‘Why does rigless also have to be riserless?’ offshore technology innovator David Wright, president of Wright’s Well Control Services, asked himself. His answer: a self-contained heave compensation system that heaves itself while running coiled tubing or wireline through the center of its dual cylinder assembly. Add in his idea for a heave compensated floor, and risers could soon be deployed from a vessel of opportunity.
Having spent many years around the moonpools of MSVs managing the deployment of subsea packages, coiled tubing and deepwater equipment for rigless and riserless interventions, I had come to accept the cost benefits of working from a vessel of opportunity along with its limitations. However, when looking to expand the service offerings available from a boat, I started questioning the age-old subsea intervention paradigm, that rigless and riserless have to be wedded concepts. Why were we limiting ourselves simply because that is the way it had always been done it in the past? Why couldn’t we have the stability and performance offered by a rig while running a riser for acid injections and stimulations, P&As, decommissioning and hydrate remediation jobs?
The answer is a new heave compensation system for running coiled tubing, wireline or other tools while two cylinders work in unison against heave from the surface and subsea. Beyond adding a riser for rigless operations, this system further breaks with convention by offering a central longitudinal conduit extending through the center of its cylinders. Where other rig-based kits externally stabilize a riser, this device is machined from its midpoint with a 6in ID pathway to allow tools to run through the center of the mechanism. The ability to have a higher degree of riser management from a vessel during bad weather scenarios will allow for continued abandonments, workovers, completions and production operations.
The heave comp as designed has a 150,000lb tension load and 20ft of stroke. After throwing the bowls and slips in a false rotary floor, setting the tension, rigging up on top of it, and attaching the riser to a BOP, coiled tubing can run through a moonpool or off the stern of a vessel via a secondary cantilever in 7500ft water depth or deeper, depending on riser weight. When that coiled tubing is moving only 3in to 4in, the operator has all the benefits of both a riser and rig.
The system has two cylinders that are aligned internally, share a common central head or base plate, and are housed in the same exoskeleton. After looking at a number of options, a fluid solution made up mostly of nitrogen gas became the medium of choice for pressurizing the heave comp’s cylinders. These nitrogen-filled cylinders are in fluid communication working in tandem to exert force against each other. While the lower cylinder absorbs the force of the surge subsea on the riser, the upper cylinder compensates for the wave motion experienced by the deployed vessel. The opposite ends of these cylinders – the top of the upper cylinder and bottom of the lower cylinder – travel through a central cavity and move up and down against a common base plate.
The same new stabilization design is also applied directly to the vessel itself. Nitrogen-filled cylinders, similar to those used for the riser provide heave compensation for a floor attached to the vessel deck. When the heave comp riser is coupled with the heave comp vessel floor, the operator has additional means to achieve rig-based performance. However, it is not my intention to replace rigs for P&As and subsea intervention. I am just building on the principle of a rig’s ability to stabilize equipment, either when running through a riser or when stabilizing the floor on which the equipment sits.
Engineered with a 2:1 safety ratio where the system’s tolerances for the riser and floor are adjustable to a predetermined pressure by manipulating the amount of nitrogen in the cylinders. Depending on multiple factors – expected load, motion, type of ship, the weight of the riser, water depth, season and changing weather conditions – the cylinders are pressured for optimal performance. The cylinders extend or retract to provide the desired length for the field conditions encountered.
The heave comp riser can also serve as a redundant system for a rig providing an additional layer of safety. Nitrogen enhances the safety of the heave comp system. It is environmentally friendly, is less likely to leak, and through a series of relief valves can provide a fluid response to force within milliseconds. The 40-80 gallon bottles of nitrogen can be pre-charged and do not need to be placed directly next to the cylinders for effective pressurization. Safety is further reinforced with hybrid emergency quick disconnects (HEQD) which allow for the system to eject from the riser for contingent or unplanned operations.
With the system in production at Parker Engineering and a number of operators looking at a first deployment, I am highly optimistic that the question ‘Why does rigless also have to be riserless?’ can soon be put to rest and the industry can begin to answer a new question: ‘What are the limits of rigless applications when a riser is in place?’ OE
David Wright is president of Wright’s Well Control Services (WWCS), established in 2006 to provide well control, P&A, and subsea intervention services for clients in the Gulf of Mexico. Wright has 25 years of global offshore experience, including work at major energy service companies and consulting roles for operators, and holds several patents for offshore technologies (OE September 2011).
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