Chock tactics
  from: Offshore Engineer
  Friday, May 16, 2008

The corrosion, erosion and fatigue issues facing the ageing offshore wells of the North Sea are focusing attention on effective maintenance and life extension of offshore installations. OE kicks off this month’s two-parter on the asset integrity front with a report on how protecting against conductor damage has been getting smarter.

North Sea structures, many of them approaching or exceeding their original design life, are continually under dynamic loading and experiencing a large number of wave-induced stress cycles with corresponding fatiguing effects, yet are likely to be required for sustained exploration and production activity for a number of years to come.

A structural integrity study* conducted a few years ago cited 83% of operators of more than 6000 wells on the UK continental shelf experiencing problems associated with structural integrity, and stated that some 10% of UKCS wells had been shut-in at some point in the previous five years on structural integrity grounds. It also predicted increasing problems in future.

Among the structural integrity issues threatening offshore structures, the well conductor system, or main outer structural tubular, is one of the components attracting attention. Certainly conductor damage is a common problem on offshore platforms, often caused by the effects of lateral wave movement. Various investigations, including a transient dynamic analysis of a typical flowline and conductor, conclude that cracks and vibration often observed on offshore platforms are caused by the fatigue effect of the motion of the conductor, and particularly the wave loading causing the conductor to rattle inside its guide barrels.

‘One of the problems is that the gap between the conductor tubing and the guides it passes through as it runs between seabed and wellhead has to be large enough to enable the conductor joint connectors to pass through, and to allow for any misalignment that might occur in fabrication of the jacket,’ explains Graham McKay, international business development manager at specialist engineering and technical services company Furmanite.

‘The larger the gap, the easier it is to install the conductor,’ he adds. ‘But on the other hand, the bigger the gap the greater the movement of the conductor within the guides. This in turn results in wellhead and flowline movement. Moreover, the impact forces when the conductor contacts a jacket guide suddenly or sharply also transmits a shock load and movement to the wellhead tree. This can cause large flowline movements and vibrations and create problems in flowline pipework and attachments, including cumulative fatigue damage.’

Solutions to address this issue have long been debated. But while it might seem that eliminating the movement is the answer, the problem has been that stopping the conductor’s movement using traditional solid rubber, wood or plastic chocks or steel shims to hold it rigidly in place can in itself create further problems.Wave induced deflection of the conductor can induce large end moments if the conductor is fixed at its guide locations and, since the jacket guide tubes and framing members are generally not designed to resist these end moments, local over-stressing and fatigue problems can occur in the jacket structure.

Intent on solving this problem, offshore contractor Amec teamed up with Furmanite back in 2001 to identify and develop a solution that resulted in a fatigue analysis and cast in-situ chocking service combining the expertise of both players, which was first put into practice on the BP Bruce platform in the UK North Sea.

Undertaking further research, Amec modelled the conductor and flowlines, and used the DNV-developed SESAM modelling wave loading program Wajac – which computes hydrostatic and hydrodynamic forces on fixed offshore structures due to environmental design loads (including wave, current, and wind loads) – to generate load-time histories for an irregular sea state. This demonstrated how the loading creates a stress issue, with corresponding wellhead displacement.

‘Further analysis identified that chocking could successfully address the problem and dramatically reduce the stresses, given that the chocks did not hold the conductor too rigidly in place,’ notes McKay.

‘It was with the results of this research that our SmartShim solution was developed. By modelling the dampening effect of varying shore hardnesses the optimum dampening characteristic and allowable annulus between the conductor and the shim could be obtained. SmartShim is highly engineered to limit lateral movement within a desired tolerance band.’

Smart solution

SmartShim consists of PVC-proofed nylon shim slips or bags, ultrasonically welded for light-tightness, and designed to provide good abrasion resistance and allow for the vertical movement of the conductor as a result of temperature changes in the riser content. These are fitted around the conductor within the guides and filled with a Furmanitedeveloped polyurethane resin that sets to form a resilient elastomer.

‘The resin is typically 80°shoreA hardness, which means the SmartShims hold the conductor sufficiently firmly in place within the guides to prevent damage, but also offer the critical elasticity to absorb the wave loading effects,’ McKay explains.

Usually four bags will be used around a conductor. These are secured in place between the conductor and guide tube using integral rods and clamps, and tubes from the manifold are attached to inlets on the bags. The two polymer components then flow to the mixing unit and adjacent pump, and the polymer is pumped to the distribution manifold, and fills the bags or shims. The process takes about three hours for a four bag system to be installed and completed, and the resin then takes approximately 24 hours to achieve full strength. Once installed SmartShim requires no long-term maintenance, McKay points out.

Being cast in-situ is a key feature of the design, he says. ‘One of the problems with traditional chocking devices such as wooden wedges or rubber blocks is that they’re difficult to keep in place. Because the SmartShim resin is mixed and injected on site and cast in-situ, it forms to fit the gap between the conductor tube and guides closely and precisely, whatever the dimensions (which may be anything from 3/4in to 8in and more), and whether or not the conductor is centred within the guides.’

Further developments have seen resins introduced in varying hardnesses and densities to meet specific requirements, from applications where moisture or oil may be present, or where low weight is preferable, to where a greater degree of movement that usual is required.

‘The mechanical properties of the polymer can effectively be tailored to the individual requirements,’ McKay underlines. ‘Damping and compression characteristics can also be altered by use of precast polymer inserts and void pockets sewn into the enclosure bag.’

First application

The inaugural application on the BP Bruce platform saw 14 slots between the 28in diameter conductor and 30in diameter guides successfully chocked at the main deck level below the wellheads, after damage to the conductor tube and flowline and wellhead displacement had been discovered and identified as being caused by wave induced loading. They had tried other traditional chocking methods and had even approached OEMs for custom-made chocks to the exact dimensions, but had not found a successful solution. A full survey and fatigue analysis was undertaken by Amec, including a structural study of the conductor and riser, and determination of corrective actions, as a result of which the SmartShims were developed and installed. The site operation lasted six days and was completed successfully to provide a lasting solution.

Since then the service, which includes an initial survey to produce a structural analysis, predict failure and identify the necessary remedial actions; production and installation of SmartShims to meet the need; and monitoring or follow up actions as required, has been applied on a number of platforms for major operators globally – both retrospectively where fatigue or damage has been identified, and as a preventative measure on a number of newbuild platforms in regions such as the Caspian.

‘Installation is typically undertaken as part of planned maintenance programmes, either while the platform is operational, or during a planned shutdown, often when our technicians are mobilised to undertake other specialist Furmanite services at the same time for optimum efficiency,’ says McKay. ‘In these instances it’s often a reactive measure, particularly in regions such as the North Sea with its ageing infrastructure, to avoid further damage and extend fatigue life. But other regions are increasingly seeing SmartShim fitted at construction stage. Nearly 200 were installed in one contract on a newbuild Caspian platform, for instance, on slots of up to 7in.’

McKay reports that the technology itself has also gone through a number of improvements since its original application on Bruce. ‘In addition to a range of resins in varying hardnesses and densities to accommodate specific performance requirements, a new pump skid has fully automated the mixing and pumping of the resin on site, providing a high degree of control and flexibility, ensuring the optimum fill pressure, and offering convenience and efficiency benefits, as well as reducing installation time.’

Subsea developments

Further development of the SmartShim led to its application subsea by a number of North Sea operators led by Shell, in late 2006 on Nelson platform in the UK sector and, more recently, StatoilHydro on the Norwegian Øseberg South platform (OE March). ‘The effect of the wave loading and movement of the conductors is amplified the higher you go and further from the source, so it’s beneficial to be able to minimise movement by installing the lower SmartShims at the lowest practicable level,’ notes McKay.

The maiden subsea installation on Nelson was undertaken at 15m below sea level. This involved further research and analysis to consider additional factors such as external water pressure, and the increased distances over which the resin had to be pumped from the deck of the diving support vessel. The subsea injection manifolds and venting arrangements were adapted accordingly.

Purpose-designed equipment was also developed, including metal support frames to keep the slips or bags in place while being filled. ‘This seems a relatively simple modification, but it makes it more compact and easier for the divers to handle,’ McKay explains. ‘The bags are installed in the new frames, and the complete assembly is installed around the conductor before filling the bags with resin. The use of a cartridge is designed to make the process diver-friendly, and so that they could easily be made ROV-deployable with minor modifications.’

Extensive trials were performed before this first subsea installation, McKay confirms. ‘We had to make sure that the polymer would travel satisfactorily through the 125m high pressure hoses and fill the slips properly and effectively, using special venting systems and resin,’ he says. ‘We also had to ensure that the process wouldn’t be compromised by the effects of the water pressure or lower temperatures. And we did onshore installation trials to test the purposedesigned support frames.’

The subsea SmartShim installation at Øseberg South, part of an underwater repair programme, involved 27 slots at 15m depth. The operator had experienced problems on the conductors caused by wave-induced displacements inside the guides, and the SmartShim installation was to prevent further damage being sustained. The operation required some 4km of hoses, 9t of resin and 30t of equipment.

Further developments now being looked at include subsea installation by ROV, as well as pipe supports and pump base dampening.

SmartShim has won a number of industry awards since its introduction. ‘As a comprehensive service, and arguably the first “scientific” approach to a common and potentially very costly fatigue and structural integrity issue, it has literally filled a gap for the offshore industry,’ concludes McKay. OE

* The structural integrity of offshore wells. 2005 report produced by Douglas- Westwood for UWG (OE June 2005).

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