The main purpose of the heavy weather launch and recovery system is to stabilize and centralize the ROV and Tether Management System (TMS) with a device called a cursor which restricts horizontal movement while transitioning through the air – sea interface (called the splash zone). The splash zone presents the greatest risk of damage to the ROV, TMS, and potentially the vessel. Large waves and high winds can cause the ROV and TMS to swing wildly, potentially impacting the vessel structure. As the vehicle is raised, this motion is amplified many times which can make it difficult if not impossible to recover the ROV in foul weather. Oceaneering designs a typical heavy weather launch and recovery system to operate in a sea state 6 environment.
Another hazard on drill rigs is the close proximity of the ROV to vessel hull mounted thrusters during entry and exit into the splash zone. Many of the new deepwater rigs are dynamically positioned using hull mounted thrusters to maintain position and orientation rather than using anchors. Due to the size and power of these vessel thrusters, ROV systems can be pulled towards and even into an operating thruster which will damage or destroy the ROV. Heavy weather launch and recovery systems move the pivot point for the umbilical from the a-frame sheave to the bottom of the vessel where the cursor will be located. This minimizes the risk of the ROV being pulled into a thruster.
Design Description
Oceaneering currently has several types of heavy weather launch and recovery systems, but they all use a cursor. The cursor is fabricated from stainless steel pipe with very few moving parts requiring low maintenance. The cursor is in the shape of an upside down bowl that encompasses the top half of the TMS and travels on a constrained path down the side of the vessel on guide wires or rigid rails. The cursor travels with the ROV and TMS until the cursor encounters a mechanical stop at the point where the wires or rails are terminated subsea. The wires or rails are terminated at the deepest possible level on the vessel structure. 1″ holes to allow the cursor to flood during submersion and drain during recovery. This combined weight of the cursor, ROV and TMS helps transit the entire package through the splash zone as quickly as possible to reduce “hesitation.” (“Hesitation” is the second or two that a standard system seems to float at the interface while the ROV and cage become flooded and begin to sink). Once the ROV and TMS have departed from the cursor they are free to descend down to the work site as controlled by the ROV crew.
Design Considerations
A number of factors need to be considered when selecting a configuration best suited for a specific application. Environmental conditions, vessel type, and the vessel limitations should serve as the base line for the selection. Space limitations and access to attachment points on the vessel lower hull or pontoons must be considered. These considerations result in the selection of one of the following configurations for heavy weather launch and recovery systems.
Guide Wire Cursor Deployment Systems
The most commonly used heavy weather launch and recovery system in Oceaneering’s fleet uses wire ropes to guide the cursor. The guide wire system is typically used on semi-submersible drilling rigs, tension leg platforms, and other similar installations. This method is useful when some horizontal movement is acceptable and there is no structure present to attach the rails. Cursor guide wire lengths are typically 150 ft. A pair of 2 inch parallel wire ropes are strung from the top of the fixed A-frame on the vessel deck to a subsea mounted lower cursor frame assembly, which is either bolted or welded to the pontoon. A specially designed and tested breakaway joint connects the wires to the lower cursor frame. This breakaway joint prevents damage to the a-frame, vessel, and most importantly personnel if a wire becomes overloaded. The guide wire system enables Oceaneering to offer a heavy weather launch and recovery system with excellent operational performance for semi-submersible drilling rigs, and other similar vessels.
Rail Cursor Deployment Systems
Commonly utilized on deep water drill ships as well as construction / intervention vessels, a removable guide rail assembly is used to constrain the cursor’s path. Typical rail lengths are 70 ft -80 ft in length. The cursor rail assembly interfaces with the vessel hull via hull mounted guide rails, subsea rail pin assembly which supports the weight of the cursor rail assembly once deployed and by topside cursor rail pad eyes which secure the top end of the rail assembly to the vessels hull. Rail systems can be used in an over the side launch and recovery system via an A-frame or through a moon pool opening with a fixed a-frame. Both of these systems provide excellent heavy weather operational performance.
Moonpool Systems
Oceaneering’s Intervention class vessels are equipped with dedicated ROV moonpools which allow operations in extreme sea conditions not normally possible in other ROV installations.
Summary
Oceaneering has numerous heavy weather launch and recovery systems installed on vessels, semi submersibles and drill ships operating worldwide. Our approach is to be involved with the customer from an early stage in the design of the vessel to facilitate and optimize the design, installation, and operation of the ROV system. The combination of the vessel’s capabilities and those of the heavy weather launch and recovery system provides our clients with more operational time and less “waiting on weather” down time, therefore being more productive.
