Riser management taking center stage as drilling moves into greater depths

The deepwater riser on the Pride Africa drillship.

Lightning can strike twice, at least in the case of deepwater dropped riser accidents. As Conoco and R&B Falcon, and Pride International address the particulars of the dissimilar drops, the industry is focusing attention on riser management as the major developing problem in deepwater exploration.

While the final cause of the accidents aboard the Deepwater Pathfinder and Pride Africa has yet to evolve, the dropped risers were apparently the result of distinct failures in complex systems - systems that some have considered over-stressed in great water depths.

A number of sources agree that the Deepwater Pathfinder accident was the result of a disk brake failure in the drawworks, while the Pride Africa had a parting in the drillline. Barry Harding with Maverick Offshore said the two symptoms are different facets of one overarching problem - deepwater riser systems are operating at or near design limits. At these depths and sea states, and the weights these vessels are supporting, something has to give. While the Pathfinder and Africa will no doubt complete repairs and get back to drilling within the next six months, this does not resolve the larger concern.

Exposure time

The deepwater riser is most prone to failure when it is in motion, either being run or pulled. Once the riser string is landed, the risk drops substantially. Moe Plaisance, Vice President and General Manager of Diamond Offshore Team Solutions called running and retrieving the riser the "Achilles' heel" of deepwater drilling. Incidents occurring when the riser is connected are virtually unheard of. If the risk lies in running and pulling the riser, it follows that this risk is greater in deepwater, where exposure time is longer. In deeper water, it takes longer to run and pull riserstrings, which are much longer, heavier, and more difficult to handle. With the added factor of currents and sea states, it easy to see how trouble can develop.

Plaisance said the riser is probably the heaviest routine load a rig will ever pull. Due to the buoyancy, the weight will remain fairly static for a long interval, adding to the difficulty and exposure.

Drillers need to run the riser as fast as possible. Sometimes this speed comes at the expense of the overall system. Quick connectors and other devices that decrease running time are becoming more common, but some of this equipment does not have the durability of conventional tools.

Stress Engineering performs detailed riser analysis for contractors and operators prior to commissioning. Chuck Miller, Principal with the company said that often "the connection is often the weak link, as far as the riser string itself. In the deepwater riser systems we have evaluated, we have mostly seen the 'flange-type' connections. But, because of the increased running times required for deepwater riser systems, it becomes desirable to use a riser connector that can minimize the time required to make-up the connection. However, care must be taken not to sacrifice fatigue and strength resistance at the expense of running time. The quick make-up connectors are generally 'dog-type' connections. A properly designed 'flange-type' connector will, in general, provide better fatigue resistant properties (i.e. lower stress concentration factors) than will a dog-type connection with a comparable strength rating. However, the flange connections require longer make-up times than do the dog-type connections," he said.

Exposure time in storm conditions also is extremely critical. The increased time required to pull the deepwater riser increases the exposure time for the entire rig. Typically, this is avoided by disconnecting the riser, hanging it off, as the rig is evacuatedDP vessels must pull the riser to the surface. "It is an exposure time that we are not comfortable with," Plaisance said.

Ship-shape versus semi

In the mid-1990s, there was a strong push into deepwater exploration. A newbuild frenzy followed the upgrading of existing vessels. With investment capital encouraging newbuild announcements, there was strong incentive for contractors to go on record as either upgrading its existing fleet or launching a series of newbuilds. The result of this rapid expansion can be seen among new and upgraded drillships, as they enter the market with long-term contracts and eyes on new water depth records.

It is common knowledge that these drillships are a less expensive alternative to the semisubmersible design. The costs are offset even more, if the drillship is a converson from an existing hull. This is one of the reasons the vast majority of newbuilds now coming onto the market are ship-shaped designs. Some reports indicate this cost saving sometimes comes with a price.

The industry seems clearly divided between the virtues of the ship-shape versus the semisubmersible design. One side argues the ship-shape does not have the stable motions of a semisubmersible and this adds to problems with riser management. Specifically, controlling heave motion and applying precision tension are more difficult with a shipshaped vessel. In addition, the semisubmersible has clearance between the waterline and the moonpool. The drillship moonpool extends below the waterline and the riser hits the sides of the moonpool in strong weather conditions. Critics say this fatigues the riser joints.

Drillship designers and proponents concede this may have been a problem in the past, but the sheer size of this new generation of vessels makes the motion characteristics very similar to a semisubmersible. As drillship size increases, heave characteristics have improved, explains Gary Casswell, Vice President, Eastern Hemisphere for Pride International.

Most of these new drillships are equipped with larger moonpools to reduce the clash with risers. But for total motions optimization, a drillship is designed to weathervane, taking the wind and waves over the bow. A semisubmerisble, by contrast, can take weather from all sides. Therefore, Casswell and others say that as long as the heading of a drillship is optimized, it will maintain motion characteristics very similar to that of a semi.

"The reason that you hear about drillships, versus semisubmersibles, is mainly a result of operating history. You can go to most operators and get a preference for semisubmersibles to drill with, or you can get a preference for drillships," said Casswell. "It's not that one is better than the other because both types are excellent deewater drilling rigs. One thing is certain: A ship needs to face the direction of the wind and the waves."

At the same time, in certain environments, a drillship is comparable to a semisubmersible. "You must consider the operating area. For example, conditions offshore West Africa are very different than conditions offshore Gulf of Mexico. The loop and eddy currents with (sometimes) opposite wind and wave direction dictate that deepwater GOM operators place a greater emphasis on the effects of weather on vessel position and riser capability. Deepwater drilling units in the GOM must have substantial riser tension capacity in order to maintain drilling operations in an area with high currents. On the contrary, currents are not a major concern in deepwater offshore West Africa," said Casswell.

Fatiguing of the riser will always be an issue. With greater frequency of running, pulling, stacking, and landing a riser system, the greater the fatigue. But according to Stress Engineering, fatigue is not the largest issues. They said storm induced fatigue damage is the same for shallow and deepwater riser systems as long as the vessel has adequate tensioning capabilities. The fact that the riser is pulled and inspected regularly can optimize the life of the system. However, a key difference between shallow and deepwater riser management is the notorious Gulf of Mexico loop current. Current flow against long risers sets up vortex induced vibration (VIV), and consequently fatigue.

Loop currents

Gulf of Mexico loop and eddy currents (provided by Horizon Marine).

The loop current is a strong flow of warm water that comes up from the Caribbean Sea, through the Yucatan Strait, then loops up into the Gulf of Mexico where it pushes north-northwest. The current can go up almost to the Mississippi Delta before it tracks back down to the southeast and eventually exits the Florida Straits and forms the Atlantic Gulf Stream. The loop current moves at speeds of 3-4 knots. A normal current flow would be under a half a knot. The loop current also spawns strong eddies, which add to the difficulty of keeping the riser in position.

"What operators are concerned about is the movement of loop current flows far into the northern Gulf. That is when they see effects on the risers and anchoring," explains Patrice Coholan, Program Manager for Boston-based Horizon Marine. "It does create havoc because certain operations are difficult or impossible when the currents are stronger than 2 knots. It hampers operations and they have to shut down or move to another site."

Horizon Marine provides a monitoring service called Eddy Watch that was started in 1984 for the offshore oil industry. The company tracks the loop current and the resulting eddies using drifting buoys (tracked via satellite as well as satellite imagery) altimetry, acoustic doppler current profile measurements from offshore rigs, and any data from surveys that are ongoing in the Gulf. This information is compiled and analyzed, and sent out in a weekly report to the operators, advising them of changes in currents and eddies.

Current prediction

Coholan said that predicting the currents accurately is extremely difficult, but it is getting better. "We have several models that allow us to do more predicting.

• "One is a forecast model based on our historical data. We compare the progress to something similar in the past.
• "We also use a feature model, which helps describe current variability within an eddy. We look at all that and give operators an idea what they can expect within the next several days or next week and then we can project out several months using the models we have," she said.

Most contractors look at the Gulf of Mexico as a fair-weather operating environment, however, contractors say loop currents can make this benign area one of the most difficult to work in. To offset the effects of loop currents, some contractors have installed fairings on riser strings. Fairings reduce the drag coefficient of the current on the riser, suppressing the vibration.

Other contractors don't bother with fairings because they take more time to install, increasing the running and pulling time which can offset the protection fairings offer.

Shakedown possibility

While some contractors and producers believe the mechanical capability of conventional riser systems is being pushed to the limit, resulting in failures, others place the blame on a simple case of shakedown problems.

"As far as pushing the equipment to the limit, the record was over 7,000 ft of water with the smaller (drill)ships. The limit was successfully pushed back then. That leads me to believe that it is shakedown of new equipment," said Dr. Ron Young, Vice President of Stress Engineering.

Casswell agreed. "We are not stretching the capacities of the equipment; we are stretching where we have been. The equipment is designed even heavier than what we really need. This is no different than our business has historically been, it is a matter of step-by-step change, as drilling moves into deepwater. Everything for deepwater is done conservatively."

Riserless future