H W Lee and R Miles examine options to keep the assets of oil operators fully functional and in a safe working order.We all know that oil majors employ risk management departments, but this has been a bit of a 'black art' where the risk management of inoperative assets were not really considered - but the failure of a vessel on a project was a major priority. There is a misappropriation of terminology that means that effort is not going into the correct area where maintenance and repairs has become a minor effort until the revenue stops. This is a totally incorrect bias. The sooner accountants looking for revenue realise that, if you have an asset, you must look after.
The number of ageing platforms and pipelines increases annually and globally, and a substantial number are facing operation beyond their intended design life. Therefore, structural integrity and risk management is an increasingly important element of the offshore operator.
Asset management programmes must perform an annual assessment of their structures and facilities and should any significant change to the structural integrity be determined then implementation of an appropriate repair or strengthening plan must be implemented, and sooner rather than later.
Ninety per cent of all offshore revenue utilises the requirement of offshore structures, and any operator who believes that the asset is failure free is adopting that well-known ostrich approach of burying one's head in the sand and hoping that it will not happen to this company.
ULO Systems LLC has been actively involved in structural repair projects since 1983. Initial works related to schemes that were implemented as a direct result of collision, impact, storms and fatigue. Recently computer modelling and re-analyses has been used to identify the need for additional strengthening. As a result of its extensive experience a range of specialist services and best practice methods exist that enable the operator to restore the structural integrity and extend the operating life of an asset. To complement these works, ULO Systems has developed a range of specialised high performance grouts, densified cements, epoxy motors and light weight sodium-silicate based grouts that may be used to meet the often demanding requirements.
A broad range of services in repair and strengthening are available that encompass specialist consultancy, design, project management, experienced engineers for supervision, together with specialist grouting services, personnel, materials and equipment.
The most common industry adopted method is welded and grouted repairs however, while a welded repair may typically restore a structure to its initial condition, if the cause of the damage was fatigue loading and this has not been eliminated, then the damage is likely to reappear. Grouted repairs are therefore widely used to provide additional strength and typical applications include:
- Filling members to prevent propagation of a dent or buckle or, to enhance the capacity.
- Sleeved repairs, filling annulus between sleeve and member to strengthen a thin walled can or to bridge over local damage.
- Leg strengthening, eg filling main pile annuli for increased stiffness.
- Clamped repairs for load transfer from existing structure into strengthening steelwork, at joints, or as part of sleeves to structural members.
- Leak sealing and plugging of redundant pipework, using hot-tap techniques.
- Reinstatement of damaged concrete, wellhead strengthening and underbase grouting of gravity base structures for void filling.
While the repair design will evolve to meet the parameters of the project, it is often important to design a solution that has minimum weight, or which occupies minimum space on the structure. In these cases, a design, which offers the maximum structural efficiency, should be adopted.
There is considerable advantage in designing repairs that require minimal in-service inspection and maintenance. The optimum solution is arrived at by a carefully balanced assessment of the relative importance of these factors.
Installation costs are often the major element in the overall cost of the repair, and hence must always be minimised. Furthermore, it is possible to adopt approaches to the structural design, where there is a trade-off between the engineering efforts required and the final cost of the repair. Options include: design for full member capacity; design for full member loads, and load sharing design.
For 'fast-track' projects, one of the first two options may be adopted but where the project schedule permits, the load sharing design approach would usually provide the most economical overall solution.
ULO Systems LLC adopts a staged approach to the development of a repair scheme from assessment, and design through to installation. To illustrate how this is implemented, a summary overview of different strengthening projects follows:
1. At a platform. The conductor guide frame had suffered severe fatigue damage from the effects of wave slam and vertical wave load component, which caused cracks in the frame members where they met the jacket's main horizontal tubular members. The design took account of the interplay between the original structure and the clamps placed upon it. By increasing the stiffness of certain of the horizontal stressed grouted clamps it was possible to reduce the loads attracted to other nodes sufficiently to avoid the need to place clamps around them.
2. During a routine underwater inspection of a platform, cracking in some of the welds connecting five brace members to the node joint were determined. The repair took the form of a three-dimensional framework surrounding the node, which was connected to the adjacent braces or the node using stressed grouted clamps. The new brace members of the framework were shop welded to the clamps or where site connections were necessary, a conventional pile/sleeve type connection was used taking advantage of weld bead shear connectors, selected to provide the maximum tolerance on final positioning and thereby facilitate installation and subsequent grouting.
3. A routine structural analysis to a platform found it was necessary to strengthen the jacket structure at a particular node connecting one of the legs to a diagonal brace. The analysis concluded that strengthening would be best achieved by installing grouted, mechanical clamps around the node, temporarily secured using spacer bolts. Specially constructed seals were installed at the open ends of the clamp and when activated, sealed the clamp annulus prior to filling with a high performance grout. When the grout had gained strength, the spacer bolts were withdrawn and post-stress applied to the clamp by means of the external stressing bolts.
4. At two different platform locations, strengthening was required to the main legs and for different reasons. At the first, while the original design was main piles with welded crown shims at the top of the jacket, shortly after installation, the structure developed a 'shimmy' or, 'wobble'. To overcome this and increase the leg stiffness, the solution implemented was to pressure grout the annulus with high performance cement. At the second location it was necessary to strengthen the legs to permit elevation of the deck. The leg-strengthening scheme incorporated external sleeving with a small annulus for subsequent grouting. Due to the large volume of infill material needed for the legs, a specialised low heat cement grout was to minimise heat of hydration. Grout filling the legs provided increased resistance to buckling and also resistance to the post tensioning forces developed by the clamp (sleeve) bolts and improved ultimate load capacity.
Ultra high performance grouts
More recently, ULO Systems LLC in association with Contec ApS with have identified the Secutec range of ultra high performance grouts (UHPG's) that may be used to enhance the integrity of structures. The UHPG's have exceptional properties with E-modulus that exceed 70 GPa (10,000 ksi) and compressive strengths that exceed 210 MPa (30,400 psi).
These physical characteristics are particularly beneficial when undertaking structural upgrades, because the UHPG may be injected into members, nodal joints or split sleeves, etc. to reinforce and enhance the integrity of the structure. In particular, because of the exceptional mechanical properties of the UHPGs it is possible to undertake repairs to caissons or piling using fibreglass sleeves or moulds because the high bond and shear strength of the UHPG will provide excellent bond and shear distribution, often eliminating the need for heavy steel clamps.
Other remedial areas where the UHPG's may be applied include corrosion protection and blast wall and firewall strengthening.
In addition, the UHPGs are utilised in the renewables/wind turbine market where the high E-modulus ensures good load transfer and the increased connection stiffness reduces the natural period or frequency of monopile structure, which because of their 'flexibility' is often a problem.
The properties and characteristics of the Secutec materials have been tested by many professionally accredited bodies and noteworthy institutions that include, Delft University of Technology (Netherlands), TNO Building and Construction Research (Netherlands), VdS (Germany), IMMB (Germany) and The Danish Technological Institute.
ULO Systems LLC entered the offshore industry in the 1970s, providing pipeline free-span correction and pile grouting services and successfully introduced to the industry the use of engineered geotextile fabric formworks ('grout bags') for pipeline stabilisation. Over the years these techniques have been developed to provide protection of subsea templates and similar structures, and to permit ROV installation and grouting, and at depths of +1000m.
Common to all projects, ULO Systems services undertake the preparation of detailed procedures to ensure complete safety for the installation of the repair, QA/QC, design and testing of specialised grouting materials, procurement and supply, through to offshore operations. ULO Systems strength lies in its experience and knowledge that enables it to engineer the optimum solution for a given situation.
ULO Systems LLC programme of prelay and post lay supports is well known by most, that any diameter/height or soil bearing can be overcome to support, protect and stabilise sub sea pipelines, cables and umbilicals, offshore structures like valve boxes and plums. What do risk management need to do to ensure enhancement of existing assets? They need to: evaluate age and production levels over past five years; evaluate cost of replacement; has against a 10 year survey and maintenance programme. Has design life been exceeded. Can we extend design life? Yes, in most cases assets installed 25-30 years ago are still in a sound status but do require more survey and maintenance.l
Enter √ at www.engineerlive.com/iogH W Lee and R Miles are with ULO Systems LLC (HFZ), Sharjah, UAE. www.ulosystems.com