Application of Offshore HDPE Pipes Route Design in North Maluku Indonesia

The lack of fresh water for the inhabitants of Maitara island is a very urgent problem to be solved. Two main factors at least must be taken into account to deliberate the right of way of subsea High-density polyethylene (HDPE) pipes, namely the hydrodynamic conditions and of a block analysis. This paper presents the study to justify the best route of subsea HDPE pipes based on hydrodynamic model analysis and concrete block strategy. The method used to analyze the best route includes 2 aspects. Firstly, the investigation method consisting of a bathymetric survey conducted by a single beam echosounder, 15 days tidal observations and seabed sediment sampling. Secondly, the hydrodynamic modelling analysis using Mike 21 FMHD and concrete block analysis, all these studies have been completed in August 2018. In the morphological behaviour analysis, three alternative routes are considered for the subsea HDPE pipes from Tidore Island to Maitara Island. The outcome of the analysis shows that the second track line option has the smallest impact by the hydrodynamic conditions, with a current speed of less than 0,5m/sec and a significant wave height of fewer than 1.2 meters. Furthermore, the uniformity of the lithology along the route is the other reason to select the second route. Finally, the concrete block analysis generated a minimum dimension of 75cm x 60cm x 30cm, and a free span of 3 meters is safe to absorb the uplift and drag forces acting on the pipe.


Introduction
Maitara Island is part of the Halmahera Islands see Figure 1A, and it is situated between Tidore and Ternate Islands and administratively is part of subdistrict North Tidore, Tidore Kepulauan Municipality, North Maluku Province Indonesia see Figure 1B. The area of Tidore Island spreads 2.04km2 (BPS, 2019) with a coastline of about 6,225km and the population reaches 2,183 (BPS 2019). The rainfall of Tidore Kepulauan Municipality is in the range of 1,503.5mm to 2,339mm (BPS, 2020). The topography of Maitara consists of hills with a slope of 2% to 15% so the inhabitant lives in the coastal area with more gentle slopes and close access to the beach. Geologically, Maitara Island is a volcanic island nonetheless the volcanic activity does not exist anymore throughout the region. Based on Geological Systematic Map (Apandi and Sudana, 1980) it could be found that the lithological conditions around the route plan consist of two types of rock unit formations, the Holocene volcanic units QHv and the Alluvium Qa coastal deposit. The lithology of the Holocene volcanic unit QHv could be found in the Maitara Island and consists of andesite breccia, andesite-basalt lava and tuff while the coastal deposits Qa are located in the Tidore Island and contain clays, silt, sand and gravel. The offshore area between Maitara and Tidore Island is still identified as the Maluku Sea within 570m to more than 1.2km width, see Figure 1C. The marine transportation in Tidore Island is supported by the Port of Rum, where the berth operating ratio (BOR) of speed boat jetties reach 75%, BOR of motorboats (small vessel) reaches 48% and the number of vessel visits reaches 47,052 (Sujarwanto, 2019) giving a good representation of the marine traffic in this area.
The issue of freshwater distribution has been investigated for few decades also in Europe (de Kok, 2002), Africa (Babah et al., 2012(Babah et al., , 2015, Asia (Mu et al., 2015) and America (Rocha, 2018) but most studies were carried out on land and deltas. It is still rare to find water distribution among the islands since there are not many countries that have a lot of islands such as Indonesia. Due to the catchment and hydrogeology limitation thus, Maitara Island in fact can not receive enough precipitation to fill its demand. Therefore, the idea of distributing the water from the outside island has appeared. Considering the marine environments, simplicity and the limitation of budget, the idea to transport fresh water from Tidore to Maitara Island without a buried system has emerged. However, the scarcity of offshore water distribution studies forced us to research the application of laying the offshore HDPE pipes. However, the hydrodynamic impact due to currents and waves and also the uplift force must be reduced to stabilize the pipes. Therefore, this study aims to identify the best route of an offshore HDPE considering the hydrodynamic forces acting on the pipe. To achieve this goal, the hydrodynamic analysis and the anchor block design will be conducted to illustrate the pipe laying application.

Acquisition data
Single-Beam Echo Sounder (SBES) Teledyne Odom Hydrographic EchoTrac Sounder CV100 was applied to acquire bathymetry data. The accuracy of the bathymetry reaches less than 10cm. The positioning and heading were used by GNSS Veripos LD 4 and processing software by EIVA Navi Suites software. The positioning accuracy was less than 10cm and the heading accuracy less than 1. Besides, 15 days of tidal measurement from 14 July to 29 July 2018 has been conducted applying Valeport Tide Master S/N58971 and manual observations to correct the bathymetry and determine the design water level. The instrumentations for tidal measurement were placed in two locations, Tidore Island (12723'5.25" E and 043'24.97" S) and Maitara Island (12722'44.6" E, 044'6.25" S). Sampling seafloor sediments have been performed by the grab method for 24 samples along the routes, see Figure 3. The grain size of sediment was defined according to the Unified Soil Classification System, American Standard Test Material ASTM D2487-06, 2006 to distinguish cohesive and non-cohesive sediments through megascopic in-situ analysis.

Hydrodynamic models and concrete block analysis
The loading on the pipe was assumed coming from both tidal current and significant wave effects for shallow and deep-water levels. Thus, the hydrodynamic analysis has been carried out using Mike 21 Flow model FM (DHI, 2012a) and Mike 21 Spectral Waves FM (DHI, 2012b). Forecasting the amplitude of current speed and wave height have been commonly used not only for the marine environment, such as in Novico et al. (2015) who gave evidence of the high amplitude of current speed in Sunda strait but also to analyze the marine energy in the straits such as in He et al. (2017), Atwater (2010). Furthermore, the wave and current forcing on the pipe body can be revealed such as Foda (1985),   Foda et al (1990), Carneiro (2013). Thus, these allhydrodynamic conditions will be elaborated within Mike 21 to screen the smallest current and wave impacting laterally on the considered pipeline routes. The simulation is preceded by the creation of the model domain such as in Figure 2, and applying the bathymetry and tidal force to generate the open boundaries.
Since the HDPE pipes will be laid on the seafloor, concrete blocks will be designed to sustain hydrodynamic forces according to literature methods as well as International Standards (Det Norske Veritas DNV, 1988;1993;2013;USACE, 2002). Besides, as an extreme design condition for the anchor block a flow current with 100 years return period is considered. No dataset of current velocity is available in this area, therefore the design flow velocity is derived by DNV (1993DNV ( , 2013, and Hale et al. (1989), where both the current and wave load can be multiplied by a safety factor of 1.1 for a shallow area to 1.4 for middle strait. Furthermore, the same return period of 100 years should be considered for the design wave. The lack of data brings us to calculate the design wave using USACE (2002), an empirical method based on the longest fetch from the installation site assuming a wind speed of 30 m.s -1 as a conservative approach.

Seafloor condition
Based on the survey campaign the morphology of the study area can be illustrated as shown in Figure 3. The shallow area situated in the middle with a water depth of less than 30m. The deepest area can be found in the north and the south of the study area reaching more than 50m. Concerning DNV (1993), this area could be classified from shallow to the deep sea. The Nearshore area at Maitara Island indicates the large distribution of the rocky bulge, coral reef and seagrass, which can be found in the southeast part of the island: red colour in Figure 3 symbolizes the shallow area where water depth is less than 4m. Based on grain size distribution analysis of 24 samples collected in-situ along the 3 alternative routes, it can be concluded that the seabed is covered by non-cohesive sediment (dominantly coarse sand) and reefal-rocky environment, as illustrated by the white circles along the routes in Figure 3. There are only 3 samples that contain cohesive sediment, namely greyish brown silt, discovered at the eastern part of the alternative 1 and 3 as represented by the yellow circles in Figure 3. Thus, due to the seafloor lithology, the second route is selected since it has a uniformity of grain size.

Pipe and route selection
The pipe category has been selected as HDPE PN20 within 110mm diameter. Design pipe density is 4.35 kg.m -3 with a minimum pipe thickness of 15.1mm. Three alternatives for the route have been evaluated by considering the length, morphology and seafloor condition, Figure 3. The lengths of the routes are, from North to South: 613m, 736m, 858m, as shown in Figure 4. Furthermore, the routes also consider the marine traffic around the area due to the presence of Rum Port in Tidore, consequently, a more distant route from the port is better to minimize the risks. Three cross-sections for each route can be seen in Figure 4, which illustrates the morphology along the routes. Furthermore, as highlighted by the vertical exaggeration in Figure 4, the first alternative has slopes in the range of 43.77% to 53.80%, the second route has slopes in the range 41.99% to 51.92%, the third route has slopes in the range 26.88% to 32.80%. This circumstance leads to choose the second route as the best route to be further calculated.

Tidal data
Two tidal stations have been constructed at Maitara and Tidore Islands and the data was collected for 15 days, see     (Thurman, 1994;Ross, 1995;Sumich, 1996). Value for the formzhal number 1.03 and 1.04 for Maitara station and Tidore station are found respectively. This means that the local tide can be classified as mixed semidiurnal, with two floods and two ebbs each day.

Hydrodynamic models
The simulation models have been implemented with the domain including Tidore and Maitara offshore area, Figure 3. Bathymetry field data were applied in the domain and the open boundary condition generated was the tidal fluctuation from Tide Model Driver (TMD) (Padman, 2005). Furthermore, the verification model as the foremost step to simulate the forecasting or hindcasting was conducted within the study. In this stage, the best simulation was identified and water elevation data were extracted then compared with the field observations data. The verification level of both    Furthermore, the selected simulation model will be used to analyze the hydrodynamic conditions along alternative routes. Both current speed and significant wave height are extracted laterally for each route to harvest the evidence of maximum hydrodynamic parameters. The highest current speed occurs at spring tide and reaches more than 1.5 m.s -1 at a location in the southern part of the study area, Figure 7. However, the current speed along the routes indicates values generally less than 1.5 m.s -1 , Table  2. Besides, the maximum significant wave height is defined at the he northern part of the simulation domain as 1.08m, Figure 7, while the highest value along the routes was less than 0.9m, Table 2.

HDPE pipe design
Pipe selection of HDPE PN20 has been considered to avoid corrosion, (Hunt et al., 2017), degradation, Da Costa et al. (2018) and contamination, Arias-Villamizar and Vázquez-Morillas (2018), and also the pipe availability. Based on hydrodynamic analysis results and selected type of HDPE PN20 pipe the safety of the proposed design can be analyzed. The stability against hydrodynamic loads, namely currents (Hale et al., 1989) and waves (Cheng et al., 2016) will be verified, including concrete block analysis.

Hydrodynamic analysis
Regarding DNV, 2013, for submarine pipeline systems standard the offshore current must be calculated in the design against a return period of at least 100 years. For this purpose, it should be conducted an extreme value analysis of offshore flow velocity by using at least 10 years of field data. Due to the lack of measurements, this parameter will be multiplied by a safety factor of 1.4 for narrow strait (middle part) and 1.1 according to common practice for design (DNV, 1993;1998;2013).
A similar analysis must be done for the waves since an extreme value analysis with 100 years return period must be considered. Since the data availability is not sufficient the wave height can be calculated by using USACE (2002) which is an empirical method base on the longest fetch from the site, assuming a wind speed of 30 m.s -1 as a conservative approach. Based on this method, the estimated wave height and wave period can be obtained. By applying 30 m.s -1 of wind speed and 6 km of fetch, Figure 8, the significant wave height can be defined as 1.8m and the period is 3 sec, Figure 9.

Offshore HDPE pipe specifications
Since an offshore HDPE pipe, PN20 will be used to distribute the freshwater, specifications for the pipe will refer to DNV (2013). HDPE pipes are in general resistant against environmental degradation, chemical impacts and abrasion-resistant, nonetheless, the HDPE pipe PN20 still must follow the standard of a minimum of pipe thickness, Table  4. Referring to the HDPE PN20 specification, Table 5 this pipe must fulfil the requirement of 12mm minimum thickness from DNV (2013) standard.

Force acting on pipes
The stability of the HDPE pipe can be analyzed by calculating both vertical and horizontal forces acting on the pipe. The inertia force (FI) and the drag The part of the pipeline/riser in the near platform (manned) area or in areas with frequent human activity. The extent of location class 2 should be based on appropriate risk analyses. If no such analyses are performed a minimum horizontal distance of 500 m shall be adopted. Minimum pipe thickness requirement Minimum 12 mm unless equivalent protection against accidental loads, other external loads and excessive corrosion are provided by other means. For diameters, less than 219 mm (8") minimum wall thickness can be less but shall be determined including the above considerations. Figure 10. Concrete block design for two HDPE PN20 pipes which has a dimension of 50cmx75cmx30cm.
force (FD) are working in a vertical direction and lifting forces (FL) works in a lateral direction. The lifting force can affect pipe stability since it will reduce the weight of the pipe. In this study, the force acting on the pipes was calculated by DNV classification, DNV (2017;2013;1988;1981). Furthermore, the on-bottom stability design of submarine pipelines is dealt with in much more detail on DNV (1988) and the hydrodynamic interaction is conducted using a Morrison formula (Trechet, 2004). Since the routes are located on shallow and deep water, so the hydrodynamic analysis takes into account both these conditions. In this case, deep water is defined by a water depth of more than 8 meters which correspond to the criteria of a ratio smaller than 0.5 between water depth and wavelength. The results of the hydrodynamic calculations are described in Table 6  and Table 7.

Concrete block design
The concrete block will be constructed to eliminate the hydrodynamic forces acting on the pipes. Therefore, the design standard such as DNV, 1988; 2017 will be applied to calculate the length of the span between the blocks and a sufficient total weight of the concrete block. The geometric design of the blocks is proposed in Figure 10. This design illustrates the double pipes within the concrete block and locked by bolts for each side. Based on Table 6 and Table 7 thus the weight of concrete block design (Ws) is such that it must compensate the distributed forces acting on pipe. For that reason, the design of   (2017). Furthermore, that reference is used to calculate the free span between the concrete blocks. Due to the safety and small dynamics amplification of the subsea pipe, the ratio between free span and pipe diameter must be less than 30 (L/D <30). Since the pipe diameter of HDPE PN20 is 110 mm, the maximum free span allowed is 3.3 meters, and in this case, a span of 3 meters will be applied, and the concrete block design can be considered as shown in Table 8 and Table 9.

Conclusion
According to this study, the lateral and vertical forces of seawater acting on the HDPE PN20 subsea pipe especially the second route can be sustained due to the on-bottom stability of the subsea pipe. Besides, the freshwater fluctuation within the pipe also can be predicted and calculated to eliminate the force working in the pipe. The analysis of results from the model simulation confirmed that the hydrodynamic forces along the pipe do not occur equally along the pipe route. The higher magnitude of the tidal current and the significant wave high arises in the eastern part near Tidore Island where the water is deeper, with the lithology shows a softer seabed than in the western part. Nevertheless, the free span between concrete blocks and the concrete block weight itself has been shown to stabilize pipes against acting forces along the pipe. Therefore, these results constitute significant evidence to be considered as a model within the application design of the offshore pipe submission.