OUT TO SEA Offshore Geotechnical Investigation GeoDrilling International Magazine Reliable, detailed and high-quality geotechnical data is needed for the design of structures both onshore and offshore. GeoDrilling International talks to geotechnical consultants and service providers about the challenges and requirements of offshore site investigations Geotechnical site investigations are a prerequisite for most structures placed on or within the seabed. The results of these offshore or near-shore investigations help inform efficient planning and design of foundations and infrastructure, as well as controlling installation risks. Traditionally, the service is used in the oil-and-gas industry, but in recent years the wind-energy sector has become increasingly active. Site investigations are also required for marine construction projects, including ports and harbours, as well as applications such as mineral prospecting and scientific research. Gardline Geosciences Roi Santos, head of geoconsultancy, and Roy Impey, technical director, explain: These developments require soil parameters to evaluate the interaction between structures and seabed in order to plan and design the installation, the operability and the removal of infrastructure. These activities would include calculating bearing capacity and sliding resistance, calculating long and short-term settlements, establishing behaviour of the foundations due to wave loading, and understanding the foundation behaviour to cyclic loading due to earthquake activity. More recently, there has also been a need for offshore investigations associated with nuclear energy projects. Nuclear plants are known to require a substantial supply of cooling water, hence their location on the coast or on the banks of rivers or lakes. There is an obvious need for investigation for the design of infrastructure built on or below the seabed, including outfall tunnels, pipelines, cable trenches and jetties. Of more significance, however, is the need for overall site characterisation encompassing an area extending several kilometres inland and out to sea. Rod Eddies of Fugro explains: The shallow transition zone is a particularly challenging environment and we have invested significant effort into plugging the data gap between land and water with a combination of intrusive and geophysical technologies.
The process of seismic risk and geohazard evaluation requires a robust ground model based on detailed and reliable understanding of stratigraphy and structure to evaluate risks such as liquefaction. To achieve this, companies such as Fugro combine land and marine surveys, as well as drilling and associated downhole testing with surface geophysical methods such as seismic reflection. Gareth Ellery of Cathie Associates adds: Of course, there are instances where geotechnical data is not acquired, such as for temporary works or wet storage at seabed. In these cases, as consultants, we often have to work with relic data or public domain information, which is not sitespecific. The risks in these cases are consequently higher. OFFSHORE ENVIRONMENT There are numerous differences between offshore and onshore geotechnical works, starting with the obvious the presence of water and its influence. Another given is the potential for challenging weather conditions offshore, which will undoubtedly affect the investigation and installation processes. When I first moved to the offshore industry over 10 years ago, the contrast with onshore was stark, Ellery comments. Everything is at least 10 times more expensive and the complexities of dealing with the harsh marine environment are demanding. Offshore site investigation consequently requires a great deal more planning and a real bespoke focus on the design of the survey campaign to ensure that the correct type, quantity and quality of data is acquired. In addition, contractors have to deal with the remoteness of work sites and the challenging soil types encountered offshore. Production rates are affected by tides, currents, sea state and wind speed. In addition, offshore investigations need to be more self-sufficient as vessels are not easily accessible when onsite. It means that any necessary repairs must be completed offshore, stock levels of spare parts must be carefully monitored, and crew members must be extra vigilant of their health and wellbeing, says Brian Bell, offshore geotechnical business unit manager at Fugro Geoservices. Santos and Impey add: The remoteness and environment of the sites mean that geotechnical teams will be permanently mobilised on the vessel and, given the costs associated, it is often more effective to operate 24/7, having two teams working on 12-hour shifts.
Generally, a geotechnical engineer and a laboratory technician work on each shift. Their main function is to evaluate sample and testing quality and provide feedback to the drilling team to adjust the drilling settings, maximising quality and data coverage. Soil conditions, in turn, can become trickier the further you move away from the shore. Santos and Impey explain that there is less energy available in the environment to transport sediment, therefore soft soils are generally encountered as water depth increases. Safe, efficient and economical foundation solutions in soft soils are quite challenging and require accurate determination of geotechnical parameters. Therefore, geotechnical investigations of soft soils require special sampling and testing techniques in order to preserve the soil integrity and obtain representative geotechnical results. Quantification and documentation of sample quality and reporting on geotechnical results uncertainty is fundamental for the design team, they say. THE SET-UP When it comes to the vessels or platforms and equipment used on offshore investigations, a primary consideration is naturally water depth. Bell expounds: Set-ups depend on the scale and complexity of the project along with the operational environment. Jack-up platforms are deployed in water depths of 0-40m and vessels in water depths of 15-3,000m or more. Ellery comments: On shallow continental-shelf seas, anchored vessels may be suitable but dynamically positioned vessels can be substantially more efficient and weather-robust. The selection of the vessel is obviously project-specific and dependent on the local supply chain and availability. Typical set-ups comprise drill-string deployment through a centrally positioned moonpool, although there is a great deal of variability in the market from cantilevered drill decks on the side of vessels of opportunity to seabed-based, remotely operated drilling systems, Ellery adds. The set-up typically also includes equipment for testing and sampling different soil and rock types. This could include cone-penetration testing (CPT) tools, push/hammer samplers and coring equipment. Fugro states that it can also complement this with other geotechnical and geophysical tools such as high pressure dilatometer (HPD) tests and wireline logging spreads. Vessels are routinely equipped with laboratories to log and test soil samples as well as provide quality control (QC) of results in the field, says Bell.
According to Gardline s experts, most of their offshore geotechnical drilling is performed using rotary drilling techniques, and soil samples are acquired using a downhole hydraulically driven tool that latches at the end of the drill string and pushes a sampler at constant speed. In-situ testing is performed with a similar downhole tool. There are a variety of push samplers designed for specific soil types aiming to maximise the recovery and the quality (piston sampler; thin- and thick-wall Shelby tubes with or without core catchers). For over-consolidated clays and rock triple-barrel coring systems are generally used. Any of these sampling and in-situ testing techniques can be swapped as the borehole progresses, so sample and in-situ data could be collected at any prescribed depth, say Santos and Impey. The drill rigs used in offshore drilling also require a heave compensation system, which aims to keep the drill head stationary by detaching the drill string from vessel motions and preventing disturbance to the ground being sampled or tested. A vessel capable of offshore geotechnical investigations will typically have a centrally mounted heave-compensated drilling derrick with a seabed re-entry frame (SBF), which also provides the reaction force for the downhole CPTU and sampling tools, they explain. Ellery inserts: Heave compensated API drilling spreads have been consistently present for years in the offshore industry and have proven to be a robust platform for the deployment of a range of investigation techniques. In shallower waters, particularly in the renewables industry, triple tube coring systems have become very popular as API drilling strings are not well suited to obtaining high-quality data in rock. Beyond these conventional, heave-compensated, surface controlled drilling techniques, remotely operated seabed drilling systems have become widely available. In deep water, these drilling systems can be 3-5 times more efficient than conventional drilling techniques. DEVELOPMENT AND INNOVATION In the offshore geotechnical sector there is an ever-increasing focus on quality and safety, as well as increased automation and reduced manpower. Among other things, automatic pipe-handling systems have come a long way over the years, as well as the monitoring of drilling equipment during operation, with rigs typically installed with measurement- while-drilling (MWD) sensors and logging equipment.
According to Fugro, one of its strengths is the ability to complement its permanent geotechnical fleet with mobile equipment that can be transported anywhere in the world for rapid mobilisation onto vessels of opportunity for individual projects or lengthy campaigns. Bell also states: Regarding tooling, the development of piggyback coring has allowed improved sample recovery and quality, while the introduction of coiled tubing for CPT operations has vastly improved production rates. For non-geotechnical applications, the introduction of large-diameter drilling using offshore vessels shows great potential by overcoming the water-depth constraints faced when using jack-up platforms. This allows construction-related drilling in much deeper water than was previously possible from a jack-up. When it comes to wind turbines, which are generally relatively light, tall and slim structures, the stiffness of foundations and cyclic behaviour of the soils become very important parameters for safe and affordable designs. Therefore, effort has been made on adapting existing traditional onshore in-situ tools to measure stiffness. Small strain measurements are generally gathered using P-S logging or seismic CPTU. Large strain measurements are usually acquired using pressure meters. Advanced laboratory programmes are conducted on undisturbed samples to study the behaviour of the soils under cyclic loading; stiffness degradation and degradation of strength must be accurately assessed, say Santos and Impey. Ellery comments: In an offshore investigation, almost all of the cost is in mobilising the vessel and drilling the borehole to retrieve soil and rock samples and to undertake CPTs. It makes sense, therefore, to gain as much information as possible from the completed borehole. Accordingly, we have seen considerable innovation and development in tools and techniques for in-situ testing. These techniques offer enhanced value from the borehole but also deliver data that is less affected by the drilling and sampling process, which causes sample disturbance. Techniques include P-S logging, high resolution acoustic televiewer logging, dilatometers and pressure meters, and adaptation to the CPT method, for example seismic CPT, T-bar and ball penetrometers. Data collection and transfer is another focus area for Fugro. Today s clients require data to be delivered faster and in more detail than ever before. The introduction of real-time monitoring of production rates and transfer/interpretation of results combined with more integrated communication means more flexible and responsive fieldworks. In this respect, offshore is no longer remote, says Bell.
As a consultancy, Cathie Associates clarifies, its research and development efforts are focused around new geotechnical design methodologies, novel foundation structures or installation methodologies, and ways of maximising the value from site-investigation data. We see GIS [geographic information systems], both on the investigation contractor and on the consultant side, as offering significant value in the delivery, presentation and interrogation of data. Certainly, in recent times, we have been very vocal in the industry about the use of advanced GIS-based ground models to drive project processes and engineering investigation data is a critical input to this. We recently authored the new MEDIN [Marine Environmental Data and Information Network] geotechnical standard in the UK, which will ensure that reported geotechnical data is in a format to be used easily in GIS with the correct metadata, explains Ellery. Focus: Wind The offshore wind industry has seen an interesting evolution of site-investigation techniques, explains Cathie Associates Gareth Ellery. This industry has experienced a transition from basic, relatively low-tech approaches to investigation to more advanced approaches, initially from jackups nearshore but then transitioning to dynamically positioned vessels offshore. A typical offshore wind-farm investigation now requires a flexible approach with more than one phase of data acquisition to limit early project spend, but still to provide sufficient data for engineering. DNV J101 is the most commonly adopted design guidance for this industry and requires a form of geotechnical investigation at the location of each structure. At preliminary stages, the structure locations aren t known, and so consultancies design investigations to address conditions anticipated by the desktop and geohazard studies and likely foundation solutions. At a later, detailed stage, data is needed for foundation design. Boreholes are drilled, typically with alternating downhole CPT and sampling, to depths often in excess of 50m below seabed. The primary downhole programme is accompanied by targeted P-S logging and seismic CPTs to understand the in-situ small strain behaviour of soils. Pressure meters or dilatometers, which have only just been adopted by heave compensated drilling contractors (2013), are used to determine the lateral stiffness and strength and also realistic
K0 values for foundation design. If rock is encountered, high-quality triple-tube coring is undertaken, frequently from a separate vessel and drill spread, with the addition of acoustic televiewer logging to understand the fabric of the rock mass for foundation design. Case Study: Wind Farm In 2015 the initial reconnaissance geotechnical survey was completed for one of the first wind farms off the west coast of the US, and the results were integrated with the geophysical results into a 3-D geomodel. The aim of the survey was to establish typical geotechnical profiles for each geological province highlighted by the geophysical interpretation to add maximum value to the overall seabed risk-management process. The geotechnical data is to be correlated with the geophysical data to confirm the findings of the engineering geological desk study, and determine the vertical and lateral variation in soil conditions. This will allow the initial geomodel for the site to be refined and aid with the engineering feasibility studies and the selection of the adequate foundation type. Seven composite boreholes across the wind-farm site, combining sampling, CPTU testing and P-S logging, were completed down to maximal depths of 70m below seabed. An advanced laboratory programme was designed and conducted at Gardline s facilities in Great Yarmouth, UK. The aims of the laboratory programme were: Validation and calibration of in-situ testing results; Enhancing the understanding of ground complexity; Evaluation of stress history; Characterise soil stress anisotropy (triaxial CAU compression/extension and DSS); Characterise cyclic behaviour (CAUcy and DSScy); Characterise small strain behaviour (bender element on triaxial cells); and Evaluation of sample quality