. Shore terminals are cheaper to construct than the equivalent ones subsea. Where existing surface Steel in contact with the air or sea water will corrode platforms exist all the processing equipment and if not protected with a film barrier of metal or plastic. pipelines are already in place and connected to a This will be a part of the engineering that is applied shore terminal. to any project. The addition of a subsea production area tied into Engineered composite materials have to be able an older platform is the most economical type of to withstand high stresses and strength for load development wit the Xmas trees, the manifold and carrying in difficult conditions. a short pipeline and a control system being all the During the early development phase of subsea capital items that are required. An example is shown production each individual well was tied into a here off a composite manifold that is designed to manifold close on the surface. This practice was flow oil, inject water and lift oil using compressed the most economical because of the short step- gas. All these facilities are controlled from a ship out distance of no more than one mile the flowlines shape surface vessel called an FPSO (Floating were small and the umbilical simple. Production System with Off-take facilities). As control systems technology got better the These systems are in use around the world where step-out distance could be increased to 10 miles production fields are remote and too far from land to however the flow assurance through small lines run a pipeline. The FPSO will separate and store the was not acceptable and production rates were being oil until an export tanker offloads it so as to deliver reduced because of bore restriction and friction. the oil to a customer somewhere around the world. 7 Section 1 - Basics of Engineering Materials & Properties Steel Angle, U, square and I beam sections and many others such as lengths of railway line are produced Steel is the basic material used for structural by using hot rolling techniques to give very precise creations or reinforcing of all critical areas of shapes and dimensions. buildings that will be subject to load or stress. It will also be used to make key components for machinery Grades and characteristics are controlled by the of all kinds and sizes. International recognition of a standard and grade e.g. API. AISI 4130.This designation directs you to Steels are engineered into various grades that are the standard and a grade within that standard. accepted and used worldwide by industry. These grades give a selection of chemical and physical Steels of different types are referred to properties and will vary one from the other but as follows; also fall into common groups. These groups will Carbon steel be identified by their alloying additives and basic Alloy steel carbon content and from this they can be selected Stainless steel for pressure containing, corrosion resistance and Many steel alloys that are used by the Oil Industry weld ability. contain elements such as Carbon this is added Many forms of steel are available with sizes for strength and hardness and creates Mild Steel. standardised worldwide. Solid round bar of 6 meters When Chrome and Molybdenum are added to long and of various geometrical shapes such as mild steel we get an alloy that is very strong and square and hexagonal are made in many sizes from performs well in a broad range of temperature from 100 mm. 0°C to plus 250°C. Tubing is made in similar sizes to bar but for Carbon is responsible for forming rust and if reduced specialised use can be very large and made in a and replaced by Chrome, Vanadium and Nickel we variety of ways. have a Stainless Steel. 8 On some occasions Stainless steel does not have violet light. the ability to meet a specification and a Corrosion The moulding process will also produce high quality Resistant Alloy (CRA) is required. All the Carbon seals to very fine tolerances such as O-rings, lip is removed and replaced by small percentages of seals and multi-ring stem packing sets for valves Titanium and copper. and chokes. This type of alloy has a very high resistance to Rubber is calibrated by its hardness value this is corrosion and will also contain pressures of up to called Shore Hardness and ranges from 10-100. 3000 BAR at temperatures from -20°C to 265°C. 60 Shore is where most fall into for oilfield use. Some of these additives can restrict certain manufacturing processes or even field operations The Blow Out Preventer (BOP) ram seals will be a for example on-site welding. certain mixture that gives them a hardness the will prevent them from extruding if the rams are closed. When these circumstances arise a crossover piece As the well gets progressively deeper these rubber is workshop made so that the site weld is a straight seals will have to change and become harder and forward similar material weld. more heat resistant. On occasions when there is no alternative to Common rubber wiper rings are used on the rig welding different or difficult materials on-site then floor to remove the drilling mud from the drill-pipe a weld procedure will be developed and approved. as it is removed from the well bore. Simultaneously to this a welder will be practiced and Drilling mud is water based until later on during the tested making him competent to do a specialised drilling phase when it will be substituted by diesel field weld. He will then be approved by a national oil and the wiper material will be changed to be body such as Lloyds Register or Den Norske Veritas resistant to it. who will issue him with an identification stamp and this he will apply to each weld he does in accordance with that procedure on the on-site job. Blind Head to Head Seals on Top Ram Each category has a number of steel types within it. Common standards used by Industry are API, ISO, Rams Dressed to Seal on 5½” Drill Pipe EN, AISI and BS. A special standard and process has been developed Rams Dressed to Seal on to 9⅝” Casing to create steel that is resistant to hydrogen embrittlement and sulphide stress cracking. By a combination of heat treatments and steel chemistry Rams Dressed to Seal on 13⅜” Casing the National Association of Corrosion Engineers (NACE) compliant steel is used in High pressure/ temperature components. Rubber Ram Nose Seal Rubber compounds Natural rubbers have a vast everyday use for sealing waterproofing or soft surfacing. They are only capable of low duty use because they will Figure 1... BOP RAM degrade over a fairly short period of time and will be perished by Ultra Violet light and heat. Rubber compounded with sulphur can be adjusted Plastics to overcome some of these inadequacies by using a process called Vulcanisation. This process will A large range of plastics have evolved over the mix, roll and heat the ingredients that are made into last few decades and this has given solutions to many forms such as sheet, seals and protection engineering problems. material. The use of P.T.F.E. (Polytetraflouroetheylene) has Components made by this process can be gained use in car and aircraft parts and when engineered to vary in hardness and resist ultra reinforced with glass or carbon fibre it becomes a 9 high duty material capable of containing pressure Composites are very strong and have good heat at elevated temperatures. This material also has and corrosion resistance and light weight. a very low friction factor similar to wet ice on wet Many attempts were made to make lightweight ice. This allows it to be used for parts or coatings assemblies such as riser for use by offshore drilling for heavily loaded parts in machinery or oil and gas rigs with little real success. valves. NASA invented a very strong resin that was used When temperatures go above 180°C in parts that to stick on tiles on the space shuttles for a re-entry have to contain pressure or move or both of these heat shield. then special proprietary elastomers have been After de-classification this resin was tried as the developed and currently will be able to function at binding agent for extreme conditions. 260°C and down to -20°C such compounds are Alfas and Kalrez. These special compounds were The results were so good that a composite drilling originally conceived for use in space vehicles and riser with a weight reduction of 40% over a steel satellites but are now in use for soft seals such riser was manufactured for use in very deep water and used at depths in excess of 2500 metres by a as O-rings because of their high temperature and less weight capable drilling rig. resistance to H2S and CO2. These man-made materials are made in many Some rubber compounds that are good for high different sections with the most popular being RSJ, temperatures perform very badly in freezing square and round tube. conditions. An example of this was the complete O-ring failure of a space shuttle fuel tank when These sections can be bonded into very light weight the temperature fell to a few degrees below zero but extremely strong structures that will protect a overnight before the launch. The outcome was a fire Christmas tree from a 50 ton snag load that could from a leak that destroyed the shuttle soon after its be delivered by deep sea fishing equipment. launch. When fibreglass gratings and hatches are fitted to this protector they will afford all that is necessary to This performance limits their use in oil production be a cover that is very ROV friendly because of its equipment because a small leak in a high pressure lightness and the fact that it is immune to marine gas line will cause extreme cold and an ice-over will growth so no added weight or obstructive organisms be the result. Compounds such as Viton can never with change the characteristics over its operational be used in such conditions. lifespan. Composites These are developing materials and as new fibres Composite is the name given to an amalgam of and resins are invented they will be put into more high tech fibres such as carbon fibres and multi-part arduous service. resins. Test Yourself 1 (a) Name 3 shapes of steel. (b) What is the common name for Iron Oxide. (c) What happens when Chrome and Molybdenum are added to mild Steel You will find the answer to Test Yourself 1 on page 68 10  Section 2 - Introduction to Materials for Subsea Systems The materials used subsea will fall into two categories, Drilling and Production. Drilling be approximately 1.50 psi per meter of depth starting During this phase of the subsea operations, energies at the ocean surface. With many wells drilled in exuded by the earth’s crust in the form of pressure 2,000 meters of water and then the well to 4,000 and temperature must be overcome. The wellbore meters you can see the pressure and stress on the is drilled in sections with the largest diameter at casing when it is run to depth is very high. Pressure the seabed and getting progressively smaller as it from inside the reservoir will not be allowed into the gets nearer to the reservoir. This wellbore will be casing as it would mean the use of much higher required to have a life of at least 25 years and will strength and heavier casing. These materials would therefore need to be lined with steel pipe called be in compliance with API 5L. casing to prevent it caving in over that time. This All steel makers use similar production methods that casing is smaller than the hole drilled for it and this are in accordance with a standard and specification. space or annulus is filled with liquid cement to bond Test pieces are taken from each and every batch of and seal it in place. steel made anywhere and these are subjected to These casing strings have to have known values testing by pulling until they stretch then break. From for chemistry, collapse, burst elongation and yield. this all the information about that melt is known This information will enable an engineering design and if correct it will be allocated a heat number and to be done that will be suitable for maintaining the when sold will have that number hard stamped on it integrity of that section. and a paper certificate will be issued. The natural pressure encountered at this point will Every heat or melt of steel is provided with a quality 11  7” Tubing Hanger record of all test data collected by the manufacturer. 30” 457 ppf 1.5” wt /309.7 ppf A test certificate is issued and the heat number 1” wt X-52 Conductor reference is hard stamped onto the steel ingot. All 7-5/8” 35.8 ppf 25Cr-125 Vam Top HC Tubing forms of product made from this (eg. Forgings, bar 7” Tubing Retrievable Safety Valve stock or plate) will carry that same original number (Non-Wequalising) and have the test certificate information available. 20” 132 ppf 0.625” wt X-65 Surface Casing The roundness and straightness of a length or joint of casing (12.7 metres or 40 feet) is also controlled by API 5L. The ends of each joint is either weld TOC (500m below 20” Casing Shoe) prepped with a bevel that is best suited to its wall thickness or with a male and female thread cut on either end. A limit of a 5 joints high stack height 7-5/8” 35.8 ppf 25Cr-125 Vam Top HC Tubing avoids weight driven distortion. Steels used by the Oil Industry will vary to accommodate the different retained fluids or 14” 82.5 ppf P110 Intermediate Casing gasses being produced and as you will see they are (10m above Gearle) individually classified against these requirements by an API trim. TOC (500m below 14” Casing Shoe) BB - Carbon or Low General Service Alloy Steel CC - Stainless Steel 7-5/8” Permanent Downhole Gauge Mandrel General Service (Dual) DD - Carbon or Low Tubing X/over, 7-5/8” x 7” Sour Service Alloy Steel 7” 32 ppf 25Cr-125 Vam Top HC Tubing EE - Carbon or Low (2 Joints) Sour Service Alloy Steel 10-3/4” x 7” Production Packer FF - Stainless Steel Sour Service 7” x 5.75” Landing Nipple, Top No-Go HH - CRA’s Tie Back Assembly (Non-Sealing) Sour Service 10-3/4” x 7” Liner Hanger / Liner Top Packer c/w 6m PBR 10-3/4” 60.7 PPF T95 Vam Top Production Low Pressure Housing Casing (30m above Intra Jurassic Uncomformity) During the drilling phase steel piping called casing will be inserted into each drilled section to line it Top Perforation Interval thereby giving a steel bore that will be resistant to caving in and seal off the raw strata. Base Perforation Interval All this steel casing will start at the wellhead that is positioned at the exact location chosen on the sea bed and reach down to the end of each section that Base Sump 7” 32 ppf 25Cr-125 Vam Top HC Liner is progressively smaller and deeper. The wellhead is in two parts a low pressure and high pressure housing. Figure 2... Wellbore Tublars Schematic The low pressure housing is made from a low alloy 12 steel and welded to a large casing size usually 30”. High Pressure Housing No special heat treatment is needed on this part The next component is the high pressure housing because it is a normal duty component of a bigger and this will be rated assembly. To contain pressure of up to 15,000 psi Running To carry heavy loads up to 8 million pounds Tool Profile Removable Lock Latch (4000 tons.) To sustain high temperatures of up to 240°C. This part is welded to the next casing size that is 20” Completion and is run, landed and locked into the inside of the Guide Base Orientation Slot previous housing. This will suspend the weight of Flowbase the 20” preventing it from floating upwards. Lock Grooves The top profile of the housing has ribs that will allow Cement Port a Blowout Preventer during drilling and a Xmas tree for production to be locked and sealed to it. Dual Socket The special VT seal area is designed to be damage Taper tolerant and uses a very carefully engineered heavy 35.76” OD ring that will deform to be compliant with the seat in the wellhead. If the ring was made too strong it Figure 3... Low Pressure Housing could deform the wellhead housing seat rendering it useless for future use. Running Tool VX Seal Area Cam Profile VX VT Tubing Hanger / VT Seal Area Lockdown Profile Seal Wickers Ratch Lockring High Strength Insert Ring Dual Socket Interface on 15K Systems Only Figure 4... High Pressure Housing 13 Casing Hanger and Seal As the wellbore increase in depth smaller casing sizes will be suspended in the high pressure housing wellhead using High alloy casing hangers for 13.3/8” and 9.5/8” sizes. Latch Profile Wicker’s Flow-By Holes.75” Particle Size Junk Trap Area 16 Square Inches High Strength Load Ring 12.31” Min. Bore Figure 5... High Alloy Casing Hanger 500k Wellhead Housing Seal Unset Seal Set Figure 6... Example of deals 14 The MS-1 seal assembly is a soft U-ring and a manufacturers who will offer equipment that will fulfil wedge that is forced onto much stronger wicker the needs of the customer and be in correct for the points they points dig into the soft seal and create tested and analysed flow of fluids and gasses from 8 annular indents. They are in fact metal seals and the well known as the service conditions. simultaneous create a lock-down mechanism. Service Conditions will show the flowing In the event of severe damage preventing a seal temperature and shut-in pressure, the percentage testing then a special SG-metallic encapsulated amounts of water, oil, and gas with the gas being elastomeric element is added to the seal lower split down into different types and their amounts assembly. This will seal on a fresh area inside the expressed in percentages except Hydrogen wellhead. Sulphide (H2S) that is given in Mole fraction because The aggregated weight of these sizes with their of its small volume. 15,000 psi test load will amount to 7 million pounds It contains the smallest known molecule called (3,500 tons) that is suspended on the red insert ring Atomic Hydrogen and this molecule is small enough inside the high pressure housing. to migrate through very thick steel section and The High Pressure Housing and its annular all metal disperse in the air. MS-1 seal is an example of how steels of different Steel that will be used for the containment of high composition and hardness and work very well pressure gas that contains H2S will need to be together to achieve a very high quality and capable specially heat treated to avoid cracking caused by seal. this gas. The items used in a subsea wellhead system Alternatively a corrosion resistant barrier of a high are designed and manufactured from materials grade of stainless steel can be applied. specified in API 17D Seventeenth Edition and will be in compliance with the criteria given for various Gate valves are very important components during types of service. all phases of the operations they are used for: The scope of API 17D is to control the commonality Abrasive mud and cuttings returns when the of equipment design and materials for all equipment well is being drilled. Sea water. Corrosive chemicals. Produced fluids and gasses. Inhibitor chemical injection. E-ring The body of oilfield valve can be made from a forging or a casting with the forging being the preferred option with the materials for every part being in accordance with API 17D. An oilfield valve will contain high pressures and “U” Lock Ring some of the aforementioned contaminants. The reason it is being used is to stop or start flow and to do this certain parts inside are required to move. A gate section will have to slide on a seat and then seal off the pressure inside it. To achieve this seal parts that are hard wearing SG Element and very stiff are used and are hard surfaced with special self hardening alloys that are high Nickel, Chrome and Vanadium content. This is usually applied by welding at the manufacturing stage and is considerable less expensive than using a solid part made from that material. Figure 7... “U” Lock Ring To remotely function subsea gate valves a very powerful hydraulic actuator is needed. 15 Production achieve, valves and chokes are required to be functioned from time to time during their life subsea After drilling is finished the well will be prepared for so a new system for sealing them had to be invented. production using steels and other material that will This was done about 2002. be required to withstand the rigours of hot corrosive The normal rubber base seal will allow the stem part fluids and gasses. of the valve mechanism to turn in the seal but when The parts used at this stage to complete the well it is a metal sealed version it has to slide through will be removable for replacement at a later date if the seal. excess wear or corrosion has taken place. These metal seals need to have a gold or platinum The steels used are normally a high strength plating to act as a lubricant since any grease would Carbon, 2.1/4% Chrome, ½% Molybdenum that has simply be pushed out of the way. been heat treated to hardness or 187-241 Brinnel Other materials used subsea will be the steel or 22 Rockwell C scale. This condition will give the sections that are used to construct all the structures steel a resistance, when in tension, to attack from that house or support the production Xmas tree, the atomic hydrogen ingress that would cause cracking manifold, export pipeline offshore host platforms and and subsequent failure. This is called controlled shore facilities. All these items need to be protected hardness. from seawater and high visibility resin paint is used This will prepare the steel for use in the life of a well for this. regardless how hostile the production fluids are all The preferred colours are white and bright yellow this is done in accordance with API 5CT and NACE. these are the most visible in ROV floodlighting to Many elastomeric components in the form of O-rings the HD underwater camera even where murky or large rubber seals are used during the production conditions from seabed silt and sand that has been completion phase for oil when medium pressure disturbed by the ROV thrusters during their routine and heat are combined with low or no contamination positioning activities. from corrosive elements will be expected. The term used here is a sweet production well and 10ft Diameter Guide Cone can be oil gas or water. Although wells that will produce gasses will be similar, seals made from rubber will not be durable enough to cope with more difficult conditions and Inner Guide they may not be deemed safe enough. In instances Cone like this a suitable metal or metallic encapsulated 30/36” Housing Receptical rubber seal must be used. Static conditions are ideal for metal seals especially where corrosive compounds can be present these are normally made from a high yield form of stainless such as Inconel. Release Mechanism When Oil or Gas production temperatures are at 250°C or more rubber will no longer be capable of Figure 8... Recoverable Guide Base doing the job have to be replaced by metal. There are two situations where metal sealing is When using any specification verify it is the needed, static and dynamic. latest issue or edition. Static seal are where very accurate sized and highly Here is an example of areas that need to be seen finished surfaces are forced together and put in a in low light conditions. This is a deep water drilling preloaded condition to avoid and movement should recoverable guide base. there be any thermal cycling take place. The guide cone is painted with the Hi-Viz yellow Dynamic metal sealing is a very difficult thing to paint to assist the driller to enter the end of a 30” 16 casing string that is being moved around by the allow an ROV complete access to the Xmas tree drilling rig and oceanic forces. valves and choke. Cocoon style covers for subsea installations are also made by the layup process. Composites and plastics subsea This is where a steel frame is covered with multiple Where a large component such as a safety cage layers of glass cloth and resin that give a non- that has to fend off fishing equipment and withstand corrode/non- snag blister. a 50 ton snagging load it must be very strong if made from steel substantial steel tubing and other Teflon is widely used as bearing surfaces. It is inert materials. in water has sufficient strength and has a low co- efficient of friction so requires no lifetime lubrication. Such an item has to be opened for access from time Additives such as Molybdenum Disulphide and to time. This will be done by lifting it from the surface powered bronze can be mixed with virgin Teflon to or by the attachment of an air bag and then opened give a very robust bearing material or lifted by an ROV. Teflon bar is used as a soft break-away component To reduce the weight but maintain the strength of that will act as a weak link to give protection. these parts they can be constructed from composite material each of which is specially made to withstand Another major use of Teflon is as a baked on coating its share of the load. that acts as a make-up lubricant for piping seals and nuts and bolts. Teflon has good corrosion protection These parts can be made using the pulltrusion and thermal properties and will function unchanged method of manufacture. This is where a die is made o o from minus 20 C to plus 180 C. for each section required RSJ, angle, channel or box sections. Mechanical Properties and Testing The die has injection points to allow resin to be Engineering materials are commonly defined and high pressure pumped in to it as threads of Kevlar specified by their properties. Of all the properties and carbon fibre are fed into the correct position of a material may possess, mechanical properties the die and pulled slowly through. As this is done a are often the most important because virtually all length of the section will be produced. This section fabrication processes and most service conditions will be very strong and give a substantial saving of involve some type of mechanical loading. Thus, in weight. selecting materials you must know the mechanical properties of a material and what they stand for. The pulltrusion method of making lightweight parts is particularly advantageous in waters in excess of The most common properties considered are 1,000 meters deep. strength, hardness, ductility, and impact resistance. A good example of this system can be seen in the Important Properties of a Material Penguin field in the North Sea where a large petal Important Properties of a Material are listed in the design can be opened and closed hydraulically to table given below. Property Meaning Hardness Resist penetration by a sharp object Toughness Resist failure by impact/energy to fracture Ductile Deforms without breaking Brittle Fails at maximum load, in contrast to ductile Stiff Resistance of an elastic body to deformation Strong Requires large force before rupture/deformation Thermal Shock Resistant Resists fracture from changes or gradients in temperature 17 Tensile Test be able to determine its strength along with how The strength of a metal is its ability to withstand much it will elongate. As you continue to pull on the action of external forces without breaking. the material until it breaks, you will obtain a good, Tensile strength, also called ultimate strength, is complete tensile profile. A curve will result showing the maximum strength developed in a metal in a how it reacted to the forces being applied. tension test. A tensile test, also known as tension Tensile Properties test, is probably the most fundamental type of mechanical test you can perform on material. The properties which may be determined by a Tensile tests are simple, relatively inexpensive, and tensile test are :- fully standardized. It is a method for determining the Proportional Limit and Modulus of Elasticity behaviour of a metal under an actual stretch loading. Tensile tests are normally taken at standardized It is found that the initial portion of the stress – strain room temperatures but may also be made at diagram is a straight line for most materials used elevated temperatures. in engineering structures or components. In this range, the stress (σ) and strain (Є) are proportional A lot of information about a material can be learnt to each other. Therefore we can write, E= σ /Є.. from tensile testing This test provides the elastic This relationship is known as Hooke’s Law. limit, elongation, yield point, yield strength, tensile strength, and the reduction in area. E is the slope of the straight line portion of the stress – strain diagram is called the Modulus of Elasticity By pulling on a material, you will be able to determine or Young’s Modulus. how the material will react to forces being applied in tension. As the material is being pulled, you will Proportional limit is the maximum stress under which Ao Af Stress Necking Ultimate tensile strength Fracture strength Fracking Yeild strength Lo Necking Af Young’s modules = slope Fracture = stress/strain Non-uniform Elastic Uniform plastic plastic deformation deformation deformation Strain Elastic Plastic strain strain Total strain Figure 9... Example of Tensile Test 18 a material will maintain a perfectly uniform rate of brittle material, the ultimate strength and breaking strain to stress. It is the highest stress at which the strength coincide. curve in a stress-strain diagram is a straight line. Ductility Proportional limit is equal to elastic limit for many metals. The ductility of a material is indicated by the amount of deformation that is possible until fracture. This is Elastic Limit determined in a tension test by two measurements. The elastic limit is defined Greatest stress that can Percent Elongation be applied to a material without causing permanent deformation. This is determined by fitting together, after fracture, Most materials can be stressed slightly above the the parts of the specimen and measuring the proportional limit without taking a permanent set. It distance between the original gauge marks. may be noted that now the curve is not straight line. Where, Lf = final gauge length and Yield Point Lo = original gauge length, usually 2 inch As the load is increased beyond the elastic limit, a stress is reached at which the material continues to Reduction in Area deform without an increase of load. It may be noted This is also determined from the broken halves of that plastic deformation takes place in the specimen the tensile specimen by measuring the minimum after elastic limit is reached. This deformation is of cross sectional area and using the following formula. a permanent type, that is, the deformation remains Where, even after load is removed. Only a few materials Ao = original cross sectional area and (notably steel) have a yield point and generally only Af = final cross sectional area under tension loading. Proof Stress Yield strength Since it is difficult to measure elastic limit, a newer Indication of maximum stress that can be developed method – that of proof stress has been developed. in a material without causing plastic deformation. Proof stress is the maximum stress a material can It is the stress at which a material exhibits a withstand without taking more than a small amount specified permanent deformation and is a practical of set. The amount is usually specified as the approximation of elastic limit. smallest that can be measured by the extensometer, Ultimate Strength namely, 0.01% in 2 inch. As with the yield strength, The ultimate strength or the tensile strength is the the preferred method of measuring proof stress is maximum stress developed by the material based by using a 0.01 % offset. on the original cross sectional area. On loading Impact Test further, a ductile material will continue to stretch Not only is it important to know how strong a metal until it fractures is in tensile strength or how ductile it is but also how Ultimate tensile strength is the value most it reacts under sudden impact is very important. frequently used from tensile test results. It is used This quality is known as toughness. for specification and quality control. An impact test is a test for determining the energy Breaking Strength absorbed in fracturing a test piece at high velocity. For a ductile material up to the ultimate strength, the Simply put it is one object striking another object at deformation is uniform along the length of the bar. a relatively high speed. At the maximum stress (ultimate strength), localized One method used is the Charpy impact test. The deformation or necking occurs in the specimen, test was developed in 1905 by the French scientist and the load falls off as the area decreases. This Georges Charpy. The test consists of breaking necking elongation is a nonuniform deformation and by one blow from a swinging pendulum, under occurs rapidly to the point of failure. The breaking conditions defined by standard, a test piece notched strength (point B in stress – strain diagram for a in the middle and supported at each end. The energy ductile material), which is determined by dividing absorbed is determined in Joules. This absorbed the breaking load by the original cross sectional energy is a measure of the impact strength of the area, is always less than the ultimate strength. For material. 19  Scale Starting Position End of Hammer Swing Specimen Anvil CHARPY IMPACT TEST Normal Cooled to -20 Cooled to -20 Cooled to -20 Brittle, Soft, Within Specification, unacceptable unacceptable Acceptable Figure 11... Charpy Impact Test 20 Hydrostatic Testing Included the product testing of API Line-Pipe is a hydrostatic test. Eash joint of pipe is subjected to an inspection hydrostatic pressure test which can be as much as 80% of the rated burst. Test pressures for seamless and welded pipe 18” OD. and smaller are held for 5 seconds minimum. Welded pipe 20” and greater are held at test pressure for 10 seconds mimimum. In all cases a no permissible leak rate standard is applied. Test Yourself 2 (a) We know the wellbore is drilled in sections. Do these sections get progressively bigger or smaller as we get nearer the reservoir? (b) Name 3 things Gate Valves are used for. (c) What is an impact test used for? You will find the answer to Test Yourself 2 on page 68 21 Section 3 - Applications of Materials Pressure containing Components from CO2, Hydrogen Sulphate (H2S) and water are formed and alloys used for these conditions are All pressure containing or highly stressed termed as Corrosion Resistant Alloys or CRA’s. components must have their design verified and materials approved before they are made and These CRA’s contain a very large percentage of tested. Nickel and Chrome for corrosion resistance and assisted by Molybdenum to cement all the atoms Material section and types are checked against the together. A small amount of Vanadium is added for Oil Industry standards such as American Petroleum strength when high pressure has to be contained. Institute (API), American Society of Mechanical Engineers (ASME Section V11) and NACE (National Hydrogen sulphide H2S is a gas deadly to man. Association of Corrosion Engineers). This would Awareness of its presence will be an odour of rotten be termed as the design verification and would be eggs. Sense of smell is immediately impaired by approved by a certifying authority such as Lloyds or H2S so the impression of it having gone away is Det Norske Veritas. where the danger lies. Important other symptoms of its presence are irritated eyes and a sore throat. Well casings will use low alloy steels for shallow A concentration of 10 parts per million inhaled for sections of around 1500 meters deep whereas the 8 hours will make the average man very sick and deeper sections will have a higher percentage of hospital treatment may be required. A few seconds Chrome and Molybdenum content to give greater of inhalation of 15% concentration means instant resistance to burst, collapse and tension loading. death. Steel grades for production tubing will be selected A substantial amount of H2S is given off by active to meet the rigours of the wells fluid or gas flow and volcanoes and surrounding vents. It is heavier than requires to have resistance to erosion from mineral air so ground depressions will fill first. particulates(sand) in these cases carbon based Primary control valves that are used in Xmas Trees alloy steel will normally be good enough. and manifolds contain dynamic components that Corrosive substances such as acidic formations have to either rotate or slide on a mating part and 22 still maintain a high pressure seal. These parts are will build up and as it does it enhances the seal in required to function up to their tested standard even a static condition preventing it from wearing out. after more than a decade of being in a static position. These seals are made from a high level CRA to To do this certain high wear and corrosion resistant avoid and corrosion taking place on the fine feather surfaces are applied to the very strong substrate of edge of the low pressure lip. the base component. This is to avoid any bending Parts such as the primary valves on a Xmas Tree under load of a surface that has this hard material or a manifold are shell tested at up to double their coating applied. maximum rated working pressure and then function Bending would create cracking that could allow the tested to full operational pressure. This will proof test leakage of high pressure gas across a gate to seat their ability to contain all the production substances. interface thereby negating the specification of zero Lubrication leak rate. When equipment is required to function in a Subsea Two proprietary materials are used for these parts environment, generally it is not easily accessed for one is a work hardening CRA called Stellite. This has routine maintenance. a number of different grades covering applications from rock shovel teeth to Oilfield valve gates, seats It is imperative that a lubricant or grease is contained and stems. The other is called Tungsten Carbide within valves and chokes that may need to be and is made by mixing powdered carbon and adjusted on an infrequent basis. tungsten together then sintering the two at very The most important things are: high temperatures. This produces a very hard alloy that requires diamond equipment to finish it to the The design is such that it will prevent any flow of required dimensions and finishes. The finished produced substance through the body otherwise it surfaces will remain in original condition for many could be dry within the first 24 hours of going into years of production or maybe even the whole life of production and invite wear or corrosion to become the field without the need for any maintenance or an issue over time. replacement. The lubricant will not dissolve in hydrocarbons and Tungsten carbide hard facing does not work well be a long polymer chain construction with a No. 2 grease consistency. on rotating surfaces therefore is confined to sliding areas only. It is not uncommon for the production temperature at the Xmas tree to be 260°C so any grease must resist Where stems are normally turned to open or close thinning or the separating of their component parts a valve or adjust a choke changes have to be and be resistant to natural solvents and H2S. High incorporated into the approved design by converting quality lubricating material such as molybdenum these from rotary to a sliding type functioned by a disulphide, graphite and castor oil are often used hydraulic actuator that’s integrated into the control for this high demand area. system. A sliding action is more reliable than rotation on a high pressure seal. Structural materials Where high pressure and temperature is present Throughout the oil industry many vital or important block construction and metal to metal sealing will structures will be part of a development. They are be a design requirement. manufactured to be a precision fit when put in place in up to 3000 meters of water this sort of accuracy Block valve construction means that as much of an means that they have to be finish machined after assembly as is practical will be encased within a fabrication or jig built. single steel block and thereby reduce the number of joints that could become a leak. The main material used is carbon structural steel the same as is used by the construction industry. Round API specifies the use of gate valve configuration tube made by rolled and welded plate or seamless for all primary production valves. Valve gate to seat drawn, square tube made by welding two pieces of sealing will be a hard faced surface on each and U channel together “I” beam or RSJ heavy angle, that these mating surfaces will be very flat (one light grating and plate are needed to make the many band) and very smooth (2 AA). structures for a typical field development. They Stems will be finished to the same standards but will vary from Xmas tree and manifold structures, will extra control on diameter and ovality. The underwater support bridges that must support high design of a seal in this area has to allow the stem to loads of piping that carry oil or liquid gas to smaller stroke through and maintain a lower pressure and and lighter ones for protection against fishing only when all movement is complete the pressure equipment. 23 The welding of these structures will be done using method instead the molten weld pool is protected by full penetration welds or fillet welds where no a shield gas of Argon or other gas mixtures. stresses will be found. Tungsten Inert gas (TIG) is a precision welding Care has to be taken to ensure certain structures technique that uses a hard tungsten electrode and a are designed to minimise weight. This is important wire rod of filler material that is dipped into the weld otherwise a normal capacity workboat may not be pool manually by the welder. This system is used for able to open inspection hatches or remove snagged root passes or thin section material especially high fishing equipment. alloy CRA’s. Xmas trees and manifolds have a protective Cladding is a very important welding process. structure to avoid damage from otter boards and It is used to give steels that have low corrosion nets. These structures are designed and tested in resistance a welded liner of high alloy CRA and conjunction with the fishing industry. thereby making savings of large sums. For many years this liner had to be done in two phase so as to The structures must resist a snag load of 50 tons overcome the dilution of the finished surface caused without deforming to where access hatches or by carbon burn in from an arc struck at nearly 90°C. emergency valves would not be operable. To guarantee the qualities of the finish surface in a Welding methods and technology one pass operation it is necessary to reach down inside a bore and weld CRA onto CRA with the A number of different welding technologies are used metallurgical bond being achieved as the molten in the oilfield each of which require the welder to weld touches the bore of the carbon steel. This hold a current certificate before he or she can carry is a much less aggressive burn in and minimises out any work. They will also need to identify any dilution. The cladding process does the same job in weld made by applying an assigned identification considerably less time and gives consistent quality number by an approved method (hard stamp, ink at a much reduced cost. pad, or vibro etch) All these welding processes need to done using Diameters of 300mm or greater can be seam welded a qualified procedure and very often they will not using the Submerged arc process where a large be able to be carried out in the field but need the diameter filler wire is burned under a cover of flux controlled environment of a workshop that has pre powder. This will exclude any oxygen from the liquid and post heat and stress relieving facilities. weld pool and prevent porosity. Friction welding is used to attach threaded ends Smaller diameters will use Stick electrodes where onto long lengths of tubing. Both the tubing and the filler wire has been coated with a hard deposit of the threaded end are rotated in opposite directions flux. This flux serves the same purpose as before. to each other in a machine, the two are brought The filler wire will be of low hydrogen content so together and when the heat caused by friction gets as to avoid the weld area being subject to possible to the correct temperature both are pushed together hydrogen embrittlement. where they make a perfect weld. Metal Inert Gas (MIG) is the preferred method of Any metal flash on inside and out are trimmed and plate or structural fillet welding and will deposit the the weld is inspected by x-ray for quality of weld and filler metal very quickly by feeding the weld pool checked for hardness to ensure it will be the proper with a thin easy to burn wire. No flux is used in this strength and not brittle. Test Yourself 3 (a) What are the 2 most important things when it comes to lubricant? (b) Name 2 types of welding. You will find the answer to Test Yourself 3 on page 68 24  Section 4 - Failure Mechanisms Corrosion its condition, during its production it will, to varying degrees shed particles into the production stream. Corrosion can come from various sources, This particulate matter will cause erosion all the way unprotected or poorly coated carbon steel will from the bottom of the well to the control equipment corrode with the formation of Iron oxide or rust. on the seabed. As the particles travel upwards they This formation reduces the steels thickness and will ultimately be travelling at supersonic speed and therefore strength and that will lead to a part failure. either as they go round a bend or squeeze through Internal corrosion inside piping can also be rust a reduced space by encountering a flow reducer but it can also come from liquids (water) or gasses and because they are contained in fluids they will (H2S/CO2) that can combine to produce acidic cut into any pipe, valve parts or choke flow reducers compounds. These compounds will etch their way that they are in contact with. through the wall section until it will leak or burst Over time this damage may be so bad that a well and cause a possible life threatening event. Where workover may need to be carried out. In the worst export risers or injection lines pass through the area case if a complete workover of the entire well from that is affected by tidal heights called the splash zone top to bottom is necessary then a large drilling rig the wet and dry of this area can cause accelerated will be needed so the work can be done under corrosion and depending on the water temperature complete well control. and salinity measures to combat this will vary from a Should a lesser job be all that is needed a smaller galvanic coating of zinc and marine paint to a sheath vessel in conjunction with an ROV could change the of a CRA that extends from substantially below and worn parts in a production choke or remove debris above the tide marks from the surrounding area of the Xmas tree or the Erosion manifold. In an oil or gas well erosion will take place. The Export lines on the seabed can also suffer source of hydrocarbons is a sand deposit or external corrosion when areas that are reservoir that has been compacted to a large degree subjected to high current runs suffer localised depending on its age and situation. Regardless of corrosion from sand contained in the water flow. 25 Damage areas like this will be picked up on a routine Tensile failure pipe inspection carried out by diver or ROV. Failures of this type are usually caused by either Metal Fatigue over stressing or insufficient manufacturing control. The over torque of bolting taking them beyond their Metal fatigue is a condition where a metal component elastic limits is a common cause of failure during or assembly is continually subjected to movement that operation or during service. until the atomic bond of the metals particles become The incorrect heat treatment of steel used in loose from each other until they finally break apart highly stressed components and the omission of in a catastrophic failure. heat treatment to expel atomic hydrogen trapped The causes of these failures have been found to beneath metal plating such cadmium or zinc can emanate from expansion and contraction, bending cause instant failure when a part is in tension during and straightening from vibrations set up by the its operational life. This type of failure is called a passing water flow creating a wide range of vortex hydrogen embrittlement failure. shedding vibrations and shaking. Brittle fracture As the water or air passes by the object an area When steel is manufactured part of the process is of low pressure behind it causes turbulence that heat treatment and that is what gives the correct vibrates the part. The frequency of the vibration is values to that steel. Some of these values relate proportional to the speed of the water or airflow. to its ability to meet its specifications for tensile strength, ductility and hardness. Where the ductility Many things are done to minimise vortex shedding. is low and hardness is high there is a chance that a Streamlining and maintaining a laminar flow brittle fracture will occur. The same situation can be characteristic will remove the low pressure at the seen on cutting tools where high hardness will resist rear of an object and the turbulence. wear but will break easily if given a shock impact. Test Yourself 4 (a) Sand particles and what, cause corossion? (b) What can cause erosion in an oil or gas well? You will find the answer to Test Yourself 4 on page 68 26  Section 5 - Cathodic Protection & Hydrogen Sulphide Cathodic Protection years or more needs Cathodic protection, we have Steel in salt water is the perfect environment for two missing factors. No seawater and no zinc. To corrosion. An excellent way of preventing this is by reach this reservoir the well casing passes through a method called cathodic protection. a section of a salt /sand rock like layer. The casing strings in the well must receive protection over a In operation we need an anode, a cathode and an long period or they will corrode away and leave electrolyte. When an assembly (the cathode) is the well unsafe. In this case an electrical current fitted with zinc bars (the anode) and is immersed positive and negative is passed into the casing in seawater (the electrolyte) a galvanic field is and stimulate the fluids in the bore will continue to produced around the assembly however one produce galvanic salts and protect the casing string. component is sacrificial. Hydrogen sulphide The Zinc bars are the sacrificial item and the amount of zinc is calculated to be sufficient for the life of Powerful internal corrosion can be produced by well field. fluids. Where produced oil and gas contain certain The anodes are smooth surfaced when new; contaminants very aggressive compounds can be however after a year or two they become pocked and produced. When we have carbon dioxide (CO2), pitted. This increases the surface area and because water and hydrogen sulphide (H2S) present in the of the apparent slow down in their reduction in size production flow all three will combine to create gives the appearance of them not working. Sulphuric acid. It is normal for 30 years of zinc to be fitted and This acid will be continuously produced and will prior to all of that being dissolved most fields are etch all the surfaces it flows through and may cause depleted. a leak or malfunction. When it is known that this situation will exist all the When a very large onshore reservoir such as the internal production tubing for the whole depth of the Arab D in Saudi Arabia with a predicted life of 150 well will be made from a CRA material. Incorporated 27 into the subsea Xmas tree will be a corrosion Strontium 90 scale within the production tubing. This inhibitor injection point. This inhibitor will be carried is a low level radio- active compound so great care along with the flow and apply a chemical coating to and proper personal protection should be taken all the internal surfaces to protect them. When the when working with or close to it. production fluids or gasses reach their processing Should the xmas tree or the production tubing point a complex separator system will remove be recovered from such a well these parts will be the sulphur to be sold as a separate commodity. sent to a de-contamination centre before going for The CO2 and H2S are flared off and the corrosion refurbishment. inhibiter is sometimes recovered so that it can then be re-used over and over again. Never inhale dust from any radio-active source. When the correct mixture of contamination and water are being produced, certain wells will produce Test Yourself 5 (a) What is Cathodic protection? (b) What needs to be present in the production flow to create Sulphuric Acid You will find the answer to Test Yourself 5 on page 68 28  Section 6 - Coatings & Surface Protection Metallic plating as the zinc dissolves in sea water it will also form complex salts that are extremely toxic to marine life. Certain components will be enhanced for their function by the application of a special metallic Since substantial surface areas will need protection coating. Valve and choke parts will have a significant and to minimise the use of metal plating special increase in their expected service life by the use of paints have been developed to ensure protection hard chrome and tungsten carbide surfacing on is given and when the paint is cured no further internal parts. pollution is given off. Where possible a change away from external These paints contain no metallic substances; they metallic coatings has taken place. This change has are resin compounds that cure and become a very meant a reduction in the use of chemicals that were long life durable surface and are completely inert a difficult to dispose of and hazardous to work with. short time after they are applied. This makes them This type of coating is also toxic to marine life. very environmentally friendly. Nuts and bolts were traditionally galvanised with The resin coatings are coloured a vivid yellow or zinc plating but they are now Teflon coated and this white to make then as visible as possible in murky gives a cleaner to apply and inert protective surface. water conditions when ROV lighting is shone on them. Another big advantage on nuts and bolts coated with Teflon is they have a built in surface lubricant In water depths of up to 150 meters or so, diver that will give a more accurate torque figure when operations are used to do certain maintenance they are made-up. work. Walk ways and hatches that are made from grating are coated with grit to prevent construction Seabed installations use large pieces of framework workers or maintenance divers from slips, trips or to provide the support of production systems such falls. as manifolds and Xmas trees. The life of such structures has to be the same as the projected Much if this work comes from damage done by life of the field and therefore the attributes of the fishing trawl boards and snagged nets. Ropes and protection used has to be of an exceptional nature. nets that are snagged on the framework can take some days for a diver or an ROV to remove. As explained before, anodes of sacrificial zinc will provide a galvanic field around the structure but 29 Section 7 - Basics of Metal Fatigue Types of material fatigue Rubber and plastic materials also suffers from failure modes especially tubing. Materials fatigue can come from a number of conditions. Perishing Synthetic rubber such as low pressure Many will emanate from simple engineering errors hose will deteriorate if left exposed to sunlight. where the wrong criteria have been used when the Bursting Cord reinforced hose used in hydraulic design life of a part was being calculated, or it has systems will burst if over-pressured. suffered misuse or has been subjected to conditions Bending When normal hoses are bent the important it is inadequate for. thing to remember is the minimum radius bend All engineering materials are made up from tiny grains and avoid kinking that would restrict flow-thru and of different elements that are then fused together damage the hose structure. by one or more of these elements. This compound is then heat treated to give characteristics that will Compliant shapes Large diameter flexible flow impart hardness, tensile strength and toughness. lines that are permanently installed in the sea for the export of production fluids and gasses are very When heat treatment is not correctly performed heavy when full and will need support to avoid too these fatigue failures can occur; much tension load occurring and causing a tensile Too hard Brittle fracture (Break with no warning) type fracture. This is avoided by maintaining a specific shape of Too soft Load fracture the hose as it rises from the seabed to the surface. (Parts indent into each other under load) Such a shape is called a compliant “S” where an Low tensile Tensile fracture “S” configuration is maintained in the water. This is (Parts not tough enough) done by using high density foam modules that are attached to the export line as it rises towards the To minimise these occurrences every batch of steel surface. is tested by its manufacturer to ensure compliance with the specification. This is done by subjecting Where a single span of large hose is supported, a test pieces to calibrated loads to verify they are mid-water arch that is made from fibreglass, steel correct. These test results will be recorded and an and high-density foam is positioned midway along identification number stamped on the material. its length (or if the length and the weight is enough 30 then two support arches can be used to reduce spoilers) at the top and running down the exterior stretch) the export line is draped over it to give of the chimney for approximately 20% of its length. support and avoid exceeding the minimum radius The fences are usually located in a helical pattern. bend. A failure of a major line is therefore avoided. The fences prevent strong vortex shedding with Vibration Many failures are attributed to vibration. low separation frequencies. The optimal pitch for Any part that extends through the ocean will be vortex shedding is a 5D pitch (5 x the diameter of subjected to tidal water twice a day as well as the stack). current runs that are often more aggressive and The frequency at which vortex shedding takes place faster and never stop or vary. As this flow passes by for a cylinder is related to the Strouhal number by a part the pressure at its front edge will be high and the following equation: St = ƒD the pressure at the back low and turbulent. V The part will then start to move back and forth between the high and low pressure areas and set Where St is the Strouhal number, f is the vortex up vibrations. This is called vortex shedding and shedding frequency, D is the diameter of the can lead to a fracture because of the continuous cylinder, and V is the flow velocity. bending straightening action. The Strouhal number depends on the body shape Vortex Shedding and on the Reynolds number. Vortex Shedding is an unsteady flow that takes place in special flow velocities (according to the In the following diagram the yellow object size and shape of the cylindrical body). In this flow, represents a cylinder with water flowing past it, vortices are created at the back of the body and the vortices are low pressure areas which cause detach periodically from either side of the body. the cylinder to move in that direction, this causes Vortex shedding is caused when a fluid flows the next vortex to form on the opposite side which past a blunt object. The fluid flow past the object causes the cylinder to oscillate back and forth. This creates alternating low-pressure vortices on the is potentially disastrous for structures such as oil downstream side of the object. The object will tend rig legs or risers. It can lead to severe fatigue and to move toward the low-pressure zone. castastrophic failure. Eventually, if the frequency of vortex shedding matches the resonance frequency of the structure, Flow the structure will begin to resonate and the Direction structure’s movement can become self-sustaining. Tall chimneys constructed of thin-walled steel tube can be sufficiently flexible that, in air flow with a speed in the critical range, vortex shedding can Oscillation Direction drive the ch