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Riser Operations and the Future of Scientific Ocean Th or collective redistirbution of any portion article of any by of this or collective redistirbution SPECIAL ISSUE FEATURE articleis has been published in Oceanography D/V Chikyu 19, Number journal of Th 4, a quarterly , Volume RISER OPERATIONS AND THE FUTURE permitted photocopy machine, only is reposting, means or other OF SCIENTIFIC OCEAN DRILLING e Oceanography Society. 2006 by Th Copyright BY DANIEL CUREWITZ AND ASAHIKO TAIRA Earth science disciplines focused on the geological record. erational means by which core recovery, investigation of climatic, ecological, or Advances in scientifi c drilling over the core preservation, borehole stability, with the approval of Th tectonic change as recorded in geological last decade have enabled progressively geophysical measurement, and borehole deposits (e.g., paleoceanography, marine better recovery and greater confi dence in monitoring operations are enhanced us- micropaleontology, paleoclimatology, the fi delity and coherence of recovered ing riser drilling (Sawyer, 1996; Kerr and gran is e Oceanography All rights Society. reserved. Permission paleomagnetism) require high-resolu- sections. The resulting interpretations Normile, 1998; Normile and Kerr, 2003). or Th e Oceanography [email protected] Send Society. to: all correspondence tion, continuous, well-preserved records and analyses have enhanced the high- for accurate analysis. Recovery of long, resolution short- and long-term records RISER DRILLING BASICS uninterrupted, relatively undisturbed of changes in the Earth system (Brewer Riser drilling involves several steps that sections has long been a primary techni- et al., 2005). are accomplished in a variety of ways de- cal challenge for any drilling operation. The adaptation of riser drilling tech- pending on specifi c technological pack- Recovered cores and geophysical mea- nology for scientifi c ocean drilling pur- ages, water depths, and bottom condi- surements form the backbone for any poses on board the D/V Chikyu (Fig- tions. First, a wide-diameter pilot hole is further data collection and data analysis. ure 1) represents a giant step forward for drilled and cased, then a specialized well- Limitations on core recovery, core condi- the scientifi c ocean drilling community. head designed to anchor a pressure sen- ted to copy this article Repu for use copy this and research. to ted in teaching tion, and sample preservation have long The D/V Chikyu is operated by the Cen- sor and shut-off valve assembly, called a been recognized as serious challenges for ter for Deep Earth Exploration of the Blow Out Preventer (BOP; Figure 2), is any reconstructions or interpretations of Japan Agency for Marine-Earth Science installed. The BOP itself is lowered to the e Oceanography Society, PO Box 1931, Rockville, MD 20849-1931, USA. and Technology. The use of this state-of- wellhead and attached using a Remotely Daniel Curewitz ([email protected]) the-art drilling and coring system as one Operated Vehicle (ROV). Lowering of is Staff Scientist, Center for Deep Earth of the principal platforms in the Inte- the BOP is accomplished by successively Exploration (CDEX), Japan Agency for grated Ocean Drilling Program (IODP) attaching riser pipe (Figure 3) to the top Marine-Earth Science and Technology will advance the science of reconstruct- of the BOP and lowering it to the sea- (JAMSTEC), Yokohama, Kanazawa, Japan. ing past environments and analyzing fl oor. Riser pipe itself comprises wide-di- Asahiko Taira is Director-General, Center Earth system evolution as recorded in ameter, high-strength pipe with external blication, systemmatic reproduction, reproduction, systemmatic blication, for Deep Earth Exploration, Japan Agency the sediments and rocks below the ocean conduits for cables and connectors to al- for Marine-Earth Science and Technology, fl oor. In the following discussion, we low control and monitoring of the BOP. Yokohama, Kanazawa, Japan. enumerate the technological and op- Once the BOP is installed at the wellhead 168 Oceanography Vol. 19, No. 4, Dec. 2006 Figure 1. (top left) Deep Sea Drilling Vessel Chikyu. Figure 2. (bottom left) Blow Out Preventer on the deck of D/V Chikyu. Figure 3. (right) Riser pipe sections racked on the deck of D/V Chikyu. and linked to the drilling vessel via the borehole inside the drillstring. Drilling etrated formations (Figure 4). riser pipes, drilling and coring (as well as mud specifi cally designed for coring op- The drilling mud is fi ltered through a any downhole logging, measurement, or erations, called “coring mud” or “bland graduated series of screens (called “shale sampling) operations begin (Brewer et coring mud,” can be used when preserva- shakers”), removing drilling cuttings and al., 2005). tion of porosity/permeability structures other fragments (e.g., borehole-wall col- The drillstring (drill pipe, drill bit, or reduced chemical contamination of lapse fragments, which are distinguished and any attached equipment, sensors, the cored formations is required. Drill- from cuttings based on physical appear- or tools) is lowered into the borehole ing/coring mud exits the drill pipe at the ance). These cuttings are then available entirely inside the riser pipe. Drilling drill bit, and returns through the riser for geological and physical properties ex- mud (a high-density, chemically inert, pipe (outside the drillstring) to the drill- amination and interpretation, including fl uidized mixture tailored to match the ing vessel, carrying with it suspended lithology, geochemistry, paleontology, ambient geological fl uid- and gas-pres- cuttings, fl uids, and gases derived from consolidation, p-wave, and porosity/den- sure conditions) is pumped into the the drilling process and from the pen- sity analysis. Gases and fl uids trapped Oceanography Vol. 19, No. 4, Dec. 2006 169 in the drilling mud are also collected for chemical analysis for both safety and scientifi c purposes. The fi ltered drilling mud is then cycled back into the drill- ing operation. As drilling progresses, the chemical composition of the drill- ing mud is altered to compensate for increased lithostatic stress, gas pressure, hydrocarbon deposits, fault zones, and/ or fractured intervals (Yano et al., 2001). RISER DRILLING ADVANTAGES Recirculation of high-density drilling fl uid through the borehole enhances the drilling and coring operations in a variety of ways (Adamson et al., 1998). First, the density of the drilling fl uid is calibrated using preexisting and real- time information about the properties of the penetrated geological formation, including gas and fl uid pressures, frac- ture density, porosity and permeability, and physical characteristics of the rocks. Drilling-fl uid density is tailored to pre- vent borehole collapse, gas blowout, or excessive fl uid fl ow in the borehole, in- creasing the safety and effi ciency of the drilling operation. Additionally, rock fragments produced by drilling and cor- ing are carried away from the drill bit by the high-density fl uid, reducing the risks of jamming the drill bit and producing a cleaner borehole, both for drilling and for later downhole operations. Tem- perature and friction at the drill bit are reduced, increasing the life span of each Figure 4. Diagrammatic depiction (modifi ed after Sawyer, 1996) of riser drilling op- bit, while increasing potential drilling erations showing the borehole, the wellhead, the BOP, and the riser pipe connecting to the drilling vessel. Drilling mud fl ows down through the drillstring, exits at the speed and enhancing both core recovery drill bit, and is returned in the annular space between the drillstring and the riser and core quality. This combination of pipe walls. advantages allows increased performance throughout the entire drilling operation, discussed below. 170 Oceanography Vol. 19, No. 4, Dec. 2006 Deeper Boreholes fragments and borehole deformation. Operations in Gas- or Fluid-Rich The circulation of high-density drill- Riser drilling also improves the ability Formations ing fl uid increases borehole stability and to ream and case completed boreholes Riser drilling using high-density drill- reduces the possibility of borehole wall by enabling rapid clearing of fragments, ing fl uid was initially developed in the collapse or constriction. Riser boreholes preventing borehole collapse, and reduc- oil industry to reduce the risks of blow- can potentially be drilled to depths of ing the risks of cavity creation that may out resulting from penetration of over- 7,000 meters or more. Reduced bit wear, occur during hole-opening operations. pressured formations. Ocean Drilling elimination and/or reduction of bit jam- These operations improve the feasibility Program (ODP) operations were, by ming or drillstring constriction, and and success rate of logging operations in necessity, restricted from drilling in areas elimination of loose fragments in the open holes (directly following drilling thought to host gas, oil, or other poten- borehole contribute to this expanded operations), and increase the viability tially pressurized fl uid deposits or con- depth range. Prior experience in scientif- and feasibility of downhole measure- duits. As a result, drilling in active fault ic ocean drilling with non-riser systems ment and monitoring operations using zones, methane-hydrate-rich regions, or indicates that boreholes are limited to either CORK (borehole seal; see Kastner any areas characterized by high possibil- ~2,000-meter depths due to the combi- et al., this issue) or CORK-like equip- ity for hydrocarbon deposits
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