The paper presents the methodology adopted for the challenging geotechnical design of suction anchors with small aspect ratio of a FPSO installed in May/June 2016 in deepwater Gulf of Guinea. The design needed to cope with anomalous soil conditions characterized by the presence of shallow hard grounds in soft normally consolidated clay deposits. Pile length was limited to mitigate the risk of penetration refusal due to the presence of hard grounds. This required a large pile diameter leading to a small length to diameter ratio (6.5 m diameter with 10.5 m penetration) which is unusual if compared to typical aspect ratios adopted in deepwater environments.
The project site is located in Angolan Deep Offshore, approximately 350 km north-northwest of Luanda and 130 km west of Soyo. Water depth around the development area ranges between 500 m and 600 m. The development consists of a Subsea Production System (SPS) tied back through Subsea Umbilical, Flow Lines and Riser systems (SURF) to a Floating Production Storage and Offloading (FPSO) unit containing the process, storage and offloading facilities. The FPSO is located in about 450 m water depth and is anchored to the seabed by 9 anchors arranged in 3 clusters of 3 lines each, with an angle of approximately 120° between each cluster and approximately 5° between each leg of the individual clusters. The FPSO has a Design Operating Life of 20 years.
The subject of this paper is the geotechnical design of the FPSO suction anchors. The geotechnical design of the anchors meets the requirements of API-RP-2SK (2005). The analyses include advanced laboratory testing interpretation, buried chain configuration, optimization of padeye position, holding capacity calculation, installation and removal analyses and soil reactions for structural design.
The holding capacity is assessed first using CAISSON_VHM program (Kay, 2010; Palix et al., 2010) considering the effect of installation tolerances (tilt and misorientation). The ultimate vertical and horizontal holding capacities are then verified by means of 3D PLAXIS finite element analyses confirming the optimal padeye position under design inclined loading. Soil reactions against the anchor wall for the operational case are derived from the 3D numerical analyses and approximated with a combination of linear trends of shear and normal stresses in equilibrium with the external loads. Finally, the study presents the monitored installation data with an interesting comparison between the predicted and the recorded field results.
Sassi, Khalida (SBM Offshore) | Zehzouh, Slimane (SBM Offshore) | Blanc, Matthieu (IFSTTAR) | Thorel, Luc (IFSTTAR) | Cathie, David (Cathie Associates) | Puech, Alain (Fugro Geoconsulting France) | Colliat-Dangus, Jean-Louis (Total)
Seabed trenching, related to large motions of relatively taut deepwater mooring systems in the long Gulf of Guinea swell, can reach the suction anchor and the chain padeye depth, thus raising serious concerns about the holding capacity of the suction anchors and the safety of the mooring systems. The potential reduction of the anchor capacity, estimated by means of 3D finite element (FE) calculations, needs to be verified by means of testing, and this is the objective of the study presented here. The paper describes the IFSTTAR geotechnical centrifuge test facility and the model soil preparation procedure for obtaining large open trenches with natural deepwater Angola clay in the swinging centrifuge. The scope of the study includes a number of holding capacity tests on suction anchors without trenches (reference cases) and with large trenches reaching the suction anchor and padeye depth. Tests were performed under vertical and inclined loading (actual case of taut deepwater moorings with significant vertical loading component), with the measurement of pore pressures at different levels below the anchor tip for verifying the effect of the trench on the reverse end bearing (passive suction) capacity. The centrifuge test results are compared with Plaxis 3D FE calculations for the purpose of comparison with previously published results. The preliminary results presented show that the REB capacity is mobilized, even with large trenches reaching the anchor and padeye depth. It is also confirmed that Plaxis 3D FE modelling offers a reliable tool for the assessment of the post-trench holding capacity of the suction anchors.
It is now recognized that seabed trenching in taut deep water moorings can reach the suction anchor and the chain padeye depth, with the safety of the mooring system becoming a concern. However, the actual formation of such trenches and their development with time are still largely unknown, as well as the relation with the characteristics and dynamics of the mooring systems. The analysis of a database of subsea inspection data from deep water Gulf of Guinea mooring systems is presented. The trench data, from in-service subsea inspections of mooring systems for five FPSO vessels and off-loading terminal buoys, comprise the measurement of trenches width and depth and of anchor-to-trench distances. Because similar metocean and soil conditions apply to the ten mooring systems, the study offers a unique database for better understanding the formation and evolution of this seabed trenching. The subsea inspections were carried out between 2014 and 2017 but the study covers a much larger time span of moorings performance as the FPSOs and off-loading buoys were installed between 17 years and 5 years ago. The trench development is studied in relation with the moorings characteristics and dynamics and is illustrated with typical features and trench geometries for the two types of floaters. The results obtained represent a key improvement for understanding this seabed trenching phenomenon and are most probably applicable to all deep water moorings of the Gulf of Guinea area. Preliminary recommendations about design aspects and possible mitigations that can be considered for limiting the seabed trenching are also addressed in the paper.
In Deep Water (DW) turbidite reservoirs in the Gulf of Guinea (GoG), waterflooding is deployed to maintain reservoir pressure and improve hydrocarbon recovery. The overall recovery from a reservoir under waterflooding is the product of displacement efficiency (DE), which is a function of remaining oil saturation (ROS) of the swept region, the vertical efficiency, and the pattern efficiency. Analysis of open hole logs from recent infill wells in the Eko field that penetrated swept intervals provided useful insight into in-situ ROS values. The found ROS of between 0.05 to 0.11 were significantly lower than the 0.2 observed from core. The resulting DE of 87% estimated on the basis of the ROS data across these swept intervals has the potential to significantly improve economic robustness of some DW projects if proven correct.
Though substantial hydrocarbon resource volumes are believed to be located within Inter Channel Thin Bed (ICTB) facies in deep-water (DW) reservoirs in the gulf of Guinea, there seems to be no dedicated effort to harness them. This is partly due to the challenges associated with how to identify, accurately characterise and ascertain producibility from ICTBs. In this paper, a workflow to identify and characterise ICTB is presented along with a review of literature that demonstrates ICTBs can be prolific producers.
Increasingly, development projects are managing biodiversity and ecosystem service risk through the application of IFC's Performance Standard 6. Central to PS6 is the identification of Critical Habitat, which in turn requires the definition of one or more Discrete Management Units. Guidance for this is limited, and DMU delineation can be especially challenging for linear infrastructure, which may have a small direct footprint but can run for long distances across many ecological zones. Options for DMU definition for linear infrastructure include A single large DMU including contiguous habitat or the administrative unit fully including the pipeline. This simple approach may be appropriate when infrastructure runs across a single landscape containing migratory or nomadic wildlife. Elsewhere, the resulting DMU may be unfeasibly large (including an entire country/province or several ecoregions and thus not helpful for mitigation planning. Defining a buffer of fixed or variable width around linear infrastructure. This is also a simple approach but usually lacks the ecological and/or management rationale required by PS6. However, this may be the only feasible approach in some circumstances, e.g. the open ocean. Buffer size should consider the infrastructure's likely area of influence, including indirect and cumulative impacts. This will be larger than the simple footprint. Defining multiple DMUs, where a buffer around the linear infrastructure intersects with coherent ecological or management units of recognised high biodiversity value. This approach allows some degree of spatial differentiation of areas that are or are not Critical Habitat, assuming that ecological or management units at a suitable spatial scale can be identified. Multiple DMUs can be combined with a ‘simple buffer’ approach for a more comprehensive area of assessment.
A single large DMU including contiguous habitat or the administrative unit fully including the pipeline. This simple approach may be appropriate when infrastructure runs across a single landscape containing migratory or nomadic wildlife. Elsewhere, the resulting DMU may be unfeasibly large (including an entire country/province or several ecoregions and thus not helpful for mitigation planning.
Defining a buffer of fixed or variable width around linear infrastructure. This is also a simple approach but usually lacks the ecological and/or management rationale required by PS6. However, this may be the only feasible approach in some circumstances, e.g. the open ocean. Buffer size should consider the infrastructure's likely area of influence, including indirect and cumulative impacts. This will be larger than the simple footprint.
Defining multiple DMUs, where a buffer around the linear infrastructure intersects with coherent ecological or management units of recognised high biodiversity value. This approach allows some degree of spatial differentiation of areas that are or are not Critical Habitat, assuming that ecological or management units at a suitable spatial scale can be identified. Multiple DMUs can be combined with a ‘simple buffer’ approach for a more comprehensive area of assessment.
DMUs specific to one or more biodiversity features are a cross-cutting approach that can be used with any of the above. This may be effective if features can easily be grouped at different spatial scales (e.g. wide-ranging mammals vs. plants confined to a particular geology). Greater complexity and possible difficulty of communication needs to be weighed against the greater specificity that can be achieved.
These approaches are illustrated with an example from Simandou, Guinea, for a 640 km long rail corridor. This uses a simple buffer (20 km each side) to identify multiple intersecting DMUs for Critical Habitat determination.
There is no one right way to define DMUs for linear infrastructure. Methods should apply ‘common sense’ taking into account the project context and bear in mind the overall aim, to allow Critical Habitat determination at an appropriate ecological/management scale. A precautionary approach is preferable, for instance to ensure buffers are wide enough to encompass potential impacts and through collective rather than individual assessment of DMUs against PS6 threshold.
Efficient removal of the drilling fluid from tubulars, screens, and near-wellbore mineral surfaces is essential to the successful completion of wells. Cleaning and removal of the non-aqueous drilling fluid (NADF) from all of the well surfaces is not an easy task. Microemulsion fluids provide a highly-successful solution to this problem.
Carefully designed and customized microemulsion fluids have been used to remediate and increase well productivity and injectivity in wells located in the Gulf of Guinea. Due to their uniform wellbore cleanup, optimum results were achieved in open-hole horizontal and highly-deviated wells. The robust microemulsion fluid systems are capable of accommodating changes in temperature, density, and salinity. They restore the water-wettability of the rock and increase injectivity or productivity of the wells.
Some water injectors and oil producers from the Gulf of Guinea exhibited low injection or production rate. The purpose of pumping the treatment downhole was to improve injectivity by removing skin damage and screen blockage by using in-situ generated microemulsions, thus allowing efficient oil production from the formation or water injection at sustainable high-injection rates into the formation.
Field data shows that injectivity and productivity increased after treatment. In some cases, the downhole pressure decreased for the injectors. Cleanup and removal of the NADF from the Gulf of Guinea wells was beyond the capabilities of conventional treatments. Laboratory tests confirmed that the in-situ microemulsion treatment removes the damage and helps to achieve the desired and predicted injection and production rates. This paper presents field data, before and after the successful microemulsion fluid treatment, and describes various laboratory tests performed prior to the Gulf of Guinea field applications.
As a major operator of subsea wells, the technological risks associated with the production phase of deepwater wells was a focus of efforts in the post-Macondo era. Shortly after this catastrophic incident, Total launched a number of in-house initiatives and in parallel joined several industry projects. Among its own, in-house initiatives, Total created the Subsea Emergency Response System Project (SERS), in order to develop and supply tools for use if an intervention was required in response to an uncontrolled hydrocarbon leak on a Total production subsea well. The SERS Project has now progressed significantly and is about to deliver its first set of tools to its operating affiliates in the Gulf of Guinea. This set of tools can be split into two systems:
The SERS Project is considered to be a significant advancement of the response capability for Totals affiliates. The design, fabrication and placement of a capping and diversion response in the Gulf of Guinea region, where Total has its greatest concentration of subsea wells, is a major improvement of Total’s capability to intervene should a subsea problem arise. The new concept of a capping and diversion system adapted directly to the constraints of a blowout on a subsea production well is an industry first. The flexibility inherent in the design provides a solution that is adaptable to various types of well kill, horizontal or vertical Xmas trees, compatible with numerous installation workover control systems and is even adaptable to a well control incident on a drilling BOP.
This paper describes these two systems and the deployment strategy developed by Total to minimize the consequences of a subsea hydrocarbon leak during the production phase.
Total is a major operator of deepwater subsea wells worldwide and particularly in the Gulf of Guinea region. Total operates around 250 subsea wells in 2014, a number which will rise to around 1000 in 10 years time with the arrival of numerous deepwater projects. In this respect, Total has an exposure to well control events not only linked to the drilling of these subsea wells, a period which lasts just a few weeks, but also to the production phase which clearly lasts for many years.