February 23, 2016
The North Sea this time of year is usually desolate of divers. With the sea temperature dropping to around 6°C and air temperatures around 4°C, the cold would be enough to put anyone off but the frequent gales and winter storms mean the it’s not only cold, it’s a dangerous place to be in the winter.
Diving time is generally restricted to the summer months and with that comes the risk of equipment break downs. With time being so important, it’s generally considered good practice to have at least 100% back up of equipment during the dive season – no one wants to lose money by losing days or even weeks due to equipment failure. Repairs are given priority over the colder periods and as such we have entered into what we know as ‘spares season’ where we have increased demand for our full MPI systems, spares and other subsea NDT supplies for companies want to ensure that everything is available for projects beginning in the spring.
It’s imperative to carry out inspection of the North Sea infrastructure. Predominantly being installed in the 1960’s and 1970’s, many structures are reaching or have already exceeded their design life. The shallow water structures, a combination of wellheads, risers and jackets, must be inspected for their integrity on a regular basis and maintained under the Design and Construction Regulations. On the United Kingdom Continental Shelf (UKCS), the mix of waves, currents and winds affects the type of jacket structure to use and the majority were designed with a 50-year design life. Extreme weather can result in buckling, instability or air gap loss in the jacket, foundations, risers and supports.
Early topside platforms in the North Sea were based on those used in the Gulf of Mexico. Nowadays, due to the harsh nature of the UKCS, specialised structures and equipment have been developed, generally formulated from carbon steel. Other materials have come into their own over the past couple of decades, especially if they are more economical, corrosion resistant or weight saving, however, these tend to be for floor grates, stair towers, etc. rather than the actual main structures. Due to the amount of welding involved on top side structures there is a greater susceptibility to corrosion, especially through-thickness issues.
Knowing whether design life can be extended means having not only an understanding of fatigue and corrosion but also quality information as to the current state of the infrastructure in question. To find out this information, inspection must be performed and analysed. Geological and geotechnical problems can also affect structures, for example, seabed scour, subsidence and slope instability can lead to deterioration in foundations. Another major issue can be fatigue; the cyclic nature of waves and wind is a known hazard in UKCS structures. These risks can result in issues with air gap loss, pile pull out or the platform itself tilting. Fatigue damage produces cracks and these are usually located around the welded connections; they can grow to the through-thickness stage and possibly lead to parts of the structure separating, breaching of water-tightness and loss of stability.
When steel is submerged the rate of corrosion can be incredible if it is not protected. Submerged cathodic protection (CP) systems, with sacrificial anode systems, are usually used to counteract this although the exact set up depends on the surrounding environment. In addition, common practice dictates that structures are built with ‘corrosion allowance’ to enhance protection, i.e. the construction allows a certain amount of material that is available to corrode without it affecting the core stability of the structure, this especially true around areas that come into contact with the mean sea level (the MSL, or average surface level of the planet’s oceans). This corrosion allowance can be around 6-12mm, particularly around the MSL areas as obviously the rate of corrosion is much greater there.
The area just above and below the sea level is known as the splash zone and it is notoriously difficult to carry out inspections here. CP systems are little use and the briny waters, oxygen exposure, wave action and debris can cause serious corrosion problems. Due to the sea spray action, non-submerged steel is usually painted for protection. Corrosion can cause loss of wall thickness, cracking, wear, buckling, loss of water-tight seals and stability loss, to name just a few jacket, in the topside structures, risers and their supports.
Impact allowance is part of the design of jackets but ship collisions or even dropping things can add to the strain on these structures. Hydrocarbon spillage can ensue should a vulnerable area be hit or there could be denting, buckling, loss of water-tight seals and stability or even the structure separating altogether! Keeping the maintenance of proper positioning, or ‘station keeping’, is also essential. To reduce fatigue and corrosion risks on riser and conductor systems or seabed connections, mooring needs to be maintained.
Of all the threats, one of the most dangerous is the possibility of fires and explosions. Hydrocarbon leaks are incorporated into the design process now, however, older structures may not have had such stringent designs – what knowledge we have now has been gained from incidents which have happened over the years. In general, although explosive blasts can be devastating, the greatest damage is likely to be caused by jet fires. These are caused by pressure build ups (around 2 bar of pressure, or more) and the hydrocarbons are released, the resulting energetic flames can result in structure overload and significant damage to every part of the structure depending on the ferocity of the fire.
The costs of inspection offshore can be considerable. There are many technologies for use in a variety of areas, however, Magnetic Particle Inspection (MPI), a technique for detecting irregularities in platforms and surrounding subsea areas, remains the industry choice because it is cost-effective, durable and flags up metal defects quickly and easily.
Fatigue damage comes in many forms but surface breaking cracks, through-thickness cracks and far-surface and embedded cracks are by far the most common. Surface-breaking cracks in metal materials, especially the jackets and submerged pipework, are usually detected underwater by Magnetic Particle Inspection (MPI) as it can be used with materials with reasonably thick coatings. It has the same principles as topside MPI procedures except hydraulic forces make removing particles easier underwater, it also doesn’t require contrast paints and obviously the environment is wet! If paint has been used on the area to be inspected, it is usually removed from the weld, weld toes and around an inch on either side of the weld. The inspection is carried out using underwater AC yokes which are turned on/off from the surface and magnetic particles are usually daylight or blue light fluorescent’s. The particles flow evenly over the surface to be inspected and once they have settled, the diver stimulates the water so that it flows through the yoke legs. Once applied and removed, the diver looks for any indications of discontinuties. If all clear, the yoke is removed and the process repeated. If the diver locates any discrepancies, the yoke is turned 90 degrees and the process repeated. Repair work is carried out on defects and the area is then tested again. If repairs are unsuccessful, replacement is necessary.
Our System 3 and System 12 MPI units are perfect for this type of inspection. The System 3 has three different magnetising techniques; prod, yoke and coil and it’s inbuilt safety systems including earth leakage trips and surface isolation transformers give peace of mind to the operator in the most arduous conditions. The System 12 is perfect for shallow water, tidal and splash zone applications and diver control for the UV Lamp and Magnetic Yoke mean no umbilical connection to the surface is required. Based on the field-proven System 3, the portable System 12 offers power in a small package. We also stock Chemetall, Circle Systems, Inc. and Johnson & Allen MPI consumables so if you’ve got inspections to carry out , we can provide you with everything you need to make the process as smooth as possible.