THE TRANSFER OF LNG IN OFFSHORE CONDITIONS. SAME SONG NEW SOUND

Transcription

1 THE TRANSFER OF LNG IN OFFSHORE CONDITIONS. SAME SONG NEW SOUND Leendert Poldervaart Business Development Manager James Ellis Business Development Manager Single Buoy Moorings Gas Business Development Group ABSTRACT This poster paper presents some of the key enabling technologies in the transfer of LNG in an offshore environment. Most of these technologies are known from the crude oil industry and adapted for the LNG industry. The main emphasis in this poster paper is placed on the: a) mooring systems; b) toroidal LNG swivel; c) vessel to vessel LNG offloading; d) vessel to terminal LNG offloading. The following abbreviations are used in the text: CALM...Catenary Anchor Leg Mooring system; FPSO...Floating Production, Storage and Offloading unit; FRU...Floating Re-gasification Unit; FSRU...Floating Storage and Re-gasification Unit; LNGFPSO...LNG Floating Production, Storage and Offloading unit; LNG FSO...LNG Floating Storage and Offloading unit; SBM...Single Buoy Moorings; SPM...Single Point Mooring; SQM...Soft Quay Mooring system; SYMO...Soft Yoke Mooring and Offloading unit; TSQM...Twin Soft Quay Mooring system; TSPM...Tower Single Point Mooring system PO-28.1

2 INTRODUCTION The ability of the offshore industry to find new solutions from old solutions remains one of its most impressive characteristics. Like certain animals, which continually sheds their skin to reveal a new shiny coat, the offshore industry continues to show an uncanny knack of being able to take an existing concept and revamp it, helping it to continually develop it into a new, improved version of its predecessors. In the world of LNG this is perhaps more true than with most industries. During the early eighties the attractiveness of LNG import and export terminals, i.e. the FSRU s and LNG FPSO units, was identified. However, only after the successful installation and proven track record of crude oil import/export terminals, LPG terminals and FPSO's, the LNG industry is now looking with great interest into these offshore terminals. In general, the LNG export or import terminals are used to moor LNGC s alongside piers or jetties to export or import LNG from/to onshore liquefaction plants. When there are no existing re-gasification plants FSRU s can be used to import gas from LNGC s. The FSRU is moored near shore, receives LNG from an LNGC, stores and re-gasifies the LNG on demand and transfers the gas to shore via a gas pipe line. The LNG FPSO can either be part of an oil development in which it liquefies associated gas or a stand alone field development of remote green gas fields. In the associated gas case, the LNG FPSO also makes remote oil developments feasible on environmental grounds, by turning the problem of associated gas disposal into a moneymaking opportunity. In the green gas field case, the LNG FPSO is an economical tool to making remote offshore gas fields economical. Like the FPSO's, the key to attractiveness of a green field LNG FPSO lies in the integration of well control, processing, storage and off-loading from the LNG FPSO, positioned over, or very near, the reservoir. It replaces the offshore platform, pipeline, liquefaction plant, storage and export system. This produces a significant opportunity to reduce cost and project time. The topsides and the vessel can be built and commissioned in a yard under controlled conditions using highly experienced staff. Once installed on site the LNG FPSO, like FPSO s, can be operated with trained personnel. Additional advantages are that after depletion of a reservoir, the barge may be moved to a different field, and that at the end of its service life, the barge can be economically decommissioned. Technically, the LNG FPSO s or FSRU s are similar to FPSO s, They consists of a basic hull with accommodations, an SPM turret mooring system, storage tanks, LNG processing or re-gasification equipment and offloading facilities. In the LNG FPSO case, the incoming gas is liquefied after pre-conditioning, and the LNG is stored at ambient pressure in thermally insulated tanks onboard the LNG FPSO. When sufficient LNG is accumulated in the storage tanks, it is offloaded to an LNGC and shipped to the customer. Similarly any produced condensates are stabilized and stored onboard to wait for offloading. The technical challenges are the marinisation of the process plant and the offloading of LNG at temperatures below -162 degrees Centigrade. PO-28.2

3 As previously mentioned, the idea of LNG import /export terminals, FSRU and LNG FPSO is not new. The main reasons for this renewed interest in LNG terminals are the: 1. Extensive experience gained with oil terminals; 2. Extensive experience and the confidence gained with oil Floating Production Storage and Offloading units (FPSOs) 3. Technology advances made over the past years and recent installed Liquefied Petroleum Gas (LPG) FPSO'S with relatively complex topsides, storage and offloading at minus 45 degrees centigrade. The LNG technology itself has become mature, and process plants and LNGC s have proven to be capable to operate in excess of their initial 20 years design life while achieving an excellent safety record. In general, extensive operational experience has been gained on terminals, FPSO operations and all aspects of the LNG production, shipping and marketing. Today, all major oil and gas production companies are studying LNG import/export terminals, FSRU s and LNG FPSO s. The FPSO mooring systems Single Buoy Moorings (SBM) a member of the SBM Offshore Group has been in the business of the design, supply and operation of Single Point Mooring (SPM) systems and Floating Production Storage and Offloading (FPSO) systems for over 30 years. SBM has also been heavily involved in the design, engineering and supply of key components (swivels) for the transfer of high pressure gas for gas injection, gas lift and for transport to/from offshore platforms or to shore. One of the first products supplied by SBM was the mooring and gas transfer system on the ARDJUNA SAKTI concrete barge for LPG refrigeration and storage in 1978 (see photograph 1). In addition to SBM's involvement in many other projects, SBM owns and operates FSO s, FPSO s an LPG FSO installed offshore CONGO and the SANHA LPG FPSO (see photograph 2) installed offshore Angola. Totally, SBM owns 22 FSO's and FPSO'S and are heavily involved in the offloading to shuttle tankers from the units SBM operates. Most of the recent FSO's and FPSO'S have been fitted with turret mooring systems. However, some of the early FSO's or FPSO's used a Rigid Arm between a standard type Catenary Anchor Leg Mooring (CALM) system with a Flexible pipe type of fluid transfer system, as shown for the ARDJUNA SAKTI FSO. As high pressure gas flexible lines are now well proven in the industry, a move towards turret type mooring systems has generally occurred except for shallow waters which usually precludes the use of catenary mooring systems due to limited chain restoring capabilities in these water depths. The turret mooring systems are composed of a rotating turret column held by an internal or external vessel s structure via a roller bearing arrangement. The vessel bound components can therefore weather-vane around the turret, which is fixed with respect to the seabed. This arrangement allows the FPSO to adopt the direction off lease resistance against waves, wind and current. A toroidal fluid swivel on top of the turret allows for the transfer of fluids across the rotating interface while the FPSO is weathervaning. The PO-28.3

4 turret column is secured to the seabed by catenary mooring legs to provide sufficient restoring to maintain the vessel within a design excursion limit. This excursion limit ensures the working of the underwater fluid transfer or riser system from sea floor to turret. Turrets concepts come in a number of different design lay outs but all use a number of catenary or in the deeper water (semi) taught mooring lines. The family of turrets includes those designed to permanently moor the vessel in 100 year storm condition, such as: 1. Simple external turret for West African conditions (see photograph 3); 2. Clamped type bow turrets; 3. Internal turrets for the more harsh environments Offshore West of Shetlands and in the Central North Sea; 4. Internal turrets for a large number of risers. For the very shallow waters, soft yoke mooring systems are frequently used as catenary anchor legs can not provide sufficient restoring. These soft yokes can be connected directly to a jacket structure fixed to the seabed, as in the case of the Shell s EA FPSO offshore Nigeria (see photograph 4). Several of these mooring systems are currently in operation. The Soft yoke system consist of a rigid mooring yoke, connected at the jacket end by a weathervaning, roll and pitch tolerant articulation joint, and at the vessel end by a large gravity counterweight held by two vertical mooring legs attached to a mooring structure mounted on the bow or stern of the vessel. When the vessel moves out of its equilibrium position due to wind, wave or current action, the weights are lifted by the two vessel mooring legs and thus creating a restoring force. The deployment of FPSO'S in certain tropical storm areas or in fields where winter ice may form has required the development of disconnectable mooring systems. These systems will enable the F(P)SO vessel to sail away from the site without external assistance, leaving the mooring system behind. Several of these FPSO systems are now in operation since the mid 1980 s. Disconnectable systems have the added advantage that the FPSO can be sailed to a shipyard for upgrade or routine inspections and statutory surveys. The LNG toroidal swivel development Al1 free weathervaning mooring systems require the transfer of one or more fluids across the interface between the "rotating" and fixed part of the FPSO. This is done via a fluid swivel which is therefore a key component in the FPSO system. Generally a typical multi-path swivel design consist of an inner fixed part and an outer rotating part which encloses toroidal shaped passage ways between the two parts. The inner and outer ring concentricity is maintained by a bearing system. Swivel stacks are, in principle, tailor made for each FPSO as requirements regarding the number of lines, path diameters, pressures and temperatures differ per application (see figure 1). The swivel stack can be arranged for handling different crude oil, water, gas, hydraulics and electricity. Each of these media is transferred through independent swivel modules, which are arranged together to form a vertical swivel stack. PO-28.4

5 Extensive research and development, full scale tests and experience have led to the availability of reliable sealing systems for all types of fluid paths. For gas swivels it is imperative to have an absolute sealing for safety reasons. The gas sealing principle is based on the use of an over pressurized back-up fluid which serves as a barrier between the sealed gas and the environment. This type of sealing arrangement has been successfully installed in several swivels up to 6000 psi and for temperatures up to 120 degrees C. During the last few years, SBM has successfully designed, fabricated and tested a 20 cryogenic LNG in-line and toroidal swivel. This success has already been demonstrated to the industry during a recent workshop and both the in-line and toroidal cryogenic LNG swivels are now ready for application. Photograph 5 shows the LNG toriodal swivel during assembly. Vessel to vessel LNG offloading The safe reliable transfer of LNG in an open sea, possibly harsh, environment has never been done. Based on the experience gained from the existing FPSO tandem and side-by-side mooring and offloading, SBM has performed studies to demonstrate, and confirmed by model tests, that open sea LNG transfer is possible today in a side-by-side mooring configuration. SBM has designed side-by-side mooring systems, like the enhanced side-by- side mooring (figure 2) and the Soft Quay Mooring system, to moor a standard trading LNGC in a side-by-side configuration in seas up to around 3.0 m significant (see figure 3). An additional benefit with the SQM mooring is that the distance between two vessels is increased for safety reasons and that the SQM provides a better fender load distribution and thus spreading of this load along LNGC hull. The fluid transfer system would use LNG loading arm structures, however, hoses might also be feasible in the future. In addition, SBM has developed in-house availability tools to calculate the side-byside mooring availability based on model tests performed. Details can be found in OTC paper 18026PP of SBM also designed a tandem mooring system using a rotating boom structure with aerial hoses (see picture 4). In this case the LNGC needs a bow manifold and bow mooring arrangement. Vessel bow loading and mooring arrangements are already used extensively in the crude oil industry. To moor a non-dedicated LNGC in harsh environments, SBM designed a Soft Yoke Mooring and Offloading (SYMO) unit. In this case the LNGC can remain moored in seas up to 5,5 m significant (see figure 10). Vessel to terminal LNG offloading SBM has performed studies to demonstrate, and has confirmed by model tests, that open sea LNG transfer is possible today between a standard trading LNGC and offshore LNG import and export terminals. SBM is working on terminals using a weathervaning LNGC and a fixed heading LNGC. The former is based on the use of floating LNG hoses. These are being developed by several companies but not yet ready for proposal. The latter type of terminal could be the Soft Quay Mooring (SQM) which is using standard LNG loading arms and is described here below. PO-28.5

6 Definitions of the systems designed by SBM 1) The FSRU A new built FSRU has been described in the text and shown in figure 2. The Unit will be used mainly as a base load terminal to supply the gas to e.g. the grid or other end user. The converted FSRU vessel might be used for isolated consumers and is shown in figure 5. 2) The LNGFPSO As described in previous text. 3) The FRU The FRU has been designed to re-gas the LNG from a side-by-side moored LNGC and to send the gas into a salt cavern storage facility or directly to shore without intermediate storage. 4) The Gas Link Barge The Gas Link Barge is shown in figure 6 and is specially developed by SBM to enable LNG supply to small and isolated markets. It allows energy users currently running on diesel or fuel oil to use cheaper and cleaner LNG without the need of large upfront investment. The system consists of a barge with re-gasification equipment and the LNG is stored in a separate LNGC. 5) The LNG FSO This LNG FSO unit is shown in figure 7. The LNG FSO unit could be used as an: a) LNG storage and export terminal to reduce the onshore LNG storage facilities and to reduce the LNG export pumping capacity; b) As a LNG hub centrally located in an area where LNG receiving facilities only require small capacity LNG supplies (see point 4 above). 6) The SQM The SQM with and without LNGC moored is shown in figure 8. The SQM systems main objective is to perform safe LNGC berthing, mooring and LNG offloading in a more harsh environment for non-dedicated, fully standard LNGC s. The SQM imitates a conventional jetty terminal to cater for a standard approach and berthing and the substantial energy absorption characteristics in the berth itself improves overall safety in a more dynamic environment. The SQM system could be used as a side-byside mooring configuration (see figure 3) or supported from individual platforms as per figure 8. Re-gas facilities, in case of an import terminal, or reliquefaction facilities, in case of an export terminal, could be installed on the central platform (see figure 9). Model tests have demonstrated that mooring can be done in sea-states between 2.5 to 3.0 m significant. 7) The TSQM The TSQM is shown in figure 9 and has the advantages that: a) The LNGC can select the preferred LNG loading side; b) There is less waiting time to moor the second vessel; PO-28.6

7 c) Two carriers can be loaded simultaneously; d) If weather permits, the TSQM could be used as a base load import facility; e) A small LNG storage tank could be inserted in the central platform. 8) The TSPM The TSPM has been designed as a free weathervaning LNG import /export terminal and is similar to the crude oil terminals. Designs are being finalized based on LNG aerial and floating hose configurations. The rotating boom structure of the TSPM could also be installed at the stern of a LNG FPSO, FSRU etc and is shown in figure 4. This rotating boom structure will be rotated onto the FSRU when not in use. The LNGC needs a bow manifold in case aerial hoses are used. 9) The SYMO The SYMO has been designed for harsh environments LNG loading or offloading using dedicated LNGCs. The SYMO can be used on a tower structure or at the stern of a FSRU or FLNG. The SYMO in figure 10 shows a re-gas facility on the tower structure. Figure 1 Typical high pressure swivel design PO-28.7

8 Figure 2 An internal turret moored new built FSRU with side-by-side moored LNGC. Figure 3 The internal turret moored FSRU with the SQM PO-28.8

9 Figure 4 Tandem mooring using LNG aerial hoses Figure 5 An external turret moored converted vessel FSRU PO-28.9

10 Figure 6 The spread moored Gas Link Barge. Figure 7 A tower soft yoke moored LNG FSO PO-28.10

11 Figure 8 The SQM with and without LNGC moored Figure 9 The TSQM with re-gas facilities on the central platform PO-28.11

12 Figure 10 The SYMO structure as an import terminal with re-gas facilities on the tower. Photograph 1 PO-28.12

13 Photograph 2 Photograph 3 The external turret mooring system PO-28.13

14 Photograph 4 The tower soft yoke mooring system Photograph 5 PO-28.14