Offshore Report

Floatover installation succeeds for Nan Bao 35-2 topsides

A challenging, float-over installation of a large topsides at CNOOC’s Nan Bao oil field in Bohai Bay has proven the method further and paved the way for additional projects

Liu Liming, Zhang Songfu, Fang Xiaoming, Chen Baojie, Hao Jun, and Alan Wang, China Offshore Oil Engineering Corp., Ltd.

Nan Bao oil field is in Block 35-2, in the middle of Bohai Bay by northern China. The site is about 100 nautical miles northeast of Tanggu. The NB35-2 oil field consists of one central production platform (CEP) with 40 slots in the North Area and one wellhead platform (WHPB) with 40 slots in the South Area. In the latter area, the water depth in which the CEP stands is 13.2 m (43.3 ft), referenced to chart data.

The offshore installations also include a 4.57-km (2.84-mi), multiphase, subsea transport pipeline, installed between the CEP and WHPB platforms to transfer produced fluid from WHPB to CEP. Crude oil from CEP is then transferred to an FPSO vessel (FPSO Century) stationed in the QHD 32-6 oil field via a subsea pipeline. First oil flowed on Sept. 10, 2005.

CEP’s topsides were fabricated at the Shengli fabrication yard in Longkou, Shangdong, China. The structure was then loaded onto launch barge CNOOC 221 (142 m × 36 m × 9.75 m) via five rows and six columns of trolley cars (LOTs) and a load-out support frame (LSF), and towed to the NB35-2 field. This was followed by float-over installation onto the preinstalled eight-legged jacket. Total load-out weight was about 8,650 t, and total float-over weight was about 7,500 t.

At the NB35-2 field, the barge was connected to a pre-installed, docking mooring system. While in stand-off position, the barge loaded with the topsides underwent final preparations for the float-over operation. The float-over procedure included the help of a sway/ surge fender system, the assistance of AHTS tugs, and operation of bow-mooring winches, stern docking winches, and two cross-lines connected to the mooring winches of a support vessel working as pulling equipment. 

The barge was positioned and docked inside the pre-installed jacket structure. The barge was then ballasted down to transfer the topside load from the barge onto the jacket substructure by using leg mating units (LMU) and deck support units (DSU) to absorb the impact load, due to barge movement. Having transferred the topside load, the barge continued ballasting until adequate clearance between the topside structure’s underside and the barge’s LSF structure had been achieved. After this, the barge was withdrawn from the platform structure.

Due to the low air gap in Bohai Bay, structural interference poses a significant challenge to float-over operations. Different from a conventional high-deck float-over, the challenge was mainly caused by some structural interference during docking and undocking operations. There are two small wellhead service platforms of EL (+)11,300, located at both the south and north sides of the CEP platform. In particular, some Christmas trees had been pre-installed at EL (+)12,500, prior to the float-over operation. This vertical interference meant that the barge could only dock from the east side of the platform with stern entry.

In addition, the low sump pump deck further limited the transverse clearance between the stabbing cones of the topsides and the sump pump deck structure to about 500 mm. This restricts the use of conventional rubber fenders Thus, wooden fenders had to be used to avoid large deformation.

During undocking, the high jacket transverse frames of EL (-) 7,000 also caused significant interference. There is only 300-800 mm vertical clearance between the underside of the lower deck girders and the top of the DSU. Furthermore, there is only 1,050 – 1,550 mm under-keel clearance between the topside of the jacket transverse frames and the barge’s bottom. Extreme caution should be taken to measure the tidal level and the barge’s draft during undocking, in case there is a collision, either between the underside of the topsides and the top of the LSF/ DSU structure, or between the barge’s bottom and the jacket transverse frames at EL (-) 7,000.

This article discusses the challenging float-over installation of the NB35-2 CEP topsides, with a total weight of about 7,500 t. It also addresses the float-over technology in detail, Fig. 1.

Fig. 1. Overview of CEP platform, just after floatover installation.

FLOAT-OVER TECHNOLOGY

For the past two decades, various float-over technologies have been developed and successfully applied to offshore installations. The “high deck” or topsides float-over methodology was initially introduced by Halliburton/ KBR in 1977 at BP’s Magnus field in the North Sea. The first float-over installation was successfully applied to an 18,600-t production platform topsides for Phillips Petroleum’s Maureen project, whose mating operation was engineered and performed by KBR UK in 1983. Following the Maureen project’s success, a string of facilities using the float-over concept followed.

Development of the float-over technique has recently included the use of versa-truss booms and strand jacks to raise the float-over decks to the required, in-place elevation at offshore sites. The versa-truss technique’s advantage is that it eliminates the need for the substructure’s open slot during docking while reducing the float-over support truss height and improving transport barge stability. KBR provided detailed design and installation services to Chevron’s LL650 Venezuela project. The versa-truss technique was successfully used in Lake Maricabo for a 5,600-t topsides on one water injection platform, and a 5,500-t topsides on one central production platform in 1999.

The strand jack lifting technique’s advantage is that it eliminates the need for a large transport truss while also improving the transport barge’s stability. Most importantly, the mating operation can be ballast-free. This can be important, where the in-field water depth is very shallow and constrains the barge size that can be employed, or the range of available barges is limited.

The float-over technique with strand jack lifting technology was successfully applied to installation of a 6,200-t topsides and a 5,200-t topsides for two platforms at Apache and PetroChina’s Zhaodong project. The site is in extremely shallow near-shore waters in western Bohai Bay, with water depths of 1.7 m (5.6 ft), as per chart data, and 4.2 m (13.8 ft) referenced to local mean sea level. This method reduces the installation barge’s size significantly, and it eases the dredging requirement for installation basins.

THE ADVANTAGES

The float-over technique is an installation method that allows offshore platform topsides to be installed as single, integrated packages without using a heavy lift crane vessel. This permits the topsides to be completed and pre-commissioned onshore prior to loadout, eliminating substantial costs associated with offshore hook-up and commissioning.

Float-over decks are especially attractive in areas where topsides weight exceeds locally available crane capacity and the cost of mobilizing larger cranes is high, such as Bohai Bay, where the largest heavy lift vessel’s maximum capacity is around 3,800 t. Originally conceived to address the problem of making heavy lifts in remote locations, float-over techniques are increasingly being applied to smaller and smaller topsides. Even in regions where suitable crane vessels are available, specifying an integrated topsides for a float-over installation opens the market to those contractors without access to such crane vessels, thereby providing a degree of additional competition during project tendering.

There are a number of reasons why the float-over method is becoming the preferred installation method for offshore decks, rather than heavy lift vessels. First, only a handful of crane vessels have the capacity to carry out heavy lifts, and their day rates are very high. The availability of such heavy lift vessels is very limited. Waiting for one suitable crane vessel to come online can cause project delays. Since the majority of heavy lift vessels is typically based in European waters, the mobilization and demobilization costs can be too costly for projects in Asian waters, especially in areas of Bohai Bay. Different from modular installations with smaller barges, the primary objective with floatover installations is to minimize costly hook-up periods offshore.

In addition, the floatover method is also particularly suited to conditions found in the waters of Bohai Bay, where water depths are shallow, typically ranging from a few meters to about 30 m. Therefore, the substructure design tends to be a conventional jacket type that favors the float-over method.

The installation engineering scope-of-work comprised conceptual design, engineering and planning the entire operation, including loadout, sea fastening, transportation and installation. Perhaps even more important in terms of ultimate cost savings for the client is early involvement during the conceptual design phase. Early design decisions for the float-over method can generate considerable savings further down the line. By being involved during the conceptual and detailed design phases, naval architects and structural engineers can provide invaluable input before construction begins. This minimizes the need for costly changes later on. Detailed planning for topsides transportation and subsequent installation also enables hook-up and commission operations to begin earlier.

The floatover method of installation was selected for the NB35-2 CEP topsides because of its primary advantage of enabling a complete, integrated deck to be installed as one unit. This allows freedom of equipment layout within the deck compared to modular designs, and also completion of testing and pre-commissioning onshore. The result is a significant reduction in overall platform cost through a shorter offshore commissioning phase without using expensive, heavy lift crane vessels, Fig. 2.

Fig. 2. The CNOOC 221 barge docks into the slot of the pre-installed jacket.

DOCKING/ MATING SYSTEM

The design of critical installation devices plays a crucial role in ensuring successful float-overs. These vertical and horizontal systems were deployed to prevent damage from wave movement. These installation devices consist of stern docking guides/ fenders, sway fenders, surge fenders/ longitudinal stoppers, LMUs and DSUs, as well as a mooring system and the assistance of AHTS tugs, etc. The latter are used during docking, mating, and undocking operations, respectively, Fig. 3.

Fig. 3. Illustrative 3D view of sway fenders, surge fenders, and LMUs.

FENDERING SYSTEMS

Three types of fendering systems were provided:

1. Type 1: Stern guide fenders
2. Type 2: Sway fenders
3. Type 3: Surge fenders. 

The stern guides are constructed to assist the barge’s initial docking into the jacket slot. The guides extend along the vessel’s stern gunwales and narrow toward the centerline. During docking, the stern guide allows an offset of the stern from the center line of the jacket legs with about 2.0 m to smooth initial entry and also protect the jacket legs.

The sway fenders extend from the stern guides and toward the surge fenders, and protect the jacket from contact with the barge hull during the docking. The wooden fenders are 650 mm × 650 mm, square, and are located alongside the barge’s gunwales until the final docking position, where the jacket legs will be located. Horizontal clearance between the jacket legs and fenders is about 100 mm at this position. The wooden fenders were used to replace the originally proposed rubber fenders. This limits the large deformation, because the transverse clearance between the stabbing cones of the topsides and the sump pump deck structure is only about 500 mm.

The surge fenders or longitudinal stoppers are constructed with a horizontal, hinged beam connected to a conical rubber fender that provides the fenders’ stiffness. This stiffness is about 100 t/m for each fender. The surge fenders should be compressed about 200 mm for the topsides to be in position over the jacket, corresponding to a compressional force of 2 × 20 t = 40 t. However, in case of favorable weather conditions, or if any misalignment is found, the position could be changed with wooden plate shims between the hinged beam and the conical rubber fenders.

LEG MATING UNIT (LMU)

Due to the vessel dynamic responses in waves, LMUs are used to buffer the impact load between support receptacles and the mating cones. DSUs are used to buffer the impact load between the deck support truss and the integrated topsides. The LMUs are specialized leg-and-deck mating units that act as shock absorbers as the vessel is ballasted down. The topside load transfers from the deck support structures onto the jacket legs. Any wave movement is then absorbed by the rubber elements in the LMUs.

First, the barge is positioned so that the topsides structure is aligned with an in-situ base structure. Then, the topsides structure is gradually installed onto the in-situ base structure by ballasting and the falling tide. During the mating process, loads need to be transferred from the barge to the in-situ base in a controlled manner by using LMUs, Fig. 4.

Fig. 4. The stabbing cone entering the LMU receptacle during mating.

LMUs consist of steel structures incorporating rubber elements to achieve specified spring rates. The specified spring rate depends on the expected loads and barge movements obtained by a nonlinear time-domain simulation.

Elastic pads are commonly used in the LMU units. Usually, the LMU comprises a mating cone, shock cells (elastomeric units), a receptacle, buffer rings and a sand jack. The mating cone is used for guiding the mating leg for insertion into the receptacle with a certain tolerance. The shock cells are used to reduce the vertical impact, due to heave, pitch and roll, while the buffer rings are used to alleviate the horizontal impact due to surge, sway and yaw. Finally, the sand jack is adapted to lower the integrated topsides into the final position after pulling out the installation barge, and to avoid steel-to-steel impact during mating.

The three major mating stages are shown in Fig. 5: 

Fig. 5. The LMU design with three different mating stages.

• The stabbing cone is engaged with the LMU receptacle, when zero load is transferred. At this stage, the top of the receptacle is about 470 mm above the topside of the outside leg casing.
• The stabbing cone is fully engaged with the LMU receptacle when 100% of the topsides load is transferred. At this stage, the top of the receptacle is about 195 mm above the topside of the outside leg casing with an elastic stroke of 27 5mm.
• The stabbing cone is fully in contact with the legs, when the sand jack is fully released. At this stage, the stabbing cone fully enters the LMU, and the deck leg and the jacket leg are fully in contact.

During the mating operation, the topsides structure is fixed by welding it to the in-situ base structure.

ANALYSIS/ RESULTS

The installation analyses were performed by using SIMO, a nonlinear time-domain program developed by Marintek as a part of DNV’s SESAM package. The nonlinear time-domain analyses were performed for several typical stages of the installation process, which covers three docking conditions, three mating conditions, and three undocking conditions. The float-over barge and the topsides are modeled as rigid bodies, with non-linear springs representing different fenders, LMUs, jacket legs and DSUs.

The hydrodynamic coefficients and wave forces used by SIMO are obtained from the recognized 3D radiation-diffraction program, WADAM, which is a part of the SESAM package. The barge’s hydrodynamic properties are obtained from the WADAM analyses of a panel model representing the geometry of the hull. In WADAM, the hull’s true shape is approximated by flat panels, by assuming a constant wave pressure over each individual panel. The added mass, damping and exciting forces and moments are calculated in the frequency domain for each specified wave direction, and stored for use in SIMO.

In SIMO, the hydrodynamic coefficients and wave excitation forces from WADAM are transformed to the time domain. The nonlinear stiffness from fenders, jacket legs and LMUs and DSUs are modeled as force-displacement relationships. Statistical extremes for the non-linear response were calculated by curve fitting to a Weibull distribution of the peaks. Main findings are as follows:

• Maximum fender load during the entire operation is about 200 t. 
• Maximum vertical stabbing cone motion during pre-mating was 0.185 m, thus yielding the required static clearance of 0.8 m. 
• Maximum horizontal stabbing cone motion is 0.51 m. The design capture radius is 0.34 m. This is not regarded as a problem, since jacket stiffness was included when the horizontal motions were calculated. The jacket leg will move together with the topsides. In addition, the wooden fenders were adopted to reduce deformation of the rubber fenders. Therefore, the LMUs’ horizontal motions are less than the capture radius.
• Maximum vertical dynamic load, excluding the static component at the most loaded LMU during load transfer, was about 445 t.
• Maximum load at the DSUs during load transfer is about 1,000 t. The sand dishes and other damping effects have been neglected in the simulation.
• Maximum vertical barge motion during post mating was 0.1 m, thus yielding the required static clearance of 0.30 m.

WEATHER WINDOW

Obviously, it was essential that exact tidal levels, sea conditions and barge motions were known during the float-over operation. The met-ocean data were used to support real-time wave and tidal measurements during the operation. The tidal data helped the installation master determine exact times for docking, mating and undocking. The real-time wave spectra data, including energy density, direction and spreading, were used throughout the float-over to distinguish swell periods and associated wave heights. This followed trends in the all-important swell conditions and helped to predict the weather window needed.

To allow the float-over barge with the topsides to dock into the jacket slot with sufficient vertical clearance to the jacket legs, a tidal constraint of about 1.58 m above LAT was imposed to fulfill the requirement for 0.8-m vertical clearance between the topsides stabbing cones and each jacket leg’s mating unit receptor. Once the topsides was in position, and the load transfer operation had started, there were no further tidal constraints on the operation.

Environmental design criteria for the docking and mating operation are given as follows:

The one-hour average mean wind is 10 m/sec at EL (+)10 m, and the surface current speed is 1.0 m/sec. In addition, the minimum visibility requirement during movement of any units involved in the operation should be 100 m. The operation is planned with sufficient lighting, so that no limitations are set on the sunlight. Conducting the mating operation during rainy weather should be avoided, so that dampness does not affect the sand jack.

Accurate weather forecasts were essential and provided by two independent, reliable forecasting sources. A meteorologist was on-site at the float-over location to assist the installation master. The float-over weather limit was moderate for May – significant wave heights of 0.5 to 1.0 m, wind speed of less than 10 m/sec, and no or very low, long-period swells. The weather window also had to extend for a minimum of 48 hr. The opening of the weather window was predicted at least three days in advance. Once opened, it persisted for the better part of 48 hr. With the help of the meteorologist, installation engineers could accelerate mooring, docking, mating and undocking procedures. This meant that the floatover could take place on an earlier tidal cycle, thus reducing overall operational time by some 12 hr.

The float-over can be divided into the following main steps:

• Connect barge to pre-deployed mooring system
• Barge and jacket preparations
• Pre-float-over operation
• Float-over operation
• Post-float-over operation

Connection to the pre-deployed mooring system, barge and jacket preparations, and pre-floatover operations are separate activities with well-defined positions/ conditions at the end of each activity – about 100-300 m from the pre-installed jacket. The float-over and post-float-over operations are also separate, in the sense that they have different weather condition constraints. However, during the float-over, the “point of no return” will be passed, hence the activity will not start unless a weather window is present.

LESSONS LEARNED

As this was COOEC’s first float-over application, there were a few lessons learned. Due to the low air gap in the areas of Bohai Bay, structural interference was a challenge to this float-over installation. A prior-to-float-over check revealed some structural interference that was corrected before float-over. However, there was still some structural interference spotted and fixed at the last minute:

• The handrails of the WHPB platform interfered with the underside piping of the sump pump deck.
• Due to miscommunication, two risers installed prior to float-over interfered with the surge fendering system. Its stopper bars had to be modified at the last minute to avoid interference.
• In addition, some sand jack valves were installed in the wrong places. This conflicted with the sway fendering system and had to be modified.
• The compression test was performed to help select the sand for sand jacks. However, actual pressure turned out to be too high. Too much sand was crushed, and it was difficult to release. A manual sand release was required.

CONCLUSIONS

The 7,500-t CEP topsides was successfully installed in a float-over in northern Bohai Bay. This float-over installation surmounted the structural interference that was due mainly to the low air gap. This successful application can benefit future float-over installations in shallow-water and benign environment areas, such as Bohai Bay. 

ACKNOWLEDGEMENT

The authors particularly thank Dr. Helge Johnsgard, Terje Muri, marine manager, and Thor Hevroy, general manager, of Aker Marine Contractors for their installation concept design, and Bureau d’Etudes for its expertise in the LMU design. The authors are also very grateful for the enthusiastic encouragement and invaluable support given by Zhang Yong, CNOOC project director, Li Ning, CNOOC senior V.P. of production, and Liu Huizhong, general manager of COOEC’s Installation Division.

BIBLIOGRAPHY

1. Chu, N., R. Newell, K. Mobbs, R. D’Souza, B. Greiner, I. Niven, and B. Surendran, “Adapting float-over installation of decks to floating platforms,” World Oil, July 2002.
2. Sigrist, J., P. Thomas and J. Naudin, “Experience in float-over integrated deck – Flexibility of the concept,” OTC paper 8618, Offshore Technology Conference, Houston, Texas, May 1998.

THE AUTHORS
	Liu Liming is V.P., Technology, China Offshore Oil Engineering Corp. (COOEC). He has engaged in design and analysis for offshore development and project management since 1975. He has earned a BS degree in architecture from Tianjin University; an MBA degree from Capital Economy & Trade University and a PhD in structural engineering from Tianjin University. He is also a nationally prominent expert on China’s High-Tech R&D 863 Program and an adjunct professor at Tianjin University.
	Zhang Songfu is V.P., production, at COOEC. He has worked as an offshore engineer, installation manager and project manager for over 35 years. He has extensive experience in offshore development and project management. Mr. Zhang earned a BS degree in ocean engineering from Tianjin University.
	Fang Xiaoming is chief engineer at COOEC. He joined the company in 1982 and has worked as field engineer, installation manager and project manager. He has more than 24 years of experience in engineering and installation of offshore platforms, pipelines and SPMs, etc. He has been certified as a National 1st Class Project Manager. Mr. Fang earned a BS degree in offshore engineering from Dalian University of Technology, and an MS degree in civil engineering from Zhejiang University.
	Chen Baojie is project manager for the Nan Bao 35-2 EPIC project. He joined COOEC in 1987 and has worked as a technical supervisor and project manager with over 19 years of experience in design and fabrication of offshore platforms. He earned a BS degree in material science from Tianjin University.
	Hao Jun is installation manager for the Nan Bao 35-2 EPIC Project and deputy manager of COOEC’s Installation Division. He has worked as an offshore engineer, technical supervisor and installation manager on many offshore projects for over 14 years. Mr. Hao has extensive experience and knowledge in offshore installation and project management. He earned a BS degree in marine engineering from Harbin Engineering University.
	Alan M. Wang is an installation engineer for the Nan Bao 35-2 EPIC project. He joined COOEC in 2004 and previously worked as a naval architect for KBR, Noble Denton and ABS. He has more than 13 years of experience in design and installation of offshore platforms and marine vessels. He earned a BS degree in naval architecture from Jiangsu University of Science & Technology; an MS degree in ocean engineering from the University of Hawaii at Manoa; and a PhD in naval architecture and marine engineering, .plus an MSE degree in mechanical engineering, from the University of Michigan.