Operators these days enjoy a far better understanding of the behaviour of deepwater risers and, consequently, can have greater confidence in their fundamental design.
This is largely the result of advances in riser monitoring technology, a field that has been pioneered relentlessly by 2H Offshore.
Going out first is never easy. It is the same in most walks of life. Front runners generally face the greatest resistance and present a natural focus for the opposition.
It is all the more remarkable, therefore, that 2H has remained at the forefront of riser monitoring after almost single handedly pioneering the concept in the mid-1990s. The company has done so by sticking to the principle that riser monitoring needs to be simple, or at least the basic hardware used to gather the raw data needs to be simple.
It was this idea that lay behind 2H’s early decision to concentrate on monitoring the way risers move as a consequence of wave action, currents and shifts in the position of the vessel or platform to which they are connected. Significantly, this motion can be recorded very easily using a combination of acceleration, angular rate and inclination sensors. These are cheap, robust, reliable and very easy to attach to the riser without removing any of the coatings or insulation around the pipe. There is also a good theoretical argument for monitoring motion. When carrying out an initial riser analysis, it is normal to set up and solve the equations that describe the riser’s motion. Thus, it makes sense to want to compare the actual motion of the riser with the assumptions made during its design.
The only drawback of motion measurements is the relative complexity of the calculations necessary to translate the raw data into pipe stress values and from there to derive the fatigue performance of the riser. However, not only did 2H engineers perfect the necessary data processing methods to achieve this, they also succeeded in developing a back-analysis technique to compute the global structural response of the riser, and hence the fatigue properties along its entire length, from motion data necessarily gathered at a series of discrete locations.
Despite the advances made by 2H, the company continued to encounter resistance from sections of the industry advocating strain measurements rather than motion measurements, mainly because of their more direct relationship to fatigue. Although 2H recognised this advantage, the company had initially been reluctant to pursue this route because of the frailty and poor reliability record of the available strain sensors and the practical problems of fixing them to the pipe.
Conscious of being seen as intransigent, and by now having gained considerable experience of the practicalities of riser monitoring, 2H set out to win over those calling for strain measurements by developing its own rugged strain sensing devices. The company subsequently patented and launched the INTEGRIstick™ and, soon afterwards, a variant on the same theme, the INTEGRIcollar™ (2H had already decided to call its motion sensor range INTEGRIpod™).
The INTEGRIstick uses strain gauges fully sealed inside a corrosionresistant metal housing to measure small changes in riser curvature in two orthogonal planes. The INTEGRIcollar uses gap sensors set within a simple steel framework to capture pipe axial and bending strains. Data from both are readily converted into pipe stress values and thence to fatigue rates. The key features of both devices are their accuracy and stability combined with excellent reliability and ruggedness. The INTEGRIstick is simply strapped to the pipe, and the INTEGRIcollar is bolted around it; both may be fitted over any coating or insulation within minutes.
The INTEGRIstick has made a strong impression on the industry and has consolidated 2H’s position at the head of the field. Earlier this year, the product won a Spotlight on New Technology Award at the annual Offshore Technology Conference in Houston, USA. Previously, it had already been approved by Chevron for use in a major riser monitoring programme centred on its deepwater Tahiti field in the Gulf of Mexico (see boxed text, Chevron Tahiti). Although still early days in terms of gathering and interpreting data, the project has demonstrated that it is possible to integrate an extensive riser monitoring system into a major offshore development without interfering with the normal project activities or the overall schedule.
Pei An, who manages the riser monitoring programme at 2H, sees this demonstration as crucial. “We sense a growing acceptance of the value of including monitoring as an integral part of new deepwater riser systems,” he says. “We are seeing a lot of interest from operators in the Gulf of Mexico and West Africa, and Brazil is emerging as a huge potential market with the acceleration in the pace of offshore development following the new, large discoveries made over the past couple of years and the Brazilian government’s decision to open up offshore developments to operators other than Petrobras.
“I am proud of the role that 2H has played in proving that riser monitoring is a practical proposition. Moreover, we have shown that it does not have to be an expensive exercise. We have stuck to our belief that riser monitoring, though a complex exercise overall, has to be simple and reliable at the point of delivery. We are seeing other companies trying to move the business upmarket with high-tech, high-cost systems. In my opinion, this is not the way to go. The value comes from the way the data is treated and the interpretation put upon it, not necessarily the way it is collected.”
Chevron Tahiti
The Chevron Tahiti riser-monitoring project is seeking to cast light on the performance of steel catenary risers: critical dynamic structures that display particularly complex fatigue behaviour. The project, which is based on a single in-field production riser, has two main objectives. The first is to capture motion and strain data, and to use them to validate the original riser design process. The second is to characterise the response of the riser to vortex induced vibration and to the extreme loads resulting from wave action and the movement of the Tahiti truss spar platform.
The monitoring system, which was designed by 2H, concentrates the sensors in the two most critical regions of the riser: the hang-off region immediately below the platform and the touchdown region at the seabed. In all, there are 20 monitoring stations spread over roughly 100 m in the hang-off region and 220 m in the touchdown region. Four of the stations have motion sensors, six have INTEGRIsticks, and 10 have a combination of both. All the sensors are wired back to the platform so that data can be gathered continuously in real time.
Significantly, as a part of the same programme, strain measurements are also being made on a 210-m section of flowline about 1.5 km away from the riser touchdown point, where the weight of the riser is reduced by buoyancy modules to promote controlled buckling. These measurements are being made using a sophisticated fibreoptic device not supplied by 2H.