Subsea Heave Compensators Bob Wilde - InterMoor Jake Ormond- InterMoor Deep Offshore Technology - 2009 Abstract In an effort to expand InterMoor s Subsea installation business and the Back of the Boat services we offer to our clients, we have discovered a need for an enhanced version of our existing Subsea Compensators. The current units that we operate work well for the sea states in the GOM but are not well suited for most installation work in other markets such as West Africa and Brazil. In these areas they experience large swells with long periods between, which encouraged us to develop a compensator that is more versatile in any situation. The new units we are developing will have a much better reaction to these sea states and offer several improvements. These units are depth compensated, subsea adjustable and can be hydraulically locked during deployment. They can also be deployed and recovered over a stern roller. This allows a much broader range of applications and vessels for which these units can be utilized. The first prototype unit (15 ton working load) has been completed and is in the final stages of testing. Engineering has already begun on two 75 ton units to be built early 2009.
Introduction The InterMoor Subsea Compensation System (ISCS) is a vertical in-line passive heave compensator. It is an installation tool designed to compensate (i.e. minimize) vertical heave during sensitive installation of subsea equipment in an offshore environment. The vertical heave source is typically generated by an installation vessel s stern motion or crane tip motion depending on deployment method. The ISCS is an in-line tool and therefore is designed to follow the payload subsea to the installation site near the sea floor. The ISCS is designed to operate in air or in water at depths up to 10,000 ft. InterMoor has investigated the various forms of compensators available. A void in the market exits for compact and easily mobilized heave compensators that can be used on a variety of installation vessels. InterMoor began a development program in mid-2007 to design, build and test a concept developed in-house for a novel heave compensation tool. In November of 2008, a successful test of the ISCS was completed offshore in the Gulf of Mexico from the M/V DMT Emerald. The purpose of this paper is to summarize the ISCS concept, the alternatives, the test program, the test results and the future of the ISCS. Alternatives The heave compensation market consists of two primary types of heave compensation systems: Passive and Active systems. Passive systems generally have some type of spring (typically pressurized gas) that is adjusted prior to deployment; the unit functions independently with no significant power requirement during the decent of the object being lowered. Active systems are compensators that sense movement of the vessel and then use external power to compensate for that movement. For example, if an active system sensed that the crane tip was rising, the compensator would pay out on the crane line an appropriate amount to compensate for the crane tip s vertical heave. The principles of passive heave compensation are described in detail later in this document. There are also variations or combinations of the two. For example: Active systems that use passive systems as the underlying system to reduce power requirements and actively controlled passive systems that make minor adjustments to the passive system to achieve higher performance.
Generally, active systems have the potential to provide the most heave compensation. Good systems can limit heave into the high 90 percentile. The cons to an active system are the higher cost and, typically, lack of portability. An active heave compensation device is normally integral to either a pedestal crane or winch, both of which are generally not easily moved. Shell Oil and Gas developed a Heave Compensation Lowering System (HCLS) that uses buoyancy modules along with chain to create a neutrally buoyant system that acts as a spring isolation device. The spring is the combination of the buoys and the chain in a lazy-s configuration. The heave of the vessel passes through the belly of the chain and the load variation seen by the buoys if very small (effectively half the wet weight of the chain picked up or paid out). The system has a very large natural period and therefore the system has a very effective spring isolation effect. There are some significant draw backs to the HCLS system, however. The system has a lot of constraints that are created by its operational limitations. The buoys sizes for large payloads become extremely large and having such large buoys subsea below installation vessels and drilling rigs is very dangerous. If the rigging fails, the impact of the buoys on the surface vessel could penetrate the hull of the vessel or drilling rig. The system cannot be deployed in less than 1000 ft of water easily because of the large lengths of chain required to make the system work correctly. The potential for entanglement during deployment is always a risk; when it occurs, catastrophic failure (i.e. loss of payload) has often followed. In summary, HCLS is a very effective method that comes with some very high associated risks. Other basic nitrogen over oil passive spring isolators exists in the market for rental or sale. These basic systems have two major shortcomings: a) They are affected by a net hydrostatic pressure on the hydraulic rods that tends to close the rods at depth. Such units are operationally limited in deep water applications. b) They are ineffective in long period seas (i.e. swell). A good passive system can achieve 80 to 90% spring isolation. The ISCS is a novel approach to nitrogen over oil spring isolators in that features have been incorporated to eliminate the rod hydrostatic effect and to obtain the very low effective spring stiffness needed to achieve spring isolation in long seas.
Spring Isolation Theory The ISCS uses the principles of spring isolation to generate a net heave compensation effect or spring isolation effect [1]. The tool is a nitrogen over oil spring dampening device. For spring isolation to occur, the natural period of the spring-mass system must be greater than the forcing heave period. Spring isolation begins to occur when the natural period of a system is 1.414 times greater than the forcing heave period. Spring isolation theory for regular waves is used to assess the performance for varying situations. Though this theory is meant for regular waves, it typically is useful to estimate nominal performance in irregular waves. The ISCS uses the theory of spring isolation to provide a tool that can allow the operator to use it in a variety of stiffness and damping modes appropriate for different situations. Spring isolation is demonstrated graphically by plotting transmissibility (TR) as a function of normalized wave period in Figure 1. The x-axis is the ratio of natural period over forcing period (Beta) and the y-axis is the ratio of the heave of the compensated payload verses the forcing heave (TR). Resonance occurs when the forcing period equals the natural period (Beta = 1). Spring isolation, defined by the isolation of heave occurs when the natural period is the square root of 2 times larger than the forcing period. An effective passive heave compensator moves the natural period at a minimum to twice the forcing period. ISCS Concept and Functionality The ISCS concept was developed with the knowledge of the shortcomings of many standard passive systems. The functionality built in to the ISCS is there to allow the installer to overcome many of the variables associated with using a passive heave compensator. It should be noted, that although the ISCS is a deployable unit that is designed to descend with the object being lowered, it is possible to have a passive compensator fixed on the deck of a vessel. Generally, the lowering wire is run through a set of sheaves connected to the compensator rods; such configurations can increase effective stroke and eliminate the need for the compensator housings to move with the wire. The difficulty associated with these systems is that they become very large, and the weight of the lowering wire becomes a variable to which the high pressure gas system must be adjusted to constantly in order to allow for the additional weight. The amount of deck
space required for such systems limits the amount of usable space on a deck and increases the amount of risk associated with handling equipment on deck. The choice to go subsea with the ISCS heave compensator eliminated many of the issues associated with a deck mounted passive heave compensator. By having the heave compensator travel subsea, the compensator is no longer impacted by the length of wire deployed and the deck space used by the equipment is much less. However, new issues arise when a hydraulic type compensator is introduced to a subsea environment. These are the typical issues with traveling subsea, but the most significant is the hydrostatic pressure and its effect on a hydraulic rod. A hydraulic/gas system requires a specific pressure charge to give the correct static load when the hydraulic rod is extended half way or mid-stroke. The hydrostatic pressure at varying depths forces certain design and operational constraints on a simple hydraulic spring damper device. Prior art heave compensators using spring isolation theory and hydraulic spring dampers do exist. The difficulties with these types of compensators are the effect that hydrostatic pressure has on the units. The hydrostatic pressure has a net effect on the piston rod calculated by the hydrostatic pressure times the piston rod area. This net load compresses the rod as the compensator is lowered to depth. Furthermore, this hydrostatic pressure limits the unit s ability to soften the spring system to achieve greater spring isolation. The limits imposed by depth effect are primarily due to the sensitivity to external pressure. The flatter the spring curve (i.e. softer), the more sensitive it is to external pressure and the greater chance that errors in mass calculations can render the heave compensator useless. The spring stiffness of the spring damper must be fairly high to allow for the collapsing force generated on the hydraulic rod. Even with a stiff spring, very deep depths become a challenge to balance the forces placed on the rod by manipulating the gas pre-charge. Thus, with standard passive compensators, one has the conflicting requirements of a soft spring for isolation purposes but a stiff spring for hydrostatic rod depth effect. The novel feature of the ISCS is the use of pressure balancing to eliminate the depth effect. A compensating cylinder is added to the tool to eliminate the depth effect. The compensating cylinder uses the piston area ratios to provide a precise amount of back pressure (at any given depth) on the low pressure side of the hydraulic cylinder to offset the load from the high pressure cylinder rod side caused by the external hydrostatic pressure. Figure 2 shows prior art solution to external pressure with the use of a tail rod. The tail rod exerts an equal force to the piston rod and for this reason eliminates
the depth effect. Unfortunately, the length of the unit is doubled with this solution. Length is considered a constraint for handling purposes and the tail rod method was not considered feasible. Using the compensator cylinder on the ISCS allows for a depth compensation to occur without adding to the length of the unit. The back pressure created by the compensating rod and piston equals the pressure created by the main rod and piston. Figure 3 is a schematic of the ISCS functionally showing how the compensating cylinder removes the hydrostatic effect on the main cylinder. With depth compensation added to the spring isolator, it now becomes feasible to increase the nitrogen volume significantly to make the spring much softer. The increase in the volume of nitrogen has the net effect of increasing the length of the natural period. The theory of spring isolation tells us that the greater the natural period of the compensator in relation to the forcing period (heave response period of the vessel) the greater the spring isolation will be. The compensating cylinder allows significantly softer spring rates and therefore much more efficient compensation and spring isolation. The ISCS has a primary accumulator and two (2) additional Air Pressure Vessels (APV). The natural period of the compensator with both APVs open is approximately 20 seconds. Spring isolation theory also suggests that dampening is helpful during situations where the forcing period is close to the natural period. The theory also suggests that damping can hinder spring isolation if the natural period is greater than 1.414 times the forcing period. The ISCS has two dampening settings that can switched subsea; the damping rates of these two settings can be adjusted by changing the size of the orifice plates. Orifice plate change out can only be done with unit on deck. The ISCS has a built in hydraulic lock. The hydraulic lock allows for safe deployment and recovery of the system that minimizes exposure of the hydraulic rod to bending stresses as it is deployed (or recovered) over the stern of the installation vessel. The ISCS has the capability to both charge and discharge the accumulator and APVs. There is enough nitrogen stored onboard the unit to make fine adjustments to the system. The high pressure tank is used to increase pressure to the system and it is also used to retract the rod in order to lock the system for recovery if necessary. There is a discharge tank that will allow for subsea venting in the circumstance that the
ambient hydrostatic pressure is higher than the system pressure. The vent tank can be discharged to sea if the hydrostatic pressure is lower than the system pressure. In summary, the ISCS has the ability for subsea adjustment that allows for variable stiffness rates, change of dampening settings and adjustment of pressure. The system is depth compensated such that it is not affected by the hydrostatic pressure on the hydraulic rod. The ISCS is robust and can be safely deployed over a stern roller or over the side with a crane. The locking mechanism allows for deployment where limited hook height is an issue. ISCS Prototype A 15 ton working load limit (WLL) ISCS prototype was built with full functionality and versatility for use as test platform and later use as a small-load operational unit. It is caged in a robust skid frame that allows the unit to be deployed over the stern roller of an anchor handling or construction vessel. The unit has a safe working load of 15 short tons and features 160 inches (4 meters) of stroke. The prototype has a built in ROV panel to allow for subsea adjustment of the ISCS as described above. The ability to make adjustments is considered critical with all of the variables involved (i.e. surface temperature, subsea temperature and margin of error in wet weight of payload). 15 Ton ISCS Prototype Offshore Test In November of 2008, InterMoor was presented with the opportunity to test the ISCS on board the M/V DMT Emerald. The test consisted of simulating a subsea load by filling an large metal basket with 3 ¼ chain. The basket dimensions were 6 ft wide x 12 ft long x 3 ft high. The total dry weight of the basket and chain was approximately 22,000 lbs or 11 short tons. The basket and compensator were both outfitted with instrumentation to record heave motions. The instrumentation was provided by Seatronics. It consisted of a subsea power supply, an accelerometer, a data logger and a ROV switch to activate the system. The data logger recorded the data string output by the accelerometer and attached a time stamp to each data point. The instrumentation for the payload (basket) was placed in a small steel frame and located in the center of the basket. The chain was laced around the instrumentation frame. The instrumentation on the ISCS was mounted inside the pyramid frame on the top of the compensator. Photos of the test setup can be viewed in Figures 4, 5, 6 and 7. Figure 8 shows the unit operating underwater.
The M/V DMT Emerald is equipped with a 100 ton multi-purpose tower and a 100 ton knuckle boom crane. The compensator test was conducted from the knuckle boom crane. The payload (basket) was rigged to the bottom of the ISCS. The top of ISCS was rigged to the knuckle boom crane. The locking device on the ISCS was utilized to keep the rod retracted to overboard the basket. Once the ISCS and basket were deployed to depth, the ROV was utilized to unlock the main hydraulic rod and make the necessary adjustments to the ROV panel. Multiple functionality tests were conducted and the ISCS was deployed a total of 3 times (on 3 separate dates). The total subsea time for all three dives added up to approximately 12 hours. The offshore test provided some very valuable data. For very small heaves, less than 1 ft peak to trough, the compensator rod did not move relative to the compensator body. At higher heave amplitudes, the compensator rod began to stoke, the unit performed very well. Compensation on the order of 80 percent was observed corresponding to a transmissibility of 0.2. The higher the heave, the more efficient the unit became. Figure 9 shows a time trace of vertical displacement of the heave compensator and the payload (basket). With the low heave (<1 ft), the transmissibility is seen to equal 1.0 (i.e. TR = 1.0). Once the heave becomes larger than 1 ft, the rod breaks the accumulated static friction (commonly referred to as stiction) at all the systems movable seal surfaces and effectively compensates the object being lowered; in this case the weight basket. The observed performance of the 15 ton ISCS compensator followed well the performance expected using spring isolation theory. Figure 10 shows a time trace of vertical velocity of the compensator and the weight basket. As can be seen from the graph, the docking velocity of the object being lowered is less than 1 ft/sec (0.3 m/s) regardless of the transmissibility. In other words, even when stiction causes the compensator and weight basket to move together, the amplitude was sufficiently low such that the peak harmonic velocity was also low. Lessons Learned Attention was paid to seal selection during the fabrication of the 15 ton ISCS. It was evident during the testing of the unit, however, that there was more static friction (or stiction) in the unit than anticipated. The stiction required a minimum inertial force before the compensator rod would begin moving. The test revealed that while heave
was nominally less than 0.5 ft single amplitude, the stiction was not generally overcome and the payload would follow the vertical movement of the ISCS in phase. Above the 0.5 ft single amplitude, the stiction was overcome and the payload and ISCS would move out of phase and the unit would perform very well. An interesting observation during the 15 ton ISCS test was that the larger the amplitude of the heave, the more efficient the ISCS became. The stiction became a smaller percentage of the total unit s resistance to changing direction when the heave was larger and thus stiction became less of a factor. The next generation of compensators will retain all of the operability of the prototype but they will be enhanced with a remote acoustic control package to eliminate the need for ROV contact with the unit. InterMoor is currently working with seal experts to achieve lower static friction characteristics in order to enhance the low heave performance and increase the efficiency of the unit overall. It should be noted, however, the even when the transmissibility of the unit was near 1.0, the maximum velocity, due to the low amplitude, was less than 1 ft/sec (0.3 m/sec); a velocity acceptable for many subsea docking operations. 75 Ton ISCS The successful completion of the 15 ton ISCS test has provided validation of design and confirmed expected performance. The system has been proven and the go-ahead to build the first 75 ton units is under consideration. The engineering work on the 75 ton ISCS was 90 percent complete at the time this paper was written in November 2008. Contingent on project approval, InterMoor anticipates delivery of it first 75 ton ISCS in June of 2009. Acknowledgments InterMoor wishes to thank Deep Marine Technology for providing access to the DMT Emerald and their assistance in the performance of the offshore tests. Reference [1] Dynamics of Structures, Ray W. Clough & Joseph Penzien; 1975 McGraw Hill Inc.
3.00 2.50 Transmissabliity, TR 2.00 1.50 1.00 Compensation begins 0.50 0.00 0.00 1.00 2.00 3.00 Period Ratio, Beta Figure 1 Transmissibility vs Beta Figure 2 Tail Rod Depth Compensator
Figure 3 Enhanced Compensator Schematic Figure 4 Compensator rigged for lowering
Figure 5 Compensator showing weight basket Figure 6 Motion sensor on compensator
Figure 7 Motion sensor in weight basket Figure 8 Compensator and basket operating
36 24 12 Heave (In) 0 2:52:59 AM 2:53:08 AM 2:53:16 AM 2:53:25 AM 2:53:34 AM 2:53:42 AM 2:53:51 AM 2:54:00 AM Heave Basket(in) Heave Forcing (in) -12-24 -36 Time Figure 9 Heave vs Time 3.00 2.00 1.00 Velocity (ft/s) 0.00 2:52:59 AM 2:53:08 AM 2:53:16 AM 2:53:25 AM 2:53:34 AM 2:53:42 AM 2:53:51 AM 2:54:00 AM Velocity Basket (ft/s) Velocity Forcing (ft/s) -1.00-2.00-3.00 Time Figure 10 Velocity vs Time