Latest advancements in drilling riser analysis technology
October 26, 2017
Wood director of drilling Donogh Lang explains how new analysis is enabling drilling operations in some of the world’s most challenging environments
Despite the ‘lower for longer’ oil price environment, operators continue to explore for, and develop, oil and gas resources in the most challenging offshore environments worldwide. Exploration or development drilling is currently ongoing or planned in deepwater locations characterised by large water depths, powerful ocean currents and high sea states.
To drill in these locations cost-effectively requires careful planning of operations to minimise non-productive time (NPT). A significant aspect of this planning involves analysis of the marine drilling riser system in order to establish the limiting environmental (or metocean) conditions for particular riser operations. Wood’s proprietary system, DeepRiser, is one such modelling tool to aid the design and analysis of drilling and top-tensioned production riser systems.
In this article, Lang explores three recent technology developments that are enabling drilling to be undertaken in some of the world’s most challenging locations.
Drilling operations in deep and ultra-deepwaters are now nearly universally carried out using dynamically positioned (DP) drilling vessels. DP vessels maintain station relative to the well, using thrusters operated by a sophisticated control system to counteract any environmental forces.
A key consideration when drilling from a DP vessel is what happens if the system develops a fault and the vessel is unable to maintain station. The types of fault that can lead to this scenario include a total loss of power, drive-off and failure of individual thrusters. Of these scenarios, usually the most critical is a total loss of power – this can lead to a drift-off event, where the vessel is pushed off-station by environmental forces.
If this situation occurs, it is necessary to disconnect the drilling riser from the well to prevent permanent damage – a process known as an emergency disconnect sequence (EDS). During the EDS, the well is first sealed in by the blow-out preventer (BOP) and the riser is then disconnected from the well at the lower marine riser package (LMRP). The timing of this sequence is crucial – the riser must be disconnected before any of the loads in the riser or the well system exceed the capacity of the equipment, to avoid permanent damage.
In practice, watch circles are used to ensure the EDS is initiated in a timely manner. Watch circles are set thresholds, in the form of horizontal distances from the well, which define when the EDS must be initiated. Usually there are two watch circles – yellow and red. The red watch circle is the last point at which the EDS must be started and the yellow is when preparations for the EDS should start.
Various approaches exist for calculating watch circle location. The simplest is to define these as a fixed percentage of the water depth. Although this approach was once commonplace, it suffers from the major drawback of not accounting for metocean conditions, and it has been shown that this could result in damage to the riser or well. For this reason it has largely fallen out of favour.
A more realistic approach is to perform a simulation of the vessel on its own to predict its drift-off path. The forces generated are calculated as functions of the wind speed and direction, the surface current speed and direction and the sea state. These are applied in a dynamic simulation that calculates the trajectory of the vessel during drift-off. The trajectory is then applied to a finite element (FE) global model of the riser system, to determine the point at which the riser must be disconnected to avoid damage (the so-called point of disconnect or POD), from which the red and yellow watch circles can be found. Although this ‘uncoupled’ approach is still sometimes used, the major disadvantage is that it does not account for the effect that the riser has on the drift-off trajectory of the vessel.
The state-of-the-art approach for watch circle calculation, and the one employed by DeepRiser, is to use a fully coupled scheme that includes both vessel and riser in a single model. Dynamic analysis is performed, in which the environmental forces acting on the vessel (and riser) are calculated and applied. The result is a prediction of the vessel drift-off trajectory that fully accounts for the effect of the riser, and calculates the POD and watch circles with the highest accuracy.
A major advantage of the fully coupled approach is that it avoids potential over-conservatism. This can be significant: especially in harsh environments, any over-conservatism in the calculation of the watch circles could result in suspended operations, which could have a substantial impact on NPT and drilling costs.
There are a number of emergency scenarios – including drift-off – that can require disconnection of the riser. The physics of an emergency disconnect are complex and there are several special factors that must be considered when simulating this scenario.
The first is the behaviour of the riser tensioner system. In an emergency disconnect, the tensioner has to be able to respond by lifting the riser enough so that the LMRP moves well clear of the BOP, while reducing the overall tension applied to the riser. All this must be achieved without damage to any of the equipment.
The tensioner system uses a number of large hydraulic cylinders energised by an array of air pressure vessels (APVs) to develop tension that is applied to the riser. An anti-recoil valve (ARV) is also situated on the hydraulic piping that connects the tensioner cylinders to the APVs (via a hydraulic accumulator). The purpose of the APV is to throttle the flow of hydraulic fluid to the tensioner cylinders during an accident, such as a riser parting or an emergency disconnect. Throttling the flow has the effect of reducing the tension in a controlled way.
Previously for drilling riser analysis, the tension applied by the riser tensioner was commonly modelled as a constant, vertical point load at the top of the riser. Such a simplistic approach was insufficient to model the behaviour of the tensioner in an emergency disconnect.
Instead, DeepRiser incorporates a detailed hydro-pneumatic model that includes the individual hydraulic and pneumatic components of the tensioner system. Each individual tensioner cylinder can be modelled independently, allowing the programme to simulate the three-dimensional response of the riser system to an emergency disconnect.
The second challenge associated with simulating an emergency disconnect event is modelling the behaviour of the drilling mud within the riser. Drilling mud is used to provide pressure control within the well and to remove drill bit cuttings. The density of the drilling mud varies, but it is generally heavier than seawater. Mud is rarely retained within the riser after disconnect – the bottom of the LMRP is open to the surrounding seawater and, because the mud is heavier than seawater, it flows out of the bottom of the LMRP. This phenomenon has a significant effect on the behaviour of the riser, as it causes a large downward drag load.
To model this accurately, DeepRiser integrates a finite volume (FV) mud flow model with the FE structural model of the riser. The fluid flow model captures the complex physics of the column of mud within the drilling riser as it collapses following emergency disconnect and the effect that this has on the riser response.
Being able to model this accurately is of particular importance for harsh deepwater environments. The water depths require the use of higher riser top tension that, when combined with vessel heave response in heavy seas, can reduce the range of metocean conditions in which the riser can be safely disconnected to very small levels. Therefore it is essential to verify that the riser can be safely handled while maintaining an economically viable operating envelope.
Wellhead and conductor
Recent developments in the drilling industry have led to an increased focus on the fatigue of the wellhead and conductor casing that can occur during drilling, workover and plug and abandonment (P&A) operations. These developments include the increased use of modern sixth-generation mobile offshore drilling units that carry larger and heavier BOP stacks, and the requirement to conduct a greater level of well intervention operations to maximise recovery from existing reservoirs.
DeepRiser includes features that allow the wellhead, conductor, casing strings, intermediate cement and soil structure to be modelled with a degree of accuracy previously not available in a global riser analysis tool. Through the use of pipe-in-pipe models, the software allows the individual strings that make up the well structure to be explicitly modelled. This provides a much higher level of accuracy than that provided by traditional ‘composite’ models, where the properties of each of the strings making up the well are combined into a single structure.
The detailed wellhead/conductor/casing model bridges the gap between the older composite models previously used for global analysis and the more intricate three-dimensional local models typically developed for component analysis using FE analysis packages. These improved predictions are essential to demonstrating the feasibility of the intended operations.
As easy-to-access resources are depleted, further exploration in harsher deepwater locations is inevitable. Ensuring exploration can be carried out in an economically sustainable way demands the use of the latest digital technologies capable of simulating drilling operations with the highest level of accuracy. Wood’s DeepRiser tool is one such technology helping to maximise the viability of drilling operations in some of world’s most challenging environments.