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Improving Geotechnical Drilling to Reach Seafloor Massive Sulfide Deposits
Rovdrill 3 Completes More Than 100 Dives on Difficult Terrain To Collect Samples for Nautilus Minerals’ Solwara 1 Deposit

AUTHORS:
Allan Spencer
Product Manager, Rovdrill
Allen Ramsey
Operations Manager, Rovdrill
Forum Energy Technologies
Houston, Texas

In 2007 and 2008, Nautilus Minerals Inc. (Toronto, Canada) carried out seafloor mineral exploration drilling and sampling programs at Solwara 1, a planned deep-sea mining project offshore Papua New Guinea, using the ROV-based conventional drilling systems Rovdrill 1 and Rovdrill 2 developed by Perry Slingsby Systems Inc. (Houston, Texas), now a Forum Energy Technologies company. In average water depths of 1,500 to 1,600 meters, these systems drilled up to 18 meters into the seafloor, recovering a number of core samples that were of sufficient quality to assay the mineral content and infer some basic characteristics and boundaries of the deposit field.

First-generation Rovdrill systems and support spreads had certain limitations. During the previous campaigns in 2007 and 2008, a number of holes were terminated prematurely—often considerably short of target depth—due to hole collapse. Steeply sloping terrains at the drill sites and reliability of the drill rig affected long-term productivity.

As a result of these operational constraints, large areas at Solwara 1 remained undrilled. The results from other geophysical investigations and surveys performed previously and concurrently with the drilling programs, particularly electromagnetic surveys, inferred that the depth of the ore body on Solwara 1 could be considerably greater than that indicated by the core sample recoveries.

The Rovdrill 3 drill module being recovered to the vessel on completion of a successful dive.


In 2010, Nautilus Minerals began another drilling program at Solwara 1 and other surrounding deposit fields. The aim was to further establish the minimum boundary for the seafloor massive sulfide (SMS) deposit field on Solwara 1, which had not been completely defined during the 2007 and 2008 campaigns. Nautilus Minerals required a system with improved drilling efficiency and capabilities to achieve a minimum penetration depth of 40 meters below the seabed, and core recovery of at least 75 percent overall, with a target of 90 percent in massive sulfide layers.

On review of the critical functional requirements for the Nautilus Minerals 2010 SMS drilling, Seafloor Geoservices (Houston) determined the attributes and capabilities of the Rovdrill 3 system were close to meeting the Nautilus requirements. In 2010, Nautilus selected Seafloor Geoservices to provide the Rovdrill 3. Forum Energy Technologies, the parent company of both Seafloor Geoservices and Perry Slingsby Systems, sponsored the development and manufacturing of the project’s equipment.


Rovdrill 3 Development
In 2008 and 2009, Perry Slingsby and Seafloor Geoservices developed the Rovdrill 3 in Houston, Texas. The impetus for this development was mainly to build upon the previously field-proven rigs, Rovdrill 1 and 2, but with considerable redesign and enhanced capabilities to address much broader subsea geotechnical investigation applications and markets other than SMS exploration.

Unlike the first-generation Rovdrill 1 and 2, the Rovdrill 3 was not designed and built in response to a specific project requirement but rather as a result of market requirements and a survey of client groups, including potential mineral exploration clients and offshore oil and gas geotechnical investigation clients. To meet Nautilus Minerals’ drilling requirements, the Rovdrill 3 required additional upgrades and modifications, principally to the subsea drill module assembly to address and overcome the limitations experienced with the previous Rovdrill systems.

Due to the extreme environmental conditions prevalent at the target drill sites (e.g., steep gradients, loose unconsolidated formations and dense clusters of hydrothermal chimneys), the primary obstacle was to ensure that the drilling module could be landed and stabilized safely and sufficiently on or as close as possible to the 50-some initial target drill sites identified by Nautilus Minerals geologists. Using previously surveyed bathymetry data, computer-based landing simulation scenarios were developed to help establish if it was possible for the drilling module to access the target drill sites, and if not, how close the module could be deployed. This was done by importing the bathymetry data into Forum Energy Technologies’ VMAX Project Simulator software in the native one-meter pixel data set in .DXF source format. The data were then converted into 3D Studio MAX (now Autodesk 3ds Max), which allowed the development team to run accurate interactive landing simulation scenarios.

This process helped produce a concept design for the drill module landing foundation geometry and dimensions. The resulting four-legged, jack-up-type foundation module assembly interfaced to the base of the drill module and deployed as a complete subsea package.

Having successfully engineered a landing foundation solution for the drilling module, the next task was to stabilize the hole collar in the upper unconsolidated zones to ensure the borings could progress into the firmer underlying mineralized zones. This was achieved using wireline drilling and sampling and deploying multiple hole casing strings in the hole’s upper zone. By utilizing wireline drilling and sampling, the risk of hole collapse was reduced significantly because the outer drill string remained in the hole during inner sampling tube deployment and recovery.

With the casing strings deployed, advanced and finally retrieved at hole termination by the drill module, the unconsolidated upper layers were effectively isolated from the remainder of the hole, eliminating downhole redrilling of collapsed materials from the upper hole that might otherwise contaminate the core samples.

Drilling Fieldwork Summary
In October 2010, the essential modifications and upgrades to the Rovdrill 3 system were completed and commissioned in Singapore. The system mobilized onto the ROV support vessel REM Etive, operated by Fugro-TSM Pty Ltd. (Perth, Australia), where it traveled to the Solwara field off Rabaul, Papua New Guinea.

The system arrived in early November 2010 and completed two weeks of sea trials before commencing the target 75-hole, 100-day drilling campaign.

Being a relatively long-term offshore campaign, a number of specific equipment and core sample handling enhancements were necessary for the vessel. These adjustments included maintaining the integrity of sample chain of custody, maximizing productivity during core extraction and handover operations, and ensuring consistently safe deployment and recovery of the approximately 20-tonne drill module, as well as on-deck core sample handling equipment.

Notably, the back deck area of the vessel was arranged in a process flow manner. From deployment and retrieval of the drill module itself, through removal, processing and hand-over of the core samples, this process carefully modeled risk assessments and human intervention scenarios. To achieve this, the team manufactured a suite of custom- designed core sample handling systems and mobilized them on the vessel’s back deck. These systems included a core sample storage rack extraction and lay-down device, a subtray extraction device, barrel lay-down and transfer tables, barrel make/break and sample extraction tables, inspection, measurement and hand-over tables, and barrel wash-down bath and rebuild tables.


Rovdrill 3 Performance
The fieldwork terminated in April 2011, and the Rovdrill system was demobilized in Singapore in early May, following transit from the field to the demobilization port.

Nautilus Minerals has not yet publically released the campaign results. However, from the drilling solution perspective the following performance indicators and milestones can be declared. The Rovdrill 3 completed more than 3,000 operational hours and 100 dives. The drill module consistently landed and successfully drilled and sampled a large number of target sites with sloping terrains in excess of 30º. The deepest hole drilled with continuous core sampling in Solwara 1 was 52 meters.

The majority of holes were successfully cased in the upper zones using multiple casing strings, all of which were successfully retrieved post-hole termination. The maximum cased depth in a single hole was 21 meters. Also, multiple target holes were drilled in a single dive without the drill module being recovered to deck during vessel relocation transits between hole sites. Finally, the longest recorded continuous duration dive with the Rovdrill 3 drill module was 112.8 hours or 4.7 days.


Conclusions
The drilling and core sampling of SMS in the Solwara deposit fields hold significant challenges in terms of drilling solution selection, deployment and operations. It is evident that the greatest success to date has been through utilizing specialized seafloorbased drilling and sampling rigs such as the Rovdrill systems.

Over the last three campaigns, there has been significant evolution in the drilling equipment, methods, procedures and results, which have allowed the exploration license holders to further extend and delineate the Solwara SMS deposit fields, bringing them closer to realizing the world’s first commercial mining operation of SMS deposits.

There is much more work still to do and a number of enhancements can be made to the seafloor drilling equipment, processes and procedures. Through experiences and lessons learned from the previous campaigns, increasing productivity and improving core recovery will be possible.

This is an exciting time for both seafloor mineral prospecting and the overall development of seafloor-based geotechnical drilling and sampling equipment and techniques. Though there are only a handful of such drilling rigs commercially available worldwide, there are significant developments ongoing and emerging.

The demonstrable success of this technology application in the mineral prospecting field can only help to bring further credibility, sustainability and interest in those seafloor-based technologies to wider applications, including the offshore oil and gas and renewable energy sectors.


References
For a list of references, contact Allan Spencer at allan.spencer@f-e-t.com.



Allan Spencer is a professional engineer at Forum Energy Technologies who has 12 years of experience in the design, project management and offshore operation of subsea remote intervention technologies. He specializes in ROV-based geotechnologies and has held a variety of senior managerial positions in the U.K. and U.S.

Allen Ramsey is a professional engineer at Forum Energy Technologies 10 years of experience in project management and the operation of specialized remotely operated and manned subsea vehicles and equipment. He has held senior positions in the U.K. as project engineer and senior project manager, and in the U.S. as operations manager.




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