The decreasing gap between technology and the it’s applicability in the oil industry has led to a rapid development of deepwater resources. Beginning with larger fields where the chances of economic success are high, to marginal fields where project economics becomes a more critical parameter, the petroleum industry has come a long way. However, the ever growing water depths and harsher environments being encountered are presently posing challenges to subsea production. Being able to develop a field and then proceeding to ensure flow for the life of the field comprises many situations where the production equipment can fail and falter or through external factors, be deemed unavailable. Some of the areas where most of the current developments in subsea production are being seen are in subsea processing, flow assurance, long term well monitoring and intervention technologies – areas that pose some of the biggest challenges to smooth operation in the deepwater environment. This research highlights the challenges to overcome in subsea production and well systems and details the advances in technology to mitigate those problems. The emphasis for this part of the research is on multiphase pumping, subsea processing, flow assurance, sustained casing pressure problems and well intervention. Furthermore, most operators realize a reduced ultimate recovery from subsea reservoirs owing to the higher backpressure imposed by longer flowlines and taller risers. This study investigates the reasons for this by developing a global energy balance and detailing measures to improve production rates and ultimate recoveries. The conclusions from this energy balance are validated by simulating a deepwater field under various subsea production scenarios.

The use of moonpools for the launch and retrieval of diving bells and remotely operated vehicles from a mother ship is well established, and the advantages of the moonpool approach are well known. The use of moonpools in Floating Production Systems is also becoming more common, as a means of allowing the marine riser to enter the vessel. The moonpool offers protection from wind and current forces and reduces the effects of angular motions of the vessel. The only problem with the moonpool concept is that the water column inside a badly designed moonpool may suffer from large and apparently unpredictable vertical surges, making operations through the moonpool hazardous, and in extreme cases threatening the seaworthiness of the vessel. The work described here shows how a moonpool design may be optimised for a particular vessel in order that such problems may be avoided. The dynamics of the problem are established, such that the water column oscillation and the forces on a subsea unit in the moonpool may be predicted. The effects of a variety of geometrical configurations are then studied, both mathematically and experimentally, in order to select configurations which allow large modifications of the moonpool response. A quantitative measure of moonpool performance is proposed, allowing individual designs to be ranked in terms of the long term expected downtime due to the moonpool. Finally, the understanding and results thus gained are integrated in order to produce a practical design procedure for a moonpool of any size, in any vessel, and in any operational area. A worked example based on a real design problem is presented in order to illustrate the practical application of the method.

The thesis deals with systems consisting of marine cables and subsea units. Such systems have wide applications in offshore subsea operations. After a general introduction, the thesis sets out to analyse both the static and dynamic behaviours of the system under various environmental and operational conditions. It endeavours to pursue a fundamental approach in order to reveal the basic characteristics of the system, in addition to developing numerical algorithms for predicting performance. The analysis of behaviour of marine cables consists of the following parts: Statics A semi-analytic approach is developed to predict the equilibrium configurations of marine cables. One-dimensional dynamics Using a coordinate transformation, the method can predict the unsteady dynamic behaviour of systems where the length of cable varies. Two-dimensional dynamics The methodology adopted in the one-dimension analysis is extended to a more general case. Three-dimensional dynamics An alternative approach based upon a lumped mass model is developed. Mathematical analysis reveals many interesting characteristics of the model. By applying modern control theory, a novel heave compensation mechanism is developed for marine systems of cables and subsea units. This mechanism involves an actively controlled winch system. A framework of optimal stochastic control is outlined for integrating all the elements of surface supported subsea operations. The thesis presents a variety of numerical examples in domenstrating the validity of the approaches adopted, along with discussions. Further developments are also recommended.

Well control in drilling operations is priority to personnel safety. Detection of kicks, or the unscheduled entry of formation fluids into the wellbore, is vital to well control. It has been determined that return flow rate is the parameter most sensitive to detecting kicks and lost circulation. One kick detection method associated with this parameter is delta flow early kick detection or simply the delta flow method. This method has limitations on floating vessels. Inaccurate readings can occur due to the heave motion of a vessel. This is a result of the sensor being downstream of the compensatory slip joint. Expansion and compression of this joint can result in return flow readings that are not representative of the actual value. Inaccurate readings could create situations in which a false kick or false lost circulation is detected. Other inaccurate readings could result in an actual kick or lost circulation situation not being detected. In the past, work has been done to address this by developing a sensor that adjusts for heave. This work supports a project aimed at removing the need for motion compensation by relocating the sensor to a location independent of this motion. A company is currently developing a delta flow early kick detection sensor to be placed at or near the seafloor. The stationary location of this sensor aims to remove the inaccuracy caused by slip joint compensation of vessel movement. This work will consist of a parametric study on the relationship of various drilling system and kick parameters at the seafloor using a well control simulator. The goal is to understand these relationships and determine the delta flow accuracy required based on a given kick size. As a result, this study found that a sensor capable of detecting a 10 barrel kick would require an accuracy of 2.4% and a 20 barrel kick would require a 4.6% accuracy for detection. This case was a shallow water, low kick intensity scenario. This accuracy and the others reported for the drilling and kick parameter ranges provide the boundaries for a well control sensor to be placed at the seafloor.

No description available.

This thesis describes novel methods to localise and estimate the burial depth of subsea cables. The limitation of the current methods is that they can measure the distance to the cable only in a small vicinity, usually up to three metres. The enhancements of the methods were investigated. The output from Neural Network algorithms was tested for its ability to enlarge the detection range and increase its precision. Following traditional methods, a new approach based on system identification and modelling was inspected. Various models were proposed and enhanced with Kalman filtering for linear models and unscented Kalman filtering for nonlinear models. In the case of subsea cable tracking, Kalman filtering requires precise knowledge of the system dynamics and associated stochastic processes. These requirements are often difficult to satisfy. To overcome this limitation this thesis proposes a novel and effective algorithm based on particle filtering. The proposed novel approach uses the whole set of sample points collected from the surface above the subsea cable. The algorithm based on a batch of samples allows to eliminate effectively the noise of the readings and estimate the position of the cable from larger distances than the current methods can do. The novel batch particle filter was implemented in different applications. Depending on the survey requirements and set-up, the method can be used on a single survey line or applied to the area covered by the survey and estimate the three-dimensional section of the cable. The algorithm was tested in a simulation of tracking by an autonomous surface vehicle. Finally, the market analysis for commercialisation of the method was conducted and a new prototype was proposed. The batch particle filter was tested on experimental data collected in different locations. The results demonstrate that the method is both practical and feasible and can successfully estimate the position of the subsea cable in shallow water.

No description available.

No description available.

No description available.

The number of subsea production systems placed in deepwater locations in the Gulf of Mexico (GOM) has increased significantly in the last ten to fifteen years. Currently, API-RP2A (2000 a,b) designates the GOM as a low seismic zone, and thus does not require seismic effects to be considered during the design process. However, there have been a number of seismic events with Richter magnitudes between 3.0 and 4.9 that have occurred in this region. As a result, questions have been raised regarding the seismic performance of deepwater subsea systems. This thesis presents an analytical parametric study where a prototype subsea structure was selected based on a survey of subsea systems. The baseline analytical model consisted of a single casing embedded in soft clay soils, which supported a lumped mass at a cantilevered height above the soil. A number of the model characteristics were varied in the parametric study to simulate the structural response of a range of subsea structures. This thesis discusses the impact of API-RP2A Zone 1 and 2 design seismic demands for the performance of subsea structures. The results from the subsequent analyses show that the stresses and deflections produced by the Zone 1 and 2 peak ground accelerations fall within the allowable limits.