Subsea Structures Research Articles

Motion Planning and Tracking of Subsea Structures

This article uses an active control dedicated to the positioning of subsea structures like flow lines. This kind of operation consists in connecting the bottom end of a very long pipeline to the wellhead, by dynamically modifying the pipeline top end position, which is linked to a Dynamically Positioned Vessel (DPV). Such long pipelines are usually called risers, because they are used to rise the drilling mud or the hydrocarbons from the wellhead to the platform. Nowadays this operation is often done manually. The use of an active control intends to reduce the operation time, and to make it possible even under bad weather conditions. The considered subsea structure can be aproximated as a cable submerged in a flow and modelled by the Bernoulli cable equation, completed with a damping factor, that linearly depends on the structure speed. This article tests previous works regarding the tracking system used to follow the reference trajectory of the motion planning considering the Euler-Bernoulli beam equation for large rotations, that is the most used model to define the dynamic behavior of this kind of structures.

Stability of offshore structures in shallow water depth

The worldwide demand for energy, and in particular fossil fuels, keeps pushing the boundaries of offshoreengineering. Oil and gas majors are conducting their exploration and production activities in remotelocations and water depths exceeding 3000 meters. Such challenging conditions call for enhancedengineering techniques to cope with the risks of collapse, fatigue and pressure containment.On the other hand, offshore structures in shallow water depth (up to 100 meter) require a different anddedicated approach. Such structures are less prone to unstable collapse, but are often subjected to higherflow velocities, induced by both tides and waves. In this paper, numerical tools and utilities to study thestability of offshore structures in shallow water depth are reviewed, and three case studies are provided.First, the Coupled Eulerian Lagrangian (CEL) approach is demonstrated to combine the effects of fluid flowon the structural response of offshore structures. This approach is used to predict fluid flow aroundsubmersible platforms and jack-up rigs.Then, a Computational Fluid Dynamics (CFD) analysis is performed to calculate the turbulent Von Karmanstreet in the wake of subsea structures. At higher Reynolds numbers, this turbulent flow can give rise tovortex shedding and hence cyclic loading. Fluid structure interaction is applied to investigate the dynamicsof submarine risers, and evaluate the susceptibility of vortex induced vibrations.As a third case study, a hydrodynamic analysis is conducted to assess the combined effects of steadycurrent and oscillatory wave-induced flow on submerged structures. At the end of this paper, such ananalysis is performed to calculate drag, lift and inertia forces on partially buried subsea pipelines.

Numerical modelling of ice ridge keel action on subsea structures

Abstract The present paper describes a numerical simulation of the keel–structure interaction where the keel geometry, ice thickness and interaction speed present a scaling ratio of 1:20. The rubble is represented by a Drucker–Prager material with cohesive softening and no dilatation. The partial differential equations are solved with the non-linear Eulerian finite element method of Abaqus Explicit V6.8.2. The results are compared to physical experiments conducted in the ice basin of the Hamburg ship model basin (HSVA). Two identical cubic subsea structures were impacted into the unconsolidated keel portion of two ice ridges with different thermal properties. The results indicate that only slight dilatation occurs, and the models are able to estimate the rubble action and deformed shape. A progressive failure of the rubble occurs. The rubble action is influenced more by the friction angle than it is in punch tests, due to higher confinement at the failure surface.

Geohazard identification: the gap between the possible and reality in geophysical surveys for the engineering industry

Offshore Contractors engaged in the business of installing pipelines and subsea structures routinely rely on third party survey data to identify and assess the risk of surface and buried geohazards. In many cases, route or site survey data are supplied to a marine Contractor with the latter having little or no input to the manner in which the geophysical and geotechnical data are acquired, processed and reported. Often, the completed survey reports indicate that neither data acquisition nor the subsequent data processing were carried out with the ultimate goal of the survey or the end user in mind. This lack of focus at the survey stage can contribute to incorrect or over-optimistic assessments of geohazard risk, with serious impact on offshore installation operations as a consequence. Two case studies from the North Sea are discussed where inappropriate data processing and reporting of geophysical data had a negative impact on offshore installation operations; these are used as a basis for a more general discussion of the underlying reasons for the production of engineering survey reports which are “not-fit-for-purpose”.

Active control of a dynamically positioned vessel for the installation of subsea structures

This article presents an active control dedicated to a re-entry problem found in the offshore oil industry. The re-entry operation consists of connecting the bottom end of a very long pipeline to the wellhead, by dynamically modifying the pipeline top-end position, which is linked to a dynamically positioned vessel (DPV). Such long pipelines are usually called risers, because they are used to make the drilling mud or the hydrocarbons rise from the wellhead to the platform. Nowadays, the re-entry operation is often done manually. The use of an active control intends to reduce the operation time and to make it possible even under bad weather conditions. The considered subsea structure can be viewed as a cable submerged in a flow and modelled by the Bernoulli's cable equation, completed with a damping factor, that linearly depends on the structure speed. After some simplifications that are justified in our context, the corresponding model turns out to be differentially flat, a useful property for control design, providing an extension of previous works.

Ontogenetic diet shifts in the reef fish Pseudanthias rubrizonatus from isolated populations on the North-West Shelf of Australia

The red-barred anthias Pseudanthias rubrizonatus is a common tropical deep reef fish species found in Australia, but little is known about its dietary preferences and trophic interactions. We examined the gut contents and stable isotope signatures (δ 13 C and δ 15 N) of P. rubrizonatus from populations on the North-West Shelf of Australia to determine differences in diet relative to site, depth and fish size. We sampled 5 fish populations from a series of sub-sea structures, from 82 to 152 m depth, which had been submerged for up to 15 yr. Gut content analysis suggested that P. rubri- zonatus displays an opportunistic feeding strategy and utilises both pelagic and benthic resources, including larval fishes, heteropods, isopods and mysids. Stable isotope analyses revealed that at all depths P. rubrizonatus underwent an ontogenetic diet shift. Values for δ 13 C in muscle ranged from -19.7‰ for small fish to -16.2‰ for larger individuals, and δ 15 N ranged from 8.2‰ for smaller fish to 13.2‰ for larger fish, indicating that a diet shift occurs at the end of juvenile development between 30 and 50 mm standard length. By simultaneously analysing gut contents and stable isotope signa- tures of the collected specimens, we have documented opportunistic dietary strategies that may assist P. rubrizonatus to colonise isolated structures.

In situ observations of Stygiomedusa gigantea in the Gulf of Mexico with a review of its global distribution and habitat

Four individuals of the large scyphozoan jellyfish Stygiomedusa gigantea were observed in the northern Gulf of Mexico over 2005–2009 by industrial remotely operated vehicles as part of the SERPENT Project. One of these observations included the symbiotic bythitid fish Thalassobathia pelagica. Prior to these observations, neither S. gigantea nor T. pelagica had been observed in, or collected from the Gulf of Mexico. In order to summarize the available information on S. gigantea, we located 110 observations obtained over 110 years (1899–2009) representing 118 individual specimens of this species from around the world. The resulting dataset confirms that this species is cosmopolitan occurring with records from all oceans except the Arctic. While the depth-range of the four Gulf of Mexico specimens was bathypelagic, there appears to be a pattern of S. gigantea occurring in mesopelagic and epipelagic depth-zones at high latitudes, particularly in the Southern Ocean and mesopelagic and bathypelagic depths at mid- and low-latitudes. This pattern may be related to the meridional vertical distribution of temperature or perhaps avoidance of light levels that could degrade porphyrin pigments. There was no evidence that this species migrates vertically. Two of the individuals in the Gulf of Mexico appeared to be actively clinging to subsea structures and we speculate that this is a consequence of its normal mode of feeding, which may entail using its large oral lobes to hold on to, and trap prey.

MA D6: A Fast-Track Deepwater Project

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper OTC 20174, ’MA D6 - Going Beyond the Limits of Deep Water Fast Track Projects,’ by Georges Michel, Willy Gauttier, and Pierre Savy, Technip, originally prepared for the 2009 Offshore Technology Conference, Houston, 4-7 May. The paper has not been peer reviewed. The MA D6 field-development project in 1100 m of water is the first deepwater development on the east coast of India. The MA D6 project has been one of the fastest deepwater projects of its size involving a large amount of technical content in a location with minimal established infrastructure. The full-length paper describes the technical aspects of the project and the resulting designs that were selected, and it outlines the logistical issues associated with this first deepwater and fast-track project in the region. Subsea System Architecture The MA D6 field is in the KG D6 Block 60 km from the east coast of India and is exposed to the eastern monsoon from April until September and to cyclones in the following months. This limits the safe and efficient operations of offshore construction vessels to a window extending from December to April. The occurrence of cyclones led the operator to select a floating production, storage, and offloading (FPSO) vessel equipped with a disconnectable turret buoy that required a riser system designed for both connected- and disconnected-turret-buoy situations. The limited working window caused the project installation to be split into two phases: one before the monsoon season of 2008 to allow first oil on arrival of the FPSO and the second one the following year for the full production system. However, the engineering design of the whole riser system was needed at the early stage of the project to address all the interferences between the various risers during installation and operations and to define all the riser equipment in a pliant-wave configuration (Fig. 1), such as buoyancy modules, riser-anchoring piles, and tethers and clamps. The field production system is based on the production of six wells through a production manifold, the injection of part of the produced gas into the reservoir by a satellite well, and the export of the gas to the future KG D6 gas-development field though a gas-export manifold (GEM). The four 8-in.-inside-diameter (ID) flexible production risers connect the FPSO to the production manifold, the four 8-in.-ID gas-export flexible risers connect the FPSO to the GEM, and the gas-injection well is connected directly to the FPSO with a 6-in.-ID flexible riser/flowline. Each subsea tree is linked to the production manifold with 6-in.-ID flexible flowlines/jumpers, and control of all the subsea structures is achieved by a dynamic-umbilical connection between the FPSO and the production manifold and with static umbilical jumpers connecting the production manifold to the seven subsea trees and the GEM.

State of the art technology for subsea tie-in and repair systems in the North Sea

The new design of subsea structures such as riser bases (RB), pipeline end terminations (PLET) and pipeline end manifolds (PLEM) has resulted in lighter and more installation-friendly solutions. PLETs and PLEMs can be installed together with the pipeline, avoiding separate installation campaigns. The RB is designed to slide on the seabed, hence it can be connected directly to the pipeline, avoiding the need for expansion spools. If dropped object and/or trawl protection is required, a range of standardised GRP protection covers is available for the protection of structures, inline/hot-tap tees and spools. A new remote controlled tie-in system—THOR—has been developed for general use. The concept offers significant cost savings when measured against the existing conventional tie-in tool based systems. The system will be available for all pipeline sizes and will be independent of connector manufacturer.A vertical system for hot-tapping and tie-in onto a hot-tap tee has been developed and installed on the Ormen Lange field. The system includes a contingency isolation plug for recovery of the isolation valve module.Active pipeline supports (APS) have been introduced to the market and are now installed on several projects. This type of structure will enable the adjustment of pipelines during and after installation, and will therefore ensure more flexibility during the design phase. The pipeline repair system developed for the North Sea has been used for 20 years and is definitively field proven through a large number of repair operations and planned work. Many repair components and their associated installation systems have been made and used over these years. Based on this knowledge we can now better recommend, to a new repair market such as Australia, which type of repair items to purchase for the local repair system. This evaluation will also identify the items or components that should be partly prepared, either on a design level only, or up until storage of forged material or completion of particular long lead items.

Scott Bader supply materials of choice for subsea structures

Scott Bader supply materials of choice for subsea structures

Flood member detection for real-time structural health monitoring of sub-sea structures ofoffshore steel oilrigs

A structural flood detection system for real-time health monitoring in the hollow sub-seamembers of new offshore steel oilrigs is presented. Field-proven flood member detectiontechniques, integrated within the concept of health monitoring, offer an alternativeto underwater nondestructive testing methods based on ultrasound and x-rays,which have been used to detect the presence of seawater in these applications,often with diverse or remote operating vehicles. The system employs a singlepiezoelectric transducer which can be permanently attached to the inner wallof every sub-sea structure and which is powered by a normally inert seawaterbattery. Upon activation, the sensor transmits ultrasonic chirp or tone encodedpulses, in the range of 21–42 kHz, to a monitoring receiver system at deck level fordecoding and identifying flooded members. Experiments have been carried outusing a jointed steel pipe structure, 7 m in length, 0.5 m in diameter and 16 mm inthickness. This structure was flooded and completely immersed in seawater. Twoapproaches to the system were considered during the investigation, depending on thecommunication channel exploited; the former utilized guided waves, on the basis ofexploiting the steel structure as a wave-guide; the latter employed underwaterultrasound, based on using the seawater as a propagation medium. Although severelosses were encountered in both approaches, the system effectively identified thesignals above the background noise. This work forms the foundation for the futuredevelopment of a system that can be used with large, commercial offshore platforms.

Continuous structural monitoring of oil rig sub-sea structures for flooded member detection using underwater ultrasound

An underwater ultrasound system for detecting flooding in the hollow sub-sea structures of offshore steel oil rigs is presented. A sensor, attached to a sub-sea structure and powered by seawater, transmits underwater ultrasound chirp -encoded signals to a real-time digital signal processing monitoring system at surface level. Experiments performed using a jointed steel pipe structure I ton in weight, with dimensions of 7 m x 0.5 m x 16 mm, completely immersed in seawater showed excellent performance using a central excitation frequency of 38 kHz, over distances of 100 m.

A new tool for the rapid remote detection of leaks from subsea pipelines during remotely operated vehicle inspections

A new 2000 m depth rated subsea sensor that can effectively, rapidly and remotely detect leaks of fluorescein dye, leak detection chemicals and hydraulic fluids from underwater structures is reported. The system utilizes ultra-bright LED technology to project a structured beam of light, at a wavelength suitable to excite the fluorescence of the target material, into the water column. The resultant fluorescence is collected and digital signal processing used to extract the intensity. The system is capable of detecting ppm concentrations of fluorescein at a range of 2.5 m in water in real time. The ability to stand-off from subsea structures, while rapidly detecting the chemicals makes the system highly suited to subsea leak inspections with remotely operated vehicles or autonomous underwater vehicles, as it allows the vehicles to be flown quickly and safely over the structure to be inspected. This increases both the speed and effectiveness of the inspection. The remote detection capability is also highly effective for probing complex underwater structures. The system has been successfully used in real subsea survey applications and has been found to be effective, user friendly and to dramatically reduce inspection times and hence costs.

Optimal setpoint chasing in dynamic positioning of deep‐water drilling and intervention vessels

Conventional controller designs for dynamic positioning of ships and floating marine structures have so far been based on the principle on automatic positioning in the horizontal-plane about desired position and heading co-ordinates defined by the operator. A three degrees-of-freedom multivariable controller either of linear or nonlinear type, normally with feedback signals from surge, sway and yaw position and velocities, has been regarded as adequate for the control objective. For floating structures with small waterplane area such as semi-submersibles, feedback from roll and pitch angular rotation velocity may also be included to avoid thrust-induced roll and pitch motions that are caused by the hydrodynamic and the geometrical couplings between the horizontal and vertical planes. However, for certain marine operations this control philosophy may not be the most appropriate approach ensuring safety and cost effectiveness. For drilling and work-over operations the main positioning objective is to minimize the bending stresses along the riser and the riser angle magnitudes at the well head on the subsea structure, and at the top joint as well. A positioning control strategy solely based on manual setting of the desired position co-ordinates may not be the most optimal solution for these applications. In this paper a new hybrid dynamic positioning controller, that also accounts for riser angle offsets and bending stresses is proposed. It is shown that a significant reduction in riser angle magnitude can be achieved. Simulations with a drilling semi-submersible demonstrate the effect of the proposed control strategy. Copyright © 2001 John Wiley & Sons, Ltd.

Optimal setpoint chasing in dynamic positioning of deep‐water drilling and intervention vessels

AbstractConventional controller designs for dynamic positioning of ships and floating marine structures have so far been based on the principle on automatic positioning in the horizontal‐plane about desired position and heading co‐ordinates defined by the operator. A three degrees‐of‐freedom multivariable controller either of linear or nonlinear type, normally with feedback signals from surge, sway and yaw position and velocities, has been regarded as adequate for the control objective. For floating structures with small waterplane area such as semi‐submersibles, feedback from roll and pitch angular rotation velocity may also be included to avoid thrust‐induced roll and pitch motions that are caused by the hydrodynamic and the geometrical couplings between the horizontal and vertical planes. However, for certain marine operations this control philosophy may not be the most appropriate approach ensuring safety and cost effectiveness. For drilling and work‐over operations the main positioning objective is to minimize the bending stresses along the riser and the riser angle magnitudes at the well head on the subsea structure, and at the top joint as well. A positioning control strategy solely based on manual setting of the desired position co‐ordinates may not be the most optimal solution for these applications. In this paper a new hybrid dynamic positioning controller, that also accounts for riser angle offsets and bending stresses is proposed. It is shown that a significant reduction in riser angle magnitude can be achieved. Simulations with a drilling semi‐submersible demonstrate the effect of the proposed control strategy. Copyright © 2001 John Wiley & Sons, Ltd.

Modelling and Control of Riser Angles and Stresses in Dynamic Positioning

Conventional controller designs for dynamic positioning of ships and floating marine structures have so far been based on the principle on automatic positioning in the horizontal-plane about some desired position and heading co-ordinates. A three degrees of freedom multivariable controller either of linear or nonlinear type, normally with feedback signals from surge, sway and yaw position and velocities, has been regarded as adequate for the control objective. For semi-submersibles feedback from roll and pitch angular rotation velocity may also be included. However, for certain marine operations this control philosophy may not be the most appropriate approach ensuring a safe and cost effective operation. For drilling and work over operations the main positioning objective is to minimise the bending stresses along the riser and the riser angle magnitudes at the well head on the subsea structure and at the top joint as well. A positioning strategy solely based on position and velocity feedback may not be the most optimal solution for these applications. In this paper a new hybrid dynamic positioning controller, that also accounts for riser angle offsets and bending stresses is proposed. It is shown that a significant reduction in riser angle magnitude can be achieved. Simulations with a drilling semi-submersible demonstrate the effect of the proposed control strategy.

Cognitive factors in the close visual and magnetic particle inspection of welds underwater.

Underwater close visual inspection (CVI) and magnetic particle inspection (MPI) are major components of the commercial diver's job of nondestructive testing and the maintenance of subsea structures. We explored the accuracy of CVI in Experiment 1 and that of MPI in Experiment 2 and observed high error rates (47% and 24%, respectively). Performance was strongly correlated with embedded figures and visual search tests and was unrelated to length of professional diving experience, formal inspection qualification, or age. Cognitive tests of memory for designs, spatial relations, dotted outlines, and block design failed to correlate with performance. Actual or potential applications of this research include more reliable inspection reporting, increased effectiveness from current inspection techniques, and directions for the refinement of subsea inspection equipment.

Numerical simulation of the ‘pull-in’ operation in submarine pipelines

‘Pull-in’ operations consist in the deflection of a pipeline, resting on the seabed, until it is ready to be connected to a subsea oil production structure. This work presents the numerical methods and strategies employed in the development of a computational system for the simulation and analysis of such operations, involving severe geometric nonlinear effects due to large displacements. The following aspects are addressed: representation of the pipeline; anisotropic friction in the soil-pipeline interaction; three-dimensional behaviour, and representation of the chain-buoy assembly in ‘off-bottom’ operations; and the adequate representation of the pull-in cable. The study of the final connection to the subsea structure is also addressed. The quality of the numerical simulation obtained from the application of this computational system is assessed in an illustrative example, where the results are compared with those obtained from an experimental small-scale model test.

ANALYSIS OF THE LATERAL DEFLECTION OF SUBMARINEPIPELINES

Lateral deflection operations consist in the deflection of a pipeline, resting on the seabed, until it is ready to be connected to a subsea oil production structure. This work presents the numerical methods and strategies employed in the development of a computational system for the simulation and analysis of such operations, involving severe geometric nonlinear effects due to large displacements. The following aspects are addressed: Representation of the pipeline; Anisotropic friction in the soil-pipeline interaction; Three- dimensional behaviour, and representation of the chain-buoy assembly in off- bottom operations; and the adequate representation of the pull-in cable. The study of the final connection to the subsea structure is also addressed. The quality of the numerical simulation obtained from the application of this computational system is assessed in an illustrative example, where the results are compared with those obtained from an experimental small-scale model test.

Subsea vehicle path planning using nonlinear programming and constructive solid geometry

Concerns the finding of a collision-free path for an unmanned underwater vehicle (UUV) and manipulator, from an initial to a goal configuration, around subsea structures. The paper concentrates on efficient searching and object representation, while removing local minima. An approach to path planning using a nonlinear programming approach is presented. The free space of the workspace is represented as a set of inequality constraints of a nonlinear programming problem, using vehicle configuration variables. The goal configuration is designed as the unique global minimum point of the objective function. The initial configuration is the start point for the nonlinear search. The numerical algorithm developed for solving the nonlinear programming problem is applied. Every immediate point generated guarantees that it is in the free space and, therefore, is collision free. Mathematical foundations for constructive solid geometry, Boolean operations and approximation techniques are developed and are used to represent the free space of the robot workspace as a set of inequalities. Techniques which guarantees convergence, efficiency and numerical robustness can be applied directly to the problem. Simulation results show its effectiveness, efficiency and potential as an online motion planner.